Effect of molecule adsorption on conductivity of selectively grown interconnected 2D-MoS2 atomically thin flakes structures

Abstract

Gas sensitivity of field-effect structures with 2D-MoS₂ channels selectively grown between Mo electrodes by Mo-CVD method has been investigated by measuring the effect of molecular adsorption from air on the device source-drain current (I_sd). The channels are composed of interconnected atomically thin MoS₂ grains, whose density and average thickness are varied by choosing two different distances (15 and 20 µm) between the Mo contacts. High response to the tested stimuli, i.e. molecule adsorption, illumination and gate voltage changes was shown. Huge, persistent photoconduction is induced by positive charge accumulation on traps most likely at grain boundaries and associated defects. I_sd increases at high vacuum, both in dark and under illumination. The relative dark current response to the change from air to high vacuum reaches up to 1000 % at the turn-on voltage. Monitored during gradual change of air pressure,I_sd was a non-monotonic function, sharply peaking at about 10^-2 mbar, suggesting molecule adsorption on different defect sites and orientations of adsorbed H₂O molecules capable of inducing the accumulation or depletion of electrons. Despite the screening of the disorder by extra electrons, #20µm sample remains 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 the measurement down to 100 K, revealing 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 the molecule adsorption, making the technology promising for easy fabrication of chemical sensors.