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

Vanishing stick-slip friction in few-layer graphenes: the thickness effect

We report the thickness dependence of intrinsic friction in few-layer graphenes, adopting molecular dynamics simulations. The friction force drops dramatically with decreasing number of layers and finally approaches zero with two or three layers. The results, which are robust over a wide range of temperature, shear velocity, and pressure are quantitatively explained by a theoretical model with regard to lateral stiffness, slip length, and maximum lateral force, which could provide a new conceptual framework for understanding stick-slip friction. The results reveal the crucial role of the dimensional effect in nanoscale friction, and could be helpful in the design of graphene-based nanodevices.

Adsorption and association of a symmetric PEO-PPO-PEO triblock copolymer on polypropylene, polyethylene, and cellulose surfaces

The association of a symmetric polyoxyethylene-polyoxypropylene-polyoxyethylene (PEO(19)-PPO(29)-PEO(19)) triblock copolymer adsorbed from aqueous solutions onto polypropylene (PP), polyethylene (PE), and cellulose surfaces was probed using Atomic Force Microscopy (AFM). Significant morphological differences between the polyolefin substrates (PP and PE) and the cellulose surfaces were observed after immersion of the films in the PEO(19)-PPO(29)-PEO(19) solutions. When the samples were scanned, while immersed in solutions of the triblock copolymer, it was revealed that the structures adsorbed on the polyolefin surfaces were smoothed by the adsorbed PEO(19)-PPO(29)-PEO(19). In contrast, those structures on the hydrophilic cellulose surfaces were sharpened. These observations were related to the roughness of the substrate and the energy of interaction between the surfaces and the PEO and PPO polymer segments. The interaction energy between each of the blocks and the surface was calculated using molecular dynamics simulations. It is speculated that the associative structures amply reported in aqueous solution at concentrations above the critical micelle concentration, CMC, are not necessarily preserved upon adsorption; instead, it appears that molecular arrangements of the anchor-buoy type and hemimicelles prevail. The reported data suggests that the roughness of the surface, as well as its degree of hydrophobicity, have a large influence on the nature of the resulting adsorbed layer. The reported observations are valuable in explaining the behavior of finishing additives and lubricants commonly used in textile and fiber processing, as well as the effect of the morphology of the boundary layers on friction and wear, especially in the case of symmetric triblock copolymers, which are commonly used as antifriction, antiwear additives.

Sliding over a phase transition

The effects of a displacive structural phase transition on sliding friction are in principle accessible to nanoscale tools such as atomic force microscopy, yet they are still surprisingly unexplored. We present model simulations demonstrating and clarifying the mechanism and potential impact of these effects. A structural order parameter inside the material will yield a contribution to stick-slip friction that is nonmonotonic as temperature crosses the phase transition, peaking at the critical T(c) where critical fluctuations are strongest, and the sliding-induced order-parameter local flips from one value to another more numerous. Accordingly, the friction below T(c) is larger when the order-parameter orientation is such that flips are more effectively triggered by the slider. The observability of these effects and their use for friction control are discussed, for future application to sliding on the surface of and ferro- or antiferrodistortive materials.

Why sliding friction of Ne and Kr monolayers is so different on the Pb(111) surface

To understand the tribological properties of Ne and Kr on Pb(111), the potential energy surfaces for sliding motion of Ne, Kr, and Xe monolayers on the Pb(111) surface are examined through density functional calculations, using either local density or self-consistent nonlocal van der Waals functionals. The calculated adsorption energy for Xe/Pb(111) agrees well with experiment, validating the present approach and parameters. Activation energies along a sliding path indicate that Ne motion is much faster than Kr and Xe on Pb(111) at T∼6  K, which explains the puzzling experimental observation.

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.

Effect of brush thickness and solvent composition on the friction force response of poly(2-(methacryloyloxy)ethylphosphorylcholine) brushes

The frictional properties of poly(2-(methacryloyloxy)ethylphosphorylcholine) (PMPC) brushes grown from planar silicon surfaces by atom transfer radical polymerization (ATRP) have been characterized using in situ friction force microscopy (FFM). The dry thicknesses of the PMPC brushes ranged from 20 to 421 nm. For brush layers with dry thicknesses greater than ca. 100 nm, the coefficient of friction decreased with increasing film thickness. For shorter brushes, the coefficient of friction varied little with brush thickness. We hypothesize that the amount of bound solvent increases as the brush length increases, causing the osmotic pressure to increase and yielding a reduced tendency for the brush layer to deform under applied load. A comparison of the force-displacement plots acquired for various PMPC brushes under water supports this hypothesis, since a greater repulsive force is measured for thicker brushes. FFM was also used to investigate the well-known co-nonsolvency behavior exhibited by PMPC chains. For a PMPC brush layer of 307 nm dry thickness, the friction force was determined as a function of the volume fraction of alcohol in alcohol/water mixtures. Unlike a previous macroscopic study, a significant increase in the coefficient of friction was observed for ethanol/water mixtures at a volume fraction of 90%. This is attributed to brush collapse due to co-nonsolvency, leading to loss of hydration of the brush chains and hence substantially reduced lubrication. Force measurements normal to the surface indicate much greater hysteresis between approaching and retraction curves under co-nonsolvency conditions. However, no such effect was observed for 2-propanol/water and methanol/water mixtures over a wide range of volume fractions, in agreement with recent ellipsometric studies of PMPC brushes.

What Part do Adhesion and Deformation Play in Fine-Scale Static and Sliding Contact?

We describe experiments involving the static or sliding contact of a 'single asperity' and a flat surface. Plastic deformation and possibly junction growth can occur even at zero applied load, as a consequence of surface forces. The conditions for this to occur, and the different modes of separation of an adhesive contact, are clarified with the help of 'maps': use of the macroscopic concepts involved, such as yield stress and work of adhesion, is sometimes questionable and evidence of the importance of nanometre-scale stepped topography is presented. Strong adhesion between hard non-metals can required thermal activation. Sliding contacts at the sub-micrometre level have shown behaviour ranging from 'unstable ploughing' to adhesive peeling, but the frictional mechanisms involved are often unclear.

Molecular Tribometry: Recent Results and Future Prospects

The method of molecular tribometry is described. Ultrathin films of liquids are confined between parallel plates, usually of atomically smooth muscovite mica, such that the apparent area of contact equals the true area of contact. This paper consists of 5 sections. First, the phenomenon of solid lubrication by liquids is described. Second, the alternative viscous response of ultrathin liquid lubricants is described; the microviscosity is considerably enhanced over that of the isotropic liquid. Third, the critical shear strength is shown to have an extreme dependence on the normal pressure. Fourth, the materials requirements for molecular tribometry are discussed. Finally, a discussion is offered of the strengths and limitations of this new tool.

Measurement of Micromechanical Properties Using Atomic Force Microscope with Capacitative

A new UHV atomic force microscope for the study of micromechanical properties is described. A capacitance technique is used, which enables simultaneous measurement of forces perpendicular and parallel to the surface (i.e., load and friction), and has low noise down to frequencies below 0.1 Hz. Preliminary results for Ir- and W-tips sliding on graphite and silicon, respectively, demonstrate the capabilities of this new instrument.

Interfacial Adhesion: Theory and Experiment

Adhesion, the binding of different materials at an interface, is of general interest to many branches of technology, e.g., micro-electronics, tribology, manufacturing, construction, etc. However, there is a lack of fundamental understanding of such diverse interfaces. In addition, experimental techniques generally have practical objectives, such as the achievement of sufficient strength to sustain mechanical or thermal effects and/or have the proper electronic properties. In addition, the theoretical description of binding at interfaces is quite limited, and a proper data base for such theoretical analysis does not exist.

This presentation will review both experimental and theoretical aspects of adhesion in nonpolymer materials. The objective will be to delineate the critical parameters needed, governing adhesion testing along with an outline of testing objectives. A distinction will be made between practical and fundamental objectives. Examples will be given where interfacial bonding may govern experimental consideration. The present status of theory will be presented along with some recommendations for future progress and needs.

Direct Observation of Friction at the Atomic Scale

An atomic force microscope has been used to measure the frictional force on a tungsten tip sliding across the basal plane of graphite at low loads < 10-4 N. The frictional force displays the 2.5 Å periodicity of the graphite surface.

Microtribological Studies by Using Atomic Force and Friction Force Microscopy and its Applications

We have used atomic force microscopy (AFM) and friction force microscopy (FFM) techniques for microtribological studies including microscale friction, nanowear, nanoscratching and nanoindentation hardness measurements. The microscale friction studies on a gold ruler sample demonstrated that the local variation in friction correspond to a change of local surface slope, and this correlation is explained by a friction mechanism. Directionality effect is also observed as the sample was scanned in either direction. Nanoscratching, nanowear and nanoindentation hardness studies were performed on single-crystal silicon. Wear rates of single crystal silicon are approximately constant for various loads and test duration. Nanoindentation hardness studies show that AFM technique allows the hardness measurements of surface monolayers and ultra thin films in multilayered structures at very shallow depths and low loads. The AFM technique has also been shown to be useful for nanofabrication.

Molecular Dynamics Simulation of Mechanical Deformation of Ultra-Thin Metal and Ceramic Films

We present an overview of the molecular dynamics computer simulation method as employed in the study of the mechanical properties of surfaces at the nanometer scale. The embedded atom method is used to model a clean metal surface and the bond-order model is used to model ceramic surfaces. The computer experiment consists of the indentation and scraping of a hard diamond-like tool into and across the surface. Results are presented for the (111) surface of copper and silver and for the (100) surface of silicon. We explicitly demonstrate in our point indentation simulations that nanoscale plasticity in metals takes place by nondislocation mechanisms, a result suggested by recent nanoindentation experiments. We also observe the surface to accommodate nearly the entire volume of the tip and the annealing out of plastic work as the tip is removed. In our orthogonal cutting simulation, we observe an interesting phenomenon: the system dynamically reorients the grain in front of the tool tip to minimize the work performed on the shear plane (i.e. the shear plane becomes an easy slip plane). Silicon transforms into an amorphous state which then flows plastically.

The Relationship Between Near-Surface Mechanical Properties, Loading Rate And Surface Chemistry

The presence of thin surface films and adsorbate layers on both metals and ceramics can cause dramatic changes in the mechanical response of the material. A similar, related, variation in tribological properties has also been observed. Though the importance of surface effects is well known and widely documented, the exact physical and chemical mechanisms that are operating remain poorly understood. The development of point probe techniques now permits the examination of mechanical and tribological properties on the same length scale as the surface films. Recently, the utilization of these testing techniques has provided a clear insight into the mechanical processes which are operating on the atomic scale. The nanoindentation results presented here show that the mechanical deformation of an individual nano-contact is a highly dynamic phenomena in which the tip-momentum on contact, as well as the loading rate during the indentation, dictate the observed mechanical properties of the material. These results indicate that the initiation of plastic deformation is dependent on the stability of atomic-size surface asperities which can be deformed irreversibly by the high stresses generated during the initial contact. Additionally, the generation of dislocations and the presence of discontinuities in the loading curve are shown to depend upon the loading rate. More significantly, it has been found that modifying the surface chemistry can cause dramatic changes in both the mode of deformation and the time-dependence of nano-scale mechanical properties. The principal conclusion that can be drawn is that the high stresses which operate over short distances make time and temperature dependent phenomena, such as diffusion and the dissipation of energy via phonons, of vital importance in determining the near-surface mechanical properties of a material. Such effects are further magnified in tribological processes where normal and tangential loading of the surface leads to the repeated making and breaking of nano-asperity contacts.

The description of friction of silicon MEMS with surface roughness: virtues and limitations of a stochastic Prandtl-Tomlinson model and the simulation of vibration-induced friction reduction

We have replaced the periodic Prandtl-Tomlinson model with an atomic-scale friction model with a random roughness term describing the surface roughness of micro-electromechanical systems (MEMS) devices with sliding surfaces. This new model is shown to exhibit the same features as previously reported experimental MEMS friction loop data. The correlation function of the surface roughness is shown to play a critical role in the modelling. It is experimentally obtained by probing the sidewall surfaces of a MEMS device flipped upright in on-chip hinges with an AFM (atomic force microscope). The addition of a modulation term to the model allows us to also simulate the effect of vibration-induced friction reduction (normal-force modulation), as a function of both vibration amplitude and frequency. The results obtained agree very well with measurement data reported previously.

Nanotribological characterization of human head hair by friction force microscopy in dry atmosphere and aqueous environment

Friction force microscopy was employed for the tribological investigation of human head hair in two different environments: a dry atmosphere and de-ionized water. The fibers were immobilized by embedding them in indium. The effects of bleaching, conditioning, and immersion in methanolic KOH were quantified in terms of the relative coefficient of friction (μ). The virgin fibers were clearly distinguished in terms of friction coefficient from the chemically damaged ones in both environments, while all categories of hair exhibited higher friction coefficients in the aqueous environment. Secondary ion mass spectroscopy was used as a complementary technique to examine the presence of fatty acids on the cuticular surface of the different categories of hair as well as the conditioner distribution. Neither bleaching nor 30 min treatment in methanolic KOH was found adequate to completely remove the fatty acids from the fibers' surface. Conditioner species were detected along the whole cuticular surface.

Regulation of local structure and composition of binary disulfide and thiol self-assembled monolayers using nanografting

Nanografting is used to create spatial confinement, which enables regulation of self-assembly reaction pathways and outcome. The degree and outcome of this regulation is revealed using binary self-assembled monolayers (SAMs) of organothiols and disulfides. In naturally grown systems, these SAMs have more complex morphology when compared with corresponding binary alkanethiol SAMs. Taller molecules form nanodomains of ellipsoidal cap in shape. These domains arrange in various irregular geometries, including 1D worm-like and 2D branches. This observation differs from binary alkanethiol SAMs, where nanodomains are separated and randomly dispersed. During nanografting, more homogeneous morphology was observed compared with naturally grown layers. By varying nanoshaving speed, the nanodomain structure can be regulated from randomly dispersed to more heterogeneous and, finally, to near natural growth. This trend is very similar to mixed alkanethiol systems, where the domain size and separation increase with increasing speed. Different from the alkanethiol systems, the observed structural variations are due to the changes in surface composition, in addition to domain size, shape, and arrangement.

Nanofriction visualized in space and time by 4D electron microscopy

In this letter, we report a novel method of visualizing nanoscale friction in space and time using ultrafast electron microscopy (UEM). The methodology is demonstrated for a nanoscale movement of a single crystal beam on a thin amorphous membrane of silicon nitride. The movement results from the elongation of the crystal beam, which is initiated by a laser (clocking) pulse, and we examined two types of beams: those that are free of friction and the others which are fixed on the substrate. From observations of image change with time we are able to decipher the nature of microscopic friction at the solid-solid interface: smooth-sliding and periodic slip-stick friction. At the molecular and nanoscale level, and when a force parallel to the surface (expansion of the beam) is applied, the force of gravity as a (perpendicular) load cannot explain the observed friction. An additional effective load being 6 orders of magnitude larger than that due to gravity is attributed to Coulombic/van der Waals adhesion at the interface. For the case under study, metal-organic crystals, the gravitational force is on the order of piconewtons whereas the static friction force is 0.5 μN and dynamic friction is 0.4 μN; typical beam expansions are 50 nm/nJ for the free beam and 10 nm/nJ for the fixed beam. The method reported here should have applications for other materials, and for elucidating the origin of periodic and chaotic friction and their relevance to the efficacy of nano(micro)-scale devices.

Local nanoscale heating modulates single-asperity friction

We demonstrate measurement and control of single-asperity friction by using cantilever probes featuring an in situ solid-state heater. The heater temperature was varied between 25 and 650 °C (tip temperatures from 25 ± 2 to 120 ± 20 °C). Heating caused friction to increase by a factor of 4 in air at ∼ 30% relative humidity, but in dry nitrogen friction decreased by ∼ 40%. Higher velocity reduced friction in ambient with no effect in dry nitrogen. These trends are attributed to thermally assisted formation of capillary bridges between the tip and substrate in air, and thermally assisted sliding in dry nitrogen. Real-time friction measurements while modulating the tip temperature revealed an energy barrier for capillary condensation of 0.40 ± 0.04 eV but with slower kinetics compared to isothermal measurements that we attribute to the distinct thermal environment that occurs when heating in real time. Controlling the presence of this nanoscale capillary and the associated control of friction and adhesion offers new opportunities for tip-based nanomanufacturing.

AFM + Raman Microscopy + SNOM + Tip-Enhanced Raman: Instrumentation and Applications

Atomic Force Microscopy (AFM) has developed into a very powerful tool for characterization of surfaces and nanoscale objects. Many physical properties of an object can be studied by AFM with nanometer-scale resolution. Local stiffness, elasticity, conductivity, capacitance, magnetization, surface potential and work function, friction, piezo response—these and many other physical properties can be studied with over 30 AFM modes. What is typically lacking in information provided by AFM studies is the chemical composition of the sample and information about its crystal structure. To obtain this information other characterization techniques are required, such as Raman and fluorescence microscopy. The Raman effect (inelastic light scattering) provides extensive information about sample chemical composition, quality of crystal structure, crystal orientation, presence of impurities and defects, and so on. Information provided by Raman and fluorescence spectroscopy is complementary to the information obtained by AFM. So it is a natural requirement in many research fields to integrate these techniques in one piece of equipment—to provide comprehensive physical, chemical, and structural characterization of the same object. Of course, for routine studies of various samples, it is important to be able to obtain AFM and Raman/fluorescence images of exactly the same sample area, preferably with the same sample scan.

Friction anisotropy of the surface of organic crystals and its impact on scanning force microscopy

The transverse component of the friction forces acting on the tip of an atomic force microscope scanning on the surface of an organic crystal was monitored as a function of the scan direction. The relation between friction and the crystallographic system is disclosed, revealing that the symmetry of the friction phenomenon is dictated by the direction of the prominent corrugations of the crystal surface. It is also illustrated that molecular-resolution images can be collected through the monitoring of the motion of the tip in a transverse direction with respect to the scan direction.

Effect of fractal disorder on static friction in the Tomlinson model

We propose a modified version of the Tomlinson model for static friction between two chains of beads. We introduce disorder in terms of vacancies in the chain, and distribute the remaining beads in a scale invariant way. For this we utilize a generalized random Cantor set. We relate the static friction force to the overlap distribution of the chains, and discuss how the distribution of the static friction force depends on the distribution of the remaining beads. For the random Cantor set we find a scaled distribution which is independent on the generation of the set.

Sliding friction of N2 on Pb(111)

Molecular dynamics simulations of the sliding friction between two thick solid slabs are performed. The upper body is formed of light N(2) particles and the substrate of heavy Pb atoms. Among the various mechanisms that are responsible for the friction, we consider the phonon-phonon interaction between the two blocks. To provide evidence of the phonon interaction, we compare two different systems. For the first we consider the substrate as formed of atoms fixed in the equilibrium (111) positions. In the second system the Pb atoms can be displaced from the ideal positions, under their mutual interactions. A comparison with recently obtained experimental data will be discussed.

High-resolution friction force microscopy under electrochemical control

We report the design and development of a friction force microscope for high-resolution studies in electrochemical environments. The design choices are motivated by the experimental requirements of atomic-scale friction measurements in liquids. The noise of the system is analyzed based on a methodology for the quantification of all the noise sources. The quantitative contribution of each noise source is analyzed in a series of lateral force measurements. Normal force detection is demonstrated in a study of the solvation potential in a confined liquid, octamethylcyclotetrasiloxane. The limitations of the timing resolution of the instrument are discussed in the context of an atomic stick-slip measurement. The instrument is capable of studying the atomic friction contrast between a bare Au(111) surface and a copper monolayer deposited at underpotential conditions in perchloric acid.

Three-dimensional atomic force microscopy - taking surface imaging to the next level

Materials properties are ultimately determined by the nature of the interactions between the atoms that form the material. On surfaces, the site-specific spatial distribution of force and energy fields governs the phenomena encountered. This article reviews recent progress in the development of a measurement mode called three-dimensional atomic force microscopy (3D-AFM) that allows the dense, three-dimensional mapping of these surface fields with atomic resolution. Based on noncontact atomic force microscopy, 3D-AFM is able to provide more detailed information on surface properties than ever before, thanks to the simultaneous multi-channel acquisition of complementary spatial data such as local energy dissipation and tunneling currents. By illustrating the results of experiments performed on graphite and pentacene, we explain how 3D-AFM data acquisition works, what challenges have to be addressed in its realization, and what type of data can be extracted from the experiments. Finally, a multitude of potential applications are discussed, with special emphasis on chemical imaging, heterogeneous catalysis, and nanotribology.

Imaging powders with the atomic force microscope: from biominerals to commercial materials

Atomically resolved images of pressed powder samples have been obtained with the atomic force microscope (AFM). The technique was successful in resolving the particle, domain, and atomic structure of pismo clam (Tivela stultorum) and sea urchin (Strongylocentrotus purpuratus) shells and of commercially available calcium carbonate (CaCO(3)) and strontium carbonate (SrCO(3)) powders. Grinding and subsequent pressing of the shells did not destroy the microstructure of these materials. The atomic-resolution imaging capabilities of AFM can be applied to polycrystalline samples by means of pressing powders with a grain size as small as 50 micrometers. These results illustrate that the AFM is a promising tool for material science and the study of biomineralization.

Direct visualization of the enzymatic digestion of a single fiber of native cellulose in an aqueous environment by atomic force microscopy

Atomic force microscopy (AFM) was used to study native cellulose films prepared from a bacterial cellulose source, Acetobacter xylinum, using a novel application of the Langmuir-Blodgett technique. These films allowed high-resolution AFM images of single fibers and their microfibril structure to be obtained. Two types of experiments were performed. First, the fibers were characterized using samples that were dried after LB deposition. Next, novel protocols that allowed us to image single fibers of cellulose in films that were never dried were developed. This procedure allowed us to perform in situ AFM imaging studies of the enzymatic hydrolysis of single cellulose fibers in solution using cellulolytic enzymes. The in situ degradation of cellulose fibers was monitored over a 9 h period using AFM. These studies provided the first direct, real-time images of the enzymatic degradation of a single cellulose fiber. We have demonstrated the tremendous potential of AFM to study the mechanism of the enzymatic digestion of cellulose and to identify the most effective enzymes for the digestion of various cellulose structures or isomorphs.

Distinguishing elastic shear deformation from friction on the surfaces of molecular crystals

Elastic deformation on the surfaces of molecular crystals can be imaged using a variant of lateral force microscopy in which the tip is scanned parallel to the cantilever axis. The shear force transverse to this direction has a distinctly different origin than the friction force as determined by the tip velocity and temperature dependence of the cantilever torque. An elastic deformation model for the tip-sample interaction predicts the crystallographic anisotropy of the transverse shear contrast, establishing its connection with the relative magnitude of the in-plane elastic tensor components.

Lattice-resolved frictional pattern probed by tailored carbon nanotubes

In this study, we demonstrate a high-resolution friction profiling technique using synchronous atomic/lateral force microscopy (AFM/LFM). The atomic resolution is achieved by our special carbon nanotube (CNT) probes made via in situ tailoring and manipulation inside an ultra-high vacuum transmission electron microscope (UHV TEM). The frictional pattern mapped on graphite displays a periodic distribution similar to the atomic (0001)-oriented graphite lattice structure. Furthermore, the electrothermal process in the UHV TEM renders a graphite-capped CNT tip, which delivers the nanotribology study within two graphite layers by the LFM measurement on graphite. The synchronous AFM and LFM images can discern a spatial shift between the atomic points and local friction maxima. We further interpret this shift as caused by the lattice distortion, which in turn induces irreversible energy dissipation. We believe this is the origin of atomic friction on the sub-nanonewton scale.

Intermittent contact scanning nonlinear dielectric microscopy

Intermittent contact scanning nonlinear dielectric microscopy (IC-SNDM) was developed as a novel technique for surface topography measurements and observation of domain structures. Domain structures on ferroelectric single crystals were observed with nanoscale resolution using IC-SNDM. The reproducibility of measurements was improved in comparison to a conventional SNDM operated under contact mode, because the tip and/or sample damage are reduced when using intermittent contact mode. The minimum loading force of the probe to provide basic performance was experimentally determined for IC-SNDM.

Friction dependence on growth conditions in epitaxial films

In investigating the growth kinetics of epitaxial films in situ by force microscopy, we have observed several instances where the lateral force contrast on the growing monolayer exhibits a strong dependence on the driving force for growth (i.e., solute concentration). We present results for three epitaxial growth systems in aqueous solutions: CaSO(3) on CaCO(3), PbSO(4) on BaSO(4), and BaSO(3) on BaSO(4). In each system, material grown at higher solute concentrations exhibits a friction higher than that of material grown at lower concentrations. These observations suggest a link between defect density and friction contrast in growing epitaxial films. An additional time-dependent behavior is observed in the CaSO(3)/CaCO(3) system, indicating an annealing process.

Quantitative kinetic measurements of the esterification of self-assembled monolayers of mercaptoundecanol by trifluoroacetic anhydride using friction force microscopy

Central to the advancement of friction force microscopy (FFM) as a quantitative surface analysis technique is the establishment of clear relationships between the organization and composition of surfaces and data acquired from FFM. As a step toward this goal, FFM has been applied to measurement of the kinetics of a surface-confined reaction, namely, the esterification of alcohol-terminated self-assembled monolayers (SAMs) by trifluoroacetic anhydride. The kinetics were studied initially using contact angle goniometry. The data obtained were found to be well described by a Langmuir adsorption isotherm. The progress of the reaction was also monitored by X-ray photoelectron spectroscopy, and comparative studies carried out using FFM. By using a simple numerical analysis of the FFM data, it has been possible to obtain quantitative information on surface composition. The resulting measurements of the fraction of derivatized molecules have been compared with compositional data obtained from XPS and contact angle goniometry, and found to be in excellent agreement, establishing unequivocally the capability of FFM as a quantitative surface analysis technique.

Comparison of frictional forces on graphene and graphite

We report on the frictional force between an SiN tip and graphene/graphite surfaces using lateral force microscopy. The cantilever we have used was made of an SiN membrane and has a low stiffness of 0.006 N m(-1). We prepared graphene flakes on a Si wafer covered with silicon oxides. The frictional force on graphene was smaller than that on the Si oxide and larger than that on graphite (multilayer of graphene). Force spectroscopy was also employed to study the van der Waals force between the graphene and the tip. Judging that the van der Waals force was also in graphite-graphene-silicon oxide order, the friction is suspected to be related to the van der Waals interactions. As the normal force acting on the surface was much weaker than the attractive force, such as the van der Waals force, the friction was independent of the normal force strength. The velocity dependency of the friction showed a logarithmic behavior which was attributed to the thermally activated stick-slip effect.

The formation of carbon nanostructures by in situ TEM mechanical nanoscale fatigue and fracture of carbon thin films

A technique to quantify in real time the microstructural changes occurring during mechanical nanoscale fatigue of ultrathin surface coatings has been developed. Cyclic nanoscale loading, with amplitudes less than 100 nm, is achieved with a mechanical probe miniaturized to fit inside a transmission electron microscope (TEM). The TEM tribological probe can be used for nanofriction and nanofatigue testing, with 3D control of the loading direction and simultaneous TEM imaging of the nano-objects. It is demonstrated that fracture of 10-20 nm thick amorphous carbon films on sharp gold asperities, by a single nanoscale shear impact, results in the formation of <10 nm diameter amorphous carbon filaments. Failure of the same carbon films after cyclic nanofatigue, however, results in the formation of carbon nanostructures with a significant degree of graphitic ordering, including a carbon onion.

Data acquisition and analysis procedures for high-resolution atomic force microscopy in three dimensions

Data acquisition and analysis procedures for noncontact atomic force microscopy that allow the recording of dense three-dimensional (3D) surface force and energy fields with atomic resolution are presented. The main obstacles for producing high-quality 3D force maps are long acquisition times that lead to data sets being distorted by drift, and tip changes. Both problems are reduced but not eliminated by low-temperature operation. The procedures presented here employ an image-by-image data acquisition scheme that cuts measurement times by avoiding repeated recording of redundant information, while allowing post-acquisition drift correction. All steps are detailed with the example of measurements performed on highly oriented pyrolytic graphite in ultrahigh vacuum at a temperature of 6 K. The area covered spans several unit cells laterally and vertically from the attractive region to where no force could be measured. The resulting fine data mesh maps piconewton forces with <7 pm lateral and<2 pm vertical resolution. From this 3D data set, two-dimensional cuts along any plane can be plotted. Cuts in a plane parallel to the sample surface show atomic resolution, while cuts along the surface normal visualize how the attractive atomic force fields extend into vacuum. At the same time, maps of the tip-sample potential energy, the lateral tip-sample forces, and the energy dissipated during cantilever oscillation can be produced with identical resolution.

Theoretical investigation of the anticorrugation effects on the tribological properties of the Xe/Cu interface

We present a molecular dynamics study of the slip time and static friction for a slab of Xe deposited on a slab of Cu. To put in evidence the role played by the phonon field of the two blocks, we compare results obtained with a substrate formed by fixed atoms with one formed by mobile atoms. In the last case the scattering between Xe and Cu mobile atoms is inelastic and there is an exchange of momentum and energy between the two blocks which produces disorder in the interface plane. This disorder favors a decrease of the static friction and a consequent increase of the slip time. We describe the interaction between Xe and Cu with a phenomenological multi-ion potential which gives rise to an anticorrugation of the charge distribution and reproduces very well the ab initio density functional calculations. Our model potential is a linear superposition of a corrugating potential and an anticorrugating one. For this reason we can study the static friction by passing from an anticorrugated to a fully corrugated system. We also investigate the slip time and we compare our results with recent experimental data measured with the quartz crystal microbalance technique.