Spin density distributions on graphene clusters and ribbons with carbene-like active sites.

Aiming to better understand the reactivity of graphene-based materials, the present work employs density functional theory that provides detailed information about spin-density distributions for single and contiguous pairs of carbene-like active sites. In order to examine the extent to which different models, methodologies, and approximations affect the outcome, our calculations employ the AIMPRO, QuantumEspresso and Gaussian program packages. Models are in the form of polycyclic aromatic hydrocarbons (PAHs) and graphene nanoribbons (GNRs), both isolated and within supercells with periodic boundary conditions. Benchmarking calculations for the phenyl radical and cation are also presented. General agreement is found among the methods and also with previous studies. A significant electron spin polarization (spin density >1.096 electron spin) on the active sites is seen in both periodic and cluster systems, but it tends to be lower for GNRs than graphene clusters. The effect of the functional seems to be much more important than the position of singularities at the edges of the GNRs. Finally, we show the interactions and effects on spin density when a single site lies at the edge of a bilayer GNR, where bonding between layers may occur under specific circumstances.

Vacancy diffusion and coalescence in graphene directed by defect strain fields.

The formation of extended defects in graphene from the coalescence of individual mobile vacancies can significantly alter its mechanical, electrical and chemical properties. We present the results of ab initio simulations which demonstrate that the strain created by multi-vacancy complexes in graphene determine their overall growth morphology when formed from the coalescence of individual mobile lattice vacancies. Using density functional theory, we map out the potential energy surface for the motion of mono-vacancies in the vicinity of multi-vacancy defects. The inhomogeneous bond strain created by the multi-vacancy complexes strongly biases the activation energy barriers for single vacancy motion over a wide area. Kinetic Monte Carlo simulations based on rates from ab initio derived activation energies are performed to investigate the dynamical evolution of single vacancies in these strain fields. The resultant coalescence processes reveal that the dominant morphology of multi-vacancy complexes will consist of vacancy lines running in the two primary crystallographic directions, and that more thermodynamically stable structures, such as holes, are kinetically inaccessible from mono-vacancy aggregation alone.