# A Palladium-Deposited Molybdenum Disulfide-Based Hydrogen Sensor at Room Temperature
## 3 Fabrication Process  
Figure 1 illustrates the comprehensive fabrication process employed in creating the Pd-deposited MoS\({}_{2}\)-based hydrogen sensor. The process initiates with the preparation of a Si wafer, onto which a layer of silicon dioxide is meticulously deposited, reaching a thickness  
of 300 nm via the wet oxidation technique. Subsequently, the crucial channel material, MoS2, is deposited utilizing chemical vapor deposition (CVD) methodology. In the CVD process, the MoS2 deposition takes place under specific conditions: the central heating zone maintains a temperature of 720 \({}^{\circ}\)C, while the upstream zone maintains a temperature of 200 \({}^{\circ}\)C. Furthermore, Ar gas is introduced at a rate of 200 standard cubic centimeters per minute (SCCM), effectively maintaining ambient pressure within the system.  
To facilitate this deposition, a combination of molybdenum trioxide powder (2 mg) and sulfur powder (100 mg) is placed in quartz tubes inserted into larger quartz tubes within a three-zone tube furnace. Throughout this process, MoS2 accumulates to a thickness of approximately 1.4 nm, constituting approximately eight layers. Following the deposition phase, a patterning step is introduced through a lithography process. Within this process, an initial cleaning procedure is conducted, involving successive treatments with acetone, an isopropyl alcohol (IPA) solution, deionized water (DI-water), and N2 gas. Subsequently, a positive photoresist (PR) is applied, forming a PR coating at an RPM of 5000 for 40 s. The coated substrate is then subjected to a hot plate at 100 \({}^{\circ}\)C for 1 min to eliminate the solvent. A mask is then strategically placed over the sample, which is subsequently exposed to ultraviolet (UV) light. Finally, the photoresist is removed utilizing a developer solution. In the final phase of the fabrication process, Pd is deposited onto the MoS2 layer through DC sputtering. The deposition parameters are precisely controlled, with Pd being deposited for 4 s, employing an Ar flow rate of 30 sccm and a power of 100 W. The resulting Pd layer exists in the form of discrete Pd particles, characterized by a thickness of 4 nm. To conclude the process, electrodes are introduced, comprising a Ti layer with a thickness of 10 nm, followed by a Au layer with a thickness of 40 nm, both deposited through DC sputtering.  
Figure 2 presents a schematic representation of the Pd-deposited MoS2-based hydrogen sensor, offering insights into its structure and key features. The thickness of the Pd layer plays a critical role in determining sensor performance. When the Pd layer is extremely thin, such as 1 nm, the Pd particles become indistinct, resulting in a limited contact area with the MoS2 layer. As a result, sensitivity is compromised, and the response time is relatively slow. Conversely, as the thickness of the Pd layer increases, the increased contact area of the Pd improves both the sensitivity and reactivity. However, deposition of the Pd into a continuous layer can lead to a decrease in sensitivity and response time. This reduction is attributed to a reduced contact area and the accumulation of an electric charge. Therefore, a balance must be struck to optimize sensor performance. To achieve the best sensing properties, the researchers in this study decided to deposit Pd particles with a thickness of 4 nm. This thickness represents an optimal compromise and was carefully selected, providing a substantial contact area while preventing the adverse effects associated with thicker Pd layers [12].  
Figure 1: The fabrication process of the Pd-deposited MoS2-based hydrogen sensor.