Orientation and stability of a bi-functional aromatic organic molecular adsorbate on silicon.

In this work we combine scanning tunneling microscopy, near-edge X-ray absorption fine structure spectroscopy, X-ray photoemission spectroscopy and density functional theory to resolve a long-standing confusion regarding the adsorption behaviour of benzonitrile on Si(001) at room temperature. We find that a trough-bridging structure is sufficient to explain adsorption at low coverages. At higher coverages when steric hindrance prevents the phenyl ring lying flat on the surface, the 2+2 cycloaddition structure dominates.

Reaction paths of phosphine dissociation on silicon (001).

Using density functional theory and guided by extensive scanning tunneling microscopy (STM) image data, we formulate a detailed mechanism for the dissociation of phosphine (PH3) molecules on the Si(001) surface at room temperature. We distinguish between a main sequence of dissociation that involves PH2+H, PH+2H, and P+3H as observable intermediates, and a secondary sequence that gives rise to PH+H, P+2H, and isolated phosphorus adatoms. The latter sequence arises because PH2 fragments are surprisingly mobile on Si(001) and can diffuse away from the third hydrogen atom that makes up the PH3 stoichiometry. Our calculated activation energies describe the competition between diffusion and dissociation pathways and hence provide a comprehensive model for the numerous adsorbate species observed in STM experiments.

Reaction pathways for pyridine adsorption on silicon (0 0 1).

Density functional theory is used to describe the reactions of chemisorption of pyridine on the silicon (0 0 1) surface. Adsorption energies of six relevant structures, and the activation energies between them are reported. We consider in detail the dative to tight-bridge transition for which conflicting results have been reported in the literature, and provide a description of the formation of inter-row chains observed in high-coverage experiments. We demonstrate that the choice of DFT functional has a considerable effect on the relative energetics and of the four DFT functionals considered, we find that the range-separated hybrid ωB97X-D functional with empirical dispersion provides the most consistent description of the experiment data.

n-type doping of germanium from phosphine: early stages resolved at the atomic level.

To understand the atomistic doping process of phosphorus in germanium, we present a combined scanning tunneling microscopy, temperature programed desorption, and density functional theory study of the reactions of phosphine with the Ge(001) surface. Combining experimental and theoretical results, we demonstrate that PH(2) + H with a footprint of one Ge dimer is the only product of room temperature chemisorption. Further dissociation requires thermal activation. At saturation coverage, PH(2) + H species self-assemble into ordered patterns leading to phosphorus coverages of up to 0.5 monolayers.

A comprehensive survey of M(2)AX phase elastic properties.

M(2)AX phases are a family of nanolaminate, ternary alloys that are composed of slabs of transition metal carbide or nitride (M(2)X) separated by single atomic layers of a main group element. In this combination, they manifest many of the beneficial properties of both ceramic and metallic compounds, making them attractive for many technological applications. We report here the results of a large scale computational survey of the elastic properties of all 240 elemental combinations using first-principles density functional theory calculations. We found correlations revealing the governing role of the A element and its interaction with the M element on the c axis compressibility and shearability of the material. The role of the X element is relatively minor, with the strongest effect seen in the in-plane constants C(11) and C(12). We identify several elemental compositions with extremal properties such as W(2)SnC, which has by far the lowest value of C(44), suggesting potential applications as a high-temperature dry lubricant.

Structure and local-equilibrium thermodynamics of the c(2x2) reconstruction of rutile TiO2 (100).

We resolve the structure of a c(2x2) reconstruction of the rutile TiO2 (100) surface using a combination of transmission electron diffraction, direct methods analysis, and density functional theory. The surface structure contains an ordered array of subsurface oxygen vacancies and is in local thermodynamic equilibrium with bulk TiO2, but not the with oxygen gas-phase environment. The transition into a bulklike (1x1) reconstruction offers insights into the time-dependent local thermodynamics of TiO2 surface reconstruction under global nonequilibrium conditions.

Single P and As dopants in the Si(001) surface.

Using first-principles density functional theory, we discuss doping of the Si(001) surface by a single substitutional phosphorus or arsenic atom. We show that there are two competing atomic structures for isolated Si-P and Si-As heterodimers, and that the donor electron is delocalized over the surface. We also show that the Si atom dangling bond of one of these heterodimer structures can be progressively charged by additional electrons. It is predicted that surface charge accumulation as a result of tip-induced band bending leads to structural and electronic changes of the Si-P and Si-As heterodimers which could be observed experimentally. Scanning tunneling microscopy (STM) measurements of the Si-P heterodimer on a n-type Si(001) surface reveal structural characteristics and a bias-voltage dependent appearance, consistent with these predictions. STM measurements for the As:Si(001) system are predicted to exhibit similar behavior to P:Si(001).

Amorphous and crystalline phases in thermal quench simulations of alumina.

The authors report molecular dynamics simulations of alumina (Al2O3) during crystallization from the melt. Using liquid quench methods, they investigate the effect of cooling rate on the structural evolution of the alpha, kappa, and the bixbyite phases. A critical temperature window is identified, where the time spent in this window is crucial in determining the extent to which the systems approach crystallinity. A strong dependence is observed between the final structure and the quench rate, which is most pronounced for the alpha phase and to lesser extent for the other phases. The results show that the different phases have different tendencies to crystallize that are determined by energetics, complexity of crystal structure, and the number of metastable states.

A theoretical and experimental study of lead substitution in calcium hydroxyapatite.

Characterization of lead substitution for calcium in hydroxyapatite (CaHA) is carried out, using experimental techniques and Density Functional theoretical (DFT) analyses. Theoretical modeling is used to obtain information of the Pb chemical environment for occupancy at either Ca(I) or Ca(II) sites of CaHA. Effects of the larger ionic radius of Pb(+2) compared to Ca(+2) are apparent in embedded cluster calculations of local chemical bonding properties. DFT periodic planewave pseudopotential studies are used to provide first-principles predictions of local structural relaxation and site preference for Pb(x)Ca(10-x)HA over the composition range x< or = 6. General characteristics of the polycrystalline material are verified by X-ray diffraction and FTIR analysis, showing the presence of a single phase of CaHA structure. A short range structure around lead is proposed in order to interpret the Pb L-edge EXAFS spectrum of the solid solution Ca(6.6)Pb(3.4)HA. In this concentration we observe that lead mainly occupies Ca(II) sites; the EXAFS fit slightly favors Pb clustering, while theory indicates the importance of Pb-Pb avoidance on site (II).

Phosphine dissociation on the Si(001) surface.

Density functional calculations are performed to identify features observed in STM experiments after phosphine (PH3) dosing of the Si(001) surface. On the basis of a comprehensive survey of possible structures, energetics, and simulated STM images, three prominent STM features are assigned to structures containing surface bound PH2, PH, and P, respectively. Collectively, the assigned features outline for the first time a detailed mechanism of PH3 dissociation and P incorporation on Si(001).

Direct observation of charge transfer at a MgO(111) surface.

Transmission electron diffraction (TED) combined with direct methods have been used to study the sqrt[3]xsqrt[3]R30 degrees reconstruction on the polar (111) surface of MgO and refine the valence charge distribution. The surface is nonstoichiometric and is terminated by a single magnesium atom. A charge-compensating electron hole is localized in the next oxygen layer and there is a nominal charge transfer from the oxygen atoms to the top magnesium atom. The partial charges that we obtain for the surface atoms are in reasonable agreement with empirical bond-valence estimations.