Electron density control in WSe₂ monolayers via photochlorination

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. Chlorine species in the lattice are validated by x-ray photoelectron spectroscopy, while density functional theory 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 integrated optoelectronics such as lateral nanopatterned p-n junctions.