Bright blue emissions on UV-excitation of LaBO 3 ( B = In , Ga , Al ) perovskite structured phosphors for commercial solid-state lighting applications

Bright blue photoluminescence (PL) was obtained from Bi-activated LaBO3 (B = In, Ga, Al) perovskite nanophosphors. A cost-effective and low-temperature chemical route was employed for preparing Bi doped LaBO3 (B=In, Ga, Al) which were then annealed at 1000 °C. The phase formation, morphological studies and luminescent properties 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 Bi doped) were investigated for promising applications in UV light-emitting diodes, variable frequency drive (VFD), field emission display (FED), and other photoelectric fields. Keywords


Introduction
Blue emissive materials have attracted considerable attention because of their vast applications in the fields of sensors, solid-state lighting technology, and light fidelity (Li-Fi) [1][2][3]. In view of such applications, design and development 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 materials requires an energy-expensive process and thus hinders the development of cost-effective blue light emitters [4]. Perhaps, in blue light-emitting phosphors, rare-earth ions like Eu 2+ and Ce 3+ 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][6][7][8]. However, phosphors doped with Eu 2+ or Ce 3+ shows some disadvantage in the view of high cost, high reabsorption and color deviation. Therefore, developing potential luminescent phosphor materials doped with non-rare earth ions as activators is more promising. Among many recent studies, it was found that Bi 3+ ion as an activator plays a crucial role in the generation of efficient blue-light emission [9][10][11][12][13]. Alternatively, Bi 3+ ions with their excited state for electron transition and emission band of Bi 3+ ion at room temperature can be rationally attributed to the 3 P1→ 1 S0 transition, which avoids the reabsorption among phosphors. It is worth mentioning that oxide-based materials are preferred over nitride thin films due to low temperatures and simple methods of preparation. Among many oxides, perovskite materials have been widely investigated for luminescence applications. For example, semiconductor 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 ferromagnetic thin films and luminescent host materials, and has high thermal stability and good dielectric properties [15][16][17]. LaGaO3 perovskite has received much attention as a leading host because of its potential use as substrate applications for phosphorus and solid oxide fuel cells [18]. In the last few years, a considerable amount of research was done on LaInO3, LaAlO3, LaGaO3 perovskite materials. A few research groups have also reported that Bi 3+ doped LaInO3 and LaGaO3 are efficient blue-emitting luminescent materials. However, to the best of our knowledge, there is no report on the optimization of Bi 3+ ions in LaInO3, LaAlO3, and LaGaO3 phosphors. Therefore, in this article, we demonstrated a comparative study for bright blue emissions on UV excitation close to industrial standards (colour coordinates: x = 0.15 and y = 0.15) obtained from Bi 3+ 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.  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 solution 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 temperature 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.% Bi 3+ , LaGaO3: 1, 1.5, 2, 2.5, 3 at.% Bi 3+ , LaAlO3: 0.5, 1, 2, 2.5, 3 at.% Bi 3+ doped nanophosphors, which were also annealed in air at 1000 °C.

Results and discussion
3.1. XRD studies XRD patterns of undoped and selected Bi 3+ doped LaInO3, LaGaO3 and LaAlO3 samples calcined at 1000 °C are shown in Fig. 1 (a-c). The diffraction peaks indicate that the calcined 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 Bi 3+ was completely dissolved in the host array, which is in good agreement with previously reported literature [19][20][21][22][23]. Typically, the crystal size of the sample is calculated 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 parameters for both pure and doped LaInO3, LaGaO3 and LaAlO3 were calculated using PowderX software and are listed in Table l.  Fig. 2 (a-c) and Bi 3+ doped samples in Fig. 2 (d-f). All crystallites are spherical in nature ranging between 90-150 nm. The higher calcinations temperature facilitates the possible rapid arrangement of crystal structure followed by coalescence of particles leading to particle agglomeration causing a considerable reduction in crystallite size of Bi 3+ 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 software 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)). ions, which is demonstrated in Fig. 5. Also, it was reported elsewhere that the energy gap of phosphor material decreased in presence of Bi 3+ ion [24][25][26]. The Bi 3+ ion renders some energy levels that have the 6s 2 valence electrons 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].

Photoluminescence and lifetime decay studies
Emission spectra of LaInO3: 0.5, 1, 2, 2.5, 3 at.% Bi 3+ , LaGaO3: 1, 1.5, 2, 2.5, 3 at.% Bi 3+ , LaAlO3: 0.5, 1, 2, 2.5, 3 at.% Bi 3+ 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 3 P1-1 S0 transition of Bi 3+ ions [28]. It was evident that the intensity of the emission band increases as the Bi 3+ concentration increases, reaching a maximum at 2.5 at.% for LaInO3: Bi 3+ , 1 at.% for LaGaO3: Bi 3+ and 2 at.% for LaAlO3: Bi 3+ samples, and then remarkably decreasing on increasing Bi 3+ content due to the concentration quenching. The concentration quenching can be triggered because the interactions 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 1 S0-3 P1 transition of Bi 3+ , 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 emission spectra trend, the highest emission intensity is observed for LaGaO3: Bi 3+ doped samples with a peak position at 375 nm on excitation with 309 nm, which is illustrated in Fig. 6 (b) [30]. The reason behind the enhanced intensity from LaGaO3:Bi 3+ doped sample is due to the energy transfer phenomena from GaO6 octahedral sites to Bi 3+ ions. To confirm this energy transfer, spectral overlap between excitation spectra of LaGaO3:Bi 3+ 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 phosphors were calculated according to the following equation:

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.