We show using scanning tunneling microscopy, spectroscopy, and ab initio
calculations that several intercalation structures exist for Na in epitaxial
graphene on SiC(0001). Intercalation takes place at room temperature and Na
electron-dopes the graphene. It intercalates in-between single-layer graphene
and the carbon-rich interfacial layer. It also penetrates beneath the
interfacial layer and decouples it to form a second graphene layer. This
decoupling is accelerated by annealing and is verified by direct Na deposition
onto the interface layer. Our observations show that intercalation in graphene
is fundamentally different than in graphite and is a versatile means of
electronic control.Comment: 10 pages text, 2 pages, references, and 4 figure page
