Intense photoluminescence in CaTiO3:Sm3+ phosphors, effect of co-doping singly, doubly and triply ionized elements and their applications in LEDs

In this work, Sm3+-doped and Sm3+/Li+/K+/Mg2+/Ba2+/Gd3+/Bi3+ co-doped CaTiO3 phosphors were synthesized by a solid-state reaction method at 1473 K. The phase of phosphors was identified to be orthorhombic with space group Pnma (62) by XRD measurements. The morphological properties of the prepared samples were studied by SEM measurements. The average crystallite and particle sizes were found to increase in the presence of modifiers and they follow the trend Li+ > Mg2+ > Gd3+ > K+ > Bi3+ > Ba2+. EDX measurements were used to verify the presence of Ca, Ti, O, Sm, K, Mg, Ba, Gd and Bi atoms in the prepared phosphor samples. The Sm3+ ion shows emission peaks at 564, 599 and 646 nm due to 4G5/2 → 6H5/2, 6H7/2 and 6H9/2 transitions upon 407 nm excitation, among which the peak situated at 599 nm has maximum emission intensity. Concentration quenching was observed above 2 mol% of Sm3+ ions in this host. However, the emission intensity of Sm3+ peaks can be enhanced using different modifier (Li+/K+/Mg2+/Ba2+/Gd3+/Bi3+) ions. It was found that the size (ionic radii) and charge compensation of the ion together play a dominant role. The enhancement is more after co-doping with smaller radius ions (Li+, Mg2+ and Gd3+), among which Li+ shows the largest enhancement. This is because ions of smaller size will be able to go closer to the activator ion and the charge imbalance causes a larger field. The CIE color coordinates, correlated color temperature (CCT) and color purity of the phosphors were calculated and show orange-red color emissions with a maximum color purity of ∼93% in the case of CaTiO3:2Sm3+/1.0Li+ phosphor. The lifetime value is increased in the presence of these ions. It is again maximum for the Li+ co-doped CaTiO3:2Sm3+ phosphor sample. Thus, the prepared phosphor samples are suitable sources for orange-red light.


Introduction
Rare earth-doped perovskite phosphor materials are chemically and thermally stable and give intense photoluminescence at suitable excitation wavelengths. [1][2][3] These materials are used for various applications such as color tunable devices, display devices, light-emitting devices, plasma display panels, temperature sensors, optical heaters, bio-imaging devices, plant growth and solar cells. [4][5][6][7][8][9][10] The rare earth ions possess ladderlike energy levels due to which they show multi-modal behaviours such as upconversion (UC), downshiing (DS) and quantum cutting (QC) depending on different excitation detection techniques. [11][12][13][14] Downshiing is a Stokes emission process, in which a high energy photon is converted into a low energy photon via different relaxation processes. Among the rare earth ions, the Sm 3+ ion emits orange-red emissions due to 4 G 5/2 / 6 H j (j = 5/2, 7/2 and 9/2) transitions under n-UV excitation. [15][16][17] Ha et al. studied the structure and photoluminescence properties of the Sm 3+ -doped CaTiO 3 phosphor and observed intense orange-red emissions due to the 4 G 5/2 / 6 H j transition under 408 nm excitation. 18 Shivaram et al. synthesized Sm 3+ -doped CaTiO 3 by a low-temperature solution combustion method and reported an intense emission peak at 601 nm due to the 4 G 5/2 / 6 H 7/2 transition under 407 nm excitation. 19 Generally, the orange/redemitting phosphors show poor luminescence efficiency as compared to green, yellow and blue-emitting phosphors and need to be improved using different sensitizer/modier ions. [20][21][22] The emission intensity of activator ions may be enhanced in two ways: the rst one is by co-doping with surface modier ions such as Li + , Na + , Sr 2+ , Ca 2+ , Ba 2+ , Mg 2+ , Bi 3+ , Gd 3+ and the second one is via energy transfer from the sensitizer to activator ions. [23][24][25][26][27][28] Cao et al. observed an enhancement in the emission intensity of Sm 3+ -doped CaTiO 3 phosphors via the addition of Na + and H 3 BO 3 . 29 Shanbhag et al. have reported the photoluminescence properties of CaTiO 3 :Sm 3+ /Li + . 30 Pamuluri et al. have tried to enhance the photoluminescence properties of Sm 3+ by energy transfer from Dy 3+ to Sm 3+ in Dy 3+ /Sm 3+ codoped Lu 3 Ga 5 O 12 nano-garnets. 31 Zhu et al. also observed an enhancement in the emission intensity of Sm 3+ ions via energy transfer from Tb 3+ to Sm 3+ in Na 3 Bi(PO) 4 . 32 Dhananjaya et al. have reported the PL properties of Eu 3+ -doped Gd 2 O 3 phosphors in the presence of alkali ions (M + = Li + , Na + and K + ). 33 They have observed that the PL intensity of phosphors is increased in the presence of alkali ions, which is due to the modication in the local crystal eld around the activator ion. In our previous work, we have observed an enhancement in the emission intensity of Eu 3+ -doped CaTiO 3 phosphors in the presence of alkali ions (Li + , Na + and K + ). 34 The enhancement in the emission intensity in the presence of alkali ions is due to the modication in crystal eld around the activator ion, which increases the average crystallite and particle size. Singh et al. have studied the photoluminescence properties of Eu 3+ -doped MSiO 3 in the presence of alkaline earth ions (M = Mg 2+ , Ca 2+ , Sr 2+ and Ba 2+ ) and found an intense emission of Eu 3+ bands due to the modication in the crystal eld around the activator ion. 35 Wang et al. reported the enhancement in the luminescence properties of SrIn 2 O 4 :Eu 3+ phosphors in the presence of Gd 3+ ions. 36 It is clear from these examples that ions of smaller as well as larger size with single, double and triple ionization state separately have been used to enhance the emission intensity of different rare earth ions in different hosts. All these ions modify the crystal eld around the activator ions due to which enhancement in emission intensity is observed. However, from all these studies, it is not clear whether the ionization state or size of the ion or both plays a dominant role in enhancing the photoluminescence emission intensity of the activator ions. Therefore, it will be interesting to study these in detail. The idea is to see whether the ionization state or size of ions and charge compensation behaviors are more effective in enhancing the emission intensity of activators.
In this work, we studied the structural and optical behaviors of Sm 3+ -doped CaTiO 3 phosphors in more detail in the presence of different types of surface modier ions like singly, doubly and triply ionized elements with smaller and larger ionic radii in a single platform and tried to realize which one is more effective. The Sm 3+ -doped CaTiO 3 and Li + /K + /Mg 2+ /Ba 2+ /Gd 3+ / Bi 3+ co-doped CaTiO 3 :2Sm 3+ phosphor materials were prepared by a solid-state reaction method at 1473 K. The XRD measurements showed that phosphor materials have an orthorhombic phase with the Pnma (62) space group. The SEM measurements were carried out to know the effect of different modier ions on the particle size of the prepared samples. The photoluminescence excitation and emission spectra of the Sm 3+doped CaTiO 3 phosphors were studied by taking l em = 599 nm and l ex = 407 nm, respectively. The concentration of Sm 3+ was optimized for optimum emission. To improve the emission intensity further, Li + /K + /Mg 2+ /Ba 2+ /Gd 3+ /Bi 3+ ions (smaller and larger ionic radii of singly, doubly and triply ionized elements) were co-doped in the CaTiO 3 :2Sm 3+ phosphor and their concentrations were varied to obtain the optimum emission. It was found that the enhancement is more in the case of smaller radius ions irrespective of their ionization state and it is optimum for Li + ions. We also calculated the CIE, CCT and color purity of the Sm 3+ ions in the presence of Li + /K + /Mg 2+ /Ba 2+ / Gd 3+ /Bi 3+ ions. The value of color purity is larger in the presence of smaller radius (Li + /Mg 2+ /Gd 3+ ) ions and they follow the trend Li + > Mg 2+ > Gd 3+ . The lifetime measurements were carried out for the 4 G 5/2 level of Sm 3+ ions in the absence and presence of these modier ions. It was found that the lifetime of the 4 G 5/2 level of Sm 3+ ions increased in the presence of modier ions and their order is s Li > s k ∼ s Gd > s Mg > s Bi > s Ba . Initially, a series of cSm 3+ (c = 1.0, 1.5, 2.0, 2.5, 3.0 and 5.0 mol%) doped CaTiO 3 phosphor samples were synthesized to nd the Sm 3+ concentration for maximum photoluminescence. The Sm 3+ ion was found to give optimum emission at 2 mol%. Further, in order to enhance the PL intensity of Sm 3+ , xLi + (where x = 0.5, 1.0 and 3.0 mol%), xK + (where x = 1.0, 3.0, 5.0 and 7.0 mol%), yMg 2+ (where y = 1.0, 3.0, 5.0 and 10 mol%), yBa 2+ (where y = 3.0, 5.0 and 10 mol%), zGd 3+ (where z = 0.1, 0.5 and 0.7 mol%) and zBi 3+ (where z = 3.0, 5.0 and 10 mol%) were co-doped in CaTiO 3 :2 mol% Sm 3+ phosphors separately to get the optimum emission of Sm 3+ ions in the presence of these ions.

Preparation
The weighed materials were carefully mixed for one hour in an agate mortar with acetone as a mixing medium. The nal mixtures were heated at 1473 K for 4 hours in a programmable electric furnace. The phosphor samples thus obtained were further crushed in an agate mortar to obtain a ne powder for further characterizations.

Instrumentation
The phase identication of phosphor materials was carried out by XRD measurements using CuK a radiation (l = 0.15406 nm) with MiniFlex600 (Rigaku, Japan). The morphology of the materials was studied by SEM using a Zeiss, Evo 18 Research system. The Fourier transform infrared measurements were done to know the phonon frequency of phosphor samples using a PerkinElmer I-Frontier system in the 400-3000 cm −1 region. The photoluminescence excitation and emission spectra of the samples were recorded using a Fluorolog-3 spectrophotometer with a 450 W xenon lamp source (Horiba Jobin Yvon). We also measured the lifetime of the 4 G 5/2 level of Sm 3+ ions in the absence and presence of Li + /K + /Mg 2+ /Ba 2+ /Gd 3+ /Bi 3+ ions using a 25 W pulsed xenon lamp attached with the same unit (only corresponding to optimized samples).
The ionic radii of Li + , Mg 2+ and Gd 3+ are 0.076, 0.072 and 0.093 nm, while the ionic radii of Ca 2+ is 0.100 nm. Therefore, on substitution of Li + , Mg 2+ and Gd 3+ ions at the Ca 2+ site, the crystal lattice shrinks due to which the XRD peaks are shied towards a higher 2q angle side. Wu et al. have also observed the shi in peaks to a higher 2q angle side on co-doping of Li + (smaller ionic radii) at the Ca 2+ site in CaTiO 3 :Eu 3+ phosphors. 37 However, the ionic radii of K + , Ba 2+ and Bi 3+ are 0.138, 0.135 and 0.103 nm, which are larger than that of Ca 2+ ionic radii. Therefore, on substitution of K + , Ba 2+ and Bi 3+ at the place of Ca 2+ the crystal lattice expands due to which the XRD peaks are shied towards a lower 2q angle side.
3.1.2. SEM and EDX measurements. The surface morphology of the prepared phosphor samples was studied by SEM measurements. Fig. 2(a)-(g) show the SEM images of CaTiO 3 :2Sm 3+ , CaTiO 3 :2Sm 3+ /1.0Li + , CaTiO 3 :2Sm 3+ /5.0K + , CaTiO 3 :2Sm 3+ /5.0Mg 2+ , CaTiO 3 :2Sm 3+ /5.0Ba 2+ , CaTiO 3 :2Sm 3+ / 0.5Gd 3+ and CaTiO 3 :2Sm 3+ /5.0Bi 3+ phosphor samples, respectively. However, the enlarged picture of a small part of these images in all the cases is shown in the right top corner. In almost all cases, particles are slightly agglomerated to each other and nearly spherical or slightly elongated in shape. It is clear from the enlarged part of the images that the particle sizes are larger in the cases of Li + , Mg 2+ and Gd 3+ compared to K + , Ba 2+ and Bi 3+ ions. The average particle size of the prepared phosphors was calculated by the SEM images from the histogram using the ImageJ soware and these values were found to be 1.  Thus, the average particles size was found to increase in the presence of all the ions and it is maximum for CaTiO 3 :2Sm 3+ /1.0Li + phosphors. The Li + /K + /Mg 2+ /Ba 2+ /Gd 3+ / Bi 3+ ion actually acts as a surface modier, which modies the local crystal eld around the Sm 3+ ions in the CaTiO 3 matrix, and therefore, improves the emission intensity in the presence of these modier ions. It is found maximum in the case of Li + ions.

Optical characterization
3.2.1. FTIR studies. The Fourier transform infrared measurements were carried out to know the phonon frequencies (vibrational bands frequencies) of the phosphor samples. The FTIR spectra of CaTiO 3 :2Sm 3+ and CaTiO 3 :2Sm 3+ /1.0Li + phosphors in the spectral region 400-3000 cm −1 are shown in Fig. 5. The vibrational bands are observed at 430 and 545 cm −1 due to Ca-O and Ti-O groups. 34,39 The spectra corresponding to modier ions (Li + ) are exactly identical except that there is a change in the intensity of the bands. The following conclusions could be drawn on the basis of these measurements. First, the phonon frequency of the phosphor materials is low, and hence, the non radiative relaxation process in this case will be poor. This means that the radiative emission in this case is expected to be high. Second, the vibrational frequency of the bands remains unchanged upon co-doping of the modier. Only their intensity is affected due to scattering of photons with the modier ion.
The Sm 3+ ions present in the ground state ( 6 H 5/2 ), are promoted to the 4 F 7/2 excited state by absorption of 407 nm photons. The Sm 3+ ions from 4 F 7/2 excited state relax nonradiatively to the 4 G 5/2 excited state, which give multitransition emission to different sublevels of the ground state, which lie in orange to red regions. The energy level diagram of Sm 3+ ions is shown in Fig. 7(a). The photoluminescence emission intensity was found to increase with the concentrations of Sm 3+ ions, and it was found optimum at 2 mol% in this host. The emission intensity was found to decrease for higher concentrations due to concentration quenching. The variation in emission intensity with different concentrations of Sm 3+ ions is shown in Fig. 7(b). Two mechanisms are generally found to involve in concentration quenching. One is the exchange interaction and the other is the multipolar interaction. The two mechanisms depend on the critical distance between the activator ions. If the value of critical distance between the two ions is #5 Å, the concentration quenching would be due to exchange interaction. However, if it is $5 Å, it would be due to multipolar interaction.
value of 17.44 Å, a value much larger than 5 Å. Hence, the concentration quenching in CaTiO 3 :cSm 3+ phosphors was found to be multipolar interaction.
The multipolar interactions can be recognized using the following formula: 34 where I is the PL emission intensity, x is the Sm 3+ ion concentration, k and b are the constants for a given host. The value of q decides the actual nature of interaction between Sm 3+ ions.
Depending on the value of q equal to 6, 8 or 10, the nature of interaction would be dipole-dipole (d-d), dipole-quadrupole (d-q) or quadrupole-quadrupole (q-q), respectively. A plot between Log(I/x) versus Log(x) for CaTiO 3 :cSm 3+ (where c = 1.0, 1.5, 2.0, 2.5, 3.0 and 5.0 mol%) phosphors with l ex = 407 nm and l em = 599 nm is shown in Fig. 7(c). The slope value of the curve was found by tting the plot Log(I/x) versus Log(x). The observed slope value (equal to −q/3) was found to be 1.04582, from which the value of q ∼ 3.13, close to 3. If the value of q is less than 6, the energy transfer is due to the interaction between the adjacent ions.   5.0 and 10 mol%), yBa 2+ (where y = 3.0, 5.0 and 10 mol%), zGd 3+ (where z = 0.1, 0.5 and 0.7 mol%) and zBi 3+ (where z = 3.0, 5.0 and 10 mol%) phosphors were monitored under excitation at 407 nm, and they are given in Fig. 8(a)-(f). The emission intensity is maximum for 1.0 mol% Li + , 5.0 mol% K + , 5.0 mol% Mg 2+ , 5.0 mol% Ba 2+ , 0.5 mol% Gd 3+ and 5.0 mol% Bi 3+ codoped CaTiO 3 :2Sm 3+ phosphors. As mentioned earlier, we selected two types of ions: one with a smaller size (Li + /Mg 2+ / Gd 3+ with singly, doubly and triply ionized states) and the other with a larger size (K + /Ba 2+ /Bi 3+ with singly, doubly and triply ionized states), and monitored the photoluminescence emission spectra under the same conditions. It was found that the emission wavelengths of the Sm 3+ ion are the same. However, the photoluminescence emission intensity is increased in the presence of all these modier ions. Moreover, it is larger in the case of ions with smaller ionic radii. The enhancement in the intensity of Sm 3+ ions follow the trend I Li + > I Mg 2+ > I Gd 3+ > I K + > I Bi 3+ > I Ba 2+. Several researchers have used these ions to improve the emission intensity of rare earth ions in different host matrices. [23][24][25][26][27] For example, Wu et al. have studied the photoluminescence properties of Eu 3+ /Li + co-doped CaTiO 3 phosphors. 37 They found that the emission intensity of Eu 3+ ions is increased in the presence of Li + ions. They have explained it to be the result of the increase in average particle size and charge compensation. Our group have also observed an enhancement in the emission intensity of Tm 3+ /Yb 3+ co-doped ZnWO 4 phosphors in the presence of Mg 2+ ions due to the modication in the local crystal eld. 40 Linga et al. have reported an enhancement in the photoluminescence intensity of (Ca 1−x−y ,Ln y ) MoO 4 :xEu 3+ (Ln = Y and Gd) phosphors in the presence of Y 3+ and Gd 3+ ions. 41 The emission intensity of CaTiO 3 :2Sm 3+ /1.0Li + , CaTiO 3 :2-Sm 3+ /5.0K + , CaTiO 3 :2Sm 3+ /5.0Mg 2+ , CaTiO 3 :2Sm 3+ /5.0Ba 2+ , CaTiO 3 :2Sm 3+ /0.5Gd 3+ and CaTiO 3 :2Sm 3+ /5.0Bi 3+ phosphors was enhanced by 6.3, 4.0, 5.1, 1.4, 5.0 and 2.5 times (for 599 nm peak) compared to the CaTiO 3 :2Sm 3+ phosphor.
These observations indicate that the ions with smaller size (ionic radii) are more effective in enhancing the emission intensity of Sm 3+ ions. This is due to the reason that ions with a smaller ionic size will be able to reach closer to activator ions. The charge compensation will create a larger eld to enhance the emission intensity. In the case of larger radius ions, whatever may be the ionization state (K + , Ba 2+ , and Bi 3+ ), they will be struck out by the host atoms/ions before they reach closer to the activator ion (i.e. Sm 3+ ) due to their larger size, and hence, the eld created will be smaller. Thus, the ionic radii of Li + , Mg 2+ and Gd 3+ ions are 0.076, 0.072 and 0.093 nm, which are smaller than the Ca 2+ ionic radii (0.100 nm) at which these ions are substituted, while the ionic radii of K + , Ba 2+ and Bi 3+ are 0.138, 0.135 and 0.103 nm, which are larger than the Ca 2+ ionic radii. In all these, Li + , Mg 2+ and Gd 3+ ions produce larger enhancement and in that Li + is the largest one. Another reason for the enhancement in emission intensity is if we compare the crystallite and particle size, they also follow the same trend, i.e. (Li + ) > (Mg 2+ ) > (Gd 3+ ) > (K + ) > (Bi 3+ ) > (Ba 2+ ). As larger the particle size, it contains a large number of activators which give large emission intensity. The increase in average crystallite and particle sizes in the presence of Li + /K + /Mg 2+ /Ba 2+ /Gd 3+ /Bi 3+ ions improves the population of the 4 G 5/2 level. Therefore, the emission intensity is enhanced in the presence of Li + /K + /Mg 2+ / Ba 2+ /Gd 3+ /Bi 3+ ions. 42 The emission intensity is maximum in the case of CaTiO 3 :2Sm 3+ /1.0Li + phosphor and it is due to its smaller ionic radii and used as a charge compensator as well as it has the largest average crystallite and particle size. The color purity of the phosphor materials is another important parameter to recognize phosphor as a good source of light for a particular color for solid-state lighting applications.
The color purity of the light source can be calculated using the following relation: 43 where ( Fig. 10(a)-(g). The decay curves of the 4 G 5/2 level of Sm 3+ ions in different cases were found to t well using a single exponential relation: 38 where I 0 and I(t) are the PL emission intensities at time zero and t seconds, respectively and 's' is the lifetime. The value of lifetime for the 4 G 5/2 level of Sm 3+ in the case of CaTiO 3 :2Sm 3+ , CaTiO 3 :2Sm 3+ /1.0Li + , CaTiO 3 :2Sm 3+ /5.0K + , CaTiO 3 :2Sm 3+ /5.0Mg 2+ , CaTiO 3 :2Sm 3+ /5.0Ba 2+ , CaTiO 3 :2Sm 3+ / 0.5Gd 3+ and CaTiO 3 :2Sm 3+ /5.0Bi 3+ phosphors were found to be 0.95, 1.02, 1.00, 0.99, 0.96, 1.00 and 0.97 ms, respectively. From this, it is clear that the value of lifetime is increased in the presence of these modiers (Li + /K + /Mg 2+ /Ba 2+ /Gd 3+ /Bi 3+ ions) and it is optimum in the case of Li + ions. The lifetime of the 4 G 5/ improve the population in 4 G 5/2 levels, and therefore, the value of lifetime of 4 G 5/2 is increased. 42 This might be one of the reasons for enhancement in the PL emission intensity.

Conclusions
Sm 3+ -doped and Sm 3+ /Li + /K + /Mg 2+ /Ba 2+ /Gd 3+ /Bi 3+ co-doped CaTiO 3 phosphors have been synthesized by a solid-state reaction method at 1473 K. The structural and morphological properties of the prepared samples were studied by XRD and SEM measurements. The average crystallite and particle sizes were found to increase in the presence of modiers and they follow the trend Li + > Mg 2+ > Gd 3+ > K + > Bi 3+ > Ba 2+ . EDX measurements were carried out to verify the elements present in the respective phosphor samples. The infrared measurements of the phosphors showed the presence of Ca-O and Ti-O vibrational bands at 430 and 545 cm −1 , respectively, indicating that the phosphor has a low phonon frequency. The emission spectra of Sm 3+ ions showed an intense emission peak at 599 nm due to 4 G 5/2 / 6 H 7/2 transition upon excitation at 407 nm wavelength and the emission intensity is maximum for 2 mol% of Sm 3+ ion. The emission intensity is quenched for a higher concentration of Sm 3+ ions. Therefore, the surface modiers were used to further enhance the emission intensity. It was found that co-doping 1.0 mol% Li + , 5.0 mol% K + , 5.0 mol% Mg 2+ , 5.0 mol% Ba 2+ , 0.5 mol% Gd 3+ and 5.0 mol% Bi 3+ ions enhances the emission intensity by 6.3, 4.0, 5.1, 1.4, 5.0 and 2.5 times respectively as compared to the 2Sm 3+ -doped CaTiO 3 phosphor. The increase in emission intensity is due to the modication of the crystal eld around the Sm 3+ ions in the CaTiO 3 host as well as the increase in average crystallite and particle sizes in the presence of these ions. It was found that the size (ionic radii) and charge compensation of the ion together play a dominant role. The enhancement is more aer co-doping with smaller radius ions (Li + , Mg 2+ and Gd 3+ ), among which Li + shows the largest enhancement. This is because ions of smaller size will be able to go closer to the activator ion and the charge imbalance causes a larger eld. The emission intensity is maximum in the case of the CaTiO 3 :2Sm 3+ /1.0Li + phosphor due to its smaller ionic radii and used as a charge compensator as well as it has the largest average crystallite and particle size. The CIE color coordinates and correlated color temperature (CCT) showed the orange-red color with a color purity as high as 93% in the case of the CaTiO 3 :2Sm 3+ /1.0Li + phosphor. The lifetime of the 4 G 5/2 level of Sm 3+ ions was also found enhanced in the presence of Li + /K + /Mg 2+ /Ba 2+ /Gd 3+ /Bi 3+ ions and it follows the trend s Li > s k ∼ s Gd > s Mg > s Bi > s Ba . From these studies, it is suggested that Sm 3+ -doped and Sm 3+ /Li + /K + /Mg 2+ /Ba 2+ /Gd 3+ / Bi 3+ co-doped CaTiO 3 samples may be suitable for display devices and for LEDs under n-UV excitation.