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The optical properties of the perovskite-type NaTaO 3 synthesized from sol-gel ͑SG͒ and solid-state ͑SS͒ methods were studied by absorption and photoluminescence ͑PL͒ techniques. We observed significant PL emission at room temperature from the SS NaTaO 3 , while the emission was negligible from the SG NaTaO 3 . Variation of the PL intensity and peak position with temperature demonstrated that the recombination of the localized exciton Ta 4+ -O − in the regular TaO 6 octahedra was the origin of the luminescence in NaTaO 3 . The Ta-O-Ta bond angle affected the delocalization of the excitons and thus, the PL behaviors of ABO 3 perovskite structures, where A is a group of I-II elements and B is a transition metal, have attracted considerable attention because of their unusual magnetic, dielectric, and luminescence properties.1-3 Recently, the luminescence properties have been extensively studied in many ABO 3 perovskite structures ͑A = Sr, Ba, K, etc., B = Nb, Ti, Ta, etc.͒, and this has provided information on the photochemical properties of ABO 3 perovskites, such as photocatalysis, photoinduced electron-transfer dynamics, and excitation energy transport.2-5 These ABO 3 perovskites exhibit a broad emission band with a large Stokes shift at low temperatures, whereas the luminescence is usually quenched at high temperatures and it is hard to detect at room temperature. Several mechanisms for the luminescence properties of ABO 3 perovskites have been proposed. They include the recombination of electrons trapped on the donors with holes trapped on the acceptors, 6 radiative transitions within the BO 6 , 7 self-trapped excitons, 2,5 and charge-transfer vibronic excitons. 8,9 Moreover, Longo et al. have attributed the origin of the visible luminescence emission in perovskite-type compounds to the localized electronic level induced in the valence band by the symmetry. 10 The semiempirical quantum chemical method of intermediate neglect of differential overlap has been used by Grigorjeva et al. for investigating the radiative recombination of correlated ͑bound͒ self-trapped electron and hole polarons in the highly polarizable ABO 3 -type matrix.11 Orhan et al. have turned to the firstprinciples theory as an appropriate tool to analyze the mechanism of the luminescence behavior in disordered ABO 3 materials.12 However, a clear understanding of the luminescence behavior of ABO 3 materials is still needed.Among the different ABO 3 materials, NaTaO 3 presents a widely versatile structure, depending on growth techniques, and the structure would influence the optical properties of NaTaO 3 .13-15 Wiegel et al. have reported the relationship between crystal structure and energy delocalization for perovskite-type alkali tantalates. 13 The excited energy is efficiently delocalized at a bond angle of TaϪOϪTa close to 180°, while the excited energy would be localized at bond angles deviating from 180°. The luminescence of ABO 3 materials at low temperatures has been explored by numerous studies. In contrast, only a limited number of...

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We report the observation of persistent photoconductivity (PPC) effect in 2H-MoSe2 layered semiconductors. The decay behavior of PPC can be well described by a stretch-exponential function. Experimental results indicate that the lattice relaxation of DX-like impurity is responsible for PPC in MoSe2. In addition, the small capture barrier created by lattice relaxation and the temperature-dependent resistance measurement exhibit that the magnitude of PPC effect in MoSe2 is small, consistent with the PPC behavior in indirect gap materials.

In this study, the excitonic luminescence behaviors of ZnO thin films in the temperature range of 10 -300 K were investigated. The photoluminescence (PL) spectrum exhibits the bound-exciton emission and the donor-acceptor recombination accompanying its multiphonon replicas at low temperatures. The observed features exhibit redshift with an increase in temperature, and the temperature dependence of the peak position was analyzed by the Varshni empirical expression. The study showed the dominant presence of bound excitonic transition below 160 K or free excitonic transition at higher temperatures for the observed PL spectrum. The free exciton emission can be observed up to room temperature. The Debye temperature of ZnO was evaluated by taking into account the elastic constants of ZnO and utilized as a parameter in the Varshni empirical expression giving an accurate description of the free exciton emission behavior up to room temperature.