Peter A. Schultz

Designing meaningful density functional theory calculations in materials science—a primer

Density functional theory (DFT) methods for calculating the quantum mechanical ground states of condensed matter systems are now a common and significant component of materials research. The growing importance of DFT reflects the development of sufficiently accurate functionals, efficient algorithms and continuing improvements in computing capabilities. As the materials problems to which DFT is applied have become large and complex, so have the sets of calculations necessary for investigating a given problem. Highly versatile, powerful codes exist to serve the practitioner, but designing useful simulations is a complicated task, involving intricate manipulation of many variables, with many pitfalls for the unwary and the inexperienced. We discuss several of the most important issues that go into designing a meaningful DFT calculation. We emphasize the necessity of investigating these issues and reporting the critical details.

Oxygen-induced restructuring of the TiO2(110) surface: a comprehensive study

We report a comprehensive experimental and theoretical study of the eVect of oxidizing a TiO 2 (110) surface at moderate temperatures. The surfaces are investigated with scanning tunneling microscopy (STM ), low-energy He+ ion scattering (LEIS ) and static secondary ion mass spectroscopy (SSIMS ). Flat (1◊1)-terminated TiO 2 (110) surfaces are obtained by sputtering and annealing in UHV at 880 K. These surfaces are exposed to oxygen gas at elevated temperatures in the range 470‐830 K. Formation of irregular networks of pseudo-hexagonal rosettes (6.5 A ˚ ◊ 6A ˚ ) and small (11:0] oriented (1◊1) islands along with {001}-oriented strands is induced at temperatures from 470 to 660 K. After annealing above 830 K, only regular (1◊1) terraces and white strands are observed. The composition of these oxygen-induced phases is quantified using 18O 2 gas in combination with LEIS and SSIMS measurements. The dependence of the restructuring process on annealing time, annealing temperature, and sample history is systematically investigated. Exposure to H 2 18O and air in the same temperature regime fails to induce the restructuring. UHV annealing of restructured, oxygen-enriched TiO 2 (110) surface smooths the surfaces and converts the rosette networks into strands and finally into the regular (1◊1) terraces. This is reported in an accompanying paper [M. Li, W. Hebenstreit, U. Diebold, Phys. Rev. B (1999), submitted ]. The rosette model is supported by first-principles density functional calculations which show a stable structure results, accompanied by significant relaxations from bulk-truncated positions. A mechanism for the dynamic processes of the formation of rosettes and (1◊1) islands is presented and the importance of these results for the surface chemistry of TiO 2 (110) surfaces is discussed.

All-atom ab initio energy minimization of the kaolinite crystal structure

Abstract Calculations that minimize the energy and optimize the geometry of all atomic coordinates for two proposed kaolinite crystal structures were performed using a first-principles, quantum chemical code based on local density functional theory. All calculations were performed using published unit-cell parameters. Inner- and interlayer H atom positions agree well with those determined by Bish (1993) from neutron diffraction data and confirm a unit cell with C1 symmetry.

Long‐range poisoning of D2 dissociative chemisorption on Pt(111) by coadsorbed K

We report detailed molecular beam studies of the effects of K adsorption on the dissociative chemisorption probabilities S0 for D2 on Pt(111). In contrast to conventional wisdom for many other molecular systems, we find that K is a very strong poison rather than promoter for H2 dissociation. S0 decreases roughly exponentially with K coverage ΘK. The effective cross section for poisoning per adsorbed K varies between 70 and 430 A2, depending upon incident energy Ei. This suggests that an extremely long‐range electronic perturbation is responsible for the poisoning. A theoretical model is developed to describe these sticking measurements. It is based on the fact that the adsorbed K lowers the work function of the surface. This enhances Pauli repulsion for the molecule‐surface interaction which, in turn, increases the barrier to dissociation. When the model is generalized to include inhomogeneous effects through a local work function, excellent agreement is obtained between the model and experiments. This mo...

Ab initio calculations of Ru, Pd, and Ag cluster structure with 55, 135, and 140 atoms

A massively parallel ab initio computer code, which uses Gaussian bases, pseudopotentials, and the local density approximation, permits the study of transition-metal systems with literally hundreds of atoms. We present total energies and relaxed geometries for Ru, Pd, and Ag clusters with N=55, 135, and 140 atoms. The N=55 and 135 clusters were chosen because of simultaneous cubo-octahedral (fcc) and icosahedral (icos) subshell closings, and we find icos geometries are preferred. Remarkably large compressions of the central atoms are observed for the icos structures (up to 6% compared with bulk interatomic spacings), while small core compressions (∼1%) are found for the fcc geometry. In contrast, large surface compressive relaxations are found for the fcc clusters (∼2%–3% in average nearest neighbor spacing), while the icos surface displays small compressions (∼1%). Energy differences between icos and fcc are smallest for Pd, and for all systems the single-particle densities of states closely resembles bu...

VALENCE AND ATOMIC SIZE DEPENDENT EXCHANGE BARRIERS IN VACANCY-MEDIATED DOPANT DIFFUSION

First-principles pseudopotential calculations of dopant-vacancy exchange barriers indicate a strong dependency on dopant valence and atomic size, in contrast to current models of vacancy-mediated dopant diffusion. First-row elements (B, C, N) are found to have exchange barriers which are an order of magnitude larger than the assumed value of 0.3 eV (the Si vacancy migration energy).

Simple intrinsic defects in gallium arsenide

We investigate the structural properties and energy levels of simple intrinsic defects in gallium arsenide. The first-principles calculations (1) apply boundary conditions appropriate to charge defects in supercells and enable quantitatively accurate predictions of defect charge transitions with a supercell approximation, (2) are demonstrated to be converged with respect to cell size and (3) assess the sensitivity to model construction to Ga pseudopotential construction (3d core or 3d valence) and density functionals (local density or generalized gradient approximation). With these factors controlled, we present the first quantitatively reliable survey of defect levels in GaAs, reassess the available literature and begin to decipher the complexity of GaAs defect chemistry. The computed defect level spectrum spans the experimental GaAs band gap, defects exhibit multiple bistabilities with (sometimes overlapping) negative-U systems, express more extensive charge states than previously anticipated and collectively suggest that our atomistic understanding of GaAs defect physics needs to be reassessed.

Theory of persistent, p-type, metallic conduction in c-GeTe

It has been known for over twenty years that rhombohedral c-germanium telluride is predicted to be a narrow gap semiconductor. However, it always displays p-type metallic conduction. This behaviour is also observed in other chalcogenide materials, including Ge2Sb2Te5, commonly used for optically and electrically switched, non-volatile memory, and so is of great interest. We present a theoretical study of the electronic structure of the perfect crystal and of the formation energies of germanium/tellurium vacancy and antisite defects in rhombohedral germanium telluride. We find that germanium vacancies are by far the most readily formed defect, independent of Fermi level and of growth ambient. Moreover, we predict that the perfect crystal is thermodynamically unstable. Thus, the predicted large equilibrium densities of the germanium vacancy of ~5 × 1019 cm−3 results in a partially filled valence band and in the observed p-type conductivity.

Ab initio calculations of adsorbate hydrogen-bond strength: ammonia on Pt(111)

Seven-layer slab results for 14 monolayer of NH3 on Pt(111) (so called α-NH3) are compared with NH3 on a 91-atom Pt cluster; we find that the latter closely mimics the extended surface. The calculations predict atop site occupancy for α-NH3 with N-down. The H-bond between α-NH3 and an additional N-down molecule (β-NH3) approaching from the gas-phase is then compared with that of two molecules in the gas phase; we discover the H-bond on the surface is almost three times stronger and the bond length appreciably shorter. Geometry relaxation then results in a 65 ± 5 degree tilt of the β-NH3 axis. Finally, slab calculations with 14ML each of α- and β-NH3 support this geometry over symmetrically coordinated β-NH3 and predict an adsorption energy in good agreement with experiment.

Fast through-bond diffusion of nitrogen in silicon

We report first-principles total energy calculations of interaction of nitrogen in silicon with silicon self-interstitials. Substitutional nitrogen captures a silicon interstitial with 3.5 eV binding energy forming a 〈100〉 split interstitial ground-state geometry, with the nitrogen forming three bonds. The low-energy migration path is through a bond bridge state having two bonds. Fast diffusion of nitrogen occurs through a pure interstitialcy mechanism: the nitrogen never has less than two bonds. Near-zero formation energy of the nitrogen interstitialcy with respect to the substitutional rationalizes the low solubility of substitutional nitrogen in silicon.