Sliding properties of Transition Metal Dichalcogenide bilayers

TL;DR

This study tackles the problem of understanding nanoscale sliding in transition metal dichalcogenide (TMD) bilayers by performing first-principles calculations with vdW-corrected DFT across a broad set of bilayers, including MoS$_2$, MoTe$_2$, WS$_2$, WSe$_2$, VS$_2$, VSe$_2$, TaS$_2$, TaSe$_2$, TiS$_2$, TiSe$_2$, HfS$_2$, ZrS$_2$, and heterostructures MoS$_2$WS$_2$ and MoS$_2$VS$_2$. The authors compute interlayer binding energies $E_b$, work of separation $W_{\\rm sep}$, and maximum sliding corrugations $W_{\\rm max}$ for 1T and 2H polytypes in R0/R180 orientations, assess sensitivity to vdW functionals (DFT-D2, DFT-D3, rVV10), and reveal that vdW interactions dominate interlayer cohesion while quantitative results depend on the chosen functional. Key findings show that most bilayers exhibit stronger bonding and greater corrugation than MoS$_2$ (with TiSe$_2$ similar to MoS$_2$, and TiS$_2$, VS$_2$, ZrS$_2$ weaker), that corrugation generally increases with chalcogen size and correlates with adhesion energy, and that $W_{\\rm max}$ scales with $W_{\\rm sep}$ roughly as $W_{\\rm max} \approx 0.35\,W_{\\rm sep}$ across systems. The work underscores the lack of a single universal trend linking tribological and electronic properties across all TMD bilayers and has implications for designing solid-state lubricants and nanoscale tribological interfaces.

Abstract

Transition-metal dichalcogenides (TMDs) are valuable as solid lubricants because of their layered structure, which allows for easy shearing along the basal planes. Using Density Functional Theory (DFT) we conducted a first-principles study of the sliding properties of several TMD bilayers: MoS$_2$, MoTe$_2$, WS$_2$, WSe$_2$, VS$_2$, VSe$_2$, TaS$_2$, TaSe$_2$, TiS$_2$, TiSe$_2$, HfS$_2$, ZrS$_2$, MoS$_2$WS$_2$, MoS$_2$VS$_2$. Given the crucial role of van der Waals (vdW) interactions in accurately describing the interlayer interactions in TMD bilayers, we employed vdW-corrected DFT functionals. Our research confirms the dominance of vdW effects by estimating the fraction of interlayer binding energy attributable to these interactions. We also examined how the choice of different vdW-corrected DFT functionals might influence quantitative results. Using MoS$_2$ as a reference TMD bilayer system, we found that most other TMD bilayers studied exhibit stronger interlayer bonds and greater corrugation. However, TiSe$_2$ shows a profile similar to MoS$_2$, while, interestingly, TiS$_2$, VS$_2$, and ZrS$_2$ are characterized by weaker bonding and lower corrugation than MoS$_2$. We explored relationships between various properties of TMD bilayers, with a particular focus on potential connections between tribological and electronic properties often characteristic of solid interfaces. To this end, we evaluated adhesion energies, work of separation, charge density redistributions in interface regions, differential charge densities, and corrugation. While corrugation and thus resistance to sliding generally tends to increase with the size of the chalcogen element and is typically proportional to the adhesion energy, the relationships between other structural, energetic, and electronic properties do not follow a single, well-defined trend.

Figures (11)

• Figure 1(a): Bulk MoS$_2$ in 1T-trigonal configuration. Upper figure: top view; lower figure: side view. The unit cells are marked with red solid lines; Mo atoms are represented in gray and S atoms in yellow.
• Figure 1(b): Bulk MoS$_2$ in 2H-octahedral configuration. Upper figure: top view; lower figure: side view. The unit cells are marked with red solid lines; Mo atoms are represented in gray and S atoms in yellow.
• Figure 1(b): Variation of the interlayer binding energy per unit-cell area, $E_b/A$, in different TMD bilayers, considering the R180 structure for 2H-octahedral configurations. Each profile is associated with the sliding of the bilayer, along the path in the $y$ direction indicated by the blue arrow in Fig. 4. Symbols correspond to calculated data; lines are just guides for the eyes.
• Figure 1(b): Variation of the interlayer binding energy per unit-cell area, $E_b/A$, in different TMD bilayers, considering the R0 structure for 1T-trigonal configurations. Each profile is associated with the sliding of the bilayer, along the path in the $y$ direction indicated by the blue arrow in Fig. 4. Symbols correspond to calculated data; lines are just guides for the eyes.
• Figure 1(b): Sliding path (indicated by the blue arrow) relative to the R0 structure in the 2H-octahedral configuration of MoS$_2$ (top view). The unit cells are marked with red solid lines; Mo atoms are represented in gray and S atoms in yellow.