Identification of bulk and surface sulfur impurities in Ti O2 by synchrotron x-ray absorption near edge structure

Synchrotron x-ray absorption near edge structure (XANES) measurements of Ti and S K edges, combined with first principles simulations, are used to characterize S-doped Ti O2 prepared by oxidative annealing of Ti S2 at various temperatures. Ti-edge XANES and x-ray powder diffraction data indicate that samples annealed above 300 °C have an anatase Ti O2 crystal structure with no trace of Ti S2 domains. S-edge XANES data reveal that the local structure seen by S atoms evolves gradually, from Ti S2 to a qualitatively different structure, as the annealing temperature is increased from 200 to 500 °C. For samples annealed at 500 °C, the spectrum appears to have features that can be assigned to S on the surface in the form of S O4 and S defects in the bulk (most likely S interstitials) of Ti O2

First-principles study of electronic structures and optical properties of Cu, Ag, and Au-doped anatase TiO2

We perform first-principles calculations to investigate the band structure, density of states, optical absorption, and the imaginary part of dielectric function of Cu, Ag, and Au-doped anatase TiO2 in 72 atoms systems. The electronic structure results show that the Cu incorporation can lead to the enhancement of d states near the uppermost of valence band, while the Ag and Au doping cause some new electronic states in band gap of TiO2. Meanwhile, it is found that the visible optical absorptions of Cu, Ag, and Au-doped TiO2, are observed by analyzing the results of optical properties,.which locate in the region of 400-1000 nm. The absorption band edges of Cu, Ag, and Au-doped TiO2 shift to the long wavelength region compared with the pure TiO2. Furthermore, according to the calculated results, we propose the optical transition mechanisms of Cu, Ag, and Au-doped TiO2, respectively. Our results show that the visible light response of TiO2 can be modulated by substitutional doping of Cu, Ag, and Au.Comment: 12 pages, 6 figures, 43 reference

Magnetic states and intervalence charge transfer of Ti and Fe defects in alpha-Al2O3: The origin of the blue in sapphire

We investigate the total energies and electronic structures of Ti-Fe pair defects in alpha-Al2O3 by carrying out density-functional-theory calculations for various configurations of spin, charge, and orientation of Ti-Fe pair. From the total energy results, we show that the high-spin state of Ti-IV-Fe-II with an anti-ferromagnetic alignment between Ti and Fe is the ground state for both edge- and face-sharing orientations. Among several meta-stable states, the high-spin state of Ti-III-Fe-III with an anti-ferromagnetic alignment is connected to the Ti-IV-Fe-II ground state. An optical transition from the highest occupied Fe defect level to the lowest unoccupied Ti leads to an excited state of the high-spin Ti-III-Fe-III configuration together with a donation of electron from Fe-II to Ti-IV. It explains that the intervalence charge transfer from Ti-IV-Fe-II to Ti-III-Fe-III is responsible for the color of blue in sapphire. In addition, the total energies of Ti-IV-Fe-II ground state are found to be quite close within 0.02 eV for both edge- and face-sharing orientations, but the optical excitation energies for edge- and face-sharing orientations are 1.99 and 1.36 eV, respectively. (C) 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.OAIID:RECH_ACHV_DSTSH_NO:T201800184RECH_ACHV_FG:RR00200001ADJUST_YN:EMP_ID:A072080CITE_RATE:6.036FILENAME:Na-Phattalung_ActaMaterialia_2018.pdfDEPT_NM:물리·천문학부EMAIL:[email protected]_YN:YFILEURL:https://srnd.snu.ac.kr/eXrepEIR/fws/file/1a5436cb-541d-4639-a5ca-429bf23e409c/linkN

Mixed-Metal Cu-Zn Thiocyanate Coordination Polymers with Melting Behavior, Glass Transition, and Tunable Electronic Properties

The solid-state mechanochemical reactions under ambient conditions of CuSCN and Zn(SCN)2 resulted in two novel materials: partially Zn-substituted α-CuSCN and a new phase CuxZny(SCN)x+2y. The reactions take place at the labile S-terminal, and both products show melting and glass transition behaviors. The optical band gap and solid-state ionization potential can be adjusted systematically by adjusting the Cu:Zn ratio. Density functional theory calculations also reveal that the Zn-substituted CuSCN structure features a complementary electronic structure of Cu 3d states at the valence band maximum (VBM) and Zn 4s states at the conduction band minimum (CBM). This work shows a new route to develop semiconductors based on coordination polymers which are becoming technologically relevant for electronic and optoelectronic applications.</p