Atomic Stick-slip Friction Research Articles

Atomic stick-slip friction as a two-dimensional thermally activated process

Widely recognized as a thermally activated process, atomic stick-slip friction has been typically explained by Prandtl-Tomlinson model with thermal activation. Despite the limited success, theoretical predictions from the classic model are primarily based on a one-dimensional (1D) assumption, which is generally not compatible with real experiments that are two-dimensional (2D) in nature. In this letter, a theoretical model based on 2D transition state theory has been derived and confirmed to be able to capture the 2D slip kinetics in atomic-scale friction experiments on crystalline surface with a hexagonal energy landscape. Moreover, we propose a reduced scheme that enables extraction of intrinsic interfacial parameters from 2D experiments approximately using the traditional 1D model. The 2D model provides a theoretical tool for understanding the rich kinetics of atomic-scale friction or other phenomena involving higher dimensional transitions.

Thickness-dependent frictional behavior of topological insulator Bi2Se3 nanoplates

Two-dimensional Bi2Se3 TIs were recently found to be the most promising room-temperature topological insulators for its relatively large bulk gap, but its surface frictional response is little investigated. Here, we prepared single-crystalline Bi2Se3 nanoplates with a lateral dimension up to ~ 1 μm and a thickness of less than 200 nm via a simple polyol method, and the molecular structure and morphology were characterized in detail using different methods. The micro-frictional behavior of Bi2Se3 nanoplates with different thickness is inventively investigated with AFM technique. The atomic stick–slip friction stemming from periodic crystal lattice, and the larger friction force of thinner nanoplates is attributed to the larger adhesion force and enhanced energy dissipation. This work has, for the first time, built the link of the behavior of topological protected surface and mechanical friction behavior of Bi2Se3.

Line Scan Reconstruction: A Viable Approach for Tracking Atomic Stick–Slip Events and True Tip Position in Atomic Force Microscopy

Since its first report on graphite, atomic stick–slip friction has been widely observed on various crystalline surfaces in atomic force microscopy (AFM) experiments. Theoretically, one can extract the key intrinsic interface parameters, e.g., functional shape of the atomic interaction potential energy, thermal hopping frequency and effective damping coefficient, from statistics of the atomic stick–slip data. However, this approach is seriously impeded by the lack of a reliable method to extract the physically meaningful statistical data. In this work, we present an easy and robust approach to reconstruct the atomic stick–slip process such that individual slip events with arbitrary slip distance can be automatically identified, while the slip forces and true tip position can be obtained simultaneously. By implementing the new scheme for both one-dimensional and two-dimensional simulated data and real AFM experiments, we clearly demonstrate that this new method can reliably resist noise interference while accurately preserving the essential thermodynamic information. The new technique enables the physical process of atomic friction to be unveiled with far more details and provides a new paradigm for examining the statistics of atomic processes in sliding interfaces.

Energy corrugation in atomic-scale friction on graphite revisited by molecular dynamics simulations

Although atomic stick–slip friction has been extensively studied since its first demonstration on graphite, the physical understanding of this dissipation-dominated phenomenon is still very limited. In this work, we perform molecular dynamics (MD) simulations to study the frictional behavior of a diamond tip sliding over a graphite surface. In contrast to the common wisdom, our MD results suggest that the energy barrier associated lateral sliding (known as energy corrugation) comes not only from interaction between the tip and the top layer of graphite but also from interactions among the deformed atomic layers of graphite. Due to the competition of these two subentries, friction on graphite can be tuned by controlling the relative adhesion of different interfaces. For relatively low tip-graphite adhesion, friction behaves normally and increases with increasing normal load. However, for relatively high tip-graphite adhesion, friction increases unusually with decreasing normal load leading to an effectively negative coefficient of friction, which is consistent with the recent experimental observations on chemically modified graphite. Our results provide a new insight into the physical origins of energy corrugation in atomic scale friction.

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.

Speed Dependence of Atomic Stick-Slip Friction in Optimally Matched Experiments and Molecular Dynamics Simulations

The atomic stick-slip behavior of a Pt tip sliding on a Au(111) surface is studied with atomic force microscopy (AFM) experiments and accelerated (i.e., reduced sliding speed) molecular dynamics (MD) simulations. The MD and AFM conditions are controlled to match, as closely as possible, the geometry and orientation, load, temperature, and compliance. We observe clear stick-slip without any damage. Comparison of both MD and AFM results with the thermally activated Prandtl-Tomlinson model shows that MD results at the highest speeds are not in the thermally activated regime. At lower speeds, within the thermally activated regime, AFM and MD provide consistent energetics, but attempt frequencies differ by orders of magnitude. Because this discrepancy lies in attempt frequencies and not energetics, atomistic details in MD simulations can be reliably used in interpreting AFM data if the MD speeds are slow enough.

Temperature Dependence of Atomic-Scale Stick-Slip Friction

We report experiments of atomic stick-slip friction on graphite as an explicit function of surface temperature between 100and 300K under ultrahigh vacuum conditions. A statistical analysis of the individual stick-slip events as a function of the velocity reveals an agreement with the thermally activated Prandtl-Tomlinson model at all temperatures. Taking into account an explicit temperature-dependence of the attempt frequency all data points collapse onto one single master curve.

Structural and frictional properties of graphene films on SiC(0001) studied by atomic force microscopy

Structural and frictional properties of single-layer and bilayer graphene films on a SiC(0001) substrate are studied by means of atomic force microscopy with atomic resolution. Friction on single-layer graphene is found to be a factor of two larger than on bilayer films for a variety of experimental situations. The friction contrast is found not to originate in differences in structural properties, in lateral contact stiffness, or in contact potential. The transition from atomic stick-slip friction to a regime of ultralow friction is found to occur at normal loads of 40 nN when the tip-sample interaction potential approaches 0.1--0.2 eV.

Low-Speed Atomistic Simulation of Stick–Slip Friction using Parallel Replica Dynamics

Atomic stick–slip friction has been predicted by molecular dynamics simulation and observed in experiments. However, direct quantitative comparison of the two has thus far not been possible because of the large difference between scanning velocities accessible to simulations and experiments. In general, the slowest sliding speeds in MD simulations are at least five orders of magnitude larger than the upper limit available to experimentalists. To take a step toward bridging this gap, we have applied parallel replica dynamics, an accelerated molecular dynamics method, to the simulation of atomic stick–slip. The method allows molecular simulations to run parallel in time in order to extend their duration, thereby enabling lower scanning velocities. We show here that this method is able to predict atomic stick–slip friction accurately and efficiently at scanning speeds several orders of magnitude slower than standard molecular dynamics simulations. The accuracy and usefulness of this method is illustrated by correct prediction of the logarithmic dependence of friction on velocity.

Atomic structure and friction of ultrathin films of KBr on Cu(100)

The frictional properties of ultrathin films of KBr on a Cu(100) substrate have been studied on the atomic scale by means of friction force microscopy. Films as thin as two atomic layers support the sliding tip and exhibit atomic stick-slip friction similar to cleaved surfaces of bulk KBr. Steps on the film are prone to wear, while substrate steps overgrown by the film are found to be stable. The interpretation of these results is supported by a detailed analysis of the structure of the films by means of atomic-resolution noncontact force microscopy. The films are found to be compressively strained by $1.0\ifmmode\pm\else\textpm\fi{}0.2\phantom{\rule{0.2em}{0ex}}%$. They exhibit a regular superstructure originating from the interference of lattice constants at the substrate-film interface, which also causes a weak modulation of the frictional response. Monolayer films of KBr are subject to significant wear when imaged in contact-mode force microscopy. Noncontact imaging confirms that these monolayers are rich in defects.

Molecular dynamics simulations on atomic friction between self-assembled monolayers: Commensurate and incommensurate sliding

Atomic friction between self-assembled monolayers (SAMs) on Au(1 1 1) has been studied through molecular dynamics simulations, with emphasis on the comparison of the performances of commensurate and incommensurate SAMs in relative sliding. Results show that the shear stress on commensurate SAMs exhibits a clean periodic pattern, manifesting the atomic stick–slip friction, while random fluctuations and a much lower average value of the shear stress are observed for incommensurate sliding. The different frictional behavior can be traced to the difference in molecule movements, especially in the collective nature of the motion. In commensurate sliding, all molecules move synchronously in each period and phase, but they swing randomly and independently for incommensurate monolayers. Simulations provide clear evidence that under the same working conditions friction forces on commensurate SAMs are always higher than those in incommensurate cases. The results also show a linear dependence of shear stress on normal pressure and logarithmic dependence on sliding velocity.

Simulations on atomic-scale friction between self-assembled monolayers: Phononic energy dissipation

Atomic-scale friction between self-assembled monolayers (SAMs) on Au (1 1 1) has been studied through molecular dynamics simulations, with emphasis on the mechanism of energy dissipation. Results show that the shear stress and chain angle on commensurate SAMs exhibit a clean periodic pattern and atomic stick–slip friction, which manifests a gradual storage and sudden release of energy. Using a simple model of two atoms, analysis shows that the atomic stick–slip originates from the dynamic instability of molecule motion. Energy has been built up during the stick, followed by a sudden separation as the equilibrium becomes unstable, and most energy dissipates at the time of slip. Moreover, the simulations reveal that more energy is stored and released in commensurate sliding, resulting in much higher friction than that in incommensurate cases. The contradictory frictional behavior can be traced to the difference in the number and strength of the Van der Waals bonds, formed in the two types of contacts.

Atomic Stick-Slip Friction Between Commensurate Self-Assembled Monolayers

Molecular dynamics (MD) simulations have been used to examine the friction between commensurate self-assembled monolayers (SAMs) on Au (111). The discrepancy between sawtoothed friction force and the discontinuous molecular movements reveals the complex dynamics of the closed-packing chains under shearing. Molecules in the lower monolayer can be divided into two groups with a phase difference of π. The periodic motion of the molecules is not synchronous with the frictional stick-slip loops, which result in a second-order valley in friction curves.

Atomic Stick-Slip Friction Between Commensurate Self- Assembled Monolayers

Molecular dynamics (MD) simulations have been used to examine the friction between commensurate self-assembled monolayers (SAMs) on Au (111). The discrepancy between sawtoothed friction force and the discontinuous molecular movements reveals the complex dynamics of the closed-packing chains under shearing. Molecules in the lower monolayer can be divided into two groups with a phase difference of π. The periodic motion of the molecules is not synchronous with the frictional stick-slip loops, which result in a second-order valley in friction curves.