Suppression of Elastic Scattering of CH₄ by Graphene Passivation of Ni(111)

We report high-resolution angular diffraction measurements of CH₄ and Ne from Ni(111) and from a graphene (Gr) layer grown on Ni(111), whose effective atomic mass is enhanced by the strong Gr-Ni interaction. The incident energies are between 43 and 68 meV for Ne, and between 62 and 108 meV for CH₄ beams. Sharp diffraction features are observed with Ne beams from both Ni(111) and Gr/Ni(111) surfaces. However, using methane beams, clear diffraction peaks are observed only from Ni(111), with broad angular distributions measured from Gr/Ni(111), as expected for classical particles, with no quantum features. This is surprising, since Ne and methane have similar masses and therefore a comparable quantum behavior is expected for the same incident energy. This effect is mainly due to the larger physisorption well of CH₄ on Gr/Ni(111), as shown by DFT calculations, in addition to the larger corrugation of the potential energy surface and the excitation of phonon modes of the graphene overlayer.