Thin films of tantalum disulfide in the 1T-polytype structural phase (1T-TaS2), a type of metallic two-dimensional (2D) transition metal dichalcogenides (TMDs), are reactive to H2 gas. Interestingly, the electrical resistance of the 1T-TaS2 thin film in the incommensurate charge–density wave (ICCDW) phase with a metallic state decreases when H2 gas is adsorbed on the 1T-TaS2 film and returns to its initial value upon desorption. In contrast, the electrical resistance of the 1T-TaS2 thin film in the nearly commensurate CDW (NCCDW) phase, which has a slight band overlap or small bandgap, does not change upon H2 gas adsorption/desorption. This difference in H2 gas reactivity is a result of differences in the electronic structures of the two 1T-TaS2 phases, namely, the ICCDW and NCCDW phases. Our experimental results indicate advantages of the use of TaS2 for H2 gas sensing compared to other semiconductor 2D-TMDs such as MoS2 and WS2, as a theoretical prediction that the metallic TaS2 captures gas molecules more easily owing to a stronger positive charge at Ta than those in Mo or W of dichalcogenides. Notably, this study is the first example of H2 gas sensing using 1T-TaS2 thin films and demonstrates the possibility of controlling the reactivity to gas by changing the electronic structure via CDW phase transitions.

The most appropriate material for a sacrificial (protection) layer to effectively stop an irradiated ion in 2D materials supported on the surface of a SiO 2 substrate was explored for changing the structure, electronic properties, and chemical activities of 2D materials by ion-implantation. We found that a NaCl protection layer covered by a Cr layer is the most suitable sacrificial layers for graphene, MoS 2 , and TaS 2 based on Raman spectroscopy and electron transport properties. The gate voltage dependence of the conductivity (resistivity) shows that irradiating graphene with B and N ions at 200 keV with a dose of 10 13 cm −2 using a Cr or NaCl sacrificial layer, introduces hole and electron carriers, respectively. Both the B and N atoms introduced by ion beam irradiation cause strain in the graphene lattice, resulting in it partially detaching from the substrate like quasi-freestanding graphene. Irradiation with B ions creates neutral impurities in graphene, producing stronger carrier scattering in the Raman scattering, and irradiation with N ions creates charged impurities which affect electron transport phenomena, respectively. The developed sacrificial layers are promising for the defect engineering of 2D materials.

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