▎ 摘　　要

We report the electrical-transport properties of a monolayer graphene device put on a 500 mu m-thick calcium fluoride (CaF2) single-crystal substrate. From the gate-voltage dependence, clear n-type behavior was observed in a graphene/CaF2 field-effect transistor. This can be understood on the basis of the strong interaction between graphene and the CaF2 substrate, where the substantial electron wavefunction overlap between graphene and fluorine atoms of CaF2 occurs. This observation indicates that the strong n-type doping to graphene is possible only through the contact of graphene with the fluorinated substrate due to the strong electron affinity of a fluorine atom. Various electrical and optical characterizations for the graphene on CaF2 substrates are provided.