Stability of extended defects on boron nitride and graphene monolayers: the role of chemical environment

We perform ab initio calculations that indicate that the relative stability of antiphase boundaries (APB) with armchair and zigzag chiralities in monolayer boron nitride (BN) is determined by the chemical potentials of the boron and nitrogen species in the synthesis process. In an N-rich environment, a zigzag APB with N-rich core is the most stable structure, while under B-rich or intrinsic growth conditions, an armchair APB with stoichiometric core is the most stable. This stability transition is shown to arise from a competition between homopolar-bond (B-B and N-N) and elastic energy costs in the core of the APBs. Moreover, in the presence of a carbon source we find that a carbon-doped zigzag APB becomes the most stable boundary near the N-rich limit. The electronic structure of the two types of APBs in BN is shown to be particularly distinct, with the zigzag APB depicting defect-like deep electronic bands in the band gap, while the armchair APB shows bulk-like shallow electronic bands.

Non-hexagonal-ring defects and structures induced by strain in graphene and in functionalized graphene

We perform {textit ab initio} calculations for the strain-induced formation of non-hexagonal-ring defects in graphene, graphane (planar CH), and graphenol (planar COH). We find that the simplest of such topological defects, the Stone-Wales defect, acts as a seed for strain-induced dissociation and multiplication of topological defects. Through the application of inhomogeneous deformations to graphene, graphane and graphenol with initially small concentrations of pentagonal and heptagonal rings, we obtain several novel stable structures that possess, at the same time, large concentrations of non-hexagonal rings (from fourfold to elevenfold) and small formation energies.

Surface dangling bond states and band-lineups in hydrogen-terminated Si, Ge, and Ge/Si nanowires

We report an ab initio study of the electronic properties of surface dangling-bond (SDB) states in hydrogen-terminated Si and Ge nanowires with diameters between 1 and 2 nm, Ge/Si nanowire heterostructures, and Si and Ge (111) surfaces. We find that the charge transition levels e(+/-) of SDB states behave as a common energy reference among Si and Ge wires and Si/Ge heterostructures, at 4.3 +/- 0.1 eV below the vacuum level. Calculations of e(+/-) for isolated atoms indicate that this nearly constant value is a periodic-table atomic property.

Gap opening in topological-defect lattices in graphene

Ab initio calculations indicate that topological-defect networks in graphene display the full variety of single-particle electronic structures, including Dirac-fermion null-gap semiconductors, as well as metallic and semiconducting systems of very low formation energies with respect to a pristine graphene sheet. Corrugation induced by the topological defects further reduces the energy and tends to reduce the density of states at the Fermi level, to widen the gaps, or even to lead to gap opening in some cases where the parent planar geometry is metallic.