Yoichi Yamaguchi

Design of novel σ*-π* conjugated polysilanes

Abstract Electronic structures of two novel σ * -π * conjugated polysilanes, poly(1,1-silole) and poly(1,1-benzo[ c ] silole), have been studied theoretically using the one-dimensional tight-binding self-consistent field-crystal orbital (SCF-CO) method at the modified neglect of diatomic overlap, Austin Model 1 (MNDO-AM1) level of approximation. It is found that the bandgaps corresponding to the σ-σ * transition of the polymers are smaller than that of the conventional polysilanes. The σ-σ * bandgaps of these two polymers are essentially determined by the lower-lying conduction band derived from the mixing of the delocalized σ * -band of the silicon skeleton and the localized π * -band of the side chains. Results suggest that the σ * -π * conjugation may be more effective for the reduction of the bandgap than the σ-π conjugation in a polysilane with aryl side chains.

A density functional theory study of the interaction of oxygen with a reduced SnO2 (110) surface

Abstract Ab initio calculations using density functional theory within the generalized gradient approximation have been performed for the interaction of oxygen with a reduced SnO 2 (110) surface. We found two species of chemisorbed O 2 that can be described as a peroxo (O 2 2− ) species for side-on type adsorption and a superoxo (O 2 − ) species for end-on type adsorption on the basis of their geometries and vibrational frequencies. We also found a low dissociation barrier of 2.0 kcal/mol for the peroxo species and high migration barriers of ∼13–18 kcal/mol for an oxygen atom produced by dissociation of O 2 on the reduced SnO 2 surface. An analysis of the electronic density of states indicates that an adsorbed oxygen atom coupled with the nearest-neighboring bridging oxygen vacant site is an O − species that displays catalytic activity on the surface.

Crystal Structure of GaN Grown on 3C-SiC Substrates by Metalorganic Vapor Phase Epitaxy

GaN films were grown on 3C-SiC substrates by metalorganic vapor phase epitaxy (MOVPE) using 1,1-dimethylhydrazine (DMHy) as the N source. The crystal structure was strongly affected by both the growth temperature and the V/III ratio. At 600° C, only hexagonal GaN films with their c-axis perpendicular to the substrate surface were grown. At 800° C, relatively high V/III ratios resulted in the growth of hexagonal GaN with their c-axis oriented in the [111] direction while cubic GaN films were obtained at lower V/III ratios. The origin of [111]-oriented hexagonal GaN is also discussed.

Strong coupling of the single excitations in the Q-like bands of phenylene-linked free-base and zinc bacteriochlorin dimers: A time-dependent density functional theory study

Time-dependent density-functional theory (TDDFT) calculations were carried out to predict the Q-like bands of the two structural isomers of the phenylene-linked free-base (FBBC) and zinc (ZnBC) bacteriochlorin dimers. The calculated singlet excitation energies and oscillator strengths for the low-lying excited states of the reference monomers, FBBC and ZnBC, are quite consistent with recent ab initio calculations. The 1,3 and 1,4-phenylene-linked dimers have monomer-like Q bands and new Q′ bands comprised of the cross-linked (i.e., charge transfer) excitations from the FBBC (ZnBC) ring to the ZnBC (FBBC) ring, whose excitation energies are slightly and considerably red-shifted by about 0.03 eV and 0.5–0.7 eV compared to the monomers, respectively. The monomer-like Qx bands are of the mixed highest occupied molecular orbital (HOMO)→lowest unoccupied molecular orbital (LUMO) excitations in the Qx bands from the different bacteriochlorin rings, whose origin is the coupling of the Qx transition dipole moments...

Electronic structure of polysilole and related compounds

The electronic structures, in the optimized geometries, of polysilacyclopentadiene (polysilole), polygermacyclopentadiene, and silole-thiophene alternating copolymer, have been studied theoretically using the one-dimensional tight-binding self-consistent field crystal orbital (SCF-CO) method. It has been found that the band gap values of these polymers are slightly smaller than those of the ordinary five-membered polyheterocycles such as polythiophene. The larger σ-polarized nature between the Group IV atom and the carbon atom might be expected to exhibit a different behaviour, compared with polythiophene, when they are doped with ann-type dopant.

Chemisorbed Oxygen Species over the (110) Face of SnO2

The electronic states of chemisorbed oxygen species on the (110) face of SnO2 and their variations caused by heat treatments and/or O2 exposure have been investigated. The reactivities of the chemisorbed oxygen species for methane oxidations were also examined.Four different chemisorbed oxygen species (O22-, O2-, O-, Ob) were observed, in addition to the lattice oxygen (O2-), on the surface of the stabilized (110) surface of SnO2 after O2 exposure. The Ob species was assumed to be the bridging oxygen at the topmost layer of the SnO2 (110) surface having no neighboring oxygen vacancies. The electronic state of Ob was converted to O- upon CH4 exposure at 473 K by coupling with newly produced vacancies at the bridging site of the SnO2 (110) surface.

Design of novel donor-acceptor polymers with low bandgaps

Abstract Quantum chemical results on the electronic and geometric structures of some novel donor-acceptor polymers containing alternating electrondonating group X (X = CH 2 , SiH 2 or GeH 2 ) and electron-accepting group Y (Y = C = CH 2 , C = O, C = CF 2 or C = C(CN) 2 ) along the conjugated cis -polyacetylene backbone, obtained on the basis of the one-dimensional tight-binding self-consistent field-crystal orbital (SCFCO) method at the MNDO-AM1 level of approximation, are reported. The optimized geometries of the polymers show a strong dependence on the nature of the electron-donating group X. Polymers derived from X = CH 2 or GeH 2 and Y = C=C (CN) 2 are predicted to have bandgap values of less than 1 eV. An analysis of their π-bond order data and the patterns of their frontier orbitals shows these two polymers to have quinoid-like electronic structures, in contrast to the benzenoid-like electronic structures for the rest of the polymers.

Theoretical study of the electronic structure and conformation of polysilole and its derivatives

The electronic structures and conformations in the optimized geometries of polysilole and its derivatives are studied theoretically, using the one-dimensional tight-binding self-consistent field crystal orbital (SCF-CO) method. It has been found that the copolymer formed by alternating aromatic silole and quinoid thiophene has a relatively small bandgap value. We observe that polysilole may show an interesting thermochromic behavior, because the conformation of polysilole is considerably easier to change at lower temperature than that of polythiophene.

SiC mask membrane for synchrotron radiation lithography

Abstract We have developed an excellent silicon carbide mask membrane with a hot wall type LPCVD using a SiH 2 Cl 2 - C 2 H 2 - H 2 reaction gas system. The internal stress can be precisely controlled by making a mixed crystal of poly β-SiC and poly Si. Surface roughness is less than about 50nm by STM measurement. The 2.0μm thick membrane shows excellent transparency of 53% by applying an AR coating. With regard to X-ray radiation durability, no change of stress is observed up to 120MJ/cm 3 of absorbed energy.

Preparation and properties of graphite grown in vapor phase

Abstract Preparation conditions and the electrical conductivity of carbon films prepared by plasma induced chemical vapor deposition (plasma CVD) of benzene were studied. It was found that the maximum electrical conductivity of carbon films was obtained when the benzene flow rate was 7 ml/hr, the substrate was stainless-steel, the substrate temperature was 1000°C and the plasma power was 40 W. The carbon films were heat-treated at various temperatures between 1500°C and 3300°C and the structure of the film obtained was analyzed by X-ray diffraction analysis and Raman spectroscopy. It was found that the carbon film prepared by plasma CVD could be graphitized at lower temperature than ordinary pyrolytic carbons. The graphite film which was obtained by the high temperature treatment (HTT) of the carbon films at 3300°C has the electrical conductivity of 2.1 × 10 4 S/cm. The highest electrical conductivity of AsF 5 intercalated graphite films was 9 × 10 5 S/cm. The stability of AsF 5 intercalated graphite films was also examined.