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Gianfranco Pacchioni - One of the best experts on this subject based on the ideXlab platform.
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Semiconductor-to-metal transition in WO3-x: Nature of the Oxygen Vacancy
Physical Review B - Condensed Matter and Materials Physics, 2011Co-Authors: Fenggong Wang, Cristiana Di Valentin, Gianfranco PacchioniAbstract:Hybrid functional density functional theory calculations of the Oxygen Vacancy (VO) in monoclinic bulk WO3 provide a coherent rationalization for the strong dependence of WO3-x electronic properties on the VO concentration and of the semiconductor-to-metal transition, a phenomenon intimately connected to the electrochromic effect. Different VO centers containing W4+, W5+, and W6+ species are expected to coexist. Optical transition levels are 0.7-1.0 eV below the conduction band minimum, in agreement with experiments. The complex nature of VO in WO3 can only be detected with methods that properly describe band gaps and polaronic distortions at defect sites.
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Electronic structure of a neutral Oxygen Vacancy in SrTiO3
2003Co-Authors: Davide Ricci, Giuseppe Bano, Gianfranco Pacchioni, Francesc IllasAbstract:The electronic structure of an isolated Oxygen Vacancy in SrTiO3 has been investigated with a variety of ab initio quantum mechanical approaches. In particular we compared pure density functional theory (DFT) approaches with the Hartree-Fock method, and with hybrid methods where the exchange term is treated in a mixed way. Both local cluster models and periodic calculations with large supercells containing up to 80 atoms have been performed. Both diamagnetic (singlet state) and paramagnetic (triplet state) solutions have been considered. We found that the formation of an O Vacancy is accompanied by the transfer of two electrons to the 3d(z(2)) orbitals of the two Ti atoms along the Ti-Vac-Ti axis. The two electrons are spin coupled and the ground state is diamagnetic. New states associated with the defect center appear in the gap just below the conduction band edge. The formation energy computed with respect to an isolated Oxygen atom in the triplet state is 9.4 eV.
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Electronic structure of an isolated Oxygen Vacancy at the TiO2(110) surface
Chemical Physics Letters, 2002Co-Authors: Thomas Bredow, Gianfranco PacchioniAbstract:The electronic structure of the rutile(110) surface after formation of an isolated Oxygen Vacancy has been studied with density functional (DF) calculations and TiO2 embedded clusters. Restricted and unrestricted open shell wave functions have been analyzed in terms of atomic spin densities and Ti core level shifts. The removal of a neutral Oxygen atom from the bridging Oxygens of the TiO2(110) surface results in two unpaired electrons localized at the 3d orbitals of neighboring 5-coordinated Ti atoms. The electronic structure of an O Vacancy on the surface of TiO2 is compared with that of the same defect in MgO and SiO2. (C) 2002 Elsevier Science B.V. All rights reserved.
T. H. Li - One of the best experts on this subject based on the ideXlab platform.
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Optical identification of Oxygen Vacancy types in SnO2 nanocrystals
Applied Physics Letters, 2013Co-Authors: Jiangwen Xu, Xinglong Wu, T. H. Li, Jinyou ShenAbstract:The Oxygen vacancies in spherical and cuboid SnO2 nanocrystals prepared by hydrothermal and laser ablation methods are investigated optically. Three Oxygen-Vacancy-related photoluminescence peaks at ∼430, ∼501, and ∼618 nm are observed, and Raman scattering and density functional calculation disclose that they originate from in-plane, sub-bridging, and bridging Oxygen vacancies, respectively. This work reveals that the photoluminescence peaks together with the Raman modes can be used to identify the Oxygen Vacancy types in SnO2 nanostructures.
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identification of Oxygen Vacancy types from raman spectra of sno2 nanocrystals
Journal of Raman Spectroscopy, 2012Co-Authors: T. H. Li, X L Wu, Jinyou ShenAbstract:Raman spectra acquired from spherical SnO2 nanocrystals prepared by pulsed laser ablation and hydrothermal synthesis exhibit three Oxygen-Vacancy-related Raman modes at 234, 573, and 618 cm−1. The peak location and intensity vary with annealing temperature under O2 finally approaching those of bulk materials. Density functional calculation discloses that the three Raman modes stem from subbridging, in-plane, and bridging Oxygen vacancies, respectively. Raman spectra can thus be used to discern different types of Oxygen vacancies in SnO2 nanocrystals. Copyright © 2012 John Wiley & Sons, Ltd.
Yanhui Lv - One of the best experts on this subject based on the ideXlab platform.
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the surface Oxygen Vacancy induced visible activity and enhanced uv activity of a zno1 x photocatalyst
Catalysis Science & Technology, 2013Co-Authors: Yanhui Lv, Xinguo G, Ruilong L ZongAbstract:A ZnO1−x photocatalyst with surface Oxygen vacancies was fabricated by the controllable reduction of H2. After surface Oxygen vacancies were introduced into the ZnO photocatalyst, a high visible-light-driven activity and photocurrent was produced. The UV activity for the degradation of MB and the photocurrent was enhanced about 2.2 times and 2.5 times, respectively. The visible-light activity resulted from the narrowed band gap due to the overlap of the valance band (VB) of the surface Oxygen Vacancy with O2p. The main active species are photoinduced holes and MB can be mineralized completely under visible-light irradiation. The overlap of the VB of the surface Oxygen Vacancy with O2p also extended the width of the VB and resulted in the increase of the separation efficiency of the photoinduced electron–hole pairs and the enhancement of the UV photoactivity greatly. The surface Oxygen Vacancy only increased the separation efficiency and did not change the photocatalytic degradation process, and the main oxidative species was still the photoinduced holes. The bulk Oxygen Vacancy can be formed via depth reduction at 700 °C for 5 h and resulted in the loss of photoactivity due to bulk defects.
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enhanced photocatalytic performance for the bipo4 x nanorod induced by surface Oxygen Vacancy
Journal of Physical Chemistry C, 2013Co-Authors: Yanhui LvAbstract:The BiPO4–x nanorod with surface Oxygen Vacancy was fabricated via vacuum deoxidation. The concentration and kind of Oxygen Vacancy could be controlled by tuning the deoxidation temperature and time in vacuum. The photocatalytic activity depended on the concentration and kind of surface Oxygen Vacancy, and the optimum photocatalytic activity and photocurrent of the BiPO4–x nanorod was about 1.5 and 2.5 times as high as that of pure BiPO4, respectively. Besides, the photocatalytic response wave range of the BiPO4–x nanorod has been expanded to more than 365 nm. The enhancement of photocatalytic activity is attributed to the high separation efficiency of photoinduced electron–hole pairs due to the broadening of the valence band (VB) induced by surface Oxygen-Vacancy states, and the extending of photoresponse is considered to be the narrowing of energy band gap resulting from the rise of the valence band maximum (VBM).
Jinyou Shen - One of the best experts on this subject based on the ideXlab platform.
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Optical identification of Oxygen Vacancy types in SnO2 nanocrystals
Applied Physics Letters, 2013Co-Authors: Jiangwen Xu, Xinglong Wu, T. H. Li, Jinyou ShenAbstract:The Oxygen vacancies in spherical and cuboid SnO2 nanocrystals prepared by hydrothermal and laser ablation methods are investigated optically. Three Oxygen-Vacancy-related photoluminescence peaks at ∼430, ∼501, and ∼618 nm are observed, and Raman scattering and density functional calculation disclose that they originate from in-plane, sub-bridging, and bridging Oxygen vacancies, respectively. This work reveals that the photoluminescence peaks together with the Raman modes can be used to identify the Oxygen Vacancy types in SnO2 nanostructures.
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identification of Oxygen Vacancy types from raman spectra of sno2 nanocrystals
Journal of Raman Spectroscopy, 2012Co-Authors: T. H. Li, X L Wu, Jinyou ShenAbstract:Raman spectra acquired from spherical SnO2 nanocrystals prepared by pulsed laser ablation and hydrothermal synthesis exhibit three Oxygen-Vacancy-related Raman modes at 234, 573, and 618 cm−1. The peak location and intensity vary with annealing temperature under O2 finally approaching those of bulk materials. Density functional calculation discloses that the three Raman modes stem from subbridging, in-plane, and bridging Oxygen vacancies, respectively. Raman spectra can thus be used to discern different types of Oxygen vacancies in SnO2 nanocrystals. Copyright © 2012 John Wiley & Sons, Ltd.
Subhashis Gangopadhyay - One of the best experts on this subject based on the ideXlab platform.
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Optimized Resistive Switching in TiO2 Nanotubes by Modulation of Oxygen Vacancy Through Chemical Reduction
IEEE Transactions on Electron Devices, 2020Co-Authors: Arnab Hazra, Ashutosh Tripathi, Souvik Kundu, Pavan Kumar Reddy Boppidi, Subhashis GangopadhyayAbstract:The resistive switching behavior of 1-D TiO2 nanotube-based resistive random access memory (ReRAM) is discussed in this article. Highly oriented TiO2 nanotubes were synthesized by anodic oxidation method on a Ti substrate, which was used as the bottom electrode. To modulate the Oxygen Vacancy (VO) in TiO2 nanotubes, hydrazine hydrate reduction was employed in the temperature range 60 °C-100 °C. After charactering the morphological, elemental, and crystallographic properties, the level of reduction in different TiO2 nanotubes array was estimated by Raman, photoluminescence, and X-ray photoelectron spectroscopies. Au/TiO2 nanotubes/Ti devices were fabricated by using TiO2 nanotubes with various levels of reductions where thin and porous Au top electrode was used to make the resistive switching faster. TiO2 Nanotubes array, reduced at 80 °C showed promising resistive switching performance with SET/RESET voltages of 2 V/1.8 V, ROFF/RON of 19 at a read voltage of 0.5 V (25 °C) and stable endurance behavior after the 100th cycle. Interestingly, reduction temperature at 60 °C and 100°C, offered degraded resistive switching within the same voltage range. All the devices showed electroforming free bipolar resistive switching. Efforts were devoted to establish the role of VO and its tuning to improve the resistive switching behavior in 1-D TiO2 nanotubes. This article systematically showcases the efficacy of 1-D metal oxide for potential ReRAM application and establishes an easy but efficient approach to improve the resistive switching by modulating Oxygen Vacancy in it.
