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.