Surface Scattering Studies of Nitric Oxide off Graphene

Greenwood, Thomas (2024) Surface Scattering Studies of Nitric Oxide off Graphene. Doctoral thesis (PhD), Manchester Metropolitan University.

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Abstract

The scattering of nitric oxide off graphene supported on gold has been investigated using a surface velocity map imaging (VMI) set-up. Nitric oxide (NO) was seeded in a helium molecular beam and directed towards the graphene surface at different velocities along the surface normal. Both the beam and the scattered NO were intersected by a laser and ionised via a (1+1) REMPI scheme at ~225 nm. The NO ions were accelerated towards an MCP/phosphor screen detector by the VMI optics. The NO was found to scatter off the graphene surface following one of two mechanisms, direct scatter and (non-thermal) trapping desorption. The directly scattered NO lost ~80% of its initial velocity to the graphene surface and the trapping desorbed NO was found to have a residence time on the order of tens to low hundreds of microseconds. Accompanying molecular dynamics simulations were carried out using the DL_POLY Classic software package. A simulations box was selected with a 120° rhombus as a base in the x-y plane of length 17.3 Å each and a z dimension perpendicular to the x-y plane of length 45 Å. The graphene surface consisted of 98 carbon atoms in a 2D lattice network and was placed on a 6 x 6 x 6 array of gold atoms. Periodic boundary conditions were applied along the x-y plane, but with no periodicity in the z dimension. Thousands of trajectories of NO molecules were directed onto the graphene along the surface normal, while varying impact position, but also speed, orientation, and rotational excitation of the nitric oxide. The simulations agreed qualitatively with the experiment. The directly scattered NO lost a large amount of its initial velocity and there was also the presence of a trapping desorption component. This work addresses a gap in previous research in the area of surface dynamics, with very few studies taking advantage of surface VMI in graphene dynamics studies and its inherent ability to obtain high levels of detail in both internal and translational energy distributions.