Bright Blue Emissions on UV-Excitation of LaBO3 (B=In, Ga, Al) Pervoskite Structured Phosphors for Commercial Solid-State Lighting Applications Chimica Techno Acta ARTICLE published by Ural Federal University 2022, vol. 9(1), No. 20229107 eISSN 2411-1414; chimicatechnoacta.ru DOI: 10.15826/chimtech.2022.9.1.07 1 of 7 Bright blue emissions on UV-excitation of LaBO3 (B=In, Ga, Al) perovskite structured phosphors for commercial solid-state lighting applications B.V. Naveen Kumar ab, T. Samuel c, Samatha Bevara d, K. Ramachandra Rao e, Satya Kamal Chirauri e* a: Department of Physics, Acharya Nagarjuna University, Guntur, India b: Shri Vishnu College of Engineering for Women(A), Bhimavaram, India c: GMR Institute of Technology, Rajam, Andhra Pradesh, India d: Chemistry Division, Vignan's Foundation for Science, Technology and Research, Guntur, Andhra Pradesh, India e: Crystal Growth & Nanoscience Research Center, Government College (A), Rajahmundry, India * Corresponding author: satyakamal.ch@gmail.com This article belongs to the regular issue. © 2022, The Authors. This article is published in open access form under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). Abstract Bright blue photoluminescence (PL) was obtained from Bi3+-activated LaBO3 (B = In, Ga, Al) perovskite nanophosphors. A cost-effective and low-temperature chemical route was employed for preparing Bi3+ doped LaBO3 (B=In, Ga, Al) which were then annealed at 1000 °C. The phase formation, morphological studies and luminescent proper- ties of the as-prepared samples were performed by X-ray diffraction (XRD), scanning electron microscopy (SEM), photoluminescence and optical absorption spectroscopy. Comparison of emission intensities, lifetime studies, energy band gaps and color purity of all samples (pure and Bi3+ doped) were investigated for promising applications in UV light-emitting diodes, variable frequency drive (VFD), field emission display (FED), and other photoelectric fields. Keywords perovskites photoluminescence phosphor quenching solid-state lightning Received: 08.07.2021 Revised: 02.01.2022 Accepted: 04.03.2022 Available online: 11.03.2022 1. Introduction Blue emissive materials have attracted considerable atten- tion because of their vast applications in the fields of sen- sors, solid-state lighting technology, and light fidelity (Li-Fi) [1–3]. In view of such applications, design and de- velopment of blue light-emitting materials is an exciting challenge. Commercially available blue phosphors like GaN and InGaN, in thin-film form, have potential use in bright blue light emitters. Preparation of such nitride ma- terials requires an energy-expensive process and thus hin- ders the development of cost-effective blue light emitters [4]. Perhaps, in blue light-emitting phosphors, rare-earth ions like Eu2+ and Ce3+ are commonly used as activator ions, owing to their 4f→5d transitions giving rise to an absorption band ranges in NUV to the blue region and a broad emission band covering blue to the red region [5–8]. However, phosphors doped with Eu2+ or Ce3+ shows some disadvantage in the view of high cost, high reabsorption and color deviation. Therefore, developing potential lumi- nescent phosphor materials doped with non-rare earth ions as activators is more promising. Among many recent studies, it was found that Bi3+ ion as an activator plays a crucial role in the generation of efficient blue-light emis- sion [9–13]. Alternatively, Bi3+ ions with their excited state for electron transition and emission band of Bi3+ ion at room temperature can be rationally attributed to the 3P1→1S0 transition, which avoids the reabsorption among phosphors. It is worth mentioning that oxide-based mate- rials are preferred over nitride thin films due to low tem- peratures and simple methods of preparation. Among many oxides, perovskite materials have been widely inves- tigated for luminescence applications. For example, semi- conductor LaInO3 revealed potential properties for the phosphor applications and as a surface for solid oxide fuel cells [14]. Subsequently, LaAlO3 has intensive applications as a substrate for superconductors, magnetic and ferro- magnetic thin films and luminescent host materials, and has high thermal stability and good dielectric properties [15–17]. LaGaO3 perovskite has received much attention as a leading host because of its potential use as substrate applica- tions for phosphorus and solid oxide fuel cells [18]. In the last http://chimicatechnoacta.ru/ https://doi.org/10.15826/chimtech.2022.9.1.07 http://creativecommons.org/licenses/by/4.0/ https://orcid.org/0000-0003-2562-244X https://crossmark.crossref.org/dialog/?doi=https://doi.org/10.15826/chimtech.2022.9.1.07&domain=pdf&date_stamp=2022-3-11 Chimica Techno Acta 2022, vol. 9(1), No. 20229107 ARTICLE 2 of 7 few years, a considerable amount of research was done on LaInO3, LaAlO3, LaGaO3 perovskite materials. A few research groups have also reported that Bi3+ doped LaInO3 and LaGaO3 are efficient blue-emitting luminescent materials. However, to the best of our knowledge, there is no report on the opti- mization of Bi3+ ions in LaInO3, LaAlO3, and LaGaO3 phos- phors. Therefore, in this article, we demonstrated a compara- tive study for bright blue emissions on UV excitation close to industrial standards (colour coordinates: x = 0.15 and y = 0.15) obtained from Bi3+ ion doped LaBO3 (B=In, Ga, Al) samples. Hence, from the photo luminescent results, these blue light emitters would be potential materials to increase the efficiency of white light-emitting solid-state devices. 2. Experimental La(NO3)36H2O [Merck, Germany], In(NO3)3H2O [Sigma- Aldrich, 99.99%], Al(NO3)3H2O [Alfa Aesar 99.99%], Gal- lium(III)nitratehydrate (Ga(NO3)3H2O) [Sigma-Aldrich, 99.99%], Bi(NO3)35H2O [Sigma-Aldrich, 99.99%], were used as starting materials. For the synthesis of pure LaInO3 nanoparticles, stoichiometric amounts of La(NO3)36H2O and In(NO3)3H2O were mixed with 20 ml of distilled water in a two-necked round bottom flask. The solution was slowly stirred and ammonium hydroxide aqueous solution was added dropwise until the clear solu- tion turned intoturbid; then the solution was maintained at 120 °C for 2 hours. The precipitate then formed in the round bottom flask was collected and thoroughly washed five times with methanol and allowed for drying. Later, the samples were heated to 1000 °C at a heating rate of 10 °C per minute in a furnace that maintained the constant temperature for 5 h, then the furnace was turned off and the samples were allowed to settle naturally at room tem- perature to cool. Finally, the samples were grounded for further investigation. The same procedure was used to prepare LaInO3: 0.5, 1, 2, 2.5, 3 at.% Bi3+, LaGaO3: 1, 1.5, 2, 2.5, 3 at.% Bi3+, LaAlO3: 0.5, 1, 2, 2.5, 3 at.% Bi3+ doped nanophosphors, which were also annealed in air at 1000 °C. 3. Results and discussion 3.1. XRD studies XRD patterns of undoped and selected Bi3+ doped LaInO3, LaGaO3 and LaAlO3 samples calcined at 1000 °C are shown in Fig. 1 (a–c). The diffraction peaks indicate that the cal- cined samples of LaInO3, LaGaO3 can be indexed in pure orthorhombic phase and LaAlO3 can be indexed in cubic or rhombohedral phase. All diffraction peaks are in good agreement with the previously reported phases of LaInO3, LaGaO3 and LaAlO3 samples, and it is observed that no second phase was detected for the samples indicating that Bi3+ was completely dissolved in the host array, which is in good agreement with previously reported literature [19–23]. Typically, the crystal size of the sample is calcu- lated from the Debye–Scherer diffraction line width using the relation d = 0.9/cos, where d is the average crystal size,  is the X-ray wave length (1.5405 Å), and  is the maximum amplitude, which is correlated to full width at half of the maximum intensity (FWHM) line, and  is the angle of diffraction. The average crystal sizes of undoped and doped LaInO3, LaGaO3 and LaAlO3 samples, which were calcined at 1000 °C, are in the range of 90–120 nm. The lattice pa- rameters for both pure and doped LaInO3, LaGaO3 and LaAlO3 were calculated using PowderX software and are listed in Table l. Table 1 Unit cell parameters of pure and doped LaInO3, LaGaO3 and LaAlO3 samples Composition a (Å) b (Å) c (Å) Volume (Å)3 LaInO3 (JCPDS: 08-0148) 5.7820 8.3360 5. 9990 289.14 LaInO3: 2.5 at.% Bi 3+ 5.7240 8.2450 5.9280 279.76 LaGaO3 (JCPDS: 024-1102) 5.4883 5.5248 7.7499 234.99 LaGaO3: 1.5 at.% Bi 3+ 5.4963 5.4688 7.7149 231.90 LaAlO3 (JCPDS: 01-085-0848) 3.7860 3.7860 3.7860 54.26 LaAlO3: 2 at.% Bi 3+ 3.7810 3.7810 3.7810 54.05 3.2. Morphological studies Fig. 2 (a–f) shows the typical scanning electron micro- scope (SEM) images of pure and LaInO3: 2.5 at.% Bi3+, LaGaO3: 1.5 at.% Bi3+, LaAlO3: 2 at.% Bi3+ doped samples. A collection of crystalline granules and particles was ob- served in the SEM images of the phosphorus. The SEM im- ages of pure LaBO3 (B = In, Ga, Al) samples are depicted in Fig. 2 (a–c) and Bi3+ doped samples in Fig. 2 (d–f). All crys- tallites are spherical in nature ranging between 90–150 nm. Fig. 1 XRD patterns of pure and Bi3+ doped LaInO3 (a), LaGaO3 (b) and LaAlO3 (c) samples calcined at 1000 °C Chimica Techno Acta 2022, vol. 9(1), No. 20229107 ARTICLE 3 of 7 Fig. 2 SEM images of pure LaBO3 (B = In, Ga, Al) (a–c) and Bi 3+ doped LaBO3 (B = In, Ga, Al) (d–f) samples The higher calcinations temperature facilitates the possi- ble rapid arrangement of crystal structure followed by coalescence of particles leading to particle agglomeration causing a considerable reduction in crystallite size of Bi3+ doped samples compared to undoped samples. To support this phenomenon, particle size distribution graphs for all the pure and doped samples were calculated using ImageJ soft- ware and are presented in Fig. 3 (a–f). The energy dispersive X-ray spectrometer (EDS) spectra used to determine the composition of the samples and show good agreement with the nominal sample compositions (Fig. 4 (a–f)). 3.3. Optical Energy bandgap calculations Fig. 5 (a–b) shows the plots of (αhυ)2 vs hυ for the as- prepared nanophosphor uses, where α is the optical absorption coefficient and hυ is the energy of the incident photon. The band gap of the optical energy (for example) can be calculated by extending the linear part of the curve (αhυ)2 = 0. Form the plots, it was found that the optical energy bandgaps are equal to 4.16 eV, 5.10 eV and 4.28 eV for LaInO3, LaGaO3 and LaAlO3 pure samples, respectively. The optimum LaInO3: 2.5 at.% Bi3+, LaGaO3: 1.5 at.% Bi3+, LaAlO3: 2 at.% Bi3+ doped samples energy bandgap values are 3.81 eV, 4.90 eV and 3.90 eV, respectively. The value of energy gap is reduced considerably on doping with Bi3+ ions, which is demonstrated in Fig. 5. Also, it was reported elsewhere that the energy gap of phosphor material de- creased in presence of Bi3+ ion [24–26]. The Bi3+ ion ren- ders some energy levels that have the 6s2 valence elec- trons together to form a continuous band. The decrease in optical energy band gap may result in an increase in the concentration of excited ions in higher energy states. Thus, doping leads to the decrease in the energy of Fermi level and, hence, a reduction in the optical band gap is observed [27]. 3.4. Photoluminescence and lifetime decay studies Emission spectra of LaInO3: 0.5, 1, 2, 2.5, 3 at.% Bi3+, LaGaO3: 1, 1.5, 2, 2.5, 3 at.% Bi3+, LaAlO3: 0.5, 1, 2, 2.5, 3 at.% Bi3+ doped samples are shown in Fig. 6 (a–c). All the samples have broad emission bands centered at 432 nm, 373 nm and 350 nm on excitation with 330 nm, 309 nm and 274 nm, respectively, which is attributed to the 3P1–1S0 transition of Bi3+ ions [28]. It was evident that the intensity of the emission band increases as the Bi3+ concentration increases, reaching a maximum at 2.5 at.% for LaInO3: Bi3+, 1 at.% for LaGaO3: Bi3+ and 2 at.% for LaAlO3: Bi3+ samples, and then remarkably decreasing on increasing Bi3+ content due to the concentration quenching. The concentration quenching can be triggered because the in- teractions between two ions increase as doping increases and the decrease in extent of the energy transfer process causes the decrease of the emission intensity [29]. The corresponding excitation spectra, a broad excitation band ranging from 300 to 500 nm with a maximum at about 330, 309, 274 nm, which was arising from 1S0–3P1 transition of Bi3+, are illustrated in supporting information (Fig. S1). Further, it is noteworthy that in the emission spectra for undoped LaGaO3 a broad band peak maximum at 430 nm is observed, which is due to the GaO6 octahedral site (Fig. S2). Of all the perovskites from the emis- sion spectra trend, the highest emission intensity is observed for LaGaO3: Bi3+ doped samples with a peak position at 375 nm on excitation with 309 nm, which is illustrated in Fig. 6 (b) [30]. Chimica Techno Acta 2022, vol. 9(1), No. 20229107 ARTICLE 4 of 7 Fig. 3 Particle size distributions of pure LaBO3 (B = In, Ga, Al) (a–) and Bi 3+ doped LaBO3 (B = In, Ga, Al) (d–f) samples Fig. 4 EDS spectra of pure LaBO3 (B = In, Ga, Al) (a–c) and Bi 3+ doped LaBO3 (B = In, Ga, Al) (d–f) samples Chimica Techno Acta 2022, vol. 9(1), No. 20229107 ARTICLE 5 of 7 Fig. 5 Optical energy band gaps of pure LaBO3 (B = In, Ga, Al) (a) and Bi 3+ doped LaBO3 (B = In, Ga, Al) (b) samples Fig. 6 Emission spectra of LaInO3: 0.5, 1, 2, 2.5, 3 at.% Bi 3+ (a), LaGaO3: 0.5, 1, 1.5, 2, 2.5 at.% Bi 3+ (b) and LaAlO3: 0.5, 1, 2, 2.5, 3 at.% Bi3+ (c) doped samples The reason behind the enhanced intensity from LaGaO3:Bi3+ doped sample is due to the energy transfer phenomena from GaO6 octahedral sites to Bi3+ ions. To confirm this energy transfer, spectral overlap between excitation spectra of LaGaO3:Bi3+ sample and emission spectra of pure LaGaO3 is demonstrated in Fig. 7. To verify the emission intensity pattern, the corresponding life time decay values (Fig. 8 (a–d)) for the prepared phos- phors were calculated according to the following equation: 𝐼 = 𝐴1𝑒 (−𝑡 𝜏1⁄ ) + 𝐴2𝑒 (−𝑡 𝜏2⁄ ), (1) where I is the luminescence intensity at the time t, τ1 and τ2 are two components of the decay time, A1 and A2 are the constants. The average decay times of the samples were found to be 215, 252, 269, 329, 273 ns of LaInO3:Bi3+ (0.5, 1, 2, 2.5, 3 at.% Bi3+), 734 ns, 806 ns and 693 ns ofLaAlO3: 0.5, 1, 2, 2.5, 3 at.% Bi3+ and for LaGaO3: 1, 1.5, 2, 2.5, 3 at.% Bi3+ samples decay life time value was found to be 702 ns and 989 ns, 745 ns, 643 ns and 567 ns, respectively. 3.5. CIE chromatic coordinates Quantification of the overall emitted colors was done with the help of Commission International De I’Eclairage (CIE) chromaticity coordinates, where any color can be expressed in terms of (x, y) color coordinates, based on emission spectra [31]. Fig. 7 Spectral overlap of pure and 1.5 at.% Bi3+ doped LaGaO3 samples Chimica Techno Acta 2022, vol. 9(1), No. 20229107 ARTICLE 6 of 7 Fig. 9 (a–c) represents the CIE coordinates of LaInO3, LaGaO3 and LaAlO3 as-prepared samples doped with Bi3+. The CIE diagram of LaInO3: 3 at.% Bi3+ lies in bright BLUE [Points (0.22, 0.18)], LaGaO3: 1 at.% Bi3+ lies in BLUE [Points (0.18, 0.15)] and LaAlO3: 1 at.% Bi3+ lies in BLUE [Points (0.26, 0.22)]. From the color coordinators, it is evi- dent that LaInO3, LaAlO3 lies in nearly blue regions, where- as LaGaO3 samples lie in the BLUE region. So, from color purity analysis, the coordinates of LaGaO3: 1 at.% Bi3+ lies very near to the industrial standard blue emission coordi- nates, i.e. (0.15, 0.15). 4. Conclusions All the samples were prepared by a cost-effective and low- temperature polyol route method and were heated to 1000 °C. From the XRD patterns and SEM, it was observed that all samples range in nanoscale in agreement with the reported data. From PL studies, it was evident that all the emission peaks of as-prepared samples are attributed to 3P1–1S0 transition of Bi3+ ions emitting strong bright blue luminescence. When excited by UV light, LaGaO3 (pure and Bi3+) samples shows emissions. The emissions from the pure sample of LaGaO3 indicates that the host lattice itself is optically self-activated, whereas when the host lattice is doped with Bi3+ the intensity of emissions are enhanced. This is beneficial for the development of optically self- activated lighting devices. From the emission spectra and the data plotted for CIE coordinates, out of all three sam- ples, LaGaO3: 1 at.% Bi3+ CIE coordinates nearly meets the standard blue emission coordinates i.e. (0.15, 0.15). Thus, in comparison with the other samples, we finally conclude that LaGaO3: 1 at.% Bi3+ would be the best blue light- emitting phosphor that may be potentially used in solid- state lighting applications. Acknowledgements The authors are very grateful to Dr. R. David Kumar, Prin- cipal, Government College (A), Rajamahendravaram, for providing the Lab facility. One of the authors, B.V. Naveen Kumar, is grateful to the principal and management of the Shri Vishnu College of Engineering for Women (A), Bhi- mavaram, India. Conflict of interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Fig. 8 Life time decay curves of LaInO3:Bi 3+ (a), LaGaO3:Bi 3+ (b), and LaAlO3:Bi 3+ (c) doped samples Fig. 9 CIE chromatic coordinates of LaInO3 (a), LaGaO3 (b) and LaAlO3 (c) samples doped with Bi 3+ Chimica Techno Acta 2022, vol. 9(1), No. 20229107 ARTICLE 7 of 7 References 1. 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