Atomic Friction Research Articles

Double-slip mechanism in atomic-scale friction: Tomlinson model at finite temperatures

We studied the atomic scale friction mechanism of a friction force microscope (FFM) by means of the dynamical simulation in the Tomlinson model at a finite temperature. Stick-slip motion of a tip atom is the intrinsic energy dissipation mechanism at low scanning velocities of a support point of FFM and the frictional behavior is classified into three modes. The single slips occur in a high friction mode but double slips occur in a low friction mode, while mixed slips of single and double occur in an intermediate friction mode. In the intermediate friction mode, the friction force shows irregular sawtooth behavior as the support point is moved. We clarify the appearance condition of these three friction modes and each dependence of the average friction force on both temperatures and the sliding velocities.

Fluctuations and jump dynamics in atomic friction experiments

Atomic stick-slip processes have been studied in detail by means of friction force microscopy with high spatial and temporal resolution. The influence of the tip-sample contact on the thermal fluctuations of the force sensor and on the dynamics of the stick-slip process are characterized. Results are compared with simulations based on an extended Tomlinson model including thermal fluctuations. A correlation between the duration of the atomic slip event and the atomic structure of the contact is established.

Thermal quantum field theory and the Casimir interaction between dielectrics

The Casimir and van der Waals interaction between two dissimilar thick dielectric plates is reconsidered on the basis of thermal quantum field theory in Matsubara formulation. We briefly review two main derivations of the Lifshitz formula in the framework of thermal quantum field theory without use of the fluctuation-dissipation theorem. A set of special conditions is formulated under which these derivations remain valid in the presence of dissipation. The low-temperature behavior of the Casimir and van der Waals interactions between dissimilar dielectrics is found analytically from the Lifshitz theory for both an idealized model of dilute dielectrics and for real dielectrics with finite static dielectric permittivities. The free energy, pressure and entropy of the Casimir and van der Waals interactions at low temperatures demonstrate the same universal dependence on the temperature as was previously discovered for ideal metals. The entropy vanishes when temperature goes to zero proving the validity of the Nernst heat theorem. This solves the long-standing problem on the consistency of the Lifshitz theory with thermodynamics in the case of dielectric plates. The obtained asymptotic expressions are compared with numerical computations for both dissimilar and similar real dielectrics and found to be in excellent agreement. The role of the zero-frequency term in Matsubara sum is investigated in the case of dielectric plates. It is shown that the inclusion of conductivity in the model of dielectric response leads to the violation of the Nernst heat theorem. The applications of this result to the topical problems of noncontact atomic friction and the Casimir interaction between real metals are discussed.

Small‐Amplitude Atomic Force Microscopy

AbstractSmall amplitude Atomic Force Microscopy (AFM) is a relatively new AFM technique which was specifically developed to perform linear measurements of nanomechanical phenomena. This is achieved by using ultra‐small cantilever amplitudes and very high sensitivity deflection sensors. Recently this technique has been used in ultra‐high vacuum (UHV) and liquid environments to measure atomic and molecular forces and dynamics with high precision. Here we focus on three examples which are interesting from a nanoengineering standpoint: Atomic energy dissipation (atomic friction), atomic‐scale contact mechanics, and nanotribology/molecular ordering in confined liquid films.

Development of a high velocity accessory for atomic force microscopy-based friction measurements

A high velocity accessory for friction force microscopy measurements for velocities up to the mm/s range was developed for a commercial stand-alone atomic force microscope (AFM). The accessory consists of a shear piezo element, which rapidly displaces the sample in the lateral direction, perpendicular to the main axis of the AFM cantilever. Friction forces, which are acquired via conventional optical beam deflection detection, can thus be measured as a function of velocity and load in controlled environment (0–40% relative humidity and 0–40°C). Using the accessory, a broad range of velocities up to several mm/s can be accessed independent of the lateral scan size up to a maximum scan size of 1000nm. The velocity dependence of friction forces and coefficients was measured on organic [poly(methylmethacrylate)], as well as inorganic [oxidized Si(100)] samples to demonstrate the feasibility and underline the importance of high velocity nanotribology using this accessory.

Direct observation of transition between stick-slip and continuous sliding in atomic friction

The distortion and gradual recovery processes after a continuous tunable laser illuminating the graphite surface in air have been observed and analyzed in detail with a scanning tunneling microscope (STM). The slow response of the tip to the laser-induced mechanical elongation is responsible for the observed phenomena, which indicate that the transition between stick-slip and continuous sliding has occurred due to the atomically modulated friction between the STM tip and the graphite substrate.

Atomic scale friction between clean graphite surfaces

We investigate atomic scale friction between clean graphite surfaces by using molecular dynamics. The simulation reproduces atomic scale stick-slip motion and a low frictional coefficient, both of which are observed in experiments using frictional force microscope. It is made clear that the microscopic origin of low frictional coefficients of graphite lies on the honeycomb structure of each layer, not only on the weak interlayer interaction as believed so far.

Tip-jump statistics of stick-slip friction

Friction maps with atomic unit cell periodicity on highly oriented graphite in ultra-high vacuum are measured with the friction force microscope. The well-known stick-slip phenomenon is observed where the tip jumps between individual equilibrium positions. We perform a statistical analysis of the frictional forces that are necessary to induce the tip jumps. The corresponding histograms give a direct fingerprint of the distribution function of the tip-jump events. The histograms depend strongly on the atomic structure of the surface, which is related to the two-dimensionality of the surface potential. We compare the experimental to the theoretical distribution function, which is based on the thermally activated Tomlinson model for atomic friction, and quantitative values for the effective energy barriers are extracted.

Local Spectroscopy and Atomic Imaging of Tunneling Current, Forces, and Dissipation on Graphite

Theory predicts that the currents in scanning tunneling microscopy (STM) and the attractive forces measured in atomic force microscopy (AFM) are directly related. Atomic images obtained in an attractive AFM mode should therefore be redundant because they should be similar to STM. Here, we show that while the distance dependence of current and force is similar for graphite, constant-height AFM and STM images differ substantially depending on the distance and bias voltage. We perform spectroscopy of the tunneling current, the frequency shift, and the damping signal at high-symmetry lattice sites of the graphite (0001) surface. The dissipation signal is about twice as sensitive to distance as the frequency shift, explained by the Prandtl-Tomlinson model of atomic friction.

Description of atomic friction as forced Brownian motion

A theoretical description of friction force microscopy experiments in terms of a forced Brownian motion model is derived on the basis of microscopic considerations. Particular emphasis is put on the discussion of the relevant state variables/collective coordinates and on a realistic description of dissipation and inertia effects by means of comparison with experimental findings. The main new prediction of the model is a non-monotonic dependence of the friction force upon the pulling velocity of the AFM-tip relative to an atomically flat surface. The region around the force maximum can be approximately described by a universal scaling law and should be observable under experimentally realistic conditions.

Key Issues of Nanotribology for Successful Nanofabrication – From Basis to C60 Molecular Bearings

We briefly review the key issues of nanotribology, particularly the mechanism of friction on graphite surface, i.e., atomic friction, flake friction, and C60 molecular bearings. For the atomic and flake frictions, frictional force maps are compared between simulations and experiments, which can be explained by stick-slip motion of the tip apex atom and flake. For C60 molecular bearings, a novel frictional mechanism realized in the graphite/C60mono/graphite system, is reported. In this system, static frictional forces have a finite value but denote that dynamical frictional forces are zero. In order to explain the mechanism, “step rotation model” of a C60 molecule is proposed by considering the effect of thermal activation, based on the concept of “single-molecular bearings.” It should be emphasized that the graphite/C60mono/graphite system is the smallest bearing in the world which has a tremendous potential for the realization of nano- and micro-machines, and has opened a new field of molecular bearings.

Atomic scale friction of nanoscale clusters

We study both the activation energy for diffusion and dynamics of nanoscale clusters of a Frenkel–Kontrova chain subject to the microscopic friction, in connection with atomic friction phenomena. We clarify the atomistic mechanism for the presence of a magic size for diffusion and show how the magic size effects appear in the friction. When a cluster is pulled by a constant driving force, the critical force strength of depinning depends crucially on the cluster size, and it is smallest at the magic size. When a cluster is pulled by a spring at a constant velocity below the critical value, clusters show a stick-and-slip motion, and the maximum force of a spring also becomes smallest at the magic size.

Transition from Stick-Slip to Continuous Sliding in Atomic Friction: Entering a New Regime of Ultralow Friction

A transition from stick-slip to continuous sliding is observed for atomically modulated friction by means of a friction force microscope. When the stick-slip instabilities cease to exist, a new regime of ultralow friction is encountered. The transition is described in the framework of the Tomlinson model using a parameter eta which relates the strength of the lateral atomic surface potential and the stiffness of the contact under study. Experimentally, this parameter can be tuned by varying the normal load on the contact. We compare our results to a recently discussed concept called superlubricity.

Friction mechanisms of graphite from a single-atomic tip to a large-area flake tip

The frictional mechanisms of graphite derived from a single-atom tip and a graphite flake tip are systematically studied. The frictional forces in both cases are approximately proportional to the loading force because of significant deformation of the graphite surface. For ``atomic friction (single-atom tip),'' the effect of water on the loading force is discussed. The deformation and effective lateral stiffness of the graphite surface under ambient conditions are evaluated using the ``extended Hertz model.'' A single-atom tip attached to the macroscopic tip gives interpretations of atomic resolution of frictional force maps even under high loading conditions. For ``flake friction (flake tip),'' the graphite flake moves on graphite such that the stacking of graphite layers is maintained. In this case, the frictional coefficient along the $[123\ifmmode\bar\else\textasciimacron\fi{}0]$ direction of the graphite(0001) surface is estimated to be approximately 0.001. The anisotropy of the frictional force as a function of the pulling direction is also shown.

Atomic Scale Friction of Amorphous Hydrogenated Carbon Thin Film in MEMS

Atomic Scale Friction of Amorphous Hydrogenated Carbon Thin Film in MEMS

A comparison of atomic force microscope friction and phase imaging for the characterization of an immiscible polystyrene/poly(methyl methacrylate) blend film

Three different forms of atomic force microscope (AFM) measurement, topography, friction force and phase imaging, have been used to investigate the surface morphology and local composition of an immiscible polystyrene (PS)/poly(methyl methacrylate) (PMMA) blend film. This sample forms discrete, micron-size domains in a continuous matrix, which is attributed to the segregation of PMMA in PS. When the samples were imaged in air, contrast in friction and phase images was caused by variations in sample topography only. When the samples were imaged under water, however, both friction and phase imaging yielded non-topographic contrast between domains. We ascribe the contrast in both of these imaging modes to preferential softening of the hydrophilic, PMMA-rich domains and to stronger tip–sample adhesive forces, highlighting the AFM's utility for probing local elastic properties and for compositional mapping of soft polymer samples.

Atomic vacancy-induced friction on the graphite surface: observation by lateral force microscopy.

Lateral force microscopy has been employed to investigate the frictional behaviour of atomic vacancies on the graphite surface. Such a study was only made possible by the controlled expansion of originally single-atom vacancies into multiatom vacancies, employing oxygen plasma etching for this purpose. Enhanced friction was observed on the vacancy regions compared with pristine areas of graphite, the origin of which is examined and discussed.

Cross-sectional Scanning Probe Microscopy of GaN-based p–n heterostructures

In this work, the structural and electrical properties of a GaN-based p–n heterostructure are studied using cross-sectional Atomic Force Microscopy, Friction Force Microscopy, Electrical Force Gradient Microscopy, and Surface Potential Microscopy. Using Atomic Force Microscopy and Friction Force Microscopy, we were able to identify and measure the thickness of the layers present in the heterostructures. The electrical conductivity type of the different layers as well as the p–n junction, and piezoelectric fields were identified and studied using Electric Force Gradient Microscopy and Surface Potential Microscopy.

Formation of nanostructures on surface of SiC by laser radiation

Formation of nanostructures on a surface of 6H–SiC:N, B under irradiation by the N 2 pulsed laser was found and studied. The nanostructures have a shape of a hill, and arise along the circular line around the focussed spot. We propose the following model for the explanation of the formation of nano-hills. The laser irradiation creates a liquid phase in the bulk under a thin solid state layer. The pressure of laser radiation (LR) together with the pressure of the liquid phase result in a bulging of the material on the surface. The results of experimental studies of the irradiated surface by Atomic Force Microscope (AFM) and Friction Force Microscope (FFM) allow us to conclude that the concentration of nitrogen in nano-hills is bigger compared with that of the unirradiated crystal. The increase of the nitrogen concentration in the hills can be explained by presence of the temperature gradient field. The 6H–SiC samples contain both N and B atoms with strongly different covalent radii, in the temperature gradient field atoms of N move towards the irradiated surface, while atoms of B—into the bulk of the crystal. The obtained results are explained by presence of the Thermogradient effect (TGE).

Model-based signal conditioning for high-speed atomic force and friction force microscopy

The imaging speed of atomic force microscopy (AFM) is limited due to the dynamics of the piezo scanner along the scanning direction. This article presents the identification and open-loop control of a piezoelectric tube scanner to enable fast imaging. By applying a model based open-loop control the dynamic behaviour of the piezo tube can be compensated. The lateral displacement error is reduced and topographical artifacts, additional cantilever deflection, modulation of the tip–sample interaction force, and modulation of the relative tip–sample velocity vanish. The open-loop controlled AFM scanner enables imaging at a line scan rate of 122 Hz, which is about 15 times faster than standard systems.