Achieving a superlubricating ohmic sliding electrical contact via a 2D heterointerface: a computational investigation.

Simultaneously achieving low friction and fine electrical conductance of sliding electrical contacts is a crucial factor but a great challenge for designing high-performance microscale and nanoscale functional devices. Through atomistic simulations, we propose an effective design strategy to obtain both low friction and high conductivity in sliding electrical contacts. By constructing graphene(Gr)/MoS2 two-dimensional (2D) heterojunctions between sliding Cu surfaces, superlubricity can be achieved with a remarkably lowered sliding energy barrier as compared to that of the homogeneous MoS2 lubricated Cu contact. Moreover, by introducing vacancy defects into MoS2 and substituting Cu with active metal Ti, the Schottky and tunneling barriers can be substantially suppressed without losing the superlubricious properties of the tribointerface. Consequently, a high conductivity ohmic contact with low sliding friction could be realized in our proposed Ti-MoS1.5-Gr-Ti system, which provides a potential strategy for tackling the well-known dilemma for high performance sliding electrical contacts.

Modeling Atomic-Scale Electrical Contact Quality Across Two-Dimensional Interfaces.

Contacting interfaces with physical isolation and weak interactions usually act as barriers for electrical conduction. The electrical contact conductance across interfaces has long been correlated with the true contact area or the "contact quantity". Much of the physical understanding of the interfacial electrical contact quality was primarily based on Landauer's theory or Richardson formulation. However, a quantitative model directly connecting contact conductance to interfacial atomistic structures still remains absent. Here, we measure the atomic-scale local electrical contact conductance instead of local electronic surface states in graphene/Ru(0001) superstructure, via atomically resolved conductive atomic force microscopy. By defining the "quality" of individual atom-atom contact as the carrier tunneling probability along the interatomic electron transport pathways, we establish a relationship between the atomic-scale contact quality and local interfacial atomistic structure. This real-space model unravels the atomic-level spatial modulation of contact conductance, and the twist angle-dependent interlayer conductance between misoriented graphene layers.

Robust microscale superlubricity under high contact pressure enabled by graphene-coated microsphere.

Superlubricity of graphite and graphene has aroused increasing interest in recent years. Yet how to obtain a long-lasting superlubricity between graphene layers, under high applied normal load in ambient atmosphere still remains a challenge but is highly desirable. Here, we report a direct measurement of sliding friction between graphene and graphene, and graphene and hexagonal boron nitride (h-BN) under high contact pressures by employing graphene-coated microsphere (GMS) probe prepared by metal-catalyst-free chemical vapour deposition. The exceptionally low and robust friction coefficient of 0.003 is accomplished under local asperity contact pressure up to 1 GPa, at arbitrary relative surface rotation angles, which is insensitive to relative humidity up to 51% RH. This ultralow friction is attributed to the sustainable overall incommensurability due to the multi-asperity contact covered with randomly oriented graphene nanograins. This realization of microscale superlubricity can be extended to the sliding between a variety of two-dimensional (2D) layers.

Tribochemical mechanism of amorphous silica asperities in aqueous environment: a reactive molecular dynamics study.

Reactive molecular dynamics (ReaxFF) simulations are used to explore the atomic-level tribochemical mechanism of amorphous silica (a-SiO2) in a nanoscale, single-asperity contact in an aqueous environment. These sliding simulations are performed in both a phosphoric acid solution and in pure water under different normal pressures. The results show that tribochemical processes have profound consequences on tribological performance. Water molecules could help avoid direct adhesive interaction between a-SiO2 surfaces in pure water under low normal load. However, formation and rupture of interfacial siloxane bonds are obviously observed under higher normal load. In phosphoric acid solution, polymerization of phosphoric acid molecules occurs, yielding oligomers under lower load, and tribochemical reactions between the molecules and the sliding surfaces could enhance wear under higher load. The bridging oxygen atoms in silica play an important role in the formation of interfacial covalent bonds, and hydrogen is found to have a weakening effect on these bonds, resulting in the rupture during shear-related loading. This work sheds light on tribochemical reactions as a mechanism for lubrication and wear in water-based or other tribological systems.

Superlubricity of two-dimensional fluorographene/MoS2 heterostructure: a first-principles study.

The atomic-scale friction of the fluorographene (FG)/MoS2 heterostructure is investigated using first-principles calculations. Due to the intrinsic lattice mismatch and formation of periodic Moiré patterns, the potential energy surface of the FG/MoS2 heterostructure is ultrasmooth and the interlayer shear strength is reduced by nearly two orders of magnitude, compared with both FG/FG and MoS2/MoS2 bilayers, entering the superlubricity regime. The size dependency of superlubricity is revealed as being based on the relationship between the emergence of Moiré patterns and the lattice mismatch ratio for heterostructures.

A shear localization mechanism for lubricity of amorphous carbon materials.

Amorphous carbon is one of the most lubricious materials known, but the mechanism is not well understood. It is counterintuitive that such a strong covalent solid could exhibit exceptional lubricity. A prevailing view is that lubricity of amorphous carbon results from chemical passivation of dangling bonds on surfaces. Here we show instead that lubricity arises from shear induced strain localization, which, instead of homogeneous deformation, dominates the shearing process. Shear localization is characterized by covalent bond reorientation, phase transformation and structural ordering preferentially in a localized region, namely tribolayer, resulting in shear weakening. We further demonstrate an anomalous pressure induced transition from stick-slip friction to continuous sliding with ultralow friction, due to gradual clustering and layering of graphitic sheets in the tribolayer. The proposed shear localization mechanism sheds light on the mechanism of superlubricity, and would enrich our understanding of lubrication mechanism of a wide variety of amorphous materials.

Tunable giant anisotropic diffusion of water sub-monolayers between graphene layers.

We investigate the in-plane confinement effect of two graphene layers on the diffusion behaviour of water sub-monolayers using molecular dynamics simulations. An unexpected fast diffusion state with giant anisotropy is observed when the two confining graphene walls have certain shifts applied to their relative positions. The phenomenon is mainly attributed to the smooth one-dimensional potential channels produced by the composition effect of the potential energy landscapes of the two graphene walls, and the concerted motion of water molecules due to hydrogen bonding. Unique duality in the diffusion mechanism is observed in the fast diffusion state, as is ballistic motion along the potential channels and Fickian diffusion across such channels. The smooth potential channels can be created in certain directions simply by shifting the confining walls, which provides a novel measure to manipulate the motion of confined molecules in real-time.

Vanishing stick-slip friction in few-layer graphenes: the thickness effect.

We report the thickness dependence of intrinsic friction in few-layer graphenes, adopting molecular dynamics simulations. The friction force drops dramatically with decreasing number of layers and finally approaches zero with two or three layers. The results, which are robust over a wide range of temperature, shear velocity, and pressure are quantitatively explained by a theoretical model with regard to lateral stiffness, slip length, and maximum lateral force, which could provide a new conceptual framework for understanding stick-slip friction. The results reveal the crucial role of the dimensional effect in nanoscale friction, and could be helpful in the design of graphene-based nanodevices.