Photodriven Transient Picosecond Top‐Layer Semiconductor to Metal Phase‐Transition in p‐Doped Molybdenum Disulfide

Abstract Visible light is shown to create a transient metallic S–Mo–S surface layer on bulk semiconducting p‐doped indirect‐bandgap 2H‐MoS₂. Optically created electron–hole pairs separate in the surface band bending region of the p‐doped semiconducting crystal causing a transient accumulation of electrons in the surface region. This triggers a reversible 2H‐semiconductor to 1T‐metal phase‐transition of the surface layer. Electron–phonon coupling of the indirect‐bandgap p‐doped 2H‐MoS₂ enables this efficient pathway even at a low density of excited electrons with a distinct optical excitation threshold and saturation behavior. This mechanism needs to be taken into consideration when describing the surface properties of illuminated p‐doped 2H‐MoS₂. In particular, light‐induced increased charge mobility and surface activation can cause and enhance the photocatalytic and photoassisted electrochemical hydrogen evolution reaction of water on 2H‐MoS₂. Generally, it opens up for a way to control not only the surface of p‐doped 2H‐MoS₂ but also related dichalcogenides and layered systems. The findings are based on the sensitivity of time‐resolved electron spectroscopy for chemical analysis with photon‐energy‐tuneable synchrotron radiation.