Hydrogenation-driven phase transition in single-layer TiSe2

Correlation between charge density wave phase transition and hydrogen adsorption in 1T-TaS2thin film devices

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. Interestingly, in the incommensurate charge-density wave (ICCDW) phase with a metallic state, the electrical resistance of the 1T-TaS₂thin film decreases when H₂is adsorbed on it and returns to its initial value upon desorption. In contrast, the electrical resistance of the film in the nearly commensurate CDW (NCCDW) phase, which has a subtle band overlap or a small bandgap, does not change upon H₂adsorption/desorption. This difference in H₂reactivity is a result of differences in the electronic structure of the two 1T-TaS₂phases, namely, the ICCDW and NCCDW phases. Compared to other semiconductor 2D-TMDs such as MoS₂and WS₂, the metallic TaS₂has been theoretically proven to capture gas molecules more easily because Ta has a stronger positive charge than Mo or W. Our experimental results provide evidence of this. Notably, this study is the first example of H₂sensing using 1T-TaS₂thin films and demonstrates the possibility of controlling the reactivity of the sensors to the gas by changing the electronic structure via CDW phase transitions.

Hydrogenated derivatives of hexacoordinated metallic Cu2Si monolayer

Herein, we carried out first-principles calculations based on density functional theory to investigate the effects of surface functionalization with hydrogen atoms on structural, dynamical and electronic properties of Cu₂Si monolayer. Pristine Cu₂Si, a metallic monolayer, has a planar hexacoordinate structure. Calculations revealed that the most favorable position of a single H atom on the Cu₂Si monolayer is at the top of a Si site. Derivatives of Cu₂Si monolayer with various H concentrations were investigated, and by performing phonon calculations, it was found that there are three stable hydrogenated structures. Specific heat of these monolayers was found to increase with the hydrogen concentration at temperatures higher than 100 K. Electronically, the hydrogenated derivatives of Cu₂Si monolayer preserve the metallic character.

Herein, we carried out first-principles calculations based on density functional theory to investigate the effects of surface functionalization with hydrogen atoms on structural, dynamical and electronic properties of Cu₂Si monolayer.