Surface band bending in semimetal Td-WTe2

Recently, spontaneous out-of-plane electric polarization and ferroelectric switching were found in WTe2 devices. On single crystal with ferroelectric property, a built-in electric field and corresponding band bending would be expected at the surface. However, such band bending in WTe2 hasn’t directly been observed experimentally. Here, by fitting angle-dependent X-ray photoelectron spectroscopy (XPS) spectra on WTe2 surface, we clearly observed downward band bending after slightly exposure to air, verifying appearance of surface polarization. Such band bending can’t be observed on pristine WTe2 surface and will disappear on fully oxidized sample. It suggests strong correlation between surface band bending and oxidation. Ionized donors from oxide species pinned at surface may contribute to the formation of surface band bending and polarization. Our study here offers new insight to figure out the microscopic origin of ferroelectric in WTe2.