53


Electronic, structural and optical properties of Gd-doped ZnO powder synthesized 
by solid-state reaction method

M. K. Gora1*, A. Kumar1, B. L. Choudhary2, S. N. Dolia1, S. Kumar1 and R. K. Singhal1

1Department of Physics, University of Rajasthan, Jaipur-302004, Rajasthan, India
2Department of Physics, BansathaliVidhyapith, Banasthali-304022, Rajasthan, India

Abstract

We explored the impact of Gd doping on the structural, electronic and optical characteristics of the 
ZnO powder. The Gd-doped ZnO (0, 2% and 5%) powder samples have been synthesized using the 
conventional solid-state reaction process with varied Gd concentrations. The XRD pattern 
confirmed that all the studied samples are in the hexagonal wurtzite crystalline structure. The 
morphology has been explored using SEM images, which exhibited an agglomerated rod-like 
particle structure. The XPS results indicate the presence of oxygen vacancies (Vo) in the Gd-doped 
ZnO samples and the Vo’s are found to increase with increasing Gd amount. According to PL 
findings, the intensity ratio of the green and ultra-violet emission peaks is found to increase from 
0.090 to 0.418 with increasing Gd-doping concentrations, confirming that Vo’s are increasing with 
Gd-doping. The UV-visible spectroscopy results reveal that the energy band gap (Eg) decreased from 
3.31 eV to 3.23 eV with increasing Gd-doping concentration. 

Keywords: Polycrystalline; Rietveld refinement; Wurtzite structure; Oxygen vacancies; 
XPS; PL

*Corresponding author e-mail: gora.phy@gmail.com

Available online at www.banglajol.info

Bangladesh J. Sci. Ind. Res. 58(1), 53-64, 2023

Introduction

Diluted magnetic semiconductor materials are used in 
optoelectronic and magneto-electronic devices that 
involve electron charge and spin (Thangeeswari et al. 
2015). When a traditional semiconductor like ZnO is 
doped with transition metal (TM) ions or rare-earth (RE) 
metals, the DMS material could realize. Numerous appli-
cations in the spintronics of zinc oxide have piqued the 
curiosity of researchers. The wide band gap energy of the 
ZnO is 3.37 eV and the exciton binding energy is 60 m eV, 
making it perfect for use in light-emitting diodes 
(Shimogaki et al. 2015; Thangeeswari et al. 2015). The 
magnetic, optical, and electrical characteristics of ZnO 
could be modified by the doping of TM ions and RE 
elements, which depend on the dopants' nature, quantity, 
and synthesis process (Deshmukh et al. 2010). In particu-
lar, ZnO is receiving much attention as a potential material 

for UV and visible light emitters (Ozgür et al. 2005; Fan 
et al. 2013). The visible emission of ZnO is because of 
several bands associated with various lattice flaws. 
Consequently, defect-related emission in ZnO displays a 
wide unstructured band spanning the blue to red spectral 
regions. Because of this uniqueness, ZnO is a promising 
phosphor for white-light emitters with a high colour 
rendering index (Fan et al. 2013). ZnO has unique 
characteristics like affordability, long-term stability, an 
environmentally friendly nature, better radiation resis-
tance, etc. Additionally, ZnO is an excellent material for 
several photonic applications when doped with a suitable 
dopant (Bahadur et al. 2007). Doping is a valuable 
method for modifying the optical characteristics of ZnO 
due to its ability to exhibit many absorption transitions in 
the visible region.

RE elements, such as Gd, may introduce additional capabili-
ties to the ZnO system because they may be capable of 
constructing combined practical devices on a single chip by 
combining magnetic and optical properties (Ohno et al. 1996; 
Tawil et al. 2011). 

Rare-earth (RE) metal doping in the dilute magnetic 
semiconductor (DMS) systems has recently received 
immense attention due to its potential usage in magneto 
electronic and photonic devices (Monteiro et al. 2006). The 
influence of Gd doping on the optical and electrical proper-
ties of ZnO is crucial for developing optoelectronic devices 
and understanding the genesis of ferromagnetism. Ma and 
Wang (2012) synthesized the Gd-doped ZnO nanocrystals 
using the thermal evaporation deposition method. According 
to their findings, Gd doping has an appreciable impact on the 
optical characteristics of the ZnO. The strong UV and blue 
emissions are detected at low Gd concentrations because Gd 
impurities introduce a mid-gap state into ZnO. In compari-
son, the UV and blue emissions reduce for high doping 
concentrations as a significant broad defect emission appears 
because of the vast numbers of defects, impurities, and 
excess Gd2O3 produced. Gd-doped ZnO nanocrystals can be 
used in optoelectronic devices. The optical and electrical 
tuning of the carrier density and optical band gap energy is 
essential for developing luminescent materials and equip-
ment based on the ZnO DMS systems. The optical band gap 
and carrier concentration are essential parameters to yield 
information about the emission wavelength. Several develop-
ment characteristics, like the synthesis methods, impurity 
concentration and substrate variety have been reported to 
influence these parameters (Wakano et al. 2001; Tan et al. 
2011; Murtaza et al.  2011).

The synthesis of DMS materials with unique optical, photo-
catalytic and magnetic characteristics seems to be the most 
promising candidate for the subsequent generation of 
electronic, optoelectronic and spintronic appliances (Kumar  
and Sahare 2014).  The low-dimension materials with imper-
fection-free lattice structures have excellent electrical and 
magnetic characteristics; purposefully made or accidental 
structural defects play a significant role in altering their 
features (Li et al. 2016; Obeid et al. 2018). Thus, the presence 
of different kinds of cation or anion vacancies and intersti-
tials in ZnO nanocrystals may change their magnetic proper-
ties, photoluminescence, and photocatalytic properties (Coey 
et al. 2005). Although certain experimental and theoretical 
findings based on DFT indicate that Gd-doped ZnO exhibits 
the room temperature ferromagnetism (RTFM) (Potzger et 
al. 2006; Aravindh et al. 2014) property, several studies have 
demonstrated that the RTFM is not present (Sambasivam et 
al. 2015). In addition to their magnetic qualities, the 4f ions 

ETH 30 kV EVO-18 Carl Zeiss, with a tilt of 0° to 60° and a 
rotation of 360°, has been used for taking SEM images.The 
defect-related states and chemical analysis of the samples 
have been determined using X-ray photoelectron spectrosco-
py. A hemispherical analyzer with a 128-channel detector and 
a monochromated, fully integrated K-alpha small-spot X-ray 
photoelectron spectrometer system of 180° double focusing 
is used in XPS (Thermo Scientific made). The UV-visible 
absorption spectroscopy (Model: Shimadzu UV-3600 
UV-VIS-NIR Spectrophotometer) was used to explore the 
prepared samples' optical absorption and band gap.
The measurements of UV-visible absorption spectra have 
been taken using the powder form of the samples. PL 
spectroscopy was used to analyze defect-related states in the 
prepared samples. Photoluminescence spectra have been 
recorded at room temperature using the fluorescence 
spectrometer (Model: Perkin Elmer FL 8500). 

Results and discussion 

XRD analysis

Fig. 1. depicts the powder XRD patterns of the Gd-doped 
ZnO (Gd = 0%, 2%, and 5%) samples. All the diffraction 
peaks detected in the ZnO sample match with the standard 
JCPDS card number (36-1451). This confirms a hexagonal 
wurtzite crystal structure of the samples.

The diffraction peaks were observed at 31.88°, 34.50°, 
36.25°, 47.72°, 56.69°, 63.03°, 66.59°, 68.17°, and 69.35°, 
which are related to the planes (100), (002), (101), (102), 

(110), (103), (200), (112), and (201), respectively (Ganesh et 
al. 2018; Bharathi et al. 2020). Further, some additional 
small peaks have been observed at 28.61°, 33.21°, and 39.14° 
with increasing Gd doping. These secondary peaks represent-
ing a crystallite phase have been identified as Gd2O3, which 
is well consistent with earlier reports (Das et al. 2012). As 
shown in Fig. 2, the highest intense peak related to the (101) 
plane shifted slightly higher angles with the increasing 
concentration of Gd doping. The shift is because of the 
substitution of bigger-sized Gd3+ (0.94 Å) ions than Zn2+ 
(0.74 Å) ions in the hexagonal wurtzite structure.  The peak 
shift may indicate that Gd3+ ions have occupied the crystallo-
graphic positions of Zn2+ ions in the ZnO host lattice, or 
itmay be because of the structural strain caused by the forma-
tion of internal compressive micro stress (Dakhel and El-Hilo 
2010; Kumar et al. 2014; Sahu et al. 2017). 

Rietveld refinements have been performed using the Full Prof 
Programme (shown in Fig. 3). The Pseudo-Voigt function has 
been used for Rietveld refinement of all the samples with the 
space group P63mc. The estimated lattice parameters have 
been listed in Table I. The lattice parameters are found to 
decrease with Gd doping, similar to those reported in earlier 
studies (Kaur et al. 2016; Obeid et al. 2019). The factors like 
the induced deficiencies (vacancies, interstitial) and the ionic 
radius of Zn and Gd ions can be responsible for the observed 
change (Kaur et al. 2016; Kumar and Thangavel 2017).

SEM analysis

Scanning electron micrographs have been used to analyze 
the morphology of the Gd-doped ZnO powder samples. The 
SEM images reveal a rod-like morphology in pure ZnO 
(Gora  et al. 2022) and the Gd-doped ZnO polycrystalline 

samples with some proof of agglomeration and phase segre-
gation, as depicted in Fig. 4 (a, b and c). Profound observa-
tion of the particles exhibits that the crystallite size is above 
100 nm. The particles were intensive and incompatibly 
distributed all over the mass. A distinct boundary between 
neighbouring crystallites can be noticed, despite the proxim-
ity of these smaller crystallites.

XPS analysis

The XPS technique is an excellent tool for determining the 
surface chemistry and electron configuration of the 
various elements present in a multicomponent material. 
The carbon C1s peak (284.6 eV) has been used as a refer-

ence to calibrate all the binding energies of the spectra. 
The survey spectrum of the Gd-doped ZnO samples is 
shown in Fig. 5. The survey spectra revealed the existence 
of O, Zn, and Gd elements without any impurity in the 
samples.

For ZnO and 2% Gd-doped ZnO powder samples, the binding 
energies of the Zn2p3/2 and Zn2p1/2 peaks are found to be 
1021.25 eV, 1044.25 eV, 1021.43 eV, and 1044.43 eV, respec-
tively. However, for the 5% Gd-doped ZnO sample, these 
peaks are at 1021.83 and 1044.83 eV energy positions. The 
small shift could be assigned to the creation of Zn interstitials 
in ZnO upon Gd doping. The binding energy gap between the 
zinc doublet peaks has been found at about 23 eV. This 
indicates that Zn in the ZnO crystal structure is in the Zn2+ 
oxidation state (Khataee et al. 2015; Gawai et al. 2019) as the 
spin-orbital splitting of Zn2p is 23 eV. This difference does 
not change with the Gd doping pointing out that Gd doping 
has no discernible effect on the chemical position of ZnO.

The O1s asymmetric XPS spectraare shown in Fig.7 (a) 
which is asymmetric in shape. The O1s XPS spectra of all 
these samples i.e. the ZnO, the Zn0.98Gd0.02O, and the 
Zn0.95Gd0.05O have been deconvoluted mainly into two Gauss-
ian peaks, as shown in Fig. 7 (b, c, and d). The lower binding 
peaks for pure and Gd-doped ZnO (Gd = 2% and 5%) 
polycrystalline samples have been detected at 529.93 eV, 
529.93 eV and 529.78 eV, respectively, which attributed to 
the lattice oxygen in the hexagonal structure of the ZnO.

The higher binding peak for the pure and the Gd-doped ZnO 
samples have been found at 531.51 eV, 531.66 eV, and 
531.49 eV, respectively, which is due to the presence of 
oxygen vacancies in the ZnO structure (Bharathi et al. 2020; 
Sukumaran et al. 2021). An additional signal centered at 
528.29 eV has been detected in the 5% Gd-doped ZnO 
powder sample, which could be ascribed to the coordination 
of oxygen in Gd-O-Zn along grain boundaries because of the 
excessive Gd dopants and also attributed to the presence of 

Gd2O3 in the sample (Yi  et al.  2017). The lattice oxygen peak 
has slightly shifted to the lower binding energy with increas-
ing Gd concentration, which has been due to the increased 
oxygen vacancies with increasing Gd doping. The estimated 
area ratios of the oxygen vacancies peak and the lattice oxygen 
peak for the samples ZnO, Zn0.98Gd0.02O, and Zn0.95Gd0.05O 
comes out to be 0.53, 1.30, and 2.49, respectively. These 
results show that the oxygen vacancies increase with increas-
ing Gd concentration.

The Fig. 8 (a, b) depicts the high-resolution Gd4d XPS 
spectra of the Gd-doped samples.In the Gd4d state, the 
spin-orbit splatted peaks are at 144.6 eV for Gd4d3/2 and 
139.6 eV for Gd4d5/2 for the 2% Gd-doped sample, whereas 
for the 5% Gd-doped ZnO sample, the spin-orbit splits at 
145.5 eV for Gd4d3/2 and 139.7 eV for Gd4d5/2 as shown in 
Fig. 8 (a, b). This indicates that Gd is present in the Gd3+ state 
in the ZnO hexagonal structure. The shift in peak values of 
Gd4d3/2 and Gd4d5/2 from 144.6 eV and 139.6 eV, respective-
ly; in the case of 2% Gd-doped sample to 145.5 eV and 139.7 
eV, respectively, in the case of 5% Gd-doped sample. This 
shift in Gd4d peaks is ascribed to the electron transfer from 
plasmonics Gd to ZnO because of the strong electronic 
interaction (covalent bond) between Gd and the oxide 
(Ahmad et al. 2011; Wang et al. 2012; Bharathi et al. 2020).

UV visible spectroscopy analysis

The UV–Visible spectroscopy is a powerful tool for estimat-
ing band gap of materials. UV–Vis absorbance spectra and 
tauc plots for the Gd-doped ZnO samples (Gd = 0% (Gora et 
al. 2022), 2%, and 5%) samples are shown in Figs.9 (a, b, c, 
and d). The spectra show a red shift with increasing Gd 
amount in the ZnO.The UV–Vis absorption intensity of the 
synthesized polycrystalline samples is found to reduce with 
the increasing Gd concentration (Mazhdi and Tafreshi 2018). 
Tauc's plot has been used to calculate the direct band gap of 
the samples (Jain et al. 2006). The direct band gap has been 
estimated using the following equation:     

Here, α is the absorption coefficient,υ is frequency, h is the 
Planck constant, A is a constant, and Eg is the energy band 
gap of the substance.

Extrapolating the linear component of the curve plotted 
among (αhυ) 2 and hυ depicted in Fig. 9 (b, c, and d) has been 
used to obtain the band gap (Farhad et al. 2018; Ghos et al. 
2021). The band gap values calculated for the Gd-doped ZnO 
samples, i.e.are 3.31 eV (Gora et al. 2022), 3.27 eV, and 
3.23eV, for the samples Gd = 0%, 2% and 5%, respectively. 
The small redshift observed in the band gap of Gd-doped 
ZnO samples is because of the changes in electronic structure 
as well as enhanced oxygen vacancies with the increasing Gd 
doping (Yi et al. 2017; Flemban et al. 2016), which is respon-
sible for the band gap reduction. 

Photoluminescence spectroscopy analysis

The structural flaws like ionic and atomic vacancies, replace-
ments and interstitials have been explored using the PL 

spectra of the synthesized polycrystalline samples. Fig. 10 (a, 
b and c) shows the deconvoluted, Gaussian-fitted PL spectra 
of the synthesized Gd-doped ZnO polycrystalline powder 
samples.

The PL spectra of the samples were recorded at room 
temperature in order to determine the types of vacancies, 
defect band emission etc. (Lin et al. 2001; Yi et al. 2017). The 
photoluminescence spectra have been deconvoluted and the 
different peaks responsible for the emission have been 

Gaussian fitted. For these samples the band edge emission 
peaksare detected at 381 nm, 384 nm, and 383 nm, for the Gd 
= 0%, 2% and 5% samples, respectively. The UV emission is 
responsible for the excitonic interaction related to the band 
edge emission. The intense violet emission exhibited in the 
Gd-doped ZnO at about 415 nm could be because ofthe 
oxygen vacancies. The exciton interaction between the holes 
in the valence band and electrons located in the interstitial 
zinc could be responsible for the violet emission peak at 
about 447 nm. Intrinsic deficiencies like interstitial zinc and 

oxygen can be attributed to the peak at 477 nm (Singh et al.  
2009; Kaur et al. 2016). The transformation between singly 
ionized oxygen deficiency and the photo-created holes (Ken-
nedy et al. 2014) or the surface imperfection and electrons 
near the conduction band, might be responsible for the green 
emission peak seen for the Gd-doped ZnOat around 525 nm. 
The green emissions are commonly attributed to theoxygen 
vacancies (Studenikin et al. 1998). For Gd-doped ZnO 
polycrystalline powder samples, the intensity ratio of the 
green and ultra-violet emission peaks is found to be 0.090 for 
ZnO, 0.204 for the 2% Gd-doped ZnO, and 0.418 for the 5% 
Gd-doped ZnO. The defect-related emissions enhanced with 
increasing Gd doping in the ZnO indicate that a large number 
of deficiencies, such as oxygen vacancies, are most usually 
related to crystal deformation (Yi et al. 2017). The results 
reveal that the Gd doping enhances the Zni and Vo flaws. It 
could be attributed to structural deformation created by Gd 
atoms with different ionic sizes than Zn in the ZnO (Ban-
dopadhyay and Mitra 2015; Sukumaran et al. 2021). Increas-
ing Gd doping in ZnO indicates an increase in defect states, 
which matches well with the XPS results.

We compared our PL results with Farhad et al. (2019), who 
prepared ZnO DC (drop casting) and ZnO nanorods. Their 
PL results detected the PL peak at 380 nm and a barely 
observable green-yellow emission peak; in our case, we find 
it at 381 nm for ZnO, 384 nm for 2% Gd-doped ZnO and 383 
nm for the 5% Gd-doped ZnO polycrystalline sample. This 
peak represented ZnO's near band edge (NBE) transition 
because of free excitons recombinations and this PL property 
indicates high crystalline quality. The luminescent character-
istics in the visible region are responsible for the different 
defects, like zinc interstitials (Zni), oxygen vacancies (Vo), 
etc., present in the ZnO crystal matrix. As a result, they 
reported that both ZnO (DC) and ZnO nanorods were found 
to be defect-free good crystalline and optical properties mate-
rials. In our case, all the prepared Gd-doped ZnO powder 
samples have been found to be with single ionized oxygen 
vacancies, which are confirmed by the green emission peak 
at 525 nm in the PL spectra.

Conclusion

The Gd-doped ZnO powder samples were synthesized by the 
solid-state reaction method. The XRD patterns affirm that Gd 
has been successfully incorporated into the ZnO lattice and 
confirm the hexagonal wurtzite structure. SEM images have 
been used to reveal the morphologies of the synthesized 
samples, which appear as rod-like structures. The XPS 
results showed that the oxygen vacancy-related states in 
Gd-doped samples increased with increasing Gd doping. PL 
findings confirmed that the defect-related states are present 

in the Gd-doped ZnO polycrystalline samples. The green 
emission and ultra-violet emission peaks' intensity ratio is 
0.418 for the 5% Gd-doped ZnO sample, which is the maxi-
mum compared to the pure ZnO and 2% Gd-doped ZnO 
polycrystalline powder sample. These findings confirmed 
that the oxygen vacancies increase as the Gd concentration 
increases and the PL results agree well with the XPS results.
The UV absorption spectra findings reveal that the band gap 
in the 5% Gd-doped ZnO sample is found to be 3.23 eV, 
which is the minimum compared to both the pure ZnO and 
the 2% Gd-doped ZnO samples, confirming that Eg is 
reduced with increasing Gd concentration.

Acknowledgment

The authors are thankful to USIC, University of Rajasthan, 
Jaipur, Rajasthan (India), for providing SEM images of the 
prepared samples. We also thank department of physics, 
BanasthaliVidhyapith, Banasthali, Jaipur, Rajasthan (India) 
for rendering XRD data.

Funding sources

This research is not financed by any agency.

Authors' contribution

Sample preparation, data analysis, manuscript composition 
and writing (Mahendra Kumar Gora); sample synthesis and 
data characterization (Arvind Kumar); data recording (Ban-
wari Lal Choudhary); data analysis and interpretation 
(Sanjay Kumar); reviewing and editing (Satya NarainDolia); 
supervision, writing, reviewing and editing (Rishi Kumar 
Singhal).

Conflicts of interest or competing interests 

All authors declare that we have no conflict of interest or 
competing interests.

Data availability

On request, data will be available.

Ethical approval

Not applicable.

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doped ZnO nanocrystals also have an attractive optical 
responsiveness. Many theoretical and experimental studies 
showed that RE-doped ZnO could reportedly absorb ultravio-
let (UV) light (Chakraborty et al.  2017). Obeid  et al. (2019) 
prepared undoped and Gd-doped ZnO nanocrystalline 
samples using the thermal decomposition method. They 
observed that the optical absorption spectra of pristine ZnO 
increased with a 6% Gd doping concentration and the 
magnetic properties changed with changing dopant 
amounts.  Their PL spectroscopy results showed the 
existence of defects, which increased with increasing Gd 
doping concentrations. They suggested that these DMS 
nanoparticles can be used for magneto-optoelectronic 
practical device applications.

Undoubtedly, the RE-doped ZnO nanostructures have been 
intensively studied recently because this is a rapidly growing 
field due to their multi-functionality and their use in possible 
new generation applications. With an aim to understand the 
effect of Gd doping in ZnO in terms of its electronic and 
optical properties we have synthesize Gd-doped ZnO powder 
series of different Gd (0%, 2%, and 5%) values. Various 
characterization techniques have been utilized to explore the 
electronic, structural and optical features of the synthesized 
powder series. Gd doping into ZnO produced changes in the 
band gap and enhanced oxygen vacancies related states in the 
compound.

Materials and methods

The Gd-doped ZnO powder samples with varying Gd (0%, 
2%, and 5%) concentrations have been synthesized using the 
conventional solid-state reaction method. A suitable amount 
of high-purity (99.999% Sigma-Aldrich) zinc oxide (ZnO) 
and gadolinium oxide (Gd2O3) powder have been used for 
their preparation. For achieving perfect homogeneity, the 
ingredients were properly pounded for ~8 hours in an agate 
mortar and pestle. The prepared material has been sintered 
for ~8h at~550°C and cooled at room temperature. Again, the 
mixture was ground for ~2 hours and sintered for ~6 hours at 
~550°C. The mixture was milled for ~4 hours before being 
turned into pellets. Finally, these pellets were sintered for 
around ~8 hours at about ~600°C. Finally, we ground the 
pellets to make the powder. The powder form of the prepared 
samples has been used for further characterization.

Characterization techniques

The crystallographic structures of the samples have been 
explored using an X-ray diffractometer (Make: Panalytical 
X'Pert PRO) using the photon energy of Cu Kα (λ= 
1.5406Å). SEM images have been used for the morphology 
study of the synthesized samples. The SEM equipment model 



Electronic, structural and optical properties of Gd-doped ZnO powder 58(1) 202354

RE elements, such as Gd, may introduce additional capabili-
ties to the ZnO system because they may be capable of 
constructing combined practical devices on a single chip by 
combining magnetic and optical properties (Ohno et al. 1996; 
Tawil et al. 2011). 

Rare-earth (RE) metal doping in the dilute magnetic 
semiconductor (DMS) systems has recently received 
immense attention due to its potential usage in magneto 
electronic and photonic devices (Monteiro et al. 2006). The 
influence of Gd doping on the optical and electrical proper-
ties of ZnO is crucial for developing optoelectronic devices 
and understanding the genesis of ferromagnetism. Ma and 
Wang (2012) synthesized the Gd-doped ZnO nanocrystals 
using the thermal evaporation deposition method. According 
to their findings, Gd doping has an appreciable impact on the 
optical characteristics of the ZnO. The strong UV and blue 
emissions are detected at low Gd concentrations because Gd 
impurities introduce a mid-gap state into ZnO. In compari-
son, the UV and blue emissions reduce for high doping 
concentrations as a significant broad defect emission appears 
because of the vast numbers of defects, impurities, and 
excess Gd2O3 produced. Gd-doped ZnO nanocrystals can be 
used in optoelectronic devices. The optical and electrical 
tuning of the carrier density and optical band gap energy is 
essential for developing luminescent materials and equip-
ment based on the ZnO DMS systems. The optical band gap 
and carrier concentration are essential parameters to yield 
information about the emission wavelength. Several develop-
ment characteristics, like the synthesis methods, impurity 
concentration and substrate variety have been reported to 
influence these parameters (Wakano et al. 2001; Tan et al. 
2011; Murtaza et al.  2011).

The synthesis of DMS materials with unique optical, photo-
catalytic and magnetic characteristics seems to be the most 
promising candidate for the subsequent generation of 
electronic, optoelectronic and spintronic appliances (Kumar  
and Sahare 2014).  The low-dimension materials with imper-
fection-free lattice structures have excellent electrical and 
magnetic characteristics; purposefully made or accidental 
structural defects play a significant role in altering their 
features (Li et al. 2016; Obeid et al. 2018). Thus, the presence 
of different kinds of cation or anion vacancies and intersti-
tials in ZnO nanocrystals may change their magnetic proper-
ties, photoluminescence, and photocatalytic properties (Coey 
et al. 2005). Although certain experimental and theoretical 
findings based on DFT indicate that Gd-doped ZnO exhibits 
the room temperature ferromagnetism (RTFM) (Potzger et 
al. 2006; Aravindh et al. 2014) property, several studies have 
demonstrated that the RTFM is not present (Sambasivam et 
al. 2015). In addition to their magnetic qualities, the 4f ions 

ETH 30 kV EVO-18 Carl Zeiss, with a tilt of 0° to 60° and a 
rotation of 360°, has been used for taking SEM images.The 
defect-related states and chemical analysis of the samples 
have been determined using X-ray photoelectron spectrosco-
py. A hemispherical analyzer with a 128-channel detector and 
a monochromated, fully integrated K-alpha small-spot X-ray 
photoelectron spectrometer system of 180° double focusing 
is used in XPS (Thermo Scientific made). The UV-visible 
absorption spectroscopy (Model: Shimadzu UV-3600 
UV-VIS-NIR Spectrophotometer) was used to explore the 
prepared samples' optical absorption and band gap.
The measurements of UV-visible absorption spectra have 
been taken using the powder form of the samples. PL 
spectroscopy was used to analyze defect-related states in the 
prepared samples. Photoluminescence spectra have been 
recorded at room temperature using the fluorescence 
spectrometer (Model: Perkin Elmer FL 8500). 

Results and discussion 

XRD analysis

Fig. 1. depicts the powder XRD patterns of the Gd-doped 
ZnO (Gd = 0%, 2%, and 5%) samples. All the diffraction 
peaks detected in the ZnO sample match with the standard 
JCPDS card number (36-1451). This confirms a hexagonal 
wurtzite crystal structure of the samples.

The diffraction peaks were observed at 31.88°, 34.50°, 
36.25°, 47.72°, 56.69°, 63.03°, 66.59°, 68.17°, and 69.35°, 
which are related to the planes (100), (002), (101), (102), 

(110), (103), (200), (112), and (201), respectively (Ganesh et 
al. 2018; Bharathi et al. 2020). Further, some additional 
small peaks have been observed at 28.61°, 33.21°, and 39.14° 
with increasing Gd doping. These secondary peaks represent-
ing a crystallite phase have been identified as Gd2O3, which 
is well consistent with earlier reports (Das et al. 2012). As 
shown in Fig. 2, the highest intense peak related to the (101) 
plane shifted slightly higher angles with the increasing 
concentration of Gd doping. The shift is because of the 
substitution of bigger-sized Gd3+ (0.94 Å) ions than Zn2+ 
(0.74 Å) ions in the hexagonal wurtzite structure.  The peak 
shift may indicate that Gd3+ ions have occupied the crystallo-
graphic positions of Zn2+ ions in the ZnO host lattice, or 
itmay be because of the structural strain caused by the forma-
tion of internal compressive micro stress (Dakhel and El-Hilo 
2010; Kumar et al. 2014; Sahu et al. 2017). 

Rietveld refinements have been performed using the Full Prof 
Programme (shown in Fig. 3). The Pseudo-Voigt function has 
been used for Rietveld refinement of all the samples with the 
space group P63mc. The estimated lattice parameters have 
been listed in Table I. The lattice parameters are found to 
decrease with Gd doping, similar to those reported in earlier 
studies (Kaur et al. 2016; Obeid et al. 2019). The factors like 
the induced deficiencies (vacancies, interstitial) and the ionic 
radius of Zn and Gd ions can be responsible for the observed 
change (Kaur et al. 2016; Kumar and Thangavel 2017).

SEM analysis

Scanning electron micrographs have been used to analyze 
the morphology of the Gd-doped ZnO powder samples. The 
SEM images reveal a rod-like morphology in pure ZnO 
(Gora  et al. 2022) and the Gd-doped ZnO polycrystalline 

samples with some proof of agglomeration and phase segre-
gation, as depicted in Fig. 4 (a, b and c). Profound observa-
tion of the particles exhibits that the crystallite size is above 
100 nm. The particles were intensive and incompatibly 
distributed all over the mass. A distinct boundary between 
neighbouring crystallites can be noticed, despite the proxim-
ity of these smaller crystallites.

XPS analysis

The XPS technique is an excellent tool for determining the 
surface chemistry and electron configuration of the 
various elements present in a multicomponent material. 
The carbon C1s peak (284.6 eV) has been used as a refer-

ence to calibrate all the binding energies of the spectra. 
The survey spectrum of the Gd-doped ZnO samples is 
shown in Fig. 5. The survey spectra revealed the existence 
of O, Zn, and Gd elements without any impurity in the 
samples.

For ZnO and 2% Gd-doped ZnO powder samples, the binding 
energies of the Zn2p3/2 and Zn2p1/2 peaks are found to be 
1021.25 eV, 1044.25 eV, 1021.43 eV, and 1044.43 eV, respec-
tively. However, for the 5% Gd-doped ZnO sample, these 
peaks are at 1021.83 and 1044.83 eV energy positions. The 
small shift could be assigned to the creation of Zn interstitials 
in ZnO upon Gd doping. The binding energy gap between the 
zinc doublet peaks has been found at about 23 eV. This 
indicates that Zn in the ZnO crystal structure is in the Zn2+ 
oxidation state (Khataee et al. 2015; Gawai et al. 2019) as the 
spin-orbital splitting of Zn2p is 23 eV. This difference does 
not change with the Gd doping pointing out that Gd doping 
has no discernible effect on the chemical position of ZnO.

The O1s asymmetric XPS spectraare shown in Fig.7 (a) 
which is asymmetric in shape. The O1s XPS spectra of all 
these samples i.e. the ZnO, the Zn0.98Gd0.02O, and the 
Zn0.95Gd0.05O have been deconvoluted mainly into two Gauss-
ian peaks, as shown in Fig. 7 (b, c, and d). The lower binding 
peaks for pure and Gd-doped ZnO (Gd = 2% and 5%) 
polycrystalline samples have been detected at 529.93 eV, 
529.93 eV and 529.78 eV, respectively, which attributed to 
the lattice oxygen in the hexagonal structure of the ZnO.

The higher binding peak for the pure and the Gd-doped ZnO 
samples have been found at 531.51 eV, 531.66 eV, and 
531.49 eV, respectively, which is due to the presence of 
oxygen vacancies in the ZnO structure (Bharathi et al. 2020; 
Sukumaran et al. 2021). An additional signal centered at 
528.29 eV has been detected in the 5% Gd-doped ZnO 
powder sample, which could be ascribed to the coordination 
of oxygen in Gd-O-Zn along grain boundaries because of the 
excessive Gd dopants and also attributed to the presence of 

Gd2O3 in the sample (Yi  et al.  2017). The lattice oxygen peak 
has slightly shifted to the lower binding energy with increas-
ing Gd concentration, which has been due to the increased 
oxygen vacancies with increasing Gd doping. The estimated 
area ratios of the oxygen vacancies peak and the lattice oxygen 
peak for the samples ZnO, Zn0.98Gd0.02O, and Zn0.95Gd0.05O 
comes out to be 0.53, 1.30, and 2.49, respectively. These 
results show that the oxygen vacancies increase with increas-
ing Gd concentration.

The Fig. 8 (a, b) depicts the high-resolution Gd4d XPS 
spectra of the Gd-doped samples.In the Gd4d state, the 
spin-orbit splatted peaks are at 144.6 eV for Gd4d3/2 and 
139.6 eV for Gd4d5/2 for the 2% Gd-doped sample, whereas 
for the 5% Gd-doped ZnO sample, the spin-orbit splits at 
145.5 eV for Gd4d3/2 and 139.7 eV for Gd4d5/2 as shown in 
Fig. 8 (a, b). This indicates that Gd is present in the Gd3+ state 
in the ZnO hexagonal structure. The shift in peak values of 
Gd4d3/2 and Gd4d5/2 from 144.6 eV and 139.6 eV, respective-
ly; in the case of 2% Gd-doped sample to 145.5 eV and 139.7 
eV, respectively, in the case of 5% Gd-doped sample. This 
shift in Gd4d peaks is ascribed to the electron transfer from 
plasmonics Gd to ZnO because of the strong electronic 
interaction (covalent bond) between Gd and the oxide 
(Ahmad et al. 2011; Wang et al. 2012; Bharathi et al. 2020).

UV visible spectroscopy analysis

The UV–Visible spectroscopy is a powerful tool for estimat-
ing band gap of materials. UV–Vis absorbance spectra and 
tauc plots for the Gd-doped ZnO samples (Gd = 0% (Gora et 
al. 2022), 2%, and 5%) samples are shown in Figs.9 (a, b, c, 
and d). The spectra show a red shift with increasing Gd 
amount in the ZnO.The UV–Vis absorption intensity of the 
synthesized polycrystalline samples is found to reduce with 
the increasing Gd concentration (Mazhdi and Tafreshi 2018). 
Tauc's plot has been used to calculate the direct band gap of 
the samples (Jain et al. 2006). The direct band gap has been 
estimated using the following equation:     

Here, α is the absorption coefficient,υ is frequency, h is the 
Planck constant, A is a constant, and Eg is the energy band 
gap of the substance.

Extrapolating the linear component of the curve plotted 
among (αhυ) 2 and hυ depicted in Fig. 9 (b, c, and d) has been 
used to obtain the band gap (Farhad et al. 2018; Ghos et al. 
2021). The band gap values calculated for the Gd-doped ZnO 
samples, i.e.are 3.31 eV (Gora et al. 2022), 3.27 eV, and 
3.23eV, for the samples Gd = 0%, 2% and 5%, respectively. 
The small redshift observed in the band gap of Gd-doped 
ZnO samples is because of the changes in electronic structure 
as well as enhanced oxygen vacancies with the increasing Gd 
doping (Yi et al. 2017; Flemban et al. 2016), which is respon-
sible for the band gap reduction. 

Photoluminescence spectroscopy analysis

The structural flaws like ionic and atomic vacancies, replace-
ments and interstitials have been explored using the PL 

spectra of the synthesized polycrystalline samples. Fig. 10 (a, 
b and c) shows the deconvoluted, Gaussian-fitted PL spectra 
of the synthesized Gd-doped ZnO polycrystalline powder 
samples.

The PL spectra of the samples were recorded at room 
temperature in order to determine the types of vacancies, 
defect band emission etc. (Lin et al. 2001; Yi et al. 2017). The 
photoluminescence spectra have been deconvoluted and the 
different peaks responsible for the emission have been 

Gaussian fitted. For these samples the band edge emission 
peaksare detected at 381 nm, 384 nm, and 383 nm, for the Gd 
= 0%, 2% and 5% samples, respectively. The UV emission is 
responsible for the excitonic interaction related to the band 
edge emission. The intense violet emission exhibited in the 
Gd-doped ZnO at about 415 nm could be because ofthe 
oxygen vacancies. The exciton interaction between the holes 
in the valence band and electrons located in the interstitial 
zinc could be responsible for the violet emission peak at 
about 447 nm. Intrinsic deficiencies like interstitial zinc and 

oxygen can be attributed to the peak at 477 nm (Singh et al.  
2009; Kaur et al. 2016). The transformation between singly 
ionized oxygen deficiency and the photo-created holes (Ken-
nedy et al. 2014) or the surface imperfection and electrons 
near the conduction band, might be responsible for the green 
emission peak seen for the Gd-doped ZnOat around 525 nm. 
The green emissions are commonly attributed to theoxygen 
vacancies (Studenikin et al. 1998). For Gd-doped ZnO 
polycrystalline powder samples, the intensity ratio of the 
green and ultra-violet emission peaks is found to be 0.090 for 
ZnO, 0.204 for the 2% Gd-doped ZnO, and 0.418 for the 5% 
Gd-doped ZnO. The defect-related emissions enhanced with 
increasing Gd doping in the ZnO indicate that a large number 
of deficiencies, such as oxygen vacancies, are most usually 
related to crystal deformation (Yi et al. 2017). The results 
reveal that the Gd doping enhances the Zni and Vo flaws. It 
could be attributed to structural deformation created by Gd 
atoms with different ionic sizes than Zn in the ZnO (Ban-
dopadhyay and Mitra 2015; Sukumaran et al. 2021). Increas-
ing Gd doping in ZnO indicates an increase in defect states, 
which matches well with the XPS results.

We compared our PL results with Farhad et al. (2019), who 
prepared ZnO DC (drop casting) and ZnO nanorods. Their 
PL results detected the PL peak at 380 nm and a barely 
observable green-yellow emission peak; in our case, we find 
it at 381 nm for ZnO, 384 nm for 2% Gd-doped ZnO and 383 
nm for the 5% Gd-doped ZnO polycrystalline sample. This 
peak represented ZnO's near band edge (NBE) transition 
because of free excitons recombinations and this PL property 
indicates high crystalline quality. The luminescent character-
istics in the visible region are responsible for the different 
defects, like zinc interstitials (Zni), oxygen vacancies (Vo), 
etc., present in the ZnO crystal matrix. As a result, they 
reported that both ZnO (DC) and ZnO nanorods were found 
to be defect-free good crystalline and optical properties mate-
rials. In our case, all the prepared Gd-doped ZnO powder 
samples have been found to be with single ionized oxygen 
vacancies, which are confirmed by the green emission peak 
at 525 nm in the PL spectra.

Conclusion

The Gd-doped ZnO powder samples were synthesized by the 
solid-state reaction method. The XRD patterns affirm that Gd 
has been successfully incorporated into the ZnO lattice and 
confirm the hexagonal wurtzite structure. SEM images have 
been used to reveal the morphologies of the synthesized 
samples, which appear as rod-like structures. The XPS 
results showed that the oxygen vacancy-related states in 
Gd-doped samples increased with increasing Gd doping. PL 
findings confirmed that the defect-related states are present 

in the Gd-doped ZnO polycrystalline samples. The green 
emission and ultra-violet emission peaks' intensity ratio is 
0.418 for the 5% Gd-doped ZnO sample, which is the maxi-
mum compared to the pure ZnO and 2% Gd-doped ZnO 
polycrystalline powder sample. These findings confirmed 
that the oxygen vacancies increase as the Gd concentration 
increases and the PL results agree well with the XPS results.
The UV absorption spectra findings reveal that the band gap 
in the 5% Gd-doped ZnO sample is found to be 3.23 eV, 
which is the minimum compared to both the pure ZnO and 
the 2% Gd-doped ZnO samples, confirming that Eg is 
reduced with increasing Gd concentration.

Acknowledgment

The authors are thankful to USIC, University of Rajasthan, 
Jaipur, Rajasthan (India), for providing SEM images of the 
prepared samples. We also thank department of physics, 
BanasthaliVidhyapith, Banasthali, Jaipur, Rajasthan (India) 
for rendering XRD data.

Funding sources

This research is not financed by any agency.

Authors' contribution

Sample preparation, data analysis, manuscript composition 
and writing (Mahendra Kumar Gora); sample synthesis and 
data characterization (Arvind Kumar); data recording (Ban-
wari Lal Choudhary); data analysis and interpretation 
(Sanjay Kumar); reviewing and editing (Satya NarainDolia); 
supervision, writing, reviewing and editing (Rishi Kumar 
Singhal).

Conflicts of interest or competing interests 

All authors declare that we have no conflict of interest or 
competing interests.

Data availability

On request, data will be available.

Ethical approval

Not applicable.

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doped ZnO nanocrystals also have an attractive optical 
responsiveness. Many theoretical and experimental studies 
showed that RE-doped ZnO could reportedly absorb ultravio-
let (UV) light (Chakraborty et al.  2017). Obeid  et al. (2019) 
prepared undoped and Gd-doped ZnO nanocrystalline 
samples using the thermal decomposition method. They 
observed that the optical absorption spectra of pristine ZnO 
increased with a 6% Gd doping concentration and the 
magnetic properties changed with changing dopant 
amounts.  Their PL spectroscopy results showed the 
existence of defects, which increased with increasing Gd 
doping concentrations. They suggested that these DMS 
nanoparticles can be used for magneto-optoelectronic 
practical device applications.

Undoubtedly, the RE-doped ZnO nanostructures have been 
intensively studied recently because this is a rapidly growing 
field due to their multi-functionality and their use in possible 
new generation applications. With an aim to understand the 
effect of Gd doping in ZnO in terms of its electronic and 
optical properties we have synthesize Gd-doped ZnO powder 
series of different Gd (0%, 2%, and 5%) values. Various 
characterization techniques have been utilized to explore the 
electronic, structural and optical features of the synthesized 
powder series. Gd doping into ZnO produced changes in the 
band gap and enhanced oxygen vacancies related states in the 
compound.

Materials and methods

The Gd-doped ZnO powder samples with varying Gd (0%, 
2%, and 5%) concentrations have been synthesized using the 
conventional solid-state reaction method. A suitable amount 
of high-purity (99.999% Sigma-Aldrich) zinc oxide (ZnO) 
and gadolinium oxide (Gd2O3) powder have been used for 
their preparation. For achieving perfect homogeneity, the 
ingredients were properly pounded for ~8 hours in an agate 
mortar and pestle. The prepared material has been sintered 
for ~8h at~550°C and cooled at room temperature. Again, the 
mixture was ground for ~2 hours and sintered for ~6 hours at 
~550°C. The mixture was milled for ~4 hours before being 
turned into pellets. Finally, these pellets were sintered for 
around ~8 hours at about ~600°C. Finally, we ground the 
pellets to make the powder. The powder form of the prepared 
samples has been used for further characterization.

Characterization techniques

The crystallographic structures of the samples have been 
explored using an X-ray diffractometer (Make: Panalytical 
X'Pert PRO) using the photon energy of Cu Kα (λ= 
1.5406Å). SEM images have been used for the morphology 
study of the synthesized samples. The SEM equipment model 



Gora, Kumar, Choudhary, Dolia, Kumar and Singhal 55

RE elements, such as Gd, may introduce additional capabili-
ties to the ZnO system because they may be capable of 
constructing combined practical devices on a single chip by 
combining magnetic and optical properties (Ohno et al. 1996; 
Tawil et al. 2011). 

Rare-earth (RE) metal doping in the dilute magnetic 
semiconductor (DMS) systems has recently received 
immense attention due to its potential usage in magneto 
electronic and photonic devices (Monteiro et al. 2006). The 
influence of Gd doping on the optical and electrical proper-
ties of ZnO is crucial for developing optoelectronic devices 
and understanding the genesis of ferromagnetism. Ma and 
Wang (2012) synthesized the Gd-doped ZnO nanocrystals 
using the thermal evaporation deposition method. According 
to their findings, Gd doping has an appreciable impact on the 
optical characteristics of the ZnO. The strong UV and blue 
emissions are detected at low Gd concentrations because Gd 
impurities introduce a mid-gap state into ZnO. In compari-
son, the UV and blue emissions reduce for high doping 
concentrations as a significant broad defect emission appears 
because of the vast numbers of defects, impurities, and 
excess Gd2O3 produced. Gd-doped ZnO nanocrystals can be 
used in optoelectronic devices. The optical and electrical 
tuning of the carrier density and optical band gap energy is 
essential for developing luminescent materials and equip-
ment based on the ZnO DMS systems. The optical band gap 
and carrier concentration are essential parameters to yield 
information about the emission wavelength. Several develop-
ment characteristics, like the synthesis methods, impurity 
concentration and substrate variety have been reported to 
influence these parameters (Wakano et al. 2001; Tan et al. 
2011; Murtaza et al.  2011).

The synthesis of DMS materials with unique optical, photo-
catalytic and magnetic characteristics seems to be the most 
promising candidate for the subsequent generation of 
electronic, optoelectronic and spintronic appliances (Kumar  
and Sahare 2014).  The low-dimension materials with imper-
fection-free lattice structures have excellent electrical and 
magnetic characteristics; purposefully made or accidental 
structural defects play a significant role in altering their 
features (Li et al. 2016; Obeid et al. 2018). Thus, the presence 
of different kinds of cation or anion vacancies and intersti-
tials in ZnO nanocrystals may change their magnetic proper-
ties, photoluminescence, and photocatalytic properties (Coey 
et al. 2005). Although certain experimental and theoretical 
findings based on DFT indicate that Gd-doped ZnO exhibits 
the room temperature ferromagnetism (RTFM) (Potzger et 
al. 2006; Aravindh et al. 2014) property, several studies have 
demonstrated that the RTFM is not present (Sambasivam et 
al. 2015). In addition to their magnetic qualities, the 4f ions 

ETH 30 kV EVO-18 Carl Zeiss, with a tilt of 0° to 60° and a 
rotation of 360°, has been used for taking SEM images.The 
defect-related states and chemical analysis of the samples 
have been determined using X-ray photoelectron spectrosco-
py. A hemispherical analyzer with a 128-channel detector and 
a monochromated, fully integrated K-alpha small-spot X-ray 
photoelectron spectrometer system of 180° double focusing 
is used in XPS (Thermo Scientific made). The UV-visible 
absorption spectroscopy (Model: Shimadzu UV-3600 
UV-VIS-NIR Spectrophotometer) was used to explore the 
prepared samples' optical absorption and band gap.
The measurements of UV-visible absorption spectra have 
been taken using the powder form of the samples. PL 
spectroscopy was used to analyze defect-related states in the 
prepared samples. Photoluminescence spectra have been 
recorded at room temperature using the fluorescence 
spectrometer (Model: Perkin Elmer FL 8500). 

Results and discussion 

XRD analysis

Fig. 1. depicts the powder XRD patterns of the Gd-doped 
ZnO (Gd = 0%, 2%, and 5%) samples. All the diffraction 
peaks detected in the ZnO sample match with the standard 
JCPDS card number (36-1451). This confirms a hexagonal 
wurtzite crystal structure of the samples.

The diffraction peaks were observed at 31.88°, 34.50°, 
36.25°, 47.72°, 56.69°, 63.03°, 66.59°, 68.17°, and 69.35°, 
which are related to the planes (100), (002), (101), (102), 

(110), (103), (200), (112), and (201), respectively (Ganesh et 
al. 2018; Bharathi et al. 2020). Further, some additional 
small peaks have been observed at 28.61°, 33.21°, and 39.14° 
with increasing Gd doping. These secondary peaks represent-
ing a crystallite phase have been identified as Gd2O3, which 
is well consistent with earlier reports (Das et al. 2012). As 
shown in Fig. 2, the highest intense peak related to the (101) 
plane shifted slightly higher angles with the increasing 
concentration of Gd doping. The shift is because of the 
substitution of bigger-sized Gd3+ (0.94 Å) ions than Zn2+ 
(0.74 Å) ions in the hexagonal wurtzite structure.  The peak 
shift may indicate that Gd3+ ions have occupied the crystallo-
graphic positions of Zn2+ ions in the ZnO host lattice, or 
itmay be because of the structural strain caused by the forma-
tion of internal compressive micro stress (Dakhel and El-Hilo 
2010; Kumar et al. 2014; Sahu et al. 2017). 

Rietveld refinements have been performed using the Full Prof 
Programme (shown in Fig. 3). The Pseudo-Voigt function has 
been used for Rietveld refinement of all the samples with the 
space group P63mc. The estimated lattice parameters have 
been listed in Table I. The lattice parameters are found to 
decrease with Gd doping, similar to those reported in earlier 
studies (Kaur et al. 2016; Obeid et al. 2019). The factors like 
the induced deficiencies (vacancies, interstitial) and the ionic 
radius of Zn and Gd ions can be responsible for the observed 
change (Kaur et al. 2016; Kumar and Thangavel 2017).

SEM analysis

Scanning electron micrographs have been used to analyze 
the morphology of the Gd-doped ZnO powder samples. The 
SEM images reveal a rod-like morphology in pure ZnO 
(Gora  et al. 2022) and the Gd-doped ZnO polycrystalline 

samples with some proof of agglomeration and phase segre-
gation, as depicted in Fig. 4 (a, b and c). Profound observa-
tion of the particles exhibits that the crystallite size is above 
100 nm. The particles were intensive and incompatibly 
distributed all over the mass. A distinct boundary between 
neighbouring crystallites can be noticed, despite the proxim-
ity of these smaller crystallites.

XPS analysis

The XPS technique is an excellent tool for determining the 
surface chemistry and electron configuration of the 
various elements present in a multicomponent material. 
The carbon C1s peak (284.6 eV) has been used as a refer-

ence to calibrate all the binding energies of the spectra. 
The survey spectrum of the Gd-doped ZnO samples is 
shown in Fig. 5. The survey spectra revealed the existence 
of O, Zn, and Gd elements without any impurity in the 
samples.

For ZnO and 2% Gd-doped ZnO powder samples, the binding 
energies of the Zn2p3/2 and Zn2p1/2 peaks are found to be 
1021.25 eV, 1044.25 eV, 1021.43 eV, and 1044.43 eV, respec-
tively. However, for the 5% Gd-doped ZnO sample, these 
peaks are at 1021.83 and 1044.83 eV energy positions. The 
small shift could be assigned to the creation of Zn interstitials 
in ZnO upon Gd doping. The binding energy gap between the 
zinc doublet peaks has been found at about 23 eV. This 
indicates that Zn in the ZnO crystal structure is in the Zn2+ 
oxidation state (Khataee et al. 2015; Gawai et al. 2019) as the 
spin-orbital splitting of Zn2p is 23 eV. This difference does 
not change with the Gd doping pointing out that Gd doping 
has no discernible effect on the chemical position of ZnO.

The O1s asymmetric XPS spectraare shown in Fig.7 (a) 
which is asymmetric in shape. The O1s XPS spectra of all 
these samples i.e. the ZnO, the Zn0.98Gd0.02O, and the 
Zn0.95Gd0.05O have been deconvoluted mainly into two Gauss-
ian peaks, as shown in Fig. 7 (b, c, and d). The lower binding 
peaks for pure and Gd-doped ZnO (Gd = 2% and 5%) 
polycrystalline samples have been detected at 529.93 eV, 
529.93 eV and 529.78 eV, respectively, which attributed to 
the lattice oxygen in the hexagonal structure of the ZnO.

The higher binding peak for the pure and the Gd-doped ZnO 
samples have been found at 531.51 eV, 531.66 eV, and 
531.49 eV, respectively, which is due to the presence of 
oxygen vacancies in the ZnO structure (Bharathi et al. 2020; 
Sukumaran et al. 2021). An additional signal centered at 
528.29 eV has been detected in the 5% Gd-doped ZnO 
powder sample, which could be ascribed to the coordination 
of oxygen in Gd-O-Zn along grain boundaries because of the 
excessive Gd dopants and also attributed to the presence of 

Gd2O3 in the sample (Yi  et al.  2017). The lattice oxygen peak 
has slightly shifted to the lower binding energy with increas-
ing Gd concentration, which has been due to the increased 
oxygen vacancies with increasing Gd doping. The estimated 
area ratios of the oxygen vacancies peak and the lattice oxygen 
peak for the samples ZnO, Zn0.98Gd0.02O, and Zn0.95Gd0.05O 
comes out to be 0.53, 1.30, and 2.49, respectively. These 
results show that the oxygen vacancies increase with increas-
ing Gd concentration.

The Fig. 8 (a, b) depicts the high-resolution Gd4d XPS 
spectra of the Gd-doped samples.In the Gd4d state, the 
spin-orbit splatted peaks are at 144.6 eV for Gd4d3/2 and 
139.6 eV for Gd4d5/2 for the 2% Gd-doped sample, whereas 
for the 5% Gd-doped ZnO sample, the spin-orbit splits at 
145.5 eV for Gd4d3/2 and 139.7 eV for Gd4d5/2 as shown in 
Fig. 8 (a, b). This indicates that Gd is present in the Gd3+ state 
in the ZnO hexagonal structure. The shift in peak values of 
Gd4d3/2 and Gd4d5/2 from 144.6 eV and 139.6 eV, respective-
ly; in the case of 2% Gd-doped sample to 145.5 eV and 139.7 
eV, respectively, in the case of 5% Gd-doped sample. This 
shift in Gd4d peaks is ascribed to the electron transfer from 
plasmonics Gd to ZnO because of the strong electronic 
interaction (covalent bond) between Gd and the oxide 
(Ahmad et al. 2011; Wang et al. 2012; Bharathi et al. 2020).

UV visible spectroscopy analysis

The UV–Visible spectroscopy is a powerful tool for estimat-
ing band gap of materials. UV–Vis absorbance spectra and 
tauc plots for the Gd-doped ZnO samples (Gd = 0% (Gora et 
al. 2022), 2%, and 5%) samples are shown in Figs.9 (a, b, c, 
and d). The spectra show a red shift with increasing Gd 
amount in the ZnO.The UV–Vis absorption intensity of the 
synthesized polycrystalline samples is found to reduce with 
the increasing Gd concentration (Mazhdi and Tafreshi 2018). 
Tauc's plot has been used to calculate the direct band gap of 
the samples (Jain et al. 2006). The direct band gap has been 
estimated using the following equation:     

Here, α is the absorption coefficient,υ is frequency, h is the 
Planck constant, A is a constant, and Eg is the energy band 
gap of the substance.

Extrapolating the linear component of the curve plotted 
among (αhυ) 2 and hυ depicted in Fig. 9 (b, c, and d) has been 
used to obtain the band gap (Farhad et al. 2018; Ghos et al. 
2021). The band gap values calculated for the Gd-doped ZnO 
samples, i.e.are 3.31 eV (Gora et al. 2022), 3.27 eV, and 
3.23eV, for the samples Gd = 0%, 2% and 5%, respectively. 
The small redshift observed in the band gap of Gd-doped 
ZnO samples is because of the changes in electronic structure 
as well as enhanced oxygen vacancies with the increasing Gd 
doping (Yi et al. 2017; Flemban et al. 2016), which is respon-
sible for the band gap reduction. 

Photoluminescence spectroscopy analysis

The structural flaws like ionic and atomic vacancies, replace-
ments and interstitials have been explored using the PL 

spectra of the synthesized polycrystalline samples. Fig. 10 (a, 
b and c) shows the deconvoluted, Gaussian-fitted PL spectra 
of the synthesized Gd-doped ZnO polycrystalline powder 
samples.

The PL spectra of the samples were recorded at room 
temperature in order to determine the types of vacancies, 
defect band emission etc. (Lin et al. 2001; Yi et al. 2017). The 
photoluminescence spectra have been deconvoluted and the 
different peaks responsible for the emission have been 

Gaussian fitted. For these samples the band edge emission 
peaksare detected at 381 nm, 384 nm, and 383 nm, for the Gd 
= 0%, 2% and 5% samples, respectively. The UV emission is 
responsible for the excitonic interaction related to the band 
edge emission. The intense violet emission exhibited in the 
Gd-doped ZnO at about 415 nm could be because ofthe 
oxygen vacancies. The exciton interaction between the holes 
in the valence band and electrons located in the interstitial 
zinc could be responsible for the violet emission peak at 
about 447 nm. Intrinsic deficiencies like interstitial zinc and 

oxygen can be attributed to the peak at 477 nm (Singh et al.  
2009; Kaur et al. 2016). The transformation between singly 
ionized oxygen deficiency and the photo-created holes (Ken-
nedy et al. 2014) or the surface imperfection and electrons 
near the conduction band, might be responsible for the green 
emission peak seen for the Gd-doped ZnOat around 525 nm. 
The green emissions are commonly attributed to theoxygen 
vacancies (Studenikin et al. 1998). For Gd-doped ZnO 
polycrystalline powder samples, the intensity ratio of the 
green and ultra-violet emission peaks is found to be 0.090 for 
ZnO, 0.204 for the 2% Gd-doped ZnO, and 0.418 for the 5% 
Gd-doped ZnO. The defect-related emissions enhanced with 
increasing Gd doping in the ZnO indicate that a large number 
of deficiencies, such as oxygen vacancies, are most usually 
related to crystal deformation (Yi et al. 2017). The results 
reveal that the Gd doping enhances the Zni and Vo flaws. It 
could be attributed to structural deformation created by Gd 
atoms with different ionic sizes than Zn in the ZnO (Ban-
dopadhyay and Mitra 2015; Sukumaran et al. 2021). Increas-
ing Gd doping in ZnO indicates an increase in defect states, 
which matches well with the XPS results.

We compared our PL results with Farhad et al. (2019), who 
prepared ZnO DC (drop casting) and ZnO nanorods. Their 
PL results detected the PL peak at 380 nm and a barely 
observable green-yellow emission peak; in our case, we find 
it at 381 nm for ZnO, 384 nm for 2% Gd-doped ZnO and 383 
nm for the 5% Gd-doped ZnO polycrystalline sample. This 
peak represented ZnO's near band edge (NBE) transition 
because of free excitons recombinations and this PL property 
indicates high crystalline quality. The luminescent character-
istics in the visible region are responsible for the different 
defects, like zinc interstitials (Zni), oxygen vacancies (Vo), 
etc., present in the ZnO crystal matrix. As a result, they 
reported that both ZnO (DC) and ZnO nanorods were found 
to be defect-free good crystalline and optical properties mate-
rials. In our case, all the prepared Gd-doped ZnO powder 
samples have been found to be with single ionized oxygen 
vacancies, which are confirmed by the green emission peak 
at 525 nm in the PL spectra.

Conclusion

The Gd-doped ZnO powder samples were synthesized by the 
solid-state reaction method. The XRD patterns affirm that Gd 
has been successfully incorporated into the ZnO lattice and 
confirm the hexagonal wurtzite structure. SEM images have 
been used to reveal the morphologies of the synthesized 
samples, which appear as rod-like structures. The XPS 
results showed that the oxygen vacancy-related states in 
Gd-doped samples increased with increasing Gd doping. PL 
findings confirmed that the defect-related states are present 

in the Gd-doped ZnO polycrystalline samples. The green 
emission and ultra-violet emission peaks' intensity ratio is 
0.418 for the 5% Gd-doped ZnO sample, which is the maxi-
mum compared to the pure ZnO and 2% Gd-doped ZnO 
polycrystalline powder sample. These findings confirmed 
that the oxygen vacancies increase as the Gd concentration 
increases and the PL results agree well with the XPS results.
The UV absorption spectra findings reveal that the band gap 
in the 5% Gd-doped ZnO sample is found to be 3.23 eV, 
which is the minimum compared to both the pure ZnO and 
the 2% Gd-doped ZnO samples, confirming that Eg is 
reduced with increasing Gd concentration.

Acknowledgment

The authors are thankful to USIC, University of Rajasthan, 
Jaipur, Rajasthan (India), for providing SEM images of the 
prepared samples. We also thank department of physics, 
BanasthaliVidhyapith, Banasthali, Jaipur, Rajasthan (India) 
for rendering XRD data.

Funding sources

This research is not financed by any agency.

Authors' contribution

Sample preparation, data analysis, manuscript composition 
and writing (Mahendra Kumar Gora); sample synthesis and 
data characterization (Arvind Kumar); data recording (Ban-
wari Lal Choudhary); data analysis and interpretation 
(Sanjay Kumar); reviewing and editing (Satya NarainDolia); 
supervision, writing, reviewing and editing (Rishi Kumar 
Singhal).

Conflicts of interest or competing interests 

All authors declare that we have no conflict of interest or 
competing interests.

Data availability

On request, data will be available.

Ethical approval

Not applicable.

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doped ZnO nanocrystals also have an attractive optical 
responsiveness. Many theoretical and experimental studies 
showed that RE-doped ZnO could reportedly absorb ultravio-
let (UV) light (Chakraborty et al.  2017). Obeid  et al. (2019) 
prepared undoped and Gd-doped ZnO nanocrystalline 
samples using the thermal decomposition method. They 
observed that the optical absorption spectra of pristine ZnO 
increased with a 6% Gd doping concentration and the 
magnetic properties changed with changing dopant 
amounts.  Their PL spectroscopy results showed the 
existence of defects, which increased with increasing Gd 
doping concentrations. They suggested that these DMS 
nanoparticles can be used for magneto-optoelectronic 
practical device applications.

Undoubtedly, the RE-doped ZnO nanostructures have been 
intensively studied recently because this is a rapidly growing 
field due to their multi-functionality and their use in possible 
new generation applications. With an aim to understand the 
effect of Gd doping in ZnO in terms of its electronic and 
optical properties we have synthesize Gd-doped ZnO powder 
series of different Gd (0%, 2%, and 5%) values. Various 
characterization techniques have been utilized to explore the 
electronic, structural and optical features of the synthesized 
powder series. Gd doping into ZnO produced changes in the 
band gap and enhanced oxygen vacancies related states in the 
compound.

Materials and methods

The Gd-doped ZnO powder samples with varying Gd (0%, 
2%, and 5%) concentrations have been synthesized using the 
conventional solid-state reaction method. A suitable amount 
of high-purity (99.999% Sigma-Aldrich) zinc oxide (ZnO) 
and gadolinium oxide (Gd2O3) powder have been used for 
their preparation. For achieving perfect homogeneity, the 
ingredients were properly pounded for ~8 hours in an agate 
mortar and pestle. The prepared material has been sintered 
for ~8h at~550°C and cooled at room temperature. Again, the 
mixture was ground for ~2 hours and sintered for ~6 hours at 
~550°C. The mixture was milled for ~4 hours before being 
turned into pellets. Finally, these pellets were sintered for 
around ~8 hours at about ~600°C. Finally, we ground the 
pellets to make the powder. The powder form of the prepared 
samples has been used for further characterization.

Characterization techniques

The crystallographic structures of the samples have been 
explored using an X-ray diffractometer (Make: Panalytical 
X'Pert PRO) using the photon energy of Cu Kα (λ= 
1.5406Å). SEM images have been used for the morphology 
study of the synthesized samples. The SEM equipment model 

Fig. 1. XRD patterns for the pure ZnO powder (Gora et 
al. 2022) and (2% and 5%) Gd-doped ZnO powder 
samples

Fig. 2. Extended XRD patterns for the (101) peak of 
undoped ZnO powder (Gora et al. 2022) and 
Gd-doped ZnO (Gd = 2% and 5%) powder 
samples



Electronic, structural and optical properties of Gd-doped ZnO powder 58(1) 202356

RE elements, such as Gd, may introduce additional capabili-
ties to the ZnO system because they may be capable of 
constructing combined practical devices on a single chip by 
combining magnetic and optical properties (Ohno et al. 1996; 
Tawil et al. 2011). 

Rare-earth (RE) metal doping in the dilute magnetic 
semiconductor (DMS) systems has recently received 
immense attention due to its potential usage in magneto 
electronic and photonic devices (Monteiro et al. 2006). The 
influence of Gd doping on the optical and electrical proper-
ties of ZnO is crucial for developing optoelectronic devices 
and understanding the genesis of ferromagnetism. Ma and 
Wang (2012) synthesized the Gd-doped ZnO nanocrystals 
using the thermal evaporation deposition method. According 
to their findings, Gd doping has an appreciable impact on the 
optical characteristics of the ZnO. The strong UV and blue 
emissions are detected at low Gd concentrations because Gd 
impurities introduce a mid-gap state into ZnO. In compari-
son, the UV and blue emissions reduce for high doping 
concentrations as a significant broad defect emission appears 
because of the vast numbers of defects, impurities, and 
excess Gd2O3 produced. Gd-doped ZnO nanocrystals can be 
used in optoelectronic devices. The optical and electrical 
tuning of the carrier density and optical band gap energy is 
essential for developing luminescent materials and equip-
ment based on the ZnO DMS systems. The optical band gap 
and carrier concentration are essential parameters to yield 
information about the emission wavelength. Several develop-
ment characteristics, like the synthesis methods, impurity 
concentration and substrate variety have been reported to 
influence these parameters (Wakano et al. 2001; Tan et al. 
2011; Murtaza et al.  2011).

The synthesis of DMS materials with unique optical, photo-
catalytic and magnetic characteristics seems to be the most 
promising candidate for the subsequent generation of 
electronic, optoelectronic and spintronic appliances (Kumar  
and Sahare 2014).  The low-dimension materials with imper-
fection-free lattice structures have excellent electrical and 
magnetic characteristics; purposefully made or accidental 
structural defects play a significant role in altering their 
features (Li et al. 2016; Obeid et al. 2018). Thus, the presence 
of different kinds of cation or anion vacancies and intersti-
tials in ZnO nanocrystals may change their magnetic proper-
ties, photoluminescence, and photocatalytic properties (Coey 
et al. 2005). Although certain experimental and theoretical 
findings based on DFT indicate that Gd-doped ZnO exhibits 
the room temperature ferromagnetism (RTFM) (Potzger et 
al. 2006; Aravindh et al. 2014) property, several studies have 
demonstrated that the RTFM is not present (Sambasivam et 
al. 2015). In addition to their magnetic qualities, the 4f ions 

ETH 30 kV EVO-18 Carl Zeiss, with a tilt of 0° to 60° and a 
rotation of 360°, has been used for taking SEM images.The 
defect-related states and chemical analysis of the samples 
have been determined using X-ray photoelectron spectrosco-
py. A hemispherical analyzer with a 128-channel detector and 
a monochromated, fully integrated K-alpha small-spot X-ray 
photoelectron spectrometer system of 180° double focusing 
is used in XPS (Thermo Scientific made). The UV-visible 
absorption spectroscopy (Model: Shimadzu UV-3600 
UV-VIS-NIR Spectrophotometer) was used to explore the 
prepared samples' optical absorption and band gap.
The measurements of UV-visible absorption spectra have 
been taken using the powder form of the samples. PL 
spectroscopy was used to analyze defect-related states in the 
prepared samples. Photoluminescence spectra have been 
recorded at room temperature using the fluorescence 
spectrometer (Model: Perkin Elmer FL 8500). 

Results and discussion 

XRD analysis

Fig. 1. depicts the powder XRD patterns of the Gd-doped 
ZnO (Gd = 0%, 2%, and 5%) samples. All the diffraction 
peaks detected in the ZnO sample match with the standard 
JCPDS card number (36-1451). This confirms a hexagonal 
wurtzite crystal structure of the samples.

The diffraction peaks were observed at 31.88°, 34.50°, 
36.25°, 47.72°, 56.69°, 63.03°, 66.59°, 68.17°, and 69.35°, 
which are related to the planes (100), (002), (101), (102), 

(110), (103), (200), (112), and (201), respectively (Ganesh et 
al. 2018; Bharathi et al. 2020). Further, some additional 
small peaks have been observed at 28.61°, 33.21°, and 39.14° 
with increasing Gd doping. These secondary peaks represent-
ing a crystallite phase have been identified as Gd2O3, which 
is well consistent with earlier reports (Das et al. 2012). As 
shown in Fig. 2, the highest intense peak related to the (101) 
plane shifted slightly higher angles with the increasing 
concentration of Gd doping. The shift is because of the 
substitution of bigger-sized Gd3+ (0.94 Å) ions than Zn2+ 
(0.74 Å) ions in the hexagonal wurtzite structure.  The peak 
shift may indicate that Gd3+ ions have occupied the crystallo-
graphic positions of Zn2+ ions in the ZnO host lattice, or 
itmay be because of the structural strain caused by the forma-
tion of internal compressive micro stress (Dakhel and El-Hilo 
2010; Kumar et al. 2014; Sahu et al. 2017). 

Rietveld refinements have been performed using the Full Prof 
Programme (shown in Fig. 3). The Pseudo-Voigt function has 
been used for Rietveld refinement of all the samples with the 
space group P63mc. The estimated lattice parameters have 
been listed in Table I. The lattice parameters are found to 
decrease with Gd doping, similar to those reported in earlier 
studies (Kaur et al. 2016; Obeid et al. 2019). The factors like 
the induced deficiencies (vacancies, interstitial) and the ionic 
radius of Zn and Gd ions can be responsible for the observed 
change (Kaur et al. 2016; Kumar and Thangavel 2017).

SEM analysis

Scanning electron micrographs have been used to analyze 
the morphology of the Gd-doped ZnO powder samples. The 
SEM images reveal a rod-like morphology in pure ZnO 
(Gora  et al. 2022) and the Gd-doped ZnO polycrystalline 

samples with some proof of agglomeration and phase segre-
gation, as depicted in Fig. 4 (a, b and c). Profound observa-
tion of the particles exhibits that the crystallite size is above 
100 nm. The particles were intensive and incompatibly 
distributed all over the mass. A distinct boundary between 
neighbouring crystallites can be noticed, despite the proxim-
ity of these smaller crystallites.

XPS analysis

The XPS technique is an excellent tool for determining the 
surface chemistry and electron configuration of the 
various elements present in a multicomponent material. 
The carbon C1s peak (284.6 eV) has been used as a refer-

ence to calibrate all the binding energies of the spectra. 
The survey spectrum of the Gd-doped ZnO samples is 
shown in Fig. 5. The survey spectra revealed the existence 
of O, Zn, and Gd elements without any impurity in the 
samples.

For ZnO and 2% Gd-doped ZnO powder samples, the binding 
energies of the Zn2p3/2 and Zn2p1/2 peaks are found to be 
1021.25 eV, 1044.25 eV, 1021.43 eV, and 1044.43 eV, respec-
tively. However, for the 5% Gd-doped ZnO sample, these 
peaks are at 1021.83 and 1044.83 eV energy positions. The 
small shift could be assigned to the creation of Zn interstitials 
in ZnO upon Gd doping. The binding energy gap between the 
zinc doublet peaks has been found at about 23 eV. This 
indicates that Zn in the ZnO crystal structure is in the Zn2+ 
oxidation state (Khataee et al. 2015; Gawai et al. 2019) as the 
spin-orbital splitting of Zn2p is 23 eV. This difference does 
not change with the Gd doping pointing out that Gd doping 
has no discernible effect on the chemical position of ZnO.

The O1s asymmetric XPS spectraare shown in Fig.7 (a) 
which is asymmetric in shape. The O1s XPS spectra of all 
these samples i.e. the ZnO, the Zn0.98Gd0.02O, and the 
Zn0.95Gd0.05O have been deconvoluted mainly into two Gauss-
ian peaks, as shown in Fig. 7 (b, c, and d). The lower binding 
peaks for pure and Gd-doped ZnO (Gd = 2% and 5%) 
polycrystalline samples have been detected at 529.93 eV, 
529.93 eV and 529.78 eV, respectively, which attributed to 
the lattice oxygen in the hexagonal structure of the ZnO.

The higher binding peak for the pure and the Gd-doped ZnO 
samples have been found at 531.51 eV, 531.66 eV, and 
531.49 eV, respectively, which is due to the presence of 
oxygen vacancies in the ZnO structure (Bharathi et al. 2020; 
Sukumaran et al. 2021). An additional signal centered at 
528.29 eV has been detected in the 5% Gd-doped ZnO 
powder sample, which could be ascribed to the coordination 
of oxygen in Gd-O-Zn along grain boundaries because of the 
excessive Gd dopants and also attributed to the presence of 

Gd2O3 in the sample (Yi  et al.  2017). The lattice oxygen peak 
has slightly shifted to the lower binding energy with increas-
ing Gd concentration, which has been due to the increased 
oxygen vacancies with increasing Gd doping. The estimated 
area ratios of the oxygen vacancies peak and the lattice oxygen 
peak for the samples ZnO, Zn0.98Gd0.02O, and Zn0.95Gd0.05O 
comes out to be 0.53, 1.30, and 2.49, respectively. These 
results show that the oxygen vacancies increase with increas-
ing Gd concentration.

The Fig. 8 (a, b) depicts the high-resolution Gd4d XPS 
spectra of the Gd-doped samples.In the Gd4d state, the 
spin-orbit splatted peaks are at 144.6 eV for Gd4d3/2 and 
139.6 eV for Gd4d5/2 for the 2% Gd-doped sample, whereas 
for the 5% Gd-doped ZnO sample, the spin-orbit splits at 
145.5 eV for Gd4d3/2 and 139.7 eV for Gd4d5/2 as shown in 
Fig. 8 (a, b). This indicates that Gd is present in the Gd3+ state 
in the ZnO hexagonal structure. The shift in peak values of 
Gd4d3/2 and Gd4d5/2 from 144.6 eV and 139.6 eV, respective-
ly; in the case of 2% Gd-doped sample to 145.5 eV and 139.7 
eV, respectively, in the case of 5% Gd-doped sample. This 
shift in Gd4d peaks is ascribed to the electron transfer from 
plasmonics Gd to ZnO because of the strong electronic 
interaction (covalent bond) between Gd and the oxide 
(Ahmad et al. 2011; Wang et al. 2012; Bharathi et al. 2020).

UV visible spectroscopy analysis

The UV–Visible spectroscopy is a powerful tool for estimat-
ing band gap of materials. UV–Vis absorbance spectra and 
tauc plots for the Gd-doped ZnO samples (Gd = 0% (Gora et 
al. 2022), 2%, and 5%) samples are shown in Figs.9 (a, b, c, 
and d). The spectra show a red shift with increasing Gd 
amount in the ZnO.The UV–Vis absorption intensity of the 
synthesized polycrystalline samples is found to reduce with 
the increasing Gd concentration (Mazhdi and Tafreshi 2018). 
Tauc's plot has been used to calculate the direct band gap of 
the samples (Jain et al. 2006). The direct band gap has been 
estimated using the following equation:     

Here, α is the absorption coefficient,υ is frequency, h is the 
Planck constant, A is a constant, and Eg is the energy band 
gap of the substance.

Extrapolating the linear component of the curve plotted 
among (αhυ) 2 and hυ depicted in Fig. 9 (b, c, and d) has been 
used to obtain the band gap (Farhad et al. 2018; Ghos et al. 
2021). The band gap values calculated for the Gd-doped ZnO 
samples, i.e.are 3.31 eV (Gora et al. 2022), 3.27 eV, and 
3.23eV, for the samples Gd = 0%, 2% and 5%, respectively. 
The small redshift observed in the band gap of Gd-doped 
ZnO samples is because of the changes in electronic structure 
as well as enhanced oxygen vacancies with the increasing Gd 
doping (Yi et al. 2017; Flemban et al. 2016), which is respon-
sible for the band gap reduction. 

Photoluminescence spectroscopy analysis

The structural flaws like ionic and atomic vacancies, replace-
ments and interstitials have been explored using the PL 

spectra of the synthesized polycrystalline samples. Fig. 10 (a, 
b and c) shows the deconvoluted, Gaussian-fitted PL spectra 
of the synthesized Gd-doped ZnO polycrystalline powder 
samples.

The PL spectra of the samples were recorded at room 
temperature in order to determine the types of vacancies, 
defect band emission etc. (Lin et al. 2001; Yi et al. 2017). The 
photoluminescence spectra have been deconvoluted and the 
different peaks responsible for the emission have been 

Gaussian fitted. For these samples the band edge emission 
peaksare detected at 381 nm, 384 nm, and 383 nm, for the Gd 
= 0%, 2% and 5% samples, respectively. The UV emission is 
responsible for the excitonic interaction related to the band 
edge emission. The intense violet emission exhibited in the 
Gd-doped ZnO at about 415 nm could be because ofthe 
oxygen vacancies. The exciton interaction between the holes 
in the valence band and electrons located in the interstitial 
zinc could be responsible for the violet emission peak at 
about 447 nm. Intrinsic deficiencies like interstitial zinc and 

oxygen can be attributed to the peak at 477 nm (Singh et al.  
2009; Kaur et al. 2016). The transformation between singly 
ionized oxygen deficiency and the photo-created holes (Ken-
nedy et al. 2014) or the surface imperfection and electrons 
near the conduction band, might be responsible for the green 
emission peak seen for the Gd-doped ZnOat around 525 nm. 
The green emissions are commonly attributed to theoxygen 
vacancies (Studenikin et al. 1998). For Gd-doped ZnO 
polycrystalline powder samples, the intensity ratio of the 
green and ultra-violet emission peaks is found to be 0.090 for 
ZnO, 0.204 for the 2% Gd-doped ZnO, and 0.418 for the 5% 
Gd-doped ZnO. The defect-related emissions enhanced with 
increasing Gd doping in the ZnO indicate that a large number 
of deficiencies, such as oxygen vacancies, are most usually 
related to crystal deformation (Yi et al. 2017). The results 
reveal that the Gd doping enhances the Zni and Vo flaws. It 
could be attributed to structural deformation created by Gd 
atoms with different ionic sizes than Zn in the ZnO (Ban-
dopadhyay and Mitra 2015; Sukumaran et al. 2021). Increas-
ing Gd doping in ZnO indicates an increase in defect states, 
which matches well with the XPS results.

We compared our PL results with Farhad et al. (2019), who 
prepared ZnO DC (drop casting) and ZnO nanorods. Their 
PL results detected the PL peak at 380 nm and a barely 
observable green-yellow emission peak; in our case, we find 
it at 381 nm for ZnO, 384 nm for 2% Gd-doped ZnO and 383 
nm for the 5% Gd-doped ZnO polycrystalline sample. This 
peak represented ZnO's near band edge (NBE) transition 
because of free excitons recombinations and this PL property 
indicates high crystalline quality. The luminescent character-
istics in the visible region are responsible for the different 
defects, like zinc interstitials (Zni), oxygen vacancies (Vo), 
etc., present in the ZnO crystal matrix. As a result, they 
reported that both ZnO (DC) and ZnO nanorods were found 
to be defect-free good crystalline and optical properties mate-
rials. In our case, all the prepared Gd-doped ZnO powder 
samples have been found to be with single ionized oxygen 
vacancies, which are confirmed by the green emission peak 
at 525 nm in the PL spectra.

Conclusion

The Gd-doped ZnO powder samples were synthesized by the 
solid-state reaction method. The XRD patterns affirm that Gd 
has been successfully incorporated into the ZnO lattice and 
confirm the hexagonal wurtzite structure. SEM images have 
been used to reveal the morphologies of the synthesized 
samples, which appear as rod-like structures. The XPS 
results showed that the oxygen vacancy-related states in 
Gd-doped samples increased with increasing Gd doping. PL 
findings confirmed that the defect-related states are present 

in the Gd-doped ZnO polycrystalline samples. The green 
emission and ultra-violet emission peaks' intensity ratio is 
0.418 for the 5% Gd-doped ZnO sample, which is the maxi-
mum compared to the pure ZnO and 2% Gd-doped ZnO 
polycrystalline powder sample. These findings confirmed 
that the oxygen vacancies increase as the Gd concentration 
increases and the PL results agree well with the XPS results.
The UV absorption spectra findings reveal that the band gap 
in the 5% Gd-doped ZnO sample is found to be 3.23 eV, 
which is the minimum compared to both the pure ZnO and 
the 2% Gd-doped ZnO samples, confirming that Eg is 
reduced with increasing Gd concentration.

Acknowledgment

The authors are thankful to USIC, University of Rajasthan, 
Jaipur, Rajasthan (India), for providing SEM images of the 
prepared samples. We also thank department of physics, 
BanasthaliVidhyapith, Banasthali, Jaipur, Rajasthan (India) 
for rendering XRD data.

Funding sources

This research is not financed by any agency.

Authors' contribution

Sample preparation, data analysis, manuscript composition 
and writing (Mahendra Kumar Gora); sample synthesis and 
data characterization (Arvind Kumar); data recording (Ban-
wari Lal Choudhary); data analysis and interpretation 
(Sanjay Kumar); reviewing and editing (Satya NarainDolia); 
supervision, writing, reviewing and editing (Rishi Kumar 
Singhal).

Conflicts of interest or competing interests 

All authors declare that we have no conflict of interest or 
competing interests.

Data availability

On request, data will be available.

Ethical approval

Not applicable.

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doped ZnO nanocrystals also have an attractive optical 
responsiveness. Many theoretical and experimental studies 
showed that RE-doped ZnO could reportedly absorb ultravio-
let (UV) light (Chakraborty et al.  2017). Obeid  et al. (2019) 
prepared undoped and Gd-doped ZnO nanocrystalline 
samples using the thermal decomposition method. They 
observed that the optical absorption spectra of pristine ZnO 
increased with a 6% Gd doping concentration and the 
magnetic properties changed with changing dopant 
amounts.  Their PL spectroscopy results showed the 
existence of defects, which increased with increasing Gd 
doping concentrations. They suggested that these DMS 
nanoparticles can be used for magneto-optoelectronic 
practical device applications.

Undoubtedly, the RE-doped ZnO nanostructures have been 
intensively studied recently because this is a rapidly growing 
field due to their multi-functionality and their use in possible 
new generation applications. With an aim to understand the 
effect of Gd doping in ZnO in terms of its electronic and 
optical properties we have synthesize Gd-doped ZnO powder 
series of different Gd (0%, 2%, and 5%) values. Various 
characterization techniques have been utilized to explore the 
electronic, structural and optical features of the synthesized 
powder series. Gd doping into ZnO produced changes in the 
band gap and enhanced oxygen vacancies related states in the 
compound.

Materials and methods

The Gd-doped ZnO powder samples with varying Gd (0%, 
2%, and 5%) concentrations have been synthesized using the 
conventional solid-state reaction method. A suitable amount 
of high-purity (99.999% Sigma-Aldrich) zinc oxide (ZnO) 
and gadolinium oxide (Gd2O3) powder have been used for 
their preparation. For achieving perfect homogeneity, the 
ingredients were properly pounded for ~8 hours in an agate 
mortar and pestle. The prepared material has been sintered 
for ~8h at~550°C and cooled at room temperature. Again, the 
mixture was ground for ~2 hours and sintered for ~6 hours at 
~550°C. The mixture was milled for ~4 hours before being 
turned into pellets. Finally, these pellets were sintered for 
around ~8 hours at about ~600°C. Finally, we ground the 
pellets to make the powder. The powder form of the prepared 
samples has been used for further characterization.

Characterization techniques

The crystallographic structures of the samples have been 
explored using an X-ray diffractometer (Make: Panalytical 
X'Pert PRO) using the photon energy of Cu Kα (λ= 
1.5406Å). SEM images have been used for the morphology 
study of the synthesized samples. The SEM equipment model 

Sample Lattice Parameter Å c/a 
a c 

ZnO 3.2505 5.2077 1.6 
2% Gd@ZnO 3.2498 5.2059 1.6 
5% Gd@ZnO 3.2490 5.2047 1.6 

Table I. Lattice parameters and c/a ratio of the  Gd-doped 
ZnO polycrystalline powder samples

Fig. 3. Rietveld refined XRD patterns for polycrystalline powder samples of (a) the pure ZnO (Gora et al.  2022), 
(b) the 2% Gd-doped ZnO and (c) the 5% Gd-doped ZnO. Observed data is shown by red dots and black solid 
line is the fitted profile. Bragg peak positions are marked by vertical green lines. The lower plot in blue is the 
residual curve



Gora, Kumar, Choudhary, Dolia, Kumar and Singhal 57

RE elements, such as Gd, may introduce additional capabili-
ties to the ZnO system because they may be capable of 
constructing combined practical devices on a single chip by 
combining magnetic and optical properties (Ohno et al. 1996; 
Tawil et al. 2011). 

Rare-earth (RE) metal doping in the dilute magnetic 
semiconductor (DMS) systems has recently received 
immense attention due to its potential usage in magneto 
electronic and photonic devices (Monteiro et al. 2006). The 
influence of Gd doping on the optical and electrical proper-
ties of ZnO is crucial for developing optoelectronic devices 
and understanding the genesis of ferromagnetism. Ma and 
Wang (2012) synthesized the Gd-doped ZnO nanocrystals 
using the thermal evaporation deposition method. According 
to their findings, Gd doping has an appreciable impact on the 
optical characteristics of the ZnO. The strong UV and blue 
emissions are detected at low Gd concentrations because Gd 
impurities introduce a mid-gap state into ZnO. In compari-
son, the UV and blue emissions reduce for high doping 
concentrations as a significant broad defect emission appears 
because of the vast numbers of defects, impurities, and 
excess Gd2O3 produced. Gd-doped ZnO nanocrystals can be 
used in optoelectronic devices. The optical and electrical 
tuning of the carrier density and optical band gap energy is 
essential for developing luminescent materials and equip-
ment based on the ZnO DMS systems. The optical band gap 
and carrier concentration are essential parameters to yield 
information about the emission wavelength. Several develop-
ment characteristics, like the synthesis methods, impurity 
concentration and substrate variety have been reported to 
influence these parameters (Wakano et al. 2001; Tan et al. 
2011; Murtaza et al.  2011).

The synthesis of DMS materials with unique optical, photo-
catalytic and magnetic characteristics seems to be the most 
promising candidate for the subsequent generation of 
electronic, optoelectronic and spintronic appliances (Kumar  
and Sahare 2014).  The low-dimension materials with imper-
fection-free lattice structures have excellent electrical and 
magnetic characteristics; purposefully made or accidental 
structural defects play a significant role in altering their 
features (Li et al. 2016; Obeid et al. 2018). Thus, the presence 
of different kinds of cation or anion vacancies and intersti-
tials in ZnO nanocrystals may change their magnetic proper-
ties, photoluminescence, and photocatalytic properties (Coey 
et al. 2005). Although certain experimental and theoretical 
findings based on DFT indicate that Gd-doped ZnO exhibits 
the room temperature ferromagnetism (RTFM) (Potzger et 
al. 2006; Aravindh et al. 2014) property, several studies have 
demonstrated that the RTFM is not present (Sambasivam et 
al. 2015). In addition to their magnetic qualities, the 4f ions 

ETH 30 kV EVO-18 Carl Zeiss, with a tilt of 0° to 60° and a 
rotation of 360°, has been used for taking SEM images.The 
defect-related states and chemical analysis of the samples 
have been determined using X-ray photoelectron spectrosco-
py. A hemispherical analyzer with a 128-channel detector and 
a monochromated, fully integrated K-alpha small-spot X-ray 
photoelectron spectrometer system of 180° double focusing 
is used in XPS (Thermo Scientific made). The UV-visible 
absorption spectroscopy (Model: Shimadzu UV-3600 
UV-VIS-NIR Spectrophotometer) was used to explore the 
prepared samples' optical absorption and band gap.
The measurements of UV-visible absorption spectra have 
been taken using the powder form of the samples. PL 
spectroscopy was used to analyze defect-related states in the 
prepared samples. Photoluminescence spectra have been 
recorded at room temperature using the fluorescence 
spectrometer (Model: Perkin Elmer FL 8500). 

Results and discussion 

XRD analysis

Fig. 1. depicts the powder XRD patterns of the Gd-doped 
ZnO (Gd = 0%, 2%, and 5%) samples. All the diffraction 
peaks detected in the ZnO sample match with the standard 
JCPDS card number (36-1451). This confirms a hexagonal 
wurtzite crystal structure of the samples.

The diffraction peaks were observed at 31.88°, 34.50°, 
36.25°, 47.72°, 56.69°, 63.03°, 66.59°, 68.17°, and 69.35°, 
which are related to the planes (100), (002), (101), (102), 

(110), (103), (200), (112), and (201), respectively (Ganesh et 
al. 2018; Bharathi et al. 2020). Further, some additional 
small peaks have been observed at 28.61°, 33.21°, and 39.14° 
with increasing Gd doping. These secondary peaks represent-
ing a crystallite phase have been identified as Gd2O3, which 
is well consistent with earlier reports (Das et al. 2012). As 
shown in Fig. 2, the highest intense peak related to the (101) 
plane shifted slightly higher angles with the increasing 
concentration of Gd doping. The shift is because of the 
substitution of bigger-sized Gd3+ (0.94 Å) ions than Zn2+ 
(0.74 Å) ions in the hexagonal wurtzite structure.  The peak 
shift may indicate that Gd3+ ions have occupied the crystallo-
graphic positions of Zn2+ ions in the ZnO host lattice, or 
itmay be because of the structural strain caused by the forma-
tion of internal compressive micro stress (Dakhel and El-Hilo 
2010; Kumar et al. 2014; Sahu et al. 2017). 

Rietveld refinements have been performed using the Full Prof 
Programme (shown in Fig. 3). The Pseudo-Voigt function has 
been used for Rietveld refinement of all the samples with the 
space group P63mc. The estimated lattice parameters have 
been listed in Table I. The lattice parameters are found to 
decrease with Gd doping, similar to those reported in earlier 
studies (Kaur et al. 2016; Obeid et al. 2019). The factors like 
the induced deficiencies (vacancies, interstitial) and the ionic 
radius of Zn and Gd ions can be responsible for the observed 
change (Kaur et al. 2016; Kumar and Thangavel 2017).

SEM analysis

Scanning electron micrographs have been used to analyze 
the morphology of the Gd-doped ZnO powder samples. The 
SEM images reveal a rod-like morphology in pure ZnO 
(Gora  et al. 2022) and the Gd-doped ZnO polycrystalline 

samples with some proof of agglomeration and phase segre-
gation, as depicted in Fig. 4 (a, b and c). Profound observa-
tion of the particles exhibits that the crystallite size is above 
100 nm. The particles were intensive and incompatibly 
distributed all over the mass. A distinct boundary between 
neighbouring crystallites can be noticed, despite the proxim-
ity of these smaller crystallites.

XPS analysis

The XPS technique is an excellent tool for determining the 
surface chemistry and electron configuration of the 
various elements present in a multicomponent material. 
The carbon C1s peak (284.6 eV) has been used as a refer-

ence to calibrate all the binding energies of the spectra. 
The survey spectrum of the Gd-doped ZnO samples is 
shown in Fig. 5. The survey spectra revealed the existence 
of O, Zn, and Gd elements without any impurity in the 
samples.

For ZnO and 2% Gd-doped ZnO powder samples, the binding 
energies of the Zn2p3/2 and Zn2p1/2 peaks are found to be 
1021.25 eV, 1044.25 eV, 1021.43 eV, and 1044.43 eV, respec-
tively. However, for the 5% Gd-doped ZnO sample, these 
peaks are at 1021.83 and 1044.83 eV energy positions. The 
small shift could be assigned to the creation of Zn interstitials 
in ZnO upon Gd doping. The binding energy gap between the 
zinc doublet peaks has been found at about 23 eV. This 
indicates that Zn in the ZnO crystal structure is in the Zn2+ 
oxidation state (Khataee et al. 2015; Gawai et al. 2019) as the 
spin-orbital splitting of Zn2p is 23 eV. This difference does 
not change with the Gd doping pointing out that Gd doping 
has no discernible effect on the chemical position of ZnO.

The O1s asymmetric XPS spectraare shown in Fig.7 (a) 
which is asymmetric in shape. The O1s XPS spectra of all 
these samples i.e. the ZnO, the Zn0.98Gd0.02O, and the 
Zn0.95Gd0.05O have been deconvoluted mainly into two Gauss-
ian peaks, as shown in Fig. 7 (b, c, and d). The lower binding 
peaks for pure and Gd-doped ZnO (Gd = 2% and 5%) 
polycrystalline samples have been detected at 529.93 eV, 
529.93 eV and 529.78 eV, respectively, which attributed to 
the lattice oxygen in the hexagonal structure of the ZnO.

The higher binding peak for the pure and the Gd-doped ZnO 
samples have been found at 531.51 eV, 531.66 eV, and 
531.49 eV, respectively, which is due to the presence of 
oxygen vacancies in the ZnO structure (Bharathi et al. 2020; 
Sukumaran et al. 2021). An additional signal centered at 
528.29 eV has been detected in the 5% Gd-doped ZnO 
powder sample, which could be ascribed to the coordination 
of oxygen in Gd-O-Zn along grain boundaries because of the 
excessive Gd dopants and also attributed to the presence of 

Gd2O3 in the sample (Yi  et al.  2017). The lattice oxygen peak 
has slightly shifted to the lower binding energy with increas-
ing Gd concentration, which has been due to the increased 
oxygen vacancies with increasing Gd doping. The estimated 
area ratios of the oxygen vacancies peak and the lattice oxygen 
peak for the samples ZnO, Zn0.98Gd0.02O, and Zn0.95Gd0.05O 
comes out to be 0.53, 1.30, and 2.49, respectively. These 
results show that the oxygen vacancies increase with increas-
ing Gd concentration.

The Fig. 8 (a, b) depicts the high-resolution Gd4d XPS 
spectra of the Gd-doped samples.In the Gd4d state, the 
spin-orbit splatted peaks are at 144.6 eV for Gd4d3/2 and 
139.6 eV for Gd4d5/2 for the 2% Gd-doped sample, whereas 
for the 5% Gd-doped ZnO sample, the spin-orbit splits at 
145.5 eV for Gd4d3/2 and 139.7 eV for Gd4d5/2 as shown in 
Fig. 8 (a, b). This indicates that Gd is present in the Gd3+ state 
in the ZnO hexagonal structure. The shift in peak values of 
Gd4d3/2 and Gd4d5/2 from 144.6 eV and 139.6 eV, respective-
ly; in the case of 2% Gd-doped sample to 145.5 eV and 139.7 
eV, respectively, in the case of 5% Gd-doped sample. This 
shift in Gd4d peaks is ascribed to the electron transfer from 
plasmonics Gd to ZnO because of the strong electronic 
interaction (covalent bond) between Gd and the oxide 
(Ahmad et al. 2011; Wang et al. 2012; Bharathi et al. 2020).

UV visible spectroscopy analysis

The UV–Visible spectroscopy is a powerful tool for estimat-
ing band gap of materials. UV–Vis absorbance spectra and 
tauc plots for the Gd-doped ZnO samples (Gd = 0% (Gora et 
al. 2022), 2%, and 5%) samples are shown in Figs.9 (a, b, c, 
and d). The spectra show a red shift with increasing Gd 
amount in the ZnO.The UV–Vis absorption intensity of the 
synthesized polycrystalline samples is found to reduce with 
the increasing Gd concentration (Mazhdi and Tafreshi 2018). 
Tauc's plot has been used to calculate the direct band gap of 
the samples (Jain et al. 2006). The direct band gap has been 
estimated using the following equation:     

Here, α is the absorption coefficient,υ is frequency, h is the 
Planck constant, A is a constant, and Eg is the energy band 
gap of the substance.

Extrapolating the linear component of the curve plotted 
among (αhυ) 2 and hυ depicted in Fig. 9 (b, c, and d) has been 
used to obtain the band gap (Farhad et al. 2018; Ghos et al. 
2021). The band gap values calculated for the Gd-doped ZnO 
samples, i.e.are 3.31 eV (Gora et al. 2022), 3.27 eV, and 
3.23eV, for the samples Gd = 0%, 2% and 5%, respectively. 
The small redshift observed in the band gap of Gd-doped 
ZnO samples is because of the changes in electronic structure 
as well as enhanced oxygen vacancies with the increasing Gd 
doping (Yi et al. 2017; Flemban et al. 2016), which is respon-
sible for the band gap reduction. 

Photoluminescence spectroscopy analysis

The structural flaws like ionic and atomic vacancies, replace-
ments and interstitials have been explored using the PL 

spectra of the synthesized polycrystalline samples. Fig. 10 (a, 
b and c) shows the deconvoluted, Gaussian-fitted PL spectra 
of the synthesized Gd-doped ZnO polycrystalline powder 
samples.

The PL spectra of the samples were recorded at room 
temperature in order to determine the types of vacancies, 
defect band emission etc. (Lin et al. 2001; Yi et al. 2017). The 
photoluminescence spectra have been deconvoluted and the 
different peaks responsible for the emission have been 

Gaussian fitted. For these samples the band edge emission 
peaksare detected at 381 nm, 384 nm, and 383 nm, for the Gd 
= 0%, 2% and 5% samples, respectively. The UV emission is 
responsible for the excitonic interaction related to the band 
edge emission. The intense violet emission exhibited in the 
Gd-doped ZnO at about 415 nm could be because ofthe 
oxygen vacancies. The exciton interaction between the holes 
in the valence band and electrons located in the interstitial 
zinc could be responsible for the violet emission peak at 
about 447 nm. Intrinsic deficiencies like interstitial zinc and 

oxygen can be attributed to the peak at 477 nm (Singh et al.  
2009; Kaur et al. 2016). The transformation between singly 
ionized oxygen deficiency and the photo-created holes (Ken-
nedy et al. 2014) or the surface imperfection and electrons 
near the conduction band, might be responsible for the green 
emission peak seen for the Gd-doped ZnOat around 525 nm. 
The green emissions are commonly attributed to theoxygen 
vacancies (Studenikin et al. 1998). For Gd-doped ZnO 
polycrystalline powder samples, the intensity ratio of the 
green and ultra-violet emission peaks is found to be 0.090 for 
ZnO, 0.204 for the 2% Gd-doped ZnO, and 0.418 for the 5% 
Gd-doped ZnO. The defect-related emissions enhanced with 
increasing Gd doping in the ZnO indicate that a large number 
of deficiencies, such as oxygen vacancies, are most usually 
related to crystal deformation (Yi et al. 2017). The results 
reveal that the Gd doping enhances the Zni and Vo flaws. It 
could be attributed to structural deformation created by Gd 
atoms with different ionic sizes than Zn in the ZnO (Ban-
dopadhyay and Mitra 2015; Sukumaran et al. 2021). Increas-
ing Gd doping in ZnO indicates an increase in defect states, 
which matches well with the XPS results.

We compared our PL results with Farhad et al. (2019), who 
prepared ZnO DC (drop casting) and ZnO nanorods. Their 
PL results detected the PL peak at 380 nm and a barely 
observable green-yellow emission peak; in our case, we find 
it at 381 nm for ZnO, 384 nm for 2% Gd-doped ZnO and 383 
nm for the 5% Gd-doped ZnO polycrystalline sample. This 
peak represented ZnO's near band edge (NBE) transition 
because of free excitons recombinations and this PL property 
indicates high crystalline quality. The luminescent character-
istics in the visible region are responsible for the different 
defects, like zinc interstitials (Zni), oxygen vacancies (Vo), 
etc., present in the ZnO crystal matrix. As a result, they 
reported that both ZnO (DC) and ZnO nanorods were found 
to be defect-free good crystalline and optical properties mate-
rials. In our case, all the prepared Gd-doped ZnO powder 
samples have been found to be with single ionized oxygen 
vacancies, which are confirmed by the green emission peak 
at 525 nm in the PL spectra.

Conclusion

The Gd-doped ZnO powder samples were synthesized by the 
solid-state reaction method. The XRD patterns affirm that Gd 
has been successfully incorporated into the ZnO lattice and 
confirm the hexagonal wurtzite structure. SEM images have 
been used to reveal the morphologies of the synthesized 
samples, which appear as rod-like structures. The XPS 
results showed that the oxygen vacancy-related states in 
Gd-doped samples increased with increasing Gd doping. PL 
findings confirmed that the defect-related states are present 

in the Gd-doped ZnO polycrystalline samples. The green 
emission and ultra-violet emission peaks' intensity ratio is 
0.418 for the 5% Gd-doped ZnO sample, which is the maxi-
mum compared to the pure ZnO and 2% Gd-doped ZnO 
polycrystalline powder sample. These findings confirmed 
that the oxygen vacancies increase as the Gd concentration 
increases and the PL results agree well with the XPS results.
The UV absorption spectra findings reveal that the band gap 
in the 5% Gd-doped ZnO sample is found to be 3.23 eV, 
which is the minimum compared to both the pure ZnO and 
the 2% Gd-doped ZnO samples, confirming that Eg is 
reduced with increasing Gd concentration.

Acknowledgment

The authors are thankful to USIC, University of Rajasthan, 
Jaipur, Rajasthan (India), for providing SEM images of the 
prepared samples. We also thank department of physics, 
BanasthaliVidhyapith, Banasthali, Jaipur, Rajasthan (India) 
for rendering XRD data.

Funding sources

This research is not financed by any agency.

Authors' contribution

Sample preparation, data analysis, manuscript composition 
and writing (Mahendra Kumar Gora); sample synthesis and 
data characterization (Arvind Kumar); data recording (Ban-
wari Lal Choudhary); data analysis and interpretation 
(Sanjay Kumar); reviewing and editing (Satya NarainDolia); 
supervision, writing, reviewing and editing (Rishi Kumar 
Singhal).

Conflicts of interest or competing interests 

All authors declare that we have no conflict of interest or 
competing interests.

Data availability

On request, data will be available.

Ethical approval

Not applicable.

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doped ZnO nanocrystals also have an attractive optical 
responsiveness. Many theoretical and experimental studies 
showed that RE-doped ZnO could reportedly absorb ultravio-
let (UV) light (Chakraborty et al.  2017). Obeid  et al. (2019) 
prepared undoped and Gd-doped ZnO nanocrystalline 
samples using the thermal decomposition method. They 
observed that the optical absorption spectra of pristine ZnO 
increased with a 6% Gd doping concentration and the 
magnetic properties changed with changing dopant 
amounts.  Their PL spectroscopy results showed the 
existence of defects, which increased with increasing Gd 
doping concentrations. They suggested that these DMS 
nanoparticles can be used for magneto-optoelectronic 
practical device applications.

Undoubtedly, the RE-doped ZnO nanostructures have been 
intensively studied recently because this is a rapidly growing 
field due to their multi-functionality and their use in possible 
new generation applications. With an aim to understand the 
effect of Gd doping in ZnO in terms of its electronic and 
optical properties we have synthesize Gd-doped ZnO powder 
series of different Gd (0%, 2%, and 5%) values. Various 
characterization techniques have been utilized to explore the 
electronic, structural and optical features of the synthesized 
powder series. Gd doping into ZnO produced changes in the 
band gap and enhanced oxygen vacancies related states in the 
compound.

Materials and methods

The Gd-doped ZnO powder samples with varying Gd (0%, 
2%, and 5%) concentrations have been synthesized using the 
conventional solid-state reaction method. A suitable amount 
of high-purity (99.999% Sigma-Aldrich) zinc oxide (ZnO) 
and gadolinium oxide (Gd2O3) powder have been used for 
their preparation. For achieving perfect homogeneity, the 
ingredients were properly pounded for ~8 hours in an agate 
mortar and pestle. The prepared material has been sintered 
for ~8h at~550°C and cooled at room temperature. Again, the 
mixture was ground for ~2 hours and sintered for ~6 hours at 
~550°C. The mixture was milled for ~4 hours before being 
turned into pellets. Finally, these pellets were sintered for 
around ~8 hours at about ~600°C. Finally, we ground the 
pellets to make the powder. The powder form of the prepared 
samples has been used for further characterization.

Characterization techniques

The crystallographic structures of the samples have been 
explored using an X-ray diffractometer (Make: Panalytical 
X'Pert PRO) using the photon energy of Cu Kα (λ= 
1.5406Å). SEM images have been used for the morphology 
study of the synthesized samples. The SEM equipment model 

  

Fig. 4. SEM images of prepared polycrystalline powder samples: (a) for the ZnO (Gora et al. 2022), (b) for the 2% 
Gd-doped ZnO and (c) for the 5% Gd- doped ZnO



Electronic, structural and optical properties of Gd-doped ZnO powder 58(1) 202358

RE elements, such as Gd, may introduce additional capabili-
ties to the ZnO system because they may be capable of 
constructing combined practical devices on a single chip by 
combining magnetic and optical properties (Ohno et al. 1996; 
Tawil et al. 2011). 

Rare-earth (RE) metal doping in the dilute magnetic 
semiconductor (DMS) systems has recently received 
immense attention due to its potential usage in magneto 
electronic and photonic devices (Monteiro et al. 2006). The 
influence of Gd doping on the optical and electrical proper-
ties of ZnO is crucial for developing optoelectronic devices 
and understanding the genesis of ferromagnetism. Ma and 
Wang (2012) synthesized the Gd-doped ZnO nanocrystals 
using the thermal evaporation deposition method. According 
to their findings, Gd doping has an appreciable impact on the 
optical characteristics of the ZnO. The strong UV and blue 
emissions are detected at low Gd concentrations because Gd 
impurities introduce a mid-gap state into ZnO. In compari-
son, the UV and blue emissions reduce for high doping 
concentrations as a significant broad defect emission appears 
because of the vast numbers of defects, impurities, and 
excess Gd2O3 produced. Gd-doped ZnO nanocrystals can be 
used in optoelectronic devices. The optical and electrical 
tuning of the carrier density and optical band gap energy is 
essential for developing luminescent materials and equip-
ment based on the ZnO DMS systems. The optical band gap 
and carrier concentration are essential parameters to yield 
information about the emission wavelength. Several develop-
ment characteristics, like the synthesis methods, impurity 
concentration and substrate variety have been reported to 
influence these parameters (Wakano et al. 2001; Tan et al. 
2011; Murtaza et al.  2011).

The synthesis of DMS materials with unique optical, photo-
catalytic and magnetic characteristics seems to be the most 
promising candidate for the subsequent generation of 
electronic, optoelectronic and spintronic appliances (Kumar  
and Sahare 2014).  The low-dimension materials with imper-
fection-free lattice structures have excellent electrical and 
magnetic characteristics; purposefully made or accidental 
structural defects play a significant role in altering their 
features (Li et al. 2016; Obeid et al. 2018). Thus, the presence 
of different kinds of cation or anion vacancies and intersti-
tials in ZnO nanocrystals may change their magnetic proper-
ties, photoluminescence, and photocatalytic properties (Coey 
et al. 2005). Although certain experimental and theoretical 
findings based on DFT indicate that Gd-doped ZnO exhibits 
the room temperature ferromagnetism (RTFM) (Potzger et 
al. 2006; Aravindh et al. 2014) property, several studies have 
demonstrated that the RTFM is not present (Sambasivam et 
al. 2015). In addition to their magnetic qualities, the 4f ions 

ETH 30 kV EVO-18 Carl Zeiss, with a tilt of 0° to 60° and a 
rotation of 360°, has been used for taking SEM images.The 
defect-related states and chemical analysis of the samples 
have been determined using X-ray photoelectron spectrosco-
py. A hemispherical analyzer with a 128-channel detector and 
a monochromated, fully integrated K-alpha small-spot X-ray 
photoelectron spectrometer system of 180° double focusing 
is used in XPS (Thermo Scientific made). The UV-visible 
absorption spectroscopy (Model: Shimadzu UV-3600 
UV-VIS-NIR Spectrophotometer) was used to explore the 
prepared samples' optical absorption and band gap.
The measurements of UV-visible absorption spectra have 
been taken using the powder form of the samples. PL 
spectroscopy was used to analyze defect-related states in the 
prepared samples. Photoluminescence spectra have been 
recorded at room temperature using the fluorescence 
spectrometer (Model: Perkin Elmer FL 8500). 

Results and discussion 

XRD analysis

Fig. 1. depicts the powder XRD patterns of the Gd-doped 
ZnO (Gd = 0%, 2%, and 5%) samples. All the diffraction 
peaks detected in the ZnO sample match with the standard 
JCPDS card number (36-1451). This confirms a hexagonal 
wurtzite crystal structure of the samples.

The diffraction peaks were observed at 31.88°, 34.50°, 
36.25°, 47.72°, 56.69°, 63.03°, 66.59°, 68.17°, and 69.35°, 
which are related to the planes (100), (002), (101), (102), 

(110), (103), (200), (112), and (201), respectively (Ganesh et 
al. 2018; Bharathi et al. 2020). Further, some additional 
small peaks have been observed at 28.61°, 33.21°, and 39.14° 
with increasing Gd doping. These secondary peaks represent-
ing a crystallite phase have been identified as Gd2O3, which 
is well consistent with earlier reports (Das et al. 2012). As 
shown in Fig. 2, the highest intense peak related to the (101) 
plane shifted slightly higher angles with the increasing 
concentration of Gd doping. The shift is because of the 
substitution of bigger-sized Gd3+ (0.94 Å) ions than Zn2+ 
(0.74 Å) ions in the hexagonal wurtzite structure.  The peak 
shift may indicate that Gd3+ ions have occupied the crystallo-
graphic positions of Zn2+ ions in the ZnO host lattice, or 
itmay be because of the structural strain caused by the forma-
tion of internal compressive micro stress (Dakhel and El-Hilo 
2010; Kumar et al. 2014; Sahu et al. 2017). 

Rietveld refinements have been performed using the Full Prof 
Programme (shown in Fig. 3). The Pseudo-Voigt function has 
been used for Rietveld refinement of all the samples with the 
space group P63mc. The estimated lattice parameters have 
been listed in Table I. The lattice parameters are found to 
decrease with Gd doping, similar to those reported in earlier 
studies (Kaur et al. 2016; Obeid et al. 2019). The factors like 
the induced deficiencies (vacancies, interstitial) and the ionic 
radius of Zn and Gd ions can be responsible for the observed 
change (Kaur et al. 2016; Kumar and Thangavel 2017).

SEM analysis

Scanning electron micrographs have been used to analyze 
the morphology of the Gd-doped ZnO powder samples. The 
SEM images reveal a rod-like morphology in pure ZnO 
(Gora  et al. 2022) and the Gd-doped ZnO polycrystalline 

samples with some proof of agglomeration and phase segre-
gation, as depicted in Fig. 4 (a, b and c). Profound observa-
tion of the particles exhibits that the crystallite size is above 
100 nm. The particles were intensive and incompatibly 
distributed all over the mass. A distinct boundary between 
neighbouring crystallites can be noticed, despite the proxim-
ity of these smaller crystallites.

XPS analysis

The XPS technique is an excellent tool for determining the 
surface chemistry and electron configuration of the 
various elements present in a multicomponent material. 
The carbon C1s peak (284.6 eV) has been used as a refer-

ence to calibrate all the binding energies of the spectra. 
The survey spectrum of the Gd-doped ZnO samples is 
shown in Fig. 5. The survey spectra revealed the existence 
of O, Zn, and Gd elements without any impurity in the 
samples.

For ZnO and 2% Gd-doped ZnO powder samples, the binding 
energies of the Zn2p3/2 and Zn2p1/2 peaks are found to be 
1021.25 eV, 1044.25 eV, 1021.43 eV, and 1044.43 eV, respec-
tively. However, for the 5% Gd-doped ZnO sample, these 
peaks are at 1021.83 and 1044.83 eV energy positions. The 
small shift could be assigned to the creation of Zn interstitials 
in ZnO upon Gd doping. The binding energy gap between the 
zinc doublet peaks has been found at about 23 eV. This 
indicates that Zn in the ZnO crystal structure is in the Zn2+ 
oxidation state (Khataee et al. 2015; Gawai et al. 2019) as the 
spin-orbital splitting of Zn2p is 23 eV. This difference does 
not change with the Gd doping pointing out that Gd doping 
has no discernible effect on the chemical position of ZnO.

The O1s asymmetric XPS spectraare shown in Fig.7 (a) 
which is asymmetric in shape. The O1s XPS spectra of all 
these samples i.e. the ZnO, the Zn0.98Gd0.02O, and the 
Zn0.95Gd0.05O have been deconvoluted mainly into two Gauss-
ian peaks, as shown in Fig. 7 (b, c, and d). The lower binding 
peaks for pure and Gd-doped ZnO (Gd = 2% and 5%) 
polycrystalline samples have been detected at 529.93 eV, 
529.93 eV and 529.78 eV, respectively, which attributed to 
the lattice oxygen in the hexagonal structure of the ZnO.

The higher binding peak for the pure and the Gd-doped ZnO 
samples have been found at 531.51 eV, 531.66 eV, and 
531.49 eV, respectively, which is due to the presence of 
oxygen vacancies in the ZnO structure (Bharathi et al. 2020; 
Sukumaran et al. 2021). An additional signal centered at 
528.29 eV has been detected in the 5% Gd-doped ZnO 
powder sample, which could be ascribed to the coordination 
of oxygen in Gd-O-Zn along grain boundaries because of the 
excessive Gd dopants and also attributed to the presence of 

Gd2O3 in the sample (Yi  et al.  2017). The lattice oxygen peak 
has slightly shifted to the lower binding energy with increas-
ing Gd concentration, which has been due to the increased 
oxygen vacancies with increasing Gd doping. The estimated 
area ratios of the oxygen vacancies peak and the lattice oxygen 
peak for the samples ZnO, Zn0.98Gd0.02O, and Zn0.95Gd0.05O 
comes out to be 0.53, 1.30, and 2.49, respectively. These 
results show that the oxygen vacancies increase with increas-
ing Gd concentration.

The Fig. 8 (a, b) depicts the high-resolution Gd4d XPS 
spectra of the Gd-doped samples.In the Gd4d state, the 
spin-orbit splatted peaks are at 144.6 eV for Gd4d3/2 and 
139.6 eV for Gd4d5/2 for the 2% Gd-doped sample, whereas 
for the 5% Gd-doped ZnO sample, the spin-orbit splits at 
145.5 eV for Gd4d3/2 and 139.7 eV for Gd4d5/2 as shown in 
Fig. 8 (a, b). This indicates that Gd is present in the Gd3+ state 
in the ZnO hexagonal structure. The shift in peak values of 
Gd4d3/2 and Gd4d5/2 from 144.6 eV and 139.6 eV, respective-
ly; in the case of 2% Gd-doped sample to 145.5 eV and 139.7 
eV, respectively, in the case of 5% Gd-doped sample. This 
shift in Gd4d peaks is ascribed to the electron transfer from 
plasmonics Gd to ZnO because of the strong electronic 
interaction (covalent bond) between Gd and the oxide 
(Ahmad et al. 2011; Wang et al. 2012; Bharathi et al. 2020).

UV visible spectroscopy analysis

The UV–Visible spectroscopy is a powerful tool for estimat-
ing band gap of materials. UV–Vis absorbance spectra and 
tauc plots for the Gd-doped ZnO samples (Gd = 0% (Gora et 
al. 2022), 2%, and 5%) samples are shown in Figs.9 (a, b, c, 
and d). The spectra show a red shift with increasing Gd 
amount in the ZnO.The UV–Vis absorption intensity of the 
synthesized polycrystalline samples is found to reduce with 
the increasing Gd concentration (Mazhdi and Tafreshi 2018). 
Tauc's plot has been used to calculate the direct band gap of 
the samples (Jain et al. 2006). The direct band gap has been 
estimated using the following equation:     

Here, α is the absorption coefficient,υ is frequency, h is the 
Planck constant, A is a constant, and Eg is the energy band 
gap of the substance.

Extrapolating the linear component of the curve plotted 
among (αhυ) 2 and hυ depicted in Fig. 9 (b, c, and d) has been 
used to obtain the band gap (Farhad et al. 2018; Ghos et al. 
2021). The band gap values calculated for the Gd-doped ZnO 
samples, i.e.are 3.31 eV (Gora et al. 2022), 3.27 eV, and 
3.23eV, for the samples Gd = 0%, 2% and 5%, respectively. 
The small redshift observed in the band gap of Gd-doped 
ZnO samples is because of the changes in electronic structure 
as well as enhanced oxygen vacancies with the increasing Gd 
doping (Yi et al. 2017; Flemban et al. 2016), which is respon-
sible for the band gap reduction. 

Photoluminescence spectroscopy analysis

The structural flaws like ionic and atomic vacancies, replace-
ments and interstitials have been explored using the PL 

spectra of the synthesized polycrystalline samples. Fig. 10 (a, 
b and c) shows the deconvoluted, Gaussian-fitted PL spectra 
of the synthesized Gd-doped ZnO polycrystalline powder 
samples.

The PL spectra of the samples were recorded at room 
temperature in order to determine the types of vacancies, 
defect band emission etc. (Lin et al. 2001; Yi et al. 2017). The 
photoluminescence spectra have been deconvoluted and the 
different peaks responsible for the emission have been 

Gaussian fitted. For these samples the band edge emission 
peaksare detected at 381 nm, 384 nm, and 383 nm, for the Gd 
= 0%, 2% and 5% samples, respectively. The UV emission is 
responsible for the excitonic interaction related to the band 
edge emission. The intense violet emission exhibited in the 
Gd-doped ZnO at about 415 nm could be because ofthe 
oxygen vacancies. The exciton interaction between the holes 
in the valence band and electrons located in the interstitial 
zinc could be responsible for the violet emission peak at 
about 447 nm. Intrinsic deficiencies like interstitial zinc and 

oxygen can be attributed to the peak at 477 nm (Singh et al.  
2009; Kaur et al. 2016). The transformation between singly 
ionized oxygen deficiency and the photo-created holes (Ken-
nedy et al. 2014) or the surface imperfection and electrons 
near the conduction band, might be responsible for the green 
emission peak seen for the Gd-doped ZnOat around 525 nm. 
The green emissions are commonly attributed to theoxygen 
vacancies (Studenikin et al. 1998). For Gd-doped ZnO 
polycrystalline powder samples, the intensity ratio of the 
green and ultra-violet emission peaks is found to be 0.090 for 
ZnO, 0.204 for the 2% Gd-doped ZnO, and 0.418 for the 5% 
Gd-doped ZnO. The defect-related emissions enhanced with 
increasing Gd doping in the ZnO indicate that a large number 
of deficiencies, such as oxygen vacancies, are most usually 
related to crystal deformation (Yi et al. 2017). The results 
reveal that the Gd doping enhances the Zni and Vo flaws. It 
could be attributed to structural deformation created by Gd 
atoms with different ionic sizes than Zn in the ZnO (Ban-
dopadhyay and Mitra 2015; Sukumaran et al. 2021). Increas-
ing Gd doping in ZnO indicates an increase in defect states, 
which matches well with the XPS results.

We compared our PL results with Farhad et al. (2019), who 
prepared ZnO DC (drop casting) and ZnO nanorods. Their 
PL results detected the PL peak at 380 nm and a barely 
observable green-yellow emission peak; in our case, we find 
it at 381 nm for ZnO, 384 nm for 2% Gd-doped ZnO and 383 
nm for the 5% Gd-doped ZnO polycrystalline sample. This 
peak represented ZnO's near band edge (NBE) transition 
because of free excitons recombinations and this PL property 
indicates high crystalline quality. The luminescent character-
istics in the visible region are responsible for the different 
defects, like zinc interstitials (Zni), oxygen vacancies (Vo), 
etc., present in the ZnO crystal matrix. As a result, they 
reported that both ZnO (DC) and ZnO nanorods were found 
to be defect-free good crystalline and optical properties mate-
rials. In our case, all the prepared Gd-doped ZnO powder 
samples have been found to be with single ionized oxygen 
vacancies, which are confirmed by the green emission peak 
at 525 nm in the PL spectra.

Conclusion

The Gd-doped ZnO powder samples were synthesized by the 
solid-state reaction method. The XRD patterns affirm that Gd 
has been successfully incorporated into the ZnO lattice and 
confirm the hexagonal wurtzite structure. SEM images have 
been used to reveal the morphologies of the synthesized 
samples, which appear as rod-like structures. The XPS 
results showed that the oxygen vacancy-related states in 
Gd-doped samples increased with increasing Gd doping. PL 
findings confirmed that the defect-related states are present 

in the Gd-doped ZnO polycrystalline samples. The green 
emission and ultra-violet emission peaks' intensity ratio is 
0.418 for the 5% Gd-doped ZnO sample, which is the maxi-
mum compared to the pure ZnO and 2% Gd-doped ZnO 
polycrystalline powder sample. These findings confirmed 
that the oxygen vacancies increase as the Gd concentration 
increases and the PL results agree well with the XPS results.
The UV absorption spectra findings reveal that the band gap 
in the 5% Gd-doped ZnO sample is found to be 3.23 eV, 
which is the minimum compared to both the pure ZnO and 
the 2% Gd-doped ZnO samples, confirming that Eg is 
reduced with increasing Gd concentration.

Acknowledgment

The authors are thankful to USIC, University of Rajasthan, 
Jaipur, Rajasthan (India), for providing SEM images of the 
prepared samples. We also thank department of physics, 
BanasthaliVidhyapith, Banasthali, Jaipur, Rajasthan (India) 
for rendering XRD data.

Funding sources

This research is not financed by any agency.

Authors' contribution

Sample preparation, data analysis, manuscript composition 
and writing (Mahendra Kumar Gora); sample synthesis and 
data characterization (Arvind Kumar); data recording (Ban-
wari Lal Choudhary); data analysis and interpretation 
(Sanjay Kumar); reviewing and editing (Satya NarainDolia); 
supervision, writing, reviewing and editing (Rishi Kumar 
Singhal).

Conflicts of interest or competing interests 

All authors declare that we have no conflict of interest or 
competing interests.

Data availability

On request, data will be available.

Ethical approval

Not applicable.

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doped ZnO nanocrystals also have an attractive optical 
responsiveness. Many theoretical and experimental studies 
showed that RE-doped ZnO could reportedly absorb ultravio-
let (UV) light (Chakraborty et al.  2017). Obeid  et al. (2019) 
prepared undoped and Gd-doped ZnO nanocrystalline 
samples using the thermal decomposition method. They 
observed that the optical absorption spectra of pristine ZnO 
increased with a 6% Gd doping concentration and the 
magnetic properties changed with changing dopant 
amounts.  Their PL spectroscopy results showed the 
existence of defects, which increased with increasing Gd 
doping concentrations. They suggested that these DMS 
nanoparticles can be used for magneto-optoelectronic 
practical device applications.

Undoubtedly, the RE-doped ZnO nanostructures have been 
intensively studied recently because this is a rapidly growing 
field due to their multi-functionality and their use in possible 
new generation applications. With an aim to understand the 
effect of Gd doping in ZnO in terms of its electronic and 
optical properties we have synthesize Gd-doped ZnO powder 
series of different Gd (0%, 2%, and 5%) values. Various 
characterization techniques have been utilized to explore the 
electronic, structural and optical features of the synthesized 
powder series. Gd doping into ZnO produced changes in the 
band gap and enhanced oxygen vacancies related states in the 
compound.

Materials and methods

The Gd-doped ZnO powder samples with varying Gd (0%, 
2%, and 5%) concentrations have been synthesized using the 
conventional solid-state reaction method. A suitable amount 
of high-purity (99.999% Sigma-Aldrich) zinc oxide (ZnO) 
and gadolinium oxide (Gd2O3) powder have been used for 
their preparation. For achieving perfect homogeneity, the 
ingredients were properly pounded for ~8 hours in an agate 
mortar and pestle. The prepared material has been sintered 
for ~8h at~550°C and cooled at room temperature. Again, the 
mixture was ground for ~2 hours and sintered for ~6 hours at 
~550°C. The mixture was milled for ~4 hours before being 
turned into pellets. Finally, these pellets were sintered for 
around ~8 hours at about ~600°C. Finally, we ground the 
pellets to make the powder. The powder form of the prepared 
samples has been used for further characterization.

Characterization techniques

The crystallographic structures of the samples have been 
explored using an X-ray diffractometer (Make: Panalytical 
X'Pert PRO) using the photon energy of Cu Kα (λ= 
1.5406Å). SEM images have been used for the morphology 
study of the synthesized samples. The SEM equipment model 

Fig. 5. The XPS survey spectra of the pure ZnO (Gora et 
al. 2022), the Zn0.98Gd0.02O and the Zn0.95Gd0. 05O 
powder samples

Fig. 6. The Zn2p core level XPS spectra of the Gd-doped 
ZnO polycrystalline powder samples

The Zn2p core level spectra of the Gd-doped ZnO samples
are depicted in Fig. 6.

αhυ = A (hυ - Eg)
 1/2



Gora, Kumar, Choudhary, Dolia, Kumar and Singhal 59

RE elements, such as Gd, may introduce additional capabili-
ties to the ZnO system because they may be capable of 
constructing combined practical devices on a single chip by 
combining magnetic and optical properties (Ohno et al. 1996; 
Tawil et al. 2011). 

Rare-earth (RE) metal doping in the dilute magnetic 
semiconductor (DMS) systems has recently received 
immense attention due to its potential usage in magneto 
electronic and photonic devices (Monteiro et al. 2006). The 
influence of Gd doping on the optical and electrical proper-
ties of ZnO is crucial for developing optoelectronic devices 
and understanding the genesis of ferromagnetism. Ma and 
Wang (2012) synthesized the Gd-doped ZnO nanocrystals 
using the thermal evaporation deposition method. According 
to their findings, Gd doping has an appreciable impact on the 
optical characteristics of the ZnO. The strong UV and blue 
emissions are detected at low Gd concentrations because Gd 
impurities introduce a mid-gap state into ZnO. In compari-
son, the UV and blue emissions reduce for high doping 
concentrations as a significant broad defect emission appears 
because of the vast numbers of defects, impurities, and 
excess Gd2O3 produced. Gd-doped ZnO nanocrystals can be 
used in optoelectronic devices. The optical and electrical 
tuning of the carrier density and optical band gap energy is 
essential for developing luminescent materials and equip-
ment based on the ZnO DMS systems. The optical band gap 
and carrier concentration are essential parameters to yield 
information about the emission wavelength. Several develop-
ment characteristics, like the synthesis methods, impurity 
concentration and substrate variety have been reported to 
influence these parameters (Wakano et al. 2001; Tan et al. 
2011; Murtaza et al.  2011).

The synthesis of DMS materials with unique optical, photo-
catalytic and magnetic characteristics seems to be the most 
promising candidate for the subsequent generation of 
electronic, optoelectronic and spintronic appliances (Kumar  
and Sahare 2014).  The low-dimension materials with imper-
fection-free lattice structures have excellent electrical and 
magnetic characteristics; purposefully made or accidental 
structural defects play a significant role in altering their 
features (Li et al. 2016; Obeid et al. 2018). Thus, the presence 
of different kinds of cation or anion vacancies and intersti-
tials in ZnO nanocrystals may change their magnetic proper-
ties, photoluminescence, and photocatalytic properties (Coey 
et al. 2005). Although certain experimental and theoretical 
findings based on DFT indicate that Gd-doped ZnO exhibits 
the room temperature ferromagnetism (RTFM) (Potzger et 
al. 2006; Aravindh et al. 2014) property, several studies have 
demonstrated that the RTFM is not present (Sambasivam et 
al. 2015). In addition to their magnetic qualities, the 4f ions 

ETH 30 kV EVO-18 Carl Zeiss, with a tilt of 0° to 60° and a 
rotation of 360°, has been used for taking SEM images.The 
defect-related states and chemical analysis of the samples 
have been determined using X-ray photoelectron spectrosco-
py. A hemispherical analyzer with a 128-channel detector and 
a monochromated, fully integrated K-alpha small-spot X-ray 
photoelectron spectrometer system of 180° double focusing 
is used in XPS (Thermo Scientific made). The UV-visible 
absorption spectroscopy (Model: Shimadzu UV-3600 
UV-VIS-NIR Spectrophotometer) was used to explore the 
prepared samples' optical absorption and band gap.
The measurements of UV-visible absorption spectra have 
been taken using the powder form of the samples. PL 
spectroscopy was used to analyze defect-related states in the 
prepared samples. Photoluminescence spectra have been 
recorded at room temperature using the fluorescence 
spectrometer (Model: Perkin Elmer FL 8500). 

Results and discussion 

XRD analysis

Fig. 1. depicts the powder XRD patterns of the Gd-doped 
ZnO (Gd = 0%, 2%, and 5%) samples. All the diffraction 
peaks detected in the ZnO sample match with the standard 
JCPDS card number (36-1451). This confirms a hexagonal 
wurtzite crystal structure of the samples.

The diffraction peaks were observed at 31.88°, 34.50°, 
36.25°, 47.72°, 56.69°, 63.03°, 66.59°, 68.17°, and 69.35°, 
which are related to the planes (100), (002), (101), (102), 

(110), (103), (200), (112), and (201), respectively (Ganesh et 
al. 2018; Bharathi et al. 2020). Further, some additional 
small peaks have been observed at 28.61°, 33.21°, and 39.14° 
with increasing Gd doping. These secondary peaks represent-
ing a crystallite phase have been identified as Gd2O3, which 
is well consistent with earlier reports (Das et al. 2012). As 
shown in Fig. 2, the highest intense peak related to the (101) 
plane shifted slightly higher angles with the increasing 
concentration of Gd doping. The shift is because of the 
substitution of bigger-sized Gd3+ (0.94 Å) ions than Zn2+ 
(0.74 Å) ions in the hexagonal wurtzite structure.  The peak 
shift may indicate that Gd3+ ions have occupied the crystallo-
graphic positions of Zn2+ ions in the ZnO host lattice, or 
itmay be because of the structural strain caused by the forma-
tion of internal compressive micro stress (Dakhel and El-Hilo 
2010; Kumar et al. 2014; Sahu et al. 2017). 

Rietveld refinements have been performed using the Full Prof 
Programme (shown in Fig. 3). The Pseudo-Voigt function has 
been used for Rietveld refinement of all the samples with the 
space group P63mc. The estimated lattice parameters have 
been listed in Table I. The lattice parameters are found to 
decrease with Gd doping, similar to those reported in earlier 
studies (Kaur et al. 2016; Obeid et al. 2019). The factors like 
the induced deficiencies (vacancies, interstitial) and the ionic 
radius of Zn and Gd ions can be responsible for the observed 
change (Kaur et al. 2016; Kumar and Thangavel 2017).

SEM analysis

Scanning electron micrographs have been used to analyze 
the morphology of the Gd-doped ZnO powder samples. The 
SEM images reveal a rod-like morphology in pure ZnO 
(Gora  et al. 2022) and the Gd-doped ZnO polycrystalline 

samples with some proof of agglomeration and phase segre-
gation, as depicted in Fig. 4 (a, b and c). Profound observa-
tion of the particles exhibits that the crystallite size is above 
100 nm. The particles were intensive and incompatibly 
distributed all over the mass. A distinct boundary between 
neighbouring crystallites can be noticed, despite the proxim-
ity of these smaller crystallites.

XPS analysis

The XPS technique is an excellent tool for determining the 
surface chemistry and electron configuration of the 
various elements present in a multicomponent material. 
The carbon C1s peak (284.6 eV) has been used as a refer-

ence to calibrate all the binding energies of the spectra. 
The survey spectrum of the Gd-doped ZnO samples is 
shown in Fig. 5. The survey spectra revealed the existence 
of O, Zn, and Gd elements without any impurity in the 
samples.

For ZnO and 2% Gd-doped ZnO powder samples, the binding 
energies of the Zn2p3/2 and Zn2p1/2 peaks are found to be 
1021.25 eV, 1044.25 eV, 1021.43 eV, and 1044.43 eV, respec-
tively. However, for the 5% Gd-doped ZnO sample, these 
peaks are at 1021.83 and 1044.83 eV energy positions. The 
small shift could be assigned to the creation of Zn interstitials 
in ZnO upon Gd doping. The binding energy gap between the 
zinc doublet peaks has been found at about 23 eV. This 
indicates that Zn in the ZnO crystal structure is in the Zn2+ 
oxidation state (Khataee et al. 2015; Gawai et al. 2019) as the 
spin-orbital splitting of Zn2p is 23 eV. This difference does 
not change with the Gd doping pointing out that Gd doping 
has no discernible effect on the chemical position of ZnO.

The O1s asymmetric XPS spectraare shown in Fig.7 (a) 
which is asymmetric in shape. The O1s XPS spectra of all 
these samples i.e. the ZnO, the Zn0.98Gd0.02O, and the 
Zn0.95Gd0.05O have been deconvoluted mainly into two Gauss-
ian peaks, as shown in Fig. 7 (b, c, and d). The lower binding 
peaks for pure and Gd-doped ZnO (Gd = 2% and 5%) 
polycrystalline samples have been detected at 529.93 eV, 
529.93 eV and 529.78 eV, respectively, which attributed to 
the lattice oxygen in the hexagonal structure of the ZnO.

The higher binding peak for the pure and the Gd-doped ZnO 
samples have been found at 531.51 eV, 531.66 eV, and 
531.49 eV, respectively, which is due to the presence of 
oxygen vacancies in the ZnO structure (Bharathi et al. 2020; 
Sukumaran et al. 2021). An additional signal centered at 
528.29 eV has been detected in the 5% Gd-doped ZnO 
powder sample, which could be ascribed to the coordination 
of oxygen in Gd-O-Zn along grain boundaries because of the 
excessive Gd dopants and also attributed to the presence of 

Gd2O3 in the sample (Yi  et al.  2017). The lattice oxygen peak 
has slightly shifted to the lower binding energy with increas-
ing Gd concentration, which has been due to the increased 
oxygen vacancies with increasing Gd doping. The estimated 
area ratios of the oxygen vacancies peak and the lattice oxygen 
peak for the samples ZnO, Zn0.98Gd0.02O, and Zn0.95Gd0.05O 
comes out to be 0.53, 1.30, and 2.49, respectively. These 
results show that the oxygen vacancies increase with increas-
ing Gd concentration.

The Fig. 8 (a, b) depicts the high-resolution Gd4d XPS 
spectra of the Gd-doped samples.In the Gd4d state, the 
spin-orbit splatted peaks are at 144.6 eV for Gd4d3/2 and 
139.6 eV for Gd4d5/2 for the 2% Gd-doped sample, whereas 
for the 5% Gd-doped ZnO sample, the spin-orbit splits at 
145.5 eV for Gd4d3/2 and 139.7 eV for Gd4d5/2 as shown in 
Fig. 8 (a, b). This indicates that Gd is present in the Gd3+ state 
in the ZnO hexagonal structure. The shift in peak values of 
Gd4d3/2 and Gd4d5/2 from 144.6 eV and 139.6 eV, respective-
ly; in the case of 2% Gd-doped sample to 145.5 eV and 139.7 
eV, respectively, in the case of 5% Gd-doped sample. This 
shift in Gd4d peaks is ascribed to the electron transfer from 
plasmonics Gd to ZnO because of the strong electronic 
interaction (covalent bond) between Gd and the oxide 
(Ahmad et al. 2011; Wang et al. 2012; Bharathi et al. 2020).

UV visible spectroscopy analysis

The UV–Visible spectroscopy is a powerful tool for estimat-
ing band gap of materials. UV–Vis absorbance spectra and 
tauc plots for the Gd-doped ZnO samples (Gd = 0% (Gora et 
al. 2022), 2%, and 5%) samples are shown in Figs.9 (a, b, c, 
and d). The spectra show a red shift with increasing Gd 
amount in the ZnO.The UV–Vis absorption intensity of the 
synthesized polycrystalline samples is found to reduce with 
the increasing Gd concentration (Mazhdi and Tafreshi 2018). 
Tauc's plot has been used to calculate the direct band gap of 
the samples (Jain et al. 2006). The direct band gap has been 
estimated using the following equation:     

Here, α is the absorption coefficient,υ is frequency, h is the 
Planck constant, A is a constant, and Eg is the energy band 
gap of the substance.

Extrapolating the linear component of the curve plotted 
among (αhυ) 2 and hυ depicted in Fig. 9 (b, c, and d) has been 
used to obtain the band gap (Farhad et al. 2018; Ghos et al. 
2021). The band gap values calculated for the Gd-doped ZnO 
samples, i.e.are 3.31 eV (Gora et al. 2022), 3.27 eV, and 
3.23eV, for the samples Gd = 0%, 2% and 5%, respectively. 
The small redshift observed in the band gap of Gd-doped 
ZnO samples is because of the changes in electronic structure 
as well as enhanced oxygen vacancies with the increasing Gd 
doping (Yi et al. 2017; Flemban et al. 2016), which is respon-
sible for the band gap reduction. 

Photoluminescence spectroscopy analysis

The structural flaws like ionic and atomic vacancies, replace-
ments and interstitials have been explored using the PL 

spectra of the synthesized polycrystalline samples. Fig. 10 (a, 
b and c) shows the deconvoluted, Gaussian-fitted PL spectra 
of the synthesized Gd-doped ZnO polycrystalline powder 
samples.

The PL spectra of the samples were recorded at room 
temperature in order to determine the types of vacancies, 
defect band emission etc. (Lin et al. 2001; Yi et al. 2017). The 
photoluminescence spectra have been deconvoluted and the 
different peaks responsible for the emission have been 

Gaussian fitted. For these samples the band edge emission 
peaksare detected at 381 nm, 384 nm, and 383 nm, for the Gd 
= 0%, 2% and 5% samples, respectively. The UV emission is 
responsible for the excitonic interaction related to the band 
edge emission. The intense violet emission exhibited in the 
Gd-doped ZnO at about 415 nm could be because ofthe 
oxygen vacancies. The exciton interaction between the holes 
in the valence band and electrons located in the interstitial 
zinc could be responsible for the violet emission peak at 
about 447 nm. Intrinsic deficiencies like interstitial zinc and 

oxygen can be attributed to the peak at 477 nm (Singh et al.  
2009; Kaur et al. 2016). The transformation between singly 
ionized oxygen deficiency and the photo-created holes (Ken-
nedy et al. 2014) or the surface imperfection and electrons 
near the conduction band, might be responsible for the green 
emission peak seen for the Gd-doped ZnOat around 525 nm. 
The green emissions are commonly attributed to theoxygen 
vacancies (Studenikin et al. 1998). For Gd-doped ZnO 
polycrystalline powder samples, the intensity ratio of the 
green and ultra-violet emission peaks is found to be 0.090 for 
ZnO, 0.204 for the 2% Gd-doped ZnO, and 0.418 for the 5% 
Gd-doped ZnO. The defect-related emissions enhanced with 
increasing Gd doping in the ZnO indicate that a large number 
of deficiencies, such as oxygen vacancies, are most usually 
related to crystal deformation (Yi et al. 2017). The results 
reveal that the Gd doping enhances the Zni and Vo flaws. It 
could be attributed to structural deformation created by Gd 
atoms with different ionic sizes than Zn in the ZnO (Ban-
dopadhyay and Mitra 2015; Sukumaran et al. 2021). Increas-
ing Gd doping in ZnO indicates an increase in defect states, 
which matches well with the XPS results.

We compared our PL results with Farhad et al. (2019), who 
prepared ZnO DC (drop casting) and ZnO nanorods. Their 
PL results detected the PL peak at 380 nm and a barely 
observable green-yellow emission peak; in our case, we find 
it at 381 nm for ZnO, 384 nm for 2% Gd-doped ZnO and 383 
nm for the 5% Gd-doped ZnO polycrystalline sample. This 
peak represented ZnO's near band edge (NBE) transition 
because of free excitons recombinations and this PL property 
indicates high crystalline quality. The luminescent character-
istics in the visible region are responsible for the different 
defects, like zinc interstitials (Zni), oxygen vacancies (Vo), 
etc., present in the ZnO crystal matrix. As a result, they 
reported that both ZnO (DC) and ZnO nanorods were found 
to be defect-free good crystalline and optical properties mate-
rials. In our case, all the prepared Gd-doped ZnO powder 
samples have been found to be with single ionized oxygen 
vacancies, which are confirmed by the green emission peak 
at 525 nm in the PL spectra.

Conclusion

The Gd-doped ZnO powder samples were synthesized by the 
solid-state reaction method. The XRD patterns affirm that Gd 
has been successfully incorporated into the ZnO lattice and 
confirm the hexagonal wurtzite structure. SEM images have 
been used to reveal the morphologies of the synthesized 
samples, which appear as rod-like structures. The XPS 
results showed that the oxygen vacancy-related states in 
Gd-doped samples increased with increasing Gd doping. PL 
findings confirmed that the defect-related states are present 

in the Gd-doped ZnO polycrystalline samples. The green 
emission and ultra-violet emission peaks' intensity ratio is 
0.418 for the 5% Gd-doped ZnO sample, which is the maxi-
mum compared to the pure ZnO and 2% Gd-doped ZnO 
polycrystalline powder sample. These findings confirmed 
that the oxygen vacancies increase as the Gd concentration 
increases and the PL results agree well with the XPS results.
The UV absorption spectra findings reveal that the band gap 
in the 5% Gd-doped ZnO sample is found to be 3.23 eV, 
which is the minimum compared to both the pure ZnO and 
the 2% Gd-doped ZnO samples, confirming that Eg is 
reduced with increasing Gd concentration.

Acknowledgment

The authors are thankful to USIC, University of Rajasthan, 
Jaipur, Rajasthan (India), for providing SEM images of the 
prepared samples. We also thank department of physics, 
BanasthaliVidhyapith, Banasthali, Jaipur, Rajasthan (India) 
for rendering XRD data.

Funding sources

This research is not financed by any agency.

Authors' contribution

Sample preparation, data analysis, manuscript composition 
and writing (Mahendra Kumar Gora); sample synthesis and 
data characterization (Arvind Kumar); data recording (Ban-
wari Lal Choudhary); data analysis and interpretation 
(Sanjay Kumar); reviewing and editing (Satya NarainDolia); 
supervision, writing, reviewing and editing (Rishi Kumar 
Singhal).

Conflicts of interest or competing interests 

All authors declare that we have no conflict of interest or 
competing interests.

Data availability

On request, data will be available.

Ethical approval

Not applicable.

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doped ZnO nanocrystals also have an attractive optical 
responsiveness. Many theoretical and experimental studies 
showed that RE-doped ZnO could reportedly absorb ultravio-
let (UV) light (Chakraborty et al.  2017). Obeid  et al. (2019) 
prepared undoped and Gd-doped ZnO nanocrystalline 
samples using the thermal decomposition method. They 
observed that the optical absorption spectra of pristine ZnO 
increased with a 6% Gd doping concentration and the 
magnetic properties changed with changing dopant 
amounts.  Their PL spectroscopy results showed the 
existence of defects, which increased with increasing Gd 
doping concentrations. They suggested that these DMS 
nanoparticles can be used for magneto-optoelectronic 
practical device applications.

Undoubtedly, the RE-doped ZnO nanostructures have been 
intensively studied recently because this is a rapidly growing 
field due to their multi-functionality and their use in possible 
new generation applications. With an aim to understand the 
effect of Gd doping in ZnO in terms of its electronic and 
optical properties we have synthesize Gd-doped ZnO powder 
series of different Gd (0%, 2%, and 5%) values. Various 
characterization techniques have been utilized to explore the 
electronic, structural and optical features of the synthesized 
powder series. Gd doping into ZnO produced changes in the 
band gap and enhanced oxygen vacancies related states in the 
compound.

Materials and methods

The Gd-doped ZnO powder samples with varying Gd (0%, 
2%, and 5%) concentrations have been synthesized using the 
conventional solid-state reaction method. A suitable amount 
of high-purity (99.999% Sigma-Aldrich) zinc oxide (ZnO) 
and gadolinium oxide (Gd2O3) powder have been used for 
their preparation. For achieving perfect homogeneity, the 
ingredients were properly pounded for ~8 hours in an agate 
mortar and pestle. The prepared material has been sintered 
for ~8h at~550°C and cooled at room temperature. Again, the 
mixture was ground for ~2 hours and sintered for ~6 hours at 
~550°C. The mixture was milled for ~4 hours before being 
turned into pellets. Finally, these pellets were sintered for 
around ~8 hours at about ~600°C. Finally, we ground the 
pellets to make the powder. The powder form of the prepared 
samples has been used for further characterization.

Characterization techniques

The crystallographic structures of the samples have been 
explored using an X-ray diffractometer (Make: Panalytical 
X'Pert PRO) using the photon energy of Cu Kα (λ= 
1.5406Å). SEM images have been used for the morphology 
study of the synthesized samples. The SEM equipment model 

 

 

Fig. 7. O1s XPS spectra: (a) asymmetric spectra for all Gd-doped ZnO bulk samples (Gd = 0%, 2% and 5%); 
(b) for the ZnO: fitted with two Gaussian peaks; (c) for the 2% Gd-doped ZnO polycrystalline sample: fitted 
with two Gaussian peaks; (d) for the 5% Gd-doped ZnO polycrystalline powder sample: fitted with three
Gaussian peaks



Electronic, structural and optical properties of Gd-doped ZnO powder60 58(1) 2023

RE elements, such as Gd, may introduce additional capabili-
ties to the ZnO system because they may be capable of 
constructing combined practical devices on a single chip by 
combining magnetic and optical properties (Ohno et al. 1996; 
Tawil et al. 2011). 

Rare-earth (RE) metal doping in the dilute magnetic 
semiconductor (DMS) systems has recently received 
immense attention due to its potential usage in magneto 
electronic and photonic devices (Monteiro et al. 2006). The 
influence of Gd doping on the optical and electrical proper-
ties of ZnO is crucial for developing optoelectronic devices 
and understanding the genesis of ferromagnetism. Ma and 
Wang (2012) synthesized the Gd-doped ZnO nanocrystals 
using the thermal evaporation deposition method. According 
to their findings, Gd doping has an appreciable impact on the 
optical characteristics of the ZnO. The strong UV and blue 
emissions are detected at low Gd concentrations because Gd 
impurities introduce a mid-gap state into ZnO. In compari-
son, the UV and blue emissions reduce for high doping 
concentrations as a significant broad defect emission appears 
because of the vast numbers of defects, impurities, and 
excess Gd2O3 produced. Gd-doped ZnO nanocrystals can be 
used in optoelectronic devices. The optical and electrical 
tuning of the carrier density and optical band gap energy is 
essential for developing luminescent materials and equip-
ment based on the ZnO DMS systems. The optical band gap 
and carrier concentration are essential parameters to yield 
information about the emission wavelength. Several develop-
ment characteristics, like the synthesis methods, impurity 
concentration and substrate variety have been reported to 
influence these parameters (Wakano et al. 2001; Tan et al. 
2011; Murtaza et al.  2011).

The synthesis of DMS materials with unique optical, photo-
catalytic and magnetic characteristics seems to be the most 
promising candidate for the subsequent generation of 
electronic, optoelectronic and spintronic appliances (Kumar  
and Sahare 2014).  The low-dimension materials with imper-
fection-free lattice structures have excellent electrical and 
magnetic characteristics; purposefully made or accidental 
structural defects play a significant role in altering their 
features (Li et al. 2016; Obeid et al. 2018). Thus, the presence 
of different kinds of cation or anion vacancies and intersti-
tials in ZnO nanocrystals may change their magnetic proper-
ties, photoluminescence, and photocatalytic properties (Coey 
et al. 2005). Although certain experimental and theoretical 
findings based on DFT indicate that Gd-doped ZnO exhibits 
the room temperature ferromagnetism (RTFM) (Potzger et 
al. 2006; Aravindh et al. 2014) property, several studies have 
demonstrated that the RTFM is not present (Sambasivam et 
al. 2015). In addition to their magnetic qualities, the 4f ions 

ETH 30 kV EVO-18 Carl Zeiss, with a tilt of 0° to 60° and a 
rotation of 360°, has been used for taking SEM images.The 
defect-related states and chemical analysis of the samples 
have been determined using X-ray photoelectron spectrosco-
py. A hemispherical analyzer with a 128-channel detector and 
a monochromated, fully integrated K-alpha small-spot X-ray 
photoelectron spectrometer system of 180° double focusing 
is used in XPS (Thermo Scientific made). The UV-visible 
absorption spectroscopy (Model: Shimadzu UV-3600 
UV-VIS-NIR Spectrophotometer) was used to explore the 
prepared samples' optical absorption and band gap.
The measurements of UV-visible absorption spectra have 
been taken using the powder form of the samples. PL 
spectroscopy was used to analyze defect-related states in the 
prepared samples. Photoluminescence spectra have been 
recorded at room temperature using the fluorescence 
spectrometer (Model: Perkin Elmer FL 8500). 

Results and discussion 

XRD analysis

Fig. 1. depicts the powder XRD patterns of the Gd-doped 
ZnO (Gd = 0%, 2%, and 5%) samples. All the diffraction 
peaks detected in the ZnO sample match with the standard 
JCPDS card number (36-1451). This confirms a hexagonal 
wurtzite crystal structure of the samples.

The diffraction peaks were observed at 31.88°, 34.50°, 
36.25°, 47.72°, 56.69°, 63.03°, 66.59°, 68.17°, and 69.35°, 
which are related to the planes (100), (002), (101), (102), 

(110), (103), (200), (112), and (201), respectively (Ganesh et 
al. 2018; Bharathi et al. 2020). Further, some additional 
small peaks have been observed at 28.61°, 33.21°, and 39.14° 
with increasing Gd doping. These secondary peaks represent-
ing a crystallite phase have been identified as Gd2O3, which 
is well consistent with earlier reports (Das et al. 2012). As 
shown in Fig. 2, the highest intense peak related to the (101) 
plane shifted slightly higher angles with the increasing 
concentration of Gd doping. The shift is because of the 
substitution of bigger-sized Gd3+ (0.94 Å) ions than Zn2+ 
(0.74 Å) ions in the hexagonal wurtzite structure.  The peak 
shift may indicate that Gd3+ ions have occupied the crystallo-
graphic positions of Zn2+ ions in the ZnO host lattice, or 
itmay be because of the structural strain caused by the forma-
tion of internal compressive micro stress (Dakhel and El-Hilo 
2010; Kumar et al. 2014; Sahu et al. 2017). 

Rietveld refinements have been performed using the Full Prof 
Programme (shown in Fig. 3). The Pseudo-Voigt function has 
been used for Rietveld refinement of all the samples with the 
space group P63mc. The estimated lattice parameters have 
been listed in Table I. The lattice parameters are found to 
decrease with Gd doping, similar to those reported in earlier 
studies (Kaur et al. 2016; Obeid et al. 2019). The factors like 
the induced deficiencies (vacancies, interstitial) and the ionic 
radius of Zn and Gd ions can be responsible for the observed 
change (Kaur et al. 2016; Kumar and Thangavel 2017).

SEM analysis

Scanning electron micrographs have been used to analyze 
the morphology of the Gd-doped ZnO powder samples. The 
SEM images reveal a rod-like morphology in pure ZnO 
(Gora  et al. 2022) and the Gd-doped ZnO polycrystalline 

samples with some proof of agglomeration and phase segre-
gation, as depicted in Fig. 4 (a, b and c). Profound observa-
tion of the particles exhibits that the crystallite size is above 
100 nm. The particles were intensive and incompatibly 
distributed all over the mass. A distinct boundary between 
neighbouring crystallites can be noticed, despite the proxim-
ity of these smaller crystallites.

XPS analysis

The XPS technique is an excellent tool for determining the 
surface chemistry and electron configuration of the 
various elements present in a multicomponent material. 
The carbon C1s peak (284.6 eV) has been used as a refer-

ence to calibrate all the binding energies of the spectra. 
The survey spectrum of the Gd-doped ZnO samples is 
shown in Fig. 5. The survey spectra revealed the existence 
of O, Zn, and Gd elements without any impurity in the 
samples.

For ZnO and 2% Gd-doped ZnO powder samples, the binding 
energies of the Zn2p3/2 and Zn2p1/2 peaks are found to be 
1021.25 eV, 1044.25 eV, 1021.43 eV, and 1044.43 eV, respec-
tively. However, for the 5% Gd-doped ZnO sample, these 
peaks are at 1021.83 and 1044.83 eV energy positions. The 
small shift could be assigned to the creation of Zn interstitials 
in ZnO upon Gd doping. The binding energy gap between the 
zinc doublet peaks has been found at about 23 eV. This 
indicates that Zn in the ZnO crystal structure is in the Zn2+ 
oxidation state (Khataee et al. 2015; Gawai et al. 2019) as the 
spin-orbital splitting of Zn2p is 23 eV. This difference does 
not change with the Gd doping pointing out that Gd doping 
has no discernible effect on the chemical position of ZnO.

The O1s asymmetric XPS spectraare shown in Fig.7 (a) 
which is asymmetric in shape. The O1s XPS spectra of all 
these samples i.e. the ZnO, the Zn0.98Gd0.02O, and the 
Zn0.95Gd0.05O have been deconvoluted mainly into two Gauss-
ian peaks, as shown in Fig. 7 (b, c, and d). The lower binding 
peaks for pure and Gd-doped ZnO (Gd = 2% and 5%) 
polycrystalline samples have been detected at 529.93 eV, 
529.93 eV and 529.78 eV, respectively, which attributed to 
the lattice oxygen in the hexagonal structure of the ZnO.

The higher binding peak for the pure and the Gd-doped ZnO 
samples have been found at 531.51 eV, 531.66 eV, and 
531.49 eV, respectively, which is due to the presence of 
oxygen vacancies in the ZnO structure (Bharathi et al. 2020; 
Sukumaran et al. 2021). An additional signal centered at 
528.29 eV has been detected in the 5% Gd-doped ZnO 
powder sample, which could be ascribed to the coordination 
of oxygen in Gd-O-Zn along grain boundaries because of the 
excessive Gd dopants and also attributed to the presence of 

Gd2O3 in the sample (Yi  et al.  2017). The lattice oxygen peak 
has slightly shifted to the lower binding energy with increas-
ing Gd concentration, which has been due to the increased 
oxygen vacancies with increasing Gd doping. The estimated 
area ratios of the oxygen vacancies peak and the lattice oxygen 
peak for the samples ZnO, Zn0.98Gd0.02O, and Zn0.95Gd0.05O 
comes out to be 0.53, 1.30, and 2.49, respectively. These 
results show that the oxygen vacancies increase with increas-
ing Gd concentration.

The Fig. 8 (a, b) depicts the high-resolution Gd4d XPS 
spectra of the Gd-doped samples.In the Gd4d state, the 
spin-orbit splatted peaks are at 144.6 eV for Gd4d3/2 and 
139.6 eV for Gd4d5/2 for the 2% Gd-doped sample, whereas 
for the 5% Gd-doped ZnO sample, the spin-orbit splits at 
145.5 eV for Gd4d3/2 and 139.7 eV for Gd4d5/2 as shown in 
Fig. 8 (a, b). This indicates that Gd is present in the Gd3+ state 
in the ZnO hexagonal structure. The shift in peak values of 
Gd4d3/2 and Gd4d5/2 from 144.6 eV and 139.6 eV, respective-
ly; in the case of 2% Gd-doped sample to 145.5 eV and 139.7 
eV, respectively, in the case of 5% Gd-doped sample. This 
shift in Gd4d peaks is ascribed to the electron transfer from 
plasmonics Gd to ZnO because of the strong electronic 
interaction (covalent bond) between Gd and the oxide 
(Ahmad et al. 2011; Wang et al. 2012; Bharathi et al. 2020).

UV visible spectroscopy analysis

The UV–Visible spectroscopy is a powerful tool for estimat-
ing band gap of materials. UV–Vis absorbance spectra and 
tauc plots for the Gd-doped ZnO samples (Gd = 0% (Gora et 
al. 2022), 2%, and 5%) samples are shown in Figs.9 (a, b, c, 
and d). The spectra show a red shift with increasing Gd 
amount in the ZnO.The UV–Vis absorption intensity of the 
synthesized polycrystalline samples is found to reduce with 
the increasing Gd concentration (Mazhdi and Tafreshi 2018). 
Tauc's plot has been used to calculate the direct band gap of 
the samples (Jain et al. 2006). The direct band gap has been 
estimated using the following equation:     

Here, α is the absorption coefficient,υ is frequency, h is the 
Planck constant, A is a constant, and Eg is the energy band 
gap of the substance.

Extrapolating the linear component of the curve plotted 
among (αhυ) 2 and hυ depicted in Fig. 9 (b, c, and d) has been 
used to obtain the band gap (Farhad et al. 2018; Ghos et al. 
2021). The band gap values calculated for the Gd-doped ZnO 
samples, i.e.are 3.31 eV (Gora et al. 2022), 3.27 eV, and 
3.23eV, for the samples Gd = 0%, 2% and 5%, respectively. 
The small redshift observed in the band gap of Gd-doped 
ZnO samples is because of the changes in electronic structure 
as well as enhanced oxygen vacancies with the increasing Gd 
doping (Yi et al. 2017; Flemban et al. 2016), which is respon-
sible for the band gap reduction. 

Photoluminescence spectroscopy analysis

The structural flaws like ionic and atomic vacancies, replace-
ments and interstitials have been explored using the PL 

spectra of the synthesized polycrystalline samples. Fig. 10 (a, 
b and c) shows the deconvoluted, Gaussian-fitted PL spectra 
of the synthesized Gd-doped ZnO polycrystalline powder 
samples.

The PL spectra of the samples were recorded at room 
temperature in order to determine the types of vacancies, 
defect band emission etc. (Lin et al. 2001; Yi et al. 2017). The 
photoluminescence spectra have been deconvoluted and the 
different peaks responsible for the emission have been 

Gaussian fitted. For these samples the band edge emission 
peaksare detected at 381 nm, 384 nm, and 383 nm, for the Gd 
= 0%, 2% and 5% samples, respectively. The UV emission is 
responsible for the excitonic interaction related to the band 
edge emission. The intense violet emission exhibited in the 
Gd-doped ZnO at about 415 nm could be because ofthe 
oxygen vacancies. The exciton interaction between the holes 
in the valence band and electrons located in the interstitial 
zinc could be responsible for the violet emission peak at 
about 447 nm. Intrinsic deficiencies like interstitial zinc and 

oxygen can be attributed to the peak at 477 nm (Singh et al.  
2009; Kaur et al. 2016). The transformation between singly 
ionized oxygen deficiency and the photo-created holes (Ken-
nedy et al. 2014) or the surface imperfection and electrons 
near the conduction band, might be responsible for the green 
emission peak seen for the Gd-doped ZnOat around 525 nm. 
The green emissions are commonly attributed to theoxygen 
vacancies (Studenikin et al. 1998). For Gd-doped ZnO 
polycrystalline powder samples, the intensity ratio of the 
green and ultra-violet emission peaks is found to be 0.090 for 
ZnO, 0.204 for the 2% Gd-doped ZnO, and 0.418 for the 5% 
Gd-doped ZnO. The defect-related emissions enhanced with 
increasing Gd doping in the ZnO indicate that a large number 
of deficiencies, such as oxygen vacancies, are most usually 
related to crystal deformation (Yi et al. 2017). The results 
reveal that the Gd doping enhances the Zni and Vo flaws. It 
could be attributed to structural deformation created by Gd 
atoms with different ionic sizes than Zn in the ZnO (Ban-
dopadhyay and Mitra 2015; Sukumaran et al. 2021). Increas-
ing Gd doping in ZnO indicates an increase in defect states, 
which matches well with the XPS results.

We compared our PL results with Farhad et al. (2019), who 
prepared ZnO DC (drop casting) and ZnO nanorods. Their 
PL results detected the PL peak at 380 nm and a barely 
observable green-yellow emission peak; in our case, we find 
it at 381 nm for ZnO, 384 nm for 2% Gd-doped ZnO and 383 
nm for the 5% Gd-doped ZnO polycrystalline sample. This 
peak represented ZnO's near band edge (NBE) transition 
because of free excitons recombinations and this PL property 
indicates high crystalline quality. The luminescent character-
istics in the visible region are responsible for the different 
defects, like zinc interstitials (Zni), oxygen vacancies (Vo), 
etc., present in the ZnO crystal matrix. As a result, they 
reported that both ZnO (DC) and ZnO nanorods were found 
to be defect-free good crystalline and optical properties mate-
rials. In our case, all the prepared Gd-doped ZnO powder 
samples have been found to be with single ionized oxygen 
vacancies, which are confirmed by the green emission peak 
at 525 nm in the PL spectra.

Conclusion

The Gd-doped ZnO powder samples were synthesized by the 
solid-state reaction method. The XRD patterns affirm that Gd 
has been successfully incorporated into the ZnO lattice and 
confirm the hexagonal wurtzite structure. SEM images have 
been used to reveal the morphologies of the synthesized 
samples, which appear as rod-like structures. The XPS 
results showed that the oxygen vacancy-related states in 
Gd-doped samples increased with increasing Gd doping. PL 
findings confirmed that the defect-related states are present 

in the Gd-doped ZnO polycrystalline samples. The green 
emission and ultra-violet emission peaks' intensity ratio is 
0.418 for the 5% Gd-doped ZnO sample, which is the maxi-
mum compared to the pure ZnO and 2% Gd-doped ZnO 
polycrystalline powder sample. These findings confirmed 
that the oxygen vacancies increase as the Gd concentration 
increases and the PL results agree well with the XPS results.
The UV absorption spectra findings reveal that the band gap 
in the 5% Gd-doped ZnO sample is found to be 3.23 eV, 
which is the minimum compared to both the pure ZnO and 
the 2% Gd-doped ZnO samples, confirming that Eg is 
reduced with increasing Gd concentration.

Acknowledgment

The authors are thankful to USIC, University of Rajasthan, 
Jaipur, Rajasthan (India), for providing SEM images of the 
prepared samples. We also thank department of physics, 
BanasthaliVidhyapith, Banasthali, Jaipur, Rajasthan (India) 
for rendering XRD data.

Funding sources

This research is not financed by any agency.

Authors' contribution

Sample preparation, data analysis, manuscript composition 
and writing (Mahendra Kumar Gora); sample synthesis and 
data characterization (Arvind Kumar); data recording (Ban-
wari Lal Choudhary); data analysis and interpretation 
(Sanjay Kumar); reviewing and editing (Satya NarainDolia); 
supervision, writing, reviewing and editing (Rishi Kumar 
Singhal).

Conflicts of interest or competing interests 

All authors declare that we have no conflict of interest or 
competing interests.

Data availability

On request, data will be available.

Ethical approval

Not applicable.

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doped ZnO nanocrystals also have an attractive optical 
responsiveness. Many theoretical and experimental studies 
showed that RE-doped ZnO could reportedly absorb ultravio-
let (UV) light (Chakraborty et al.  2017). Obeid  et al. (2019) 
prepared undoped and Gd-doped ZnO nanocrystalline 
samples using the thermal decomposition method. They 
observed that the optical absorption spectra of pristine ZnO 
increased with a 6% Gd doping concentration and the 
magnetic properties changed with changing dopant 
amounts.  Their PL spectroscopy results showed the 
existence of defects, which increased with increasing Gd 
doping concentrations. They suggested that these DMS 
nanoparticles can be used for magneto-optoelectronic 
practical device applications.

Undoubtedly, the RE-doped ZnO nanostructures have been 
intensively studied recently because this is a rapidly growing 
field due to their multi-functionality and their use in possible 
new generation applications. With an aim to understand the 
effect of Gd doping in ZnO in terms of its electronic and 
optical properties we have synthesize Gd-doped ZnO powder 
series of different Gd (0%, 2%, and 5%) values. Various 
characterization techniques have been utilized to explore the 
electronic, structural and optical features of the synthesized 
powder series. Gd doping into ZnO produced changes in the 
band gap and enhanced oxygen vacancies related states in the 
compound.

Materials and methods

The Gd-doped ZnO powder samples with varying Gd (0%, 
2%, and 5%) concentrations have been synthesized using the 
conventional solid-state reaction method. A suitable amount 
of high-purity (99.999% Sigma-Aldrich) zinc oxide (ZnO) 
and gadolinium oxide (Gd2O3) powder have been used for 
their preparation. For achieving perfect homogeneity, the 
ingredients were properly pounded for ~8 hours in an agate 
mortar and pestle. The prepared material has been sintered 
for ~8h at~550°C and cooled at room temperature. Again, the 
mixture was ground for ~2 hours and sintered for ~6 hours at 
~550°C. The mixture was milled for ~4 hours before being 
turned into pellets. Finally, these pellets were sintered for 
around ~8 hours at about ~600°C. Finally, we ground the 
pellets to make the powder. The powder form of the prepared 
samples has been used for further characterization.

Characterization techniques

The crystallographic structures of the samples have been 
explored using an X-ray diffractometer (Make: Panalytical 
X'Pert PRO) using the photon energy of Cu Kα (λ= 
1.5406Å). SEM images have been used for the morphology 
study of the synthesized samples. The SEM equipment model 

Fig.  8. The Gd4dXPS spectra for (a) 2% Gd-doped ZnO bulk sample and (b) 5% Gd-doped ZnO bulk sample

Fig. 9. (a) UV–visible spectra of Gd-doped ZnO (pure ZnO (Gora et al. 2022); Gd = 2% and 5%) and Tauc plots 
of the synthesized powder samples: (b) for the ZnO (Gora et al. 2022), (c) for the 2% Gd-doped ZnO and (d) for 
the 5% Gd-doped ZnO



RE elements, such as Gd, may introduce additional capabili-
ties to the ZnO system because they may be capable of 
constructing combined practical devices on a single chip by 
combining magnetic and optical properties (Ohno et al. 1996; 
Tawil et al. 2011). 

Rare-earth (RE) metal doping in the dilute magnetic 
semiconductor (DMS) systems has recently received 
immense attention due to its potential usage in magneto 
electronic and photonic devices (Monteiro et al. 2006). The 
influence of Gd doping on the optical and electrical proper-
ties of ZnO is crucial for developing optoelectronic devices 
and understanding the genesis of ferromagnetism. Ma and 
Wang (2012) synthesized the Gd-doped ZnO nanocrystals 
using the thermal evaporation deposition method. According 
to their findings, Gd doping has an appreciable impact on the 
optical characteristics of the ZnO. The strong UV and blue 
emissions are detected at low Gd concentrations because Gd 
impurities introduce a mid-gap state into ZnO. In compari-
son, the UV and blue emissions reduce for high doping 
concentrations as a significant broad defect emission appears 
because of the vast numbers of defects, impurities, and 
excess Gd2O3 produced. Gd-doped ZnO nanocrystals can be 
used in optoelectronic devices. The optical and electrical 
tuning of the carrier density and optical band gap energy is 
essential for developing luminescent materials and equip-
ment based on the ZnO DMS systems. The optical band gap 
and carrier concentration are essential parameters to yield 
information about the emission wavelength. Several develop-
ment characteristics, like the synthesis methods, impurity 
concentration and substrate variety have been reported to 
influence these parameters (Wakano et al. 2001; Tan et al. 
2011; Murtaza et al.  2011).

The synthesis of DMS materials with unique optical, photo-
catalytic and magnetic characteristics seems to be the most 
promising candidate for the subsequent generation of 
electronic, optoelectronic and spintronic appliances (Kumar  
and Sahare 2014).  The low-dimension materials with imper-
fection-free lattice structures have excellent electrical and 
magnetic characteristics; purposefully made or accidental 
structural defects play a significant role in altering their 
features (Li et al. 2016; Obeid et al. 2018). Thus, the presence 
of different kinds of cation or anion vacancies and intersti-
tials in ZnO nanocrystals may change their magnetic proper-
ties, photoluminescence, and photocatalytic properties (Coey 
et al. 2005). Although certain experimental and theoretical 
findings based on DFT indicate that Gd-doped ZnO exhibits 
the room temperature ferromagnetism (RTFM) (Potzger et 
al. 2006; Aravindh et al. 2014) property, several studies have 
demonstrated that the RTFM is not present (Sambasivam et 
al. 2015). In addition to their magnetic qualities, the 4f ions 

ETH 30 kV EVO-18 Carl Zeiss, with a tilt of 0° to 60° and a 
rotation of 360°, has been used for taking SEM images.The 
defect-related states and chemical analysis of the samples 
have been determined using X-ray photoelectron spectrosco-
py. A hemispherical analyzer with a 128-channel detector and 
a monochromated, fully integrated K-alpha small-spot X-ray 
photoelectron spectrometer system of 180° double focusing 
is used in XPS (Thermo Scientific made). The UV-visible 
absorption spectroscopy (Model: Shimadzu UV-3600 
UV-VIS-NIR Spectrophotometer) was used to explore the 
prepared samples' optical absorption and band gap.
The measurements of UV-visible absorption spectra have 
been taken using the powder form of the samples. PL 
spectroscopy was used to analyze defect-related states in the 
prepared samples. Photoluminescence spectra have been 
recorded at room temperature using the fluorescence 
spectrometer (Model: Perkin Elmer FL 8500). 

Results and discussion 

XRD analysis

Fig. 1. depicts the powder XRD patterns of the Gd-doped 
ZnO (Gd = 0%, 2%, and 5%) samples. All the diffraction 
peaks detected in the ZnO sample match with the standard 
JCPDS card number (36-1451). This confirms a hexagonal 
wurtzite crystal structure of the samples.

The diffraction peaks were observed at 31.88°, 34.50°, 
36.25°, 47.72°, 56.69°, 63.03°, 66.59°, 68.17°, and 69.35°, 
which are related to the planes (100), (002), (101), (102), 

(110), (103), (200), (112), and (201), respectively (Ganesh et 
al. 2018; Bharathi et al. 2020). Further, some additional 
small peaks have been observed at 28.61°, 33.21°, and 39.14° 
with increasing Gd doping. These secondary peaks represent-
ing a crystallite phase have been identified as Gd2O3, which 
is well consistent with earlier reports (Das et al. 2012). As 
shown in Fig. 2, the highest intense peak related to the (101) 
plane shifted slightly higher angles with the increasing 
concentration of Gd doping. The shift is because of the 
substitution of bigger-sized Gd3+ (0.94 Å) ions than Zn2+ 
(0.74 Å) ions in the hexagonal wurtzite structure.  The peak 
shift may indicate that Gd3+ ions have occupied the crystallo-
graphic positions of Zn2+ ions in the ZnO host lattice, or 
itmay be because of the structural strain caused by the forma-
tion of internal compressive micro stress (Dakhel and El-Hilo 
2010; Kumar et al. 2014; Sahu et al. 2017). 

Rietveld refinements have been performed using the Full Prof 
Programme (shown in Fig. 3). The Pseudo-Voigt function has 
been used for Rietveld refinement of all the samples with the 
space group P63mc. The estimated lattice parameters have 
been listed in Table I. The lattice parameters are found to 
decrease with Gd doping, similar to those reported in earlier 
studies (Kaur et al. 2016; Obeid et al. 2019). The factors like 
the induced deficiencies (vacancies, interstitial) and the ionic 
radius of Zn and Gd ions can be responsible for the observed 
change (Kaur et al. 2016; Kumar and Thangavel 2017).

SEM analysis

Scanning electron micrographs have been used to analyze 
the morphology of the Gd-doped ZnO powder samples. The 
SEM images reveal a rod-like morphology in pure ZnO 
(Gora  et al. 2022) and the Gd-doped ZnO polycrystalline 

samples with some proof of agglomeration and phase segre-
gation, as depicted in Fig. 4 (a, b and c). Profound observa-
tion of the particles exhibits that the crystallite size is above 
100 nm. The particles were intensive and incompatibly 
distributed all over the mass. A distinct boundary between 
neighbouring crystallites can be noticed, despite the proxim-
ity of these smaller crystallites.

XPS analysis

The XPS technique is an excellent tool for determining the 
surface chemistry and electron configuration of the 
various elements present in a multicomponent material. 
The carbon C1s peak (284.6 eV) has been used as a refer-

ence to calibrate all the binding energies of the spectra. 
The survey spectrum of the Gd-doped ZnO samples is 
shown in Fig. 5. The survey spectra revealed the existence 
of O, Zn, and Gd elements without any impurity in the 
samples.

For ZnO and 2% Gd-doped ZnO powder samples, the binding 
energies of the Zn2p3/2 and Zn2p1/2 peaks are found to be 
1021.25 eV, 1044.25 eV, 1021.43 eV, and 1044.43 eV, respec-
tively. However, for the 5% Gd-doped ZnO sample, these 
peaks are at 1021.83 and 1044.83 eV energy positions. The 
small shift could be assigned to the creation of Zn interstitials 
in ZnO upon Gd doping. The binding energy gap between the 
zinc doublet peaks has been found at about 23 eV. This 
indicates that Zn in the ZnO crystal structure is in the Zn2+ 
oxidation state (Khataee et al. 2015; Gawai et al. 2019) as the 
spin-orbital splitting of Zn2p is 23 eV. This difference does 
not change with the Gd doping pointing out that Gd doping 
has no discernible effect on the chemical position of ZnO.

The O1s asymmetric XPS spectraare shown in Fig.7 (a) 
which is asymmetric in shape. The O1s XPS spectra of all 
these samples i.e. the ZnO, the Zn0.98Gd0.02O, and the 
Zn0.95Gd0.05O have been deconvoluted mainly into two Gauss-
ian peaks, as shown in Fig. 7 (b, c, and d). The lower binding 
peaks for pure and Gd-doped ZnO (Gd = 2% and 5%) 
polycrystalline samples have been detected at 529.93 eV, 
529.93 eV and 529.78 eV, respectively, which attributed to 
the lattice oxygen in the hexagonal structure of the ZnO.

The higher binding peak for the pure and the Gd-doped ZnO 
samples have been found at 531.51 eV, 531.66 eV, and 
531.49 eV, respectively, which is due to the presence of 
oxygen vacancies in the ZnO structure (Bharathi et al. 2020; 
Sukumaran et al. 2021). An additional signal centered at 
528.29 eV has been detected in the 5% Gd-doped ZnO 
powder sample, which could be ascribed to the coordination 
of oxygen in Gd-O-Zn along grain boundaries because of the 
excessive Gd dopants and also attributed to the presence of 

Gd2O3 in the sample (Yi  et al.  2017). The lattice oxygen peak 
has slightly shifted to the lower binding energy with increas-
ing Gd concentration, which has been due to the increased 
oxygen vacancies with increasing Gd doping. The estimated 
area ratios of the oxygen vacancies peak and the lattice oxygen 
peak for the samples ZnO, Zn0.98Gd0.02O, and Zn0.95Gd0.05O 
comes out to be 0.53, 1.30, and 2.49, respectively. These 
results show that the oxygen vacancies increase with increas-
ing Gd concentration.

The Fig. 8 (a, b) depicts the high-resolution Gd4d XPS 
spectra of the Gd-doped samples.In the Gd4d state, the 
spin-orbit splatted peaks are at 144.6 eV for Gd4d3/2 and 
139.6 eV for Gd4d5/2 for the 2% Gd-doped sample, whereas 
for the 5% Gd-doped ZnO sample, the spin-orbit splits at 
145.5 eV for Gd4d3/2 and 139.7 eV for Gd4d5/2 as shown in 
Fig. 8 (a, b). This indicates that Gd is present in the Gd3+ state 
in the ZnO hexagonal structure. The shift in peak values of 
Gd4d3/2 and Gd4d5/2 from 144.6 eV and 139.6 eV, respective-
ly; in the case of 2% Gd-doped sample to 145.5 eV and 139.7 
eV, respectively, in the case of 5% Gd-doped sample. This 
shift in Gd4d peaks is ascribed to the electron transfer from 
plasmonics Gd to ZnO because of the strong electronic 
interaction (covalent bond) between Gd and the oxide 
(Ahmad et al. 2011; Wang et al. 2012; Bharathi et al. 2020).

UV visible spectroscopy analysis

The UV–Visible spectroscopy is a powerful tool for estimat-
ing band gap of materials. UV–Vis absorbance spectra and 
tauc plots for the Gd-doped ZnO samples (Gd = 0% (Gora et 
al. 2022), 2%, and 5%) samples are shown in Figs.9 (a, b, c, 
and d). The spectra show a red shift with increasing Gd 
amount in the ZnO.The UV–Vis absorption intensity of the 
synthesized polycrystalline samples is found to reduce with 
the increasing Gd concentration (Mazhdi and Tafreshi 2018). 
Tauc's plot has been used to calculate the direct band gap of 
the samples (Jain et al. 2006). The direct band gap has been 
estimated using the following equation:     

Here, α is the absorption coefficient,υ is frequency, h is the 
Planck constant, A is a constant, and Eg is the energy band 
gap of the substance.

Extrapolating the linear component of the curve plotted 
among (αhυ) 2 and hυ depicted in Fig. 9 (b, c, and d) has been 
used to obtain the band gap (Farhad et al. 2018; Ghos et al. 
2021). The band gap values calculated for the Gd-doped ZnO 
samples, i.e.are 3.31 eV (Gora et al. 2022), 3.27 eV, and 
3.23eV, for the samples Gd = 0%, 2% and 5%, respectively. 
The small redshift observed in the band gap of Gd-doped 
ZnO samples is because of the changes in electronic structure 
as well as enhanced oxygen vacancies with the increasing Gd 
doping (Yi et al. 2017; Flemban et al. 2016), which is respon-
sible for the band gap reduction. 

Photoluminescence spectroscopy analysis

The structural flaws like ionic and atomic vacancies, replace-
ments and interstitials have been explored using the PL 

spectra of the synthesized polycrystalline samples. Fig. 10 (a, 
b and c) shows the deconvoluted, Gaussian-fitted PL spectra 
of the synthesized Gd-doped ZnO polycrystalline powder 
samples.

The PL spectra of the samples were recorded at room 
temperature in order to determine the types of vacancies, 
defect band emission etc. (Lin et al. 2001; Yi et al. 2017). The 
photoluminescence spectra have been deconvoluted and the 
different peaks responsible for the emission have been 

Gaussian fitted. For these samples the band edge emission 
peaksare detected at 381 nm, 384 nm, and 383 nm, for the Gd 
= 0%, 2% and 5% samples, respectively. The UV emission is 
responsible for the excitonic interaction related to the band 
edge emission. The intense violet emission exhibited in the 
Gd-doped ZnO at about 415 nm could be because ofthe 
oxygen vacancies. The exciton interaction between the holes 
in the valence band and electrons located in the interstitial 
zinc could be responsible for the violet emission peak at 
about 447 nm. Intrinsic deficiencies like interstitial zinc and 

oxygen can be attributed to the peak at 477 nm (Singh et al.  
2009; Kaur et al. 2016). The transformation between singly 
ionized oxygen deficiency and the photo-created holes (Ken-
nedy et al. 2014) or the surface imperfection and electrons 
near the conduction band, might be responsible for the green 
emission peak seen for the Gd-doped ZnOat around 525 nm. 
The green emissions are commonly attributed to theoxygen 
vacancies (Studenikin et al. 1998). For Gd-doped ZnO 
polycrystalline powder samples, the intensity ratio of the 
green and ultra-violet emission peaks is found to be 0.090 for 
ZnO, 0.204 for the 2% Gd-doped ZnO, and 0.418 for the 5% 
Gd-doped ZnO. The defect-related emissions enhanced with 
increasing Gd doping in the ZnO indicate that a large number 
of deficiencies, such as oxygen vacancies, are most usually 
related to crystal deformation (Yi et al. 2017). The results 
reveal that the Gd doping enhances the Zni and Vo flaws. It 
could be attributed to structural deformation created by Gd 
atoms with different ionic sizes than Zn in the ZnO (Ban-
dopadhyay and Mitra 2015; Sukumaran et al. 2021). Increas-
ing Gd doping in ZnO indicates an increase in defect states, 
which matches well with the XPS results.

We compared our PL results with Farhad et al. (2019), who 
prepared ZnO DC (drop casting) and ZnO nanorods. Their 
PL results detected the PL peak at 380 nm and a barely 
observable green-yellow emission peak; in our case, we find 
it at 381 nm for ZnO, 384 nm for 2% Gd-doped ZnO and 383 
nm for the 5% Gd-doped ZnO polycrystalline sample. This 
peak represented ZnO's near band edge (NBE) transition 
because of free excitons recombinations and this PL property 
indicates high crystalline quality. The luminescent character-
istics in the visible region are responsible for the different 
defects, like zinc interstitials (Zni), oxygen vacancies (Vo), 
etc., present in the ZnO crystal matrix. As a result, they 
reported that both ZnO (DC) and ZnO nanorods were found 
to be defect-free good crystalline and optical properties mate-
rials. In our case, all the prepared Gd-doped ZnO powder 
samples have been found to be with single ionized oxygen 
vacancies, which are confirmed by the green emission peak 
at 525 nm in the PL spectra.

Conclusion

The Gd-doped ZnO powder samples were synthesized by the 
solid-state reaction method. The XRD patterns affirm that Gd 
has been successfully incorporated into the ZnO lattice and 
confirm the hexagonal wurtzite structure. SEM images have 
been used to reveal the morphologies of the synthesized 
samples, which appear as rod-like structures. The XPS 
results showed that the oxygen vacancy-related states in 
Gd-doped samples increased with increasing Gd doping. PL 
findings confirmed that the defect-related states are present 

in the Gd-doped ZnO polycrystalline samples. The green 
emission and ultra-violet emission peaks' intensity ratio is 
0.418 for the 5% Gd-doped ZnO sample, which is the maxi-
mum compared to the pure ZnO and 2% Gd-doped ZnO 
polycrystalline powder sample. These findings confirmed 
that the oxygen vacancies increase as the Gd concentration 
increases and the PL results agree well with the XPS results.
The UV absorption spectra findings reveal that the band gap 
in the 5% Gd-doped ZnO sample is found to be 3.23 eV, 
which is the minimum compared to both the pure ZnO and 
the 2% Gd-doped ZnO samples, confirming that Eg is 
reduced with increasing Gd concentration.

Acknowledgment

The authors are thankful to USIC, University of Rajasthan, 
Jaipur, Rajasthan (India), for providing SEM images of the 
prepared samples. We also thank department of physics, 
BanasthaliVidhyapith, Banasthali, Jaipur, Rajasthan (India) 
for rendering XRD data.

Funding sources

This research is not financed by any agency.

Authors' contribution

Sample preparation, data analysis, manuscript composition 
and writing (Mahendra Kumar Gora); sample synthesis and 
data characterization (Arvind Kumar); data recording (Ban-
wari Lal Choudhary); data analysis and interpretation 
(Sanjay Kumar); reviewing and editing (Satya NarainDolia); 
supervision, writing, reviewing and editing (Rishi Kumar 
Singhal).

Conflicts of interest or competing interests 

All authors declare that we have no conflict of interest or 
competing interests.

Data availability

On request, data will be available.

Ethical approval

Not applicable.

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doped ZnO nanocrystals also have an attractive optical 
responsiveness. Many theoretical and experimental studies 
showed that RE-doped ZnO could reportedly absorb ultravio-
let (UV) light (Chakraborty et al.  2017). Obeid  et al. (2019) 
prepared undoped and Gd-doped ZnO nanocrystalline 
samples using the thermal decomposition method. They 
observed that the optical absorption spectra of pristine ZnO 
increased with a 6% Gd doping concentration and the 
magnetic properties changed with changing dopant 
amounts.  Their PL spectroscopy results showed the 
existence of defects, which increased with increasing Gd 
doping concentrations. They suggested that these DMS 
nanoparticles can be used for magneto-optoelectronic 
practical device applications.

Undoubtedly, the RE-doped ZnO nanostructures have been 
intensively studied recently because this is a rapidly growing 
field due to their multi-functionality and their use in possible 
new generation applications. With an aim to understand the 
effect of Gd doping in ZnO in terms of its electronic and 
optical properties we have synthesize Gd-doped ZnO powder 
series of different Gd (0%, 2%, and 5%) values. Various 
characterization techniques have been utilized to explore the 
electronic, structural and optical features of the synthesized 
powder series. Gd doping into ZnO produced changes in the 
band gap and enhanced oxygen vacancies related states in the 
compound.

Materials and methods

The Gd-doped ZnO powder samples with varying Gd (0%, 
2%, and 5%) concentrations have been synthesized using the 
conventional solid-state reaction method. A suitable amount 
of high-purity (99.999% Sigma-Aldrich) zinc oxide (ZnO) 
and gadolinium oxide (Gd2O3) powder have been used for 
their preparation. For achieving perfect homogeneity, the 
ingredients were properly pounded for ~8 hours in an agate 
mortar and pestle. The prepared material has been sintered 
for ~8h at~550°C and cooled at room temperature. Again, the 
mixture was ground for ~2 hours and sintered for ~6 hours at 
~550°C. The mixture was milled for ~4 hours before being 
turned into pellets. Finally, these pellets were sintered for 
around ~8 hours at about ~600°C. Finally, we ground the 
pellets to make the powder. The powder form of the prepared 
samples has been used for further characterization.

Characterization techniques

The crystallographic structures of the samples have been 
explored using an X-ray diffractometer (Make: Panalytical 
X'Pert PRO) using the photon energy of Cu Kα (λ= 
1.5406Å). SEM images have been used for the morphology 
study of the synthesized samples. The SEM equipment model 

Fig. 10. PL spectra: (a) for ZnO (Gora et al., 2022), (b) for 
2% Gd-doped ZnO (c) for 5% Gd-doped ZnO powder 
sample



RE elements, such as Gd, may introduce additional capabili-
ties to the ZnO system because they may be capable of 
constructing combined practical devices on a single chip by 
combining magnetic and optical properties (Ohno et al. 1996; 
Tawil et al. 2011). 

Rare-earth (RE) metal doping in the dilute magnetic 
semiconductor (DMS) systems has recently received 
immense attention due to its potential usage in magneto 
electronic and photonic devices (Monteiro et al. 2006). The 
influence of Gd doping on the optical and electrical proper-
ties of ZnO is crucial for developing optoelectronic devices 
and understanding the genesis of ferromagnetism. Ma and 
Wang (2012) synthesized the Gd-doped ZnO nanocrystals 
using the thermal evaporation deposition method. According 
to their findings, Gd doping has an appreciable impact on the 
optical characteristics of the ZnO. The strong UV and blue 
emissions are detected at low Gd concentrations because Gd 
impurities introduce a mid-gap state into ZnO. In compari-
son, the UV and blue emissions reduce for high doping 
concentrations as a significant broad defect emission appears 
because of the vast numbers of defects, impurities, and 
excess Gd2O3 produced. Gd-doped ZnO nanocrystals can be 
used in optoelectronic devices. The optical and electrical 
tuning of the carrier density and optical band gap energy is 
essential for developing luminescent materials and equip-
ment based on the ZnO DMS systems. The optical band gap 
and carrier concentration are essential parameters to yield 
information about the emission wavelength. Several develop-
ment characteristics, like the synthesis methods, impurity 
concentration and substrate variety have been reported to 
influence these parameters (Wakano et al. 2001; Tan et al. 
2011; Murtaza et al.  2011).

The synthesis of DMS materials with unique optical, photo-
catalytic and magnetic characteristics seems to be the most 
promising candidate for the subsequent generation of 
electronic, optoelectronic and spintronic appliances (Kumar  
and Sahare 2014).  The low-dimension materials with imper-
fection-free lattice structures have excellent electrical and 
magnetic characteristics; purposefully made or accidental 
structural defects play a significant role in altering their 
features (Li et al. 2016; Obeid et al. 2018). Thus, the presence 
of different kinds of cation or anion vacancies and intersti-
tials in ZnO nanocrystals may change their magnetic proper-
ties, photoluminescence, and photocatalytic properties (Coey 
et al. 2005). Although certain experimental and theoretical 
findings based on DFT indicate that Gd-doped ZnO exhibits 
the room temperature ferromagnetism (RTFM) (Potzger et 
al. 2006; Aravindh et al. 2014) property, several studies have 
demonstrated that the RTFM is not present (Sambasivam et 
al. 2015). In addition to their magnetic qualities, the 4f ions 

ETH 30 kV EVO-18 Carl Zeiss, with a tilt of 0° to 60° and a 
rotation of 360°, has been used for taking SEM images.The 
defect-related states and chemical analysis of the samples 
have been determined using X-ray photoelectron spectrosco-
py. A hemispherical analyzer with a 128-channel detector and 
a monochromated, fully integrated K-alpha small-spot X-ray 
photoelectron spectrometer system of 180° double focusing 
is used in XPS (Thermo Scientific made). The UV-visible 
absorption spectroscopy (Model: Shimadzu UV-3600 
UV-VIS-NIR Spectrophotometer) was used to explore the 
prepared samples' optical absorption and band gap.
The measurements of UV-visible absorption spectra have 
been taken using the powder form of the samples. PL 
spectroscopy was used to analyze defect-related states in the 
prepared samples. Photoluminescence spectra have been 
recorded at room temperature using the fluorescence 
spectrometer (Model: Perkin Elmer FL 8500). 

Results and discussion 

XRD analysis

Fig. 1. depicts the powder XRD patterns of the Gd-doped 
ZnO (Gd = 0%, 2%, and 5%) samples. All the diffraction 
peaks detected in the ZnO sample match with the standard 
JCPDS card number (36-1451). This confirms a hexagonal 
wurtzite crystal structure of the samples.

The diffraction peaks were observed at 31.88°, 34.50°, 
36.25°, 47.72°, 56.69°, 63.03°, 66.59°, 68.17°, and 69.35°, 
which are related to the planes (100), (002), (101), (102), 

(110), (103), (200), (112), and (201), respectively (Ganesh et 
al. 2018; Bharathi et al. 2020). Further, some additional 
small peaks have been observed at 28.61°, 33.21°, and 39.14° 
with increasing Gd doping. These secondary peaks represent-
ing a crystallite phase have been identified as Gd2O3, which 
is well consistent with earlier reports (Das et al. 2012). As 
shown in Fig. 2, the highest intense peak related to the (101) 
plane shifted slightly higher angles with the increasing 
concentration of Gd doping. The shift is because of the 
substitution of bigger-sized Gd3+ (0.94 Å) ions than Zn2+ 
(0.74 Å) ions in the hexagonal wurtzite structure.  The peak 
shift may indicate that Gd3+ ions have occupied the crystallo-
graphic positions of Zn2+ ions in the ZnO host lattice, or 
itmay be because of the structural strain caused by the forma-
tion of internal compressive micro stress (Dakhel and El-Hilo 
2010; Kumar et al. 2014; Sahu et al. 2017). 

Rietveld refinements have been performed using the Full Prof 
Programme (shown in Fig. 3). The Pseudo-Voigt function has 
been used for Rietveld refinement of all the samples with the 
space group P63mc. The estimated lattice parameters have 
been listed in Table I. The lattice parameters are found to 
decrease with Gd doping, similar to those reported in earlier 
studies (Kaur et al. 2016; Obeid et al. 2019). The factors like 
the induced deficiencies (vacancies, interstitial) and the ionic 
radius of Zn and Gd ions can be responsible for the observed 
change (Kaur et al. 2016; Kumar and Thangavel 2017).

SEM analysis

Scanning electron micrographs have been used to analyze 
the morphology of the Gd-doped ZnO powder samples. The 
SEM images reveal a rod-like morphology in pure ZnO 
(Gora  et al. 2022) and the Gd-doped ZnO polycrystalline 

samples with some proof of agglomeration and phase segre-
gation, as depicted in Fig. 4 (a, b and c). Profound observa-
tion of the particles exhibits that the crystallite size is above 
100 nm. The particles were intensive and incompatibly 
distributed all over the mass. A distinct boundary between 
neighbouring crystallites can be noticed, despite the proxim-
ity of these smaller crystallites.

XPS analysis

The XPS technique is an excellent tool for determining the 
surface chemistry and electron configuration of the 
various elements present in a multicomponent material. 
The carbon C1s peak (284.6 eV) has been used as a refer-

ence to calibrate all the binding energies of the spectra. 
The survey spectrum of the Gd-doped ZnO samples is 
shown in Fig. 5. The survey spectra revealed the existence 
of O, Zn, and Gd elements without any impurity in the 
samples.

For ZnO and 2% Gd-doped ZnO powder samples, the binding 
energies of the Zn2p3/2 and Zn2p1/2 peaks are found to be 
1021.25 eV, 1044.25 eV, 1021.43 eV, and 1044.43 eV, respec-
tively. However, for the 5% Gd-doped ZnO sample, these 
peaks are at 1021.83 and 1044.83 eV energy positions. The 
small shift could be assigned to the creation of Zn interstitials 
in ZnO upon Gd doping. The binding energy gap between the 
zinc doublet peaks has been found at about 23 eV. This 
indicates that Zn in the ZnO crystal structure is in the Zn2+ 
oxidation state (Khataee et al. 2015; Gawai et al. 2019) as the 
spin-orbital splitting of Zn2p is 23 eV. This difference does 
not change with the Gd doping pointing out that Gd doping 
has no discernible effect on the chemical position of ZnO.

The O1s asymmetric XPS spectraare shown in Fig.7 (a) 
which is asymmetric in shape. The O1s XPS spectra of all 
these samples i.e. the ZnO, the Zn0.98Gd0.02O, and the 
Zn0.95Gd0.05O have been deconvoluted mainly into two Gauss-
ian peaks, as shown in Fig. 7 (b, c, and d). The lower binding 
peaks for pure and Gd-doped ZnO (Gd = 2% and 5%) 
polycrystalline samples have been detected at 529.93 eV, 
529.93 eV and 529.78 eV, respectively, which attributed to 
the lattice oxygen in the hexagonal structure of the ZnO.

The higher binding peak for the pure and the Gd-doped ZnO 
samples have been found at 531.51 eV, 531.66 eV, and 
531.49 eV, respectively, which is due to the presence of 
oxygen vacancies in the ZnO structure (Bharathi et al. 2020; 
Sukumaran et al. 2021). An additional signal centered at 
528.29 eV has been detected in the 5% Gd-doped ZnO 
powder sample, which could be ascribed to the coordination 
of oxygen in Gd-O-Zn along grain boundaries because of the 
excessive Gd dopants and also attributed to the presence of 

Gd2O3 in the sample (Yi  et al.  2017). The lattice oxygen peak 
has slightly shifted to the lower binding energy with increas-
ing Gd concentration, which has been due to the increased 
oxygen vacancies with increasing Gd doping. The estimated 
area ratios of the oxygen vacancies peak and the lattice oxygen 
peak for the samples ZnO, Zn0.98Gd0.02O, and Zn0.95Gd0.05O 
comes out to be 0.53, 1.30, and 2.49, respectively. These 
results show that the oxygen vacancies increase with increas-
ing Gd concentration.

The Fig. 8 (a, b) depicts the high-resolution Gd4d XPS 
spectra of the Gd-doped samples.In the Gd4d state, the 
spin-orbit splatted peaks are at 144.6 eV for Gd4d3/2 and 
139.6 eV for Gd4d5/2 for the 2% Gd-doped sample, whereas 
for the 5% Gd-doped ZnO sample, the spin-orbit splits at 
145.5 eV for Gd4d3/2 and 139.7 eV for Gd4d5/2 as shown in 
Fig. 8 (a, b). This indicates that Gd is present in the Gd3+ state 
in the ZnO hexagonal structure. The shift in peak values of 
Gd4d3/2 and Gd4d5/2 from 144.6 eV and 139.6 eV, respective-
ly; in the case of 2% Gd-doped sample to 145.5 eV and 139.7 
eV, respectively, in the case of 5% Gd-doped sample. This 
shift in Gd4d peaks is ascribed to the electron transfer from 
plasmonics Gd to ZnO because of the strong electronic 
interaction (covalent bond) between Gd and the oxide 
(Ahmad et al. 2011; Wang et al. 2012; Bharathi et al. 2020).

UV visible spectroscopy analysis

The UV–Visible spectroscopy is a powerful tool for estimat-
ing band gap of materials. UV–Vis absorbance spectra and 
tauc plots for the Gd-doped ZnO samples (Gd = 0% (Gora et 
al. 2022), 2%, and 5%) samples are shown in Figs.9 (a, b, c, 
and d). The spectra show a red shift with increasing Gd 
amount in the ZnO.The UV–Vis absorption intensity of the 
synthesized polycrystalline samples is found to reduce with 
the increasing Gd concentration (Mazhdi and Tafreshi 2018). 
Tauc's plot has been used to calculate the direct band gap of 
the samples (Jain et al. 2006). The direct band gap has been 
estimated using the following equation:     

Here, α is the absorption coefficient,υ is frequency, h is the 
Planck constant, A is a constant, and Eg is the energy band 
gap of the substance.

Extrapolating the linear component of the curve plotted 
among (αhυ) 2 and hυ depicted in Fig. 9 (b, c, and d) has been 
used to obtain the band gap (Farhad et al. 2018; Ghos et al. 
2021). The band gap values calculated for the Gd-doped ZnO 
samples, i.e.are 3.31 eV (Gora et al. 2022), 3.27 eV, and 
3.23eV, for the samples Gd = 0%, 2% and 5%, respectively. 
The small redshift observed in the band gap of Gd-doped 
ZnO samples is because of the changes in electronic structure 
as well as enhanced oxygen vacancies with the increasing Gd 
doping (Yi et al. 2017; Flemban et al. 2016), which is respon-
sible for the band gap reduction. 

Photoluminescence spectroscopy analysis

The structural flaws like ionic and atomic vacancies, replace-
ments and interstitials have been explored using the PL 

spectra of the synthesized polycrystalline samples. Fig. 10 (a, 
b and c) shows the deconvoluted, Gaussian-fitted PL spectra 
of the synthesized Gd-doped ZnO polycrystalline powder 
samples.

The PL spectra of the samples were recorded at room 
temperature in order to determine the types of vacancies, 
defect band emission etc. (Lin et al. 2001; Yi et al. 2017). The 
photoluminescence spectra have been deconvoluted and the 
different peaks responsible for the emission have been 

Gaussian fitted. For these samples the band edge emission 
peaksare detected at 381 nm, 384 nm, and 383 nm, for the Gd 
= 0%, 2% and 5% samples, respectively. The UV emission is 
responsible for the excitonic interaction related to the band 
edge emission. The intense violet emission exhibited in the 
Gd-doped ZnO at about 415 nm could be because ofthe 
oxygen vacancies. The exciton interaction between the holes 
in the valence band and electrons located in the interstitial 
zinc could be responsible for the violet emission peak at 
about 447 nm. Intrinsic deficiencies like interstitial zinc and 

oxygen can be attributed to the peak at 477 nm (Singh et al.  
2009; Kaur et al. 2016). The transformation between singly 
ionized oxygen deficiency and the photo-created holes (Ken-
nedy et al. 2014) or the surface imperfection and electrons 
near the conduction band, might be responsible for the green 
emission peak seen for the Gd-doped ZnOat around 525 nm. 
The green emissions are commonly attributed to theoxygen 
vacancies (Studenikin et al. 1998). For Gd-doped ZnO 
polycrystalline powder samples, the intensity ratio of the 
green and ultra-violet emission peaks is found to be 0.090 for 
ZnO, 0.204 for the 2% Gd-doped ZnO, and 0.418 for the 5% 
Gd-doped ZnO. The defect-related emissions enhanced with 
increasing Gd doping in the ZnO indicate that a large number 
of deficiencies, such as oxygen vacancies, are most usually 
related to crystal deformation (Yi et al. 2017). The results 
reveal that the Gd doping enhances the Zni and Vo flaws. It 
could be attributed to structural deformation created by Gd 
atoms with different ionic sizes than Zn in the ZnO (Ban-
dopadhyay and Mitra 2015; Sukumaran et al. 2021). Increas-
ing Gd doping in ZnO indicates an increase in defect states, 
which matches well with the XPS results.

We compared our PL results with Farhad et al. (2019), who 
prepared ZnO DC (drop casting) and ZnO nanorods. Their 
PL results detected the PL peak at 380 nm and a barely 
observable green-yellow emission peak; in our case, we find 
it at 381 nm for ZnO, 384 nm for 2% Gd-doped ZnO and 383 
nm for the 5% Gd-doped ZnO polycrystalline sample. This 
peak represented ZnO's near band edge (NBE) transition 
because of free excitons recombinations and this PL property 
indicates high crystalline quality. The luminescent character-
istics in the visible region are responsible for the different 
defects, like zinc interstitials (Zni), oxygen vacancies (Vo), 
etc., present in the ZnO crystal matrix. As a result, they 
reported that both ZnO (DC) and ZnO nanorods were found 
to be defect-free good crystalline and optical properties mate-
rials. In our case, all the prepared Gd-doped ZnO powder 
samples have been found to be with single ionized oxygen 
vacancies, which are confirmed by the green emission peak 
at 525 nm in the PL spectra.

Conclusion

The Gd-doped ZnO powder samples were synthesized by the 
solid-state reaction method. The XRD patterns affirm that Gd 
has been successfully incorporated into the ZnO lattice and 
confirm the hexagonal wurtzite structure. SEM images have 
been used to reveal the morphologies of the synthesized 
samples, which appear as rod-like structures. The XPS 
results showed that the oxygen vacancy-related states in 
Gd-doped samples increased with increasing Gd doping. PL 
findings confirmed that the defect-related states are present 

in the Gd-doped ZnO polycrystalline samples. The green 
emission and ultra-violet emission peaks' intensity ratio is 
0.418 for the 5% Gd-doped ZnO sample, which is the maxi-
mum compared to the pure ZnO and 2% Gd-doped ZnO 
polycrystalline powder sample. These findings confirmed 
that the oxygen vacancies increase as the Gd concentration 
increases and the PL results agree well with the XPS results.
The UV absorption spectra findings reveal that the band gap 
in the 5% Gd-doped ZnO sample is found to be 3.23 eV, 
which is the minimum compared to both the pure ZnO and 
the 2% Gd-doped ZnO samples, confirming that Eg is 
reduced with increasing Gd concentration.

Acknowledgment

The authors are thankful to USIC, University of Rajasthan, 
Jaipur, Rajasthan (India), for providing SEM images of the 
prepared samples. We also thank department of physics, 
BanasthaliVidhyapith, Banasthali, Jaipur, Rajasthan (India) 
for rendering XRD data.

Funding sources

This research is not financed by any agency.

Authors' contribution

Sample preparation, data analysis, manuscript composition 
and writing (Mahendra Kumar Gora); sample synthesis and 
data characterization (Arvind Kumar); data recording (Ban-
wari Lal Choudhary); data analysis and interpretation 
(Sanjay Kumar); reviewing and editing (Satya NarainDolia); 
supervision, writing, reviewing and editing (Rishi Kumar 
Singhal).

Conflicts of interest or competing interests 

All authors declare that we have no conflict of interest or 
competing interests.

Data availability

On request, data will be available.

Ethical approval

Not applicable.

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doped ZnO nanocrystals also have an attractive optical 
responsiveness. Many theoretical and experimental studies 
showed that RE-doped ZnO could reportedly absorb ultravio-
let (UV) light (Chakraborty et al.  2017). Obeid  et al. (2019) 
prepared undoped and Gd-doped ZnO nanocrystalline 
samples using the thermal decomposition method. They 
observed that the optical absorption spectra of pristine ZnO 
increased with a 6% Gd doping concentration and the 
magnetic properties changed with changing dopant 
amounts.  Their PL spectroscopy results showed the 
existence of defects, which increased with increasing Gd 
doping concentrations. They suggested that these DMS 
nanoparticles can be used for magneto-optoelectronic 
practical device applications.

Undoubtedly, the RE-doped ZnO nanostructures have been 
intensively studied recently because this is a rapidly growing 
field due to their multi-functionality and their use in possible 
new generation applications. With an aim to understand the 
effect of Gd doping in ZnO in terms of its electronic and 
optical properties we have synthesize Gd-doped ZnO powder 
series of different Gd (0%, 2%, and 5%) values. Various 
characterization techniques have been utilized to explore the 
electronic, structural and optical features of the synthesized 
powder series. Gd doping into ZnO produced changes in the 
band gap and enhanced oxygen vacancies related states in the 
compound.

Materials and methods

The Gd-doped ZnO powder samples with varying Gd (0%, 
2%, and 5%) concentrations have been synthesized using the 
conventional solid-state reaction method. A suitable amount 
of high-purity (99.999% Sigma-Aldrich) zinc oxide (ZnO) 
and gadolinium oxide (Gd2O3) powder have been used for 
their preparation. For achieving perfect homogeneity, the 
ingredients were properly pounded for ~8 hours in an agate 
mortar and pestle. The prepared material has been sintered 
for ~8h at~550°C and cooled at room temperature. Again, the 
mixture was ground for ~2 hours and sintered for ~6 hours at 
~550°C. The mixture was milled for ~4 hours before being 
turned into pellets. Finally, these pellets were sintered for 
around ~8 hours at about ~600°C. Finally, we ground the 
pellets to make the powder. The powder form of the prepared 
samples has been used for further characterization.

Characterization techniques

The crystallographic structures of the samples have been 
explored using an X-ray diffractometer (Make: Panalytical 
X'Pert PRO) using the photon energy of Cu Kα (λ= 
1.5406Å). SEM images have been used for the morphology 
study of the synthesized samples. The SEM equipment model 



RE elements, such as Gd, may introduce additional capabili-
ties to the ZnO system because they may be capable of 
constructing combined practical devices on a single chip by 
combining magnetic and optical properties (Ohno et al. 1996; 
Tawil et al. 2011). 

Rare-earth (RE) metal doping in the dilute magnetic 
semiconductor (DMS) systems has recently received 
immense attention due to its potential usage in magneto 
electronic and photonic devices (Monteiro et al. 2006). The 
influence of Gd doping on the optical and electrical proper-
ties of ZnO is crucial for developing optoelectronic devices 
and understanding the genesis of ferromagnetism. Ma and 
Wang (2012) synthesized the Gd-doped ZnO nanocrystals 
using the thermal evaporation deposition method. According 
to their findings, Gd doping has an appreciable impact on the 
optical characteristics of the ZnO. The strong UV and blue 
emissions are detected at low Gd concentrations because Gd 
impurities introduce a mid-gap state into ZnO. In compari-
son, the UV and blue emissions reduce for high doping 
concentrations as a significant broad defect emission appears 
because of the vast numbers of defects, impurities, and 
excess Gd2O3 produced. Gd-doped ZnO nanocrystals can be 
used in optoelectronic devices. The optical and electrical 
tuning of the carrier density and optical band gap energy is 
essential for developing luminescent materials and equip-
ment based on the ZnO DMS systems. The optical band gap 
and carrier concentration are essential parameters to yield 
information about the emission wavelength. Several develop-
ment characteristics, like the synthesis methods, impurity 
concentration and substrate variety have been reported to 
influence these parameters (Wakano et al. 2001; Tan et al. 
2011; Murtaza et al.  2011).

The synthesis of DMS materials with unique optical, photo-
catalytic and magnetic characteristics seems to be the most 
promising candidate for the subsequent generation of 
electronic, optoelectronic and spintronic appliances (Kumar  
and Sahare 2014).  The low-dimension materials with imper-
fection-free lattice structures have excellent electrical and 
magnetic characteristics; purposefully made or accidental 
structural defects play a significant role in altering their 
features (Li et al. 2016; Obeid et al. 2018). Thus, the presence 
of different kinds of cation or anion vacancies and intersti-
tials in ZnO nanocrystals may change their magnetic proper-
ties, photoluminescence, and photocatalytic properties (Coey 
et al. 2005). Although certain experimental and theoretical 
findings based on DFT indicate that Gd-doped ZnO exhibits 
the room temperature ferromagnetism (RTFM) (Potzger et 
al. 2006; Aravindh et al. 2014) property, several studies have 
demonstrated that the RTFM is not present (Sambasivam et 
al. 2015). In addition to their magnetic qualities, the 4f ions 

ETH 30 kV EVO-18 Carl Zeiss, with a tilt of 0° to 60° and a 
rotation of 360°, has been used for taking SEM images.The 
defect-related states and chemical analysis of the samples 
have been determined using X-ray photoelectron spectrosco-
py. A hemispherical analyzer with a 128-channel detector and 
a monochromated, fully integrated K-alpha small-spot X-ray 
photoelectron spectrometer system of 180° double focusing 
is used in XPS (Thermo Scientific made). The UV-visible 
absorption spectroscopy (Model: Shimadzu UV-3600 
UV-VIS-NIR Spectrophotometer) was used to explore the 
prepared samples' optical absorption and band gap.
The measurements of UV-visible absorption spectra have 
been taken using the powder form of the samples. PL 
spectroscopy was used to analyze defect-related states in the 
prepared samples. Photoluminescence spectra have been 
recorded at room temperature using the fluorescence 
spectrometer (Model: Perkin Elmer FL 8500). 

Results and discussion 

XRD analysis

Fig. 1. depicts the powder XRD patterns of the Gd-doped 
ZnO (Gd = 0%, 2%, and 5%) samples. All the diffraction 
peaks detected in the ZnO sample match with the standard 
JCPDS card number (36-1451). This confirms a hexagonal 
wurtzite crystal structure of the samples.

The diffraction peaks were observed at 31.88°, 34.50°, 
36.25°, 47.72°, 56.69°, 63.03°, 66.59°, 68.17°, and 69.35°, 
which are related to the planes (100), (002), (101), (102), 

(110), (103), (200), (112), and (201), respectively (Ganesh et 
al. 2018; Bharathi et al. 2020). Further, some additional 
small peaks have been observed at 28.61°, 33.21°, and 39.14° 
with increasing Gd doping. These secondary peaks represent-
ing a crystallite phase have been identified as Gd2O3, which 
is well consistent with earlier reports (Das et al. 2012). As 
shown in Fig. 2, the highest intense peak related to the (101) 
plane shifted slightly higher angles with the increasing 
concentration of Gd doping. The shift is because of the 
substitution of bigger-sized Gd3+ (0.94 Å) ions than Zn2+ 
(0.74 Å) ions in the hexagonal wurtzite structure.  The peak 
shift may indicate that Gd3+ ions have occupied the crystallo-
graphic positions of Zn2+ ions in the ZnO host lattice, or 
itmay be because of the structural strain caused by the forma-
tion of internal compressive micro stress (Dakhel and El-Hilo 
2010; Kumar et al. 2014; Sahu et al. 2017). 

Rietveld refinements have been performed using the Full Prof 
Programme (shown in Fig. 3). The Pseudo-Voigt function has 
been used for Rietveld refinement of all the samples with the 
space group P63mc. The estimated lattice parameters have 
been listed in Table I. The lattice parameters are found to 
decrease with Gd doping, similar to those reported in earlier 
studies (Kaur et al. 2016; Obeid et al. 2019). The factors like 
the induced deficiencies (vacancies, interstitial) and the ionic 
radius of Zn and Gd ions can be responsible for the observed 
change (Kaur et al. 2016; Kumar and Thangavel 2017).

SEM analysis

Scanning electron micrographs have been used to analyze 
the morphology of the Gd-doped ZnO powder samples. The 
SEM images reveal a rod-like morphology in pure ZnO 
(Gora  et al. 2022) and the Gd-doped ZnO polycrystalline 

samples with some proof of agglomeration and phase segre-
gation, as depicted in Fig. 4 (a, b and c). Profound observa-
tion of the particles exhibits that the crystallite size is above 
100 nm. The particles were intensive and incompatibly 
distributed all over the mass. A distinct boundary between 
neighbouring crystallites can be noticed, despite the proxim-
ity of these smaller crystallites.

XPS analysis

The XPS technique is an excellent tool for determining the 
surface chemistry and electron configuration of the 
various elements present in a multicomponent material. 
The carbon C1s peak (284.6 eV) has been used as a refer-

ence to calibrate all the binding energies of the spectra. 
The survey spectrum of the Gd-doped ZnO samples is 
shown in Fig. 5. The survey spectra revealed the existence 
of O, Zn, and Gd elements without any impurity in the 
samples.

For ZnO and 2% Gd-doped ZnO powder samples, the binding 
energies of the Zn2p3/2 and Zn2p1/2 peaks are found to be 
1021.25 eV, 1044.25 eV, 1021.43 eV, and 1044.43 eV, respec-
tively. However, for the 5% Gd-doped ZnO sample, these 
peaks are at 1021.83 and 1044.83 eV energy positions. The 
small shift could be assigned to the creation of Zn interstitials 
in ZnO upon Gd doping. The binding energy gap between the 
zinc doublet peaks has been found at about 23 eV. This 
indicates that Zn in the ZnO crystal structure is in the Zn2+ 
oxidation state (Khataee et al. 2015; Gawai et al. 2019) as the 
spin-orbital splitting of Zn2p is 23 eV. This difference does 
not change with the Gd doping pointing out that Gd doping 
has no discernible effect on the chemical position of ZnO.

The O1s asymmetric XPS spectraare shown in Fig.7 (a) 
which is asymmetric in shape. The O1s XPS spectra of all 
these samples i.e. the ZnO, the Zn0.98Gd0.02O, and the 
Zn0.95Gd0.05O have been deconvoluted mainly into two Gauss-
ian peaks, as shown in Fig. 7 (b, c, and d). The lower binding 
peaks for pure and Gd-doped ZnO (Gd = 2% and 5%) 
polycrystalline samples have been detected at 529.93 eV, 
529.93 eV and 529.78 eV, respectively, which attributed to 
the lattice oxygen in the hexagonal structure of the ZnO.

The higher binding peak for the pure and the Gd-doped ZnO 
samples have been found at 531.51 eV, 531.66 eV, and 
531.49 eV, respectively, which is due to the presence of 
oxygen vacancies in the ZnO structure (Bharathi et al. 2020; 
Sukumaran et al. 2021). An additional signal centered at 
528.29 eV has been detected in the 5% Gd-doped ZnO 
powder sample, which could be ascribed to the coordination 
of oxygen in Gd-O-Zn along grain boundaries because of the 
excessive Gd dopants and also attributed to the presence of 

Gd2O3 in the sample (Yi  et al.  2017). The lattice oxygen peak 
has slightly shifted to the lower binding energy with increas-
ing Gd concentration, which has been due to the increased 
oxygen vacancies with increasing Gd doping. The estimated 
area ratios of the oxygen vacancies peak and the lattice oxygen 
peak for the samples ZnO, Zn0.98Gd0.02O, and Zn0.95Gd0.05O 
comes out to be 0.53, 1.30, and 2.49, respectively. These 
results show that the oxygen vacancies increase with increas-
ing Gd concentration.

The Fig. 8 (a, b) depicts the high-resolution Gd4d XPS 
spectra of the Gd-doped samples.In the Gd4d state, the 
spin-orbit splatted peaks are at 144.6 eV for Gd4d3/2 and 
139.6 eV for Gd4d5/2 for the 2% Gd-doped sample, whereas 
for the 5% Gd-doped ZnO sample, the spin-orbit splits at 
145.5 eV for Gd4d3/2 and 139.7 eV for Gd4d5/2 as shown in 
Fig. 8 (a, b). This indicates that Gd is present in the Gd3+ state 
in the ZnO hexagonal structure. The shift in peak values of 
Gd4d3/2 and Gd4d5/2 from 144.6 eV and 139.6 eV, respective-
ly; in the case of 2% Gd-doped sample to 145.5 eV and 139.7 
eV, respectively, in the case of 5% Gd-doped sample. This 
shift in Gd4d peaks is ascribed to the electron transfer from 
plasmonics Gd to ZnO because of the strong electronic 
interaction (covalent bond) between Gd and the oxide 
(Ahmad et al. 2011; Wang et al. 2012; Bharathi et al. 2020).

UV visible spectroscopy analysis

The UV–Visible spectroscopy is a powerful tool for estimat-
ing band gap of materials. UV–Vis absorbance spectra and 
tauc plots for the Gd-doped ZnO samples (Gd = 0% (Gora et 
al. 2022), 2%, and 5%) samples are shown in Figs.9 (a, b, c, 
and d). The spectra show a red shift with increasing Gd 
amount in the ZnO.The UV–Vis absorption intensity of the 
synthesized polycrystalline samples is found to reduce with 
the increasing Gd concentration (Mazhdi and Tafreshi 2018). 
Tauc's plot has been used to calculate the direct band gap of 
the samples (Jain et al. 2006). The direct band gap has been 
estimated using the following equation:     

Here, α is the absorption coefficient,υ is frequency, h is the 
Planck constant, A is a constant, and Eg is the energy band 
gap of the substance.

Extrapolating the linear component of the curve plotted 
among (αhυ) 2 and hυ depicted in Fig. 9 (b, c, and d) has been 
used to obtain the band gap (Farhad et al. 2018; Ghos et al. 
2021). The band gap values calculated for the Gd-doped ZnO 
samples, i.e.are 3.31 eV (Gora et al. 2022), 3.27 eV, and 
3.23eV, for the samples Gd = 0%, 2% and 5%, respectively. 
The small redshift observed in the band gap of Gd-doped 
ZnO samples is because of the changes in electronic structure 
as well as enhanced oxygen vacancies with the increasing Gd 
doping (Yi et al. 2017; Flemban et al. 2016), which is respon-
sible for the band gap reduction. 

Photoluminescence spectroscopy analysis

The structural flaws like ionic and atomic vacancies, replace-
ments and interstitials have been explored using the PL 

spectra of the synthesized polycrystalline samples. Fig. 10 (a, 
b and c) shows the deconvoluted, Gaussian-fitted PL spectra 
of the synthesized Gd-doped ZnO polycrystalline powder 
samples.

The PL spectra of the samples were recorded at room 
temperature in order to determine the types of vacancies, 
defect band emission etc. (Lin et al. 2001; Yi et al. 2017). The 
photoluminescence spectra have been deconvoluted and the 
different peaks responsible for the emission have been 

Gaussian fitted. For these samples the band edge emission 
peaksare detected at 381 nm, 384 nm, and 383 nm, for the Gd 
= 0%, 2% and 5% samples, respectively. The UV emission is 
responsible for the excitonic interaction related to the band 
edge emission. The intense violet emission exhibited in the 
Gd-doped ZnO at about 415 nm could be because ofthe 
oxygen vacancies. The exciton interaction between the holes 
in the valence band and electrons located in the interstitial 
zinc could be responsible for the violet emission peak at 
about 447 nm. Intrinsic deficiencies like interstitial zinc and 

oxygen can be attributed to the peak at 477 nm (Singh et al.  
2009; Kaur et al. 2016). The transformation between singly 
ionized oxygen deficiency and the photo-created holes (Ken-
nedy et al. 2014) or the surface imperfection and electrons 
near the conduction band, might be responsible for the green 
emission peak seen for the Gd-doped ZnOat around 525 nm. 
The green emissions are commonly attributed to theoxygen 
vacancies (Studenikin et al. 1998). For Gd-doped ZnO 
polycrystalline powder samples, the intensity ratio of the 
green and ultra-violet emission peaks is found to be 0.090 for 
ZnO, 0.204 for the 2% Gd-doped ZnO, and 0.418 for the 5% 
Gd-doped ZnO. The defect-related emissions enhanced with 
increasing Gd doping in the ZnO indicate that a large number 
of deficiencies, such as oxygen vacancies, are most usually 
related to crystal deformation (Yi et al. 2017). The results 
reveal that the Gd doping enhances the Zni and Vo flaws. It 
could be attributed to structural deformation created by Gd 
atoms with different ionic sizes than Zn in the ZnO (Ban-
dopadhyay and Mitra 2015; Sukumaran et al. 2021). Increas-
ing Gd doping in ZnO indicates an increase in defect states, 
which matches well with the XPS results.

We compared our PL results with Farhad et al. (2019), who 
prepared ZnO DC (drop casting) and ZnO nanorods. Their 
PL results detected the PL peak at 380 nm and a barely 
observable green-yellow emission peak; in our case, we find 
it at 381 nm for ZnO, 384 nm for 2% Gd-doped ZnO and 383 
nm for the 5% Gd-doped ZnO polycrystalline sample. This 
peak represented ZnO's near band edge (NBE) transition 
because of free excitons recombinations and this PL property 
indicates high crystalline quality. The luminescent character-
istics in the visible region are responsible for the different 
defects, like zinc interstitials (Zni), oxygen vacancies (Vo), 
etc., present in the ZnO crystal matrix. As a result, they 
reported that both ZnO (DC) and ZnO nanorods were found 
to be defect-free good crystalline and optical properties mate-
rials. In our case, all the prepared Gd-doped ZnO powder 
samples have been found to be with single ionized oxygen 
vacancies, which are confirmed by the green emission peak 
at 525 nm in the PL spectra.

Conclusion

The Gd-doped ZnO powder samples were synthesized by the 
solid-state reaction method. The XRD patterns affirm that Gd 
has been successfully incorporated into the ZnO lattice and 
confirm the hexagonal wurtzite structure. SEM images have 
been used to reveal the morphologies of the synthesized 
samples, which appear as rod-like structures. The XPS 
results showed that the oxygen vacancy-related states in 
Gd-doped samples increased with increasing Gd doping. PL 
findings confirmed that the defect-related states are present 

in the Gd-doped ZnO polycrystalline samples. The green 
emission and ultra-violet emission peaks' intensity ratio is 
0.418 for the 5% Gd-doped ZnO sample, which is the maxi-
mum compared to the pure ZnO and 2% Gd-doped ZnO 
polycrystalline powder sample. These findings confirmed 
that the oxygen vacancies increase as the Gd concentration 
increases and the PL results agree well with the XPS results.
The UV absorption spectra findings reveal that the band gap 
in the 5% Gd-doped ZnO sample is found to be 3.23 eV, 
which is the minimum compared to both the pure ZnO and 
the 2% Gd-doped ZnO samples, confirming that Eg is 
reduced with increasing Gd concentration.

Acknowledgment

The authors are thankful to USIC, University of Rajasthan, 
Jaipur, Rajasthan (India), for providing SEM images of the 
prepared samples. We also thank department of physics, 
BanasthaliVidhyapith, Banasthali, Jaipur, Rajasthan (India) 
for rendering XRD data.

Funding sources

This research is not financed by any agency.

Authors' contribution

Sample preparation, data analysis, manuscript composition 
and writing (Mahendra Kumar Gora); sample synthesis and 
data characterization (Arvind Kumar); data recording (Ban-
wari Lal Choudhary); data analysis and interpretation 
(Sanjay Kumar); reviewing and editing (Satya NarainDolia); 
supervision, writing, reviewing and editing (Rishi Kumar 
Singhal).

Conflicts of interest or competing interests 

All authors declare that we have no conflict of interest or 
competing interests.

Data availability

On request, data will be available.

Ethical approval

Not applicable.

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doped ZnO nanocrystals also have an attractive optical 
responsiveness. Many theoretical and experimental studies 
showed that RE-doped ZnO could reportedly absorb ultravio-
let (UV) light (Chakraborty et al.  2017). Obeid  et al. (2019) 
prepared undoped and Gd-doped ZnO nanocrystalline 
samples using the thermal decomposition method. They 
observed that the optical absorption spectra of pristine ZnO 
increased with a 6% Gd doping concentration and the 
magnetic properties changed with changing dopant 
amounts.  Their PL spectroscopy results showed the 
existence of defects, which increased with increasing Gd 
doping concentrations. They suggested that these DMS 
nanoparticles can be used for magneto-optoelectronic 
practical device applications.

Undoubtedly, the RE-doped ZnO nanostructures have been 
intensively studied recently because this is a rapidly growing 
field due to their multi-functionality and their use in possible 
new generation applications. With an aim to understand the 
effect of Gd doping in ZnO in terms of its electronic and 
optical properties we have synthesize Gd-doped ZnO powder 
series of different Gd (0%, 2%, and 5%) values. Various 
characterization techniques have been utilized to explore the 
electronic, structural and optical features of the synthesized 
powder series. Gd doping into ZnO produced changes in the 
band gap and enhanced oxygen vacancies related states in the 
compound.

Materials and methods

The Gd-doped ZnO powder samples with varying Gd (0%, 
2%, and 5%) concentrations have been synthesized using the 
conventional solid-state reaction method. A suitable amount 
of high-purity (99.999% Sigma-Aldrich) zinc oxide (ZnO) 
and gadolinium oxide (Gd2O3) powder have been used for 
their preparation. For achieving perfect homogeneity, the 
ingredients were properly pounded for ~8 hours in an agate 
mortar and pestle. The prepared material has been sintered 
for ~8h at~550°C and cooled at room temperature. Again, the 
mixture was ground for ~2 hours and sintered for ~6 hours at 
~550°C. The mixture was milled for ~4 hours before being 
turned into pellets. Finally, these pellets were sintered for 
around ~8 hours at about ~600°C. Finally, we ground the 
pellets to make the powder. The powder form of the prepared 
samples has been used for further characterization.

Characterization techniques

The crystallographic structures of the samples have been 
explored using an X-ray diffractometer (Make: Panalytical 
X'Pert PRO) using the photon energy of Cu Kα (λ= 
1.5406Å). SEM images have been used for the morphology 
study of the synthesized samples. The SEM equipment model 



RE elements, such as Gd, may introduce additional capabili-
ties to the ZnO system because they may be capable of 
constructing combined practical devices on a single chip by 
combining magnetic and optical properties (Ohno et al. 1996; 
Tawil et al. 2011). 

Rare-earth (RE) metal doping in the dilute magnetic 
semiconductor (DMS) systems has recently received 
immense attention due to its potential usage in magneto 
electronic and photonic devices (Monteiro et al. 2006). The 
influence of Gd doping on the optical and electrical proper-
ties of ZnO is crucial for developing optoelectronic devices 
and understanding the genesis of ferromagnetism. Ma and 
Wang (2012) synthesized the Gd-doped ZnO nanocrystals 
using the thermal evaporation deposition method. According 
to their findings, Gd doping has an appreciable impact on the 
optical characteristics of the ZnO. The strong UV and blue 
emissions are detected at low Gd concentrations because Gd 
impurities introduce a mid-gap state into ZnO. In compari-
son, the UV and blue emissions reduce for high doping 
concentrations as a significant broad defect emission appears 
because of the vast numbers of defects, impurities, and 
excess Gd2O3 produced. Gd-doped ZnO nanocrystals can be 
used in optoelectronic devices. The optical and electrical 
tuning of the carrier density and optical band gap energy is 
essential for developing luminescent materials and equip-
ment based on the ZnO DMS systems. The optical band gap 
and carrier concentration are essential parameters to yield 
information about the emission wavelength. Several develop-
ment characteristics, like the synthesis methods, impurity 
concentration and substrate variety have been reported to 
influence these parameters (Wakano et al. 2001; Tan et al. 
2011; Murtaza et al.  2011).

The synthesis of DMS materials with unique optical, photo-
catalytic and magnetic characteristics seems to be the most 
promising candidate for the subsequent generation of 
electronic, optoelectronic and spintronic appliances (Kumar  
and Sahare 2014).  The low-dimension materials with imper-
fection-free lattice structures have excellent electrical and 
magnetic characteristics; purposefully made or accidental 
structural defects play a significant role in altering their 
features (Li et al. 2016; Obeid et al. 2018). Thus, the presence 
of different kinds of cation or anion vacancies and intersti-
tials in ZnO nanocrystals may change their magnetic proper-
ties, photoluminescence, and photocatalytic properties (Coey 
et al. 2005). Although certain experimental and theoretical 
findings based on DFT indicate that Gd-doped ZnO exhibits 
the room temperature ferromagnetism (RTFM) (Potzger et 
al. 2006; Aravindh et al. 2014) property, several studies have 
demonstrated that the RTFM is not present (Sambasivam et 
al. 2015). In addition to their magnetic qualities, the 4f ions 

ETH 30 kV EVO-18 Carl Zeiss, with a tilt of 0° to 60° and a 
rotation of 360°, has been used for taking SEM images.The 
defect-related states and chemical analysis of the samples 
have been determined using X-ray photoelectron spectrosco-
py. A hemispherical analyzer with a 128-channel detector and 
a monochromated, fully integrated K-alpha small-spot X-ray 
photoelectron spectrometer system of 180° double focusing 
is used in XPS (Thermo Scientific made). The UV-visible 
absorption spectroscopy (Model: Shimadzu UV-3600 
UV-VIS-NIR Spectrophotometer) was used to explore the 
prepared samples' optical absorption and band gap.
The measurements of UV-visible absorption spectra have 
been taken using the powder form of the samples. PL 
spectroscopy was used to analyze defect-related states in the 
prepared samples. Photoluminescence spectra have been 
recorded at room temperature using the fluorescence 
spectrometer (Model: Perkin Elmer FL 8500). 

Results and discussion 

XRD analysis

Fig. 1. depicts the powder XRD patterns of the Gd-doped 
ZnO (Gd = 0%, 2%, and 5%) samples. All the diffraction 
peaks detected in the ZnO sample match with the standard 
JCPDS card number (36-1451). This confirms a hexagonal 
wurtzite crystal structure of the samples.

The diffraction peaks were observed at 31.88°, 34.50°, 
36.25°, 47.72°, 56.69°, 63.03°, 66.59°, 68.17°, and 69.35°, 
which are related to the planes (100), (002), (101), (102), 

(110), (103), (200), (112), and (201), respectively (Ganesh et 
al. 2018; Bharathi et al. 2020). Further, some additional 
small peaks have been observed at 28.61°, 33.21°, and 39.14° 
with increasing Gd doping. These secondary peaks represent-
ing a crystallite phase have been identified as Gd2O3, which 
is well consistent with earlier reports (Das et al. 2012). As 
shown in Fig. 2, the highest intense peak related to the (101) 
plane shifted slightly higher angles with the increasing 
concentration of Gd doping. The shift is because of the 
substitution of bigger-sized Gd3+ (0.94 Å) ions than Zn2+ 
(0.74 Å) ions in the hexagonal wurtzite structure.  The peak 
shift may indicate that Gd3+ ions have occupied the crystallo-
graphic positions of Zn2+ ions in the ZnO host lattice, or 
itmay be because of the structural strain caused by the forma-
tion of internal compressive micro stress (Dakhel and El-Hilo 
2010; Kumar et al. 2014; Sahu et al. 2017). 

Rietveld refinements have been performed using the Full Prof 
Programme (shown in Fig. 3). The Pseudo-Voigt function has 
been used for Rietveld refinement of all the samples with the 
space group P63mc. The estimated lattice parameters have 
been listed in Table I. The lattice parameters are found to 
decrease with Gd doping, similar to those reported in earlier 
studies (Kaur et al. 2016; Obeid et al. 2019). The factors like 
the induced deficiencies (vacancies, interstitial) and the ionic 
radius of Zn and Gd ions can be responsible for the observed 
change (Kaur et al. 2016; Kumar and Thangavel 2017).

SEM analysis

Scanning electron micrographs have been used to analyze 
the morphology of the Gd-doped ZnO powder samples. The 
SEM images reveal a rod-like morphology in pure ZnO 
(Gora  et al. 2022) and the Gd-doped ZnO polycrystalline 

samples with some proof of agglomeration and phase segre-
gation, as depicted in Fig. 4 (a, b and c). Profound observa-
tion of the particles exhibits that the crystallite size is above 
100 nm. The particles were intensive and incompatibly 
distributed all over the mass. A distinct boundary between 
neighbouring crystallites can be noticed, despite the proxim-
ity of these smaller crystallites.

XPS analysis

The XPS technique is an excellent tool for determining the 
surface chemistry and electron configuration of the 
various elements present in a multicomponent material. 
The carbon C1s peak (284.6 eV) has been used as a refer-

ence to calibrate all the binding energies of the spectra. 
The survey spectrum of the Gd-doped ZnO samples is 
shown in Fig. 5. The survey spectra revealed the existence 
of O, Zn, and Gd elements without any impurity in the 
samples.

For ZnO and 2% Gd-doped ZnO powder samples, the binding 
energies of the Zn2p3/2 and Zn2p1/2 peaks are found to be 
1021.25 eV, 1044.25 eV, 1021.43 eV, and 1044.43 eV, respec-
tively. However, for the 5% Gd-doped ZnO sample, these 
peaks are at 1021.83 and 1044.83 eV energy positions. The 
small shift could be assigned to the creation of Zn interstitials 
in ZnO upon Gd doping. The binding energy gap between the 
zinc doublet peaks has been found at about 23 eV. This 
indicates that Zn in the ZnO crystal structure is in the Zn2+ 
oxidation state (Khataee et al. 2015; Gawai et al. 2019) as the 
spin-orbital splitting of Zn2p is 23 eV. This difference does 
not change with the Gd doping pointing out that Gd doping 
has no discernible effect on the chemical position of ZnO.

The O1s asymmetric XPS spectraare shown in Fig.7 (a) 
which is asymmetric in shape. The O1s XPS spectra of all 
these samples i.e. the ZnO, the Zn0.98Gd0.02O, and the 
Zn0.95Gd0.05O have been deconvoluted mainly into two Gauss-
ian peaks, as shown in Fig. 7 (b, c, and d). The lower binding 
peaks for pure and Gd-doped ZnO (Gd = 2% and 5%) 
polycrystalline samples have been detected at 529.93 eV, 
529.93 eV and 529.78 eV, respectively, which attributed to 
the lattice oxygen in the hexagonal structure of the ZnO.

The higher binding peak for the pure and the Gd-doped ZnO 
samples have been found at 531.51 eV, 531.66 eV, and 
531.49 eV, respectively, which is due to the presence of 
oxygen vacancies in the ZnO structure (Bharathi et al. 2020; 
Sukumaran et al. 2021). An additional signal centered at 
528.29 eV has been detected in the 5% Gd-doped ZnO 
powder sample, which could be ascribed to the coordination 
of oxygen in Gd-O-Zn along grain boundaries because of the 
excessive Gd dopants and also attributed to the presence of 

Gd2O3 in the sample (Yi  et al.  2017). The lattice oxygen peak 
has slightly shifted to the lower binding energy with increas-
ing Gd concentration, which has been due to the increased 
oxygen vacancies with increasing Gd doping. The estimated 
area ratios of the oxygen vacancies peak and the lattice oxygen 
peak for the samples ZnO, Zn0.98Gd0.02O, and Zn0.95Gd0.05O 
comes out to be 0.53, 1.30, and 2.49, respectively. These 
results show that the oxygen vacancies increase with increas-
ing Gd concentration.

The Fig. 8 (a, b) depicts the high-resolution Gd4d XPS 
spectra of the Gd-doped samples.In the Gd4d state, the 
spin-orbit splatted peaks are at 144.6 eV for Gd4d3/2 and 
139.6 eV for Gd4d5/2 for the 2% Gd-doped sample, whereas 
for the 5% Gd-doped ZnO sample, the spin-orbit splits at 
145.5 eV for Gd4d3/2 and 139.7 eV for Gd4d5/2 as shown in 
Fig. 8 (a, b). This indicates that Gd is present in the Gd3+ state 
in the ZnO hexagonal structure. The shift in peak values of 
Gd4d3/2 and Gd4d5/2 from 144.6 eV and 139.6 eV, respective-
ly; in the case of 2% Gd-doped sample to 145.5 eV and 139.7 
eV, respectively, in the case of 5% Gd-doped sample. This 
shift in Gd4d peaks is ascribed to the electron transfer from 
plasmonics Gd to ZnO because of the strong electronic 
interaction (covalent bond) between Gd and the oxide 
(Ahmad et al. 2011; Wang et al. 2012; Bharathi et al. 2020).

UV visible spectroscopy analysis

The UV–Visible spectroscopy is a powerful tool for estimat-
ing band gap of materials. UV–Vis absorbance spectra and 
tauc plots for the Gd-doped ZnO samples (Gd = 0% (Gora et 
al. 2022), 2%, and 5%) samples are shown in Figs.9 (a, b, c, 
and d). The spectra show a red shift with increasing Gd 
amount in the ZnO.The UV–Vis absorption intensity of the 
synthesized polycrystalline samples is found to reduce with 
the increasing Gd concentration (Mazhdi and Tafreshi 2018). 
Tauc's plot has been used to calculate the direct band gap of 
the samples (Jain et al. 2006). The direct band gap has been 
estimated using the following equation:     

Here, α is the absorption coefficient,υ is frequency, h is the 
Planck constant, A is a constant, and Eg is the energy band 
gap of the substance.

Extrapolating the linear component of the curve plotted 
among (αhυ) 2 and hυ depicted in Fig. 9 (b, c, and d) has been 
used to obtain the band gap (Farhad et al. 2018; Ghos et al. 
2021). The band gap values calculated for the Gd-doped ZnO 
samples, i.e.are 3.31 eV (Gora et al. 2022), 3.27 eV, and 
3.23eV, for the samples Gd = 0%, 2% and 5%, respectively. 
The small redshift observed in the band gap of Gd-doped 
ZnO samples is because of the changes in electronic structure 
as well as enhanced oxygen vacancies with the increasing Gd 
doping (Yi et al. 2017; Flemban et al. 2016), which is respon-
sible for the band gap reduction. 

Photoluminescence spectroscopy analysis

The structural flaws like ionic and atomic vacancies, replace-
ments and interstitials have been explored using the PL 

spectra of the synthesized polycrystalline samples. Fig. 10 (a, 
b and c) shows the deconvoluted, Gaussian-fitted PL spectra 
of the synthesized Gd-doped ZnO polycrystalline powder 
samples.

The PL spectra of the samples were recorded at room 
temperature in order to determine the types of vacancies, 
defect band emission etc. (Lin et al. 2001; Yi et al. 2017). The 
photoluminescence spectra have been deconvoluted and the 
different peaks responsible for the emission have been 

Gaussian fitted. For these samples the band edge emission 
peaksare detected at 381 nm, 384 nm, and 383 nm, for the Gd 
= 0%, 2% and 5% samples, respectively. The UV emission is 
responsible for the excitonic interaction related to the band 
edge emission. The intense violet emission exhibited in the 
Gd-doped ZnO at about 415 nm could be because ofthe 
oxygen vacancies. The exciton interaction between the holes 
in the valence band and electrons located in the interstitial 
zinc could be responsible for the violet emission peak at 
about 447 nm. Intrinsic deficiencies like interstitial zinc and 

oxygen can be attributed to the peak at 477 nm (Singh et al.  
2009; Kaur et al. 2016). The transformation between singly 
ionized oxygen deficiency and the photo-created holes (Ken-
nedy et al. 2014) or the surface imperfection and electrons 
near the conduction band, might be responsible for the green 
emission peak seen for the Gd-doped ZnOat around 525 nm. 
The green emissions are commonly attributed to theoxygen 
vacancies (Studenikin et al. 1998). For Gd-doped ZnO 
polycrystalline powder samples, the intensity ratio of the 
green and ultra-violet emission peaks is found to be 0.090 for 
ZnO, 0.204 for the 2% Gd-doped ZnO, and 0.418 for the 5% 
Gd-doped ZnO. The defect-related emissions enhanced with 
increasing Gd doping in the ZnO indicate that a large number 
of deficiencies, such as oxygen vacancies, are most usually 
related to crystal deformation (Yi et al. 2017). The results 
reveal that the Gd doping enhances the Zni and Vo flaws. It 
could be attributed to structural deformation created by Gd 
atoms with different ionic sizes than Zn in the ZnO (Ban-
dopadhyay and Mitra 2015; Sukumaran et al. 2021). Increas-
ing Gd doping in ZnO indicates an increase in defect states, 
which matches well with the XPS results.

We compared our PL results with Farhad et al. (2019), who 
prepared ZnO DC (drop casting) and ZnO nanorods. Their 
PL results detected the PL peak at 380 nm and a barely 
observable green-yellow emission peak; in our case, we find 
it at 381 nm for ZnO, 384 nm for 2% Gd-doped ZnO and 383 
nm for the 5% Gd-doped ZnO polycrystalline sample. This 
peak represented ZnO's near band edge (NBE) transition 
because of free excitons recombinations and this PL property 
indicates high crystalline quality. The luminescent character-
istics in the visible region are responsible for the different 
defects, like zinc interstitials (Zni), oxygen vacancies (Vo), 
etc., present in the ZnO crystal matrix. As a result, they 
reported that both ZnO (DC) and ZnO nanorods were found 
to be defect-free good crystalline and optical properties mate-
rials. In our case, all the prepared Gd-doped ZnO powder 
samples have been found to be with single ionized oxygen 
vacancies, which are confirmed by the green emission peak 
at 525 nm in the PL spectra.

Conclusion

The Gd-doped ZnO powder samples were synthesized by the 
solid-state reaction method. The XRD patterns affirm that Gd 
has been successfully incorporated into the ZnO lattice and 
confirm the hexagonal wurtzite structure. SEM images have 
been used to reveal the morphologies of the synthesized 
samples, which appear as rod-like structures. The XPS 
results showed that the oxygen vacancy-related states in 
Gd-doped samples increased with increasing Gd doping. PL 
findings confirmed that the defect-related states are present 

in the Gd-doped ZnO polycrystalline samples. The green 
emission and ultra-violet emission peaks' intensity ratio is 
0.418 for the 5% Gd-doped ZnO sample, which is the maxi-
mum compared to the pure ZnO and 2% Gd-doped ZnO 
polycrystalline powder sample. These findings confirmed 
that the oxygen vacancies increase as the Gd concentration 
increases and the PL results agree well with the XPS results.
The UV absorption spectra findings reveal that the band gap 
in the 5% Gd-doped ZnO sample is found to be 3.23 eV, 
which is the minimum compared to both the pure ZnO and 
the 2% Gd-doped ZnO samples, confirming that Eg is 
reduced with increasing Gd concentration.

Acknowledgment

The authors are thankful to USIC, University of Rajasthan, 
Jaipur, Rajasthan (India), for providing SEM images of the 
prepared samples. We also thank department of physics, 
BanasthaliVidhyapith, Banasthali, Jaipur, Rajasthan (India) 
for rendering XRD data.

Funding sources

This research is not financed by any agency.

Authors' contribution

Sample preparation, data analysis, manuscript composition 
and writing (Mahendra Kumar Gora); sample synthesis and 
data characterization (Arvind Kumar); data recording (Ban-
wari Lal Choudhary); data analysis and interpretation 
(Sanjay Kumar); reviewing and editing (Satya NarainDolia); 
supervision, writing, reviewing and editing (Rishi Kumar 
Singhal).

Conflicts of interest or competing interests 

All authors declare that we have no conflict of interest or 
competing interests.

Data availability

On request, data will be available.

Ethical approval

Not applicable.

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doped ZnO nanocrystals also have an attractive optical 
responsiveness. Many theoretical and experimental studies 
showed that RE-doped ZnO could reportedly absorb ultravio-
let (UV) light (Chakraborty et al.  2017). Obeid  et al. (2019) 
prepared undoped and Gd-doped ZnO nanocrystalline 
samples using the thermal decomposition method. They 
observed that the optical absorption spectra of pristine ZnO 
increased with a 6% Gd doping concentration and the 
magnetic properties changed with changing dopant 
amounts.  Their PL spectroscopy results showed the 
existence of defects, which increased with increasing Gd 
doping concentrations. They suggested that these DMS 
nanoparticles can be used for magneto-optoelectronic 
practical device applications.

Undoubtedly, the RE-doped ZnO nanostructures have been 
intensively studied recently because this is a rapidly growing 
field due to their multi-functionality and their use in possible 
new generation applications. With an aim to understand the 
effect of Gd doping in ZnO in terms of its electronic and 
optical properties we have synthesize Gd-doped ZnO powder 
series of different Gd (0%, 2%, and 5%) values. Various 
characterization techniques have been utilized to explore the 
electronic, structural and optical features of the synthesized 
powder series. Gd doping into ZnO produced changes in the 
band gap and enhanced oxygen vacancies related states in the 
compound.

Materials and methods

The Gd-doped ZnO powder samples with varying Gd (0%, 
2%, and 5%) concentrations have been synthesized using the 
conventional solid-state reaction method. A suitable amount 
of high-purity (99.999% Sigma-Aldrich) zinc oxide (ZnO) 
and gadolinium oxide (Gd2O3) powder have been used for 
their preparation. For achieving perfect homogeneity, the 
ingredients were properly pounded for ~8 hours in an agate 
mortar and pestle. The prepared material has been sintered 
for ~8h at~550°C and cooled at room temperature. Again, the 
mixture was ground for ~2 hours and sintered for ~6 hours at 
~550°C. The mixture was milled for ~4 hours before being 
turned into pellets. Finally, these pellets were sintered for 
around ~8 hours at about ~600°C. Finally, we ground the 
pellets to make the powder. The powder form of the prepared 
samples has been used for further characterization.

Characterization techniques

The crystallographic structures of the samples have been 
explored using an X-ray diffractometer (Make: Panalytical 
X'Pert PRO) using the photon energy of Cu Kα (λ= 
1.5406Å). SEM images have been used for the morphology 
study of the synthesized samples. The SEM equipment model