Interaction of gases with monolayer WS2: an in situ spectroscopy study

The optical and electronic properties of two-dimensional (2D) materials can be tuned through physical and chemical adsorption of gases. They are also ideal sensor platforms, where charge transfer from the adsorbate can induce a measurable change in the electrical resistance within a device configuration. While 2D materials-based gas sensors exhibit high sensitivity, questions exist regarding the direction of charge transfer and the role of lattice defects during sensing. Here we measured the dynamics of adsorption of NO2 and NH3 on monolayer WS2 using in situ photoluminescence (PL) and resonance Raman spectroscopy. Experiments were conducted across a temperature range of 25-250 degrees C and gas concentrations between 5-650 ppm. The PL emission energies blue- and red-shifted when exposed to NO2 and NH3, respectively, and the magnitude of the shift depended on the gas concentration as well as the temperature down to the lowest concentration of 5 ppm. Analysis of the adsorption kinetics revealed an exponential increase in the intensities of the trion peaks with temperature, with apparent activation energies similar to barriers for migration of sulfur vacancies in the WS2 lattice. The corresponding Resonance Raman spectra allowed the simultaneous measurement of the defect-induced LA mode. A positive correlation between the defect densities and the shifts in the PL emission energies establish lattice defects such as sulfur vacancies as the preferential sites for gas adsorption. Moreover, an increase in defect densities with temperature in the presence of NO2 and NH3 suggests that these gases may also play a role in the creation of lattice defects. Our study provides key mechanistic insights into gas adsorption on monolayer WS2, and highlights the potential for future development of spectroscopy-based gas sensors based on 2D materials.