Effect of molecular adsorption on the conductivity of selectively grown, interconnected 2D-MoS2 atomically thin flake structures

Abstract

The gas sensitivity of field-effect structures with 2D-MoS₂ channels selectively grown between Mo electrodes using the Mo-CVD method was investigated by measuring the effect of molecular adsorption from air on the device source-drain current (I_sd). The channels were composed of interconnected atomically thin MoS₂ grains, with their density and average thickness varied by choosing two different distances (15 and 20 μm) between the Mo contacts. A high response to the tested stimuli, including molecule adsorption, illumination and gate voltage changes, was observed. A significant, persistent photoconduction was induced by positive charge accumulation on traps, most likely at grain boundaries and associated defects. I_sd increased under high vacuum, both in the dark and under illumination. The relative dark current response to the transition from air to high vacuum reached up to 1000% at the turn-on voltage. When monitored during the gradual change in air pressure, I_sd exhibited a non-monotonic function, sharply peaking at about 10⁻² mbar, suggesting molecular adsorption on different defect sites and orientations of adsorbed H₂O molecules, which were capable of inducing electron accumulation or depletion. Despite the screening of disorder by extra electrons, the #20 μm sample remained more sensitive to air molecules on its surface. The high vacuum state was also investigated by annealing devices at temperatures up to 340 K in high vacuum, followed by measurements down to 100 K. This revealed thermally stimulated currents and activation energies of trapping electronic states assigned to sulfur vacancies (230 meV) and other shallow levels (85–120 meV), possibly due to natural impurities, grain boundaries or disorder defects. The results demonstrate the high sensitivity of these devices to molecular adsorption, making the technology promising for the easy fabrication of chemical sensors.