Abstract
We have developed a lithography free technique for the fabrication of two-dimensional material based devices for electrical characterization. We fabricated few-layer and multilayer WS2 devices using a transmission electron microscope (TEM) grid as a shadow mask, and its transport characteristics were studied by electrical measurements. WS2 samples were synthesized by first depositing WO3 followed by sulfurization and characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), and Raman spectroscopy. Each sample was exposed to hydrazine at varying pressures, and their electrical resistances were monitored during adsorption (exposing to hydrazine vapor) and subsequent desorption (by pumping). The WS2 sample consisting of two layers showed a decrease of resistance upon exposure to hydrazine vapor and showed complete reversibility upon pumping. The WS2 sample with three layers showed a decrease of resistance during exposure but showed only partial recovery during desorption. In contrast, the multilayered (12 layers) WS2 sample showed an initial decrease followed by a continued increase of the resistance upon exposure to hydrazine with little or no reversibility upon pumping. The charge transfer from N2H4 to WS2 is believed to be responsible for the decrease of the resistance. Trapping of N2H4 molecules within the multilayers of WS2 causing charge redistribution and possible chemical reactions may be responsible for the increase in resistance during the adsorption and complete irreversibility of resistance during desorption. The experimental results are explained with the help of computational calculations performed by employing the density functional theory (DFT) framework, as implemented in the Vienna Ab-initio Simulation Package (VASP).