We review the most recent advances in tribological studies of 2D materials, with particular focus on the unique mechanisms underlying their novel friction and wear behaviors. Based on the fundamental ...understandings, the impacts of atomic structures of shearing interfaces and environmental factors are summarized and various strategies for achieving friction modulation and superlubricity are discussed. Finally, prospects toward practical applications of 2D materials in engineering mechanical systems are presented.
Display omitted
Two-dimensional (2D) materials are crystalline materials made of a single or a few layers of atoms. They have been an active research subject in recent years because of their unique physical and chemical properties. In particular, 2D materials such as graphene, hexagonal BN, and MoS2 exhibit some of the lowest friction coefficients and wear rates, making them attractive for enhancing the efficiency, durability, and environmental compatibility of future mechanical systems. This review will focus on recent advances in the tribology of 2D materials. Starting from general physical characteristics, the essential friction and wear behavior of 2D materials together with the associated mechanisms are reviewed for both interlayer and surface sliding. Influences of the atomic structures of the slip interfaces and environmental factors are discussed, with special attention given to various strategies for achieving friction modulation and superlubricity. Finally, the emerging engineering applications of 2D materials, as well as future prospects, are summarized.
Droplet impacting and bouncing off solid surface plays a vital role in various biological/physiological processes and engineering applications. However, due to a lack of accurate control of force ...transmission, the maneuver of the droplet movement and energy conversion is rather primitive. Here we show that the translational motion of an impacting droplet can be converted to gyration, with a maximum rotational speed exceeding 7300 revolutions per minute, through heterogeneous surface wettability regulation. The gyration behavior is enabled by the synergetic effect of the asymmetric pinning forces originated from surface heterogeneity and the excess surface energy of the spreading droplet after impact. The findings open a promising avenue for delicate control of liquid motion as well as actuating of solids.
Photochemical solution-phase reactions have been widely applied for the syntheses of nanocrystals. In particular, tuning of the nucleation and growth of solids has been a major area of focus. Here we ...demonstrate a facile approach to generate atomically dispersed platinum via photochemical reduction of frozen chloroplatinic acid solution using ultraviolet light. Using this iced-photochemical reduction, the aggregation of atoms is prevented, and single atoms are successfully stabilized. The platinum atoms are deposited on various substrates, including mesoporous carbon, graphene, carbon nanotubes, titanium dioxide nanoparticles, and zinc oxide nanowires. The atomically dispersed platinum on mesoporous carbon exhibits efficient catalytic activity for the electrochemical hydrogen evolution reaction, with an overpotential of only 65 mV at a current density of 100 mA cm
and long-time durability (>10 h), superior to state-of-the-art platinum/carbon. This iced-photochemical reduction may be extended to other single atoms, for example gold and silver, as demonstrated in this study.
Earthquakes have long been recognized as being the result of stick-slip frictional instabilities. Over the past few decades, laboratory studies of rock friction have elucidated many aspects of ...tectonic fault zone processes and earthquake phenomena. Typically, the static friction of rocks grows logarithmically with time when they are held in stationary contact, but the mechanism responsible for this strengthening is not understood. This time-dependent increase of frictional strength, or frictional ageing, is one manifestation of the 'evolution effect' in rate and state friction theory. A prevailing view is that the time dependence of rock friction results from increases in contact area caused by creep of contacting asperities. Here we present the results of atomic force microscopy experiments that instead show that frictional ageing arises from the formation of interfacial chemical bonds, and the large magnitude of ageing at the nanometre scale is quantitatively consistent with what is required to explain observations in macroscopic rock friction experiments. The relative magnitude of the evolution effect compared with that of the 'direct effect'--the dependence of friction on instantaneous changes in slip velocity--determine whether unstable slip, leading to earthquakes, is possible. Understanding the mechanism underlying the evolution effect would enable us to formulate physically based frictional constitutive laws, rather than the current empirically based 'laws', allowing more confident extrapolation to natural faults.
Mechanically driven light generation is an exciting and under‐exploited phenomenon with a variety of possible practical applications. However, the current driving mode of mechanoluminescence (ML) ...devices needs strong stimuli. Here, a flexible sensitive ML device via nanodopant elasticity modulus modification is introduced. Rigid ZnS:M2+(Mn/Cu)@Al2O3 microparticles are dispersed into soft poly(dimethylsiloxane) (PDMS) film and printed out to form flexible devices. For various flexible and sensitive scenes, SiO2 nanoparticles are adopted to adjust the elasticity modulus of the PDMS matrix. The doped nanoparticles can concentrate stress to ZnS:M2+(Mn/Cu)@Al2O3 microparticles and achieve intense ML under weak stimuli of the moving skin. The printed nano‐/microparticle‐doped matrix film can achieve skin‐driven ML, which can be adopted to present fetching augmented animations expressions. The printable ML film, amenable to large areas, low‐cost manufacturing, and mechanical softness will be versatile on stress visualization, luminescent sensors, and open definitely new functional skin with novel augmented animations expressions, the photonic skin.
A flexible, sensitive mechanoluminescence (ML) device is demonstrated via matrix elasticity modulus modification. Rigid ZnS:M2+(Mn/Cu)@Al2O3 microparticles are dispersed into soft poly(dimethylsiloxane) (PDMS) film and printed for preparing flexible devices. Via a SiO2‐nanoparticle dopant, the ML intensity for small strain is significantly increased. The ML devices achieve intense ML under the weak stimuli of moving skin, which will be significant for photonic‐skin devices.
Abstract
Realizing programmable assembly and reconfiguration of small objects holds promise for technologically-significant applications in such fields as micromechanical systems, biomedical devices, ...and metamaterials. Although capillary forces have been successfully explored to assemble objects with specific shapes into ordered structures on the liquid surface, reconfiguring these assembled structures on demand remains a challenge. Here we report a strategy, bioinspired by
Anurida maritima
, to actively reconfigure assembled structures with well-defined selectivity, directionality, robustness, and restorability. This approach, taking advantage of optocapillarity induced by photodeformation of floating liquid crystal polymer actuators, not only achieves programmable and reconfigurable two-dimensional assembly, but also uniquely enables the formation of three-dimensional structures with tunable architectures and topologies across multiple fluid interfaces. This work demonstrates a versatile approach to tailor capillary interaction by optics, as well as a straightforward bottom-up fabrication platform for a wide range of applications.
Frictional Characteristics of Atomically Thin Sheets Lee, Changgu; Li, Qunyang; Kalb, William ...
Science (American Association for the Advancement of Science),
04/2010, Volume:
328, Issue:
5974
Journal Article
Peer reviewed
Using friction force microscopy, we compared the nanoscale frictional characteristics of atomically thin sheets of graphene, molybdenum disulfide (MoS₂), niobium diselenide, and hexagonal boron ...nitride exfoliated onto a weakly adherent substrate (silicon oxide) to those of their bulk counterparts. Measurements down to single atomic sheets revealed that friction monotonically increased as the number of layers decreased for all four materials. Suspended graphene membranes showed the same trend, but binding the graphene strongly to a mica surface suppressed the trend. Tip-sample adhesion forces were indistinguishable for all thicknesses and substrate arrangements. Both graphene and MoS₂ exhibited atomic lattice stick-slip friction, with the thinnest sheets possessing a sliding-length-dependent increase in static friction. These observations, coupled with finite element modeling, suggest that the trend arises from the thinner sheets' increased susceptibility to out-of-plane elastic deformation. The generality of the results indicates that this may be a universal characteristic of nanoscale friction for atomically thin materials weakly bound to substrates.
Abstract
Although layered van der Waals (vdW) materials involve vast interface areas that are often subject to contamination, vdW interactions between layers may squeeze interfacial contaminants into ...nanopockets. More intriguingly, those nanopockets could spontaneously coalesce into larger ones, which are easier to be squeezed out the atomic channels. Such unusual phenomena have been thought of as an Ostwald ripening process that is driven by the capillarity of the confined liquid. The underlying mechanism, however, is unclear as the crucial role played by the sheet’s elasticity has not been previously appreciated. Here, we demonstrate the coalescence of separated nanopockets and propose a cleaning mechanism in which both elastic and capillary forces are at play. We elucidate this mechanism in terms of control of the nanopocket morphology and the coalescence of nanopockets via a mechanical stretch. Besides, we demonstrate that bilayer graphene interfaces excel in self-renewal phenomena.
The growth of wafer-scale single-crystal two-dimensional transition metal dichalcogenides (TMDs) on insulating substrates is critically important for a variety of high-end applications1–4. Although ...the epitaxial growth of wafer-scale graphene and hexagonal boron nitride on metal surfaces has been reported5–8, these techniques are not applicable for growing TMDs on insulating substrates because of substantial differences in growth kinetics. Thus, despite great efforts9–20, the direct growth of wafer-scale single-crystal TMDs on insulating substrates is yet to be realized. Here we report the successful epitaxial growth of two-inch single-crystal WS2 monolayer films on vicinal a-plane sapphire surfaces. In-depth characterizations and theoretical calculations reveal that the epitaxy is driven by a dual-coupling-guided mechanism, where the sapphire plane–WS2 interaction leads to two preferred antiparallel orientations of the WS2 crystal, and sapphire step edge–WS2 interaction breaks the symmetry of the antiparallel orientations. These two interactions result in the unidirectional alignment of nearly all the WS2 islands. The unidirectional alignment and seamless stitching of WS2 islands are illustrated via multiscale characterization techniques; the high quality of WS2 monolayers is further evidenced by a photoluminescent circular helicity of ~55%, comparable to that of exfoliated WS2 flakes. Our findings offer the opportunity to boost the production of wafer-scale single crystals of a broad range of two-dimensional materials on insulators, paving the way to applications in integrated devices.A dual-coupling-guided growth mechanism enables the realization of wafer-scale single-crystal WS2 on vicinal a-plane sapphire.
Using friction force microscopy, we have investigated the frictional behavior of graphene deposited on various substrates as well as over micro‐fabricated wells. Both graphene on SiO2/Si substrates ...and graphene freely suspended over the wells showed a trend of increasing friction with decreasing number of atomic layers of graphene. However, this trend with thickness was absent for graphene deposited on mica, where the graphene is strongly bonded to the substrate. Measurements together with a mechanics model suggest that mechanical confinement to the substrate plays an important role in the frictional behavior of these atomically thin graphite sheets. Loosely bound or suspended graphene sheets can pucker in the out‐of‐plane direction due to tip‐graphene adhesion. This increases contact area, and also allows further deformation of the graphene when sliding, leading to higher friction. Since thinner samples have lower bending stiffness, the puckering effect and frictional resistance are greater. However, if the graphene is strongly bound to the substrate, the puckering effect will be suppressed and no thickness dependence should be observed. The results can provide potentially useful guidelines in the rational design and use of graphene for nano‐mechanical applications, including nano‐lubricants and components in micro‐ and nano‐devices.
PDF
