BIBECHANA Vol. 21, No. 3, December 2024, 213-220 ISSN 2091-0762 (Print), 2382-5340 (Online) Journal homepage: http://nepjol.info/index.php/BIBECHANA Publisher: Dept. of Phys., Mahendra Morang A. M. Campus (Tribhuvan University) Biratnagar Photoluminescence properties of Tm3+/Ho3+/Cr3+-co-doped Y3AlGa4O12 nano-garnet phosphors for visible-NIR LED applications Ramadevi Nepal1, Jyothi Prasad G.M.2,3, Praveena Ravipati3,∗, B.D. Joshi4,∗∗, D. Sujatha5 1Central Department of Physics, Tribhuvan University, Kirtipur, Kathmandu, Nepal. 2Andhra University, Visakhapatnam -530003, India. 3Department of Physics, Gayatri Vidya Parishad College of Engineering (A), Visakhapatnam- 530048, India. 4Department of Physics, Siddhanath Science Campus, Tribhuvan University, Mahendranagar, Nepal. 5GVSM Govt. Degree College, Ulavapadu, SPSR Nellore – 523292, India. ∗Corresponding author: Email: praveena@gvpce.ac.in ∗∗Corresponding author: Email:bhawani.joshi@snsc.tu.edu.np Abstract Trivalent lanthanide (Ln3+; Ln = Tm, Ho, Cr)-doped Y3AlGa4O12 nano-garnet phosphor powders with varying Cr3+ ion concentration (0.5, 1.0, 2.0 and 3.0 mol%) were prepared using sol-gel synthesis. The prepared powders were characterized by X-ray powder diffraction (XRD), Raman and photoluminescence spectroscopic techniques. Phase purity, structure and crystallite size have been estimated from the XRD results. Raman spectra showed the vibra- tional analysis of the prepared powders. Excitation spectrum showed a broad band centred at 354 nm when monitored at 712 nm. Under 360 nm excitation, emission spectra showed a broad band with characteristic peaks of Tm3+, Ho3+ and Cr3+ ions. The intensity of peaks and full width at half maximum were increased up to 2.0 mol% Cr3+ ion and then decreased. All the decay curves exhibited non-exponential nature with an average lifetime of 0.302 ms. The decay curves were found to be insensitive to the Cr3+ ion concentration. The CIE colour co-ordinates were located in the orange-red region of the CIE diagram with correlated colour temperature 2261 K. The results showed that the present phosphors were suitable for the solid state light emitting diode applications. Keywords Tm, Ho, Cr, YAGG, garnets, phosphors, light emitting diode Article information Manuscript received: February 19, 2024; Revised: April 23, 2024; Accepted: April 26, 2024 DOI https://doi.org/10.3126/bibechana.v21i3.62845 This work is licensed under the Creative Commons CC BY-NC License. https://creativecommons. org/licenses/by-nc/4.0/ 213 http://nepjol.info/index.php/BIBECHANA praveena@gvpce.ac.in bhawani.joshi@snsc.tu.edu.np https://doi.org/10.3126/bibechana.v21i3.62845 https://creativecommons.org/licenses/by-nc/4.0/ https://creativecommons.org/licenses/by-nc/4.0/ Ramadevi Nepal et al./ BIBECHANA 21 (2024) 213-220 214 1 Introduction Nanomaterials like nanowires or nano-phosphors can improve electron and photon management within the light emitting diodes (LEDs) structure, leading to reduced energy consumption and in- creased luminous efficiency. Nanoparticles provide a high level of tunability, allowing for the cus- tomization of various properties such as colour, con- ductivity, and thermal management. This flexi- bility enables the tailoring of LED materials to meet specific application requirements, leading to advancements in diverse fields, including lighting, displays, and optoelectronics. Nanoparticles enable the miniaturization of LED components, allowing for the creation of smaller, more compact devices. This is particularly important in the development of flexible and bendable LED displays, where the use of nanomaterials helps to maintain the required structural integrity while achieving the desired flex- ibility. Recently, research on nano-phosphors has re- ceived significant attention as these are efficient lu- minescent materials for LEDs, field emission dis- plays and plasma displays. Phosphor-converted LEDs has excellent properties such as less produc- tion cost and simple device structure [1]. Further, near infrared (NIR) broadband sources are very im- portant for non-invasive medical diagnostics, non- destructive food measurement, bioimaging, night- vision technologies, light converters and optical am- plifiers [2–6]. The conventional incandescent bulbs and halogen lamps face different problems like low efficiency, short lifetime, high working temperatures and large size which limit their applications [7]. On the other hand, NIR-emitting LEDs have great ad- vantages like high efficiency, long lifetime, small in size, etc. Therefore, NIR broad band sources with these features are highly desired in wide range of applications [8]. NIR phosphor-converted LEDs contains a In- GaN blue LED chip with NIR-emitting phosphors which can bring together the great advantages of InGaN LED chips including higher thermal sta- bility, low manufacturing cost and high luminous efficiency [9] [10]. Besides, broadband and light converters can be realized by the careful selection of host material and emitting ion of NIR phos- phors [11]. Trivalent lanthanide (Ln3+) ions have been extensively employed as emitting ions for var- ious phosphor-converted LEDs. Among the vari- ous Ln3+ ions, Tm3+ is one of the most interesting ions as it provides blue and red emissions whereas Ho3+ ion is famous for its green emission. There- fore, many researchers have focused on the devel- opment of single phased phosphors co-doped with both Tm3+ and Ho3+ ions and studied the energy transfer between Tm3+ and Ho3+ ions excited by NIR wavelength [12, 13]. In our previous work, ul- traviolet excited (UV) Tm3+ and Ho3+ co-doped Y3Al4GaO12 phosphors were prepared and stud- ied their luminescence and energy transfer prop- erties for white LED applications [14]. In the present work, Tm3+/Ho3+:Y3AlGa4O12 (YAGG) phosphors co-doped with Cr3+ ions were prepared in order to broaden and extend the spectral profile in to the NIR region for broadband source applica- tions. Cr3+ ion is the interesting ion to dope into the inorganic materials owing to its deep red colours (~ 700 nm) and narrow band emissions due to the spin-forbidden 2E →4A2 transition or broadband emission (650-1600 nm) due to the spin-allowed 4T2→4A2 transition, which strongly depends on the surrounding crystal-field strength given by the host lattices [15–17]. Garnets have been chosen as the host materials for this purpose owing to their excellent properties like high thermal conductivity, high chemical sta- bility and exhibit intense luminescence when doped with Ln3+ ions [18]. Ln3+:Y3Al5O12 (YAG) is the well-known commercial phosphor for LED applica- tions [19] . However, the search for the advanced phosphors with better multiple properties for dis- play applications is still remaining as challenging task. In this regard, the chemical composition of the garnets can be changed by replacing the part of the Al3+ ions with the Ga3+ or Fe3+ or Ge3+ ions and/or Y3+ ions with Lu3+ or Gd3+ ions. When the Al3+ ions are replaced by larger Ga3+ ions, the distance between dodecahedral lattice sites (where the Ln3+ ion is substituted) is going to be increased. This increase in the distance between the Ln3+ ions tends to decrease the interaction be- tween them in the host matrix and in turn reduces the concentration quenching [20]. This would en- hance the luminescence intensity and efficiency as well. For instance, it was predicted theoretically that the Ga3+ garnets show better luminescence properties when compared to Al3+ garnets [21]. The same was also observed by Praveena et al., [22] where the luminescence intensity of Dy3+-doped Y3AlGa4O12 nano-garnets are higher than that of Y3Al4GaO12 nano-garnets. Hence, the authors are motivated to investigate the luminescence proper- ties of the present Tm3+/Ho3+/Cr3+:YAGG phos- phors by varying the Cr3+ ion concentration. To the best of author’s knowledge, no report is found on the current study. 2 Experimental technique In the present study the phosphors were synthe- sised by the well-know sol-gel method [23]. Sol-gel synthesis is a versatile method for producing phos- phors with tunable properties, including structural and optical characteristics. Optimizing synthesis Ramadevi Nepal et al./ BIBECHANA 21 (2024) 213-220 215 parameters can significantly impact the final prod- uct’s properties. For instance, the choice of precur- sors can affect the chemical composition, dopant concentration and crystal structure of the phos- phor which can influence the luminescence prop- erties. The solvent affects the solubility of precur- sors and the rate of hydrolysis and condensation reactions. It can also impact the porosity, homo- geneity and surface area of the phosphor. The pH of the so-gel solution can influence the hydrolysis and condensation reactions, affecting the size, mor- phology, and crystallinity of the phosphor particles. pH adjustment can also help control the distribu- tion of dopants in the host matrix. The tempera- ture and duration of the sol-gel process can affect the kinetics of the reaction, influencing the phase composition, crystallinity, and particle size of the phosphor. Higher temperatures and longer reaction times generally lead to larger, more crystalline par- ticles. The drying and calcinations processes can impact the phase purity, crystallinity, and lumines- cence properties of the phosphor. Controlled drying and calcinations can help prevent phase transfor- mations and enhance the phosphor’s optical prop- erties. The concentration of dopants in the sol-gel solution can affect the luminescence efficiency and colour of the phosphor. Optimizing the dopant con- centration is crucial for achieving the desired opti- cal properties. The addition of additives and sur- factants can influence the morphology, particle size, and dispersion of the phosphor. This can also af- fect the luminescence efficiency and stability of the phosphor. Post-treatment processes, such as an- nealing or surface modification, can further enhance the structural and optical properties of the phos- phor. These processes can help to improve the crys- tallinity, phase purity, and luminescence efficiency of the phosphor. Therefore, optimizing sol-gel syn- thesis parameters is essential for tailoring the struc- tural and optical properties of phosphors. Consid- ering aforementioned parameters, the following op- timizing conditions were taken into consideration after performing several experiments. YAGG nano-crystalline powders tri-doped with dopant concentrations of 0.1 Tm3+/0.3 Ho3+/x Cr3+ ions (where x= 0.5, 1.0, 2.0, 3.0 mol%) were prepared [23]. Nitrate forms of the precursors were dissolved into the 25 ml of 1M HNO3 under stirring to get ‘sol’. pH is maintained at around 1. Cit- ric acid and PEG were added to the solution in the ratio metal:citric acid:PEG is 1:2:8. Continued stir- ring for 2 h till the ‘gel’ is obtained and then heated at 90 oC for 40 h. No other additives and surfac- tants were used. Then, the obtained yellow colour gel was first heated at 500 oC for 4 h and later at 950 oC for 16 h in an electronic furnace with air at- mosphere. The final product is in the white colour nano-crystalline powder form. XRD patterns were recorded using X-ray diffractometer (Philips PW 1830) using Cu K radia- tion (1.5406 Å). Confocal micro-Raman spectrom- eter (Horiba Jobin Yvon Lab RAM-HR 800) was used to record the Raman spectrum. Excitation, emission and decay spectra were recorded by using JOBIN YVON Flurolog – 3 spectrofluorimeter with a 450 W xenon flash lamp. All the measurements were carried out at the similar conditions. 3 Results and Discussion 3.1 X-ray diffraction XRD profiles of the tri-doped YAGG garnet nano- crystalline powders are displayed in the Fig. 1. All the diffraction peaks were well matched with the standard JCPDS data card no. 89-6661 that cor- responds to space group of Ia-3d cubic structure. No impurity peak is identified. This indicates that the formation of single phase high purity phosphors and successful incorporation of dopant ions without causing any disruptions in the crystal structure of YAGG. The well resolved sharp peaks suggest that the synthesized samples are highly crystalline in na- ture. From Fig. 1, it is clear that the crystallinity is increased with increasing Cr3+ ion concentra- tion. The crystallite size of the powder samples was estimated using Debye-Scherrer’s equation Dhkl = kλ/βcosϑ, where D is the average crystallite size, k is the shape factor (0.9), is the wavelength of the X- rays, is the full width at half maximum (FWHM) and is the diffraction angle. The crystallite size is increased from 13.8 to 23.7 nm when the Cr3+ concentration is increased from 0.5 to 3 mol%. The crystallite size was slightly reduced upon adding the Cr3+ into the Tm,Ho-doped YAGG host [14] which represents the enhanced surface area. Figure 1: XRD profiles of the tri-doped YAGG nano-crystalline powders. Ramadevi Nepal et al./ BIBECHANA 21 (2024) 213-220 216 3.2 Raman spectra The Raman spectrum of 1.0 mol% Cr3+ tri-doped YAGG samples is shown in Fig. 2. The high fre- quency modes (800–1000 cm-1) are associated to symmetric and asymmetric internal stretching vi- brations of rigid AlO4/GaO4 tetrahedra and the modes lying between 450–800 cm-1 are assigned to bending motions of these tetrahedra. The remain- ing lattice modes (150– 450cm-1) involve rotations and translations of the AlO4/GaO4 groups, octahe- drally co-ordinated trivalent cations and dodecahe- drally co-ordinated trivalent cations [22,24,25]. Figure 2: Raman spectrum of tri-doped YAGG nano-crystalline powder. 3.3 Excitation It is well-known that Cr3+ has 2E, 4A2, 4T2 and 4T1 energy levels come from the 4F term in its d3 configuration [26, 27]. The excitation and lumines- cence properties of the Cr3+ ion depend on the rel- ative positions of the 4T2 and 2E levels. 4T2 state is above the 2E level when the Cr3+ ion locates in the intermediate-field site that results R-line emis- sion whereas 4T2 is under the 2E level when the Cr3+ ion locates in the weak-field site. Figure 3 shows the excitation spectrum of 1 mol% Cr3+ tri- doped YAGG sample monitoring at emission wave- length of 712 nm. The spectrum consists of broad- band in the region 300-450 nm which corresponds to 4A2→4T1 (4F) transition of Cr3+ ion [28]. From Fig. 3 it is confirmed that the present samples can be excited by near UV and visible radiation that is matching with InGaN LED chip. Figure 3: Excitation spectra of tri-doped YAGG sample. 3.4 Photoluminescence Figure 4 shows the photoluminescence spectra of tri-doped YAGG nano-crystalline powders excited with the wavelength of 360 nm. The spectra have a broad band in the region 450-800 nm with sharp characteristic peaks. The peaks are located at 482, 523, 557, 582, 612, 640 and 712 nm. The peak at 482 nm corresponds to 1G4→3H6 transition of Tm3+ ion, peaks at 523 and 557 nm correspond to 5F4, 5S2→5I8 transitions of Ho3+ ion, peaks at 582 and 612 nm correspond to 4T2→4A2 transi- tion of Cr3+ ion, peak at 640 nm correspond to 1G4→3F4 transition of Tm3+ ion and peak at 712 nm correspond to 2E →4A2 transition of Cr3+ ion [14,28,30]. Rai et. al., [31] also noticed the 581 and 700 nm emissions of Cr3+ in LaVO4 host upon ex- cited with wavelengths of 428 and 467 nm. Yao et. al., [32] also observed the similar type of sharp lines at higher wavelengths side of the 2E→4A2 transi- tion corresponding to Nd3+ ion when it is co-doped with Cr3+ ion into the Ca3Sc2SiO12 host. Chen et. al., [33] reported the Yb3+/Ln3+/Cr3+ (Ln = Er, Ho) doped transparent glass ceramics: crystalliza- tion, Ln3+ sensitized Cr3+ upconversion emission and multimodal temperature sensing. An intense Cr3+ upconversion luminescence assigned to the 2E→4A2 transition was observed upon 980 nm laser excitation via energy transfer from Yb3+ sensitizers to Er3+ activators/bridging-centers and finally to Cr3+ emitting centers. Zhang et. al., [34] observed the enhanced photoluminescence of Gd3Al4GaO12 (GAGG):Cr3+ by energy transfer from co-doped Dy3+ ions. From Fig. 4, it is observed that the luminescence intensity is slightly increased up to 2 mol% of Cr3+ and then decreased which represents the optimum concentration is 2.0 mol%. From Fig. 4, it is noticed that the intensity variation with respect to Cr3+ ion is very less and hence these Ramadevi Nepal et al./ BIBECHANA 21 (2024) 213-220 217 are stable phosphors. The FWHM values of the emission band are found to be 75.3, 77.3, 88.1 and 81.2 nm for 0.5, 1.0, 2.0 and 3.0 mol% of Cr3+ ion, respectively. These values are also slightly in- creased up to 2.0 mol% and then decreased. Hence, the optimised concentration is 2.0 mol%. Fur- ther, it is observed that the FWHM is higher than that of YAGG:0.5Tm,1.5Ho host (~65 nm) [14] and MgAlGa0.7B0.3O4:Cr3+(29 nm) at lower concentra- tions [35]. Thus, with the addition of Cr3+ ion to the YAGG:Tm/Ho host increased the FWHM value. Therefore, from the luminescence spectra it is concluded that these phosphor powders can be useful for the broadband visible-NIR light emitting diode applications. Figure 4: The photoluminescence spectra of tri- doped YAGG nano-crystalline powders. 3.5 Decay spectra Figure 5 shows the decay curves monitoring at 582 nm emission and excited by 360 nm wavelength. All the decay curves exhibit non-exponential na- ture but insensitive to the Cr3+ ion concentration. The non-exponential nature indicates the energy transfer processes are involved among Tm3+, Ho3+ and Cr3+ ions. The insensitive nature of decay curves over Cr3+ ion concentration represents the absence of concentration quenching in these phos- phors, which enhances the luminescence efficiency. The average lifetime is found to be around 0.302 ms which is comparable with that of Cr3+:LaVO4 host (0.4503 ms) [31], higher than that of Cr3+-doped garnets like YGG (0.240 ms), GGG (0.160 ms), YSGG (0.140 ms), GSAG (0.150 ms) and GSGG (0.120 ms) [36] and Cr3+:LuScO3 (0.023 ms) hosts [37]. Figure 5: The decay profiles of tri-doped YAGG phosphors excited at 360 nm wavelength. 3.6 Energy transfer among Tm3+,Ho3+ and Cr3+ The energy transfer among Tm3+, Ho3+ and Cr3+ ions in the YAGG host can be elucidated through the schematic energy level diagram depicted in Fig. 6. When subjected to 360 nm excitation, first the Tm3+ ions get excited from 3H6 to the 1D2 level. Subsequently, these ions undergo non-radiative re- laxation (NR) to the lower 1G4 level. Following this relaxation, radiative decay of Tm3+ ions to the ground state takes place, emitting photons at wave- lengths 482 nm and 640 nm, corresponding to the 1G4→3H6 and 1G4→3F4 transitions, respectively. During this process, a fraction of the energy is transferred from the excited Tm3+ ions to adjacent ground state Ho3+ ions through energy transfer (ET1): 1D2(Tm) + 5I8(Ho)→3F4(Tm) + 5G6(Ho). This phenomenon arises due to pronounced spectral overlap between the Tm3+ emission (1D2→3F4) and Ho3+ excitation (5I8→5G6) in the vicinity of 460 nm [14]. Subsequently, the excited Ho3+ ions undergo non-radiative relaxation to lower levels, fol- lowed by radiative transitions, 5F4→5I8 (523 nm) and 5S2→5I8 (557 nm). Concurrently, Cr3+ ions are also elevated to the 4T1 level from 4A2 upon 360 nm excitation. These ions undergo non-radiative tran- sitions to lower levels, populating the 2E and 4T2 levels. Notably, a strong yellow emission at 582 nm and a relatively weak red emission at 612 nm are ob- served from the Cr3+ energy levels, corresponding to the 4T2→4A2 transition, along with NIR emis- sion at 712 nm associated with the 2E→4A2 tran- sition of Cr3+ ions. In addition to this, Cr3+ ions transfer a portion of their energy to adjacent Ho3+ ions (ET2), populating the 5F4 and 5S2 excited lev- els of Ho3+. This energy transfer is reflected in the emission spectra (Fig. 4), where the intensity at 523 nm and 577 nm increases with increase in Cr3+ ion Ramadevi Nepal et al./ BIBECHANA 21 (2024) 213-220 218 concentration up to 2.0 mol% and then decreases. Figure 6: Schematic partial energy level diagram of YAGG:Tm,Ho,Cr nano-phosphors. Solid lines with arrow represent the transitions between the energy levels, zigzag lines indicate non-raditative transi- tions (NR), and ET represents energy transfer. 3.7 CIE colour co-ordinates Commission International de I’Eclairage (CIE) colour co-ordinates and chromaticity diagrams pro- vide a globally recognized and standardized system for describing colours accurately [38]. This con- sistency is vital for industries to ensure colour fi- delity across various materials. These co-ordinates and diagrams are used to study colour percep- tion, human vision and lighting technologies. Fig- ure 7(a) display the CIE chromaticity diagram and 7(b) represent the corresponding spectral profile of the 2.0 mol% Cr3+ tri-doped YAGG phosphor. The colour co-ordinates (x, y = 0.5304, 0.4611) are located in the orange-red region with corre- lated colour temperature 2261 K. This represents the present phosphor gives a warm light [39]. The colour co-ordinates in the present host are closer to the YAGG:0.5Tm,1.5Ho [14], as the ratio of Tm/Ho concentration is same, but shifted towards orange- red side with the addition of Cr3+ ion. An LED de- vice fabricated with the GAGG:0.1Cr3+,0.01Dy3+ phosphor and a 450 nm blue chip showed the CIE coordinates at (0.6387, 0.2873) [34]. It appears as a milky white light in the LED device and provides a bright purplish-red emission driven by a current of 20 mA. It gives a strong red emission and yields a luminous efficacy of 27.8 lmW-1. In addition, chlorophyll pigment in plants absorbs blue (400-500 nm) and red (600-700 nm) light to help photosyn- thetic and phototropic processes [40]. Phytochrome in plants changes its state to active form in the red light (600-700 nm, peaking at 660 nm) and inactive form in the far-red light (600-780 nm, peaking at 730 nm) [41]. These two pigments respond to red light in particular to support several functionalities in plants. Therefore, the Tm/Ho/Cr-doped YAGG nano-crystalline powders can also be used in the plant growth LED applications. (a) (b) Figure 7: (a) CIE diagram (b) corresponding spectral profile of 2.0 mol% Cr3+ samples. 4 Conclusions Tm3+/Ho3+/Cr3+ tri-doped YAGG nano- crystalline powders were prepared by sol-gel method and are characterized by XRD, Raman and photoluminescence measurements. The XRD results confirmed the formation of single phased cubic garnet structure. The Raman spectrum pro- vided the vibrational analysis of the present sam- ples. Excitation spectrum showed that these phos- phors can be successfully excited with near UV LED light. The photoluminescence spectra showed the broad band in the visible and NIR regions with characteristic peaks of Tm3+/Ho3+/Cr3+ions. The addition of Cr3+ ion increased the FWHM value. The decay curves were invariant with the Cr3+ ion Ramadevi Nepal et al./ BIBECHANA 21 (2024) 213-220 219 concentration which indicates that the absence of concentration quenching. The CIE co-ordinates fell in the orange-red region. The results showed that the present phosphors have potential applications in the visible-NIR light emitting broadband sources and plant growth LED technology. Acknowledgements The authors are highly thankful to the Gaya- tri Vidya Parishad College of Engineering (A), Visakhapatnam for providing partial financial sup- port. Ramadevi also thankful to Prof. Dr. Leela Pradhan, Amrit Science Campus, Tribhuvan Uni- versity and the Kailali Multiple Campus, Farwest University, Dhangadhi, Kailali, for extending the Laboratory facilities. References [1] E. Fred et al. Solid-state light sources getting smart. Science, 308(5726):1274–1278, 2003. [2] S. Dadgar et al. Optical spectroscopic sens- ing of tumor hypoxia. Journal of Biomedical Optics, 23(6):067001–067001, 2018. [3] J. Ma et al. Advanced techniques for hyper- spectral imaging in the food industry: princi- ples and recent applications. Annual Review of Food Science and Technology, 10:197–220, 2019. [4] L. Zhou et al. Spectral properties and en- ergy transfer of a potential solar energy con- verter. Chemistry of Materials, 28(10):2834– 2843, 2016. [5] G. Hong et al. Near-infrared fluorophores for biomedical imaging. Nature Biomedical Engi- neering, 1(1):0010, 2017. [6] K. Chrzanowski et al. Review of night vi- sion technology. Opto-Electronics Review, 21(2):153–181, 2013. [7] T. Pulli et al. Advantages of white led lamps and new detector technology in photometry. Light: Science Applications, 4(8):e332–e332, 2015. [8] R. Filippo et al. Leds: sources and intrin- sically bandwidth-limited detectors. Sensors, 17(7):1673–1684, 2017. [9] L. Zhang et al. A high efficiency broad- band near-infrared Ca2LuZr2Al3O12:Cr3+ gar- net phosphor for blue LED chips. Journal of Material Chemistry C, 6(18):4967–4976, 2018. [10] P.F. Pereira et al. Red, green, and blue (RGB) emission doped Y3Al5O12 (YAG) phosphors prepared by non-hydrolytic sol-gel route. Jour- nal of Luminescence, 130:488–493, 2010. [11] C. Liu et al. Synthesis, crystal structure, and enhanced luminescence of garnet type Ca3Ga2Ge3O12:Cr3+ by codoping Bi3+. Jour- nal of American Ceramic Society, 98(6):1870– 1876, 2015. [12] S.D. Jackson. Efficient Tm3+, Ho3+-co-doped silica fibre laser diode pumped at 1150 nm. Op- tics communications, 281(18):3837–3840, 2008. [13] N. Liu et al. Bright white up-conversion emis- sion from Yb3+/Tm3+/Ho3+ tri-doped M- AgGd(WO4)2 phosphors. Journal of Lumines- cence, 152:182–187, 2014. [14] R. Praveena et al. Photoluminescence prop- erties of Ho3+/Tm3+-doped YAGG nano- crystalline powders. Optical Materials, 72:666– 672, 2017. [15] Y. Li et al. Long persistent and photo- stimulated luminescence in Cr3+-doped Zn– Ga–Sn–O phosphors for deep and reproducible tissue imaging. Journal of Material Chemistry C, 2(15):2657–2663, 2014. [16] Z. Pan et al. Sunlight-activated long-persistent luminescence in the near-infrared from Cr3+- doped zinc gallogermanates. Naturematerials, 11(1):58–63, 2012. [17] B. Qiao et al. Study on ZnGa2O4:Cr3+ ac pow- der electroluminescent device. Materials Let- ters, 61(2):401–404, 2007. [18] V. Lupei. RE3+ emission in garnets: multi- sites, energy transfer and quantum efficiency. Optical Materials, 19(1):95–107, 2002. [19] P.F. Pereira et al. Red, green and blue (RGB) emission doped Y3Al5O12 (YAG) phosphors prepared by non-hydrolytic sol–gel route. Journal of Luminescence, 130:488–493, 210. [20] T.H. Allik et al. Crystallography, spec- troscopic analysis, and lasing properties of Nd3+:Y3Sc2Al3O12. Physical Review B, 41(1):21–30, 1990. [21] L.H. Spangler et al. A computational study of host effects on Er3+ upconversion and self quenching efficiency in ten garnets. Journal of Applied Physics, 79(10):573–577, 1996. [22] R. Praveena et al. White light generation from Dy3+-doped yttrium aluminium gallium mixed garnet nano-powders. Journal of Lumines- cence, 170:262–270, 2016. Ramadevi Nepal et al./ BIBECHANA 21 (2024) 213-220 220 [23] R. Kasuya et al. Glycothermal synthesis and photoluminescence of YAG: Ce3+ nano- phosphors. Journal of Alloys and Compounds, 408(1-2):820–823, 2006. [24] K. Papagelis et al. Lattice dynamical prop- erties of the rare earth aluminum garnets (RE3Al5O12). Physica Status Solidi (b), 233(1):134–150, 2002. [25] K. Papagelis and S. Ves. Vibrational properties of the rare earth aluminum garnets. Journal of Applied Physics, 94(11):6491, 2003. [26] I. Hernandez et al. Inorganic chemistry. Inor- ganic Chemistry, 47:10288–10298, 2008. [27] M.N. Sanz-Ortiz et al. Origin of the 2E⇐⇒4T2 fano resonance in Cr3+-doped LiCaAlF6: Pressure-induced excited-state crossover. Physical Review B, 81(4):045114, 2010. [28] J. Zhou et al. Synthesis and near-infrared lu- minescence of La3GaGe5O16:Cr3+ phosphors. RSC Advances, 4(86):46313–46318, 2014. [29] Z. Jia et al. Strategies to approach high per- formance in Cr3+-doped phosphors for high- power NIR-LED light sources. Light: Science Applications, 9:86, 2020. [30] H. Yu et al. Broadband near-infrared emis- sion of K3ScF6:Cr3+ phosphors for night vision imaging system sources. Chemical Engineering Journal, 417:129271, 2021. [31] Z. Jia et al. Structural and photoluminescence properties of Cr3+doped LaVo4 phosphor. Light: Science Applications, 129(9):106904– 106915, 2022. [32] L. Yao et al. Broadband emission of single- phase Ca3Sc2Si3O12:Cr3+/Ln3+ (lLn= Nd, yb, ce) phosphors for novel solid-state light sources with visible to near-infrared light output. Ce- ramics International, 45:14249–14255, 2019. [33] D. Chen et al. Yb3+/Ln3+/Cr3+ (Ln = Er, Ho) doped transparent glass ceramics: crystallization, ln3+ sensitized Cr3+ upconver- sion emission and multi-modal temperature sensing. Journal of Material Chemistry C, 5:11769–11780, 2017. [34] Y. Zhang et al. Enhanced photolumines- cence of Gd3Al4GaO12:Cr3+ by energy trans- fers from Co-doped Dy3+ Nanomaterials, 12:4183, 2022. [35] C. Zhong et al. High output power and high quantum efficiency in novel nir phosphor MgAlGa0.7B0.3O4:Cr3+ with pro- found FWHM variation. Advanced Materials, 36(10):2309500, 2023. [36] M. Yamaga et al. Temperature dependence of the lifetime of Cr3+ luminescence in garnet crystals i. Applied Physics B, 50(4):425–431, 1990. [37] J. Li et al. Growth, spectroscopic properties, and energy levels of Cr3+ doped LuScO3 crys- tal. Optical Material Express, 12(8):3071–3080, 2022. [38] Y. Guo et al. A red-emitting perovskite-type SrLa(1x)MgTaO6:xEu3+ for white LED appli- cation. Journal of Luminescence, 167:381–385, 2015. [39] L. Wu et al. Luminescence and energy trans- fer of a color tunable phosphor: Dy3+-, Tm3+- , and Eu3+-coactivated KSr4(BO3)3 for warm white UV LEDs. Journal of Materials Chem- istry, 22:6463–6470, 2012. [40] J. Xiang et al. Enhancement of red emis- sion and site analysis in Eu2+ doped new-type structure Ba3CaK(PO4)3 for plant growth white LEDs. Chemical Engineering Journal, 356:236–244, 2019. [41] Q. Sun et al. Synthesis and photolu- minescence properties of deep red-emitting CaGdAlO4:Mn4+ phosphors for plant growth LEDs. Journal of Luminescence, 203:371–375, 2018. Introduction Experimental technique Results and Discussion X-ray diffraction Raman spectra Excitation Photoluminescence Decay spectra Energy transfer among Tm3+,Ho3+ and Cr3+ CIE colour co-ordinates Conclusions