Focus on Advances in Surface and Interface Science 2011

Transparent conducting oxides (TCOs) pose a number of serious challenges. In addition to the pursuit of high-quality single crystals and thin films, their application has to be preceded by a thorough understanding of their peculiar electronic structure. It is of fundamental interest to understand why these materials, transparent up to the UV spectral regime, behave also as conductors. Here we investigate In₂O₃ and Ga₂O₃, two binary oxides, which show the smallest and largest optical gaps among conventional n-type TCOs. The investigations on the electronic structure were performed on high-quality n-type single crystals showing carrier densities of ∼10¹⁹ cm⁻³ (In₂O₃) and ∼10¹⁷ cm⁻³ (Ga₂O₃). The subjects addressed for both materials are: the determination of the band structure along high-symmetry directions and fundamental gaps by angular resolved photoemission (ARPES). We also address the orbital character of the valence- and conduction-band regions by exploiting photoemission cross sections in x-ray photoemission (XPS) and by x-ray absorption (XAS). The observations are discussed with reference to calculations of the electronic structure and the experimental results on thin films.

An organic dye-sensitized solar cell consisting of a squaraine molecule attached to a TiO₂ surface is modeled using first-principles molecular dynamics and time-dependent density functional theory. The system is surrounded by solvent molecules that are treated at the same level of theory as the dye molecule and the surface. The effect of the solvent on optical properties is investigated by computing many absorption spectra for various configurations along a molecular dynamics trajectory. It is shown that the dynamical effects induced by thermal fluctuations have a strong effect on the optical properties and that satisfactory agreement with experiments is achieved only when those thermal effects are accounted for explicitly.

Isostructural and heterostructural pseudobinary Mg_xZn_1−xO and Cd_xZn_1−xO alloys are studied by combining the wurtzite and the rocksalt polymorphs within a cluster expansion. The computationally demanding calculation of the quasiparticle electronic structure has been achieved for all cluster cells of the expansion using the recently developed HSE03+G₀W₀ scheme. These results are used to compute the configurational averages for the fundamental band gaps and the densities of states. A strongly nonlinear behavior of the band gaps is observed and it is quantified by means of the corresponding bowing parameters for both material systems. In order to calculate the macroscopic dielectric functions, including excitonic and local-field effects for iso- and heterostructural Mg_xZn_1−xO alloys as well as wurtzite (wz) Cd_xZn_1−xO, the Bethe–Salpeter equation has been solved for each of the corresponding clusters. The respective configurational averages indicate that the composition-dependent variation in the exciton peaks allows conclusions on the alloy composition and preparation conditions.

Magnetic single-atom contacts have been controllably fabricated with a scanning tunnelling microscope. A voltage-dependent spin valve effect with conductance variations of ≈40% is reproducibly observed from contacts comprising a Cr-covered tip and Co and Cr atoms on ferromagnetic nanoscale islands on W(110) with opposite magnetization. The spin-dependent conductances are interpreted from first-principles calculations in terms of the orbital character of the relevant electronic states of the junction.

During the exothermic adsorption of molecules at solid surfaces, dissipation of the released energy occurs via the excitation of electronic and phononic degrees of freedom. For metallic substrates, the role of the non-adiabatic electronic excitation channel has been controversially discussed, as the absence of a band gap could favour an easy coupling to a manifold of electron–hole pairs of arbitrarily low energies. We analyse this situation for the highly exothermic showcase system of molecular oxygen dissociating at Pd(100), using time-dependent perturbation theory applied to first-principles electronic-structure calculations. For a range of different trajectories of impinging O₂ molecules, we compute largely varying electron–hole pair spectra, which underlines the necessity to consider the high-dimensionality of the surface dynamical process when assessing the total energy loss into this dissipation channel. Despite the high Pd density of states at the Fermi level, the concomitant non-adiabatic energy losses nevertheless never exceed about 5% of the available chemisorption energy. While this supports an electronically adiabatic description of the predominant heat dissipation into the phononic system, we critically discuss the non-adiabatic excitations in the context of the O₂ spin transition during the dissociation process.