Abstract
Density functional theory- (DFT-) based ab initio calculations were used to investigate the surface-to-surface interaction and frictional behavior of two hydrogenated C(100) dimer surfaces. A monolayer of hydrogen atoms was applied to the fully relaxed C(100)2x1 surface having rows of C=C dimers with a bond length of 1.39 Å. The obtained C(100)2x1-H surfaces (C–H bond length 1.15 Å) were placed in a large vacuum space and translated toward each other. A cohesive state at a surface separation of 4.32 Å that is stabilized by approximately 0.42 eV was observed. An increase in the charge separation in the surface dimer was calculated at this separation having a 0.04 e transfer from the hydrogen atom to the carbon atom. The Mayer bond orders were calculated for the C–C and C–H bonds and were found to be 0.962 and 0.947, respectively.σC–H bonds did not change substantially from the fully separated state. A significant decrease in the electron density difference between the hydrogen atoms on opposite surfaces was seen and assigned to the effects of Pauli repulsion. The surfaces were translated relative to each other in the (100) plane, and the friction force was obtained as a function of slab spacing, which yielded a 0.157 coefficient of friction.
Highlights
Carbon-based surface films are ubiquitous as frictional barriers that lower the wear rates of interacting bodies
DFT calculations using the local density approximation (LDA) formalism reveal a cohesive state between two C(100)2x1 surfaces passivated with a monolayer of hydrogen
This state is characterized by strong charge transfer from the surface hydrogen atoms to the first layer of carbon atoms in the slab and a relative strengthening of the surface C–C of the dimer
Summary
Carbon-based surface films are ubiquitous as frictional barriers that lower the wear rates of interacting bodies. The subject of this report is the application of density functional theory- (DFT-) based ab initio calculations to the interaction of hydrogenated C(100)2x1 surfaces by probing the electronic and bonding structure changes during the frictional interaction of these surfaces. As preparation for the study of surface-to-surface interactions we assembled a C(100)2x1 slab model of the diamond surface (before hydrogenation) and calculated its relaxed geometry and electronic states (model and results are not shown as this study is focused on the hydrogenated surface) This model contained eight layers of carbon atoms (not including the dimer) in a large vacuum space that was set at forty times the layer spacing to ensure minimal effects of slabto-slab interactions. The outlined changes in this surface electronic structure from one dominated by olefinic character, and its resultant facile chemistry, to a dominantly aliphatic surface, makes further surface chemistry difficult and contributes to the known stability of this surface
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