Electronic excitations in ZnWO4 and ZnxNi1−x WO4 (x = 0.1 − 0.9) using VUV synchrotron radiation

Abstract

Abstract:The photoluminescence spectra and luminescence excitation spectra of pure microcrystalline and nano-sized ZnWO 4 as well as the Zn Ni 1− WO 4 solid solutions were studied using vacuum ultraviolet (VUV) synchrotron radiation. The samples were also characterized by x-ray powder diffraction. We found that: (i) the shape of the photoluminescence band at 2.5 eV, being due to radiative electron transitions within the [WO 6 ] 6− anions, becomes modulated by the optical absorption of Ni 2+ ions in the Zn Ni 1− WO 4 solid solutions; and (ii) no significant change in the excitation spectra of Zn 0 9 Ni 0 1 WO 4 is observed compared to pure ZnWO 4 . At the same time, a shift of the excitonic bands to smaller energies and a set of peaks, attributed to the one-electron transitions from the top of the valence band to quasi-localized states, were observed in the excitation spectrum of nano-sized ZnWO 4 .PACS (2008)

UV-VUV synchrotron radiation spectroscopy of NiWO4

“…In NiWO 4 , the same photoluminescence band has irregular asymmetric shape, which is close to that observed previously in solid solutions Zn c Ni 1-c WO 4 [32]. The origin of such band shape can be attributed [32] to the self-absorption effect, i.e., to a modulation of optical absorption by the intense intra-ion transition within Ni 2+ (3d 8 ) ions from the ground state 3 A 2g to the excited state 3 T 1 [44]. The maximum of the photoluminescence band in nanoNiWO 4 is located at 2.7 eV, thus being shifted by ~0.32 eV to higher energy compared to nano-ZnWO 4 [45].…”

“…The origin of the band has been previously assigned to radiative electron transitions within the [WO 6 ] 6-molecular complex [42,43]. In NiWO 4 , the same photoluminescence band has irregular asymmetric shape, which is close to that observed previously in solid solutions Zn c Ni 1-c WO 4 [32]. The origin of such band shape can be attributed [32] to the self-absorption effect, i.e., to a modulation of optical absorption by the intense intra-ion transition within Ni 2+ (3d 8 ) ions from the ground state 3 A 2g to the excited state 3 T 1 [44].…”

“…2). They differ from that for ZnWO 4 from [32] due to the difference of about 1 eV in the band gaps: E g = 3.6 eV [46] for NiWO 4 , but E g = 4.6 eV for ZnWO 4 [47]. As a result, the strong excitonic band clearly visible at ~4.5 eV for ZnWO 4 [32] is not observed in the present data for NiWO 4 due to the spectrometer range limitations: the excitonic band is expected to be located below 3.7 eV.…”

“…The tungstate sediment was subsequently washed, filtrated and, after drying, annealed in air for 4 h at 80 °C (nanocrystalline sample with crystallites size below 2 nm) and 900 °C (microcrystalline sample). More details can be found in [32].…”

“…The experiments were carried out at SUPERLUMI beamline (HASYLAB at DESY, Hamburg) using 4-20 eV synchrotron radiation from the DORIS storage ring for excitation [24]. This experimental set-up is a unique tool for investigations of different types of wide band gap materials [24][25][26][27][28][29][30][31][32][33]. The interpretation of the photoluminescence and excitation spectra is given based on the comparative analysis with isomorphous ZnWO 4 tungstate [32] and using the results of LCAO calculations [22] and the Ni(Zn) K-edge and W L 3 -edge x-ray absorption spectroscopy studies.…”