Molecular Dynamics Simulations of Nitric Oxide Scattering Off Graphene

Abstract

We performed classical molecular dynamics simulations to model the scattering process of nitric oxide, NO, off graphene supported on gold. This is motivated by our desire to probe the energy transfer in collisions with graphene. Since many of these collision systems comprising of graphene and small molecules have been shown to scatter non‐reactively, classical molecular dynamics appear to describe such systems sufficiently. We directed thousands of trajectories of NO molecules onto graphene along the surface normal, while varying impact position, but also speed, orientation, and rotational excitation of the nitric oxide, and compare the results with experimental data. While experiment and theory do not match quantitatively, we observe agreement that the relative amount of kinetic energy lost during the collision increases with increasing initial kinetic energy of the NO. Furthermore, while at higher collision energies, all NO molecules lose some energy, and the vast majority of NO is scattered back, in contrast at low impact energies, the fraction of those nitric oxide molecules that are trapped at the surface increases, and some NO molecules even gain some kinetic energy during the collision process. The collision energy seems to preferentially go into the collective motion of the carbon atoms in the graphene sheet.

Velocity-Selected Rotational State Distributions of Nitric Oxide Scattered off Graphene Revealed by Surface-Velocity Map Imaging

“…It appears as if the NO molecules transfer much of their translational energy to the graphene substrate in just a few collisions, but it is possible that only those with a cold(er) rotational temperature are scattered back, while those that are (partly) thermalizing may become trapped, leading to the relatively cold rotational state distribution of the trapping− desorption component. Our previous MD simulations for NO scattering off graphene can shed some light onto that, 35 though they do not compare quantitatively and do not reproduce the two well-separated scattered components observed here (directly scattered and trapping−desorption). In the simulations, the majority of NO scatters off graphene directly, undergoing only a single collision, but there is a small fraction of NO molecules that interact with the surface for only a couple of picoseconds before desorbing again (though these do not show up as a separate component in our time-of-flight profiles), presumably undergoing too few collisions to rotationally thermalize.…”

“…The accompanying classical molecular dynamics simulations have been described previously. 35 The gold substrate was formed of a 6 × 6 × 6 array of gold atoms, and 98 carbon atoms were positioned in a hexagonal 2D network in the x – y plane on top of the gold substrate. Periodic boundary conditions were applied along the x – y plane but with no periodicity in the z dimension.…”