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Tunable optical properties of multiphase ZnO–V2O5 polycrystalline powders
Scripta Materialia xxx (2015) xxx–xxx
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Tunable optical properties of multiphase ZnO–V2O5 polycrystalline powders E. Senthil Kumar a,b,⇑, F. Bellarmine a,1, M. Ramanjaneyulu a,1, Divya Deepthi a, M.S. Ramachandra Rao a,⇑ a b
Department of Physics, Nano Functional Materials Technology Centre and Materials Science Research Centre, Indian Institute of Technology Madras, Chennai 600036, India Department of Materials Science and Engineering, Yonsei University, Seoul 120-749, Republic of Korea
a r t i c l e
i n f o
Article history: Received 24 April 2015 Revised 19 May 2015 Accepted 19 May 2015 Available online xxxx Keywords: Zinc oxide Optical properties Zinc vanadate White emission
a b s t r a c t Bright yellow and white-yellow light emitting ZnO–V2O5 polycrystalline powders are synthesized by engineering zinc oxide and different zinc vanadate phases. Emission characteristics strongly depend on different zinc vanadate phases. The bright yellow emission is originated from the 3T2 ? 1A1 and 3 T1 ? 1A1 transitions in the VO4 tetrahedra of zinc vanadate phase. White-yellow emission is the combination of blue and green emissions due to native point defects of ZnO and the yellow and red emission from zinc vanadate. Ó 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Design and synthesis of ZnO in bulk, thin films and nanostructures form have been of increasing interest due to its excellent optical properties in the near UV and visible region [1–3]. It is well known that non-stoichiometric ZnO exhibits violet, blue and green luminescence due to native point defects, such as zinc interstitials (Zni), zinc vacancies (Znv) and oxygen vacancies (Ov) respectively [4–8]. The luminescent properties of ZnO can be engineered in various ways like doping, alloying and/or by growing nanostructures with controlled defects [9–11]. For example, Eu doped ZnO nanoparticles showed interesting white light emission [12]. The exciting white emission in this system was attributed to the combined luminescence of ZnO and intra-4f transitions of Eu3+ ions [13]. Nanostructures of ZnO–Al2O3 and ZnO–SiO2 nanocomposites have shown white light emission and were mainly attributed to the oxygen defects (Ov) at the surface of the ZnO nanostructures [14,15]. Hybrid nanostructures of polyvinyl alcohol/ZnO have also shown white light emission at room temperature [16]. Mordkovich et al. have shown bright luminescent phases in ZnO:W, ZnO:V, ZnO:(W, Mg) and ZnO:(Y, Eu) binary and ternary systems grown by combinatorial pulsed laser deposition methodology [17]. In ZnO–V2O5 system, different vanadate phases, viz., ZnV2O6, Zn2V2O7, Zn3V2O8 and Zn4V2O9, give fascinating broad band emissions from green to yellow, due to the charge transfer (CT) of an ⇑ Corresponding authors. E-mail addresses: [email protected] (E. Senthil Kumar), [email protected] (M.S. Ramachandra Rao). 1 Equal contribution.
electron from the oxygen 2p orbital to the vacant 3d orbital of V5+ in tetrahedral VO4 with Td symmetry [18]. In particular, Zn2V2O7 and Zn3V2O8 have characteristic crystal structures with dimerized and isolated VO4 tetrahedra respectively that show interesting luminescent properties [17,18]. Hence it is possible to tune the luminescence characteristics of the zinc vanadate phases by mixing with blue and green emitting ZnO to produce white light emission. We hardly find any report on the luminescent properties of ZnO–V2O5 based phosphors in polycrystalline powder form that can be used with blue LEDs for white light emission. Hence in this paper we have made a systematic attempt to develop near UV excitable bright yellow and white-yellow light emitting ZnO–V2O5 multiphase system, which can be used for white LEDs applications in near future. (1 x) ZnO–xV2O5 based polycrystalline powders were synthesized using conventional solid-state reaction method. The V2O5 mole fraction, x, in the polycrystalline samples was varied from 10, 15, 20, 25, 28, 30 to 33 mol% and are denoted as ZV10, ZV15, ZV20, ZV25, ZV28, ZV30 and ZV33 respectively. For example ZV33 represents ZnO–V2O5 system with V2O5 mole fraction of 0.33. Stoichiometric amounts of ZnO and V2O5 were ground thoroughly and heated at 500 °C for 12 h in air in alumina crucibles. Then the powder samples were ground again and heated at 650 °C for 12 h. Presence of different crystalline phases in the as prepared samples was identified by X-ray diffraction (XRD, PANalytical) using Cu Ka radiation (1.5418 Å). Diffuse reflectance spectra (DRS) of the as synthesized powders were measured with Ocean optics (USB 2000) UV–Vis spectrometer using Ba2SO4 as a
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Fig. 1. XRD patterns of ZnO–V2O5 polycrystalline powders with V concentration varying from 10 to 33 mol%. Solid squares and circles represent Zn2V2O7 and Zn3V2O8 phases respectively. ZnO phase is indicated by the letter ‘‘z’’.
standard reference. Room temperature photoluminescence (PL) spectra of the samples were recorded using HORIBA JOBIN-YVON (Fluorolog-3-11) spectrofluorometer with xenon lamp (450 W) as the excitation source. The PL spectra were recorded at different excitation wavelengths. Microstructure of the polycrystalline powders were recorded using high resolution scanning electron microscope, (HRSEM: FEI, Quant 400). Chemical compositions of the powder samples were analyzed using energy dispersive X-ray spectrometry (EDS). Fig. 1 shows XRD patterns of the ZnO–V2O5 polycrystalline powder samples with V concentration varying from 10 to 33 mol%. Peaks corresponding to wurtzite ZnO are clearly observed for the samples with V concentration 628 mol%. At lower V concentration, ZnO phase is dominant with three prominent peaks; (1 0 0), (0 0 2) and (1 0 1). As the V concentration increases, intensity
of the ZnO phase diminishes and completely disappears for ZV33. Additional reflections corresponding to other phases are also observed in all the samples. A careful analysis of the XRD patterns reveals that the additional peaks belong to different zinc vanadate phases. All the ZnO–V2O5 samples show mainly two zinc vanadate phases Zn2V2O7 and Zn3V2O8. In the XRD spectra, solid squares and solid circles represent Zn2V2O7 and Zn3V2O8 phases respectively. ZnO phase is represented by the symbol ‘‘z’’. The Zn2V2O7 crystallizes into monoclinic structure and the Zn3V2O8 has orthorhombic crystal structure. The VO4 tetrahedra in Zn2V2O7 are linked through corner oxygen atom forming dimers. On the other hand, the VO4 tetrahedra are isolated in Zn3V2O8 phase [15]. At lower V concentration (<20 mol%), the Zn3V2O8 phase dominates along with ZnO and only minor peaks corresponding to Zn2V2O7 are visible. However above 28 mol% of V concentration, the presence of ZnO and Zn3V2O8 phases are minimal and only Zn2V2O7 phase is found to dominate. An interesting observation is that for ZV28 sample, which later showed broad white-yellow luminescence, both the Zn2V2O7 and Zn3V2O8 phases are dominant. The HRSEM analysis reveals that the ZnO–V2O5 polycrystalline powders exhibit assorted micro and nanostructured morphologies. These structures consist of rods, discs with hexagonal symmetry and some irregular structured particles. Fig. 2(a–e) shows the effect of V concentration on the morphology of the ZnO–V2O5 system. Polar nature of ZnO allows the particle to grow along c-axis (0 0 0 1) with a rod like features with a length of 1–2 lm. With increase in V concentration beyond 20 mol%, no hexagonal symmetry is observed in the particles and the samples exhibit only some polyhedral and irregular agglomerated morphology (Fig. 2d and f). This is in agreement with XRD, which showed no detectable hexagonal ZnO phase in the sample ZV33. Composition analysis of different elements like Zn, V and O was carried out using energy dispersive spectroscopy (EDS). All the samples showed reduction in overall V concentration compared to initial value. We found that the Zn/V ratio decreases as the V concentration increases. However, at higher V concentrations (P28%), we have observed more V loss in the samples. Fig. 2f shows the example EDS
Fig. 2. High-magnification HRSEM images of the ZnO–V2O5 powders (a) ZV10, (b) ZV15, (c) ZV20, (d) ZV28 and (e) ZV33. (f) Energy dispersive spectrum of the sample ZV28.
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Fig. 3. (a) Room temperature DR spectra of the ZnO–V2O5 samples showing the band edge around 375 nm. It is evident from the figure that the absorption edge of ZV33 is around 418 nm. (b) Plot of the derivative of DR intensity dIR/k vs wavelength (k) showing the absorption edge of the samples, and the inset shows the magnified view of the absorption edge.
Fig. 4. Room temperature PL spectra of the ZnO–V2O5 samples excited at (a) 325 nm (b) 365 nm (c) 385 nm (d) 395 nm. (e) De-convoluted PL spectrum of ZV28, excited at 365 nm, showing emission peaks at blue, green, yellow and red region. (f) Schematics of the one electron charge transfer in VO4 tetrahedra in a vanadate phase depicting 3 T2 ? 1A1 and 3T1 ? 1A1 transitions that causes yellow and red emissions respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
spectrum of the ZV28 sample. For ZV28, the observed atomic percent of Zn, V and O are 28.74%, 18.55% and 52.71%. Fig. 3a shows the diffuse reflectance (DR) spectra of the unintentionally doped ZnO (reference) and the as prepared ZnO–V2O5 samples. Prior to the measurement, the DR intensity was calibrated to 100% reflection for Barium sulfate (Ba2SO4) reference sample. All
the spectra were recorded in the range of 250–800 nm. A sharp fall in DR intensity (IR) around the optical band gap is observed only for the reference ZnO. All other samples showed broadening at the optical band gap revealing the heterogeneous nature. There is an onset of absorption around 418 nm in the spectra for V concentration P20 mol%. The red circle indicates the absorption edges
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of zinc vanadate phases. The arrow around 560 nm indicates the d– d transitions of the V2+ ion in the tetrahedral crystal field [18,19]. Fig. 3b shows plots of the derivative of DR intensity, dIR/k, versus wavelength (k) of ZnO–V2O5 phases that clearly show that the absorption edges become broader with increase in V concentration. As the V concentration increases, a slight blue shift, from 384 nm (pure ZnO) to 376 nm (ZV25), is observed for these samples (see inset of Fig. 3b). This reveals that a significant amount of V is doped into ZnO lattice. Small peak around 418 nm, for V concentration P25 mol%, corresponds to the absorption edge of zinc vanadate phases [17,18]. Sample ZV33 shows the absorption edge only at 418 nm and no trace of ZnO absorption edge was found. This is in good agreement with XRD spectra, which showed no evidence of ZnO phase for ZV33 sample. Fig. 4a–d shows the normalized room temperature PL spectra of the ZnO–V2O5 samples recorded with different excitation wavelengths in the range of 325–395 nm. An intense visible emission is observed only for the excitation wavelengths of 325 nm and 365 nm. No visible emission is observed for the excitation wavelengths higher than 395 nm. Interestingly, we have observed that the peak position and integrated PL intensity in the visible region is a characteristic of the V concentration and type of zinc vanadate phases present in the ZnO–V2O5 system. Moreover, the emission can be tuned from bright yellow to whitish-yellow by carefully controlling the ratio (ZnO:Zn2V2O7:Zn3V2O8) of vanadate phases in ZnO–V2O5 system. For example, samples with low V concentrations (V 6 15%) exhibit relatively narrow visible emission spectrum. On the other hand, samples with V P 30% exhibited strong yellow emission. A very broad white-yellow emission is observed only for the V concentrations 20 mol% and 28 mol%. In particular, the integrated PL intensity of the sample ZV28 is higher for all the excitation wavelengths. It is worth noting that the luminescent characteristic of the sample ZV28 covers the entire visible region compared with other samples. For this sample, we have observed that both Zn2V2O7 and Zn3V2O8 phases show nearly equal amounts in quantity along with ZnO phase. Hence 28 mol% of V2O5 in ZnO is the optimum composition for white-yellow emission at the excitation wavelengths of 325 nm and 365 nm. Other concentrations like 30 and 33 mol% of V2O5 generally give bright yellow emission. Fig. 4e shows the room temperature PL spectrum of the ZV28 excited at 365 nm. In order to further understand the emission spectrum of the sample ZV28, we have de-convoluted the visible region. The de-convoluted visible region of the emission spectrum mainly consists of four peaks centered around 480 nm, 530 nm, 592 nm and 649 nm whose full width at half maxima (FWHM) are 55 nm, 67 nm, 75 nm and 94 nm respectively. The integrated PL intensity ratio of the blue:green:yellow:red peaks are 0.53:1.00:0.90:0.76. This clearly reveals that the multiple emission centers present in the system. It is well known that native point defects in ZnO produce distinct visible luminescence peaks. The very common point defect in ZnO is zinc interstitials (Zni), which also has very low formation energy. Energy levels of the Zni are very close to the conduction band. In contrast to Zni, zinc vacancies (Znv) form acceptor levels close to valence band. Hence the blue emission peak around 480 nm is a result of the radiative electronic transition from Zni to Znv [4]. The green emission around 530 nm is attributed to the transition from conduction band edge to deep levels associated with oxygen vacancies (Ov) [4,6–8]. Generally the characteristic luminescence of the vanadate phosphors is attributed to the 3T2 ? 1A1 and 3T1 ? 1A1 transitions in the VO4 tetrahedra as depicted in Fig. 4f. The emission at 592 nm
corresponds to the 3T2 ? 1A1 and 649 nm can be attributed to the 3T1 ? 1A1 transition [17,18,20]. Hence, the observed strong white-yellow luminesce is a combination of emission peaks due to native point defects of ZnO and characteristic transitions of zinc vanadate phases. These results suggest that a near UV excitable strong white-yellow emission can be obtained by carefully optimizing the different zinc vanadate and ZnO phases in a multiphase polycrystalline powder. In summary, luminescent characteristics of the multiphase ZnO–V2O5 polycrystalline powders are studied. The luminescence intensity and the emission wavelengths were correlated with the type of zinc vanadate phase present in the system. We have demonstrated that for efficient broad white-yellow emission both the vanadate phases (Zn2V2O7 and Zn3V2O8) should be present in equal proportions along with ZnO. This method suggests a processing methodology for the synthesis of near UV excitable white light emitting phosphors by carefully controlling the concentration of different zinc vanadate phases in the system. Our results reveal that characteristic yellow and red emission peaks of zinc vanadate phases along with the blue and green emission associated with native point defects of ZnO are responsible for the strong white-yellow emission. These materials can find potential applications in phosphor converted white LEDs. The authors acknowledge Sophisticated Analytical Instrument Facility (SAIF), IIT Madras for the PL measurement. We also acknowledge Department of Science and Technology (DST), India for the financial support [Grant Nos: SR/NM/NAT-02/2005, SR/S2/CMP-0076/2010] for the establishment of Nano Functional Materials Technology Centre (NFMTC) and PL facility in IIT Madras, and for the Indo-Japan (DST-JSPS) joint research project [Grant No: DST/INT/JAP/P-47/08].
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Scripta Materialia journal homepage: www.elsevier.com/locate/scriptamat
Tunable optical properties of multiphase ZnO–V2O5 polycrystalline powders E. Senthil Kumar a,b,⇑, F. Bellarmine a,1, M. Ramanjaneyulu a,1, Divya Deepthi a, M.S. Ramachandra Rao a,⇑ a b
Department of Physics, Nano Functional Materials Technology Centre and Materials Science Research Centre, Indian Institute of Technology Madras, Chennai 600036, India Department of Materials Science and Engineering, Yonsei University, Seoul 120-749, Republic of Korea
a r t i c l e
i n f o
Article history: Received 24 April 2015 Revised 19 May 2015 Accepted 19 May 2015 Available online xxxx Keywords: Zinc oxide Optical properties Zinc vanadate White emission
a b s t r a c t Bright yellow and white-yellow light emitting ZnO–V2O5 polycrystalline powders are synthesized by engineering zinc oxide and different zinc vanadate phases. Emission characteristics strongly depend on different zinc vanadate phases. The bright yellow emission is originated from the 3T2 ? 1A1 and 3 T1 ? 1A1 transitions in the VO4 tetrahedra of zinc vanadate phase. White-yellow emission is the combination of blue and green emissions due to native point defects of ZnO and the yellow and red emission from zinc vanadate. Ó 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Design and synthesis of ZnO in bulk, thin films and nanostructures form have been of increasing interest due to its excellent optical properties in the near UV and visible region [1–3]. It is well known that non-stoichiometric ZnO exhibits violet, blue and green luminescence due to native point defects, such as zinc interstitials (Zni), zinc vacancies (Znv) and oxygen vacancies (Ov) respectively [4–8]. The luminescent properties of ZnO can be engineered in various ways like doping, alloying and/or by growing nanostructures with controlled defects [9–11]. For example, Eu doped ZnO nanoparticles showed interesting white light emission [12]. The exciting white emission in this system was attributed to the combined luminescence of ZnO and intra-4f transitions of Eu3+ ions [13]. Nanostructures of ZnO–Al2O3 and ZnO–SiO2 nanocomposites have shown white light emission and were mainly attributed to the oxygen defects (Ov) at the surface of the ZnO nanostructures [14,15]. Hybrid nanostructures of polyvinyl alcohol/ZnO have also shown white light emission at room temperature [16]. Mordkovich et al. have shown bright luminescent phases in ZnO:W, ZnO:V, ZnO:(W, Mg) and ZnO:(Y, Eu) binary and ternary systems grown by combinatorial pulsed laser deposition methodology [17]. In ZnO–V2O5 system, different vanadate phases, viz., ZnV2O6, Zn2V2O7, Zn3V2O8 and Zn4V2O9, give fascinating broad band emissions from green to yellow, due to the charge transfer (CT) of an ⇑ Corresponding authors. E-mail addresses: [email protected] (E. Senthil Kumar), [email protected] (M.S. Ramachandra Rao). 1 Equal contribution.
electron from the oxygen 2p orbital to the vacant 3d orbital of V5+ in tetrahedral VO4 with Td symmetry [18]. In particular, Zn2V2O7 and Zn3V2O8 have characteristic crystal structures with dimerized and isolated VO4 tetrahedra respectively that show interesting luminescent properties [17,18]. Hence it is possible to tune the luminescence characteristics of the zinc vanadate phases by mixing with blue and green emitting ZnO to produce white light emission. We hardly find any report on the luminescent properties of ZnO–V2O5 based phosphors in polycrystalline powder form that can be used with blue LEDs for white light emission. Hence in this paper we have made a systematic attempt to develop near UV excitable bright yellow and white-yellow light emitting ZnO–V2O5 multiphase system, which can be used for white LEDs applications in near future. (1 x) ZnO–xV2O5 based polycrystalline powders were synthesized using conventional solid-state reaction method. The V2O5 mole fraction, x, in the polycrystalline samples was varied from 10, 15, 20, 25, 28, 30 to 33 mol% and are denoted as ZV10, ZV15, ZV20, ZV25, ZV28, ZV30 and ZV33 respectively. For example ZV33 represents ZnO–V2O5 system with V2O5 mole fraction of 0.33. Stoichiometric amounts of ZnO and V2O5 were ground thoroughly and heated at 500 °C for 12 h in air in alumina crucibles. Then the powder samples were ground again and heated at 650 °C for 12 h. Presence of different crystalline phases in the as prepared samples was identified by X-ray diffraction (XRD, PANalytical) using Cu Ka radiation (1.5418 Å). Diffuse reflectance spectra (DRS) of the as synthesized powders were measured with Ocean optics (USB 2000) UV–Vis spectrometer using Ba2SO4 as a
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Fig. 1. XRD patterns of ZnO–V2O5 polycrystalline powders with V concentration varying from 10 to 33 mol%. Solid squares and circles represent Zn2V2O7 and Zn3V2O8 phases respectively. ZnO phase is indicated by the letter ‘‘z’’.
standard reference. Room temperature photoluminescence (PL) spectra of the samples were recorded using HORIBA JOBIN-YVON (Fluorolog-3-11) spectrofluorometer with xenon lamp (450 W) as the excitation source. The PL spectra were recorded at different excitation wavelengths. Microstructure of the polycrystalline powders were recorded using high resolution scanning electron microscope, (HRSEM: FEI, Quant 400). Chemical compositions of the powder samples were analyzed using energy dispersive X-ray spectrometry (EDS). Fig. 1 shows XRD patterns of the ZnO–V2O5 polycrystalline powder samples with V concentration varying from 10 to 33 mol%. Peaks corresponding to wurtzite ZnO are clearly observed for the samples with V concentration 628 mol%. At lower V concentration, ZnO phase is dominant with three prominent peaks; (1 0 0), (0 0 2) and (1 0 1). As the V concentration increases, intensity
of the ZnO phase diminishes and completely disappears for ZV33. Additional reflections corresponding to other phases are also observed in all the samples. A careful analysis of the XRD patterns reveals that the additional peaks belong to different zinc vanadate phases. All the ZnO–V2O5 samples show mainly two zinc vanadate phases Zn2V2O7 and Zn3V2O8. In the XRD spectra, solid squares and solid circles represent Zn2V2O7 and Zn3V2O8 phases respectively. ZnO phase is represented by the symbol ‘‘z’’. The Zn2V2O7 crystallizes into monoclinic structure and the Zn3V2O8 has orthorhombic crystal structure. The VO4 tetrahedra in Zn2V2O7 are linked through corner oxygen atom forming dimers. On the other hand, the VO4 tetrahedra are isolated in Zn3V2O8 phase [15]. At lower V concentration (<20 mol%), the Zn3V2O8 phase dominates along with ZnO and only minor peaks corresponding to Zn2V2O7 are visible. However above 28 mol% of V concentration, the presence of ZnO and Zn3V2O8 phases are minimal and only Zn2V2O7 phase is found to dominate. An interesting observation is that for ZV28 sample, which later showed broad white-yellow luminescence, both the Zn2V2O7 and Zn3V2O8 phases are dominant. The HRSEM analysis reveals that the ZnO–V2O5 polycrystalline powders exhibit assorted micro and nanostructured morphologies. These structures consist of rods, discs with hexagonal symmetry and some irregular structured particles. Fig. 2(a–e) shows the effect of V concentration on the morphology of the ZnO–V2O5 system. Polar nature of ZnO allows the particle to grow along c-axis (0 0 0 1) with a rod like features with a length of 1–2 lm. With increase in V concentration beyond 20 mol%, no hexagonal symmetry is observed in the particles and the samples exhibit only some polyhedral and irregular agglomerated morphology (Fig. 2d and f). This is in agreement with XRD, which showed no detectable hexagonal ZnO phase in the sample ZV33. Composition analysis of different elements like Zn, V and O was carried out using energy dispersive spectroscopy (EDS). All the samples showed reduction in overall V concentration compared to initial value. We found that the Zn/V ratio decreases as the V concentration increases. However, at higher V concentrations (P28%), we have observed more V loss in the samples. Fig. 2f shows the example EDS
Fig. 2. High-magnification HRSEM images of the ZnO–V2O5 powders (a) ZV10, (b) ZV15, (c) ZV20, (d) ZV28 and (e) ZV33. (f) Energy dispersive spectrum of the sample ZV28.
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Fig. 3. (a) Room temperature DR spectra of the ZnO–V2O5 samples showing the band edge around 375 nm. It is evident from the figure that the absorption edge of ZV33 is around 418 nm. (b) Plot of the derivative of DR intensity dIR/k vs wavelength (k) showing the absorption edge of the samples, and the inset shows the magnified view of the absorption edge.
Fig. 4. Room temperature PL spectra of the ZnO–V2O5 samples excited at (a) 325 nm (b) 365 nm (c) 385 nm (d) 395 nm. (e) De-convoluted PL spectrum of ZV28, excited at 365 nm, showing emission peaks at blue, green, yellow and red region. (f) Schematics of the one electron charge transfer in VO4 tetrahedra in a vanadate phase depicting 3 T2 ? 1A1 and 3T1 ? 1A1 transitions that causes yellow and red emissions respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
spectrum of the ZV28 sample. For ZV28, the observed atomic percent of Zn, V and O are 28.74%, 18.55% and 52.71%. Fig. 3a shows the diffuse reflectance (DR) spectra of the unintentionally doped ZnO (reference) and the as prepared ZnO–V2O5 samples. Prior to the measurement, the DR intensity was calibrated to 100% reflection for Barium sulfate (Ba2SO4) reference sample. All
the spectra were recorded in the range of 250–800 nm. A sharp fall in DR intensity (IR) around the optical band gap is observed only for the reference ZnO. All other samples showed broadening at the optical band gap revealing the heterogeneous nature. There is an onset of absorption around 418 nm in the spectra for V concentration P20 mol%. The red circle indicates the absorption edges
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of zinc vanadate phases. The arrow around 560 nm indicates the d– d transitions of the V2+ ion in the tetrahedral crystal field [18,19]. Fig. 3b shows plots of the derivative of DR intensity, dIR/k, versus wavelength (k) of ZnO–V2O5 phases that clearly show that the absorption edges become broader with increase in V concentration. As the V concentration increases, a slight blue shift, from 384 nm (pure ZnO) to 376 nm (ZV25), is observed for these samples (see inset of Fig. 3b). This reveals that a significant amount of V is doped into ZnO lattice. Small peak around 418 nm, for V concentration P25 mol%, corresponds to the absorption edge of zinc vanadate phases [17,18]. Sample ZV33 shows the absorption edge only at 418 nm and no trace of ZnO absorption edge was found. This is in good agreement with XRD spectra, which showed no evidence of ZnO phase for ZV33 sample. Fig. 4a–d shows the normalized room temperature PL spectra of the ZnO–V2O5 samples recorded with different excitation wavelengths in the range of 325–395 nm. An intense visible emission is observed only for the excitation wavelengths of 325 nm and 365 nm. No visible emission is observed for the excitation wavelengths higher than 395 nm. Interestingly, we have observed that the peak position and integrated PL intensity in the visible region is a characteristic of the V concentration and type of zinc vanadate phases present in the ZnO–V2O5 system. Moreover, the emission can be tuned from bright yellow to whitish-yellow by carefully controlling the ratio (ZnO:Zn2V2O7:Zn3V2O8) of vanadate phases in ZnO–V2O5 system. For example, samples with low V concentrations (V 6 15%) exhibit relatively narrow visible emission spectrum. On the other hand, samples with V P 30% exhibited strong yellow emission. A very broad white-yellow emission is observed only for the V concentrations 20 mol% and 28 mol%. In particular, the integrated PL intensity of the sample ZV28 is higher for all the excitation wavelengths. It is worth noting that the luminescent characteristic of the sample ZV28 covers the entire visible region compared with other samples. For this sample, we have observed that both Zn2V2O7 and Zn3V2O8 phases show nearly equal amounts in quantity along with ZnO phase. Hence 28 mol% of V2O5 in ZnO is the optimum composition for white-yellow emission at the excitation wavelengths of 325 nm and 365 nm. Other concentrations like 30 and 33 mol% of V2O5 generally give bright yellow emission. Fig. 4e shows the room temperature PL spectrum of the ZV28 excited at 365 nm. In order to further understand the emission spectrum of the sample ZV28, we have de-convoluted the visible region. The de-convoluted visible region of the emission spectrum mainly consists of four peaks centered around 480 nm, 530 nm, 592 nm and 649 nm whose full width at half maxima (FWHM) are 55 nm, 67 nm, 75 nm and 94 nm respectively. The integrated PL intensity ratio of the blue:green:yellow:red peaks are 0.53:1.00:0.90:0.76. This clearly reveals that the multiple emission centers present in the system. It is well known that native point defects in ZnO produce distinct visible luminescence peaks. The very common point defect in ZnO is zinc interstitials (Zni), which also has very low formation energy. Energy levels of the Zni are very close to the conduction band. In contrast to Zni, zinc vacancies (Znv) form acceptor levels close to valence band. Hence the blue emission peak around 480 nm is a result of the radiative electronic transition from Zni to Znv [4]. The green emission around 530 nm is attributed to the transition from conduction band edge to deep levels associated with oxygen vacancies (Ov) [4,6–8]. Generally the characteristic luminescence of the vanadate phosphors is attributed to the 3T2 ? 1A1 and 3T1 ? 1A1 transitions in the VO4 tetrahedra as depicted in Fig. 4f. The emission at 592 nm
corresponds to the 3T2 ? 1A1 and 649 nm can be attributed to the 3T1 ? 1A1 transition [17,18,20]. Hence, the observed strong white-yellow luminesce is a combination of emission peaks due to native point defects of ZnO and characteristic transitions of zinc vanadate phases. These results suggest that a near UV excitable strong white-yellow emission can be obtained by carefully optimizing the different zinc vanadate and ZnO phases in a multiphase polycrystalline powder. In summary, luminescent characteristics of the multiphase ZnO–V2O5 polycrystalline powders are studied. The luminescence intensity and the emission wavelengths were correlated with the type of zinc vanadate phase present in the system. We have demonstrated that for efficient broad white-yellow emission both the vanadate phases (Zn2V2O7 and Zn3V2O8) should be present in equal proportions along with ZnO. This method suggests a processing methodology for the synthesis of near UV excitable white light emitting phosphors by carefully controlling the concentration of different zinc vanadate phases in the system. Our results reveal that characteristic yellow and red emission peaks of zinc vanadate phases along with the blue and green emission associated with native point defects of ZnO are responsible for the strong white-yellow emission. These materials can find potential applications in phosphor converted white LEDs. The authors acknowledge Sophisticated Analytical Instrument Facility (SAIF), IIT Madras for the PL measurement. We also acknowledge Department of Science and Technology (DST), India for the financial support [Grant Nos: SR/NM/NAT-02/2005, SR/S2/CMP-0076/2010] for the establishment of Nano Functional Materials Technology Centre (NFMTC) and PL facility in IIT Madras, and for the Indo-Japan (DST-JSPS) joint research project [Grant No: DST/INT/JAP/P-47/08].
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Please cite this article in press as: E. Senthil Kumar et al., Scripta Mater. (2015), http://dx.doi.org/10.1016/j.scriptamat.2015.05.042