3.1. The structures of the As, Ge atom-doped monolayer WX2
The crystal structure of the monolayer WX2 (S, Se, Te) studied in this work was shown in Figure. 1, the optimized lattice constants of WX2 were a, b = 12.76 Å and c = 20.56 Å, and the band gap of WS2, WSe2 and WTe2 was 1.954 eV (Experiment value = 1.95 eV), 1.627 eV (Experiment value = 1.58 eV) and 1.282 eV (Experiment value = 1.1 eV), which were in good agreement with the reported results [37–39]. Figure. 1 showed the partial charge density for valence band maximum (VBM) and conduction band minimum (CBM) respectively. Our analysis showed that the states near the VBM mainly arise from Se atom, and the states near the CBM originate mainly from the W atom with some contribution. We thought the wider the distance between two peaks, the larger band gap value near the 0 Fermi level. (WTe2 < WSe2 < WS2).
3.2. The adsorption of SO2 gas molecules on As, Ge doped monolayer WX2
Figure. 3 showed transition metal (As, Ge) doped monolayer WX2 (X = S, Se, Te) adsorbing SO2 gas molecule. The pure WX2 represented pristine monolayer WX2 (X = S, Se, Te), the WVs represented pure WX2 (X = S, Se, Te) lost a X atom and the As, Ge/WX2 represented As atom doped monolayer WX2 and Ge atom doped monolayer WX2. At the beginning of the computational study, it was necessary to ensure the accuracy and reliability of the calculation method in this study. The calculated S-O bond lengths r(H) = 1.488Å and SO2 θ(H-C-H) ∠O-S-O angles = 118.537°, respectively. After adsorption, compared with the pure monolayer WX2 and transition metal (As, Ge) doped MoS2 monolayer, the atoms (As, Ge) doped monolayer WX2 was stronger than the pure monolayer WX2, In the adsorption of SO2 gas molecule, the SO2 gas molecule was chemically or physically bonded to the Ge, As doped monolayer WX2, so we thought that the X-doped monolayer WX2 had a great effect. The Q values of the SO2 gas molecule adsorbed on monolayer Ge-WX2 were − 0.264 e, -0.325 e and − 0.371 e, respectively, indicating that the charge transfers between WTe2 and these materials were more noticeable than monolayer WSe2 and WS2. Ge atom occupy more charge transfers in atom element doping. In addition, Ge atom doped monolayer WX2 to connected one S atoms, and we found that Ge atom doped WX2 had better adsorption effect.
As shown in Figure. 3, It was clear that metal atom doped monolayer WSe2 would reduce the band gap, different elements added to new line above the fermi energy to adjust band width, the band gap of Ge/WS2 was 0.099 eV, which presented metal attribute, others were semiconductor materials. See in Figure. 4, more specifically, the valence bands near the Fermi level was mainly derived from the d orbitals of W atoms near the doped metal atom. We had seen the four electrostatic potential of systems, and the surface of X-WS2 had various colors, the darker color represented the more charge accumulation, The real space charge at the top of the valence band was only locally distributed in the monolayer at the bottom, while the real space charge at the top of the conduction band was only locally distributed in the monolayer. Obviously, the colors of Ge and As atom were darker and the charge transfers were stronger.
As shown in Figure. 5 the electronic properties of the adsorption system induced by the interaction between the adsorbed SO2 molecule and the pristine WSe2, MVSe, As-WSe2 and Ge-WSe2 monolayer. Our result shown that the band gap strength increases in the order of Ge-WSe2 < MVSe < pristine WSe2 < As-WSe2 with high band gap of 0.107 eV, 1.091 eV, 1.156 eV and 1.597 eV, respectively. according to the different element doped the WSe2, the band gap had the changed, similar to the doping structure of the WS2, it was also the smallest band after Ge doping. See in Figure. 6, for DoS of structure, we find that causes the peak between − 15 eV and − 12 eV was the interaction of SO2 and Se atom, and the peak between − 6 eV and 0 eV was due to the interaction of SO2 and Se atom, and the peak between 0 eV and 2.5 eV was owing to the interaction of SO2 and W atom. The orbital hybrid occurs at the deep level position caused small interfacial charge transfer, which indicates that the interaction between CO2 and WSe2 monolayer was weak. We could see the four electrostatic potential of systems, only SO2-Ge/WSe2 had a covalent bond was formed from SO2 gas molecule and Ge atom, and the Ge/WSe2 had strong charge transfers for SO2 gas molecule. Therefore, we thought that Ge/WSe2 had a good adsorption effect on SO2 gas molecule.
We had studied the adsorption of SO2 molecule on the pristine WTe2, MVTe, As-WTe2 and Ge-WTe2 monolayers. The resulted for the SO2 adsorption on the pristine WTe2 monolayer was presented firstly. Similar to WS2 and WSe2 adsorption systems, the interaction between SO2 and pure WTe2monolayer was so weak, which indicated that the obtained energy favorable configurations belong to physisorption. The adsorption configurations of SO2 molecule on the four kinds of WTe2 monolayers were illustrated in Figure. 7. Compared to SO2 adsorption on monolayer WS2 and WSe2, the band gap of monolayer WTe2 was decreased, but the monolayer Ge/WTe2 was getting bigger from 0.099 eV to 0.178 eV. Obviously, after doping, all the electronic structures had one more line. All the band gaps were above the fermi energy, only Ge doped monolayer WTe2, a line appeared between 0 eV to 0.5 eV, and decreased the band gap of SO2-Ge/WTe2. We had been seen the four electrostatic potential of systems, it had been found that as a bridge, the Ge atom could represent the adsorbent or contribute electrons together with WTe2. In other cases, the Ge atom could also be combined with a gas molecule to inject electrons into the material.
3.3. The work function of SO2-As, Ge/WX2
To further understand the effect of the adsorbed molecule on different WSe2 nanosheets, the work functions Φ of all adsorbed systems and the bare sheets had been calculated shown in Figure. 9. The work functions were calculated according to the reaction equations:
Φ = V (∞) - EF (2)
where Φ, V (∞), and EF were the work function, electrostatic potential at the vacuum level and the Fermi energy of the WX2 nanosheets, respectively. Due to the structure was changed under the different metal doping, the work function had changed more and more, the work function of a metal was expressed as the minimum energy required for an electron whose initial energy was equal to the Fermi level to escape from the interior of the metal into the vacuum. The size of the work function marked the strength of electrons bound in the metal. The larger the work function, the less likely the electrons were to leave the metal. The work function of SO2/pristine, X vacancy, As and Ge doped monolayer WTe2 was lower than any form of monolayer WS2 and WSe2, their work function had been decreased, among them, the work function of SO2/Ge-WTe2 gas molecule was the lowest (5.06 eV), and its metal escape work was the easiest to reach, so the adsorption effect of SO2 on Ge-WTe2 was better.
Figure. 12 shown the more parameters under the different strain (0–8%), including the distance between Ge atom and S atom (dGe−S), adsorption energy (Ea) and the rate of change of distance between Ge atom and S atom (Rd). The Rd was defined as:
Rd =| d′Ge−S – dGe−S| / ( dGe−S ) (3)
where dAg−N and d′Ag−N are the distance between S atom and Ge atom of Ge/WTe2 monolayer before and after adsorption of SO2, respectively.
As shown in Figure. 12, Ea of SO2-Ge/WTe2 monolayer system briefly decreased with the rising strain from 0–8%. It also shown charge transfers (the middle corner of Figure. 12) from − 0.371 e to -0.355 e. It cleared that adsorption energy of SO2 on Ge/WT2 monolayer could be reduced by the application of strain, thus the performance of SO2 gas molecule sensor based on strained Ge/WTe2 monolayer could also be weaken.
3.5. Recovery time
In principle, the process of air molecules (oxygen molecules in the air) adsorbing on the surface of the sample and returning electrons to the semiconductor valence band. This period of time is called the recovery time . The recovery time indicates the desorption speed of the gas sensor to the measured gas, also known as the desorption time. Similarly, I hope that this time will be as soon as possible. Since this time cannot be zero, in principle, the time required for the gas sensor to recover from the detection gas to 10% of the resistance in normal air is defined as the recovery time, which is represented by trec . The initial state was the configuration with the SO2 adsorbed on monolayer Ge-WX2. As a result, their desorption energy barrier was equal to their own adsorption energy for these three systems, which were − 0.45eV, -0.89eV and − 1.1 eV, respectively. According to the formula, the smaller the adsorption energy, the faster the recovery time.