MODELING DISPERSION INTERACTIONS ON METAL SURFACES USING THE EXCHANGE-HOLE DIPOLE MOMENT

Abstract

Graphene is a two-dimensional material possessing unique electrical and physi- cal properties. London dispersion interactions play a significant role in its adsorp- tion and friction on metal surfaces. Accurate computational modeling of these processes is complicated by the fact that conventional density-functional methods do not include the proper physics to describe dispersion interactions. Model- ing dispersion between a metal surface and substrate is found to be particularly complex as models based on properties of the free metal atoms alone cause the interaction strength to be over-estimated. As an alternative, the exchange-hole dipole moment (XDM) method is a density-dependent dispersion correction that has previously been shown to model dispersion interactions accurately for both molecules and solids. In this thesis, we first test the validity of XDM for model- ing surface-substrate dispersion interactions for a set of small aromatic molecules physisorbed on noble-metal (111) surfaces. Upon validation of XDM for molec- ular adsorption, we investigated interfaces of single and double-layered graphene on selected transition-metal surfaces in two rotational orientations. Our results show that thermal effects greatly affect the potential energy surface for graphene on a nickel surface. In general, the rotational orientation significantly affects graphene interlayer distances and interactions and there is an energetic preference for substrate alignment. Our results also demonstrate that metal substrates af- fect interlayer distances and exfoliation energies for bilayer graphene systems that chemisorb to metal surfaces. The sliding of multi-layered graphene is investigated in detail for copper surfaces. It is shown that the energetically preferred sliding interface is between graphene and the copper surface, even when subjected to an applied constraint. Our results consistently demonstrate that XDM captures the proper physics required to model surface dispersion interactions.