Luminescent Properties of Doped Zinc Aluminate ... - ACS Publications

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J. Phys. Chem. C 2009, 113, 16954–16961

Luminescent Properties of Doped Zinc Aluminate and Zinc Gallate White Light Emitting Nanophosphors Prepared via Sonochemical Method Dimple P. Dutta,*,† R. Ghildiyal,‡ and A. K. Tyagi*,† Chemistry DiVision, Bhabha Atomic Research Centre, Mumbai 400085, India, and Indian Diamond Institute, Surat 395008, India

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ReceiVed: June 16, 2009; ReVised Manuscript ReceiVed: August 25, 2009

Novel single, double and triple doped ZnAl2O4:M and ZnGa2O4:M (where M ) Dy3+, Tb3+, Eu3+ and Mn2+) nanophosphors were synthesized through a simple sonochemical process and characterized by using X-ray diffraction (XRD), transmission electron microscopy (TEM) and photoluminescence (PL) spectrophotometry. The results of XRD and TEM show that resultant nanoparticles are single phase and have spherical shape. The doped samples showed multicolor emission on single wavelength excitation. Energy transfer was observed from host to the dopant ions. Characteristic blue and yellow emission from Dy3+ ions, green from Tb3+ ions and red from Eu3+ ions were observed. However, the Mn2+ ions in zinc gallate host yielded red emission which has been assigned to charge-transfer deexcitation associated with Mn2+ ion. The CIE coordinates of ZnGa2O4:Dy3+(2%):Tb3+(0.5%):Mn2+(2.5%) and ZnAl2O4:Dy3+(1.5%):Tb3+(0.25%):Eu3+(0.25%) nanophosphors are (0.304,0.310) and (0.312, 0.307), respectively, which lie in the white light region of the chromaticity diagram. 1. Introduction There is considerable interest in obtaining efficient white light emitting phosphors as a replacement for conventional light sources.1 Various methods have been explored to get efficient solid state sources for white light generation. Blue LEDs are used to pump one or more visible light-emitting phosphors integrated into the phosphor-converted LED (pc-LED) package. Such pc-LEDs are designed to leak some of the blue light beyond the phosphor to generate the blue portion of the spectrum, while phosphor converts the remainder of the blue light into the red and green portions of the spectrum. Ultraviolet LEDs are also used to pump a combination of red, green and blue phosphors in such a way that none of the pump LED light is allowed to escape. Hence, the availability of high quality phosphors operating under UV excitation is of prime importance for better performance of such LEDs. Recently, ZnAl2O4 and ZnGa2O4 have attracted attention as an important phosphor host material for applications in thin film electroluminescent displays, mechano-optical stress sensors, and stress imaging devices.2-6 The optical band gap of polycrystalline ZnAl2O4 semiconductors is 3.8 eV,7 which indicates that zinc aluminate in the polycrystalline form is transparent for light possessing wavelengths >320 nm. Rare earth ions are widely used as activators in different hosts due to their high fluorescence efficiencies and very narrow line fluorescence bands. Zinc aluminate doped with rare earth metal ions has been investigated most frequently because of the unique luminescent properties resulting from its stability and high emission quantum yields.8-10 This rare earth doped material produces efficient visible emissions in a 4f shell, which is, to a large extent, insensitive to the influence of its surroundings attributed to the shielding effect of the outer 5s and 5p orbitals.11 The optical bandgap of * Corresponding authors. E-mail: [email protected], [email protected]. Tel: +91 22 25592308. Fax: +91 22 25505151. † Bhabha Atomic Research Centre. ‡ Indian Diamond Institute.

ZnGa2O4 is approximately 4.4 eV. Like other wide bandgap semiconductors, ZnGa2O4 exhibits a blue emission under excitation by both ultraviolet light and low-voltage electrons.12 Activation with Mn2+, Eu3+, Tb3+ ions shifts the emission to green or red. The characteristics of phosphor particles required for information displays are spherical morphology, fine particle size, high density/low porosity, narrow size distribution, high crystallinity and homogeneous distribution of dopant activator ions throughout the host material. A spherical shape and clean surface of phosphor particles minimize the scattering of light on the phosphor surface and maximize the packing density of phosphor particles, thereby enhancing the efficiency of information displays. Bulk and monodispersed nanostructures of ZnAl2O4 have been synthesized via a solid-state reaction of zinc and aluminum oxides above 800 °C13,14 or via wet chemical routes such as coprecipitation,15,16 hydrothermal,17 and sol-gel18 aluminum oxide template methods.19 Rare earth activated ZnAl2O4 phosphors can be prepared by a variety of techniques, e.g., the hydrothermal method,17 spray pyrolysis,10 Pechini’s method20 and combustion synthesis.21 ZnGa2O4 is mostly prepared by a solid-state reaction between two metal compounds such as zinc oxide and gallium oxide,7 and the flux method using Li3PO4 at high temperatures above 1200 °C.22,23 Nanosized ZnGa2O4 spinel powders have been synthesized from gallium and zinc salts under hydrothermal conditions at 150-240 °C27-29 and under atmospheric conditions below 90 °C.27 Nonaqueous sol-gel methods for synthesis of nano ZnGa2O4 have also been reported.28 However, none of them have reported on the application of these materials as white light emitting phosphors. In this manuscript, we report the synthesis of nanocrystalline single, double and triple doped ZnAl2O4:M (M ) Dy3+, Tb3+ and Eu3+) and ZnGa2O4:M (M ) Dy3+, Tb3+ and Eu3+/Mn2+), by sonochemical method. Sonochemical synthesis is based on acoustic cavitation resulting from the continuous formation, growth and implosive collapse of the bubbles in a liquid.29,30 In

10.1021/jp905631g CCC: $40.75  2009 American Chemical Society Published on Web 09/10/2009

ZnAl2O4:M and ZnGa2O4:M Nanophosphors

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our study, Dy3+, Tb3+ and Eu3+/Mn2+ ions were single, double and triple doped in ZnAl2O4 and ZnGa2O4 host lattice using sonochemical technique. By altering the individual doping concentrations in the triple doped samples, the desired photoluminescence emission at selected wavelength could be obtained. The highlight of the work is the synthesis of a novel composition of nanocrystalline ZnAl2O4:Dy3+:Tb3+:Eu3+ and ZnGa2O4:Dy3+: Tb3+:Mn2+ phosphors which simultaneously emits blue, green, yellow and red light from its active region on exciting the host and have very impressive CIE and CCT values. Such codoped nanocrystalline phosphors are expected to have tremendous applications as tunable multicolor displays and in white light emitting UV-LEDs. 2. Experimental section 2.1. Synthesis. All the reactions were carried out at room temperature under ambient conditions. High purity zinc chloride [ZnCl2], aluminum chloride [AlCl3], gallium nitrate [Ga(NO3)3 · xH2O], manganese acetate [Mn(OAc)2 · 4H2O], europium nitrate [Eu(NO3)3 · 6H2O], dysprosium nitrate [Dy(NO3)3 · 6H2O] and terbium nitrate [Tb(NO3)3 · 4H2O] were obtained from commercial sources (Aldrich). AR grade ammonia solution was used. Preparation of Zinc Aluminate Nanoparticles. An aqueous solution of ZnCl2 (1 g, 7.33 mmol) and AlCl3 (1.97 g, 14.67 mmol) was made basic by adding ∼2 mL of ammonia and was exposed to high intensity ultrasound under ambient temperature for 90 min. When the reaction was completed, a white precipitate was obtained. The precipitate was separated by centrifuging at 10,000 rpm for 5 min. It was washed with water and dried in air. The dried powder was heated in a furnace at 475 °C for 4 h. The final product was in the form of white powder. Preparation of Zinc Gallate Nanoparticles. ZnCl2 (0.35 g, 2.56 mmol) and Ga(NO3)3 · xH2O (1.5 g, 5.1 mmol) were dissolved in 60 mL of distilled water, and the solution was made basic by the addition of ammonia. The system was then irradiated with high-intensity (100 W/cm2) ultrasonic waves operating at 20 kHz, under air at room temperature for 1.5 h. This was done by the direct immersion of a titanium horn (13 mm diameter) to a depth of 6 cm in the solution. When the reaction was complete, a white precipitate was obtained, which was separated by centrifuging at 10,000 rpm for 5 min. It was washed with water and dried in air. It was heated in a furnace at 475 °C for different time periods (2 h and 4 h). The final product was in the form of white powder. Preparation of Doped Zinc Aluminate and Zinc Gallate Nanoparticles. The method is similar to that reported for zinc aluminate and zinc gallate nanoparticles, but here a stoichiometric amount of dysprosium/europium/terbium nitrate or manganese acetate was also added to the reaction mixture. We have prepared ZnAl2O4:Dy3+(1.9%):Tb3+(0.1%), ZnAl2O4:Dy3+(1.5%):Tb3+(0.25%):Eu3+(0.25%), ZnGa2O4:Dy3+(2%), ZnGa2O4: Eu3+(1%):Dy3+(2%), ZnGa2O4:Eu3+(0.5%):Dy3+(2%), ZnGa2O4: Dy3+(2%):Tb3+(0.5%):Mn2+(1.5%) and ZnGa2O4:Dy3+(2%): Tb3+(0.5%):Mn2+(2.5%) nanoparticles using the sonochemical technique.The products obtained were subjected to thermal treatment, and resultant residues were characterized by XRD. 2.2. Characterization. X-ray diffraction (XRD) measurements were carried out on a Philips Instrument, operating with Cu KR radiation (λ ) 1.5406 Å) and employing a scan rate of 0.02°/s in the scattering angular range (2θ) of 10° to 80°. Silicon was used as an external standard for correction due to instrumental broadening. The lattice parameters were calculated

Figure 1. Powder XRD of ZnGa2O4 obtained after (a) 2 and (b) 4 h furnace heating of the sonolysed product at 475 °C.

Figure 2. Powder XRD of (a) ZnGa2O4:Dy3+(2%), (b) ZnGa2O4: Dy3+(2%):Eu3+(2%) and (c) ZnGa2O4:Dy3+(2%):Tb3+(0.5%):Mn2+(2.5%) nanoparticles obtained after 4 h furnace heating of the sonolysed product at 475 °C.

from the least-squares fitting of the diffraction peaks using POWDERX program. The average crystallite size was calculated from the diffraction line width based on Scherrer’s relation: d ) 0.9λ/(B cos θ), where λ denotes the wavelength of X-rays and B is the corrected full width at half maximum (fwhm). Conventional TEM micrographs were recorded on JEOL 2000FX. The particulates obtained, were dispersed in methanol solution and then deposited on the carbon coated copper grids for TEM/SAED studies. The luminescence measurements were carried out at room temperature with a resolution of 3 nm, using a Hitachi Instrument (F-4010) having a 150 W Xe lamp as the excitation source. Powder samples were mixed with methanol, spread over a glass plate, dried and mounted inside the sample holder. The CIE color coordinates (x, y) were calculated from the equidistant wavelength method based on the photoluminescence emission spectra.31,32 3. Results and Discussion 3.1. Structural Characterization. Figure 1 illustrates the diffraction patterns of the sonication products for different time periods of furnace heating (2 h and 4 h) for undoped ZnGa2O4. It is clear that the very broad features detected for the 2 h furnace heated product (Figure 1a) are due to the partly amorphous nature of the product and also due to the very small particle size. The diffraction peaks at 2θ ) 35.7°, 57.4° and 63.08° are close to the reported data for ZnGa2O4 (JC-PDS card 71-0843). The very broad nature of the diffraction peaks implies that nanocrystalline ZnGa2O4 was formed. The products obtained after 4 h of heating (Figure 1b) show diffraction peaks at 2θ ) 30.0°, 35.7°, 53.9°, 57.4° and 63.08°, which clearly match that of cubic ZnGa2O4. Using the Scherrer equation, the sizes of

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TABLE 1: Lattice Parameters and Size of Undoped and Doped Zinc Aluminate/Gallate Nanoparticles sample

diameter of nanoparticles from XRD (nm)

lattice parameter (Å)

diameter of nanoparticles from TEM (nm)

ZnAl2O4 ZnAl2O4:Dy3+(1.9%):Tb3+(0.1%) ZnAl2O4:Dy3+(1.5%):Tb3+(0.25%):Eu3+(0.25%) ZnGa2O4 ZnGa2O4:Dy3+(2%) ZnGa2O4:Dy3+(2%):Eu3+(0.5%) ZnGa2O4:Dy3+(2%):Eu3+(1%) ZnGa2O4:Dy3+(2%):Tb3+(0.5%):Mn2+(1.5%) ZnGa2O4:Dy3+(2%):Tb3+(0.5%):Mn2+(2.5%)

∼21 ∼28 ∼25 ∼5 ∼5 ∼5 ∼5 ∼6 ∼7

8.091(2) 8.346(5) 8.405(3) 8.330(5) 8.862(2) 8.868(2) 8.913(5) 8.878(3) 8.894(2)

20-25 25-30 25-30 4-6 3-6 4-6 4-6 4-7 4-6

the ZnGa2O4 nanoparticles were found to be ∼3 and ∼5 nm for the 2 and 4 h furnace heated products, respectively. It can be clearly seen that the particle size and the crystallinity of the nanoparticles significantly increase with an increase in the time of heat treatment. Figures 2a-c show the XRD patterns for the doped ZnGa2O4 nanoparticles obtained after heat treatment of the sonolysed products at 475 °C under argon. A representative XRD pattern for the ZnGa2O4:2%Dy3+ particles has been shown in Figure 2a. With incorporation of the rare earth ions, the lattice undergoes distortion as revealed by the broadening of diffraction peaks corresponding to that of undoped ZnGa2O4 nanoparticles obtained by the same method (Table 1). The lattice parameter for ZnGa2O4:Dy3+(2%) was found to be a ) 8.862(2) Å as compared to a ) 8.330(5) Å for undoped ZnGa2O4. This indicates that Dy3+ has got incorporated into the ZnGa2O4 lattice, as the ionic radius of Dy3+ is slightly larger than that of Ga3+. Similar broadening of peaks was also observed for the ZnGa2O4: Dy3+(2%):Eu3+(2%) and ZnGa2O4:Dy3+(2%):Tb3+(0.5%):Mn2+(2.5%) nanoparticles as shown in Figures 2b and 2c, respectively. With increase in the doping concentration of the rare earth ions, there is an increase in the lattice broadening. The diffraction patterns of the doped and undoped ZnAl2O4 (Figure 3) belong to the cubic spinel system which is in good agreement with the literature (JCPDS No. 82-1043). Based on the line width of the diffraction peaks, average particle size has been found to be around 20 nm for undoped ZnAl2O4 and around 25 nm for the doped ZnAl2O4. The structure and morphology of the above nanoparticles have been further analyzed by TEM. Figure 4 shows the representative bright-field TEM image of the 4 h heat treated ZnGa2O4: Dy3+(2%):Tb3+(0.5%):Mn2+(2.5%) nanoparticle sample along

Figure 3. Powder XRD of (a) ZnAl2O4 (b) ZnAl2O4:Dy3+(1.9%): Tb3+(0.1%) and (c) ZnAl2O4:Dy3+(1.5%):Tb3+(0.25%):Eu3+(0.25%) nanoparticles obtained after 4 h furnace heating of the sonolysed product at 475 °C.

with its SAED. The nanoparticles are mostly spherical and have a mean diameter of ∼5 nm along with a standard deviation of 1.1 nm, which is in excellent agreement with that obtained from XRD. The SAED pattern confirmed the nanocrystalline nature of the particles. The indexing of the concentric rings corresponds to cubic ZnGa2O4 crystal structure. 3.2. Photoluminescence in Zinc Gallate Nanophosphors. The emission spectra of bulk ZnGa2O4 show a blue emission band around 470 nm in the range of 300 and 600 nm. It originates from the self-activation center of the octahedral GaO6 group in the spinel lattices and the Ga3+ ions combine with UV-generated free electrons produced in oxygen vacancies.33-42 It has also been reported that the emission of ZnGa2O4 is influenced by factors such as particle size, shape and crystallinity.33-36 In the case of our ZnGa2O4 nanoparticles, obtained after furnace heating at 475 °C for 4 h on exciting at 356 nm, the emission spectra (Figure 5) show a broad emission band in the range of 400 and 600 nm and a maximum emission peak at 469 nm with a shoulder at 437 nm. Similar results have been recently reported for ZnGa2O4 nanocrystals prepared by combining thermolysis of single molecular precursor with the sol-gel method in the presence of a capping agent.43 However, in their case, the emission had the highest intensity at 450 nm with a shoulder at 470 nm. The excitation spectrum corresponding to 469 nm emission is shown as an inset of Figure 5. The pattern shows a sharp peak around 356 nm and is due to the charge transfer between Ga3+ and O2- ions. The photoluminescence emission spectrum of the doped ZnGa2O4:Dy3+(2%) nanoparticles is shown in Figure 6. It shows the emission from the host, and superimposed on it are the characteristic fluorescence transitions 4F9/2 f 6H15/2 and 4F9/2 f 6H13/2 of Dy3+ ions at 480 nm and 573 nm respectively. However, the Dy3+ emissions were weak in these doped nanoparticles. The excitation spectrum of the Dy3+doped sample at λem ) 480 nm (shown as inset of Figure 6) consists of a strong broad excitation band extending from 255 to 420 nm with peak maximum at 360 nm and a convolution of several weak lines at 368, 376, 386 and 394 nm in the longer wavelength region, which are due to the f-f transitions of Dy3+ within its 4 f9 ground-state configuration.44 Similar weak lines are also observed in the case of the excitation spectrum recorded at λem ) 573 nm. The origin of the strong band can be correlated taking into consideration the excitation of the host. The excitation spectrum of pure ZnGa2O4 is also composed of a strong band ranging from 320 to 370 nm with a maximum at 356 nm. This indicates that the two strong excitation bands have the same origin, i.e., from the host lattice. The emission spectrum showing a strong broad band from the host and weak peaks of Dy3+ dopants suggests that minimum energy transfer has occurred from the host lattice to the Dy3+ luminescence center. This is in contrast to the results obtained when Dy3+ ions have been



Figure 4. TEM micrograph and SAED pattern of ZnGa2O4:Dy3+(2%):Tb3+(0.5%):Mn2+(2.5%) particles obtained after 4 h of furnace heating at 475 °C.

Figure 5. Emission spectrum of ZnGa2O4 nanoparticles at λexc ) 356 nm. Inset shows the corresponding excitation spectrum.

Figure 6. Emission spectra monitored at λexc ) 360 nm for ZnGa2O4: Dy3+(2%) nanoparticles. Inset shows the excitation spectrum for λem ) 480 nm.

doped in bulk ZnGa2O4.45 No improvement was observed in the emission intensity from Dy3+ ions on changing the dopant concentrations from 1 to 5 atom %. It is known that spectral overlap and distance R significantly affect energy-transfer rate. In the present case, there is only a small spectral overlap between the ZnGa2O4 emission band (Ga being the sensitizer) and the 4 f9 intraconfigurational absorption lines of Dy3+ (being the activator); the latter are forbidden by the parity selection rule. Moreover, the strong absorption band of the activator lies outside

the zinc gallate emission band. This makes the energy transfer by electric multipolar interactions less probable. The observation of low emission intensity from the Dy3+ ions suggests poor energy transfer from host which might have occurred due to exchange interactions. Also due to surface inhomogeneity which is commonly observed in case of nanoparticles, the emission from Dy3+ dopant gets relatively quenched as compared to their bulk counterpart. To evaluate the material performance on color luminescent emission, the Commission International del’Eclairage (CIE) chromaticity coordinates x, y were obtained for the undoped and 2 mol % Dy3+ ion doped ZnGa2O4 nanoparticles. The CIE coordinates of the balanced white light region of the chromaticity diagram lie in the range x ) 0.28 to 0.35, y ) 0.30 to 0.37. For ZnGa2O4 and ZnGa2O4:Dy3+(2%) nanoparticles, the CIE coordinates were found to be x ) 0.211, y ) 0.243 and x ) 0.223, y ) 0.250, respectively. The colors obtained were predominantly blue, which is expected since in both cases the emissions are dominated by that emanating from the host. Nanocrystalline ZnGa2O4 was then codoped with Eu3+ ions with the intention of mixing some red component in the emission. In the case of codoping, the concentration of doping of the activator ion was 2 mol % of Dy3+ and 1 mol %/2 mol % of Eu3+, thus keeping the total doping level up to a maximum of 4 mol % only. The PL spectra of the ZnGa2O4:Eu3+:Dy3+ samples are shown in Figure 7. Strong emissions from Dy3+ at 480 and 573 nm along with a relatively moderate emission at 612 nm from Eu3+ were observed. The emission of Eu3+ is due to the transition between the excited 5D0 state and the 7F2 ground state. Weak emissions were also observed at 586 and 595 nm corresponding to the 5D0 f 7F0 and 5D0 f 7F1 transitions of Eu3+ respectively. The PLE spectra have been monitored at the blue peak of 480 nm, and the red peak of 612 nm consists of a broad band from 250 to 400 nm on which several small peaks can be seen. The band shows a maximum at around 360 nm originating from the host, and the weak peaks are due to the f-f transitions of Dy3+ within its 4f9 ground-state configuration. The excitation spectra monitored at 612 nm show a broad band from 250 to 350 nm, which is ascribed to the charge-transfer band (CTB) of Eu3+-O2- together with absorption of ZnGa2O4 host lattice,

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Figure 7. Emission spectra monitored at λexc ) 340 nm for (a) ZnGa2O4:Dy3+(2%):Eu3+(1%) and (b) ZnGa2O4:Dy3+(2%):Eu3+(2%) nanoparticles.

Figure 8. Emission spectra monitored at λexc ) 340 nm for (a) ZnGa2O4:Dy3+(2%):Tb3+(0.5%):Mn2+(1.5%) and (b) ZnGa2O4:Dy3+(2%): Tb3+(0.5%):Mn2+(2.5%) nanoparticles.

and a number of sharp excitation peaks in the region from 350 to 550 nm which are associated with the f-f transition of Eu3+.46,47 The CTB has a close relationship with the crystal field of the host material. Eu3+ ions occupy the Ga3+ sites in ZnGa2O4 host lattice. The ionic radius of Eu3+ ion is 107 pm, and the ionic radius of Ga3+ ion is only 62 pm. The crystal field may be perturbed by the substitution of Eu3+ ions for the Ga3+ ions since the Oh and the zinc occupied Td sites share common oxygen. On increasing the Eu3+ concentration at fixed Dy3+ concentration from ZnGa2O4:Dy3+(2%):Eu3+(1%) to ZnGa2O4: Dy3+(2%):Eu3+(2%), the CIE coordinates changed negligibly from x ) 0.234, y ) 0.259 to x ) 0.239, y ) 0.264. The corresponding CCT value showed a decrease from 34282 K to 25224 K. Doping of Eu3+ in the ZnGa2O4:Dy3+(2%) matrix leads to a slight decrease in the blue emission intensity and minor increase in the red emission intensity (Figure 7). This behavior suggests negligible energy transfer from the blue emission band to the red absorption bands. To get a white emitting phosphor using zinc gallate as host, it is essential to offset its strong blue emission with an adequate proportion of the other primary colors, i.e., green and red. The dysprosium doping in our ZnGa2O4 nanoparticles gave weak emissions of the Dy3+ dopants in blue and yellow regions. The yellow emission was too weak to balance the strong blue emission emanating from the host. Addition of Eu3+ to ZnGa2O4: Dy3+(2%) shifted the CIE coordinates to some extent, but since the red emission in this case was quite sharp, it was clearly no match for the broad and strong blue emission from ZnGa2O4 nanoparticles. In an effort to get white light from zinc gallate phosphor, we codoped the ZnGa2O4:Dy3+(2%) nanoparticles with Tb3+ and Mn2+ ions. The former is known to give green emission. The choice of Mn2+ was due to the fact that it has been reported to have two independent luminescence channels (at 520 and 650 nm) when doped in spinel compounds like MgAl2O4.48 The luminescence around 520 nm was assigned to transition from the lowest electronic excited state 4T1 to the ground state 6A1 of Mn2+ (3d)5 ion. The broad emission at 650 nm was triggered by the band edge excitation and was assigned similarly to the charge-transfer process associated with the manganese ion. The shorter wavelength emission (520 nm) was induced by the longer wavelength pumping (450 nm) while the longer wavelength emission (650 nm) was brought about by the shorter wavelength pumping (310 nm). The emission spectra obtained from the ZnGa2O4:Dy3+(2%):Tb3+(0.5%):Mn2+(1.5%) and ZnGa2O4:Dy3+(2%):Tb3+(0.5%):Mn2+(2.5%) nanoparticles on excitation at 340 nm are shown in Figure 8. Apart from emissions from the host and Dy3+ ions, strong broad red

emissions from Mn2+ dopants have been observed between 650 to 700 nm. A very weak green emission was also seen as 519 nm which originates from the 4T1 f 6A1 transition of Mn2+. The emission at 544 nm is due to the transition between the excited 5D4 state and the 7F5 ground state of Tb3+ ion. Previous reports on ZnGa2O4:Mn2+ powder samples have shown predominantly green emissions.49,50 It has been reported very recently that when certain trivalent rare-earth ions are codoped in CaGa2S4:Mn2+ as sensitizers, the Mn red emission of the compound is strikingly enhanced.51 The observance of a predominantly red emission from the rare earth codoped ZnGa2O4 nanoparticles in our case can also be explained similarly. On increasing the Mn2+ doping concentration, the red emission intensity showed an increase. The emission process at 650 and 750 nm has been assigned to charge-transfer deexcitation associated with Mn2+ ion.48 Since it is inherently broad in nature, it balances out the strong blue emission of the zinc gallate host. The CIE coordinates for ZnGa2O4:Dy3+(2%): Tb3+(0.5%):Mn2+(1.5%) and ZnGa2O4:Dy3+(2%):Tb3+(0.5%): Mn2+(2.5%) nanoparticles were found to be x ) 0.265, y ) 0.285 and x ) 0.304, y ) 0.310, respectively, with CCT values of 12298 K and 7251 K. The corresponding chromaticity diagram is shown in Figure 9. The ZnGa2O4:Dy3+(2%): Tb3+(0.5%):Mn2+(2.5%) nanophosphor has CIE coordinates in the white light region of the chromaticity diagram. Hence, it has tremendous potential application as a cool white light emitting phosphor for UV LEDs. 3.3. Photoluminescence in Zinc Aluminate Nanophosphors. The PLE spectrum of the Dy3+ doped ZnAl2O4 sample consists of a moderate excitation band extending from 260 to 300 nm with peak maximum at 280 nm from the fundamental band-to-band excitations and a strong broad band with convolution of several lines (at 360, 365, 380 and 390 nm) due to the f-f transitions of Dy3+ within its 4f9 ground-state configuration. The corresponding PL spectrum for the doped samples when excited at 380 nm is shown in Figure 10. The broad emission band in all cases is dominated by two main peaks, the strongest one in the blue region centered at 480 nm (4F9/2 f 6H15/2) and a less strong one in the yellow region centered at 571 nm (4F9/2 f 6H13/2).52 A relatively weak emission at 543 nm due to the transition between the excited 5D4 state and the 7F5 ground state of Tb3+ was also observed. The broad band at ∼506 nm can be attributed to the intrinsic luminescence of the host lattice. The optical properties of the nanomaterials are often influenced by the structure of the matrix and the synthesis technique. In the present case, due to the sonochemical process, defects should be produced inevitably because of the partially incomplete



Figure 9. Chromaticity diagram for (1) ZnGa2O4:Dy3+(2%):Tb3+(0.5%):Mn2+(1.5%) and (2) ZnGa2O4:Dy3+(2%):Tb3+(0.5%):Mn2+(2.5%) nanoparticles.

Figure 10. PL emission spectrum at λexc ) 380 nm for ZnAl2O4: Dy3+(1.9%):Tb3+(0.1%) nanoparticles.

Figure 11. PL emission spectra monitored at λexc ) 380 nm for ZnGa2O4:Dy3+(1.5%):Tb3+(0.25%):Eu3+(0.25%) nanoparticles.

TABLE 2: CIE and CCT Values of ZnAl2O4:Dy3+(1.9%):Tb3+(0.1%) Nanoparticles for Various λexc

TABLE 3: CIE and CCT Values of ZnAl2O4:Dy3+(1.5%):Tb3+(0.25%):Eu3+(0.25%) Nanoparticles for Various λexc

excitation wavelength (nm) 360 380 390

x 0.245 0.248 0.235

y 0.291 0.303 0.310

CCT (K)

excitation wavelength (nm)

x

y

CCT (K)

14806 12867 13991

364 380 396

0.305 0.312 0.259

0.293 0.307 0.272

7475 6689 15329

crystallization. Besides this, the cation disorder in spinel structure is considerable so that there exist a great number of defects, which can serve as electron and/or hole traps.53 The integral intensity of the blue emission (4F9/2-6H15/2) is stronger than that of the yellow emission (4F9/2-6H13/2). This spectral property of Dy3+ provides some information on the site occupation of Dy3+ in the host lattice. It is well-known that the 4 F9/2-6H13/2 yellow emission of Dy3+ is a hypersensitive transition which is strongly influenced by the crystal-field environment.54-56 When Dy3+ is located at a low-symmetry local site (without an inversion symmetry), the yellow emission

dominates the PL spectrum. The blue emission (4F9/2-6H13/2) is stronger than the yellow one when Dy3+ is located at a highsymmetry local site (with inversion symmetry). In the host lattice, Zn2+ ions occupy tetrahedral sites (Td point symmetry without inversion center) coordinated by four oxygen atoms and Ga3+ ions occupy octahedral sites (Oh point symmetry with inversion center) coordinated by six oxygen atoms. In our case it can be seen from Figure 10 that the blue emission dominates, which indicates Dy3+ replacing Ga3+ in the spinel-structured host. Moreover, the ionic radius of Dy3+ (0.0912 nm) is more compatible with Ga3+ (0.062 nm) for six coordination than that for four coordination. Though the ionic radius of Zn2+ is 0.074

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Figure 12. Chromaticity diagram for ZnAl2O4:Dy3+(1.5%):Tb3+(0.25%):Eu3+(0.25%) nanoparticles with λexc A ) 380 nm, B ) 364 nm, C ) 396 nm.

nm for six coordination, Dy3+ ion rarely substitutes at its tetrahedral site due to charge imbalance. This is in contrast to the earlier report on ZnAl2O4:Dy3+ synthesized using combustion technique, where most of the Dy3+ ions were located at the surface of ZnAl2O4 with few of them entering into the lattice.57 The PL emissions of the ZnAl2O4:Dy3+(1.9%): Tb3+(0.1%) nanoparticles were gauged employing the CIE coordinates. The CIE coordinates with corresponding CCT for different λexc are shown in Table 2. It indicates that all the emissions lie predominantly in the greenish blue region. In the case of the ZnAl2O4:Dy3+(1.9%):Tb3+(0.1%) nanoparticles, the color emitted is very cool as indicated by the CCT values. To make it warmer and with CIE coordinates more toward the white light region, nanocrystalline ZnAl2O4:Dy3+:Tb3+ was then codoped with Eu3+ ions with the intention of mixing some red component in the emission, and photoluminescence (PL) studies were done on the sample. ZnAl2O4:Dy3+(1.5%):Tb3+(0.25%):Eu3+(0.25%) nanoparticles were synthesized using the sonochemical technique. The dopant ratios were selected after much deliberation to ensure an overall emission that would yield CIE coordinates near to white light. To offset the predominantly blue emission from the ZnAl2O4: Dy3+(1.9%):Tb3+(0.1%) nanoparticles, Dy3+ concentration was reduced while the Tb3+ concentration was increased to get in more of green. The red component was expected to be obtained using Eu3+ dopant. The total dopant concentration was kept fixed at 2% as in the previous case. The excitation spectrum for the red luminescence of the ZnAl2O4:Dy3+(1.5%):Tb3+(0.25%): Eu3+(0.25%) nanoparticles (the emission wavelength considered was 611 nm) showed a band centered at 287 nm. This band is originated by transitions to the charge-transfer state due to europium-oxygen interactions. A number of sharp excitation peaks in the region from 320 to 400 nm are also observed which are associated with the f-f transition of Eu3+. The PL emission spectrum of ZnAl2O4:Tb3+:Dy3+:Eu3+ sample at λexc of 380 nm is shown in Figure 11. It is possible to distinguish four emission

bands centered at ∼595, 611, 644 and 686 nm which correspond to 5D0 f 7F1, 5D0 f 7F2, 5D0 f 7F3 and 5D0 f 7F4 transitions of Eu3+ ions, respectively. The 5D0 f 7F1 band at 590 nm is a magnetic dipole transition and hardly changes with the crystal field strength around the Eu3+ ion, while the 5D0 f 7F2 band at 617 nm is an electric dipole transition of the 4f shell and is highly sensitive to structural changes and environmental effects in the vicinity of the Eu3+ ion. The fluorescence intensity ratio of 5D0 f 7F2 to 5D0 f 7F1 transitions, generally called the asymmetry ratio, gives a measure of the degree of distortion from the inversion symmetry of the local environment of the Eu3+ ion in the matrix. Therefore, a large asymmetry ratio value indicates strong electric fields of low symmetry at the Eu3+ ions. However, in our case, the asymmetry ration is less than 1.5 and hence does not suggest a large amount of distortion from inversion symmetry as seen in the case of ZnAl2O4:Eu nanorods.58 The emissions from Dy3+ ions centered at 480 nm (4F9/2 f 6H15/2) and a less strong one in the yellow region centered at 571 nm (4F9/2 f 6H13/2) are also clearly discernible. The emission bands observed at 495 and 543 nm are due to the 5D4 f 7F6 and 5D4 f 7F5 transitions of Tb3+ ions. In this case too the broad band at ∼506 nm is due to the intrinsic luminescence of the host lattice. However, its intensity is much lower compared to that observed in the case of ZnAl2O4:Dy3+(1.9%):Tb3+(0.1%) nanoparticles. The color emission of the ZnAl2O4:Dy3+(1.5%): Tb3+(0.25%):Eu3+(0.25%) nanoparticles was evaluated. CIE coordinates with corresponding CCT for different λexc are shown in Table 3. The chromaticity diagram shown in Figure 12 indicates that all the emissions with λexc ) 364 and 380 nm lie predominantly in the white region. From the diagram it is evident that closest to the white light region is the emission obtained from the ZnAl2O4:Dy3+(1.5%):Tb3+(0.25%): Eu3+(0.25%) nanoparticles with λexc ) 380 nm, and its CCT value (6689 K) is also close to that of noon daylight (6500 K, commonly known as D65 point).

ZnAl2O4:M and ZnGa2O4:M Nanophosphors 4. Conclusion An efficient method for the synthesis of doped ZnAl2O4 and ZnGa2O4 nanophosphors using sonochemical technique has been established. Extensive photoluminescence studies were done on numerous single, double and triple doped ZnAl2O4:M (M ) Dy3+, Tb3+ and Eu3+) and ZnGa2O4:M (M ) Dy3+, Tb3+ and Eu3+/Mn2+) nanoparticles of various compositions. Multicolor emissions at selected wavelengths could be obtained by altering the doping concentration in the triple doped sample. Based on the CIE coordinates and the CCT values, it could be inferred that ZnGa2O4:Dy3+(2%):Tb3+(0.5%):Mn2+(2.5%) nanophosphor, when excited at 340 nm, emits light with CIE coordinates of x ) 0.304, y ) 0.310 and a CCT value of 7251 K corresponding to cool white light. For the zinc aluminate host, ZnAl2O4: Dy3+(1.5%):Tb3+(0.25%):Eu3+(0.25%) nanoparticles with λexc ) 380 nm was the best choice. In this case, the CCT value is close to that of noon daylight. For zinc gallate phosphors, the predominant blue emission of the host could be offset only with Mn2+ doping whereas in the case of zinc aluminate, line emissions from Eu3+ dopants were sufficient to give a resultant white light emission. The results thus provide a practical basis for the design of white light illumination sources by blending this kind of phosphor with an appropriate UV emitter. References and Notes (1) Dutta, D. P.; Tyagi, A. K. Solid State Phenom. 2009, 155, 113. (2) Minami, T.; Kuroi, Y.; Miyata, T.; Yamada, H.; Takata, S. J. Lumin. 1997, 72, 997. (3) Kim, J. S.; Lee, S. G.; Park, H. L.; Park, J. Y.; Han, S. D. Mater. Lett. 2004, 58, 1354. (4) Matsui, H.; Xu, C. N.; Tateyama, H. Appl. Phys. Lett. 2001, 78, 1068. (5) Martinez-Sanchez, E.; Garcia-Hipolito, M.; Guzman, J.; RamosBrito, F.; Santoyo Salazar, J.; Martinez-Martinez, R.; Alvarez-Fregoso, O.; Ramos-Cortes, M. I.; Mendez-Delgado, J. J.; Falcony, C. Phys. Status Solidi A 2005, 202, 102. (6) Lou, Z.; Hao, J. Appl. Phys. A: Mater. Sci. Process. 2005, 80, 151. (7) Sampath, S. K.; Cordor, J. F. J. Am. Ceram. Soc. 1998, 81, 649. (8) Zawadzki, M.; Wrzyszcz, J.; Strek, W.; Hreniak, D. J. Alloys Comp. 2001, 323-324, 279. (9) Gu, F.; Wang, S. F.; Lu, M. K.; Qi, Y. X.; Zhou, G. J.; Xu, D.; Yuan, D. R. Opt. Mater. 2004, 25, 59. (10) Lou, Z.; Hao, J. Thin Solid Films 2004, 450, 334. (11) Dieke, G. H. Spectra and Energy LeVels of Rare Earth Ions in Crystals; Crosswhite, H. M., Crosswhite, H., Eds.; Interscience Publishers: New York, 1968. (12) Jeong, I. K.; Park, H. L.; Mho, S. Solid State Commun. 1997, 105, 179. (13) Hong, W. S.; DeJonghe, L. C.; Yang, X.; Rahaman, M. N. J. Am. Ceram. Soc. 1995, 78, 3217. (14) Keller, J. T.; Agrawal, D. K.; McKinstry, H. A. AdV. Ceram. Mater. 1988, 3, 420. (15) Yuan, F. L.; Hu, P.; Yin, C. L.; Huang, S. L.; Li, J. L. J. Mater. Chem. 2003, 13, 634. (16) Valenzuela, M. A.; Jacobs, J. P.; Bosch, P.; Reije, S.; Zapata, B.; Brongersma, H. H. Appl. Catal., A 1997, 148, 315. (17) Zawadzki, M.; Wrzyszcz, J. Mater. Res. Bull. 2000, 35, 109. (18) Mathur, S.; Veith, M.; Haas, M.; Shen, H.; Lecerf, N.; Huch, V.; Hu¨fner, S.; Haberkorn, R.; Beck, H. P.; Jilavi, M. J. Am. Ceram. Soc. 2001, 84, 1921. (19) Wang, Y.; Wu, K. J. Am. Chem. Soc. 2005, 127, 9686. (20) Xu, Z.; Li, Y.; Liu, Z.; Xiong, Z. Mater. Sci. Eng. B 2004, 110, 3002. (21) Barros, B. S.; Costa, A. C. F. M.; Kiminami, R. H. A. G.; Gama, L. J. Metastable Nanocryst. Mater. 2004, 20-21, 325.

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