The evolving quality of frictional contact with graphene.

Graphite and other lamellar materials are used as dry lubricants for macroscale metallic sliding components and high-pressure contacts. It has been shown experimentally that monolayer graphene exhibits higher friction than multilayer graphene and graphite, and that this friction increases with continued sliding, but the mechanism behind this remains subject to debate. It has long been conjectured that the true contact area between two rough bodies controls interfacial friction. The true contact area, defined for example by the number of atoms within the range of interatomic forces, is difficult to visualize directly but characterizes the quantity of contact. However, there is emerging evidence that, for a given pair of materials, the quality of the contact can change, and that this can also strongly affect interfacial friction. Recently, it has been found that the frictional behaviour of two-dimensional materials exhibits traits unlike those of conventional bulk materials. This includes the abovementioned finding that for few-layer two-dimensional materials the static friction force gradually strengthens for a few initial atomic periods before reaching a constant value. Such transient behaviour, and the associated enhancement of steady-state friction, diminishes as the number of two-dimensional layers increases, and was observed only when the two-dimensional material was loosely adhering to a substrate. This layer-dependent transient phenomenon has not been captured by any simulations. Here, using atomistic simulations, we reproduce the experimental observations of layer-dependent friction and transient frictional strengthening on graphene. Atomic force analysis reveals that the evolution of static friction is a manifestation of the natural tendency for thinner and less-constrained graphene to re-adjust its configuration as a direct consequence of its greater flexibility. That is, the tip atoms become more strongly pinned, and show greater synchrony in their stick-slip behaviour. While the quantity of atomic-scale contacts (true contact area) evolves, the quality (in this case, the local pinning state of individual atoms and the overall commensurability) also evolves in frictional sliding on graphene. Moreover, the effects can be tuned by pre-wrinkling. The evolving contact quality is critical for explaining the time-dependent friction of configurationally flexible interfaces.

Dynamics of atomic stick-slip friction examined with atomic force microscopy and atomistic simulations at overlapping speeds.

Atomic force microscopy (AFM) and atomistic simulations of atomic friction with silicon oxide tips sliding on Au(111) are conducted at overlapping speeds. Experimental data unambiguously reveal a stick-slip friction plateau above a critical scanning speed, in agreement with the thermally activated Prandtl-Tomlinson (PTT) model. However, friction in experiments is larger than in simulations. PTT energetic parameters for the two are comparable, with minor differences attributable to the contact area's influence on the barrier to slip. Recognizing that the attempt frequency may be determined by thermal vibrations of the larger AFM tip mass or instrument noise fully resolves the discrepancy. Thus, atomic stick-slip is well described by the PTT model if sources of slip-assisting energy are accounted for.

Fluorination of graphene enhances friction due to increased corrugation.

The addition of a single sheet of carbon atoms in the form of graphene can drastically alter friction between a nanoscale probe tip and a surface. Here, for the first time we show that friction can be altered over a wide range by fluorination. Specifically, the friction force between silicon atomic force microscopy tips and monolayer fluorinated graphene can range from 5-9 times higher than for graphene. While consistent with previous reports, the combined interpretation from our experiments and molecular dynamics simulations allows us to propose a novel mechanism: that the dramatic friction enhancement results from increased corrugation of the interfacial potential due to the strong local charge concentrated at fluorine sites, consistent with the Prandtl-Tomlinson model. The monotonic increase of friction with fluorination in experiments also demonstrates that friction force measurements provide a sensitive local probe of the degree of fluorination. Additionally, we found a transition from ordered to disordered atomic stick-slip upon fluorination, suggesting that fluorination proceeds in a spatially random manner.

Frictional behavior of atomically thin sheets: hexagonal-shaped graphene islands grown on copper by chemical vapor deposition.

Single asperity friction experiments using atomic force microscopy (AFM) have been conducted on chemical vapor deposited (CVD) graphene grown on polycrystalline copper foils. Graphene substantially lowers the friction force experienced by the sliding asperity of a silicon AFM tip compared to the surrounding oxidized copper surface by a factor ranging from 1.5 to 7 over loads from the adhesive minimum up to 80 nN. No damage to the graphene was observed over this range, showing that friction force microscopy serves as a facile, high contrast probe for identifying the presence of graphene on Cu. Consistent with studies of epitaxially grown, thermally grown, and mechanically exfoliated graphene films, the friction force measured between the tip and these CVD-prepared films depends on the number of layers of graphene present on the surface and reduces friction in comparison to the substrate. Friction results on graphene indicate that the layer-dependent friction properties result from puckering of the graphene sheet around the sliding tip. Substantial hysteresis in the normal force dependence of friction is observed with repeated scanning without breaking contact with a graphene-covered region. Because of the hysteresis, friction measured on graphene changes with time and maximum applied force, unless the tip slides over the edge of the graphene island or contact with the surface is broken. These results also indicate that relatively weak binding forces exist between the copper foil and these CVD-grown graphene sheets.

Optical methods for determining thicknesses of few-layer graphene flakes.

Optical microscopy (OM) methods have been commonly used as a convenient means for locating and identifying few-layer graphene (FLG) on SiO2/Si substrates. However, it is less clear how reliably optical images of FLG could be used to determine the sample thickness. In this work, various OM methods based on color differences and color contrasts are presented and their reliabilities are evaluated. Our analysis shows that these color-based OM methods depend sensitively on certain parameters of the measuring system, particularly the light source and the reference substrate. These parameters have usually been overlooked and less controlled in routine experiments. From evaluating the performance of these OM methods with both virtual and real FLG samples, we propose some practical guidelines for minimizing the impact of these less-controlled experimental parameters and provide a user-friendly MATLAB script for facilitating the implementation.