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.

http://science.sciencemag.org/content/sci/328/5974/76.full.pdf

Frictional Characteristics of Atomically Thin Sheets

Organic (opto)electronic materials have received considerable attention due to their applications in thin-film-transistors, light-emitting diodes, solar cells, sensors, photorefractive devices, and many others. The technological promises include low cost of these materials and the possibility of their room-temperature deposition from solution on large-area and/or flexible substrates. The article reviews the current understanding of the physical mechanisms that determine the (opto)electronic properties of high-performance organic materials. The focus of the review is on photoinduced processes and on electronic properties important for optoelectronic applications relying on charge carrier photogeneration. Additionally, it highlights the capabilities of various experimental techniques for characterization of these materials, summarizes top-of-the-line device performance, and outlines recent trends in the further development of the field. The properties of materials based both on small molecules and on conjug...

Organic Optoelectronic Materials: Mechanisms and Applications

Asymmetric catalysis at chiral metal surfaces.

The special mechanical properties of nanoparticles allow for novel applications in many fields, e.g., surface engineering, tribology and nanomanufacturing/nanofabrication. In this review, the basic physics of the relevant interfacial forces to nanoparticles and the main measuring techniques are briefly introduced first. Then, the theories and important results of the mechanical properties between nanoparticles or the nanoparticles acting on a surface, e.g., hardness, elastic modulus, adhesion and friction, as well as movement laws are surveyed. Afterwards, several of the main applications of nanoparticles as a result of their special mechanical properties, including lubricant additives, nanoparticles in nanomanufacturing and nanoparticle reinforced composite coating, are introduced. A brief summary and the future outlook are also given in the final part. (Some figures may appear in colour only in the online journal)

https://iopscience.iop.org/article/10.1088/0022-3727/47/1/013001/pdf

Mechanical properties of nanoparticles: basics and applications

The physics of sliding friction is gaining impulse from nanoscale and mesoscale experiments, simulations, and theoretical modeling This Colloquium reviews some recent developments in modeling and in atomistic simulation of friction, covering open-ended directions, unconventional nanofrictional systems, and unsolved problems

/pdf/colloquium-modeling-friction-from-nanoscale-to-mesoscale-2gi3akfv3u.pdf

Colloquium : Modeling friction: from nanoscale to mesoscale

The temperature dependence of the kinetic friction between a highly oriented pyrolytic graphite surface and a silicon nitride probe tip has been evaluated through atomic force microscopy measurements performed under an ultra-high vacuum environment over the temperature range 140–750 K. As temperature increases from 140 to 400 K a sharp decrease in friction is observed. A relatively weaker dependence on temperature is observed in the friction measured between 400 and 750 K. Collectively, these results obtained from a fundamental interface are consistent with an activated mechanism of energy dissipation during sliding.

Thermally Activated Friction

Atomic scale frictional forces encountered as a function of temperature for the contact of a Si3N4 probe tip and the basal plane of MoS2 have been measured with atomic force microcopy over the temperature range 100-500 K. Friction is observed to increase exponentially with decreasing temperature from 500 to 220 K. An Arrhenius analysis of the temperature dependent friction over this range yields an effective activation energy of approximately 0.3 eV for the thermally activated stick-slip motion of the probe tip on this surface. As temperature is reduced further below 220 K, a distinct transition to a largely athermal behavior is detected and is shown to result from the onset of interfacial wear, entailing an alternative energy dissipation pathway.

Transition from thermal to athermal friction under cryogenic conditions.

To explore the environmental dependence of friction for solid lubricants containing molybdenum disulfide (MoS2), we have investigated friction on the basal plane of single-crystal MoS2 with atomic force microscopy (AFM) as a function of relative humidity (RH) and tip composition. For both a bare Si3N4 tip and a MoS2-coated tip, changes in interfacial friction are observed with increasing relative humidity, however, with markedly different behaviors. For sliding contacts involving bare Si3N4 tips, the friction coefficient is observed to increase with increasing RH, from 0% to the point of water saturation. For Si3N4 tips precoated with MoS2 particles, friction appears to be relatively insensitive to increasing RH in the range of 0−40%. However, above 40% RH, a drastic increase in friction is observed and is accompanied by evidence for interfacial wear provided in images of the basal plane following the friction measurements. A comparison to the tribological properties of the basal plane of highly oriented ...

The role of water in modifying friction within MoS2 sliding interfaces.

The tribology of molybdenum disulfide (MoS2)–Sb2O3–C films was tested under a variety of environmental conditions (ambient 50% RH, 10−7 Torr vacuum, 150 Torr oxygen, and 8 Torr water) and correlated with the composition of the surface composition expressed while sliding. High friction and low friction modes of behavior were detected. The lowest coefficient of friction, 0.06, was achieved under vacuum, while sliding in 8 Torr water and ambient conditions both yielded the highest value of 0.15. Water vapor was determined to be the environmental species responsible for high friction performance. XPS evaluations revealed a preferential expression of MoS2 at the surface of wear tracks produced under vacuum and an increase in Sb2O3 concentration in wear tracks produced in ambient air (50% RH). In addition, wear tracks produced by sliding in vacuum exhibited the lowest surface roughness as compared to those produced in other environments, consistent with the picture of low friction originating from well-ordered MoS2 layers produced through sliding in vacuum.

Environmental Effects on the Tribology and Microstructure of MoS2–Sb2O3–C Films

Adsorption of three ketones, R-3-methyl-cyclohexanone, cyclohexanone, and 2-hexanone, on the chiral Cu(6 4 3) surface has been investigated with scanning tunneling microscopy (STM) under ultrahigh vacuum conditions. The measurements reveal that the clean Cu(6 4 3) surface is composed of a well-ordered and evenly spaced step and terrace structure. In addition, the measurements indicate a high mobility of Cu atoms along the steps of the surface held at room temperature. Upon adsorption at room temperature, all the three ketones form ordered structures on Cu(6 4 3) consisting of one molecule per terrace width with a spacing along the step direction commensurate with the ideal kink spacing. The ordered structures of cyclohexanone are consistent with the reconstruction of the Cu(6 4 3) to a surface composed of (11 7 5) (major) and (5 3 2) (minor) facets. This reconstruction comprises an effective reduction in terrace width while preserving the kinked step structure.

Xueying Zhao

Papers

Thermally Activated Friction

Transition from thermal to athermal friction under cryogenic conditions.

The role of water in modifying friction within MoS2 sliding interfaces.

Environmental Effects on the Tribology and Microstructure of MoS2–Sb2O3–C Films

Ordered adsorption of ketones on Cu(6 4 3) revealed by scanning tunneling microscopy