  Diffusion and drift of a graphene flake on a graphite surface are analyzed. A
potential energy relief of the graphene flake is computed using ab initio and
empirical calculations. Based on the analysis of this relief, different
mechanisms of diffusion and drift of the graphene flake on the graphite surface
are considered. A new mechanism of diffusion and drift of the flake is
proposed. According to the proposed mechanism, rotational transition of the
flake from commensurate to incommensurate state takes place with subsequent
simultaneous rotation and translational motion until a commensurate state is
reached again, and so on. Analytic expressions for the diffusion coefficient
and mobility of the flake corresponding to different mechanisms are derived in
wide ranges of temperatures and sizes of the flake. The molecular dynamics
simulations and estimates based on ab initio and empirical calculations
demonstrate that the proposed mechanism can be dominant under certain
conditions. The influence of structural defects on the diffusion of the flake
is examined on the basis of calculations of the potential energy relief and
molecular dynamics simulations. The methods of control over the diffusion and
drift of graphene components in nanoelectromechanical systems are discussed.
The possibility to experimentally determine the barriers to relative motion of
graphene layers based on the study of diffusion of a graphene flake is
considered. The results obtained can be also applied to polycyclic aromatic
molecules on graphene and should be qualitatively valid for a set of
commensurate adsorbate-adsorbent systems.
