High-Performance Chemical Sensing Using Schottky-Contacted Chemical Vapor Deposition Grown Monolayer MoS₂ Transistors

Trace chemical detection is important for a wide range of practical applications. Recently emerged two-dimensional (2D) crystals offer unique advantages as potential sensing materials with high sensitivity, owing to their very high surface-to-bulk atom ratios and semiconducting properties. Here, we report the first use of Schottky-contacted chemical vapor deposition grown monolayer MoS₂ as high-performance room temperature chemical sensors. The Schottky-contacted MoS₂ transistors show current changes by 2–3 orders of magnitude upon exposure to very low concentrations of NO₂ and NH₃. Specifically, the MoS₂ sensors show clear detection of NO₂ and NH₃ down to 20 ppb and 1 ppm, respectively. We attribute the observed high sensitivity to both well-known charger transfer mechanism and, more importantly, the Schottky barrier modulation upon analyte molecule adsorption, the latter of which is made possible by the Schottky contacts in the transistors and is not reported previously for MoS₂ sensors. This study shows the potential of 2D semiconductors as high-performance sensors and also benefits the fundamental studies of interfacial phenomena and interactions between chemical species and monolayer 2D semiconductors

Reversible Semiconducting-to-Metallic Phase Transition in Chemical Vapor Deposition Grown Monolayer WSe₂ and Applications for Devices

Two-dimensional (2D) semiconducting monolayer transition metal dichalcogenides (TMDCs) have stimulated lots of interest because they are direct bandgap materials that have reasonably good mobility values. However, contact between most metals and semiconducting TMDCs like 2H phase WSe₂ are highly resistive, thus degrading the performance of field effect transistors (FETs) fabricated with WSe₂ as active channel materials. Recently, a phase engineering concept of 2D MoS₂ materials was developed, with improved device performance. Here, we applied this method to chemical vapor deposition (CVD) grown monolayer 2H-WSe₂ and demonstrated semiconducting-to-metallic phase transition in atomically thin WSe₂. We have also shown that metallic phase WSe₂ can be converted back to semiconducting phase, demonstrating the reversibility of this phase transition. In addition, we fabricated FETs based on these CVD-grown WSe₂ flakes with phase-engineered metallic 1T-WSe₂ as contact regions and intact semiconducting 2H-WSe₂ as active channel materials. The device performance is substantially improved with metallic phase source/drain electrodes, showing on/off current ratios of 10⁷ and mobilities up to 66 cm²/V·s for monolayer WSe₂. These results further suggest that phase engineering can be a generic strategy to improve device performance for many kinds of 2D TMDC materials