Photoresponse of O2 plasma treated WS2 monolayers synthesized by chemical vapor deposition

Abstract

WS2 monolayers synthesized by chemical vapor deposition underwent low-power O2 plasma treatment, and the effect of this treatment was studied in this work. The results show that the WS2 photoresponse is quenched due to the treatment and subsequently vanishes for more extended periods of plasma exposure. Raman spectra of the treated WS2 monolayers show that the position of the LA(M) mode is blueshifted and has a larger FWHM when compared with pristine samples. Since the LA(M) phonon can be used as a figure of merit to infer indirectly a density of defects in the lattice, these results show an increase in defects upon treatment times. At the same time, the main E2g and A1g peak positions remain nearly unchanged for short plasma exposures, indicating that there is an intrinsic defect-related process rather than a strain-related change. X-ray photoelectron spectroscopy results reveal the presence of O impurities incorporated into lattice passivating sulfur vacancies, while atomic force microscopy confirms that the sample creates cracks at the micrometer scale. Our results indicate that the pure oxygen plasma treatment significantly reduces the photoresponse of WS2 monolayers by increasing the density of defects.