Robert V. Kasowski

PbS in polymers. From molecules to bulk solids

The transition of PbS from molecular to bulk form has been observed in polymer films. As the particle size decreases the band gap shifts to the blue and eventually approaches the transition energy of the first allowed excited state, X→A, of a PbS molecule. Discrete absorption bands also appear. The electron‐hole‐in‐a‐box model with effective mass approximation cannot explain the observed size dependence. We have developed two theoretical models, both including the effect of band nonparabolicity, that successfully explain the observed size dependence down to about 25 A.

Temperature dependence of the electronic structure of oxides : MgO, MgAl2O4 and Al2O3

We have studied the room temperature optical reflectivity of MgO, MgAl2O4, and α-Al2O3 from 5 to 40eV using a novel spectrophotometer with a laser plasma light source. Structure in the imaginary component of the dielectric response is analysed using critical point line shapes, and the origins of the major transitions in MgO and MgAl2O4 are determined using an ab initio pseudofunction band structure calculation of MgO. The exciton reflectivity has been studied in the three materials at temperatures between 300 and 1500 K, and exciton-phonon coupling appears to increase from MgO to α-Al2O3. The temperature dependence of the higher lying interband transitions in MgO has been determined to 1100 K, and we find that while the temperature dependence of the onset transitions at Γ and X are nearly identical (− 1.22meV/K at Γ), higher lying transitions have very different temperature dependences. Furthermore with increasing temperature the X point valence band separation increases at a rate of 0.38meV/K, while the conduction band separation at X decreases at −0.41meV/K.

Cu deposition on Al2O3 and AlN surfaces: Electronic structure and bonding

We report a photoelectron spectroscopy study of the interaction of copper with Al2O3 and AlN model systems processed under ultrahigh vacuum conditions and compared the intrinsic electronic interactions between the two cases. The evolution of the electronic structure and bonding of Cu to AlN has been further studied using ab initio total energy pseudofunction techniques.

Band structure of metallic pyrochlore ruthenates Bi2Ru2O7 and Pb2Ru2O6.5

The band structure of Bi2Ru2O7 and Pb2Ru2O6.5 has been computed self‐consistently from first principles for the first time by the pseudofunction method. We discover that the 6s bands of Bi and Pb are very deep and unlikely to contribute to the metallic behavior as previously believed. The unoccupied 6p bands, however, are only several eV above the Fermi energy and are mixed with the Ru 4d band at the Fermi surface via the framework O atoms, leading to band conduction and delocalized magnetic moments. The predicted location of the 6s bands and the location and width of the O 2p band are confirmed by synchrotron radiation and ultraviolet electron spectroscopy of single crystals.

Electronic structure of pure and doped orthorhombic La2CuO4

Abstract The electronic structure of orthorhombic La2CuO4 has been investigated by first principles pseudofunction band calculations and group theoretical analysis. We find that pure as well as doped compounds remain metallic at all finite temperatures as a conseqence of the Cmca (D182h) space group symmetry. The experimentally observed rapid rise in resistivity below 30°K suggests a structural transition to a lower symmetry space group that could be driven electronically or magnetically. One possible candidate is monoclinic C2/m (C32h) which is a subgroup of Cmca and can be obtained by distorting the CuO bonds or rotating the CuO6 octahedra. Implications for superconductivity are discussed.

Electronic properties of polyacetylene, polyethylene, and polytetrafluoroethylene

The extended muffin tin orbitals technique was applied to study electronic properties of polyethylene (PE), polytetrafluoroethylene (PTFE), and polyacetylene (PA). The computed optical gap is 9 and 6.2 eV, respectively, for PE and PTFE whereas it is 1.2 to 1.7 eV for PA depending on the isomeric forms. Three dimensional charge density contours are also presented.

Theoretical and experimental studies on Cu metallization of Al2O3

Abstract The electronic structure and bonding of Cu to Al 2 O 3 has been studied both theoretically and experimentally. Ab initio total-energy pseudofunction calculations indicate that Cu initially bonds to surface O atoms instead of surface Al atoms. Ultraviolet photoemission spectra (UPS) for low coverage of Cu on Al 2 O 3 films support the theory in that good agreement between theory and experiment is only obtained for Cu bonded to surface O.

Muffin tin orbitals in open structures

Abstract The LCMTO method, which for muffin tin potentials is equivalent to the KKR method of band theory, has been applied to Si using a non-spherical cellular potential. With a basis of 9 MTOs per atom and one nonlinear variational parameter the energy bands compare within 15 mRy to those obtained with 220 OPWs. Close relation to both LCAO and pseudopotential methods is exhibited.

Vacuum ultraviolet, photoemission and theoretical studies of the electronic structure of Al2O3 up to 1000°C

Abstract The high temperature properties of Al2O3 are important to the processing of this material, for understanding its conductivity and mass transport behavior for sintering, and to the high temperature applications of the material as an optical and structural material. We report the results of a study of the electronic structure of single crystal α-Al2O3 using results of vacuum ultraviolet and X-ray photoelectron spectroscopies, and pseudofunction band structure calculations of the room- and high-temperature electronic structure, incorporating the lattice expansion and electron—phonon interaction. The optical direct band gap is found to decrease linearly from 8.8 eV at room temperature to 7.2 eV at 1763 K with a temperature coefficient of −1.1 meV/K. The effect of thermal expansion and the electron-phonon interaction, on the band gap, is found to be linear in this temperature range. The valence-band width is constant with temperature, while the unoccupied conduction band states broaden and shift toward the valence band.

Summary Abstract: A study of room‐temperature Cu–Al2O3 and Cu–AlN interfacial reactions

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