Field Effect Transistor Gas Sensors Based on Mechanically Exfoliated Van der Waals Materials

Impact of Molecule-Molecule Interactions When Discerning Low-Concentration Hazardous Gas Mixtures

Gas sensor arrays are versatile and powerful tools for gas detection and analysis, enabling a wide range of applications across numerous industries. Critically, the accuracy and reliability of these arrays depend on the distinct gas sensing behavior or selectivity of the individual component gas sensors. However, studies of such arrays often consider only overly idealized scenarios, and the interaction between gas molecules is not typically considered in such studies. Here, based on first-principles calculations and direct experimental demonstrations, we show that interactions between gas molecules at the surface can play a significant role. We found that NO2 and NH3 molecules can be expected to align together to form dimers due to the strong interaction between NH3 and NO2 at the Fermi level, which enhances the adsorption capability and sensitivity of MoS2. Compared with the gas sensing performance of MoS2 for either NO2 or NH3 alone, a faster response is observed for sensing the NO2 and NH3 gas mixtures. Enhanced sensitivity, however, is achieved only at lower carrier densities with an appropriate concentration ratio between NO2 and NH3. These results not only provide evidence of the pronounced effect of gas molecule interactions but also suggest an approach for discerning gases.

Twisted van der Waals Quantum Materials: Fundamentals, Tunability, and Applications

Twisted van der Waals (vdW) quantum materials have emerged as a rapidly developing field of two-dimensional (2D) semiconductors. These materials establish a new central research area and provide a promising platform for studying quantum phenomena and investigating the engineering of novel optoelectronic properties such as single photon emission, nonlinear optical response, magnon physics, and topological superconductivity. These captivating electronic and optical properties result from, and can be tailored by, the interlayer coupling using moiré patterns formed by vertically stacking atomic layers with controlled angle misorientation or lattice mismatch. Their outstanding properties and the high degree of tunability position them as compelling building blocks for both compact quantum-enabled devices and classical optoelectronics. This paper offers a comprehensive review of recent advancements in the understanding and manipulation of twisted van der Waals structures and presents a survey of the state-of-the-art research on moiré superlattices, encompassing interdisciplinary interests. It delves into fundamental theories, synthesis and fabrication, and visualization techniques, and the wide range of novel physical phenomena exhibited by these structures, with a focus on their potential for practical device integration in applications ranging from quantum information to biosensors, and including classical optoelectronics such as modulators, light emitting diodes, lasers, and photodetectors. It highlights the unique ability of moiré superlattices to connect multiple disciplines, covering chemistry, electronics, optics, photonics, magnetism, topological and quantum physics. This comprehensive review provides a valuable resource for researchers interested in moiré superlattices, shedding light on their fundamental characteristics and their potential for transformative applications in various fields.

Enhanced NO2 Sensitivity of Vertically Stacked van der Waals Heterostructure Gas Sensor and Its Remarkable Electric and Mechanical Tunability

Nanodevices based on van der Waals heterostructures have been predicted, and shown, to have unprecedented operational principles and functionalities that hold promise for highly sensitive and selective gas sensors with rapid response times and minimal power consumption. In this study, we fabricated gas sensors based on vertical MoS2/WS2 van der Waals heterostructures and investigated their gas sensing capabilities. Compared with individual MoS2 or WS2 gas sensors, the MoS2/WS2 van der Waals heterostructure gas sensors are shown to have enhanced sensitivity, faster response times, rapid recovery, and a notable selectivity, especially toward NO2. In combination with a theoretical model, we show that it is important to take into account created trapped states (flat bands) induced by the adsorption of gas molecules, which capture charges and alter the inherent built-in potential of van der Waals heterostructure gas sensors. Additionally, we note that the performance of these MoS2/WS2 heterostructure gas sensors could be further enhanced using electrical gating and mechanical strain. Our findings highlight the importance of understanding the effects of altered built-in potentials arising from gas molecule adsorption induced flat bands, thus offering a way to enhance the gas sensing performance of van der Waals heterostructure gas sensors.