Effect of metallicity on interlayer frictional potential energy of transition metal dichalcogenides under normal load

Van der Waals two-dimensional (2D) transition-metal dichalcogenides (TMDs) exhibit great potential in solid tribology design. Understanding the mechanism at the electronic level is essential for the design of low-friction-coefficient 2D materials. Here, via a comparative study of metallic ${d}^{1}$ and semiconducting ${d}^{2}$ TMD bilayers (BLs), we find that their frictional potential energy under normal load is correlated to their metallicity; namely, the metallic TMD has a lower frictional potential energy. The underlying reason is that the metallicity or semiconductivity in TMD energy band structures results in different interlayer quasibonding (QB) interactions. For the metallic ${d}^{1} \mathrm{Nb}{\mathrm{S}}_{2}$ monolayer, there is a half-filled band; in a BL, the interlayer QB alters the half-filled band into two partial-filled bands that have different numbers of electrons, leading to a charge transfer between the two partial-filled bands. The charge transfer between bands effectively weakens the Pauli repulsion between the metallic TMD layers and ultimately leads to a significant decrease in the slope of the frictional potential energy with the increase of normal load. In contrast, for semiconducting ${d}^{2} \mathrm{Mo}{\mathrm{S}}_{2}$, there is no such charge-transfer effect to reduce the interlayer Pauli repulsion, and the slope of frictional potential energy with normal load is larger than the metallic TMDs. Our findings provide an approach for designing lower friction coefficients with metallic van der Waals layered materials.