Efficiency of electron doping to monolayer hexagonal boron nitride by alkali metals

We investigated electron doping of monolayer hexagonal boron nitride (hBN) on metallic substrates by doping alkali metals (AMs). The valence band maximum (VBM) of hBN/Rh after doping with Li and Cs was directly observed using angle-resolved photoemission spectroscopy in a wide wavenumber space. The valence band shift resulting from doping confirms the vacuum level alignment model. Furthermore, when the same AM is used, the resultant binding energy of VBM was almost identical regardless of the substrate, even if it differs by $∼1$ eV before doping, which we found by comparison with the literature. This independence from the substrate is explained by an extension of the vacuum level alignment model, wherein the VBM is determined by the work function of the AM when it intercalates to the interface as well as adsorbs on surfaces of hBN. It means that the doping of Cs, which has the lowest work function, gives the deepest binding energy at VBM. This is evaluated as 5.7 eV in our experiments.