  Our first-principles calculations show that an energy gap around 0.12-0.25 eV
can be engineered in epitaxial graphene on SiC(0001) through the non-covalent
intercalation of transition- or alkali-metals, yet originated from the distinct
mechanisms. The former is attributed to the combined effects of metal induced
perpendicular electric field and interaction while the latter is solely
attributed to the built-in electric field. A great advantage of this scheme is
that the gap size is almost independent of the gate voltage up to 1 V/nm, thus
reserving the electric means to tune the Fermi level of graphene when
configured as field-effect transistors. Given the recent progress in
experimental techniques on the intercalated graphene, our findings provide a
practical way to incorporate the graphene with the current semiconductor
industry.
