Spectral properties of Dy3+ doped ZnAl2O4 phosphor

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Vinay Kumar. 1. , R. J. Choudhary. 2 and D. M. Phase. 2. 1Department of Physics, Shri Mata Vaishno Devi University, Katra-182320 (J&K) India. 2UGC DAE ...

Spectral properties of Dy

3+

doped ZnAl2O4 phosphor

Ram Prakash, Sandeep Kumar, Rubby Mahajan, Pooja Khajuria, Vinay Kumar, R. J. Choudhary, and D. M. Phase

Citation: AIP Conference Proceedings 1953, 030040 (2018); doi: 10.1063/1.5032375 View online: https://doi.org/10.1063/1.5032375 View Table of Contents: http://aip.scitation.org/toc/apc/1953/1 Published by the American Institute of Physics

Spectral Properties Of Dy3+ Doped ZnAl2O4 Phosphor Ram Prakash1, a), Sandeep Kumar1, Rubby Mahajan1, Pooja Khajuria1, Vinay Kumar1, R. J. Choudhary2 and D. M. Phase2 1

Department of Physics, Shri Mata Vaishno Devi University, Katra-182320 (J&K) India 2 UGC DAE Consortium for Scientific Research, Indore-452001, India a) Corresponding author: [email protected]; [email protected]

Abstract. Herein, Dy3+ doped ZnAl2O4 phosphor was synthesized by the solution combustion method. The synthesized phosphor was characterized by X-ray diffraction (XRD), photoluminescence (PL) spectroscopy, UV-Vis spectroscopy and X-ray photoelectron spectroscopy (XPS). The phase purity of the phosphor was confirmed by the XRD studies that showed cubic symmetry of the synthesized phosphor. Under UV excitation (388 nm) the PL emission spectrum of the phosphor shows characteristic transition from the Dy 3+ ion. A band gap of 5.2 eV was estimated from the diffused reflectance spectroscopy. The surface properties of the phosphor were studied using the X-ray photoelectron spectroscopy.

INTRODUCTION The synthesis and characterization of rare earth ion doped materials have become an interesting area for the material scientists in discovering the new luminescent materials and exploring their applications in the white light emitting diodes (LEDs), solar cells, solid state lasers, lamps, etc [1-4]. Two methods are known for producing white light first one is by flashing the light from blue LED on yellow light emitting phosphor while in the second case primary tricolor (red, green and blue) phosphor are excited by near UV-LED chips [5]. These methods have certain shortcomings such as high color tolerance and low color rendering index (CRI). Thus, in order to overcome these issues, different host materials doped with rare earth (RE) ions have been studied to explore novel phosphors for white LED [5]. In this context, the photoluminescence and optical properties of Dy3+ doped zinc aluminate (ZnAl2O4) phosphor have been studied. ZnAl2O4 is a well known semiconductor with wide band gap that belongs to a class of mixed-metal oxides knows as spinels (AB2O4) where A and B are divalent and trivalent cations. ZnAl 2O4 crystallize in either cubic normal or inverse spinel structure depending on the method of preparation. It is used in numerous applications sensor technology, optoelectronics, information display technology because of optical and hydrophobic properties along with high chemical and thermal stability [6-9]. Earlier Tshabalala et al. [7] have studied the luminescent properties of ZnAl2O4:Ce3+, Tb3+ phosphor.

EXPERIMENTAL Dy3+ doped ZnAl2O4 phosphors (0.5 mol. %) were prepared by the solution combustion method at 570 ⁰C by taking aluminium nitrate nonahydrate [Al (NO3)3.9H2O], zinc nitrate hexahydrate [Zn (NO3)2.6H2O], urea (CH4N2O) and dysprosium oxide (Dy2O3) from Merck Chemicals, India. The precursors were weighed according to the balanced chemical reaction given below in Eq. (1). The synthesis method used for synthesis is same as reported in our previous reports [10-11].

2nd International Conference on Condensed Matter and Applied Physics (ICC 2017) AIP Conf. Proc. 1953, 030040-1–030040-4; https://doi.org/10.1063/1.5032375 Published by AIP Publishing. 978-0-7354-1648-2/$30.00

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The crystalline nature of the final product was examined by X-ray diffraction (XRD) measurements using a standard diffractometer (Bruker D2 Phaser). The spectral features like photoluminescence excitation and emission (in phosphorescence mode) spectra were measured using a Cary-Eclipse spectrofluorometer (Agilent) equipped with a 150 Watt Xenon lamp as an excitation source. The diffused reflectance spectrum was recorded using integrating sphere assembly attached with Shimadzu UV-2600 Double beam spectrophotometer in the spectral region 190-1400 nm. The X-ray photoelectron spectroscopy (XPS) measurement was performed using Omicron energy analyzer (EA125) with Al Kα (1486.6 eV) X-ray source. All these characterizations were carried out at room temperature.

RESULTS AND DISCUSSION X-ray Diffraction The XRD patterns of the Dy3+ doped ZnAl2O4 nanophosphor along with the standard JCPDS data are shown in Fig. 1. All the XRD peaks are well matched with the standard pattern (JCPDS card no. 821403) that belongs to the cubic phase of ZnAl2O4. The diffraction peak observed at ~34.5 ° is attributed to impurities such as aluminum oxides, hydroxides and oxyhydroxides [7]

3+

ZnAl2O4:Dy

JCPDS 821043

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2 Theta (degree) FIGURE 1. XRD patterns of Dy3+ doped ZnAl2O4 (0.5 mol. %) phosphor.

Spectral Studies Fig. 2 (a) and 2(b) depicts the photoluminescence excitation (PLE) and emission spectra of Dy 3+ doped ZnAl2O4 phosphor at room temperature. The excitation spectrum is recorded at an emission wavelength of 579 nm. In the PLE spectrum the broad band observed in the vicinity of 200-250 nm is attributed to the charge transfer (CT) band of O2--Dy3+ interaction in the host material, whereas the peaks observed at 353 nm, 388 nm, and 426 nm that corresponds to the 6H15/2 → 4M15/2, 4P7/2, 6H15/2 → 4I13/2, 4F7/2, 6H15/2 → 4G11/2 transitions of Dy3+ ions, respectively. The PL emission spectrum of the phosphor excited under 388 nm wavelength is shown in Fig. 2(b). Under this excitation the emission spectrum shows three peaks at ~490 nm, ~579 nm and ~673 nm [6]. The main emission band at ~579 nm corresponds to the 4F9/2 → 6H13/2 transition known as electric dipole transition (yellow band) and at ~483 nm corresponds 4F9/2 → 6H15/2 transition which is known as magnetic dipole transition (blue band), which is less intense than the electric dipole transition. The other weak band at ~673 nm in emission spectrum corresponds to the 4 F9/2 → 6H11/2 of Dy3+ ion [12-13]. The 4F9/2 → 6H13/2 transition of Dy3+ is a well-known hypersensitive transition, which is strongly influenced by the crystal-field environment. In the present case the yellow emission (electric dipole

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transition) dominates the blue emission (magnetic dipole transition). Inset of Figure 2(b) shows the CIE 1931 chromaticity diagram for the phosphor under 388 nm excitation. From figure it is clearly seen that the CIE coordinates (x, y) of (0.385, 0.409) corresponds to the yellowish white region of the color gamut.

FIGURE 2 (a) PL Excitation spectrum of ZnAl2O4: Dy3+ phosphor. (b) PL Emission spectrum of ZnAl2O4: Dy3+ phosphor. Inset shows the CIE 1931 chromaticity diagram for phosphor under 388 nm excitation.

The diffuse reflectance spectrum of ZnAl2O4:Dy3+ phosphor in the wavelength ranging from 190-1400 nm is shown in Fig. 3. The spectrum shows absorption edge in lower wavelength region along with some inhomogeneous broadened bands at higher wavelengths centered at 795, 895, 1074 and 1270 nm. The observed bands correspond to the absorption transitions from the 6H15/2 ground state to 6F5/2, 6F7/2, 6F9/2+6H7/2, 6F11/2+6H9/2 excited states of the Dy3+ ion. The bandgap of the phosphor was calculated from the diffuse reflectance spectrum using the Kubleka-Munk Theory [14] and is shown in the inset of Figure 3. From this theory a band gap of 5.2 eV was estimated for the ZnAl2O4:Dy3+ phosphor.

FIGURE 3. Diffused reflectance spectrum of ZnAl2O4: Dy3+ phosphor. Inset shows the band gap of the phosphor.

Surface Studies In order to understand the surface properties of the phosphor, the X-ray photoelectron spectroscopy (XPS) measurements have been performed. Figure 4 depicts the X-ray photoelectron survey scan of the phosphor. From the

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wide scan XPS spectrum it is observed that only Zn, Al, O and Dy are present in the phosphor with minor contribution from C. The presence of C in the spectrum is expected because phosphor was exposed to the environment before the measurements. The position of observed photoemission peaks are marked at their respective binding energies in the spectrum. The core levels scans of Zn-2p, Al-2s, O-1s and Dy-4d have been also performed (not shown here).

FIGURE 4. XPS survey scan spectrum of ZnAl2O4: Dy3+ phosphor.

CONCLUSIONS To conclude, Dy3+ doped ZnAl2O4 phosphors were synthesized by the solution combustion technique at 570 ⁰C. The XRD study confirms the cubic structure of the phosphor. PL excitation and emission results show that characteristics 4f-4f transitions of Dy3+ ion are responsible for the luminescence properties in the phosphor. The CIE coordinates (x, y) of (0.385, 0.409) corresponds to the yellowish white region of the color gamut. The potential of phosphor under UV excitation may be explored as near UV WLED phosphor in the solid state lighting.

ACKNOWLEDGMENTS SK heartfully thanks UGC-DAE CSR, Indore, India for fellowship under CRS project (No. CSR-IC-BL-12/CRS109-2014-15/1205).

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