Hexagonal molybdenum diselenide (MoSe₂) multilayers were grown by chemical vapor deposition (CVD). A relatively high pressure (>760 Torr) was used during the CVD growth to achieve multilayers by creating multiple nuclei based on the two-dimensional crystal growth model. Our CVD-grown multilayer MoSe₂ thin-film transistors (TFTs) show p-type-dominant ambipolar behaviors, which are attributed to the formation of Se vacancies generated at the decomposition temperature (650 °C) after the CVD growth for 10 min. Our MoSe₂ TFT with a reasonably high field-effect mobility (10 cm²/V · s) exhibits a high photoresponsivity (93.7 A/W) and a fast photoresponse time (τ_rise ~ 0.4 s) under the illumination of light, which demonstrates the practical feasibility of multilayer MoSe₂ TFTs for photodetector applications.

Layered semiconductors with atomic thicknesses are becoming increasingly important as active elements in high-performance electronic devices owing to their high carrier mobilities, large surface-to-volume ratios, and rapid electrical responses to their surrounding environments. Here, we report the first implementation of a highly sensitive chemical-vapor-deposition-grown multilayer MoSe₂ field-effect transistor (FET) in a NO₂ gas sensor. This sensor exhibited ultra-high sensitivity (S = ca. 1,907 for NO₂ at 300 ppm), real-time response, and rapid on–off switching. The high sensitivity of our MoSe₂ gas sensor is attributed to changes in the gap states near the valence band induced by the NO₂ gas absorbed in the MoSe₂, which leads to a significant increase in hole current in the off-state regime. Device modeling and quantum transport simulations revealed that the variation of gap states with NO₂ concentration is the key mechanism in a MoSe₂ FET-based NO₂ gas sensor. This comprehensive study, which addresses material growth, device fabrication, characterization, and device simulations, not only indicates the utility of MoSe₂ FETs for high-performance chemical sensors, but also establishes a fundamental understanding of how surface chemistry influences carrier transport in layered semiconductor devices.