Atomistic Frictional Properties of the C(100)2x1-H Surface

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 1.39Å. The obtained C(100)2x1-H surfaces (C-H bond length 1.15Å) were placed in a large vacuum space and translated toward each other. A cohesive state at a surface separation of 4.32Å 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. C-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.

A first‐principles investigation on interactions between various surface‐terminated diamond films

“…Thus, from the aspects of computational efficiency and accuracy, the slab thickness of the surface‐terminated diamond surface model includes eight carbon atom layers (also eight C atoms in each layer) and two termination layers. In the previous computational studies, eight layers even six layers of carbon atoms were also proved thick enough to represent the diamond surfaces. For the H‐terminated diamond (100) surface, the reconstructed 2 × 1 structure is also the most stable structure, the same as that of the clean surface .…”

Perfluoropolyethers: Development of an All-Atom Force Field for Molecular Simulations and Validation with New Experimental Vapor Pressures and Liquid Densities

“…f 1 −f 4 are zero for the vast majority of systems studied with OPLS-AA, 35,36,41 but in this work, some nonzero values are reported. Unlike their hydrogenated counterparts, PFPEs assume conformations that are asymmetric about ether (i.e., C−O) bonds 33,34 and thus phase angles are required to accurately model all dihedrals with central CF n −O bonds. To fit dihedral energy profiles, leastsquares regression was performed using the open-source Multidisciplinary Design Analysis and Optimization (Open-MDAO) framework written in Python 47 to minimize the differences in relative energies between ab initio structures and energies calculated according to eq 2; in some cases, one or more Fourier terms were removed during this process because they were not needed to accurately reproduce the curve.…”

“…42 This observation is consistent with experiments and prior QM calculations, which showed that the C−O bond in perfluorodimethyl ether (CF 3 −O−CF 3 ) is shortened (r 0 ≈ 1.37 Å) compared to dimethyl ether (CH 3 − O−CH 3 ) (r 0 ≈ 1.43 Å). 33,34 Another example is the C F −O−C F angle, whose equilibrium value (θ 0 ) was increased to 121.4°f rom the equilibrium value of 109.5°used to model C H −O−C H angles in hydrogenated ethers. 42 This result is also consistent with experiments and prior QM calculations, which have shown that the C−O−C angle in perfluorodimethyl ether is significantly increased (θ 0 ≈ 120°) compared to dimethyl ether (θ 0 ≈ 112°).…”

“…Note that the fitting procedure requires all energetic components of E total with the exception of E dihedral (see eq 1) to be calculated from the force field itself; these energies were calculated using the E nonbonded parameters from model #1 and optimized E bond and E angle parameters. As previously mentioned, PFPEs adopt conformations that are asymmetric about ether (i.e., C−O) bonds, 33,34 so in order to accurately model dihedrals with central CF n −O bonds using eq 2, nonzero phase angles (f 1 −f 4 ) are required. For example, if we consider the most energetically favorable conformations of dimethyl ether ( Figure 3A) and perfluorodimethyl ether ( Figure 3B), the H− C H −O−C H dihedral in dimethyl ether assumes a staggered conformation as expected (i.e., hydrogen atoms on the proximal carbon are arranged to maximize their distances from each other and the other carbon), while the F−C F −O−C F dihedral in perfluorodimethyl ether increases by a further ∼17°; this result is consistent with the experimental observation of 14°, 33 and prior QM calculations which predict values ranging from 15 to 17°.…”

“…22 Tani et al studied the conformational behavior of various PFPE thin-films on hydrogenated carbon surfaces using the DREIDING force field and determined that molecular structure (i.e., backbone type and end group functionalization) could impact lubricant film thickness, 23,24 surface coverage, 24,25 and adhesive properties. 26 However, these AA force fields were not derived to specifically model PFPEs and have not been sufficiently validated for use with PFPEs, which is of particular concern given that, unlike molecules containing related functional groups (e.g., hydrogenated ethers, perfluoroalkanes, and perfluoroalcohols), PFPEs tend to adopt asymmetric conformations about ether (i.e., C−O) bonds, 33,34 which may significantly impact fundamental molecular properties. While Li et al previously derived a united-atom (UA) potential specifically for PFPEs, 2,3 to date, no such all-atom force field has been developed.…”