Abstract:
We use ab initio density functional calculations to study hydrogen-induced disintegration of single- and multi-wall carbon fullerenes and nanotubes. Our results indicate that hydrogen atoms preferentially chemisorb along lines in sp2 bonded carbon nanostructures, locally weakening the carbon bonds and releasing stress. For particular structural arrangements, hydrogen helps to relieve the accumulated stress by inducing step-wise local cleavage leading to disintegration of the outermost wall.

Abstract:
We investigate a previously unknown phase of phosphorus that shares its layered structure and high stability with the black phosphorus allotrope. We find the in-plane hexagonal structure and bulk layer stacking of this structure, which we call `blue phosphorus', to be related to graphite. Unlike graphite and black phosphorus, blue phosphorus displays a wide fundamental band gap and should exfoliate easily to form quasi-2D structures suitable for electronic applications. We study a likely transformation pathway from black to blue phosphorus and discuss possible ways to synthesize the new structure.

Abstract:
We use ab initio density functional calculations to study the effect of electron and hole doping on the equilibrium geometry and electronic structure of C, Si, and S monatomic wires. Independent of doping, all these nanowires are found to be metallic. In absence of doping, C wires are straight, whereas Si and S wires display a zigzag structure. Besides two preferred bond angles of 60 deg and 120 deg in Si wires, we find an additional metastable bond angle of 90 deg in S wires. The equilibrium geometry and electronic structure of these nanowires is shown to change drastically upon electron and hole doping.

Abstract:
Carrier injection into carbon nanotubes and graphene nanoribbons, contacted by a metal coating over an arbitrary length, is studied by various means: Minimal models allow for exact analytic solutions which can be transferred to the original system with high precision. Microscopic ab initio calculations of the electronic structure at the carbon-metal interface allow us to extract -- for Ti and Pd as contacting materials -- realistic parameters, which are then used in large scale tight-binding models for transport calculations. The results are shown to be robust against nonepitaxially grown electrodes and general disorder at the interface, as well as various refinements of the model.

Abstract:
We use ab initio density functional calculations to study the chemical functionalization of single-wall carbon nanotubes and graphene monolayers by silyl (SiH3) radicals and hydrogen. We find that silyl radicals form strong covalent bonds with graphene and nanotube walls, causing local structural relaxations that enhance the sp3 character of these graphitic nanostructures. Silylation transforms all carbon nanotubes into semiconductors, independent of their chirality. Calculated vibrational spectra suggest that specific frequency shifts can be used as a signature of successful silylation.

Abstract:
We use ab initio density functional theory to study the equilibrium geometry and electronic structure of few-layer grey arsenic. In contrast to the bulk structure that is semimetallic, few-layer grey As displays a significant band gap that depends sensitively on the number of layers, in-layer strain, layer stacking and inter-layer spacing. A metal-semiconductor transition can be introduced by changing the number of layers or the in-layer strain. We interpret this transition by an abrupt change in the spatial distribution of electronic states near the top of the valence band.

Abstract:
Using Monte Carlo simulations, we investigate the possibility of an orientational melting transition within a "rope" of (10,10) carbon nanotubes. When twisting nanotubes bundle up during the synthesis, orientational dislocations or twistons arise from the competition between the anisotropic inter-tube interactions, which tend to align neighboring tubes, and the torsion rigidity that tends to keep individual tubes straight. We map the energetics of a rope containing twistons onto a lattice gas model and find that the onset of a free "diffusion" of twistons, corresponding to orientational melting, occurs at T_OM > 160 K.

Abstract:
We combine {\em ab initio} density functional theory (DFT) structural studies with DFT-based nonequilibrium Green function calculations to investigate how the presence of non-hexagonal rings affects electronic transport in graphitic structures. We find that infinite monolayers, finite-width nanoribbons and nanotubes formed of 5-8 haeckelite with only 5- and 8-membered rings are generally more conductive than their graphene-based counterparts. Presence of haeckelite defect lines in the perfect graphitic structure, a model of grain boundaries in CVD-grown graphene, increases the electronic conductivity and renders it highly anisotropic.

Abstract:
Combining time-dependent density functional calculations for electrons with molecular dynamics simulations for ions, we investigate the dynamics of excited carriers in a (3,3) carbon nanotube at different temperatures. Following an hv=6.8 eV photoexcitation, the carrier decay is initially dominated by efficient electron-electron scattering. At room temperature, the excitation gap is reduced to nearly half its initial value after ~230 fs, where coupling to phonons starts dominating the decay. We show that the onset point and damping rate in the phonon regime change with initial ion velocities, a manifestation of temperature dependent electron-phonon coupling.

Abstract:
As a way to further improve the electronic properties of group V layered semiconductors, we propose to form in-layer 2D heterostructures of black phosphorus and grey arsenic. We use \textit{ab initio} density functional theory to optimize the geometry, determine the electronic structure, and identify the most stable allotropes as a function of composition. Since pure black phosphorus and pure grey arsenic monolayers differ in their equilibrium structure, we predict a structural transition and a change in frontier states, including a change from a direct-gap to an indirect-gap semiconductor, with changing composition.