Strain-engineered S-HfSe2 monolayer as a promising gas sensor for detecting NH3: A first-principles study

The development of high-performance gas sensing materials is one of the development trends of new gas sensor technology. In this work, in order to predict the gas-sensitive characteristics of HfSe and its potential as a gas-sensitive material, the interactions of nonmetallic element (O, S, Te) doped HfSe monolayer and small molecules (NH and O) have been studied by first-principles based on density functional theory. The results show that the adsorption of NH and O on pristine HfSe monolayer is weak, and the adsorption strength can be significantly improved by doping O. And O-HfSe is chemical adsorption to O with large adsorption energy and transfer charge, and the band gap of O[sbnd]HfSe disappears after adsorbing O, indicating that the adsorption of O has a significant effect on the electrical properties of the substrate. These mean that O is difficult to recover from the substrate surface, thus preventing O-HfSe from developing into a sensitive material for O detection. After doping S, the charge transfers and adsorption strength to NH are the largest, but it is still small. So, the strain effect on the S-HfSe/NH adsorption system is also studied. The results indicate that the adsorption strength of S-HfSe to NH can be enhanced by stretching S-HfSe along x-axis. After absorbing NH, the conductivity of x-axis strained S-HfSe changes, which suggest its sensitivity. And the predicted recovery times of S-HfSe surfaces with ε=4%, 6% and 8% are 0.027 s, 1.153 s and 102.467 s, respectively, which suggests that the S-HfSe monolayer has the potential to be developed as a sensitive material for NH detection. These adsorption mechanism studies can also serve as a theoretical foundation for the experimental design of gas-sensing materials.