Friction between van der Waals Solids during Lattice Directed Sliding

Macroscale Superlubricity on Nanoscale Graphene Moiré Structure-Assembled Surface via Counterface Hydrogen Modulation

Interlayer incommensurateness slippage is an excellent pathway to realize superlubricity of van der Waals materials; however, it is instable and heavily depends on twisted angle and super-smooth substrate which pose great challenges for the practical application of superlubricity. Here, macroscale superlubricity (0.001) is reported on countless nanoscale graphene moiré structure (GMS)-assembled surface via counterface hydrogen (H) modulation. The GMS-assembled surface is formed on grinding balls via sphere-triggered strain engineering. By the H modulation of counterface diamond-like carbon (25 at.% H), the wear of GMS-assembled surface is significantly reduced and a steadily superlubric sliding interface between them is achieved, based on assembly face charge depletion and H-induced assembly edge weakening. Furthermore, the superlubricity between GMS-assembled and DLC25 surfaces holds true in wide ranges of normal load (7-11 N), sliding velocity (0.5-27 cm -1s), contact area (0.4×104-3.7×104 µm2), and contact pressure (0.19-1.82 GPa). Atomistic simulations confirm the preferential formation of GMS on a sphere, and demonstrate the superlubricity on GMS-assembled surface via counterface H modulation. The results provide an efficient tribo-pairing strategy to achieve robust superlubricity, which is of significance for the engineering application of superlubricity.

van der Waals Epitaxy, Superlubricity, and Polarization of the 2D Ferroelectric SnS

Tin monosulfide (SnS) is a two-dimensional layered semiconductor that exhibits in-plane ferroelectric order at very small thicknesses and is of interest in highly scaled devices. Here we report the epitaxial growth of SnS on hexagonal boron nitride (hBN) using a pulsed metal-organic chemical vapor deposition process. Lattice matching is observed between the SnS(100) and hBN{11̅0} planes, with no evidence of strain. Atomic force microscopy reveals superlubricity along the commensurate direction of the SnS/hBN interface, and first-principles calculations suggest that friction is controlled by the edges of the SnS islands, rather than interface interactions. Differential phase contrast imaging detects remnant polarization in SnS islands with domains that are not dictated by step-edges in the SnS. The growth of ferroelectric SnS on high quality hBN substrates is a promising step toward electrically switchable ferroelectric semiconducting devices.

Tunable, Wide-Temperature, and Macroscale Superlubricity Enabled by Nanoscale Van Der Waals Heterojunction-to-Homojunction Transformation

Achieving macroscale superlubricity of van der Waals (vdW) nanopowders is particularly challenging, due to the difficulty in forming ordered junctions before friction and the friction-induced complex contact restructuration among multiple nanometer-sized junctions. Here, a facile way is reported to achieve vdW nanopowder-to-heterojunction conversion by graphene edge-oxygen (GEO) incorporation. The GEO effectively weakens the out-of-plane edge-edge and in-plane plane-edge states of the vdW nanopowder, leading to a coexistent structure of nanoscale homojunctions and heterojunctions on the grinding balls. When sliding on diamond-like carbon surfaces, the ball-supported structure governs macroscale superlubricity by heterojunction-to-homojunction transformation among the countless nanoscale junctions. Furthermore, the transformation guides the tunable design of superlubricity, achieving superlubricity (µ ≈ 0.005) at wide ranges of load, velocity, and temperature (-200 to 300 °C). Atomistic simulations reveal the GEO-enhanced conversion of vdW nanopowder to heterojunctions and demonstrate the heterojunction-to-homojunction transformation superlubricity mechanism. The findings are of significance for the macroscopic scale-up and engineering application of structural superlubricity.

Wrapping-Trapping versus Extraction Mechanism of Bactericidal Activity of MoS2 Nanosheets against Staphylococcus aureus Bacterial Membrane

The promising broad-spectrum antibacterial activity of two-dimensional molybdenum disulfide (2D MoS₂) has been widely recognized in the past decade. However, a comprehensive understanding of how the antibacterial pathways opted by the MoS₂ nanosheets varies with change in lipid compositions of different bacterial strains is imperative to harness their full antibacterial potential and remains unexplored thus far. Herein, we present an atomistic molecular dynamics (MD) study to investigate the distinct modes of antibacterial action of MoS₂ nanosheets against Staphylococcus aureus (S. aureus) under varying conditions. We observed that the freely dispersed nanosheets readily adhered to the bacterial membrane outer surface and opted for an unconventional surface directed "wrapping-trapping" mechanism at physiological temperature (i.e., 310 K). The adsorbed nanosheets mildly influenced the membrane structure by originating a compact packing of the lipid molecules present in its direct contact. Interestingly, these surface adsorbed nanosheets exhibited extensive phospholipid extraction to their surface, thereby inducing transmembrane water passage analogous to the cellular leakage, even at a slight increment of 20 K in the temperature. The strong van der Waals interactions between lipid fatty acyl tails and MoS₂ basal planes were primarily responsible for this destructive phospholipid extraction. In addition, the MoS₂ nanosheets bound to an imaginary substrate, controlling their vertical alignment, demonstrated a "nano-knives" action by spontaneously piercing inside the membrane core through their sharp corner, subsequently causing localized lipid ordering in their vicinity. The larger nanosheet produced a more profound deteriorating impact in all of the observed mechanisms. Keeping the existing knowledge about the bactericidal activity of 2D MoS₂ in view, our study concludes that their antibacterial activity is strongly governed by the lipid composition of the bacterial membrane and can be intensified either by controlling the nanosheet vertical alignment or by moderately warming up the systems.

Sliding Friction and Superlubricity of Colloidal AFM Probes Coated by Tribo-Induced Graphitic Transfer Layers

Colloidal probe atomic force microscopy (AFM) allows us to explore sliding friction phenomena in graphite contacts of nominal lateral size up to hundreds of nanometers. It is known that contact formation involves tribo-induced material transfer of graphite flakes from the graphitic substrate to the colloidal probe. In this context, sliding states with nearly vanishing friction, i.e., superlubricity, may set in. A comprehensive investigation of the transfer layer properties is mandatory to ascertain the origin of superlubricity. Here we explore the friction response of micrometric beads, of different size and pristine surface roughness, sliding on graphite under ambient conditions. We show that such tribosystems undergo a robust transition toward a low-adhesion, low-friction state dominated by mechanical interactions at one dominant tribo-induced nanocontact. Friction force spectroscopy reveals that the nanocontact can be superlubric or dissipative, in fact undergoing a load-driven transition from dissipative stick-slip to continuous superlubric sliding. This behavior is excellently described by the thermally activated, single-asperity Prandtl-Tomlinson model. Our results indicate that upon formation of the transfer layer, friction depends on the energy landscape experienced by the topographically highest tribo-induced nanoasperity. We consistently find larger dissipation when the tribo-induced nanoasperity is slid against surfaces with higher atomic corrugation than graphite, like MoS₂ and WS₂, in prototypical van der Waals layered heterojunctions.

MoS2 nanosheet induced destructive alterations in the Escherichia coli bacterial membrane

Two dimensional molybdenum disulfide (MoS₂) nanosheets have recently gained wide recognition for their efficient broad-spectrum antibacterial activity complemented with great biocompatibility and minimal bacterial resistance inducing capabilities. However, despite the numerous investigations, the molecular level interactions at the nano-bio interface responsible for their bactericidal activity remain obscure. Herein, through an atomistic molecular dynamics study, we attempt to seek an in-depth understanding of the atomic level details of the underlying mechanism of their antibacterial action against the Escherichia coli (E. coli) bacterial membrane. Our study reveals a two-step MoS₂ nanosheet interaction pathway with the bacterial membrane. The nanosheets spontaneously adhere to the membrane surface and prompt vigorous phospholipid extraction majorly via strong van der Waals interactions with lipid hydrophobic tails. The lipid extraction process originates a significant water intrusion in the bilayer hydrophobic region, signifying the onset of cytoplasmic leakage under realistic conditions. Further, a synergistic effect of lipid-lipid self-interactions and lipid-MoS₂ dispersion interactions drags the nanosheet to completely immerse in the bilayer hydrophobic core. The embedded nanosheets induce a layerwise structural rearrangement of the membrane lipids in their vicinity, thus altering the structural and dynamic features of the membrane in a localized manner by (i) increasing the lipid fatty acyl tail ordering and (ii) alleviating the lipid lateral dynamics. The detrimental efficacy of the nanosheets can be magnified by enlarging the nanosheet size or by increasing the nanosheet concentration. Our study concludes that the MoS₂ nanosheets can exhibit their antibacterial action through destructive phospholipid extraction as well as by altering the morphology of the membrane by embedding in the membrane core.

Tribo-Piezoelectricity in Group III Nitride Bilayers: A Density Functional Theory Investigation

The notable out-of-plane piezoelectric effect caused by the large electronegativity of the constituent elements makes two-dimensional (2D) group III nitrides appealing for nanoscale energy-harvesting applications. Here, we demonstrate by extensive density functional theory investigations that the vertical piezoelectricity is enhanced significantly in 2D XN (X = B, Al, Ga) bilayers due to in-plane interlayer sliding. The sliding operation generates tribological energy from the vertical resistance force between the monolayers. A maximum shear strength between the monolayers of 1-25 GPa is recorded during vertical sliding. We elucidate the tribo-piezoelectricity generation mechanism of XN bilayers using the tribological energy conversion to overcome the interfacial sliding barrier. The strongest out-of-plane piezoelectricity is found when the bilayers are in the A-A stacking arrangement. Any reduction in the interlayer distance between group III nitride bilayers enhances out-of-plane polarization due to the increase in sliding energy resistances, leading to an increased inductive voltage output. An induced voltage of ∼3.5 V is achieved during vertical compressive sliding of the upper layer. Using these phenomena, we present a compression-slide XN bilayer nanogenerator strategy capable of tuning the produced tribo-piezoelectric energy through sliding and compression.

100 km wear-free sliding achieved by microscale superlubric graphite/DLC heterojunctions under ambient conditions

Wear-free sliding between two contacted solid surfaces is the ultimate goal in the effort to extend the lifetime of mechanical devices, especially when it comes to inventing new types of micro-electromechanical systems where wear is often a major obstacle. Here we report experimental observations of wear-free sliding for a micrometer-sized graphite flake on a diamond-like-carbon (DLC) surface under ambient conditions with speeds up to 2.5 m/s, and over a distance of 100 km. The coefficient of friction (COF) between the microscale graphite flake, a van der Waals (vdW) layered material and DLC, a non-vdW-layered material, is measured to be of the order of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} }{}${10^{ - 3}}$\end{document}, which belongs to the superlubric regime. Such ultra-low COFs are also demonstrated for a microscale graphite flake sliding on six other kinds of non-vdW-layered materials with sub-nanometer roughness. With a synergistic analysis approach, we reveal the underlying mechanism to be the combination of interfacial vdW interaction, atomic-smooth interfaces and the low normal stiffness of the graphite flake. These features guarantee a persistent full contact of the interface with weak interaction, which contributes to the ultra-low COFs. Together with the extremely high in-plane strength of graphene, wear-free sliding is achieved. Our results broaden the scope of superlubricity and promote its wider application in the future.

Correlation between morphology and local mechanical and electrical properties of van der Waals heterostructures

Properties of van der Waals (vdW) heterostructures strongly depend on the quality of the interface between two dimensional (2D) layers. Instead of having atomically flat, clean, and chemically inert interfaces without dangling bonds, top-down vdW heterostructures are associated with bubbles and intercalated layers (ILs) which trap contaminations appeared during fabrication process. We investigate their influence on local electrical and mechanical properties of MoS₂/WS₂heterostructures using atomic force microscopy (AFM) based methods. It is demonstrated that domains containing bubbles and ILs are locally softer, with increased friction and energy dissipation. Since they prevent sharp interfaces and efficient charge transfer between 2D layers, electrical current and contact potential difference are strongly decreased. In order to reestablish a close contact between MoS₂and WS₂layers, vdW heterostructures were locally flattened by scanning with AFM tip in contact mode or just locally pressed with an increased normal load. Subsequent electrical measurements reveal that the contact potential difference between two layers strongly increases due to enabled charge transfer, while localI/Vcurves exhibit increased conductivity without undesired potential barriers.

Strong tribo-piezoelectric effect in bilayer indium nitride (InN)

The high electronegativity between the atoms of two-dimensional (2D) group-III nitrides makes them attractive to demonstrating a strong out-of-plane piezo-electricity effect. Energy harvesting devices can be predicted by cultivating such salient piezoelectric features. This work explores the tribo-piezoelectric properties of 2D-indium nitride (InN) as a promising candidate in nanogenerator applications by means of first-principles calculations. In-plane interlayer sliding between two InN monolayers leads to a noticeable rise of vertical piezoelectricity. The vertical resistance between the InN bilayer renders tribological energy by the sliding effect. During the vertical sliding, a shear strength of 6.6-9.7 GPa is observed between the monolayers. The structure can be used as a tribo-piezoelectric transducer to extract force and stress from the generated out-of-plane tribo-piezoelectric energy. The A-A stacking of the bilayer InN elucidates the highest out-of-plane piezoelectricity. Any decrease in the interlayer distance between the monolayers improves the out-of-plane polarization and thus, increases the inductive voltage generation. Vertical compression of bilayer InN produces an inductive voltage in the range of 0.146-0.196 V. Utilizing such a phenomenon, an InN-based bilayer compression-sliding nanogenerator is proposed, which can tune the generated tribo-piezoelectric energy by compressing the interlayer distance between the InN monolayers. The considered model can render a maximum output power density of ~ 73 mWcm-2 upon vertical sliding.

Robust Superlubricity and Moiré Lattice's Size Dependence on Friction between Graphdiyne Layers

Structural superlubricity is a fascinating physical phenomenon that plays a significant role in many scientific and technological fields. Here, we report the robust superlubricating state achieved on the interface of relatively rotated graphdiyne (GDY) bilayers; such an interface with ultralow friction is formed at nearly arbitrary rotation angles and sustained at temperatures up to 300 K. We also identified the reverse correlation between the friction coefficient and size of the Moiré lattice formed on the surface of the incommensurate stacked GDY bilayers, particularly in a small size range. Our investigations show that the ultralow friction and the reduction of the friction coefficient with the increase in size of the Moiré lattice are closely related to the interfacial energetics and charge density as well as the atomic arrangement. Our findings enable the development of a new solid lubricant with novel superlubricating properties, which facilitate precise modulation of the friction at the interface between two incommensurate contacting crystalline surfaces.

One-Dimensional Lateral Force Anisotropy at the Atomic Scale in Sliding Single Molecules on a Surface

Using a q+ atomic force microscopy at low temperature, a sexiphenyl molecule is slid across an atomically flat Ag(111) surface along the direction parallel to its molecular axis and sideways to the axis. Despite identical contact area and underlying surface geometry, the lateral force required to move the molecule in the direction parallel to its molecular axis is found to be about half of that required to move it sideways. The origin of the lateral force anisotropy observed here is traced to the one-dimensional shape of the molecule, which is further confirmed by molecular dynamics simulations. We also demonstrate that scanning tunneling microscopy can be used to determine the comparative lateral force qualitatively. The observed one-dimensional lateral force anisotropy may have important implications in atomic scale frictional phenomena on materials surfaces.

Computational Prediction of Superlubric Layered Heterojunctions

Structural superlubricity has attracted increasing interest in modern tribology. However, experimental identification of superlubric interfaces among the vast number of heterojunctions is a trial-and-error and time-consuming approach. In this work, based on the requirements on the in-plane stiffnesses of layered materials and the interfacial interactions at the sliding incommensurate interfaces of heterojunctions for structural superlubricity, we propose criteria for predicting structural superlubricity between heterojunctions. Based on these criteria, we identify 61 heterojunctions with potential superlubricity features from 208 candidates by screening the data of first-principles calculations. This work provides a universal route for accelerating the discovery of new superlubric heterojunctions.

Two-dimensional talc as a van der Waals material for solid lubrication at the nanoscale

Talc is a van der Waals and naturally abundant mineral with the chemical formula Mg₃Si₄O₁₀(OH)₂. Two-dimensional (2D) talc could be an alternative to hBN as van der Waals dielectric in 2D heterostructures. Furthermore, due to its good mechanical and frictional properties, 2D talc could be integrated into various hybrid microelectromechanical systems, or used as a functional filler in polymers. However, properties of talcas one of the main representatives of the phyllosilicate (sheet silicates) group are almost completely unexplored when ultrathin crystalline films and monolayers are considered. We investigate 2D talc flakes down to single layer thickness and reveal their efficiency for solid lubrication at the nanoscale. We demonstrate by atomic force microscopy based methods and contact angle measurements that several nanometer thick talc flakes have all properties necessary for efficient lubrication: a low adhesion, hydrophobic nature, and a low friction coefficient of 0.10 ± 0.02. Compared to the silicon-dioxide substrate, 2D talc flakes reduce friction by more than a factor of five, adhesion by around 20%, and energy dissipation by around 7%. Considering our findings, together with the natural abundance of talc, we put forward that 2D talc can be a cost-effective solid lubricant in micro- and nano-mechanical devices.

Nanoscale Friction Behavior of Transition-Metal Dichalcogenides: Role of the Chalcogenide

Despite extensive research on the tribological properties of MoS₂, the frictional characteristics of other members of the transition-metal dichalcogenide (TMD) family have remained relatively unexplored. To understand the effect of the chalcogen on the tribological behavior of these materials and gain broader general insights into the factors controlling friction at the nanoscale, we compared the friction force behavior for a nanoscale single asperity sliding on MoS₂, MoSe₂, and MoTe₂ in both bulk and monolayer forms through a combination of atomic force microscopy experiments and molecular dynamics simulations. Experiments and simulations showed that, under otherwise identical conditions, MoS₂ has the highest friction among these materials and MoTe₂ has the lowest. Simulations complemented by theoretical analysis based on the Prandtl-Tomlinson model revealed that the observed friction contrast between the TMDs was attributable to their lattice constants, which differed depending on the chalcogen. While the corrugation amplitudes of the energy landscapes are similar for all three materials, larger lattice constants permit the tip to slide more easily across correspondingly wider saddle points in the potential energy landscape. These results emphasize the critical role of the lattice constant, which can be the determining factor for frictional behavior at the nanoscale.

Structural Superlubricity Based on Crystalline Materials

Herein, structural superlubricity, a fascinating phenomenon where the friction is ultralow due to the lateral interaction cancellation resulted from incommensurate contact crystalline surfaces, is reviewed. Various kinds of nano- and microscale materials such as 2D materials, metals, and compounds are used for the fabrication. For homogeneous frictional pairs, superlow friction forces exist in most relative orientations with incommensurate configuration. Heterojunctions bear no resemblance to homogeneous contact, since the lattice constants are naturally mismatched which leads to a robust structural superlubricity with any orientation of the two different surfaces. A discussion on the perspectives of this field is also provided to meet the existing challenges and chart the future.

Indirect-To-Direct Band Gap Transition of One-Dimensional V2Se9: Theoretical Study with Dispersion Energy Correction

Recently, we synthesized a one-dimensional (1D) structure of V2Se9. The 1D V2Se9 resembles another 1D material, Nb2Se9, which is expected to have a direct band gap. To determine the potential applications of this material, we calculated the band structures of 1D and bulk V2Se9 using density functional theory by varying the number of chains and comparing their band structures and electronic properties with those of Nb2Se9. The results showed that a small number of V2Se9 chains have a direct band gap, whereas bulk V2Se9 possesses an indirect band gap, like Nb2Se9. We expect that V2Se9 nanowires with diameters less than ∼20 Å would have direct band gaps. This indirect-to-direct band gap transition could lead to potential optoelectronic applications for this 1D material because materials with direct band gaps can absorb photons without being disturbed by phonons.

Advances in Atomic Force Microscopy: Weakly Perturbative Imaging of the Interfacial Water

The structure and dynamics of interfacial water, determined by the water-interface interactions, are important for a wide range of applied fields and natural processes, such as water diffusion (Kim et al., 2013), electrochemistry (Markovic, 2013), heterogeneous catalysis (Over et al., 2000), and lubrication (Zilibotti et al., 2013). The precise understanding of water-interface interactions largely relies on the development of atomic-scale experimental techniques (Guo et al., 2014) and computational methods (Hapala et al., 2014b). Scanning probe microscopy has been extensively applied to probe interfacial water in many interdisciplinary fields (Ichii et al., 2012; Shiotari and Sugimoto, 2017; Peng et al., 2018a). In this perspective, we review the recent progress in the noncontact atomic force microscopy (nc-AFM) imaging and AFM simulation techniques and discuss how the newly developed techniques are applied to study the properties of interfacial water. The nc-AFM with the quadrupole-like CO-terminated tip can achieve ultrahigh-resolution imaging of the interfacial water on different surfaces, trace the reconstruction of H-bonding network and determine the intrinsic structures of the weakly bonded water clusters and even their metastable states. In the end, we present an outlook on the directions of future AFM studies of interfacial water as well as the challenges faced by this field.

Origin of Nanoscale Friction Contrast between Supported Graphene, MoS2, and a Graphene/MoS2 Heterostructure

Ultralow friction can be achieved with 2D materials, particularly graphene and MoS₂. The nanotribological properties of these different 2D materials have been measured in previous atomic force microscope (AFM) experiments sequentially, precluding immediate and direct comparison of their frictional behavior. Here, friction is characterized at the nanoscale using AFM experiments with the same tip sliding over graphene, MoS₂, and a graphene/MoS₂ heterostructure in a single measurement, repeated hundreds of times, and also measured with a slowly varying normal force. The same material systems are simulated using molecular dynamics (MD) and analyzed using density functional theory (DFT) calculations. In both experiments and MD simulations, graphene consistently exhibits lower friction than the MoS₂ monolayer and the heterostructure. In some cases, friction on the heterostructure is lower than that on the MoS₂ monolayer. Quasi-static MD simulations and DFT calculations show that the origin of the friction contrast is the difference in energy barriers for a tip sliding across each of the three surfaces.

Sliding Interaction for Coated Asperity with Power-Law Hardening Elastic-Plastic Coatings

Sliding between asperities occurs inevitably in the friction pair, which affects the efficiency and reliability in both lubricated and non-lubricated conditions. In this work, the contact parameters in the coated asperity sliding process are studied, and the universal expressions of the average contact force and the friction coefficient are obtained. The effect of the interference between asperities, the material and geometrical parameters including the Young’s modulus ratio and yield strength ratio of the coating and substrate, and the hardening exponent and thickness of the coating on the average contact forces and friction coefficient is considered. It shows both normal and tangential contact forces increase with the increasing interference, increasing Young’s modulus ratio, decreasing yield strength ratio, and decreasing coating thickness; while the trend is different for the effect of the hardening exponent of the coating. The normal force increases and the tangential force decreases as the hardening exponent increases. Based on this, the influence of these parameters on the effective friction coefficient is obtained further. It reveals that the friction coefficient increases as the interference and Young’s modulus ratio enlarge and decreases as the yield strength ratio, the coating’s hardening exponent, and thickness increase. The universal expressions for the contact force and friction coefficient in the sliding process are obtained. This work might give some useful results to help choose the optimum coatings for specific substrates to reduce friction in cases where the asperity contact exists, especially in the focused field of the journal bearing in the marine engine under poor lubrication conditions.

Molecules on rails: friction anisotropy and preferential sliding directions of organic nanocrystallites on two-dimensional materials

Two-dimensional (2D) materials are envisaged as ultra-thin solid lubricants for nanomechanical systems. So far, their frictional properties at the nanoscale have been studied by standard friction force microscopy. However, lateral manipulation of nanoparticles is a more suitable method to study the dependence of friction on the crystallography of two contacting surfaces. Still, such experiments are lacking. In this study, we combine atomic force microscopy (AFM) based lateral manipulation and molecular dynamics simulations in order to investigate the movements of organic needle-like nanocrystallites grown by van der Waals epitaxy on graphene and hexagonal boron nitride. We observe that nanoneedle fragments - when pushed by an AFM tip - do not move along the original pushing directions. Instead, they slide on the 2D materials preferentially along the needles' growth directions, which act as invisible rails along commensurate directions. Further, when the nanocrystallites were rotated by applying a torque with the AFM tip across the preferential sliding directions, we find an increase of the torsional signal of the AFM cantilever. We demonstrate in conjunction with simulations that both, the significant friction anisotropy and preferential sliding directions are determined by the complex epitaxial relation and arise from the commensurate and incommensurate states between the organic nanocrystallites and the 2D materials.

Robust microscale superlubricity in graphite/hexagonal boron nitride layered heterojunctions

Structural superlubricity is a fascinating tribological phenomenon, in which the lateral interactions between two incommensurate contacting surfaces are effectively cancelled resulting in ultralow sliding friction. Here we report the experimental realization of robust superlubricity in microscale monocrystalline heterojunctions, which constitutes an important step towards the macroscopic scale-up of superlubricity. The results for interfaces between graphite and hexagonal boron nitride clearly demonstrate that structural superlubricity persists even when the aligned contact sustains external loads under ambient conditions. The observed frictional anisotropy in the heterojunctions is found to be orders of magnitude smaller than that measured for their homogeneous counterparts. Atomistic simulations reveal that the underlying frictional mechanisms in the two cases originate from completely different dynamical regimes. Our results are expected to be of a general nature and should be applicable to other van der Waals heterostructures.

Anisotropic Polymer Adsorption on Molybdenite Basal and Edge Surfaces and Interaction Mechanism With Air Bubbles

The anisotropic surface characteristics and interaction mechanisms of molybdenite (MoS₂) basal and edge planes have attracted much research interest in many interfacial processes such as froth flotation. In this work, the adsorption of a polymer depressant [i.e., carboxymethyl cellulose (CMC)] on both MoS₂ basal and edge surfaces as well as their interaction mechanisms with air bubbles have been characterized by atomic force microscope (AFM) imaging and quantitative force measurements. AFM imaging showed that the polymer coverage on the basal plane increased with elevating polymer concentration, with the formation of a compact polymer layer at 100 ppm CMC; however, the polymer adsorption was much weaker on the edge plane. The anisotropy in polymer adsorption on MoS₂ basal and edge surfaces coincided with water contact angle results. Direct force measurements using CMC functionalized AFM tips revealed that the adhesion on the basal plane was about an order of magnitude higher than that on the edge plane, supporting the anisotropic CMC adsorption behaviors. Such adhesion difference could be attributed to their difference in surface hydrophobicity and surface charge, with weakened hydrophobic attraction and strengthened electrostatic repulsion between the polymers and edge plane. Force measurements using a bubble probe AFM showed that air bubble could attach to the basal plane during approach, which could be effectively inhibited after polymer adsorption. The edge surface, due to the negligible polymer adsorption, showed similar interaction behaviors with air bubbles before and after polymer treatment. This work provides useful information on the adsorption of polymers on MoS₂ basal/edge surfaces as well as their interaction mechanism with air bubbles at the nanoscale, with implications for the design and development of effective polymer additives to mediate the bubble attachment on solid particles with anisotropic surface properties in mineral flotation and other engineering processes.

Structural superlubricity in graphite flakes assembled under ambient conditions

To date, structural superlubricity in microscale contacts is mostly observed in intrinsic graphite flakes that are cleaved by shearing from HOPG mesas in situ or friction pairs assembled in vacuum due to the high requirement of ultra clean interface for superlubricity, which severely limits their practical applications. Herein, we report observations on microscale structural superlubricity in graphite flake pairs assembled under ambient conditions, where contaminants are inevitably present at the interfaces. For such friction pairs, we find a novel running-in phenomenon, where the friction decreases with reciprocating motions, but no morphological or chemical changes can be observed. The underlying mechanism for the new running-in process is revealed to be the removal of third bodies confined between the surfaces. Our results improve the understanding of microscale superlubricity and may help extend the practical applications of superlubricity.

Recent highlights in nanoscale and mesoscale friction

Friction is the oldest branch of non-equilibrium condensed matter physics and, at the same time, the least established at the fundamental level. A full understanding and control of friction is increasingly recognized to involve all relevant size and time scales. We review here some recent advances on the research focusing of nano- and mesoscale tribology phenomena. These advances are currently pursued in a multifaceted approach starting from the fundamental atomic-scale friction and mechanical control of specific single-asperity combinations, e.g., nanoclusters on layered materials, then scaling up to the meso/microscale of extended, occasionally lubricated, interfaces and driven trapped optical systems, and eventually up to the macroscale. Currently, this "hot" research field is leading to new technological advances in the area of engineering and materials science.

Friction and work function oscillatory behavior for an even and odd number of layers in polycrystalline MoS2

A large effort is underway to investigate the properties of two-dimensional (2D) materials for their potential to become building blocks in a variety of integrated nanodevices. In particular, the ability to understand the relationship between friction, adhesion, electric charges and defects in 2D materials is of key importance for their assembly and use in nano-electro-mechanical and energy harvesting systems. Here, we report on a new oscillatory behavior of nanoscopic friction in continuous polycrystalline MoS2 films for an odd and even number of atomic layers, where odd layers show higher friction and lower work function. Friction force microscopy combined with Kelvin probe force microscopy and X-ray photoelectron spectroscopy demonstrates that an enhanced adsorption of charges and OH molecules is at the origin of the observed increase in friction for 1 and 3 polycrystalline MoS2 layers. In polycrystalline films with an odd number of layers, each crystalline nano-grain carries a dipole due to the MoS2 piezoelectricity, therefore charged molecules adsorb at the grain boundaries all over the surface of the continuous MoS2 film. Their displacement during the sliding of a nano-size tip gives rise to the observed enhanced dissipation and larger nanoscale friction for odd layer-numbers. Similarly, charged adsorbed molecules are responsible for the work function decrease in odd layer-number.

Enhanced Lubrication and Photocatalytic Degradation of Liquid Paraffin by Hollow MoS2 Microspheres

Nowadays, with the rapid development of environmental protection awareness, the demand for the emergence of a green counterpart of lubricant additive plays a more and more important role in reducing friction and wear as the times require. In this paper, full-hollow and semihollow molybdenum disulfide (MoS₂) microspheres were prepared via a hydrothermal method and were characterized and confirmed by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). According to our results, both fully hollow and semihollow MoS₂ microspheres possessed excellent lubrication-enhancing effects for liquid paraffin (LP), while full-hollow samples after friction provided better photocatalytic degradation properties than semihollow samples after friction. Related analysis indicated that curved layer opened structures with more rim and edge sites, bigger surface area, and narrower band gap made full-hollow MoS₂ samples achieve a better photocatalytic level. Thus, it was a sustainable solution for both lubrication-enhancing and photocatalytic degradation functions during different stages of the usage of lubricating oils, which suggests a potential strategy for achieving environmentally friendly developments.