Highly Sensitive and Selective Defect WS2 Chemical Sensor for Detecting HCHO Toxic Gases

The gas sensitivity of the W defect in WS2 (VW/WS2) to five toxic gases—HCHO, CH4, CH3HO, CH3OH, and CH3CH3—has been examined in this article. These five gases were adsorbed on the VW/WS2 surface, and the band, density of state (DOS), charge density difference (CDD), work function (W), current–voltage (I–V) characteristic, and sensitivity of adsorption systems were determined. Interestingly, for HCHO-VW/WS2, the energy level contribution of HCHO is closer to the Fermi level, the charge transfer (B) is the largest (0.104 e), the increase in W is more obvious than other adsorption systems, the slope of the I–V characteristic changes more obviously, and the calculated sensitivity is the highest. To sum up, VW/WS2 is more sensitive to HCHO. In conclusion, VW/WS2 has a great deal of promise for producing HCHO chemical sensors due to its high sensitivity and selectivity for HCHO, which can aid in the precise and efficient detection of toxic gases.


Introduction
The majority of volatile organic compounds (VOCs) such as HCHO, CH 4 , CH 3 CHO, CH 3 OH, and CH 3 CH 3 are hazardous to human health and safety, the environment, and cause cancer [1].In order to identify harmful gases and lessen their damage, we need devices.Chemical sensors are commonly used to detect and analyze chemical substances and can measure the existence and specific concentration of experimental substances.Researchers have been continuously exploring and innovating chemical sensors to address the issue of harmful gas detection and monitoring in various domains.
In the current period of rapid scientific and technological advancement, chemical sensors are progressively becoming essential instruments across a wide range of fields.These sensors have the ability to detect sensitive signals and accurately identify chemicals, thus affecting our life, health, and environment.For instance, the use of chemical sensors is significant in the field of environmental protection.A number of toxic gases, including CO and SO 2 , are present in the atmosphere and are extremely dangerous to both human health and the environment.Scientists can track and identify the concentration of these dangerous gases in real time and take appropriate action to protect the environment by inventing high-performance chemical sensors.Furthermore, the use of chemical sensors in the industrial sector is very important.Consider a chemical plant as an example.During the production process, several hazardous gases are emitted, including H 2 S and NH 3 .High-performance chemical sensors allow workers to detect and identify the presence of dangerous gases in real time, allowing them to avoid potential hazards that could endanger their health.Furthermore, the medical industry is also involved in the application of chemical sensors.For instance, certain harmful gases, such as NO, will be created during respiratory therapy and will negatively affect the health of patients.Medical personnel can monitor the concentration of these dangerous gases in real time and take appropriate action Sensors 2024, 24, 762 3 of 14 V W /WS 2 is able to identify toxic gas with a very low concentration of HCHO [23].Because HCHO is frequently damaging to human health at low concentrations, this is crucial for safeguarding both the environment and people.Second, selectivity refers to V W /WS 2 's ability to recognize variations in various gases and solely react to the toxic gas (HCHO) [24].Due to these benefits, V W /WS 2 is a valuable tool for environmental monitoring, industrial safety, and personal safety.It also makes it easier to recognize and manage toxic gases, which is particularly useful for HCHO detection [25].

Computational Methods
We calculated the density functional theory (DFT) by using the Vienna ab initio simulation software package 5.4.4 (VASP).The exchange-correlation interaction and the electron-ion interaction are described, respectively, by the extended gradient approximation of Perdew Burke Ernzerhof (PBE) and the projection-enhanced wave approach.In order to assure system stability, we built a 4 × 4 × 1 supercell model and a vacuum layer of 20 Å was added in the z direction for WS 2 , defect WS 2 , and the adsorption systems.For both structural and electronic structure optimization calculations, the sampling grid in the K space of the Brillouin zone is 9 × 9 × 1, the energy convergence criterion is 10 −7 eV, the ideal convergence threshold is to guarantee that the force acting on atoms is less than 10 −3 eV/Å, and the cutoff energy is set to 500 eV.
The adsorption energy (E ab ) between molecules was further evaluated.The energy generated when free monolayers combine to form adsorption systems is known as the E ab , and it is expressed as [26][27][28][29]: The energy released during the construction of the adsorption system is represented by the symbol E abs in the above equation.The total energy of the adsorption systems is also indicated by the symbol E adsorbedsystem .Additionally, the total energy of V W /WS 2 and gas molecules is indicated by E monolayer and E targetmolecule .Lastly, the unit area under consideration is indicated by the sign S.
By utilizing computed differences in charge density difference (CDD) [30] and charge transfer (B) [31], we can analyze the transfer of charges between the adsorption systems.This formula can be expressed as follows: By employing computed variations in charge density (ρ h , ρ 1 , and ρ 2 ), we can examine the B mechanisms between the V W /WS 2 and gas molecules.Here, ρ h represents the charge density of the adsorption systems, while ρ 1 and ρ 2 correspond to the charge densities of the V W /WS 2 and gas molecules.We studied the I-V characteristic of the adsorption systems, which was obtained by calculating the current at different bias voltages.The current at a specific bias voltage can be obtained using the following equation [32]: where , and E R F are the Fermi energy level and Fermi distribution at the equilibrium state of the left and right electrodes, respectively.T(E; V b ) is the transmission probability function spectrum for electrons.
Lastly, we used the following formula to evaluate the adsorption systems' sensitivity [33]: where G is the conductance of adsorption systems, and G 0 is the conductance of V W /WS 2 .We used Vienna ab initio simulation package (VASP) software to calculate the electronic properties of the systems, such as energy band, density of state (DOS), CDD, and B, and used Nanodcal 2023A software to calculate the electrical characteristics of I-V [34,35].

Results and Discussion
The focus of our research is toxic gases, and HCHO, CH 4 , CH 3 CHO, CH 3 OH, and CH 3 CH 3 are the most common toxic gases.There is still a technical gap in the field of detection.So, we adsorbed five gas molecules, HCHO, CH 4 , CH 3 CHO, CH 3 OH, and CH 3 CH 3 , on WS 2 and optimized their structures.The adsorption distance (H) of the adsorption systems before optimization is 3 Å, the optimized H values are 2.94 Å, 2.80 Å, 2.78 Å, 2.49 Å, and 2.52 Å in Table 1, respectively.We also calculated the E ab of the adsorption systems and determined the most stable model, as shown in Figure 1.Next, the energy bands of the adsorption systems were calculated, as shown in Figure 2. We can see that the energy bands have hardly changed, so the adsorption of gas molecules has little effect on the electronic properties of WS 2 .Gas molecules are physically adsorbed.In order to further investigate the physical adsorption of gas molecules, we calculated the DOS; the total DOS of the adsorption system is shown by the gray line in Figure 3, the DOS of WS 2 is represented by the blue line.Compared with the DOS of WS 2 , the adsorption systems show a DOS contribution of gas molecules near the Fermi level and the DOS of gas molecules is represented by the red line.The DOS of CH 4 and CH 3 CH 3 disappeared near the Fermi level.The DOS of CH 3 OH and CH 3 CHO is further away from the Fermi level, while the DOS of HCHO is closest to the Fermi level.Therefore, HCHO adsorption has the greatest impact on the electronic properties near the Fermi level compared to other gas adsorption.Figure 4 is the CDD we calculated; the sky-blue area denotes the charge accumulation, and the purple area denotes the charge depletion.The electric field is usually formed by the charge accumulation, and the charge depletion leads to the flow and transport of charges, which in turn affects the electrical characteristics of materials.There is a difference between the energy level of the adsorbed substance and the energy level of the surface, and the movement of electrons leads to the resonance of electrons or the change in local electron density, thus causing B in the adsorption systems.We can see that there is B between the gas molecule and WS 2 .In Table 1, combined with the B of the adsorption systems (0.107 e, 0.092 e, 0.088 e, 0.091 e, and 0.089 e, respectively), the B of the HCHO-WS 2 system is the largest, which is consistent with DOS.Next, we calculated the W of the WS 2 adsorption systems, which are 5.72 eV, 5.53 eV, 5.01 eV, 5.15 eV, and 5.53 eV, respectively.Compared with the W of WS 2 (5.39 eV), the W of HCHO-WS 2 increased the most significantly, while the W of CH 3 CHO-WS 2 and CH 3 OH-WS 2 decreased.The W of CH 4 -WS 2 and CH 3 CH 3 -WS 2 show a slight increase.So, when detecting gas, WS 2 is more likely to react with HCHO.In summary, WS 2 has higher sensitivity and selectivity towards HCHO.An important research object in the study of 2D materials is vacancy defects.By adding vacancy defects in certain ways, scientists can investigate how defects affect electrical conductivity, optical absorption, electronic structure, and other aspects.Researchers can gain a deeper understanding of the relationship between a material's structure and properties, as well as expand the potential applications of 2D materials in areas like catalysis, sensing, and electrical devices through vacancy defects.Based on the WS 2 model, we removed the W atom to construct V W /WS 2 , and similarly removed the S atom to construct S defect WS 2 (V S /WS 2 ), the model diagram of which is displayed in Figure 5a.Next, we computed the E ad of these two structures after fully relaxing them.The E ad of V W /WS 2 is −17.33 eV, and that of V S /WS 2 is −7.15 eV, as Figure 5b illustrates.We decided to focus our subsequent research on V W /WS 2 since the more negative adsorption energy it has, the more stable it becomes [36].To gain more insight into this structure's thermal stability, we computed the ab initio molecular dynamics (AIMD) at 300 K [37][38][39].V W /WS 2 exhibits thermal stability, as evidenced by Figure 6b, where the total energy tends to stabilize across the simulation duration and the crystal structure is free of deformation and bond breaking.We computed the band structure [3] and DOS of V W /WS 2 in order to comprehend its electronic properties.V W /WS 2 is a direct band gap semiconductor with a band gap value of 0.379 eV, as can be seen in the band diagram in Figure 6c.The Fermi level is situated between the conduction band and the valence band, V W /WS 2 is a semiconductor, and the 4f level of the W atom supplies the valence band maximum and conduction band minimum, which correlates to the energy band, as can be seen in the DOS diagram of Figure 6d.
atom to construct VW/WS2, and similarly removed the S atom to construct S defect WS2 (VS/WS2), the model diagram of which is displayed in Figure 5a.Next, we computed the Ead of these two structures after fully relaxing them.The Ead of VW/WS2 is −17.33 eV, and that of VS/WS2 is −7.15 eV, as Figure 5b illustrates.We decided to focus our subsequent research on VW/WS2 since the more negative adsorption energy it has, the more stable it becomes [36].To gain more insight into this structure's thermal stability, we computed the ab initio molecular dynamics (AIMD) at 300 K [37][38][39].VW/WS2 exhibits thermal stability, as evidenced by Figure 6b, where the total energy tends to stabilize across the simulation duration and the crystal structure is free of deformation and bond breaking.We computed the band structure [3] and DOS of VW/WS2 in order to comprehend its electronic properties.VW/WS2 is a direct band gap semiconductor with a band gap value of 0.379 eV, as can be seen in the band diagram in Figure 6c.The Fermi level is situated between the conduction band and the valence band, VW/WS2 is a semiconductor, and the 4f level of the W atom supplies the valence band maximum and conduction band minimum, which correlates to the energy band, as can be seen in the DOS diagram of Figure 6d.atom to construct VW/WS2, and similarly removed the S atom to construct S defect WS2 (VS/WS2), the model diagram of which is displayed in Figure 5a.Next, we computed the Ead of these two structures after fully relaxing them.The Ead of VW/WS2 is −17.33 eV, and that of VS/WS2 is −7.15 eV, as Figure 5b illustrates.We decided to focus our subsequent research on VW/WS2 since the more negative adsorption energy it has, the more stable it becomes [36].To gain more insight into this structure's thermal stability, we computed the ab initio molecular dynamics (AIMD) at 300 K [37][38][39].VW/WS2 exhibits thermal stability, as evidenced by Figure 6b, where the total energy tends to stabilize across the simulation duration and the crystal structure is free of deformation and bond breaking.We computed the band structure [3] and DOS of VW/WS2 in order to comprehend its electronic properties.VW/WS2 is a direct band gap semiconductor with a band gap value of 0.379 eV, as can be seen in the band diagram in Figure 6c.The Fermi level is situated between the conduction band and the valence band, VW/WS2 is a semiconductor, and the 4f level of the W atom supplies the valence band maximum and conduction band minimum, which correlates to the energy band, as can be seen in the DOS diagram of Figure 6d.The H of the adsorption systems before optimization is 3 Å.In Table 2, the most stable model (the H values are 2.92 Å, 2.79 Å, 2.65 Å, 2.67 Å, and 2.51 Å, respectively) for V W /WS 2 to adsorb the five toxic gases-HCHO, CH 4 , CH 3 CHO, CH 3 OH, and CH 3 CH 3 -is depicted in Figure 7 [11,40].The band gap values of V W /WS 2 and the five adsorption systems are displayed in Figure 8 [41].We can see that compared with V W /WS 2 the band gap undergoes certain changes after adsorbing gas molecules.The DOSs of adsorption systems are displayed in Figure 9. Compared with other gas molecules, due to the structure and electronic configuration of HCHO, its oxygen atom may introduce more electrons, and some special electronic states may be introduced near the Fermi level, resulting in the formation of a local electronic state near the Fermi level.The DOS of HCHO is closer to the Fermi level and its contribution is greater near the Fermi level.The DOSs of CH 4 and CH 3 CH 3 disappear near Fermi level, while the DOSs of CH 3 CHO and CH 3 OH are far away from Fermi level and their contribution is smaller.Therefore, V W /WS 2 has high sensitivity and selectivity for HCHO.The CDD of the adsorption systems is shown in Figure 10.We can see the accumulation and dissipation of charge in the adsorption systems, which shows that the conductivity of V W /WS 2 has changed to some extent after adsorbing gas molecules.In order to describe which gas molecules V W /WS 2 is most sensitive to, we calculated B. As shown in Table 2, HCHO-V W /WS 2 has the highest B, which shows that the conductivity of HCHO-V W /WS 2 has the most obvious change, and V W /WS 2 has high sensitivity and selectivity to HCHO [12,[41][42][43].The H of the adsorption systems before optimization is 3 Å.In Table 2, the most stable model (the H values are 2.92 Å, 2.79 Å, 2.65 Å, 2.67 Å, and 2.51 Å, respectively) for VW/WS2 to adsorb the five toxic gases-HCHO, CH4, CH3CHO, CH3OH, and CH3CH3-is depicted in Figure 7 [11,40].The band gap values of VW/WS2 and the five adsorption systems are displayed in Figure 8 [41].We can see that compared with VW/WS2 the band gap undergoes certain changes after adsorbing gas molecules.The DOSs of adsorption systems are displayed in Figure 9. Compared with other gas molecules, due to the structure and electronic configuration of HCHO, its oxygen atom may introduce more electrons, and some special electronic states may be introduced near the Fermi level, resulting in the formation of a local electronic state near the Fermi level.The DOS of HCHO is closer to the Fermi level and its contribution is greater near the Fermi level.The DOSs of CH4 and CH3CH3 disappear near Fermi level, while the DOSs of CH3CHO and CH3OH are far away from Fermi level and their contribution is smaller.Therefore, VW/WS2 has high sensitivity and selectivity for HCHO.The CDD of the adsorption systems is shown in Figure 10.We can see the accumulation and dissipation of charge in the adsorption systems, which shows that the conductivity of VW/WS2 has changed to some extent after adsorbing gas molecules.In order to describe which gas molecules VW/WS2 is most sensitive to, we calculated B. As shown in Table 2, HCHO-VW/WS2 has the highest B, which shows that the conductivity of HCHO-VW/WS2 has the most obvious change, and VW/WS2 has high sensitivity and selectivity to HCHO [12,[41][42][43].The relationship between W and the sensitivity of chemical sensors is mainly reflected in the change in electronic structure caused by the interaction between materials and target molecules.This relationship is very important for understanding the working mechanism of the sensor, optimizing the sensor's performance, and designing a more selective and sensitive sensor.Additionally, we computed the W of five adsorption systems and the V W /WS 2 [44].∆W represents the change in the W after adsorption, as seen in Table 2.The ∆W of the CH 4 -V W /WS 2 (0.005 eV) and CH 3 CH 3 -V W /WS 2 (0.008 eV) systems show a minor increase, and the CH 3 HO-V W /WS 2 and CH 3 OH-V W systems show a decrease [45,46].The W clearly increases for the HCHO-V W /WS 2 system, and the change value is 0.205 eV, which is noticeably larger than for other adsorption systems.This indicates that V W /WS 2 has high sensitivity and selectivity for HCHO and that HCHO will interact with V W /WS 2 more readily when it is in contact with experimental gas.Device models can be used to predict the performance of electronic devices, including I-V characteristic, power consumption, and speed [47].We constructed device models, as Figure 11a illustrates.The gadget is separated into a center scattering zone and left and right electrodes.The buffer layer can adjust the charge distribution in the device and prevent excessive charge from accumulating in a certain area, thus maintaining the stability and reliability of the device.The buffer layer can also block the diffusion of external impurities, protect the internal purity of the device, and reduce the negative impact on the performance of the device, so, at the intersection of the electrodes and the central scattering region, we inserted three buffer layers to prevent any interference of the device center [48].We utilized Materials studio software to compute the I-V characteristic of five adsorption systems and V W /WS 2 , which were represented by the I-V curve, so that we could clearly see the change in conductivity.The I-V curve is displayed in Figure 11b-f, where the gray lines in each figure correspond to the V W /WS 2 I-V curve.We can observe that the current starts to increase when a bias voltage of 1.8 V is applied [49].Compared with the I-V curve of BC 6 N adsorption systems [33], the slope change of the I-V curve after V W /WS 2 adsorbed gas molecules is more obvious, and the conductivity change is more sensitive when detecting gas molecules.Figure 11b shows us that hardly any current passes through HCHO-V W /WS 2 when the bias voltage is less than 1.8 V.The current increases dramatically when the applied bias voltage rises above the threshold value (1.8 V).The current of 1.14 × 10 −7 A is reached at 2.4 V, which is much greater than that of V W /WS 2 (7.99 × 10 −8 A).Following adsorption, V W /WS 2 is clearly sensitive to HCHO, as seen by the sharply altered I-V curve, higher slope (under the bias voltage of 1.8 V~2.4 V), and clearly increased conductivity.The I-V curves of the four types of adsorption systems in Figure 11c-f have altered somewhat when compared to the I-V curve of the V W /WS 2 .By examining the slope, we can also observe that V W /WS 2 has the maximum sensitivity to HCHO, as seen by the I-V curves of the five adsorption systems (which is compatible with the W calculation results).It can also demonstrate the selectivity of the sensor, allowing us to differentiate HCHO from other gases through the observation of the conductivity shift.To validate the work function and I-V curve results, we computed V W /WS 2 sensitivity at 1.8 V, 2.0 V, 2.2 V, and 2.4 V bias voltages [50,51].Table 3 displays the results of the calculation [52].The sensitivity of V W /WS 2 to HCHO, CH 4 , CH 3 HO, CH 3 OH, and CH 3 CH 3 is 98.1%, 96.7%, 88.9%, 94.9%, and 96.0%, respectively, when a bias voltage of 1.8 V is applied to the adsorption systems.This indicates that V W /WS 2 has high sensitivity to these five gases at a voltage of 1.8 V.The sensitivity of Pt-NiS 2 to HCHO is −99.2% [53], and the sensitivity of V W /WS 2 to HCHO is −98.1% under 1.8 V, so it is necessary to study further.Significantly higher than that of other adsorbed gas molecules, the sensor's sensitivity to HCHO is 108.0%and 42.9% at 2.2 V and 2.4 V, respectively [54][55][56].This suggests that V W /WS 2 can selectively detect HCHO, and the calculated results are in agreement with those of W and the I-V characteristic.The relationship between W and the sensitivity of chemical sensors is mainly reflected in the change in electronic structure caused by the interaction between materials and target molecules.This relationship is very important for understanding the working mechanism of the sensor, optimizing the sensor's performance, and designing a more selective and sensitive sensor.Additionally, we computed the W of five adsorption systems and the VW/WS2 [44].ΔW represents the change in the W after adsorption, as seen in Table 2.The ΔW of the CH4-VW/WS2 (0.005 eV) and CH3CH3-VW/WS2 (0.008 eV) systems show a

Conclusions
This research uses a series of simulations to study the gas sensitivity of V W /WS 2 to HCHO, CH 4 , CH 3 HO, CH 3 OH, and CH 3 CH 3 .Initially, we used first-principles calculations to determine the system's electronic properties.According to the DOS, HCHO contributes more than other gas molecules to an energy level close to the Fermi level.The HCHO-V W /WS 2 B is the biggest (0.104 e).The system's W was then computed, and the HCHO system's W clearly rose.We also calculated the I-V characteristic.It was observed that in the case of adsorbed HCHO, the corresponding conductivity increased most dramatically and the slope increased most noticeably once the bias voltage rose beyond the threshold voltage.Therefore, V W /WS 2 can solve the cross-sensitivity and other defects of general chemical sensors, and then develop into HCHO chemical sensors with high sensitivity and selectivity, which can be further integrated into flexible wearable devices to realize real-time monitoring of individual health and environmental factors.

Figure 4 .
Figure 4.The CDD of (a) HCHO, (b) CH4, (c) CH3CHO, (d) CH3OH, and (e) CH3CH3 adsorbed on WS2 with the isosurface value of 3 × 10 −4 e Å −1 .The W and S atoms are colored in blue and grey, the skyblue area denotes the accumulation of electrons, and the purple area denotes the depletion of electrons.

Figure 4 .
Figure 4.The CDD of (a) HCHO, (b) CH4, (c) CH3CHO, (d) CH3OH, and (e) CH3CH3 adsorbed on WS2 with the isosurface value of 3 × 10 −4 e Å −1 .The W and S atoms are colored in blue and grey, the skyblue area denotes the accumulation of electrons, and the purple area denotes the depletion of electrons.

Figure 4 .
Figure 4.The CDD of (a) HCHO, (b) CH 4 , (c) CH 3 CHO, (d) CH 3 OH, and (e) CH 3 CH 3 adsorbed on WS 2 with the isosurface value of 3 × 10 −4 e Å −1 .The W and S atoms are colored in blue and grey, the sky-blue area denotes the accumulation of electrons, and the purple area denotes the depletion of electrons.

Figure 5 .
Figure 5.The (a) configuration and (b) adsorption energy of VW/WS2 and VS/WS2, the W and S atoms are colored in blue and grey.

Figure 6 .
Figure 6.The (a) configuration, (b) graph of total energy and simulation time, (c) band structure, and (d) DOS of VW/WS2, the W and S atoms are colored in blue and grey.

Figure 5 .
Figure 5.The (a) configuration and (b) adsorption energy of V W /WS 2 and V S /WS 2 , the W and S atoms are colored in blue and grey.

Figure 5 .
Figure 5.The (a) configuration and (b) adsorption energy of VW/WS2 and VS/WS2, the W and S atoms are colored in blue and grey.

Figure 6 .
Figure 6.The (a) configuration, (b) graph of total energy and simulation time, (c) band structure, and (d) DOS of VW/WS2, the W and S atoms are colored in blue and grey.

Figure 6 .
Figure 6.The (a) configuration, (b) graph of total energy and simulation time, (c) band structure, and (d) DOS of V W /WS 2 , the W and S atoms are colored in blue and grey.

Figure 7 .
Figure 7.The most stable adsorption configurations of (a) HCHO, (b) CH 4 , (c) CH 3 CHO, (d) CH 3 OH, and (e) CH 3 CH 3 adsorbed on V W /WS 2 , the W and S atoms are colored in blue and grey.

Sensors 2024 , 15 Figure 10 .
Figure 10.The CDD of (a) HCHO, (b) CH4, (c) CH3CHO, (d) CH3OH, and (e) CH3CH3 adsorbed on VW/WS2.The W and S atoms are colored in blue and grey,the sky-blue area denotes the accumulation of electrons, and the purple area denotes the depletion of electrons.The value of the isosurface is set to 3 × 10 −4 e Å −1 .

Figure 10 .Figure 11 .
Figure 10.The CDD of (a) HCHO, (b) CH 4 , (c) CH 3 CHO, (d) CH 3 OH, and (e) CH 3 CH 3 adsorbed on V W /WS 2 .The W and S atoms are colored in blue and grey, the sky-blue area denotes the accumulation of electrons, and the purple area denotes the depletion of electrons.The value of the isosurface is set to 3 × 10 −4 e Å −1 .

Table 1 .
The E ad , the H, band gap (E g ), B, and W for WS 2 , and HCHO, CH 4 , CH 3 CHO, CH 3 OH, and CH 3 CH 3 adsorbed on WS 2 .

Table 1 .
The Ead, the H, band gap (Eg), B, and W for WS2, and HCHO, CH4, CH3CHO, CH3OH, and CH3CH3 adsorbed on WS2.An important research object in the study of 2D materials is vacancy defects.By adding vacancy defects in certain ways, scientists can investigate how defects affect electrical conductivity, optical absorption, electronic structure, and other aspects.Researchers can gain a

Table 2 .
The E ad , H, E g , and B for HCHO, CH 4 , CH 3 CHO, CH 3 OH, and CH 3 CH 3 adsorbed on V W /WS 2 .

Table 3 .
The sensitivity (%) of the sensor based on V W /WS 2 with left and right electrodes toward HCHO, CH 4 , CH 3 CHO, CH 3 OH, and CH 3 CH 3 at bias voltage of 1.8 V, 2.0 V, 2.2 V, and 2.4 V. Negative (positive) sensitivity means that the current of the sensor dropped (enhanced) after interaction with the adsorption.