For the relevant properties of pristine and doped (Si, P, Se, Te, As) monolayer WS₂ before and after the adsorption of CO, CO₂, N₂, NO, NO₂ and O₂, density functional theory (DFT) calculations are made. Calculation results reveal that the monolayer WS₂ doped with P and As atoms can be substrate materials for NO and NO₂ gas sensors. However, after the subsequent CDD and ELF calculations, it is found that P-doped monolayer WS₂ adsorbs NO and NO₂ in a chemical way, while As-doped monolayer WS₂ adsorbs NO and NO₂ in a physical way. Also, the charge transfer between As-doped monolayer WS₂ and NO is relatively small and not easily detected. Besides, As-doped monolayer WS₂ system exhibits greater differences in optical properties (the imaginary part of reflectivity and dielectric function) before and after the adsorption of NO₂ gas than before and after adsorption of NO gas. These differences in optical properties assist sensor devices in making gas adsorption-related judgments. Through the analysis of the recovery time, DOS and PDOS, As-doped monolayer WS₂ is also verified to be a promising NO₂ sensing material, whose recovery time is calculated to be as short as 0.169 ms at 300 K.

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 HfSe₂ and its potential as a gas-sensitive material, the interactions of nonmetallic element (O, S, Te) doped HfSe₂ monolayer and small molecules (NH₃ and O₃) have been studied by first-principles based on density functional theory. The results show that the adsorption of NH₃ and O₃ on pristine HfSe₂ monolayer is weak, and the adsorption strength can be significantly improved by doping O. And O-HfSe₂ is chemical adsorption to O₃ with large adsorption energy and transfer charge, and the band gap of O[sbnd]HfSe₂ disappears after adsorbing O₃, indicating that the adsorption of O₃ has a significant effect on the electrical properties of the substrate. These mean that O₃ is difficult to recover from the substrate surface, thus preventing O-HfSe₂ from developing into a sensitive material for O₃ detection. After doping S, the charge transfers and adsorption strength to NH₃ are the largest, but it is still small. So, the strain effect on the S-HfSe₂/NH₃ adsorption system is also studied. The results indicate that the adsorption strength of S-HfSe₂ to NH₃ can be enhanced by stretching S-HfSe₂ along x-axis. After absorbing NH₃, the conductivity of x-axis strained S-HfSe₂ changes, which suggest its sensitivity. And the predicted recovery times of S-HfSe₂ surfaces with ε_x=4%, 6% and 8% are 0.027 s, 1.153 s and 102.467 s, respectively, which suggests that the S-HfSe₂ monolayer has the potential to be developed as a sensitive material for NH₃ detection. These adsorption mechanism studies can also serve as a theoretical foundation for the experimental design of gas-sensing materials.

As the concentration of acetone (C₃H₆O) in exhaled gas of diabetics is significantly higher than that of healthy people. Here, the sensing performance of X doped MoSe₂ (X–MoSe₂, X = Ti, Ni and Cu) for acetone is studied theoretically. It is found that Ti–MoSe₂ shows absolute advantages in both adsorption energy and charge transfer. Additionally, the changes of bandgap for C₃H₆O/Ti–MoSe₂ in the adsorption process are the largest in all adsorption systems, indicating it will produce the largest electrical signal that can be detected. Besides, the co-adsorption system (consisting of C₃H₆O, H₂O, CO₂, N₂ and O₂) will be more stable after O₂ is removed and the adsorption site occupied by acetone restricts the contact of other disturbing gases with Ti–MoSe₂. Importantly, comparing with the reported acetone sensing materials ((110) face of SnO₂, (MgO)₁₂-graphene and oxygen-plasma-treated ZnO (ZnO–O)), Ti–MoSe₂ demonstrates its superiority in terms of the absolute value of charge transfer (0.37 e) and adsorption energy (2.42 eV). All these results show that Ti–MoSe₂ is expected to become the reliable sensing material for acetone and has enormous potential for the application in noninvasive and rapid detection of type-1 diabetes.

Recently, the application of phosphorene structure analogues in gas sensors has been a hot research topic since the appearance of phosphorene. SnSe monolayer as one of them has been proved to be much more stable properties than phosphorene. Based on the density-functional theory, the interaction between gas molecules (CO, CO ₂ , O ₂ , NO, NH ₃ , SO ₂ and NO ₂ ) and SnSe monolayer are theoretically investigated by first-principles calculation. Macroscopically, gas molecules selective adsorption of SnSe monolayer is analyzed by molecular dynamics. Compared to CO, CO ₂ , O ₂ , SnSe monolayer performs stronger affinity for SO ₂ and NO ₂ , which possesses appropriate adsorption energies (−6.000 eV and −0.759 eV) and elevated charge transfers (−0.239 e and −0.328 e). SnSe monolayer chemical adsorption of NO ₂ , while physically adsorbing SO ₂ , is more suitable for the adsorption mode of SO ₂ sensors. Surprisingly, the adsorption amount of SO ₂ is 6 times that of NO ₂ . Therefore, the adsorption of SO ₂ is more likely to occur compared to other gas molecules. For a mixed environment of SO ₂ and NO ₂ , the adsorption quantity of SO ₂ is not significantly affected, while the adsorption of NO ₂ is inhibited. Therefore, the SnSe monolayer could be a promising candidate as SO ₂ sensors with high selectivity and sensitivity.

Sensitivity and selectivity are important factors for Tl₂O monolayer to be the sensitive material. In this work, the sensing performance of NO₂ on pure and X-Tl₂O (X = In, Cd, Y, Pb, Ga, Si) sheets has been detailed investigated by means of DFT. The results show that the doped systems have a better sensing performance for NO₂ and it is most evident in the In-Tl₂O substrate. And In-Tl₂O has a unique response to NO₂ with appropriate adsorption energy (−1.79 eV) and charge transfer (−0.51 e) which are far more than B-C₃N (−1.15 eV, 0.44 e). Besides, although In-Tl₂O monolayer and SO₂ have formed the chemical bond, the adsorption effect of NO₂ on the substrate hardly changes when the co-adsorption of SO₂ and NO₂ occurs on it. In addition, the reflectivity and optical absorption of the gas/In-Tl₂O (gas = NO, NO₂ and SO₂) adsorption system are calculated, and the results indicate optical absorption and reflectivity of NO₂/In-Tl₂O system are all far greater than that of other systems in the visible and near infrared regions. So it is easy to be distinguished by infrared detection. These all show that the In-Tl₂O is an excellent sensing material for NO₂ detection.