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A promising novel orange–red emitting SrZnV2O7:Sm3+ nanophosphor for phosphor-converted white LEDs with near-ultraviolet excitation
Journal of Physics and Chemistry of Solids 89 (2016) 45–52
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A promising novel orange–red emitting SrZnV2O7:Sm3 þ nanophosphor for phosphor-converted white LEDs with near-ultraviolet excitation Mandeep Dalal a, V.B. Taxak a, Sangeeta Chahar a, Avni Khatkar b, S.P. Khatkar a,n a b
Department of Chemistry, Maharshi Dayanand University, Rohtak 124001, India University Institute of Engineering and Technology, Maharshi Dayanand University, Rohtak 124001, India
art ic l e i nf o
a b s t r a c t
Article history: Received 12 September 2015 Accepted 31 October 2015 Available online 2 November 2015
A novel trivalent samarium doped SrZnV2O7 nanophosphors was developed via urea assisted solution combustion method using metal nitrates as initial raw materials. The qualitative and quantitative phase analysis was carried out using Rietveld refinement technique. It was found to crystallize in monoclinic lattice with the P121/n1 (14) space group. The photoluminescent spectral study of SrZnV2O7:Sm3 þ revealed that the excitation of 405 nm yields the characteristic emission peaks at 569, 599, 640 and 702 nm due to 4G5/2-6H5/2, 4G5/2-6H7/2, 4G5/2-6H9/2 and 4G5/2-6H11/2 respectively. The optimum concentration of Sm3 þ ion in SrZnV2O7 for best luminescence was found to be 2 mol%. The luminescence intensity was further enhanced by incorporating compensator charge R þ (R ¼Li, Na, and K) into the SrZnV2O7:0.02Sm3 þ nanophosphor. The critical distance for non-radiative energy transfer was calculated to be 26.64 Å. Dipole–dipole (d–d) interactions were ascribed as the major factor responsible for concentration quenching arising from the over-doping of the activator ions. The results indicate that these nanophosphors are suitable candidate for PC-WLEDs using near UV excitation. & 2015 Elsevier Ltd. All rights reserved.
Keywords: A. Optical materials B. Chemical synthesis C. X-ray diffraction C. Electron microscopy D. Luminescence
1. Introduction Recently, opto-electronic study of rare earth doped inorganic host materials emerged out as one of the most proficient field in material science and nanotechnology [1–5]. Additionally, significant research efforts have been devoted to the exploration of lightemitting diodes due to their promising candidature in white light generation for solid state lightning (SSL) applications [6–9]. White LEDs exhibit excellent advantages such as more economical power consumption, higher durability, longer life, being mercury-free and less pollution as compared to the conventional halogen and incandescent lamp [10,11]. Nowadays, phosphor converted white light-emitting diodes (PC-WLEDs) are being fabricated either by blue GaN LED chip (λex ¼ 450–470 nm) covered by a yellowish phosphor coating or by near-ultraviolet (NUV) LED chips (λex ¼350– 420 nm) coated with red–blue–green (RGB) tricolor phosphors [12– 15]. However, white LEDs using GaN based chip system have poor color rendering index and low chromatic stability under different driving currents [16–18]. Therefore, near UV-LEDs coated with RGB phosphor offer effective and attractive solution for the problem of white light emission with lower color temperature [19]. n
Corresponding author. E-mail address: [email protected] (S.P. Khatkar).
http://dx.doi.org/10.1016/j.jpcs.2015.10.017 0022-3697/& 2015 Elsevier Ltd. All rights reserved.
To synthesize one of the indispensable component in RGB tricolor luminescent materials for NUV-WLEDs, red or orange–red emitting Eu3 þ and Sm3 þ doped phosphors have been extensively exploited due to the sharp spectral bands arising from intra-configrutional f–f transitions in activator ion [20–23]. In order to fabricate Ln3 þ activated nanophosphor, a numerous range of host materials have been explored, but SrZnV2O7 is still a novel host compound to the research field. To the best of the authors knowledge, the only attempt of rare earth doping (SrZnV2O7:Yb3 þ ) is reported by Guan et al. to convert the ultraviolet light into near infra-red region. This work is executed in order to study the photoluminescent down-conversion from ultraviolet to visible light via Sm3 þ doping in the SrZnV2O7 host lattice [24]. Vanadates form an important class of oxides in inorganic chemistry because of the diverse structural alignment of the vanadium–oxygen unit. Pyrovanadates contain discrete V2O74 complex ions units with both vanadium atoms sharing one corner [25–27]. The monoclinic host, SrZnV2O7 has lattice constant a (Å)¼7.4115, b (Å)¼ 6.6895, c (Å)¼ 11.9610, V¼589.73 Å3 and P121/n1 (14) space group with four formula units per unit cell [28,29]. Out of the several wet chemical methods such as hydrothermal synthesis [30], sol gel processing [31] and solution combustion synthesis [32], we have used the urea assisted solution combustion route to synthesis SrZn1 xSmxV2O7 nanophosphors. This efficient
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phosphor processing method has a number of advantages; such as inexpensive raw materials, low synthesis temperature, a relatively simple preparation process and homogenous product with fine particle size [33]. The Rietveld refinement of SrZn1 xSmxV2O7 reveals the monoclinic lattice with P121/n1 space group [34]. The critical distance for energy transfer and the variation of luminescence intensity after the optimal concentration (2 mol%) reveals the mechanism behind the concentration quenching phenomena. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) are used to study Surface morphological properties. The Scherrer's equation has been used to calculate the particle size which in turn further confirmed from SEM and TEM micrographs. Finally, the effect of charge compensation R þ (R¼ Li, Na, and K) on luminescence intensity has been also studied. The photoluminiscent excitation and emission spectra of synthesised samples suggest their potential application in phosphor converted white LEDs with NUV excitation.
2. Experimental 2.1. Material and synthesis A series of rare earth doped SrZn1-xV2O7:xSm3 þ (x¼0.005–0.03) and SrZn0.96V2O7:0.02Sm3 þ , 0.02R þ (R¼Li, Na, and K) nanophosphors were synthesized using solution combustion method in which constituent raw materials were high purity Sr(NO3)2, Zn(NO3)2 6H2O, Sm(NO3)3 6H2O, NH4VO3, KNO3, NaNO3, LiNO3 and urea. The starting materials were weighed in stoichiometric proportion and dissolved in minimum quantity of deionized water in a 400 ml capacity pyrex beaker. The amount of the urea was calculated using the total oxidizing and reducing valencies of the oxidizer and the fuel [35]. Finally the solution was placed into a preheated furnace maintained at 500 °C. The starting material undergoes rapid dehydration and foaming followed by decomposition, producing combustible gases. These volatile combustible gases ignite and burn with a flame yielding voluminous solid. Urea was oxidized by nitrate ions and served as a fuel for propellant reaction. The products were then cooled to room temperature and pulverized. A fraction of all the products were calcined at 600 °C and 700 °C for 3 h and kept in desiccator for further characterization.
preferred over the backscattered electron imaging (BSEI) due to its higher surface sensitivity and greater resolution. TEM images were obtained using the electron beam accelerated at 80 kV with a magnification of 105. SEM and TEM analysis was also used to confirm the particle size in nano range as given by the X-ray diffraction studies. The photoluminescence excitation and emission spectra (scanning rate: 1200 nm min 1, PMT voltage: 400 V, widths of the excitation slit and emission slit: 2.5 nm) and fluorescence decay curve and color coordinates of the SrZn1 xSmxV2O7 nanocrystals in the ultraviolet–visible region were recorded and investigated at room temperature using a Hitachi F-7000 fluorescence spectrophotometer with Xe-lamp as the excitation source.
3. Results and discussion 3.1. Crystal structure Rietveld refinement method was used to analyse the crystal structure and phase identification of the sample as-prepared, calcined at 600 °C and 700 °C. Fig. 1 depicts that the as-prepared sample is a mixture of two phases Sr(NO3)2 (JCPDS Card No.¼761375) and Zn2V2O7 (JCPDS Card No.¼29-1396) [40,41]. The black dot line is the experimental diffraction data observed for the sample as prepared at 500 °C whereas solid blue, green and red lines are the simulated pattern data from Zn2V2O7, Sr(NO3)2 and Zn2V2O7 þSr(NO3)2 respectively. The quantitative and qualitative phase study of the sample calcined at 600 °C shows that it was a mixture of Zn2V2O7 (JCPDS Card No.¼76-1375) and SrZnV2O7 (JCPDS Card No.¼51-0131) as shown in Fig. 2 [28,29,41]. The black dot line and solid red line are the experimental pattern observed for the sample calcined at 600 °C and simulated XRD profile for the mixture (Zn2V2O7 þ SrZnV2O7) respectively. Solid green and blue lines are the simulated diffraction data from phases SrZnV2O7 and Zn2V2O7 respectively. The Rietveld refinement of the samples with different Sm3 þ concentration calcined at 700 °C confirmed that SrZn1 xSmxV2O7 crystallizes in single phase with almost zero impurity. Fig. 3 shows the observed (blue dot line) and calculated (solid black line) X-ray diffraction (XRD) profiles along with their difference at bottom for the Rietveld refinement of for SrZn0.98Sm0.02V2O7 at room temperature with λ ¼ 0.1540562 nm.
2.2. Materials characterization The crystal phases of SrZn1 xSmxV2O7, as-prepared and calcined at different temperatures were characterized using a high resolution Rigaku Ultima-IV X-ray powder (XRD) diffractometer fitted with a duel position graphite monochromator in the diffracted beam. The diffraction pattern was recorded from 2θ ¼10° to 80° at a scanning speed of 2° min 1 using Cu Kα radiation at 40 kV tube voltage and 40 mA tube current. The qualitative and quantitative phase analysis of samples as prepared, calcined at 600 °C and calcined at 700 °C was also carried out using Rietveld refinement technique by MAUD program for Reitveld refinement [36–39]. The particle size was calculated by Scherrer equation using the X-ray diffraction pattern. The Fourier transform infrared (FT-IR) spectra were recorded in the transmission mode using a Perkin-Elmer FT-IR/RZX spectrometer in the spectral range of 4000–400 cm 1 following KBr pellet technique with a resolution of 0.5 cm 1. The background correction has been made in the FTIR experiment. The complete FT-IR spectrum is consisted of 64 scans. The surface morphological properties were studied using Jeol JSM-6510 scanning electron microscope (SEM) and Tecnai G2 FEI transmission electron microscope (TEM). In order to obtain the clear SEM micrographs, secondary electron imaging (SEI) was
Fig. 1. Rietveld refinement of Sr(NO3)2 þ Zn2V2O7 with sample as-prepared. χ¼ 2.69, Rwp (%) ¼14.12, Rp (%) ¼13.04, and Rexp (%)¼ 5.24.
M. Dalal et al. / Journal of Physics and Chemistry of Solids 89 (2016) 45–52
Fig. 2. Rietveld refinement of SrZnV2O7 þ Zn2V2O7 with sample calcined at 600 °C. χ ¼1.48, Rwp (%) ¼7.19, Rp (%) ¼6.70, and Rexp (%) ¼4.82.
Fig. 3. Rietveld refinement of SrZn0.98Sm0.02V2O7 with sample calcined at 700 °C for three hours. χ ¼1.48, Rwp (%) ¼7.19, Rp (%) ¼6.70, and Rexp (%) ¼4.82.
Table 1 Comparison of crystal structure data of SrZn0.98Sm0.02V2O7 nanophosphors with standard SrZnV2O7. Formula
SrZnV2O7 (Published)
SrZn0.98Sm0.02V2O7 (This work)
Formula weight Symmetry Space group a (Å) b (Å) c (Å) α ¼ γ (degree) β (degree) Volume (Å3) Z Density
366.88 Monoclinic P121/n1 7.425(2) 6.697(2) 11.977(5) 90 95.98 592.3 4 4.1138
370.37 Monoclinic P121/n1 7.4297 6.7015 11.9849 90 95.957 593.5 4 4.1246
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Fig. 4. XRD profile of SrZn1 xSmxV2O7 (x¼ 0.005–0.03) and SrZn0.96V2O7:0.02Sm3 þ , 0.02R þ (R¼ K, Na, and Li) nanophosphors alongwith standard data of SrZnV2O7 (JCPDS Card No.: 76-0759).
The result confirms that SrZn0.98Sm0.02V2O7 crystallizes into a monoclinic structure with the space group P121/n1 (14). The lattice parameters were determined to be a ¼7.4297 Å, b ¼6.7015 Å, c¼ 11.9849 Å, α ¼90°, β ¼95.957°, γ ¼90°, V¼593.5 Å3 and Z¼4 and the refinement finally converged to Rp ¼ 6.70%, Rwp ¼ 7.19% and χ2 ¼ 2.19 [28,29]. The comparison of the crystal structure data of SrZn0.98Sm0.02V2O7 with standard host is shown in the Table 1. X-ray diffraction profiles of SrZn1 xV2O7:xSm3 þ (x ¼0.005–0.03) and SrZn0.96V2O7:0.02Sm3 þ , 0.02R þ (R¼K, Na, and Li) along with the diffraction peaks of standard host is shown in Fig. 4 (JCPDS Card No.¼51-0131). The results clearly indicate that monovalent R þ and trivalent Sm3 þ ions are very well dissolved in SrZnV2O7 host lattice without any major structural default. We argue that the Sm3 þ ions are expected to replace Zn2 þ ions randomly in SrZnV2O7 host lattice. This can be understood in term of the increment in unit cell volume from 592.3 to 593.5 in process of dopant incorporation due to larger ionic radius of Sm3 þ than Zn2 þ . The crystal structure of SrZn0.98Sm0.02V2O7 is shown in the inset of Fig. 3 which clearly portrays that divalent zinc and trivalent samarium have same coordinative environment with slightly distorted trigonal bipyramidal geometry. The crystallites size was calculated with the help of the Scherrer's equation, D ¼0.941λ/ β cos θ, where D is the average grain size, λ the X-ray wavelength (0.1548 nm), θ and β are the diffraction angle and full width at half-maximum (FWHM, in radian) of an observed peak, respectively. The average particle size (D) for SrZn0.98Sm0.02V2O7 was found to be 67 nm [42,43]. 3.2. Infrared spectroscopy The FT-IR spectra of SrZn0.98Sm0.02V2O7 nanophosphor calcined at 700 °C is shown in Fig. 5. It can be seen that the major absorption bands are in the range of 450–950 cm 1 due to bending and stretching vibrations of the VO43 polyhedral anionic unit [44,45]. The absence of absorption peaks due to residual NO3 or any other additional phase confirms the complete crystallization of SrZn0.98Sm0.02V2O7 nanophosphor. The characteristic bending and stretching vibrational bands of water molecule at 1600 cm 1 and 3400 cm 1 are also absent due to the calcination of the sample at 700 °C for 3 h. The infra-red bands in the region 930–675 cm 1 may be ascribed to V–O stretching vibrations and the O–V–O
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Fig. 5. FT-IR spectrum of SrZn0.98Sm0.02V2O7 powder calcined at 700 °C temperature. Fig. 7. TEM micrographs of SrZn0.98Sm0.02V2O7 powder calcined at 700 °C.
asymmetric bending vibrations can be successfully accounted for the IR band at 556 cm 1.
3.3. Morphological characteristics The SEM and TEM analysis have been used obtain grain size and surface morphological properties of SrZnV2O7:Sm3 þ sample. Fig. 6 depicts the SEM micrograph of SrZn0.98Sm0.02V2O7 calcined at 700 °C. The morphology of the particles seems to be spherical or semi-spherical with very minor extant of agglomeration. Fig. 7, presenting the TEM image of SrZn0.98Sm0.02V2O7 calcined at 700 °C, clearly shows the smoothly surfaced spherical particles with small deviation in size factor. After considering both of the microscopic studies, we propose a typical grain size in the range of 60–95 nm, which in turn was also supported by the Scherrer's equation. Nano-scaled phosphor materials have larger surface to volume ratio and less internal scattering as compared to the bulk system, resulting in the enhancement of luminescence efficiency with lower non-radiative emission [46,47].
Fig. 8. Excitation spectrum of SrZn0.98Sm0.02V2O7 sample calcined 700 °C corresponding to a λem of 599 nm.
3.4. Optical properties
Fig. 6. SEM micrographs of SrZn0.98Sm0.02V2O7 powder calcined at 700 °C.
Fig. 8 depicts the excitation spectral characteristics of SrZn0.98Sm0.02V2O7 nanophosphor calcined at 700 °C monitored with 599 nm emission wavelength (4G5/2-6H7/2). The spectrum includes a broad band in ultraviolet region from 300 nm to 380 nm with a maximum at 341 nm wavelength and some sharp f–f excitation peaks in the longer wavelength region of 390 nm to 500 nm. The broad peak at 341 nm is attributed to host absorption band (HAB) corresponding to the charge transfer transition from ground state 1A1 to the excited state 1T1 within tetrahedrally coordinated VO43 group of mixed metal double pyrovanadate [48,49]. The sharp excitation peaks in the longer wavelength region corresponds to the characteristic f–f transitions of samarium ions within its incompletely filled subshell. The peaks present at 398 nm, 405 nm, 418 nm, 469 nm and 482 nm may be attributed to 6H5/2-4K11/2, 6H5/2-4M19/2, 6H5/2-4I13/2 and 6H5/2-4G7/2 transitions of Sm3 þ ion, respectively [50]. The room temperature photoluminescence emission spectra of
M. Dalal et al. / Journal of Physics and Chemistry of Solids 89 (2016) 45–52
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Fig. 9. Energy level diagram for Sm3 þ doped SrZnV2O7 nanophosphor.
SrZn0.98Sm0.02V2O7 calcined at 700 °C was monitored at 405 nm excitation. After excitation at the 4K11/2 level, the non-radiative relaxation of Sm3 þ ions leads to a sufficient population built up of the 4G5/2 energy level. The characteristic emission bands at 569, 599, 640 and 702 nm can be ascribed to 4G5/2-6H5/2, 4G5/2-6H7/2, 4G5/2-6H9/2 and 4 G5/2-6H11/2 transitions respectively. Fig. 9 portrays schematic energy level diagram for the energy transfer (ET) process, leading to the down-conversion (DC) of near-ultraviolet radiation. Furthermore, the strongest transition (4G5/2-6H7/2) with ΔJ¼ 71 is magnetic dipole allowed but is electrical dipole dominated. The spin and parity forbiddance of these f–f transitions are removed via spin–orbit mixing and non- centrosymmetric electric environment, respectively [51,52]. In SrZnV2O7 host lattice, the only asymmetric polyhedral site (Trigonal bypyramidal ZnO5) is occupied by Sm3 þ ions in a randomly distributed manner. Fig. 10 (a) depicts the variation of photoluminescence intensity of SrZn0.98Sm0.02V2O7 nanophosphor as a function of temperature. After excitation at 405 nm, it is observed that PL intensity increases with increase in the calcination temperature; the maximum intensity was noticed for 700 °C which may be attributed to removal of any additional phases like Zn2V2O7 with less optical efficiency. The dependence of photoluminescence intensity on Sm3 þ concentration in SrZn1 xV2O7:xSm3 þ (x¼0.005–0.03) samples calcined at 700 °C is shown in Fig. 10 (b). The PL emission intensity first increases with the increasing dopant concentration, reaching a maximum value at 2 mol% and then starts decreasing due to concentration quenching caused by the enhancement of non-radiative cross relaxation between neighboring Sm3þ ions. The efficient excitation in the range of 350– 420 nm suggests their potential applications in white light emitting diodes based on NUV-LED chips. Fig. 11 presents the variation of PL emission intensity at 599 nm (4G5/2 -6H7/2) as a function of Sm3 þ molar concentration. The observed decrease in photoluminescence intensity after 2 mol% (optimal molar concentration) gives a clear insight of the energy transfer mechanism between neighboring Sm3 þ ions. The critical energy transfer distance (Rc) can be calculated using Eq. (1):
Fig. 10. (a) Emission spectra of SrZn0.98Sm0.02V2O7 showing relative photoluminescent intensity as a function of temperature and (b) Emission spectra of SrZn1 xSmxV2O7 (x¼ 0.005–0.03) showing variation of emission intensity as a function of Sm3 þ concentrations, λex ¼ 405 nm.
⎡ 3V ⎤1/3 Rc = 2 ⎢ ⎥ ⎣ 4Πx c N ⎦
(1)
where N is the number of Z cations in the unit cell, xc is the optimal concentration and V is the volume of the unit cell. By taking, N ¼4, xc ¼ 0.02 and V¼ 593.5 Å3; the critical distance for energy transfer was found to be 26.64 Å [53]. For non-radiative energy transfer, radiation re-absorption and exchange interactions can be ruled out on the basis of absence of excitation-emission spectral overlap and considerably large critical distance, respectively [54]. Hence energy transfer process must be controlled by multipolar interactions as governed by the Dexter's theory [55]. Eq. (2) can be used for the multipolar interactions between same activator ions:
I −1 = K ⎡⎣ 1 + β (x)Q /3 ⎤⎦ x
(2)
where x is the activator concentration which is greater than the critical concentration, Q is the constant of multipolar interaction and is equal to 6, 8, 10 for dipole–dipole (d–d), dipole–quadrupole (d–q) and quadrupole–quadrupole (q–q) interactions, respectively.
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Fig. 11. Variation of the emission intensity at 599 nm as a function of Sm3 þ concentration in SrZn1 xSmxV2O7 nanophosphor.
Fig. 13. The luminescence decay curve for the 599 nm (4G5/2-6H7/2 of Sm3 þ ) emission of SrZn1 xSmxV2O7 (x ¼ 0.005–0.03) nanocrystals.
Table 2 CIE 1931 chromaticity index and photoluminescence lifetime for SrZn1 xSmxV2O7 (x ¼0.005–0.03) nanophosphors.
Fig. 12. Plot of log (I/x) as a function of log (x) in SrZnV2O7:Sm3 þ nanophosphor.
K and β are constants for the same excitation condition for the given host. The Plot of log (I/x) as a function of log (x) is a straight line with a slope of 1.97482, yielding a Q value equals to 5.92446 E6 (Fig. 12). Therefore, dipole–dipole interactions are predominantly responsible for the concentration quenching in Sm3 þ doped SrZnV2O7 nanophosphor. The luminescence decay curves for the emission at 599 nm (4G5/2 -6H7/2 of Sm3 þ ) in SrZn1 xSmxV2O7 nanophosphors are shown in Fig. 13. It can be seen that the decay curves can be well fitted to a single-exponential function as governed by Eq. (3):
I = I0 exp ( − t /τ)
(3)
where τ is the decay time for radiative emission, I0 and I are the luminescence intensities at time 0 and t respectively. Table 2 presents the emission life time values (ms) for SrZn1 xSmxV2O7 nanoparticles obtained from single exponential fits in logarithmic scale. The mono-exponential behavior of the decay profiles revealed that the Sm3 þ ions are occupying only one type of lattice site inside the host matrix without any cluster formation [56,57]. The photoluminescence lifetime (ms) decreases from 2.89 to 1.53 ms as the Sm3 þ concentrations increases from 0.005 to 3.0 mol% which may be attributed to the increase in non radiative emission. The increase of dopant concentration supports the non-
Sm3 þ concentration (mol%)
Color coordinates (x,y)
Lifetime (ms)
0.5 1 1.5 2.0 2.5 3.0
(0.5465, 0.4491) (0.5479, 0.4472) (0.5501, 0.4469) (0.5517, 0.4448) (0.5528, 0.4432) (0.5515, 0.4449)
2.89 2.63 2.40 2.12 1.70 1.53
radiative decay which may be credited either to the loss of local symmetry around the dopant or to the segregation of Sm3 þ ions [58,59]. The fluorescence decay time of SrZn1 xSmxV2O7 (x ¼0.005–0.03) nanophosphors decrease sharply after 2 mol% which can be explained in term of the concentration quenching due to cross relaxation process between two neighboring Sm3 þ ions. The photoluminescence emission spectra of SrZn0.96V2O7: 0.02Sm3 þ and SrZn0.96V2O7:0.02Sm3 þ , 0.02R þ (R¼Li, Na, and K) nanophosphor, upon excitation at 405 nm, revealed the effect of charge compensation as shown in Fig. 14. The replacement of Zn2 þ ions with Sm3 þ could impart slight lattice distortion and charge imbalance, inducing a boost in the non-radiative emission and slightly quenched luminescence [60–62]. The incorporation of one alkali metal R þ ion for each trivalent Sm3 þ doping might eliminate the charge imbalance and lattice distortion yielding enhanced luminescence efficiency. Furthermore, Li þ was found as the most efficient charge compensator which may be attributed to the similar ionic radius of Li þ and Zn2 þ [63]. The Commission International De I'Eclairage (CIE 1931) chromaticity diagram was used to analyze and confirm the emission color of SrZn1 x SmxV2O7 (x ¼0.005–0.03) and nanophosphor calcined at 700 °C (excited at 405 nm). All the color coordinates (x, y) fall in orange– red region and remains almost unshifted as shown in Table 2. Refering to the previous reports of orange–red phosphor for PC-WLEDs [NaCaBO3:Sm3 þ (0.57, 0.42), Na3YSi2O7:Sm3 þ (0.5449, 0.4472), LaMgAl11O19:Sm3 þ (0.578, 0.420)]; our sample SrZn0.96 V2O7:0.02Sm3 þ , 0.02Li þ synthesized using solution combustion method exhibits well comparable CIE values (0.5629, 0.4312) as depicted in Fig. 15 [50,64,65].
M. Dalal et al. / Journal of Physics and Chemistry of Solids 89 (2016) 45–52
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Among alkali metal cations, Li þ ions were found to be the best charge compensator for SrZnV2O7:0.02Sm3 þ nanophosphor as far as luminescence intensity is concerned. These nanophosphors find the potential applications in phosphor-converted white LEDs with near-ultraviolet excitation.
Acknowledgment One of the authors, Mandeep Dalal gratefully acknowledge the financial support in the form of junior research fellowship (JRF) from Council of Scientific and Industrial Research (CSIR), New Delhi, India (Award No: 09/382(0150)/2012-EMR-I).
References
Fig. 14. Emission spectra of SrZnV2O7:0.02Sm3 þ ,0.02R þ (R¼ Li, Na, and K) showing relative photoluminescent intensity as a function of different compensator charges.
Fig. 15. CIE chromaticity diagram for SrZn0.98Sm0.02V2O7 nanophosphor.
4. Conclusion In conclusion, SrZn1 xV2O7:xSm3 þ (x ¼0.005–0.03) and SrZn0.96V2O7:0.02Sm3 þ , 0.02R þ (R ¼Li, Na, and K) nanophosphors have been synthesized successfully via urea assisted solution combustion route; unveiling a potential entry to the class of RGB components for PC-WLEDs. The structural and optical analysis reveals the coordinative environment of the dopant and energy transfer behavior between various spectroscopic states of Sm3 þ ion. The effect of doping on the structural parameters helped out in understanding the opto-electronic properties. Upon excitation at 405 nm, a pure and intense orange–red emission has been observed at 599 nm corresponding to 4G5/2-6H7/2 transition of Sm3 þ ion. Nano-scaling of the particles suppressed the internal scattering and in turn enhanced the luminescence efficiency. Dexter's analysis clearly explained the concentration quenching arising from dipole–dipole cross relaxation phenomena in SrZn1 xV2O7:xSm3 þ (x ¼0.005–0.03) nanophosphor after 2 mol%.
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Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
A promising novel orange–red emitting SrZnV2O7:Sm3 þ nanophosphor for phosphor-converted white LEDs with near-ultraviolet excitation Mandeep Dalal a, V.B. Taxak a, Sangeeta Chahar a, Avni Khatkar b, S.P. Khatkar a,n a b
Department of Chemistry, Maharshi Dayanand University, Rohtak 124001, India University Institute of Engineering and Technology, Maharshi Dayanand University, Rohtak 124001, India
art ic l e i nf o
a b s t r a c t
Article history: Received 12 September 2015 Accepted 31 October 2015 Available online 2 November 2015
A novel trivalent samarium doped SrZnV2O7 nanophosphors was developed via urea assisted solution combustion method using metal nitrates as initial raw materials. The qualitative and quantitative phase analysis was carried out using Rietveld refinement technique. It was found to crystallize in monoclinic lattice with the P121/n1 (14) space group. The photoluminescent spectral study of SrZnV2O7:Sm3 þ revealed that the excitation of 405 nm yields the characteristic emission peaks at 569, 599, 640 and 702 nm due to 4G5/2-6H5/2, 4G5/2-6H7/2, 4G5/2-6H9/2 and 4G5/2-6H11/2 respectively. The optimum concentration of Sm3 þ ion in SrZnV2O7 for best luminescence was found to be 2 mol%. The luminescence intensity was further enhanced by incorporating compensator charge R þ (R ¼Li, Na, and K) into the SrZnV2O7:0.02Sm3 þ nanophosphor. The critical distance for non-radiative energy transfer was calculated to be 26.64 Å. Dipole–dipole (d–d) interactions were ascribed as the major factor responsible for concentration quenching arising from the over-doping of the activator ions. The results indicate that these nanophosphors are suitable candidate for PC-WLEDs using near UV excitation. & 2015 Elsevier Ltd. All rights reserved.
Keywords: A. Optical materials B. Chemical synthesis C. X-ray diffraction C. Electron microscopy D. Luminescence
1. Introduction Recently, opto-electronic study of rare earth doped inorganic host materials emerged out as one of the most proficient field in material science and nanotechnology [1–5]. Additionally, significant research efforts have been devoted to the exploration of lightemitting diodes due to their promising candidature in white light generation for solid state lightning (SSL) applications [6–9]. White LEDs exhibit excellent advantages such as more economical power consumption, higher durability, longer life, being mercury-free and less pollution as compared to the conventional halogen and incandescent lamp [10,11]. Nowadays, phosphor converted white light-emitting diodes (PC-WLEDs) are being fabricated either by blue GaN LED chip (λex ¼ 450–470 nm) covered by a yellowish phosphor coating or by near-ultraviolet (NUV) LED chips (λex ¼350– 420 nm) coated with red–blue–green (RGB) tricolor phosphors [12– 15]. However, white LEDs using GaN based chip system have poor color rendering index and low chromatic stability under different driving currents [16–18]. Therefore, near UV-LEDs coated with RGB phosphor offer effective and attractive solution for the problem of white light emission with lower color temperature [19]. n
Corresponding author. E-mail address: [email protected] (S.P. Khatkar).
http://dx.doi.org/10.1016/j.jpcs.2015.10.017 0022-3697/& 2015 Elsevier Ltd. All rights reserved.
To synthesize one of the indispensable component in RGB tricolor luminescent materials for NUV-WLEDs, red or orange–red emitting Eu3 þ and Sm3 þ doped phosphors have been extensively exploited due to the sharp spectral bands arising from intra-configrutional f–f transitions in activator ion [20–23]. In order to fabricate Ln3 þ activated nanophosphor, a numerous range of host materials have been explored, but SrZnV2O7 is still a novel host compound to the research field. To the best of the authors knowledge, the only attempt of rare earth doping (SrZnV2O7:Yb3 þ ) is reported by Guan et al. to convert the ultraviolet light into near infra-red region. This work is executed in order to study the photoluminescent down-conversion from ultraviolet to visible light via Sm3 þ doping in the SrZnV2O7 host lattice [24]. Vanadates form an important class of oxides in inorganic chemistry because of the diverse structural alignment of the vanadium–oxygen unit. Pyrovanadates contain discrete V2O74 complex ions units with both vanadium atoms sharing one corner [25–27]. The monoclinic host, SrZnV2O7 has lattice constant a (Å)¼7.4115, b (Å)¼ 6.6895, c (Å)¼ 11.9610, V¼589.73 Å3 and P121/n1 (14) space group with four formula units per unit cell [28,29]. Out of the several wet chemical methods such as hydrothermal synthesis [30], sol gel processing [31] and solution combustion synthesis [32], we have used the urea assisted solution combustion route to synthesis SrZn1 xSmxV2O7 nanophosphors. This efficient
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phosphor processing method has a number of advantages; such as inexpensive raw materials, low synthesis temperature, a relatively simple preparation process and homogenous product with fine particle size [33]. The Rietveld refinement of SrZn1 xSmxV2O7 reveals the monoclinic lattice with P121/n1 space group [34]. The critical distance for energy transfer and the variation of luminescence intensity after the optimal concentration (2 mol%) reveals the mechanism behind the concentration quenching phenomena. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) are used to study Surface morphological properties. The Scherrer's equation has been used to calculate the particle size which in turn further confirmed from SEM and TEM micrographs. Finally, the effect of charge compensation R þ (R¼ Li, Na, and K) on luminescence intensity has been also studied. The photoluminiscent excitation and emission spectra of synthesised samples suggest their potential application in phosphor converted white LEDs with NUV excitation.
2. Experimental 2.1. Material and synthesis A series of rare earth doped SrZn1-xV2O7:xSm3 þ (x¼0.005–0.03) and SrZn0.96V2O7:0.02Sm3 þ , 0.02R þ (R¼Li, Na, and K) nanophosphors were synthesized using solution combustion method in which constituent raw materials were high purity Sr(NO3)2, Zn(NO3)2 6H2O, Sm(NO3)3 6H2O, NH4VO3, KNO3, NaNO3, LiNO3 and urea. The starting materials were weighed in stoichiometric proportion and dissolved in minimum quantity of deionized water in a 400 ml capacity pyrex beaker. The amount of the urea was calculated using the total oxidizing and reducing valencies of the oxidizer and the fuel [35]. Finally the solution was placed into a preheated furnace maintained at 500 °C. The starting material undergoes rapid dehydration and foaming followed by decomposition, producing combustible gases. These volatile combustible gases ignite and burn with a flame yielding voluminous solid. Urea was oxidized by nitrate ions and served as a fuel for propellant reaction. The products were then cooled to room temperature and pulverized. A fraction of all the products were calcined at 600 °C and 700 °C for 3 h and kept in desiccator for further characterization.
preferred over the backscattered electron imaging (BSEI) due to its higher surface sensitivity and greater resolution. TEM images were obtained using the electron beam accelerated at 80 kV with a magnification of 105. SEM and TEM analysis was also used to confirm the particle size in nano range as given by the X-ray diffraction studies. The photoluminescence excitation and emission spectra (scanning rate: 1200 nm min 1, PMT voltage: 400 V, widths of the excitation slit and emission slit: 2.5 nm) and fluorescence decay curve and color coordinates of the SrZn1 xSmxV2O7 nanocrystals in the ultraviolet–visible region were recorded and investigated at room temperature using a Hitachi F-7000 fluorescence spectrophotometer with Xe-lamp as the excitation source.
3. Results and discussion 3.1. Crystal structure Rietveld refinement method was used to analyse the crystal structure and phase identification of the sample as-prepared, calcined at 600 °C and 700 °C. Fig. 1 depicts that the as-prepared sample is a mixture of two phases Sr(NO3)2 (JCPDS Card No.¼761375) and Zn2V2O7 (JCPDS Card No.¼29-1396) [40,41]. The black dot line is the experimental diffraction data observed for the sample as prepared at 500 °C whereas solid blue, green and red lines are the simulated pattern data from Zn2V2O7, Sr(NO3)2 and Zn2V2O7 þSr(NO3)2 respectively. The quantitative and qualitative phase study of the sample calcined at 600 °C shows that it was a mixture of Zn2V2O7 (JCPDS Card No.¼76-1375) and SrZnV2O7 (JCPDS Card No.¼51-0131) as shown in Fig. 2 [28,29,41]. The black dot line and solid red line are the experimental pattern observed for the sample calcined at 600 °C and simulated XRD profile for the mixture (Zn2V2O7 þ SrZnV2O7) respectively. Solid green and blue lines are the simulated diffraction data from phases SrZnV2O7 and Zn2V2O7 respectively. The Rietveld refinement of the samples with different Sm3 þ concentration calcined at 700 °C confirmed that SrZn1 xSmxV2O7 crystallizes in single phase with almost zero impurity. Fig. 3 shows the observed (blue dot line) and calculated (solid black line) X-ray diffraction (XRD) profiles along with their difference at bottom for the Rietveld refinement of for SrZn0.98Sm0.02V2O7 at room temperature with λ ¼ 0.1540562 nm.
2.2. Materials characterization The crystal phases of SrZn1 xSmxV2O7, as-prepared and calcined at different temperatures were characterized using a high resolution Rigaku Ultima-IV X-ray powder (XRD) diffractometer fitted with a duel position graphite monochromator in the diffracted beam. The diffraction pattern was recorded from 2θ ¼10° to 80° at a scanning speed of 2° min 1 using Cu Kα radiation at 40 kV tube voltage and 40 mA tube current. The qualitative and quantitative phase analysis of samples as prepared, calcined at 600 °C and calcined at 700 °C was also carried out using Rietveld refinement technique by MAUD program for Reitveld refinement [36–39]. The particle size was calculated by Scherrer equation using the X-ray diffraction pattern. The Fourier transform infrared (FT-IR) spectra were recorded in the transmission mode using a Perkin-Elmer FT-IR/RZX spectrometer in the spectral range of 4000–400 cm 1 following KBr pellet technique with a resolution of 0.5 cm 1. The background correction has been made in the FTIR experiment. The complete FT-IR spectrum is consisted of 64 scans. The surface morphological properties were studied using Jeol JSM-6510 scanning electron microscope (SEM) and Tecnai G2 FEI transmission electron microscope (TEM). In order to obtain the clear SEM micrographs, secondary electron imaging (SEI) was
Fig. 1. Rietveld refinement of Sr(NO3)2 þ Zn2V2O7 with sample as-prepared. χ¼ 2.69, Rwp (%) ¼14.12, Rp (%) ¼13.04, and Rexp (%)¼ 5.24.
M. Dalal et al. / Journal of Physics and Chemistry of Solids 89 (2016) 45–52
Fig. 2. Rietveld refinement of SrZnV2O7 þ Zn2V2O7 with sample calcined at 600 °C. χ ¼1.48, Rwp (%) ¼7.19, Rp (%) ¼6.70, and Rexp (%) ¼4.82.
Fig. 3. Rietveld refinement of SrZn0.98Sm0.02V2O7 with sample calcined at 700 °C for three hours. χ ¼1.48, Rwp (%) ¼7.19, Rp (%) ¼6.70, and Rexp (%) ¼4.82.
Table 1 Comparison of crystal structure data of SrZn0.98Sm0.02V2O7 nanophosphors with standard SrZnV2O7. Formula
SrZnV2O7 (Published)
SrZn0.98Sm0.02V2O7 (This work)
Formula weight Symmetry Space group a (Å) b (Å) c (Å) α ¼ γ (degree) β (degree) Volume (Å3) Z Density
366.88 Monoclinic P121/n1 7.425(2) 6.697(2) 11.977(5) 90 95.98 592.3 4 4.1138
370.37 Monoclinic P121/n1 7.4297 6.7015 11.9849 90 95.957 593.5 4 4.1246
47
Fig. 4. XRD profile of SrZn1 xSmxV2O7 (x¼ 0.005–0.03) and SrZn0.96V2O7:0.02Sm3 þ , 0.02R þ (R¼ K, Na, and Li) nanophosphors alongwith standard data of SrZnV2O7 (JCPDS Card No.: 76-0759).
The result confirms that SrZn0.98Sm0.02V2O7 crystallizes into a monoclinic structure with the space group P121/n1 (14). The lattice parameters were determined to be a ¼7.4297 Å, b ¼6.7015 Å, c¼ 11.9849 Å, α ¼90°, β ¼95.957°, γ ¼90°, V¼593.5 Å3 and Z¼4 and the refinement finally converged to Rp ¼ 6.70%, Rwp ¼ 7.19% and χ2 ¼ 2.19 [28,29]. The comparison of the crystal structure data of SrZn0.98Sm0.02V2O7 with standard host is shown in the Table 1. X-ray diffraction profiles of SrZn1 xV2O7:xSm3 þ (x ¼0.005–0.03) and SrZn0.96V2O7:0.02Sm3 þ , 0.02R þ (R¼K, Na, and Li) along with the diffraction peaks of standard host is shown in Fig. 4 (JCPDS Card No.¼51-0131). The results clearly indicate that monovalent R þ and trivalent Sm3 þ ions are very well dissolved in SrZnV2O7 host lattice without any major structural default. We argue that the Sm3 þ ions are expected to replace Zn2 þ ions randomly in SrZnV2O7 host lattice. This can be understood in term of the increment in unit cell volume from 592.3 to 593.5 in process of dopant incorporation due to larger ionic radius of Sm3 þ than Zn2 þ . The crystal structure of SrZn0.98Sm0.02V2O7 is shown in the inset of Fig. 3 which clearly portrays that divalent zinc and trivalent samarium have same coordinative environment with slightly distorted trigonal bipyramidal geometry. The crystallites size was calculated with the help of the Scherrer's equation, D ¼0.941λ/ β cos θ, where D is the average grain size, λ the X-ray wavelength (0.1548 nm), θ and β are the diffraction angle and full width at half-maximum (FWHM, in radian) of an observed peak, respectively. The average particle size (D) for SrZn0.98Sm0.02V2O7 was found to be 67 nm [42,43]. 3.2. Infrared spectroscopy The FT-IR spectra of SrZn0.98Sm0.02V2O7 nanophosphor calcined at 700 °C is shown in Fig. 5. It can be seen that the major absorption bands are in the range of 450–950 cm 1 due to bending and stretching vibrations of the VO43 polyhedral anionic unit [44,45]. The absence of absorption peaks due to residual NO3 or any other additional phase confirms the complete crystallization of SrZn0.98Sm0.02V2O7 nanophosphor. The characteristic bending and stretching vibrational bands of water molecule at 1600 cm 1 and 3400 cm 1 are also absent due to the calcination of the sample at 700 °C for 3 h. The infra-red bands in the region 930–675 cm 1 may be ascribed to V–O stretching vibrations and the O–V–O
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M. Dalal et al. / Journal of Physics and Chemistry of Solids 89 (2016) 45–52
Fig. 5. FT-IR spectrum of SrZn0.98Sm0.02V2O7 powder calcined at 700 °C temperature. Fig. 7. TEM micrographs of SrZn0.98Sm0.02V2O7 powder calcined at 700 °C.
asymmetric bending vibrations can be successfully accounted for the IR band at 556 cm 1.
3.3. Morphological characteristics The SEM and TEM analysis have been used obtain grain size and surface morphological properties of SrZnV2O7:Sm3 þ sample. Fig. 6 depicts the SEM micrograph of SrZn0.98Sm0.02V2O7 calcined at 700 °C. The morphology of the particles seems to be spherical or semi-spherical with very minor extant of agglomeration. Fig. 7, presenting the TEM image of SrZn0.98Sm0.02V2O7 calcined at 700 °C, clearly shows the smoothly surfaced spherical particles with small deviation in size factor. After considering both of the microscopic studies, we propose a typical grain size in the range of 60–95 nm, which in turn was also supported by the Scherrer's equation. Nano-scaled phosphor materials have larger surface to volume ratio and less internal scattering as compared to the bulk system, resulting in the enhancement of luminescence efficiency with lower non-radiative emission [46,47].
Fig. 8. Excitation spectrum of SrZn0.98Sm0.02V2O7 sample calcined 700 °C corresponding to a λem of 599 nm.
3.4. Optical properties
Fig. 6. SEM micrographs of SrZn0.98Sm0.02V2O7 powder calcined at 700 °C.
Fig. 8 depicts the excitation spectral characteristics of SrZn0.98Sm0.02V2O7 nanophosphor calcined at 700 °C monitored with 599 nm emission wavelength (4G5/2-6H7/2). The spectrum includes a broad band in ultraviolet region from 300 nm to 380 nm with a maximum at 341 nm wavelength and some sharp f–f excitation peaks in the longer wavelength region of 390 nm to 500 nm. The broad peak at 341 nm is attributed to host absorption band (HAB) corresponding to the charge transfer transition from ground state 1A1 to the excited state 1T1 within tetrahedrally coordinated VO43 group of mixed metal double pyrovanadate [48,49]. The sharp excitation peaks in the longer wavelength region corresponds to the characteristic f–f transitions of samarium ions within its incompletely filled subshell. The peaks present at 398 nm, 405 nm, 418 nm, 469 nm and 482 nm may be attributed to 6H5/2-4K11/2, 6H5/2-4M19/2, 6H5/2-4I13/2 and 6H5/2-4G7/2 transitions of Sm3 þ ion, respectively [50]. The room temperature photoluminescence emission spectra of
M. Dalal et al. / Journal of Physics and Chemistry of Solids 89 (2016) 45–52
49
Fig. 9. Energy level diagram for Sm3 þ doped SrZnV2O7 nanophosphor.
SrZn0.98Sm0.02V2O7 calcined at 700 °C was monitored at 405 nm excitation. After excitation at the 4K11/2 level, the non-radiative relaxation of Sm3 þ ions leads to a sufficient population built up of the 4G5/2 energy level. The characteristic emission bands at 569, 599, 640 and 702 nm can be ascribed to 4G5/2-6H5/2, 4G5/2-6H7/2, 4G5/2-6H9/2 and 4 G5/2-6H11/2 transitions respectively. Fig. 9 portrays schematic energy level diagram for the energy transfer (ET) process, leading to the down-conversion (DC) of near-ultraviolet radiation. Furthermore, the strongest transition (4G5/2-6H7/2) with ΔJ¼ 71 is magnetic dipole allowed but is electrical dipole dominated. The spin and parity forbiddance of these f–f transitions are removed via spin–orbit mixing and non- centrosymmetric electric environment, respectively [51,52]. In SrZnV2O7 host lattice, the only asymmetric polyhedral site (Trigonal bypyramidal ZnO5) is occupied by Sm3 þ ions in a randomly distributed manner. Fig. 10 (a) depicts the variation of photoluminescence intensity of SrZn0.98Sm0.02V2O7 nanophosphor as a function of temperature. After excitation at 405 nm, it is observed that PL intensity increases with increase in the calcination temperature; the maximum intensity was noticed for 700 °C which may be attributed to removal of any additional phases like Zn2V2O7 with less optical efficiency. The dependence of photoluminescence intensity on Sm3 þ concentration in SrZn1 xV2O7:xSm3 þ (x¼0.005–0.03) samples calcined at 700 °C is shown in Fig. 10 (b). The PL emission intensity first increases with the increasing dopant concentration, reaching a maximum value at 2 mol% and then starts decreasing due to concentration quenching caused by the enhancement of non-radiative cross relaxation between neighboring Sm3þ ions. The efficient excitation in the range of 350– 420 nm suggests their potential applications in white light emitting diodes based on NUV-LED chips. Fig. 11 presents the variation of PL emission intensity at 599 nm (4G5/2 -6H7/2) as a function of Sm3 þ molar concentration. The observed decrease in photoluminescence intensity after 2 mol% (optimal molar concentration) gives a clear insight of the energy transfer mechanism between neighboring Sm3 þ ions. The critical energy transfer distance (Rc) can be calculated using Eq. (1):
Fig. 10. (a) Emission spectra of SrZn0.98Sm0.02V2O7 showing relative photoluminescent intensity as a function of temperature and (b) Emission spectra of SrZn1 xSmxV2O7 (x¼ 0.005–0.03) showing variation of emission intensity as a function of Sm3 þ concentrations, λex ¼ 405 nm.
⎡ 3V ⎤1/3 Rc = 2 ⎢ ⎥ ⎣ 4Πx c N ⎦
(1)
where N is the number of Z cations in the unit cell, xc is the optimal concentration and V is the volume of the unit cell. By taking, N ¼4, xc ¼ 0.02 and V¼ 593.5 Å3; the critical distance for energy transfer was found to be 26.64 Å [53]. For non-radiative energy transfer, radiation re-absorption and exchange interactions can be ruled out on the basis of absence of excitation-emission spectral overlap and considerably large critical distance, respectively [54]. Hence energy transfer process must be controlled by multipolar interactions as governed by the Dexter's theory [55]. Eq. (2) can be used for the multipolar interactions between same activator ions:
I −1 = K ⎡⎣ 1 + β (x)Q /3 ⎤⎦ x
(2)
where x is the activator concentration which is greater than the critical concentration, Q is the constant of multipolar interaction and is equal to 6, 8, 10 for dipole–dipole (d–d), dipole–quadrupole (d–q) and quadrupole–quadrupole (q–q) interactions, respectively.
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M. Dalal et al. / Journal of Physics and Chemistry of Solids 89 (2016) 45–52
Fig. 11. Variation of the emission intensity at 599 nm as a function of Sm3 þ concentration in SrZn1 xSmxV2O7 nanophosphor.
Fig. 13. The luminescence decay curve for the 599 nm (4G5/2-6H7/2 of Sm3 þ ) emission of SrZn1 xSmxV2O7 (x ¼ 0.005–0.03) nanocrystals.
Table 2 CIE 1931 chromaticity index and photoluminescence lifetime for SrZn1 xSmxV2O7 (x ¼0.005–0.03) nanophosphors.
Fig. 12. Plot of log (I/x) as a function of log (x) in SrZnV2O7:Sm3 þ nanophosphor.
K and β are constants for the same excitation condition for the given host. The Plot of log (I/x) as a function of log (x) is a straight line with a slope of 1.97482, yielding a Q value equals to 5.92446 E6 (Fig. 12). Therefore, dipole–dipole interactions are predominantly responsible for the concentration quenching in Sm3 þ doped SrZnV2O7 nanophosphor. The luminescence decay curves for the emission at 599 nm (4G5/2 -6H7/2 of Sm3 þ ) in SrZn1 xSmxV2O7 nanophosphors are shown in Fig. 13. It can be seen that the decay curves can be well fitted to a single-exponential function as governed by Eq. (3):
I = I0 exp ( − t /τ)
(3)
where τ is the decay time for radiative emission, I0 and I are the luminescence intensities at time 0 and t respectively. Table 2 presents the emission life time values (ms) for SrZn1 xSmxV2O7 nanoparticles obtained from single exponential fits in logarithmic scale. The mono-exponential behavior of the decay profiles revealed that the Sm3 þ ions are occupying only one type of lattice site inside the host matrix without any cluster formation [56,57]. The photoluminescence lifetime (ms) decreases from 2.89 to 1.53 ms as the Sm3 þ concentrations increases from 0.005 to 3.0 mol% which may be attributed to the increase in non radiative emission. The increase of dopant concentration supports the non-
Sm3 þ concentration (mol%)
Color coordinates (x,y)
Lifetime (ms)
0.5 1 1.5 2.0 2.5 3.0
(0.5465, 0.4491) (0.5479, 0.4472) (0.5501, 0.4469) (0.5517, 0.4448) (0.5528, 0.4432) (0.5515, 0.4449)
2.89 2.63 2.40 2.12 1.70 1.53
radiative decay which may be credited either to the loss of local symmetry around the dopant or to the segregation of Sm3 þ ions [58,59]. The fluorescence decay time of SrZn1 xSmxV2O7 (x ¼0.005–0.03) nanophosphors decrease sharply after 2 mol% which can be explained in term of the concentration quenching due to cross relaxation process between two neighboring Sm3 þ ions. The photoluminescence emission spectra of SrZn0.96V2O7: 0.02Sm3 þ and SrZn0.96V2O7:0.02Sm3 þ , 0.02R þ (R¼Li, Na, and K) nanophosphor, upon excitation at 405 nm, revealed the effect of charge compensation as shown in Fig. 14. The replacement of Zn2 þ ions with Sm3 þ could impart slight lattice distortion and charge imbalance, inducing a boost in the non-radiative emission and slightly quenched luminescence [60–62]. The incorporation of one alkali metal R þ ion for each trivalent Sm3 þ doping might eliminate the charge imbalance and lattice distortion yielding enhanced luminescence efficiency. Furthermore, Li þ was found as the most efficient charge compensator which may be attributed to the similar ionic radius of Li þ and Zn2 þ [63]. The Commission International De I'Eclairage (CIE 1931) chromaticity diagram was used to analyze and confirm the emission color of SrZn1 x SmxV2O7 (x ¼0.005–0.03) and nanophosphor calcined at 700 °C (excited at 405 nm). All the color coordinates (x, y) fall in orange– red region and remains almost unshifted as shown in Table 2. Refering to the previous reports of orange–red phosphor for PC-WLEDs [NaCaBO3:Sm3 þ (0.57, 0.42), Na3YSi2O7:Sm3 þ (0.5449, 0.4472), LaMgAl11O19:Sm3 þ (0.578, 0.420)]; our sample SrZn0.96 V2O7:0.02Sm3 þ , 0.02Li þ synthesized using solution combustion method exhibits well comparable CIE values (0.5629, 0.4312) as depicted in Fig. 15 [50,64,65].
M. Dalal et al. / Journal of Physics and Chemistry of Solids 89 (2016) 45–52
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Among alkali metal cations, Li þ ions were found to be the best charge compensator for SrZnV2O7:0.02Sm3 þ nanophosphor as far as luminescence intensity is concerned. These nanophosphors find the potential applications in phosphor-converted white LEDs with near-ultraviolet excitation.
Acknowledgment One of the authors, Mandeep Dalal gratefully acknowledge the financial support in the form of junior research fellowship (JRF) from Council of Scientific and Industrial Research (CSIR), New Delhi, India (Award No: 09/382(0150)/2012-EMR-I).
References
Fig. 14. Emission spectra of SrZnV2O7:0.02Sm3 þ ,0.02R þ (R¼ Li, Na, and K) showing relative photoluminescent intensity as a function of different compensator charges.
Fig. 15. CIE chromaticity diagram for SrZn0.98Sm0.02V2O7 nanophosphor.
4. Conclusion In conclusion, SrZn1 xV2O7:xSm3 þ (x ¼0.005–0.03) and SrZn0.96V2O7:0.02Sm3 þ , 0.02R þ (R ¼Li, Na, and K) nanophosphors have been synthesized successfully via urea assisted solution combustion route; unveiling a potential entry to the class of RGB components for PC-WLEDs. The structural and optical analysis reveals the coordinative environment of the dopant and energy transfer behavior between various spectroscopic states of Sm3 þ ion. The effect of doping on the structural parameters helped out in understanding the opto-electronic properties. Upon excitation at 405 nm, a pure and intense orange–red emission has been observed at 599 nm corresponding to 4G5/2-6H7/2 transition of Sm3 þ ion. Nano-scaling of the particles suppressed the internal scattering and in turn enhanced the luminescence efficiency. Dexter's analysis clearly explained the concentration quenching arising from dipole–dipole cross relaxation phenomena in SrZn1 xV2O7:xSm3 þ (x ¼0.005–0.03) nanophosphor after 2 mol%.
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