Tailoring Nanoscale Friction in MX₂ Transition Metal Dichalcogenides

Abstract

Lattice dynamics of MX2 transition metal dichalcogenides (M = Mo, W; X = S, Se, Te) have been studied with density functional theory techniques to control the macroscopic tribological behavior. Long-range van der Waals forces have been modeled with Grimme correction to capture the interlayer interactions. A new lattice dynamic metric, named cophonicity, is proposed and used in combination with electronic and geometric descriptors to relate the stability of the lattice distortions with the electro-structural features of the system. The cophonicity analysis shows that the distortion modes relevant to the microscopic friction can be controlled by tuning the relative M/X atomic contributions to the phonon density of states. Guidelines on how to engineer macroscopic friction at nanoscale are formulated, and finally applied to design a new Ti-doped MoS2 phase with enhanced tribologic properties.

Vibrational contributions to intrinsic friction in charged transition metal dichalcogenides

“…In our previous works, [20][21][22] we already pointed out how few lowfrequency phonon modes are directly connected with the relative sliding of adjacent layers. These consist of relative layer glides and out-of-phase vertical (parallel toĉ) shifts of subsequent layers, eventually combined with intralayer motions ( Figure 2); we will refer to such modes as sliding and compressive modes.…”

“…18 We also take into account Van der Waals interactions using the Grimme correction, 19 that is able to reproduce the structural features, as we reported in previous works and references therein. [20][21][22] The Brillouin zone is sampled with a minimum of a 5 × 5 × 3 k-point mesh and plane wave cutoff of 550 eV. We consider neutral as well as charged systems, with charge q = 0, ±0.01, ±0.03 ± 0.05, ±0.1, ±0.2, ±0.3, ±0.5, ±0.7, ±1.0|e|/cell, where one "cell" is a 2 × 2 × 2 supercell of the reference unit cell.…”

“…We now proceed with the study of the details of the electronic distribution. To this aim, we first perform a charge Bader analysis in all the considered systems, and consider the difference q B = q M − q X , where q M and q X are the Bader charges of the X and M atomic species, respectively (Figure 8a); we then calculate the C M,X M-X bond covalency 34 and the C ph (M-X) cophonicity 20 (Figure 8b-c). The M-X pair cophonicity is a measure of how the M and X atomic species contribute to form the phonon states in the considered range, and has been applied to the study of the electro-structural coupling in distinct classes of materials.…”

On the lubricity of transition metal dichalcogenides: an ab initio study

“…Conversely, it has been predicted by our group that the presence of titanium (replacing Mo atoms) within MoS 2 may enhance the tribological performance of the material. 55 By using this information to deepen our understanding of the synergy between interlayer interactions and frictional behaviour, we highlight the powerful predictive capabilities of in silico methods during the material design process. Indeed, it is the ultimate goal of any solid-state theoretician to be able to predict new materials with properties that surpass predecessors, without requiring input from experiment.…”

“…[47][48][49][50] Calculations were performed using projectoraugmented wave pseudopotentials 51,52 and the Perdew-BurkeErnzerhof functional, 53 with vdW interactions accounted for using the DFT-D2 method of Grimme; 54 this methodology has been demonstrated as being robust and reliable, accurately reproducing fundamental structural features. 55 Brillouin zone integrations utilised k-point grids generated automatically using the Monkhorst-Pack scheme: 56 a 15×15×15 grid was used for (bulk, 2H phase) unit cell optimisations, which was subsequently reduced to a 15×15×1 grid for the homogeneous MX 2 bilayers. Owing to the significant increase in the size of the unit cell of the incommensurate * bilayers, a 7×7×1 grid was employed for angles of 38.2 (Mo×14; S×28) and 92.2 (Mo×26; S×52) degrees, and a 5×5×1 grid for an angle of 17.9 degrees (Mo×62; S×124), in order to render them computationally tractable.…”