Semiconducting two-dimensional transition-metal dichalcogenides (TMDs) have garnered a great deal of interest owing to their large surface-to-volume ratios compared to traditional three-dimensional (3D) semiconductors. One such TMD is tungsten diselenide (WSe2), which at monolayer (1L) exhibits a direct bandgap and tunable charge transport behavior. This, in combination with its large surface-to-volume ratio, has made WSe2 a heavily researched material, including for gas sensing applications. However, the synthesis of monolayer WSe2 has proven to be challenging. In this study, we demonstrate the halide-assisted low-pressure chemical vapor deposition (HA-LPCVD) of high crystalline-quality, monolayer WSe2. The average synthesized WSe2 crystallite possessed an edge length of ~ 30 microns. After synthesis, the properties of the WSe2 membranes were characterized using Raman and photoluminescence spectroscopy. For device fabrication, Cr contacts were deposited atop the WSe2 membranes using photolithography with a dark-field mask and a metal lift-off process with electron-beam evaporation. After device fabrication, we then proceeded to use these devices for gas sensing applications, where two terminal electronic transport measurements allowed us to monitor the modulation in the WSe2 transport characteristics with incoming gas flow. The I-V response to target gases N2 and CO2 were recorded to investigate the effects of surface adsorption mechanisms of extrinsic molecules on our material’s conducting behavior.
Understanding gas flow behavior is crucial in the ability of materials to sense toxic gasses for environmental, industrial, safety, agriculture, and related applications. Conventional metal oxide-based gas sensors such as SnO2 and TiO2, suffer from poor stability at higher operating temperatures, and can degrade with exposure to higher levels of humidity. Two - dimensional (2D) materials serve as a promising candidate for gas sensing studies because of their large surface area-tovolume ratio, ease of surface functionalization, and the relative simplicity in their integration with microelectronics fabrication approaches within a planar 3D device architecture. In particular, the microstructure of solution-processed graphene appears to be well-suited to surface adsorption processes, as one of the governing mechanisms in devices that are candidates for gas sensing, given its more porous microstructure compared to vapor synthesized materials. In our work we report on here, we measure the electrical transport characteristics of ink jet printed graphene to incoming gas flow, specifically to N2 and CO2. This work serves as a prototypical platform to study the device characteristics of solution processed graphene and other 2D materials for more exotic gases in the future for gas sensing applications.
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