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

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 phasetransition 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 radiatio