Sensing performance of two-dimensional WTe<sub>2</sub>-based gas sensors

Since the discovery of graphene, graphene-based gas sensors have been widely studied, but the inherent zero band gap of graphene limits the response sensitivity of gas sensors. Transition metal dichalcogenides (TMDs) are ideal materials for designing nanoscaled highly-sensitive gas sensors due to their moderate band gaps, large surface-to-volume ratios and high carrier mobilities. Tungsten ditelluride (WTe₂), as an important member of TMDs family, has outstanding advantages such as high specific surface area, excellent selectivity, and fast response. The WTe₂ has quite a high carrier mobility and thus can provide a great response speed for gas sensor compared with graphene, which motivates us to further explore WTe₂ as a promising sensing material. Recent studies have reported that monolayered and multilayered WTe₂ films have been successfully synthesized, and the precise control of the number of atomic layers of monolayered WTe₂ has been achieved. In this work, by density functional theory calculation, we examine the most stable adsorption configuration, adsorption energy, charge transfer, electrical and magnetic properties for each of the gas molecules (CO, CO₂, NH₃, NO and NO₂) adsorbed on WTe₂ monolayer. The results show that all the adsorptions of these gas molecules are physical adsorptions, and the adsorption energy of nitrogen-based gas is smaller than that of carbon-based gas, indicating that WTe₂ is more sensitive to the adsorption of N-based gas molecules. The adsorption of NH₃ behaves as a charge donor with electron obtained from WTe₂ monolayer. The adsorption of CO, CO₂, NO, and NO₂ are charge acceptors, which accept charges from the WTe₂ monolayer. Moreover, compared with the adsorption of CO, CO₂ and NH₃ gas molecules, the adsorption of NO and NO₂ gas molecules introduce impurity states near the Fermi level, which are mainly contributed by the N p orbital and O p orbital. In addition, the adsorption of NO and NO₂ induce magnetic moments of 0.99 <i>μ</i>_B and 0.80 <i>μ</i>_B, respectively. The results obtained in this work not only conduce to further understanding the charge transfer mechanism of gas molecules adsorbed on WTe₂ monolayer, but also indicate the promising prospects of developing WTe₂-based ultra-sensitivity gas sensing nanodevices.