# A Palladium-Deposited Molybdenum Disulfide-Based Hydrogen Sensor at Room Temperature
## 4 Results
### Measurement Result  
In the Measurement Result section, we present the spectrum results, analyzing the structural characteristics of MoS\({}_{2}\), and the sensor responses at varying H\({}_{2}\) concentrations. Additionally, a table is incorporated for comparison with other studies. The structural properties of MoS\({}_{2}\) are precisely analyzed through Raman and photoluminescence (PL) spectroscopy, providing important insights into the properties of materials. Figure 4a shows the Raman spectrum, an important indicator for analyzing the structural properties of MoS\({}_{2}\). Two major peaks are important in the Raman spectrum. The first peak, indicated by E1 \({}_{\text{2g}}\), is related to the crystal structure in the form of MoS\({}_{2}\). The position and intensity of the E1 \({}_{\text{2g}}\) peak are greatly influenced by both the crystal structure and the thickness of MoS\({}_{2}\). The second peak, designated as A\({}_{\text{1g}}\), is related to interatomic vibration in the material. A\({}_{\text{1g}}\) is also influenced by crystal structure and thickness, and A\({}_{\text{1g}}\) is an indicator of material quality. In the Raman spectrum, the E1 \({}_{\text{2g}}\) and A\({}_{\text{1g}}\) peaks of the MoS\({}_{2}\) monolayer were observed at 382 cm\({}^{-1}\) and 402 cm\({}^{-1}\), respectively, and the E1 \({}_{\text{2g}}\) and A\({}_{\text{1g}}\) peaks of the multilayer MoS\({}_{2}\) were observed at 380 cm\({}^{-1}\) and 407 cm\({}^{-1}\), respectively. The E1 \({}_{\text{2g}}\) peak corresponds to the in-plane vibration mode, which is the vibration of Mo atoms and S atoms parallel to the layer, and the A\({}_{\text{1g}}\) peak corresponds to the out-of-plane vibration mode, which is the vibration of S atoms perpendicular to the layer. The distance between the two peaks represents the number of layers present in the MoS\({}_{2}\) crystal, and as the number of layers increases, the distance between the two peaks increases due to interlayer vibration. Figure 4b is a PL spectrum graph of MoS\({}_{2}\) obtained at a laser wavelength of 532 nm. The PL spectrum of MoS\({}_{2}\) provides essential information on electronic states and structural properties. In the PL spectrum, the peak position reflects the bandgap of MoS\({}_{2}\). The peak of the monolayer MoS\({}_{2}\) is located at 660 nm. The peak of the multilayer MoS\({}_{2}\) is located at 995 nm. Applying the conversion relationship between photon energy and  
Figure 4: (a) Raman spectrum of MoS\({}_{2}\); (b) PL spectrum of MoS\({}_{2}\) at a 532 nm laser wavelength. Figure 3: The mechanism of the Pd\(-\)deposited MoS\({}_{2}-\)based hydrogen sensor. laser wavelength, the bandgap of the monolayer MoS\({}_{2}\) is calculated as 1.87 eV, and the bandgap of the multilayer MoS\({}_{2}\) is calculated as 1.24 eV. This corresponds to a generally known bandgap range of MoS\({}_{2}\). The intensity of the PL peak represents the efficiency of the light-emitting process occurring in MoS\({}_{2}\)[13]. Figure 5 shows the sensitivity of the Pd-deposited MoS\({}_{2}\)-based hydrogen sensor according to the H\({}_{2}\) concentration at room temperature. Figure 5a shows a H\({}_{2}\) concentration of 1% and an average sensitivity of 1.34. Increasing the H\({}_{2}\) concentration to 3% in Figure 5b increases the average sensitivity to 2.17, and, finally, the results for the 4% H\({}_{2}\) concentration in Figure 5c show the average sensitivity to be 2.77. The response time was 4 to 5 s. Response time is defined as the time when the sensor's total current changes by 90%. Sensitivity is defined as S = Ig/I0. Ig is the current after injecting the H\({}_{2}\) gas, and I0 is the current without H\({}_{2}\). The fast response time was caused by the high carrier mobility of MoS\({}_{2}\) and the simplification of the process due to the simple structure. Typical MoS\({}_{2}\)-based sensors have high operating temperatures. However, the sensor showed changes in current to H\({}_{2}\) at room temperature.