Selective Growth of Two-Dimensional Heterostructures of Gallium Selenide on Monolayer Graphene and the Thickness Dependent p- and n-Type Nature

GaSe crystals were grown on graphene domains with few-layer graphene (FLG) grains at the centers of larger monolayer graphene (MLG) grains. We found that GaSe are selectively grown on the MLG and not on the adjacent FLG and the oxidized Si substrates. Nucleation of GaSe was preferentially occurred at the steps of FLG/MLG and MLG/SiO2 because of the presence of dangling bonds/graphene edges as supported by density function theory (DFT) calculation. We also evidenced that wrinkles on graphene were not the preferred nucleation site for GaSe if there is no dangling bond. Subsequent growth of the GaSe nuclei on MLG was favorable due to the higher migration tendency of adatoms on the MLG, as supported by DFT calculation, which promoted lateral growth of larger GaSe. The surface roughness and defects on SiO2 may also promote nucleation of GaSe on MLG. We further investigated the work functions of the GaSe/graphene heterostructures using Kelvin probe force microscopy. We have detected a unique thickness-dependent work function of GaSe on MLG, which suggests for a shift of Fermi level due to n-type to p-type conversion. This is a promising route to prepare GaSe p-n junction on MLG and an approach to match the work function of GaSe and MLG by controlling the Schottky barrier height for application in electrical devices.