Ultrafast charge transfer to graphene monolayers

One possibility to directly measure the charge transfer time (τ_CT ) of adsorbed species on surfaces is the core hole clock method, in which the transfer time of an excited electron localized on an adsorbate to the substrate is determined by a quantitative analysis of the corresponding core hole decay spectra.
For epitaxial graphene, we have shown that charge transfer from resonantly excited adsorbed Ar atoms to the surfaces of Gr monolayers grown on various substrates is generally quite fast but varies widely for different substrates.
The CT rate is strongly accelerated by Gr coupling to the metallic substrates, which is caused by orbital mixing between empty metal states and Gr states in the relevant energy range. However, even for the cases that are usually considered to be bound only by vdW forces (Pt; hills on Ru) CT is much faster than on decoupled Gr. This may mean that even in

these cases there is some hybridization or that there is an influence of distance as for decoupling atomic and molecular layers. For decoupled monolayers, where the intrinsic states above the Gr surface are expected to determine the CT and the charge has to spread in 2D, the CT is much slower, even though the empty surface DOS is high at the Ar^c+ 4s energy as well.