Disentangling oxygen and water vapor effects on optoelectronic properties of monolayer tungsten disulfide

Abstract

By understanding how the environmental composition impacts the optoelectronic properties of transition metal dichalcogenide monolayers, we demonstrate that simple photoluminescence (PL) measurements of tungsten disulfide (WS₂) monolayers can differentiate relative humidity environments. In this paper, we examine the PL and photoconductivity of chemical vapor deposition grown WS₂ monolayers under three carefully controlled environments: inert gas (N₂), dry air (O₂ in N₂), and humid nitrogen (H₂O vapor in N₂). The WS₂ PL is measured as a function of 532 nm laser power and exposure time and can be decomposed into the exciton, trion, and lower energy state(s) contributions. Under continuous illumination in either O₂ or H₂O vapor environment, we find dramatic (and reversible) increases in PL intensity relative to the PL in an inert environment. The PL bathochromically shifts in an O₂ environment and is dominated by increased trion emission and diminished exciton emission. In contrast, the WS₂ PL increase in a H₂O environment results from an overall increase in emission from all spectral components where the exciton contribution dominates. The drastic increases in PL are anticorrelated with corresponding decreases in photoconductivity, as measured by time-resolved microwave conductivity. The results suggest that both O₂ and H₂O react photochemically with the WS₂ monolayer surface, modifying the optoelectronic properties, but do so via distinct pathways. Thus, we use these optoelectronic differences to differentiate the amount of humidity in the air, which we show with 0%, 40%, and 80% relative humidity environments. This deeper understanding of how ambient conditions impact WS₂ monolayers enables novel humidity sensors as well as a better understanding of the correlation between TMDC surface chemistry, light emission, and photoconductivity. Moreover, these WS₂ measurements highlight the importance of considering the impact of the local environment on reported results.