Enhanced photoluminescence properties of BaAl2O4: Ce3+/Li+ yellow phosphors

Ce3+/Li+-activated barium aluminate phosphor (BAO) was synthesized by conventional solid-state reaction method. The crystal structure of the synthesized phosphor was analyzed by X-ray diffraction (XRD) and Raman spectroscopy analysis. Fourier transformed infrared spectrum results revealed the characteristic vibration bands present in the synthesized phosphor. Surface composition analysis of the prepared samples was examined using X-ray photoelectron spectroscopy (XPS). Photoluminescence emission band observed at 589 nm was assigned to 5D excited level corresponding to 2F5/2 transition in yellow region under the excitation wavelength of 320 nm. Yellow light emission was confirmed by the Commission Internationale de L’Eclairage (CIE) chromatic coordinate graph. The synthesized phosphors BAO: 0.5Ce3+, BAO: 0.5Ce3+, 0.1Li+ displayed a color purity and lifetime decay of 78.4%, 81.3% and 4.333 ns, 4.738 ns, respectively.


Introduction
Alkaline earth aluminates doped with rare earth ions have attracted considerable attention as a research topic owing to the properties like long-afterglow phosphorescence, fast scintillation due to their thermal & chemical stability, high quantum e ciency [1] and their potential applications in the areas of safety improvement, radiation detection and light emitting diodes (LEDs) [2]. Right now, due to low cost and simple fabrication, the combination of yellow phosphors (YAG: Ce) and a blue LED has been the popular route for white light generation [3]. Vasilica Tucureanu et al., reported enhancement of optical properties of Ce 3+ doped YAG prepared by co-precipitate method [4]. D. Jia et al., reported long persistent BaAl 2 O 4 : Ce 3+ phosphor with persistence luminescence longer than 10 h [5]. M. Ayvacikli reported copper-doped barium aluminate synthesized by the conventional ceramic method, emitting green light [6]. A rare-earth trivalent metal ion cerium (Ce 3+ ) doped aluminate based phosphors synthesized by Foka et.al., exhibited photoluminescence with emission peaks in the long UV region [7]. Cerium  ; sigma) were mixed homogeneously using agate mortar for 30 min. Small amount of ethanol was added in order to ensure homogeneity mixing of the starting composition. Then the mixed powder was transferred into alumina crucibles and placed inside a box type furnace and the temperature was increased at a heating rate of 5°C/min. During the process, the synthesized temperature was maintained at 1100 º C for 12 hrs. The obtained powders were reground and heated at 1100ºC for 12hr in a reducing atmosphere where carbon powder was used as a reduction agent. Later the obtained phosphors were subjected to different characterizations.
Crystalline phase of the synthesized material was identi ed by X-ray diffraction (XRD; PANalytical) analysis with Cu-Kα 1 radiation of wavelength (λ = 1.5406 Å), working voltage 40 kV and current 40 mA with step size 0.026. The X-ray data were analyzed by Rietveld Re nement using full proof package.
Raman spectroscopy was performed on the synthesized BaAl 2 O 4 compound using a laser with wavelength of 532 nm in the range of 100 cm − 1 to 900 cm − 1 . The obtained Raman data was smoothed using a Savitzky-Golay lter and the base line was corrected using an adaptive iteratively reweighted penalized least square (airPLS) algorithm [12]. Fourier transformed infrared (FTIR) spectroscopy (Bruker, Alpha T) was used to perform vibrational spectroscopic analysis of the synthesized phosphors. The composition pro le of the sample was recorded using X-ray Photoelectron Spectroscopy (XPS) (ULVAC-PHI, PHI 5000 version probe III). The photoluminescence (PL) spectra of the synthesized phosphor were recorded by spectro uorometer (Shimadzu, RF-5301PC, Japan) using a xenon lamp as the excitation source. Fluorescence lifetime decay measurements was observed DeltaPro Tm uorescence lifetime system (HORBIA Scienti c) using diode laser as a excitation source.
The F 2g (1) Raman mode observed at ~ 244 cm − 1 along with a shoulder observed at 199 cm − 1 are assigned to the translation of the tetrahedron in the crystal structure, which is typically occupied by Ba 2+ ions [16]. The E g mode observed at ~ 415 cm − 1 has been attributed to the movement of oxygen atoms along the T-O bonds, (i.e.) bonds between the tetrahedral sites, occupied by Ba 2+ ions, and oxygen [15]. A shoulder can be observed on the E g mode at ~ 395 cm − 1 . This can be attributed to the cation disordering caused by the bending mode of Al ions on the tetrahedral sites [15]. There isn't much work discussing the origins of the F 2g (2) peak observed at ~ 591 cm − 1 . It has been assigned to the symmetric bending of oxygen with respect to the cations at the tetrahedral sites [16]. Shoulders to the left and right of the F 2g (2) mode can be observed in Fig. 3. Further examination would yield information about the origin of the disorders causing the breakdown of Raman selection rules. The nature of the octahedral cation dictates the position of the F 2g (3) mode, observed at 691 cm − 1 [15]. However, few authors have assigned this mode to the asymmetric bending or stretching of oxygen with respect to the tetrahedral cation [16]. Group theory analysis predicts the occurrence of an acoustic mode A 1g and its wavenumber is likely to be > F 2g (3) [15]. Chromates and ferrites are known to exhibit intense A1g modes while in contrast, aluminates (BaAl 2 O 4 ) are known to exhibit weak A 1g peaks. A week peak can be observed at ~ 760 cm − 1 from Fig. 3 corresponding to A 1g mode. . Ce 3d composed of two multiplets to spin-orbit split 3d 5/2 and 3d 5/2 , respectively. The binding energy peaks at 906 eV, 901.54 eV (Ce 4+ 3d 5/2 ) and peaks at 896.49 eV, 890.14 eV (Ce 4+ 3d 3/2 ) is assigned to Ce 3d. Four photoelectron signal peaks at 898.4 eV, 904.2 eV and 884.4 eV, 887.5 eV is assigned to Ce 3+ 3d 5/2 and Ce 3+ 3d 3/2 [13,19,20]. nm. Excitation spectrum at 320 nm ( 2 F 5/2 -excited 5D states) corresponds to the transition to 4f-5d from the ground state of Ce 3+ ion. Generally, Ce 3+ has two ground states of 2 F 5/2 and 2 F 7/2 due to the spinorbit interaction [21].  Fig. 7. The phosphor exhibits broad peak centered at 589 nm which signi es a transition from excited level 5D to 2 F 5/2 [22]. The transitions of the yellow light emission of the Ce 3+ rare-earth element belongs to the electric dipole transition which is tuned by Ce 3+ ion site [23].
The critical energy distance (R c ) can be calculated using Blasse Eq. (1) [24]. The interaction between Ce 3+ ions is stimulated by increasing doping concentration but non radiative energy transfer between the doping ions can lead to either electric multipolar or exchange interactions. Generally, exchange interactions take place a smaller value of R c (< 5 Å), otherwise electric multipolar interactions dominate. For BAO: Ce 3+ phosphors the value of R c was found to be 15.83 Å, herein the electric multipolar interactions take place [13,25]. To enhance the luminescence intensity of barium aluminates, different concentration of Li + ions are doped into highest luminescence of BAO: 0.5Ce 3+ host matrix. Researchers reported the successful enhancement of luminous intensity by co doping Li + with CaO: Ce 3+ , CaTiO: Dy, and Ca 2 SrAl 2 O 6 : Ce 3+ , Mn 3+ , SrAl2O4:Eu 2+ , Ce 3+ matrix [26,27,28,29]. It is noted that the luminescence intensity can be further increased at suitable Li + ion concentration. Initially the luminescence intensity increases and then decreases with the increase of Li + concentration. Li + ion doped into BAO: xCe 3+ host lattice, the Li + ion radii (76 pm) in turn lead to occupy the defect site due the smaller ionic radius, which changes the symmetry of local environment of Ce 3+ in crystal lattice and prevents the relaxation between Ce 3+ ion. As a result, the luminescence intensity is enhanced by co-activator Li + ion [10,30].
Exact emission color of phosphors for both BAO: x Ce 3+ (x = 0.1, 0.5 and 1 M.W %) and BAO: 0.5 Ce 3+ , y Li + (y = 0.1, 0.5 and 1 M.W %) was identi ed using the Commission International del'Eclairage (CIE) chromaticity coordinates (X, Y) graph as shown in Figure. 8, 9, respectively. It can be seen that the emission is located in yellow region [31]. From the above results, BAO: 0.5Ce 3+ , 0.1Li + phosphor can be considered as a proper source for emitting yellow light under 320 nm excitation wavelength. Calculated CIE chromaticity coordinates of the phosphors is shown in inset of Figs. 8 & 9. The color purity of the synthesized phosphors is found by given expression. (2). [32] --------------- (2) Here, x, y is the chromaticity coordinate 0.5% Ce 3+  Here, x e = 0.332, y e = 0.186, is the epicenter  Figure   11 results show a decrease lifetime with increase in Li + ion concentration. The curves are tted with biexpotential function indicated with red line in the spectra.
The average lifetime is calculated using the Eq. (6) given below: The From Luminescence decay spectra, the lifetimes of 0.5% Ce 3+ ions increase with increase of Li + ion, which is due to the energy transfer from the Ce 3+ to Li + ions. The energy transfers e ciency (η ET ) was found using Eq. (7) [35].

Conclusion
Light emission display in Yellow to orange region was con rmed by CCT value and CIE chromaticity graph of phosphors BAO: xCe 3+ (y = 0.1, 0.5 and 1 M.W %) and BAO: 0.5%Ce 3+ , y% Li + (y = 0.1, 0.5 and 1 M.W %) synthesized by solid state reaction method. The synthesized phosphor emitted yellow (589 nm) light when excited with a radiation of wavelength 320 nm due to excited 5D states to 2 F 5/2 transition. The observed lifetime decay of synthesized phosphors BAO: xCe 3+ and BAO: 0.5%Ce 3+ , y% Li + are 4.33, 4.78 ns respectively. In conclusion, the yellow light source BAO: 0.5%Ce 3+ , 0.1% Li + phosphor may be a crucial phosphor as a solid state lighting devices application.