K. Seino

Ground‐ and excited‐state properties of DNA base molecules from plane‐wave calculations using ultrasoft pseudopotentials

We present equilibrium geometries, vibrational modes, dipole moments, ionization energies, electron affinities, and optical absorption spectra of the DNA base molecules adenine, thymine, guanine, and cytosine calculated from first principles. The comparison of our results with experimental data and results obtained by using quantum chemistry methods show that in specific cases gradient‐corrected density‐functional theory (DFT‐GGA) calculations using ultrasoft pseudopotentials and a plane‐wave basis may be a numerically efficient and accurate alternative to methods employing localized orbitals for the expansion of the electron wave functions.

Efficiency limits of Si/SiO2 quantum well solar cells from first-principles calculations

In order to investigate the applicability of new photovoltaic absorber materials, we show how to use first-principles calculations combined with device simulations to determine the efficiency limits of solar cells made from SiO2/Si superlattices and from coaxial ZnO/ZnS nanowires. Efficiency limits are calculated for ideal systems according to the Shockley–Queisser theory but also for more realistic devices with finite mobilities, nonradiative lifetimes, and absorption coefficients. Thereby, we identify the critical values for mobility and lifetime that are required for efficient single junction as well as tandem solar cells.

Structure and energetics of Ga-rich GaAs(001) surfaces

The atomic structures and energies of Ga-rich GaAs(0 0 1) surface reconstructions are examined by means of firstprinciples total-energy calculations based on a real-space multigrid method. Our calculations confirm the existence of the novel f(4 � 2) structure suggested by Lee et al. [Phys. Rev. Lett. 85 (2000) 3890]. (4 � 6) surface reconstructions suggested to explain STM experiments are found to be unstable. The calculations indicate that the adsorption of Ga adatoms in the trenches of the f(4 � 2) surface could possibly explain the observed structures. The diffusion of Ga/As adatoms on the Ga-rich GaAs surface is predicted to be anisotropic and should preferably take place parallel to the [1 1 0]/[1 � 0] direction, respectively. 2002 Elsevier Science B.V. All rights reserved.

Metallic Properties of the Si(111) − 5 × 2 − Au Surface from Infrared Plasmon Polaritons and Ab Initio Theory

The metal-atom chains on the Si(111) - 5 × 2 - Au surface represent an exceedingly interesting system for the understanding of one-dimensional electrical interconnects. While other metal-atom chain structures on silicon suffer from metal-to-insulator transitions, Si(111) - 5 × 2 - Au stays metallic at least down to 20 K as we have proven by the anisotropic absorption from localized plasmon polaritons in the infrared. A quantitative analysis of the infrared plasmonic signal done here for the first time yields valuable band structure information in agreement with the theoretically derived data. The experimental and theoretical results are consistently explained in the framework of the atomic geometry, electronic structure, and IR spectra of the recent Kwon-Kang model.

Oxidation- and organic-molecule-induced changes of the Si surface optical anisotropy: ab initio predictions

In the last couple of years there has been much methodological and computational progress in the modelling of optical properties from first principles .R ef lectance anisotropy spectra (RAS) can now be calculated with true predictive power and can thus be used to draw conclusions directly on the surface geometry. In the present work we study two potentially very interesting applications for RAS: the oxidation of Si(001) and the functionalization of the Si surface with organic molecules. Our calculations confirm experimental indications that the polarity of the interface-induced optical anisotropy is reversed layer by layer with increasing oxide thickness. The oscillation of the RAS amplitude should thus allow for the quantitative monitoring of the vertical progression of the oxidation. Our results for Si(001) surfaces modified by cyclopentene and 9,10-phenanthrenequinone adsorption show a strong sensitivity of the RAS signal with respect to the adsorption geometry. Comparison with experimental data shows that cyclopentene most probabl ya dsorbs vi aa cycloaddition reaction with the Si surface dimers, while phenanthrenequinone seems to adsorb across two Si dimers.

Metal-to-Insulator Transition in Au Chains on Si(111)-5×2-Au by Band Filling: Infrared Plasmonic Signal and Ab Initio Band Structure Calculation

The Si(111)-5×2-Au surface is increasingly of interest because it is one of the rare atomic chain systems with quasi-one-dimensional properties. For the deposition of 0.7 monolayers of Au, these chains are metallic. Upon the evaporation of an additional submonolayer amount of gold, the surface becomes insulating but keeps the 5×2 symmetry. This metal-to-insulator transition was in situ monitored based on the infrared plasmonic signal change with coverage. The phase transition is theoretically explained by total-energy and band-structure calculations. Accordingly, it can be understood in terms of the occupation of the originally half-filled one-dimensional band at the Fermi level. By annealing the system, the additional gold is removed from the surface and the plasmonic signal is recovered, which underlines the stability of the metallic structure. So, recent results on the infrared plasmonic signals of the Si(111)-5 × 2-Au surface are supported. The understanding of potential one-dimensional electrical interconnects is improved.

Coverage-dependent geometries of nanowires on Ge(0 0 1)-Au surfaces: modification of trenches.

Despite intense research the microscopic atomic structure of Au-induced nanowires on Ge(0 0 1) substrates is still under discussion. We analyse a variety of structural models for Au-induced nanowires on the Ge(0 0 1) surface using first-principles calculations. Here we focus on subridge modifications at higher Au coverages and study geometries based on the giant missing row model with Ge-Ge dimers in the grooves between the nanowires due to replacing them by Ge-Au heterodimers or Au-Au homodimers. Stable geometries are predicted for higher Au coverages, which however have only a minor influence on the electronic structure. The findings are interpreted that the Au coverage and the actual geometry may vary in the various experiments according to the preparation conditions.

Anomalous Water Optical Absorption: Large-Scale First-Principles Simulations

The optical spectrum of water is not well-understood. For example, the main absorption peak shifts upwards by 1.3 eV upon condensation of gas-phase water monomers, which is contrary to the behaviour expected from aggregation-induced broadening of molecular levels. We investigate theoretically the effects of electron-electron and electron-hole correlation, finding that condensation leads to delocalisation of the exciton onto nearby hydrogen-bonded molecules. This reduces its binding energy and has a dramatic impact on the line shape. The calculated spectrum is in excellent agreement with experiment.

DNA Base Properties from First Principles Plane-Wave Calculations

We present equilibrium geometries, dipole moments, ionization energies and electron affinities of the DNA base molecules adenine, thymine, guanine, and cytosine calculated from first principles. The comparison of our results with experimental data and results obtained by using quantum chemistry methods shows that gradient-corrected density-functional theory (DFT-GGA) calculations using ultra-soft pseudopotentials and a plane-wave basis are a numerically efficient and accurate alternative to methods employing localized orbitals for the expansion of the electron wave functions.

Optical Response of Semiconductor Surfaces and Molecules Calculated from First Principles

First-principles pseudopotential calculations on the optical anisotropy of GaAs(OOl) surfaces and on the optical absorption of of DNA base molecules are presented. It is found that both electronic surface states as well as surface-perturbed bulk wave functions contribute to the optical anisotropy of GaAs(OOl). The latter contributions are modified by surface electric fields, giving rise to signals which are both reconstruction and electric field dependent. Pronounced differences in the absorption behavior of the DNA base molecules suggest the possibility of a base discrimination by means of single molecule spectroscopy.