Overcoming the response instability of MoS2 humidity sensors by hydrochloric acid surface treatment

The synthesis of MoS₂ with chemical vapor deposition (CVD) using sodium molybdate (Na₂MoO₄) as the Mo precursor produces a big number of large flakes (∼100-300 μm) compared to other CVD methods that use different precursors. In this work, humidity sensors based on MoS₂ are developed, whereby MoS₂ is grown using this Mo precursor in an aqueous solution form. The final devices exhibit a response-switching during operation under high (>50%) relative humidity conditions, due to the presence of Na₂MoO₄ residues on their surface. By decreasing the concentration of the aqueous Mo precursor during the CVD process we partially diminish the switching effect, as the Na₂MoO₄ residue is reduced To completely overcome this issue, we present a post-fabrication surface treatment using hydrochloric acid that removes the Na₂MoO₄ residue from the devices' surface. Rinsing the devices with an HCl solution results in the elimination of the response-switching effect and the sensors demonstrate a constant positive response from the initial operation steps.