UV light-activated gas mixture sensing by ink-printed WS2 layer

We fabricated a sensing layer from ink-printed WS₂ flakes and utilized it for UV-activated gas sensing. The optical imaging of the structure made by repeated printing revealed the continuous layer comprising sub-µm flakes, confirmed independently by small-area AFM images (1×1 µm²). The activity of the sensing surface was investigated locally via AFM scanning of the surface with a polarized probing tip. The results indicated that the applied UV light amplifies the existing conducting paths in the dark. These hot spots are associated with the sensing activity of the WS₂ surface (local adsorption-desorption centers). Gas sensing experiments revealed that the DC resistance of the WS₂ sensor changes in the opposite direction for increasing concentrations of NO₂ and NH_3, which correlates with the electron-accepting and electron-donating properties of these species. On the contrary, low-frequency noise intensifies gradually in both gases, and relative changes in noise responses are higher than DC resistance responses for all investigated concentrations. The lowest detection limit obtained was 103 ppb from DC responses for NO₂ and 168 ppb from noise responses for NH₃. The studies of sensing responses for mixtures of the mentioned target gases revealed that the amplitude of resistance fluctuations is not a direct summation of spectra obtained for pure compounds. Such an effect observed for mixed gases indicates that the intermittent reactions between both species before adsorbing at the sensing surface or in the adsorption centers impact their detection.