Electronic and geometric structure of graphene/SiC(0001) decoupled by lithium intercalation

Graphene formation on top of SiC(0001) by decoupling the carbon buffer layer through lithium intercalation is investigated. Low-energy electron diffraction and core-level photoemission spectroscopy results show that graphene formation already occurs at room temperature, and that the interface morphology is improved after thermal annealing. Angle-resolved photoemission spectroscopy (ARPES) shows that the resulting graphene layer is strongly n-type doped, and in spite of the decoupling by lithium intercalation, a persistent interaction with the substrate imposes a superperiodicity on the graphene band structure that modulates the pi band intensity and gives rise to quasi-(2 x 2) pi replica bands. Through a comparison of the ARPES-derived band structure with density-functional-theory calculations, we assign the observed bands to SiC-derived states and interface-related ones; this assignment permits us to establish that the intercalated lithium occupies the T4 site on the topmost SiC layer.