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Valence Bands (valence + bands)
Selected AbstractsThe electronic and optical properties of oligo(trans -1,2-di(2-thienyl)-1,3-butadiene): A theoretical studyINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 13 2008Nei Marçal Abstract In the present work we investigated the theoretical electronic structure of poly(trans -1,4-di(2-thienyl)-1,3-butadiene) (PTB) and determined the optical properties of its neutral and doped oligomers. Geometrical optimizations were at the semiempirical level by using the Austin method 1 (AM1). The band structure of , electrons regarding to the neutral PTB polymer was obtained by using a tight-binding Hamiltonian. The densities of electronic states (DOS) for neutral and doped copolymers were calculated by using the negative factor counting technique. The spatial charge distribution of the oligomeric chain was also analyzed. The energy of the electronic transitions and their associated oscillator strength values were calculated for the neutral, double, and single charged oligomers to determine the UV,vis absorption spectra. The calculations were performed using the intermediate neglect of differential overlap Hamiltonian in combination with the single configuration-interaction technique in order to include correlation effects. The band gap obtained in the PTB was about 2.101 eV for the optics absorption and 1.73 eV for the DOS. The bipolaron states appear in the gap, about 0.57 eV and 0.48 eV below and above the conduction and valence bands, respectively. When the dopants concentration is increased the DOS showed that the energy gap tends to vanish, which may lead to semiconductor,metal transition. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008 [source] Temperature effects on the UV,Vis electronic spectrum of trans-stilbeneINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 4-5 2001S. P. Kwasniewski Abstract The ultraviolet (UV),Visible absorption spectrum of trans-stilbene (tS) is computed at different temperatures by coupling molecular dynamics (MD) simulations with the classical MM3 force field to ZINDO/S-CIS calculations of vertical excitation energies and transition dipole moments. The selection of a large number of structures along the MD trajectories enables a consistent treatment of temperature effects in the vacuum, whereas the ZINDO/S-CIS calculations permit a reliable treatment of electron correlation and relaxation, taking account of multistate interactions in the final state. Thermal motions are found to alter very differently the width and shape of bands. Structural alterations such as the stretching and the torsion of the vinyl single and double bonds very strongly influence the appearance of the first valence state, pertaining to the highest occupied and lowest unoccupied molecular orbital (HOMO,LUMO) transition. At temperatures less than 400 K, these are found to yield a merely Gaussian and very pronounced thermal broadening of the related band (A), up to nearly 30 nm, together with a minor blue shift of its maximum ,max. In contrast, a red shift by several nanometers occurs due to thermal motions for the remaining three valence bands. As can be expected, the broadening intensifies at higher temperatures, and for the A-band, becomes markedly asymmetric when T exceeds 400 K. The combination of MD(MM3) and ZINDO/S-CIS computations enables also consistent calculations of hot bands, which are forbidden by symmetry at 0 K. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001 [source] DFT calculations on the electronic structures of BiOX (X = F, Cl, Br, I) photocatalysts with and without semicore Bi 5d statesJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 2 2009Wen Lai Huang Abstract The electronic structures of BiOX (X = F, Cl, Br, I) photocatalysts have been calculated with and without Bi 5d states using the experimental lattice parameters, via the plane-wave pseudopotential method based on density functional theory (DFT). BiOF exhibits a direct band gap of 3.22 or 3.12 eV corresponding to the adoption of Bi 5d states or not. The indirect band gaps of BiOCl, BiOBr, and BiOI are 2.80, 2.36, and 1.75 eV, respectively, if calculated with Bi 5d states, whereas the absence of Bi 5d states reduces them to 2.59, 2.13, and 1.53 eV successively. The calculated gap characteristics and the falling trend of gap width with the increasing X atomic number agree with the experimental results, despite the common DFT underestimation of gap values. The shapes of valence-band tops and conduction-band bottoms are almost independent of the involvement of Bi 5d states. The indirect characteristic becomes more remarkable, and the conduction-band bottom flattens in the sequence of BiOCl, BiOBr, and BiOI. Both O 2p and X np (n = 2, 3, 4, and 5 for X = F, Cl, Br, and I, respectively) states dominate the valence bands, whereas Bi 6p states contribute the most to the conduction bands. With the growing X atomic number, the localized X np states shift closer toward the valence-band tops, and the valence and conduction bandwidths evolve in opposite trends. Atomic and bond populations have also been explored to elucidate the atomic interactions, along with the spatial distribution of orbital density. © 2008 Wiley Periodicals, Inc. J Comput Chem, 2009 [source] The electronic and electrochemical properties of the TiFe1,xNix alloysPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 1 2003A. Szajek Abstract Mechanical alloying (MA) process was introduced to produce nanocrystalline TiFe1,xNix alloys (0 , x , 1). XRD analysis showed that, firstly, after 25 h of milling, the starting mixture of the elements had decomposed into an amorphous phase and, secondly, the annealing in high purity argon at 750 °C for 0.5 h led to formation of the CsCl-type (B2) structures with a crystallite sizes of about 30 nm. These materials, used as negative electrodes for Ni,MH batteries, showed an increase in discharge capacity with a maximum for x = 3/4. The band structure has been studied by the Tight Binding version of the Linear Muffin-Tin method in the Atomic Sphere Approximation (TB LMTO ASA). Increasing content of Ni atoms intensified charge transfer from Ti atoms, extended valence bands and increased the values of the densities of electronic states at the Fermi level. [source] Electron localization and emission mechanism in wurtzite (Al, In, Ga)N alloysPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 1 2010Qihang Liu Abstract The electronic structures of wurtzite InGaN and AlGaN alloys are investigated using the first-principle density functional theory calculation. The results indicate that some short In,N,In atomic chains and small In,N atomic condensates composed of a few In and N atoms can be randomly formed in InGaN alloys. The electrons at the top of valence bands can be effectively localized in the vicinity of the In,N,In zigzag chains (weak localization) and the In,N atomic condensates (strong localization). These localized electrons extremely enhance the emission efficiency of InGaN alloys. [source] Electronic structure of binary and ternary components of CdTe:O thin filmsPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue S1 2004E. Menéndez-Proupin Abstract We report first-principles calculations of the electronic structure of the simplest compounds that may be present in Cd,Te,O mixtures: CdTe, CdO, ,-TeO2, CdTeO3 and Cd3TeO6. The calculations are carried out in the Local Density Approximation (LDA) and predict the insulating character of these compounds, underestimating the optical bandgaps by nearly 1 eV, as usual for LDA. In the four oxides, the top valence bands originate mainly from the O 2p atomic levels. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Kinetics of optically excited charge carriers and defects in hydrogenated amorphous silicon and related alloysPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 5 2004J. Whitaker Abstract Electron spin resonance and optically induced electron spin resonance measurements on hydrogenated amorphous silicon and germanium are useful probes of charge carriers in localized electronic states near the edges of the conduction and valence bands (band-tail states) and of defects, such as silicon and germanium dangling bonds. We review recent results on the recombination kinetics of charge carriers trapped in band-tail states. We also review recent results on the kinetics of metastable optically induced germanium dangling bonds in hydrogenated amorphous germanium. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Spectroscopic Characteristics of Differently Produced Single-Walled Carbon NanotubesCHEMPHYSCHEM, Issue 13 2009Zhongrui Li Prof. Dr. Abstract Single-walled carbon nanotubes (SWNTs) synthesized with different methods are investigated by using multiple characterization techniques, including Raman scattering, optical absorption, and X-ray absorption near edge structure, along with X-ray photoemission by following the total valence bands and C 1s core-level spectra. Four different SWNT materials (produced by arc discharge, HiPco, laser ablation, and CoMoCat methods) contain nanotubes with diameters ranging from 0.7 to 2.8 nm. The diameter distribution and the composition of metallic and semiconducting tubes of the SWNT materials are strongly affected by the synthesis method. Similar sp2 hybridization of carbon in the oxygenated SWNT structure can be found, but different surface functionalities are introduced while the tubes are processed. All the SWNTs demonstrate stronger plasmon resonance excitations and lower electron binding energy than graphite and multiwalled carbon nanotubes. These SWNT materials also exhibit different valence-band X-ray photoemission features, which are considerably affected by the nanotube diameter distribution and metallic/semiconducting composition. [source] | |||||||||||||