Sliding Properties of MoS₂ Layers: Load and Interlayer Orientation Effects

Abstract

Among the members of the transition metal dichalcogenides (TMD) family, molybdenum disulfide has the most consolidated application outcomes in tribological fields. However, despite the growing usage as nanostructured solid lubricant due to its lamellar structure, little is known about the atomistic interactions taking place at the interface between two MoS2 sliding layers, especially at high loads. By means of ab initio modeling of the static potential energy surface and charge distribution analysis, we demonstrate how electrostatic interactions, negligible in comparison with van der Waals and Pauli contributions at zero load, progressively affect the sliding motion at increasing loads. As such, they discriminate the relative stability and the frictional behavior of bilayers where the two monolayers defining the interface have a different relative orientation. In particular, for antiparallel sliding layers we observed a load-induced increase of both the depth of the minima and the height of the energy barriers compared to parallel ones, which may have important consequences for the fabrication of more efficient ultralow friction devices at the nanoscale.Among the members of the transition metal dichalcogenides (TMD) family, molybdenum disulfide has the most consolidated application outcomes in tribological fields. However, despite the growing usage as nanostructured solid lubricant due to its lamellar structure, little is known about the atomistic interactions taking place at the interface between two MoS2 sliding layers, especially at high loads. By means of ab initio modeling of the static potential energy surface and charge distribution analysis, we demonstrate how electrostatic interactions, negligible in comparison with van der Waals and Pauli contributions at zero load, progressively affect the sliding motion at increasing loads. As such, they discriminate the relative stability and the frictional behavior of bilayers where the two monolayers defining the interface have a different relative orientation. In particular, for antiparallel sliding layers we observed a load-induced increase of both the depth of the minima and the height of the energy barriers compared to parallel ones, which may have important consequences for the fabrication of more efficient ultralow friction devices at the nanoscale

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

“…In Table 3 we also report calculated nanomechanical properties (E bind , γ and τ y ) that underpin bilayer shear behaviour. Values for MoS 2 are in good agreement with previous DFT results reported by Levita et al 32,33 Focusing on MoX 2 stoichiometries, the electronic contributions to the interlayer attraction (repulsion) at the global energy minimum (maximum) are evident in the charge density difference plots: § presented in Fig 5 are the charge density difference isosurfaces for the global energy minimum and maximum of MoS 2 , and the corresponding plane-averaged charge density profiles for all MoX 2 stoichiometries are shown in Fig 6. For extra clarity, individual charge density profiles for each TMD, showing atomic positions within the cell, can be found in the ESI. Further detailed analysis of the potential energy vs. charge density relationship is also provided in the ESI, in which we detail the changes in potential energy vs. the integral of charge density accumulation at the interface for each unique geometry.…”

“…In concurrence with previous density functional calculations, 32,33 generating the sliding potential energy profile entails calculating the potential energy as the origin of the upper MX 2 layer is shifted along the long-diagonal of the unit cell, corresponding to the [1100] direction, represented by distance 'y' in Fig 2. Incommensurate bilayers were attained using accidental angular commensurations.…”

“…32 y) and the global potential energy maximum (dy = 3.68 † A is unit cell cross-sectional area; γ is defined as the energy per unit area required to separate the layers from equilibrium to infinity ‡ f max is the maximum lateral force experienced during sliding along 'y', i.e., steepest PES gradient: f(y) = dE/dy; τ y is defined as the maximum load applied parallel to the face of the material that the material can resist prior to sliding Å; 2 3 y). Although rendered structures are only shown for MoS 2 , the same general structure vs. energetics relationship is consistent for all bilayer stoichiometries considered herein.…”

“…All potential energy profiles also exhibit two energy maxima; these are attributable to stacking arrangements in which the X atoms (i) reside at an intermediate bridging position (dy = 0.92 Å or 1 6 y for MoS 2 ) or (ii) directly atop one another (dy = 3.68 Å or 2 3 y for MoS 2 ), where repulsion between chalcogen atom lone pairs is maximised. 32,33 Interlayer distances for the energetically important points along each profile are reported in Table 2. For all MX 2 systems, it can be seen that energy minima (maxima) correlate with reduced (increased) interlayer distances, as expected.…”

“…[42][43][44][45] The aim of this paper is twofold. Firstly, we have performed a study similar to that reported by Levita et al, 32 but enlarging it to include the whole TMD family (i.e., compounds with general formula MX 2 , with M = Mo, W and X = S, Se, Te). For each compound we calculated the potential energy surface (PES), enabling the estimation of the maximum static force that is necessary to be applied to a layer in order to observe sliding.…”