Formation of monolayer charge density waves and anomalous edge doping in Na doped bulk VSe$_2$

International audienceAlkali atom doping is an efficient way to induce charge transfer and Fermi level tuning in layered materials through intercalation.However, there is a general lack of microscopic understanding of the effect of doping inhomogeneity in geometric and electronic aspects. Here we report surface doping of a bulk VSe$_2$ crystal by sodium. Na atoms form intercalated subsurface islands that modify the electronic phase of the top layer of VSe2. In addition to n-doping, the charge density wave of the intercalated VSe2 surface layer changes from the ( 4×4 ) bulk phase to the ( √3 ×√7 ) known in monolayer phase of VSe$_2$. Surprisingly, an electronic state at the edges of Na-intercalated area shift anomalously upward in energy as detected by scanning tunneling spectroscopy. This is explained by a local gating effect resulting from local dipoles at the edges. Our study illustrates a clear example of intercalation effect that should be general in alkali-intercalated bulk layered materials