Veronika Brázdová

Automatic data distribution and load balancing with space-filling curves: implementation in CONQUEST.

We present an automatic, spatially local data distribution and load balancing scheme applicable to many-body problems running on parallel architectures. The particle distribution is based on spatial decomposition of the simulation cell. A one-dimensional Hilbert curve is mapped onto the three-dimensional real space cell, which reduces the dimensionality of the problem and provides a way to assign different spatially local parts of the cell to each processor. The scheme is independent of the number of processors. It can be used for both ordered and disordered structures and does not depend on the dimensionality or shape of the system. Details of implementation in the linear-scaling density functional code CONQUEST, as well as several case studies of systems of various complexity, containing up to 55 755 particles, are given.

Pseudo-atomic orbitals as basis sets for the O(N) DFT code CONQUEST

Various aspects of the implementation of pseudo-atomic orbitals (PAOs) as basis functions for the linear scaling CONQUEST code are presented. Preliminary results for the assignment of a large set of PAOs to a smaller space of support functions are encouraging, and an important related proof on the necessary symmetry of the support functions is shown. Details of the generation and integration schemes for the PAOs are also given.

Investigating individual arsenic dopant atoms in silicon using low-temperature scanning tunnelling microscopy.

We study subsurface arsenic dopants in a hydrogen-terminated Si(001) sample at 77 K, using scanning tunnelling microscopy and spectroscopy. We observe a number of different dopant-related features that fall into two classes, which we call As1 and As2. When imaged in occupied states, the As1 features appear as anisotropic protrusions superimposed on the silicon surface topography and have maximum intensities lying along particular crystallographic orientations. In empty-state images the features all exhibit long-range circular protrusions. The images are consistent with buried dopants that are in the electrically neutral (D0) charge state when imaged in filled states, but become positively charged (D+) through electrostatic ionization when imaged under empty-state conditions, similar to previous observations of acceptors in GaAs. Density functional theory calculations predict that As dopants in the third layer of the sample induce two states lying just below the conduction-band edge, which hybridize with the surface structure creating features with the surface symmetry consistent with our STM images. The As2 features have the surprising characteristic of appearing as a protrusion in filled-state images and an isotropic depression in empty-state images, suggesting they are negatively charged at all biases. We discuss the possible origins of this feature.

Nucleation of gold atoms on vanadyl-terminated V2O3(0001)

The adsorption of Au atoms on a vanadyl-terminated V2O3 film grown on Au(111) is studied by means of low-temperature STM and DFT+U calculations. The adatoms preferentially bind in an O Au O bridge configuration between two adjacent V O groups. Missing vanadyl groups that have been identified as the characteristic surface defect do not offer an attractive binding environment and are only sparsely occupied with Au. On the other hand, varying concentrations of V2O3 bulk defects that modulate the oxide electronic structure are found to affect the spatial distribution of Au atoms on the oxide surface.

Site-Dependent Ambipolar Charge States Induced by Group V Atoms in a Silicon Surface

We report that solitary bismuth and antimony atoms, incorporated at Si(111) surfaces, induce either positive or negative charge states depending on the site of the surface reconstruction in which they are located. This is in stark contrast to the hydrogenic donors formed by group V atoms in silicon bulk crystal and therefore has strong implications for the design and fabrication of future highly scaled electronic devices. Using scanning tunnelling microscopy (STM) and density functional theory (DFT) we determine the reconstructions formed by different group V atoms in the Si(111)2 × 1 surface. Based on these reconstructions a model is presented that explains the polarity as well as the location of the observed charges in the surface. Using locally resolved scanning tunnelling spectroscopy we are furthermore able to map out the spatial extent over which a donor atom influences the unoccupied surface and bulk electronic states near the Fermi-level. The results presented here therefore not only show that a dopant atom can induce both positive and negative charges but also reveal the nature of the local electronic structure in the region of the silicon surface where an individual donor atom is present.

Fast Exfoliation and Functionalisation of Two-Dimensional Crystalline Carbon Nitride by Framework Charging

2D layered graphitic carbon nitride nanosheets offer tunable electronic and chemical properties. However, exfoliation and functionalisation of gCN for specific applications remains challenging. We report a scalable one-pot reductive method to produce solutions of single and few layer 2D gCN nanosheets with excellent stability in a high mass yield (35%) from polytriazine imide. High resolution imaging confirms the intact crystalline structure and identifies an AB stacking. The first successful deliberate organic functionalisation of dissolved gCN is illustrated, providing a general route to adjust their properties.

Hydrogen adsorption and diffusion around Si(0 0 1)/Si(1 1 0) corners in nanostructures

While the diffusion of hydrogen on silicon surfaces has been relatively well characterized, both experimentally and theoretically, diffusion around corners between surfaces, as will be found on nanowires and nanostructures, has not been studied. Motivated by nanostructure fabrication by Patterned Atomic Layer Epitaxy, we present a density functional theory study of the diffusion of hydrogen around the edge formed by the orthogonal (0 0 1) and (1 1 0) surfaces in silicon. We find that the barrier from (0 0 1) to (1 1 0) is approximately 0.3 eV lower than from (1 1 0) to (0 0 1), and that it is comparable to diffusion between rows on a clean surface, with no significant effect on the hydrogen patterns at the growth temperatures used.