  Two-dimensional (2D) WS2 films were deposited on SiO2 wafers, and the related
interfacial properties were investigated by high-resolution x-ray photoelectron
spectroscopy (XPS) and first-principles calculations. Using the direct
(indirect) method, the valence band offset (VBO) at monolayer WS2/SiO2
interface was found to be 3.97 eV (3.86 eV), and the conduction band offset
(CBO) was 2.70 eV (2.81 eV). Furthermore, the VBO (CBO) at bulk WS2/SiO2
interface is found to be about 0.48 eV (0.33 eV) larger due to the interlayer
orbital coupling and splitting of valence and conduction band edges. Therefore,
the WS2/SiO2 heterostructure has a Type I energy-band alignment. The band
offsets obtained experimentally and theoretically are consistent except the
narrower theoretical bandgap of SiO2. The theoretical calculations further
reveal a binding energy of 75 meV per S atom and the totally separated partial
density of states, indicating a weak interaction and negligible Fermi level
pinning effect between WS2 monolayer and SiO2 surface. Our combined
experimental and theoretical results provide proof of the sufficient VBOs and
CBOs and weak interaction in 2D WS2/SiO2 heterostructures.
