Alkali Metal Doped Mono- and Bilayer Graphene: Band Structure, Raman Spectrum and Electronic Transport

Abstract

We use ultra-high vacuum (UHV) Raman spectroscopy in tandem with angle-resolved photoemission (ARPES) to investigate the doping-dependent Raman spectrum of epitaxially grown graphene. The evolution of the Raman spectra from pristine to heavily Cs doped graphene up to a carrier concentration of 4.4x10^14 cm-2 is investigated [1]. Renormalization effects reduce the electronic bandwidth at this doping level when graphene is at the onset of the Lifshitz transition. Ultraviolet (UV) light allows to probe the optical transition at the saddle point in the Brillouin zone achieving resonance Raman conditions in close vicinity to the van Hove singularity in the joint density of states. The position of the Raman G band of fully doped graphene/Ir(111) shifts down by 60 cm-1 and the G band asymmetry of Cs doped epitaxial graphene on Ir(111) assumes an unusual strong Fano asymmetry opposite to that of the G band of doped graphene on insulators. We compare these results on Cs doped monolayer graphene to studies of Cs doped bilayer graphene. Growth of bilayer graphene is realized by intercalation of carbon atoms under monolayer graphene using molecular beam epitaxy. We probe each step of the intercalation process using UHV Raman which reveals distinct changes in the linshape. Finally, the graphene is transferred from the metal substrate onto Si wafers and probed by UHV electronic transport during alkali metal deposition. Our work demonstrates high sensitivity towards sensing of alkali atoms and a giant decrease of the four-point resistance by three orders of magnitude upon Cs doping. [1] M.G.Hell at al. Nano Letters 2018 18 (9), 6045-6056 Figure 1