Atomic-scale friction of a tungsten tip on a graphite surface

Predictions and observations of multiple slip modes in atomic-scale friction

Using the quasistatic Tomlinson model as a simple representation of an atomic force microscope, conditions for transitions in atomic-scale friction behavior from smooth sliding to single slips and then multiple slip regimes are derived based on energy minimization. The calculations predict and give a general explanation for transitions between different stick-slip regimes in the limit of low damping. The predictions are consistent with experimental observations of these transitions.

Two-dimensional stick-slip on a soft elastic polymer: pattern generation using atomic force microscopy

It has been demonstrated that it is possible to create laterally differentiated frictional patterning and three-dimensional structures using an atomic force microscope (AFM) probe on the surface of a soft elastic polymer, poly(dimethylsiloxane) (PDMS). The resulting effect of contact mode imaging at low loading forces (<100 nN), observed in the lateral force mode, revealed a homogeneous pattern on the PDMS surface exhibiting higher friction. With higher loading forces ([Formula: see text] nN) the effect is non-uniform, resulting in structures with depths on the nanometre scale. The topographic and frictional data revealed stick-slip responses in both the fast (orthogonal to the long axis of the lever) and slow (parallel to the long axis of the lever) directions of probe travel from scanning in a raster pattern. The stick-slip events are manifested in the form of a series of shallow channels spaced evenly apart on the polymer surface. Detailed friction loop analysis acquired during the manipulation process showed that the lateral force changed according to the strength of trapping of the tip with the polymer surface exhibiting significant in-plane deformation due to lateral forces being imposed. An incremental increase in the initial loading force resulted in an increase in in-plane displacement and a greater spacing between the stick lines/channels in the slow-scan direction. A decrease in channel length in the fast-scan direction is also observed as a result of an increase in static friction with normal force, resulting in greater surface deformation and shorter track length for sliding friction.

Friction of Mixed and Single-Component Aromatic Monolayers in Contacts of Different Adhesive Strength

Friction force microscopy has been used to study single-component and mixed self-assembled monolayers of aminothiophenol and thiophenol on gold. The friction forces and transition pressures of mixed monolayers were intermediate to the ones of single-component monolayers, and varied systematically with composition. The strength of the adhesion was altered by working in dry N2 gas or in ethanol. In all systems studied, low adhesion (in ethanol) resulted in a linear dependence of the friction on load already at low loads, whereas high adhesion (in dry N2) gave an apparent area-dependence. However, for a given monolayer composition, similar transition pressures were observed in dry N2 and in ethanol, suggesting that the overall monolayer structure was not strongly altered by the presence of ethanol. Similar observations were made for very close-packed monolayers of octadecanethiol.

Friction and Adhesion between C60 Single Crystal Surfaces and AFM Tips: Effects of the Orientational Phase Transition

We have investigated the nanotribological properties of C60 single crystal (111) and (100) surfaces around its orientational order-disorder phase transition temperature, approximately 260 K, by atomic force microscopy and frictional force microscopy (AFM/FFM) in high vacuum. Results show that for both surfaces across the phase transition temperature, the friction force and the adhesive force between a C60 coated AFM tip and the C60 crystal surfaces exhibit discontinuous behavior. The friction force within the applied external load range in the low temperature phase is significantly larger than that in the high temperature phase, with no obvious change in the slope of the friction force curves (the friction coefficient) in the low and high temperature phases. The abrupt change in friction was found to be caused mainly by the abrupt change in adhesion, which, in turn, can be qualitatively understood through changes in the van der Waals interaction and the short-range Coulomb interaction associated with the structural changes across the phase transition. Compared to most other degrees of freedom, the rotation of C60 molecules was found to have little effect on friction and is an ineffective energy dissipation channel.

Enhanced Lubricity in Mixed Alkanethiol Monolayers

The frictional properties of two-component mixed alkanethiol monolayers were investigated with frictional force microscopy. With large chain length differences, the two types of molecules segregated into large islands and reveal similar frictional properties as those of single-component monolayers. With small chain length differences, the completely mixed monolayers display a decreased friction as compared with single-component monolayers. This observation indicates that development of gauche kinks in the top portion of the mixed monolayers can enhance lubrication rather than increase friction, perhaps due to suppression of other energy dissipation channels.

Friction model for the velocity dependence of nanoscale friction

The velocity dependence of nanoscale friction is studied for the first time over a wide range of velocities between 1 microm s(-1) and 10 mm s(-1) on large scan lengths of 2 and 25 microm. High sliding velocities are achieved by modifying an existing commercial atomic force microscope (AFM) setup with a custom calibrated nanopositioning piezo stage. The friction and adhesive force dependences on velocity are studied on four different sample surfaces, namely dry (unlubricated), hydrophilic Si(100); dry, partially hydrophobic diamond-like carbon (DLC); a partially hydrophobic self-assembled monolayer (SAM) of hexadecanethiol (HDT); and liquid perfluoropolyether lubricant, Z-15. The friction force values are seen to reverse beyond a certain critical velocity for all the sample surfaces studied. A comprehensive friction model is developed to explain the velocity dependence of nanoscale friction, taking into consideration the contributions of adhesion at the tip-sample interface, high impact velocity-related deformation at the contacting asperities and atomic scale stick-slip. A molecular spring model is used for explaining the velocity dependence of friction force for HDT.

Non-contact microsphere–surface adhesion measurement via acoustic base excitations

A non-contact adhesion measurement technique based on acoustic base excitation and laser interferometery has been introduced and demonstrated. The vibrational motion of 21.4-microm polystyrene latex particles (PSL) microspheres on surfaces were excited in the frequency range of 0-3.5 MHz, and their axial displacement responses were measured by an interferometer. It is shown that the rolling motion is dominant compared to the axial displacement of the bond. Using a formula for the rotational moment resistance of the particle-surface adhesion bond and the equation of rotational motion, the natural frequency of the rotational motion is related to the work of adhesion of the particle and substrate materials. The substrate materials used in the experiments include copper, aluminum, tantalum, and silicon. Measured work of adhesion values are compared to the data reported in the literature and good agreement is found.

Ab Initio Studies on Nanoscale Friction between Graphite Layers: Effect of Model Size and Level of Theory

Ab initio methods were used to investigate the nanoscale friction between two graphite layers placed in contact. The interaction energies were calculated for four two-layer models in series, C(6(n+1))2H(6n+1))-C(6)(n)2H(6)(n) with n = 1, 2, 3, and 4, and additionally for C(54)H(18)-C(6)H(6) and C(150)H(30)-C(6)H(6). The study was done with the Hartree-Fock method using basis sets 3-21G and 6-31G and with the second-order Møller-Plesset theory using basis set 6-31G. A density functional method (B3PW91) was also tested for reference purposes. The main interest was how the model size and level of theory affect the nanoscale friction coefficient. Most of the calculated friction coefficients fell within the range of values of 0.07-0.14.

Rate description of the stick-slip motion in friction force microscopy experiments

During the stick-slip motion of an atomic force microscope tip contacting with a uniformly moving atomically clean surface, the force developed in the cantilever spring performs random sawtoothlike oscillations resulting from the thermally activated transitions of the tip from one surface site to the next. Using escape rate theory, the probability distribution of forces is calculated numerically to deduce the time-average lateral force as a function of pulling velocity. A transcendental equation for the average force is proposed and its approximate solution is obtained. The accuracy of this analytic approximation is demonstrated via comparison with the numerical results. The analogous force-velocity relations existing in the literature are shown to be the limiting cases of low and high cantilever spring constants of our analytic approximation.

Combined AFM and confocal fluorescence microscope for applications in bio-nanotechnology

We present a custom-designed atomic force fluorescence microscope (AFFM), which can perform simultaneous optical and topographic measurements with single molecule sensitivity throughout the whole visible to near-infrared spectral region. Integration of atomic force microscopy (AFM) and confocal fluorescence microscopy combines the high-resolution topographical imaging of AFM with the reliable (bio)-chemical identification capability of optical methods. The AFFM is equipped with a spectrograph enabling combined topographic and fluorescence spectral imaging, which significantly enhances discrimination of spectroscopically distinct objects. The modular design allows easy switching between different modes of operation such as tip-scanning, sample-scanning or mechanical manipulation, all of which are combined with synchronous optical detection. We demonstrate that coupling the AFM with the fluorescence microscope does not compromise its ability to image with a high spatial resolution. Examples of several modes of operation of the AFFM are shown using two-dimensional crystals and membranes containing light-harvesting complexes from the photosynthetic bacterium Rhodobacter sphaeroides.

Spectroscopy of the shear force interaction in scanning near-field optical microscopy

Shear force detection is a common method of tip-sample distance control in scanning near-field optical microscopy. Shear force is the force acting on a laterally oscillating probe tip near a surface. Despite its frequent use, the nature of the interaction between tip and sample surface is a matter of debate. In order to investigate the problem, approach curves, i.e. amplitude and phase of the tip oscillation as a function of the tip-sample distance, are studied in terms of a harmonic oscillator model. The extracted force and damping constants are influenced by the substrate material. The character of the interaction ranges from elastic to dissipative. The interaction range is of atomic dimensions with a sharp onset. Between a metal-coated tip and a Cu sample, a power law for the force-distance curve is observed.

X-modulation: instrumentation and optimization

The measurement of lateral force in response to small amplitude lateral oscillations of a sample using a scanning probe microscope (X-modulation) is presented as an effective means of identifying key qualitative differences in the nanomechanical behavior of solid surfaces. To study the surface behavior in detail, it is critical that the instrument have sufficient flexibility. Computer-assisted measurement automation achieved using LabVIEW makes an X-modulation experiment more flexible and convenient. Even though further refinement of the modulation technique is necessary in both theory and experiment to obtain a complete picture, the systematic approach made possible by this sort of instrumentation is valuable for the study of mechanical properties of the surface.

Friction and second-order phase transitions

A microscopic model is studied numerically to describe wearless dry friction without thermal fluctuations between atomically flat contact interfaces. The analysis is based on a double-chain model with a Lennard-Jones interaction between the chains which are the respective upper flexible monolayers of the rigid bulk systems. Whereas below a critical interaction strength epsilon(c) the system exhibits a frictionless state, it offers static friction above epsilon(c) . Introducing an appropriate order parameter function we demonstrate the analogy of the critical behavior to a phase transition of second order. The order parameter is related to a hull function describing uniquely the incommensurate ground state of the model. The breakdown of analyticity of the hull function is identified with the phase transition. Critical exponents are calculated and the validity of finite-size scaling is displayed.

Diameter oscillation of axonemes in sea-urchin sperm flagella

The 9 + 2 configuration of axonemes is one of the most conserved structures of eukaryotic organelles. Evidence so far has confirmed that bending of cilia and flagella is the result of active sliding of microtubules induced by dynein arms. If the conformational change of dynein motors, which would be a key step of force generation, is occurring in a three-dimensional manner, we can easily expect that the microtubule sliding should contain some transverse component, i.e., a motion in a direction at a right angle to the longitudinal axis of axonemes. Using a modified technique of atomic force microscopy, we found such transverse motion is actually occurring in an oscillatory manner when the axonemes of sea-urchin sperm flagella were adhered onto glass substrates. The motion was adenosine triphosphate-dependent and the observed frequency of oscillation was similar to that of oscillatory sliding of microtubules that had been shown to reflect the physiological activity of dynein arms (S. Kamimura and R. Kamiya. 1989. Nature: 340:476-478; 1992. J. Cell Biol. 116:1443-1454). Maximal amplitude of the diameter oscillation was around 10 nm, which was within a range of morphological change observed with electron microscopy (F. D. Warner. 1978. J. Cell Biol. 77:R19-R26; N. C. Zanetti, D. R. Mitchell, and F. D. Warner. 1979. J. Cell Biol. 80:573-588).

A torsional resonance mode AFM for in-plane tip surface interactions

Changing the method of tip/sample interaction leads to contact, tapping and other dynamic imaging modes in atomic force microscopy (AFM) feedback controls. A common characteristic of these feedback controls is that the primary control signals are based on flexural deflection of the cantilever probes, statically or dynamically. We introduce a new AFM mode using the torsional resonance amplitude (or phase) to control the feedback loop and maintain the tip/surface relative position through lateral interaction. The torsional resonance mode (TRmode ) provides complementary information to tapping mode for surface imaging and studies. The nature of tip/surface interaction of the TRmode facilitates phase measurements to resolve the in-plane anisotropy of materials as well as measurements of dynamic friction at nanometer scale. TRmode can image surfaces interleaved with TappingMode with the same probe and in the same area. In this way we are able to probe samples dynamically in both vertical and lateral dimensions with high sensitivity to local mechanical and tribological properties. The benefit of TRmode has been proven in studies of water adsorption on HOPG surface steps. TR phase data yields approximately 20 times stronger contrast than tapping phase at step edges, revealing detailed structures that cannot be resolved in tapping mode imaging. The effect of sample rotation relative to the torsional oscillation axis of the cantilever on TR phase contrast has been observed. Tip wear studies of TRmode demonstrated that the interaction forces between tip and sample could be controlled for minimum tip damage by the feedback loop.

SURFACE CHEMISTRY AND TRIBOLOGY OF MEMS

The microscopic length scale and high surface-to-volume ratio, characteristic of microelectro-mechanical systems (MEMS), dictate that surface properties are of paramount importance. This review deals with the effects of surface chemical treatments on tribological properties (adhesion, friction, and wear) of MEMS devices. After a brief review of materials and processes that are utilized in MEMS technology, the relevant tribological and chemical issues are discussed. Various MEMS microinstruments are discussed, which are commonly employed to perform adhesion, friction, and wear measurements. The effects of different surface treatments on the reported tribological properties are discussed.

Reproducible lateral force microscopy measurements for quantitative comparisons of the frictional and chemical properties of nanostructures

Atomic force microscopy (AFM) is a widely used technique for characterizing the topography and frictional properties of nanostructures. Inherent misalignments between the AFM cantilever and the feedback hardware lead to crosstalk between topography data and lateral force microscopy (LFM) data. Because the degree of crosstalk depends on the positioning of the cantilever, LFM and topography data of the same structure can vary from one experiment to the next. For nanostructures with large LFM contrast, errors as large as 50% in topography and LFM can be observed. This paper describes an empirical strategy for correcting this alignment error. The technique is used to characterize the frictional properties of scanning probe-induced oxide nanostructures and the hydrogen-terminated Si(111) surfaces on which they are patterned. Reproducible differences in the frictional properties of the oxide nanostructures patterned on HF-treated and NH4F-treated Si(111) surfaces are observed and attributed to the mixed-hydride versus monohydride termination of each surface. The observed frictional contrast is consistent with known differences in surface reactivity and demonstrates how LFM measurements can provide insight into the frictional and chemical properties of nanostructures

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.

Superlubricity of graphite

Using a home-built frictional force microscope that is able to detect forces in three dimensions with a lateral force resolution down to 15 pN, we have studied the energy dissipation between a tungsten tip sliding over a graphite surface in dry contact. By measuring atomic-scale friction as a function of the rotational angle between two contacting bodies, we show that the origin of the ultralow friction of graphite lies in the incommensurability between rotated graphite layers, an effect proposed under the name of "superlubricity" [Phys. Rev. B 41, 11 837 (1990)]].

Nanoscopic liquid bridges exposed to a torsional strain

In this paper we investigate the response to a torsional strain of a molecularly thin film of spherically symmetric molecules confined to a chemically heterogeneous slit pore by means of Monte Carlo simulations in the grand canonical ensemble. The slit pore comprises two identical plane-parallel solid substrates, the fluid-substrate interaction is purely repulsive except for elliptic regions attracting fluid molecules. Under favorable thermodynamic conditions the confined film consists of fluid bridges where the molecules are preferentially adsorbed by the attractive elliptic regions, and span the gap between the opposite substrate surfaces. By rotating the upper substrate while holding the lower one in position, bridge phases can be exposed to a torsional strain 0< or =theta< or =pi/2 and the associated torsional stress T(theta) of the (fluidic) bridge phases can be calculated from molecular expressions. The obtained stress curve T(theta)(theta) is qualitatively similar to the one characteristic of sheared confined films: as the torsion strain increases, T(theta) rises to a maximum (yield point) and then decays monotonically to zero. By changing the ellipses' aspect ratio while keeping their area constant, we also investigate the influence of the attractive elliptic patterns' shape on T(theta)(theta).

Measurement of mobility of adsorbed colloids by lateral force microscopy

The surface mobility of colloidal latex particles adsorbed on mica was measured by moving the particles with an AFM tip in the lateral force microscopy mode. The same particle was repeatedly scanned while the normal force was gradually increased, until the particle was displaced from its location on the substrate. The lateral (friction) force curve obtained for that scan was then used to determine the force needed to displace the particle. The data accumulated for approximately 100 particles indicate a wide distribution in the lateral force required. However, the data show that the mean lateral force is proportional to the particle diameter, with the effect of electrostatic interactions on the mobility of adsorbed particles seen to be weak. These results are consistent with classical theories of friction in macroscopic systems.

Effect of C60 molecular rotation on nanotribology

The effect of C60 molecular rotation on the nanotribological properties of C60 single crystal surfaces has been studied by atomic/frictional force microscopy. The orientational order-disorder phase transition, in which the high temperature C60 free rotation is reduced to a low temperature hindered rotation, is shown to give rise to an abrupt change in friction and adhesion. This change in frictional force is quantitatively consistent with the observed change in adhesion. The similar slopes of the friction versus load curves in both phases indicate that the friction coefficient in the two phases remains about the same. Hence the C60 rotation does not provide an additional energy dissipation channel in the friction process.

Friction traced to the single atom

Friction is caused by dissipative lateral forces that act between macroscopic objects. An improved understanding of friction is therefore expected from measurements of dissipative lateral forces acting between individual atoms. Here we establish atomic resolution of both conservative and dissipative forces by lateral force microscopy, presenting the resolution of atomic defects. The interaction between a single-tip atom that is oscillated parallel to an Si(111)-(7 x 7) surface is measured. A dissipation energy of up to 4 eV per oscillation cycle is found. The dissipation is explained by a "plucking action of one atom on to the other" as described by G. A. Tomlinson in 1929 [Tomlinson, G. A. (1929) Phil. Mag. 7, 905-939].

Abrasive wear on the atomic scale

A scanning force microscope in ultrahigh vacuum has been used to realize and detect atomic-scale abrasion on KBr(001). The continuous time evolution of the lateral force under scratching reveals that the wear mechanism is due to the removal and the rearrangement of single ion pairs. The debris is reorganized in regular terraces with the same periodicity and orientation as the unscratched surface, as in local epitaxial growth. The applied load has a strong influence on the abrasive process, whereas the scan velocity is less relevant.

Effect of temperature on friction observed between a Si3N4 tip and a dodecanethiol self-assembled monolayer on Au(1 1 1)

Recent computational studies (Phys. Rev. Lett. 70 (1993) 1960; Phys. Rev. B 62 (2000) 17055) predicted that friction of ordered organic monolayer had characteristic dependence on temperature, where the maximum friction was observed around rotator transition point of the monolayer. This remained to be confirmed experimentally. Using a friction force microscope (FFM) combined with a temperature regulation module, we attempted to investigate such dependence on temperature (130 K-room temperature) on a self-assembled monolayer of dodecanethiol prepared on Au(1 1 1). The observed friction showed strong dependence on temperature and good agreement with the computational prediction.

Self-sensing piezoresistive cantilever and its magnetic force microscopy applications

A newly developed Si self-sensing piezoresistive cantilever is presented. Si piezoresistive cantilevers for scanning microscopy are fabricated by Si micro-machining technique. The sensitivity of the piezoresistive cantilever is comparable to the current laser detecting system. Topographic images are successfully obtained with the piezoresistive cantilever and some comparisons are made with the laser detecting system. Furthermore, the magnetic film (Co-Cr-Pt) is coated on the tip of the piezoresistive cantilever for magnetic force microscopy (MFM) application. The magnetic images are successfully obtained with the self-sensing MFM piezoresistive cantilever. The self-sensing piezoresistive cantilevers have been successfully applied in scanning probe microscopy and MFM.

Developments for inverted atomic force microscopy

Atomic force microscopy (AFM) has been used to study a wide range of systems. Chemically and biologically modified probes have extended AFM by coupling chemical and biological information with the physical measurements. In an effort to further expand the capabilities of modified AFM probes, previous studies investigated the use of an inverted AFM design (i-AFM), wherein a microfabricated tip array is used to image a cantilever-supported sample. This report details developments in cantilever and tip array fabrication which are aimed at improving the applicability and performance of this i-AFM design. Using an epoxy-based procedure, commercial cantilevers were modified with a series of standard substrates, including template-stripped gold, highly oriented pyrolytic graphite, and mica. The samples on these cantilevers were imaged with i-AFM, and lateral force images are obtained. This paper demonstrates the first use of i-AFM for measuring friction.

Mapping of lateral vibration of the tip in atomic force microscopy at the torsional resonance of the cantilever

Lateral vibration of the tip in atomic force microscopy was mapped at the torsional resonance of the cantilever by attaching a shear piezo element at the base of the cantilever or under the sample. Fixed frequency excitation and self-excitation of torsional motion were implemented. The lateral vibration utilized as measured by an optical lever was in the order of 10 pm to 3 nm, and its frequency approximately 450 kHz for a contact-mode silicon nitride cantilever. The amplitude and phase of the torsional motion of the cantilever was measured by a lock-in-amplifier or a rectifier and plotted in x and y as the sample was raster scanned. The imaging technique gave contrast between graphite terraces, self-assembled monolayer domains, silicon and silicon dioxide, graphite and mica. Changing contrast was observed as silicon islands oxidized in atmosphere, showing that the imaging technique can detect change in lateral tip mobility due to changes occurring near the surface. Torsional self-excitation showed nanometric features of self-assembled monolayer islands due to different lateral dissipation. Dependence of torsional resonance frequency on excitation amplitude, and contrast change due to driving frequency around resonance were observed.