  Modulation of the Fermi level using an ultraviolet (UV)-assisted
photochemical method is demonstrated in tungsten diselenide monolayers.
Systematic shifts and relative intensities between charged and neutral exciton
species indicate a progressive and controllable decrease of the electron
density and switch tungsten diselenide from n-type to a p-type semiconductor.
The presence of chlorine in the 2D crystal shifts the Fermi level closer to the
valence band while the effect can be only partially reversible via continuous
wave laser rastering process. The presence of chlorine species in the lattice
is validated by X-ray photoelectron spectroscopy (XPS), and density functional
theory (DFT) calculations predict that adsorption of chlorine on the selenium
vacancy sites leads to p-type doping. The results of our study indicate that
photochemical techniques have the potential to enhance the performance of
various 2D materials, making them suitable for potential applications in
optoelectronics.
