Long-range surface reconstruction: Si(110)-(16 x 2).

A variety of reconstruction models is studied for the Si(110)-(16 x 2) surface using first-principles calculations. Assuming appropriate rebonding of edge atoms and surface chains buckled in antiphase, we show that steps along the [112] direction yielding a trench indeed lower the surface energy. We explain the long-range surface reconstruction and develop a geometry model based on steps, adatoms, tetramers, and interstitials. The model is able to explain the stripes of paired pentagons seen obviously in empty-state scanning tunneling microscopy images.

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.

Excitation energies and radiative lifetimes of Ge1-xSix nanocrystals: alloying versus confinement effects.

The composition dependence of the optical and structural properties of Ge1-xSix nanocrystals is investigated by means of ab initio total-energy and electronic-structure calculations. A trimodal distribution of the Ge-Ge, Ge-Si, and Si-Si bond lengths is found. The pair-excitation energies and the Stokes shift are calculated taking into account many-body and alloying effects. They show a distinct nonlinear behavior with changing composition. The radiative lifetime decreases exponentially with increasing Ge molar fraction. The theoretical results explain recent photoluminescence measurements. They show that composition and confinement effects can be discussed nearly separately.

Origin of the different reconstructions of diamond, Si, and Ge(111) surfaces.

Ab initio calculations of the 2x1, c(2x8), and 7x7 reconstructions of the diamond, Si, and Ge(111) surfaces are reported. The pi-bonded chain, adatom, and dimer-adatom-stacking fault models are studied to understand the driving forces for a certain reconstruction. The resulting energetics, geometries, and band structures are compared for the elemental semiconductors with different atomic sizes, and chemical trends are derived. We show why the lowest-energy reconstructions are different for the group-IV materials considered.