Control of energy dissipation in sliding low-dimensional materials

Abstract

Frictional forces acting during the relative motion of nanosurfaces are the cause of energy loss and wear which limit an efficient assembly and yield of atomic-scale devices. In this research, we investigate the microscopic origin of the dissipative processes as a result of the frictional response, with the aim to control them in a subtle way. We recast the study of friction in terms of phonon modes of the system at the equilibrium, with no need to resort to dynamics simulations. As a case study, we here consider layer sliding in transition metal dichalcogenides thin films. We find that the population of specific atomic orbitals and the relative contribution of the atomic type to selected system vibrations are the crucial quantities which determine the frictional response in tribological conditions. A reduced amount of energy dissipation is found when the bond character is more ionic and the layer sliding is realized by a faster motion of the chalcogen atoms. The individuated relevant parameters governing the energy dissipation can be used as descriptors in high-throughput calculations or machine learning engines to screen databases of frictional materials. The presented framework is general and can be promptly extended to the design of tribological materials with targeted frictional response, irrespective of the chemistry and atomic topology.

Effect of electric fields in low-dimensional materials: Nanofrictional response as a case study

“…( 2) we can infer that, if the effect of the electric field is to halve the frequency, the resulting frictional force is four times smaller than the initial one. We are aware that this is an approximation because phonon coupling should be taken into account explicitly [52][53][54]64]. A rigorous derivation of the force in terms of phonon contributions requires the explicit expression of the energy in terms of single phonons and the relative coupling, also including the effect of the temperature; however, this falls out of the scope of the present discussion and we plan to investigate it in a future work.…”

“…For a convenient visualization of this effect, we report the projections of selected charge density differences in the Supplemental Material [39]. The charge redistribution may affect the M-X bond covalency, as it has already been observed in TMD-based systems [54]. In order to quantify this effect, we calculate the bond covalency C M,X in terms of atomic participation to the electronic density of states [66]; we then consider the relation among C M,X and the polarizations involving the d orbitals (Fig.…”

“…In the present work, we focus on the latter by considering pristine compounds lacking in irregularities or defects. We already observed that relative atomic motions can be geometrically described by linear combination of phonon eigendisplacements [51,52]; those with the largest coefficients in the combination are named sliding modes and keep active the layer sliding as long as they own enough energy [53,54]. The sliding modes can be classically seen as restoring forces which prevent the layer to displace from the equilibrium position, such forces being proportional to the square of the mode frequency.…”

“…The breathing modes are associated to a restoring force which 155433-2 regulates the interlayer distance: A small force corresponds to a large allowed variation (compression/dilation) of the layer separation and facilitates the lateral shift. Both sliding and breathing motions then occur at the same time whenever two layers are displaced, no matter if the displacement is done slowly and in a reversible way (case of static friction) or rapidly (case of dynamic friction); in terms of the phonon description, this corresponds to the phonon coupling between both kind of modes, leading to dissipative processes which are active until the relative layer displacement occurs [53,54]. By tuning the phonon frequency we can then control the intrinsic frictional response; to this aim, in what follows, we will focus on both the sliding and the breathing modes.…”

Friction under active control in systems with tailored degrees of freedom

“…They studied layer sliding in transition metal dichalcogenide thin films. It was observed that it is possible to tune energy dissipation due to friction by fine-tuning the phonon coupling [199]. They found that when the bond character was more ionic, the energy dissipation reduced.…”