Recent ab initio calculations without spin [P. Koskinen et al., Phys. Rev. Lett. 101, 115502 (2008)] predict that the zigzag edges of graphene should be reconstructed, albeit with an energy barrier to be overcome. After verifying that spin-polarized calculations give qualitatively the same result, we study the mechanism and the free energy of the reconstruction of the zigzag edges from a periodic hexagon structure (zz) to an alternate pentagon-heptagon structure [zz(57)] using the empirical long-range carbon bond order potential II (LCBOPII) potential. The zz(57) edges are stabilized by an almost triple bond similar to that of the armchair edges, and we propose a way to account for this quantum mechanical effect in the LCBOPII potential. Aside from that, the reconstructed edge is flat as a result of tensile edge stress. The reconstruction occurs spontaneously in molecular dynamics simulations at high temperature, leading to the identification of a reaction coordinate for the reconstruction that allows us to calculate the free-energy barrier by using Monte Carlo simulations and umbrella sampling. At room temperature, we find a free-energy barrier of 0.83 eV for the first transformations of two hexagons to a pentagon-heptagon pair that is higher than the one for a fully reconstructed edge and increasing with temperature.

Using density functional theory, we study proton permeation through graphene and hexagonal boron nitride. We consider several factors influencing the barriers for permeation, including structural optimization, the role of the solvent, surface curvature and proton transport through hydrogenated samples. Furthermore, we discuss the ground state charge transfer from the membrane to the proton and the strong tendency for bond formation. If the process is assumed to be slow we find that none of these effects lead to a satisfactory answer to the observed discrepancies between theory and experiment.

We present an extended Tersoff potential for boron nitride (BN-ExTeP) for application in large scale atomistic simulations. BN-ExTeP accurately describes the main low energy B, N, and BN structures and yields quantitatively correct trends in the bonding as a function of coordination. The proposed extension of the bond order, added to improve the dependence of bonding on the chemical environment, leads to an accurate description of point defects in hexagonal BN (h-BN) and cubic BN (c-BN). We have implemented this potential in the molecular dynamics LAMMPS code and used it to determine some basic properties of pristine 2D h-BN and the elastic properties of defective h-BN as a function of defect density at zero temperature. Our results show that there is a strong correlation between the size of the static corrugation induced by the defects and the weakening of the in-plane elastic moduli.

Density-functional-theory based calculations of two single-walled carbon nanotubes of different chirality settle open issues on the sidewall chemisorption of atomic oxygen at low concentrations. Ether groups are the thermodynamically favored configurations. If kinetically trapped in epoxide groups, oxygen introduces characteristic new levels in the gap of the nanotube that are detected with scanning tunneling spectroscopy experiments. Discrepancies with previous predictions are shown to originate from the inadequacy of previous models to describe low-concentration oxygen adsorbated on nanotubes.

Contact-passivated sensor devices allow one to measure the response of individual ultraclean single-walled carbon nanotubes to 1 ppm NO2, and show that the activation energies for desorption from nanotubes of diameters in the 1.5–3.5 nm range are of the order of 1 eV. DFT calculations based on several exchange-correlation functionals are presented and critically examined. The nature of the molecular binding is thus clarified for NO2, N2O4, and NO3, and also the dependence on the size of the nanotube. The binding strength of physisorbed NO3 is consistent with the experimental data on desorption.