Generation of defects in model lubricant monolayers and their contribution to energy dissipation in friction

The structural, mechanical (friction) and spectroscopic properties of model lubricant films made of self-assembled and Langmuir–Blodgett monolayers on quartz, mica and gold have been investigated with atomic force microscopy, the surface forces apparatus and sum-frequency generation. In these films, the molecules tend to form densely packed structures, with the alkane chains mostly vertical and parallel to each other. The SFG results suggest that under moderate pressures of a few tens of MPa, the methyl end group of the alkane chains is rotated to accommodate a terminal gauche distortion. The molecule, however, retains its upright close-packed structure with a lattice periodicity when ordered, which can be resolved by AFM. At pressures above 0.1 GPa, changes in the form of collective molecular tilts take place that lower the height of the monolayer. Only certain angles of tilt are allowed that are explained by the interlocking of methylene units in neighboring chains. The discrete angular tilts are accompanied by increases in friction. A model based on the van der Waals attractive energy between chains is used to explain the stability of the films and to estimate the cohesive energy changes during tilt and, from that, the increases in friction force.     

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.     

Characterization of mixed self-assembled monolayers for immobilization of streptavidin using chemical force microscopy

Mixed self-assembled monolayers (SAMs) to immobilize streptavidin on a gold surface were investigated by measuring the pull-off force between an AFM tip and a biotin-modified surface using CFM. Biotin-LC-NHS was modified on SAMs prepared from a mixed solution of cystamine and MEOH. Increased pull-off forces between the AFM tip and the surface were observed with an increased cystamine mole fraction in the solution. Streptavidin was immobilized onto biotin-LC-NHS modified mixed SAMs and analyzed by tapping AFM. Also, the formation of mixed SAMs containing MUOH and MBDA was confirmed using CFM. The measured pull-off forces on the only MBDA surface were larger than those on the surface with MUOH. These results can be applied to determine an optimal mixing ratio of MUOH and MBDA SAMs that reduces non-specific streptavidin binding onto a surface.     

Tribological properties of asperity arrays coated with self-assembled monolayers

The effects of a self-assembled monolayer (SAM) coating on the friction and pull-off forces were determined by using two-dimensional asperity arrays on silicon wafers. The arrays were coated with SAM composed of one of five different alkylchlorsilanes. First, two-dimensional asperity arrays were created by using a focussed ion beam (FIB) system to mill patterns on silicon plates. Each silicon plate had different patterns of equally spaced asperities. Each pattern (5 × 5 μm²) had a different radius of curvature of the asperity peaks, ranging from about 200 to 2500 nm. Then, each silicon plate was immersed in a solution of a different alkylchlorsilane in hexane (either hexyltrichlorosilane, octyltrichlorosilane, dodecyltrichlorosilane, tetradecyltrichlorosilane, or octadecyltrichlorosilane), thus coating the asperity arrays with SAM. The friction and pull-off forces on the SAM-coated arrays were measured by using an atomic force microscope (AFM) that had a square flat probe. The pull-off force for SAM-coated silicon was roughly proportional to the radius of curvature of the asperity peaks. The magnitude of the pull-off force corresponded approximately to the capillary force calculated by using the contact angle of water on the surface of SAM. The friction coefficient correlated with the inverse of the alkyl-chain length of the SAM.     

Atomic friction studies on well-defined surfaces

Atomic friction studies have been performed by means of a friction force microscope (FFM) in ultrahigh vacuum, where well-defined surfaces can be prepared. A home-built FFM allows us to study lateral forces as low as 0.05 nN using rectangular silicon cantilevers. Furthermore, comparison with dissipation measurements performed in non-contact mode are possible. Recent experimental results are presented and discussed in the framework of a one-dimensional Tomlinson model which includes thermal activation. Atomic-scale stick–slip processes on a metallic surface could be repeatedly measured on Cu(111), while the Cu(100) surface was distorted by the tip during the scanning process. A logarithmic velocity dependence of atomic friction has been measured on Cu(111) and NaCl(100) for low scanning velocities. The dissipation found in stick–slip measurements is compared to the power loss detected in dynamic non-contact measurement.     

Shear-induced mechanochromism in polydiacetylene monolayers

We use atomic force microscopy to actuate and characterize the nanoscale mechanochromism of polydiacetylene monolayers on atomically-flat silicon oxide substrates. We find explicit evidence that the irreversible blue-to-red transformation is caused by shear forces exerted normal to the polydiacetylene polymer backbone. The anisotropic probe-induced transformation is characterized by a significant change in the tilt orientation of the side chains with respect to the surface normal. We discuss preliminary molecular dynamics simulations and electronic structure calculations on twelve-unit polydiacetylene oligomers that allow us to correlate the transformation with bond-angle changes in the conjugated polymer backbone.     

Simulations of friction anisotropy on ordered organic monolayer

A method of molecular dynamics is used to investigate friction anisotropy observed on a hexagonally packed organic monolayer of straight-chain molecules, which tilt in a specific direction. A rigid gold slider with a single atomic protuberance is used as a model of a typical atomic force microscope tip apex. The friction anisotropy is observed at 50 K, which is below the melting point of rotation around a long axis of the molecule. The anisotropic frictional behavior is that sliding in directions normal to the direction of the collective tilt of the molecules results in the maximum friction force. The origin of the anisotropy is attributed to anisotropy in lateral compliance in the monolayer.     

Dynamic friction measurements with an atomic force microscope on polymer surfaces

The combination of scanning friction force microscopy (SFFM) and lock‐in techniques leads to dynamic SFFM (DSFFM) and provides great advantages in friction force studies with sub‐micrometre resolution. In this paper are presented measurements on thin adsorbed organic films on polymers (polymer blend of 75% poly(allylaminehydrochloride) (PAA) and 25% poly(diallyl‐dimethylammonium chloride) (PDDAC)) and on mica (as a reference). The amplitude and phase response as a function of the excitation amplitude can be explained on hard surfaces by a simple static and dynamic friction model. This model allows us further to distinguish static friction forces and kinetic friction forces in a quantitative way. Furthermore, we demonstrate the use of these spectra to determine the correct modulation amplitude of the excitation to achieve the optimal friction contrasts directly. Polymer data suggest that the viscoelastic shear flow under the atomic force microscope (AFM) tip is responsible for the shape of the phase and amplitude spectrum. Lastly, we demonstrate that DSFFM is a useful technique for surface characterisation in situations where SFFM may not be adequate.     

Microscopic Studies of Friction and Wear at the Benzotriazole/Copper Interface

Interfacial friction and topographic changes at copper surfaces have been measured in situ with AFM in nitric acid and in acidic and neutral solutions of benzotriazole (BTA), a known corrosion inhibitor. In addition, changes in the thickness of the copper film have been measured ex situ using stylus profilometry as a function of solution treatment. These measurements demonstrate isotropic dissolution in 0.10 M HNO₃ solutions and little change in neutral solutions of BTA. However in acidified solutions of BTA, the formation of a substantial reaction overlayer, the presence of higher interfacial friction, and the tip-mediated, localized dissolution of copper interface is observed. These measurements indicate that BTA plays a multifunctional role under acidic conditions. In the absence of mechanical action, BTA acts to passivate the surface from isotropic etching under acidic conditions. In regions of interfacial contact, the dissolution of a BTA-Cu reaction layer leads to the localized removal of copper through a tip-mediated process.     

Immobilization of iron storage protein on a gold electrode based on self-assembled monolayers

Ferritin is a globular protein consisting of 24 subunits to form a hollow shell and is capable of storing iron in the cavity. Findings that the naturally existing iron core of ferritin can be readily extracted and replaced with a variety of electroactive materials make ferritin suitable for biosensor and biofuel cell applications. The immobilization of ferritin on the electrode surface is essential for various bioelectronic applications. In this work, based on self-assembled monolayers, ferritin was immobilized on a gold electrode through two different methods: chemisorption of thiolated ferritin onto bare gold electrodes and covalent binding of ferritin to succinimidyl alkanedisulfide-modified Au electrodes. Effects of experimental conditions on the ferritin immobilization were investigated. The ferritin immobilized on the gold electrode was characterized by atomic force microscopy and cyclic voltammetry.     

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 . 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.     

Adhesion and friction properties of hydrogenated amorphous carbon films measured by atomic force microscopy

Atomic force microscopy has been used to measure adhesion and friction forces at the interface between an oxidized metal probe tip and amorphous carbon films of varying hydrogen contents (12.3–39.0 atomic percent hydrogen). The interface of an oxide surface and a hard carbon coating models the unlubricated head-disk interface of current hard disk products. Adhesion forces normalized by the radius of curvature of the contacting tip range from 1.09 to 8.53 N/m. Coefficients of friction values, measured as the slope of the friction versus load plot, range from 0.33 to 0.87. A trend of increasing adhesion forces and coefficients of friction is observed for increasing hydrogen content in the films. We attribute the increase in adhesion and friction to increases in the surface free energy of the carbon films with the incorporation of hydrogen.     

Large friction anisotropy of a polydiacetylene monolayer

Friction force microscopy measurements of a polydiacetylene monolayer film reveal a 300% friction anisotropy that is correlated with the film structure. The film consists of a monolayer of the red form of N‐(2‐ethanol)‐10,12‐pentacosadiynamide, prepared on a Langmuir trough and deposited on a mica substrate. As confirmed by atomic force microscopy and fluorescence microscopy, the monolayer consists of domains of linearly oriented conjugated backbones with pendant hydrocarbon side chains above and below the backbones. Maximum friction occurs when the sliding direction is perpendicular to the backbones. We propose that this effect is due to anisotropic film stiffness, which is a result of anisotropic side chain packing and/or anisotropic stiffness of the backbone itself. Friction anisotropy is therefore a sensitive, optically‐independent indicator of polymer backbone direction and monolayer structural properties. This revised version was published online in September 2006 with corrections to the Cover Date.     

Observation of proportionality between friction and contact area at the nanometer scale

The nanotribological properties of a hydrogen‐terminated diamond(111)/tungsten‐carbide interface have been studied using ultra‐high vacuum atomic force microscopy. Both friction and local contact conductance were measured as a function of applied load. The contact conductance experiments provide a direct and independent way of determining the contact area between the conductive tungsten‐carbide AFM tip and the doped diamond sample. We demonstrate that the friction force is directly proportional to the real area of contact at the nanometer‐scale. Furthermore, the relation between the contact area and load for this extremely hard heterocontact is found to be in excellent agreement with the Derjaguin–Müller–Toporov continuum mechanics model. This revised version was published online in September 2006 with corrections to the Cover Date.     

Frictional dynamics of perfluorinated self-assembled monolayers on amorphous SiO<Subscript>2</Subscript>

Silicon micromachines in microelectromechanical systems (MEMS) are coated with self-assembled monolayers (SAMs) in order to reduce the wear and stiction that are commonplace during operation. Recently, perfluorinated SAMs have been the focus of attention because they have better processing properties than hydrocarbon SAMs. In this study, we perform molecular dynamics simulations that model adhesive contact and friction for perfluorinated alkylsilane (Si(OH)₃(CF₂)₁₀CF₃) self-assembled monolayers (SAMs), which are commonly used in MEMS devices. Amorphous silica is used as the substrate for the SAMs in the simulations. The frictional behavior is investigated as a function of applied pressure (50 MPa–1 GPa) for a shear velocity of 2 m/s and compared to recent simulation results of hydrocarbon alkylsilane SAMs. The microscopic friction coefficient for the perfluorinated SAMs is the same as was measured for the hydrocarbon SAMs, but the shear stress is slightly larger than in the case of the hydrocarbon SAMs on amorphous silica.     

Quantitative secondary ion mass spectrometry imaging of self-assembled monolayer films for electron beam dose mapping in the environmental scanning electron microscope

Fluorinated alkanethiol self-assembled monolayers (SAM) films immobilized on gold substrates have been used as electron-sensitive resists to map quantitatively the spatial distribution of the primary electronbeam scattering in an environmental scanning electron microscope (ESEM). In this procedure, a series of electron dose standards are prepared by exposing a SAM film to electron bombardment in well-defined regions at different levels of electron dose. Microbeam secondary ion mass spectrometry (SIMS) using Cs⁺ bombardment is then used to image the F^- secondary ion signal from these areas. From the reduction in F^- intensity as a function of increasing electron dose, a calibration curve is generated that allows conversion of secondary ion signal to electron dose on a pixel-by-pixel basis. Using this calibration, electron dose images can be prepared that quantitatively map the electron scattering distribution in the ESEM with micrometer spatial resolution. The SIMS imaging technique may also be used to explore other aspects of electron-surface interactions in the ESEM.     

Macro- and nanotribological properties of organosilane monolayers prepared by a chemical vapor adsorption method on silicon substrates

Organosilane monolayers are novel ultrathin films used to control the physicochemical properties, such as friction and wear, of solid surfaces. In this study, the authors prepared alkylsilane and fluoroalkylsilane monolayers with a series of chain lengths by a chemical vapor adsorption method. The monolayers tribological properties were investigated by lateral force microscope (LFM) and friction tester. LFM nanoscale measurements of tribological properties showed that alkylsilane monolayer gave lower lateral force than the Si substrate surface. The lateral force decreased as the length of the alkyl chain increased. On the macroscale, friction test revealed that the organosilane monolayers gave lower dynamic friction coefficients than the Si substrate surface in air at room temperature. The longer the alkyl chain, the greater the wear resistance of the organosilane monolayers. Friction experiments using tetradecane as a lubricant showed better tribological properties than were obtained in air. Furthermore, microscopically line-patterned two-component organosilane monolayers were prepared and their macroscopic friction behavior was investigated. Even though the height difference between the two-components was less than 1 nm, friction force anisotropy between the parallel and perpendicular directions against the line pattern was observed.     

Tribological Properties of Self-Assembled Monolayers and Their Substrates Under Various Humid Environments

Using friction force microscopy (FFM) under controlled environments, we have systematically investigated the humidity effect on the frictional properties of two important classes of self-assembled monolayers (SAMs), i.e., N-octadecyltrimethoxysilane (OTE, CH₃(CH₂)₁₇Si(OCH₃)₃) on SiO₂(OTE/SiO₂), and N-alkanethiols on Au(111), together with their respective substrates. Experimental results show that both OTE and alkylthiol SAMs can decrease the friction force between a Si₃N₄ atomic force microscope (AFM) tip and substrates. The nearly humidity-independent friction of the two kinds of SAMs indicates that these SAMs are ideal lubricants in applications of micro-electro-mechanical systems (MEMS) under different environments. The humidity dependence—as the humidity increases, the friction first increases and then decreases—of the two substrates, SiO₂ and Au(111), can be explained by the adsorption of water. The decrease in the friction at high humidity is attributed to the low viscosity in the multilayers of water, while the increase in the friction at low humidity can be explained by the high viscosity between the water monolayer and the surfaces (AFM tip and sample), possibly due to the confinement effects. The effect of modification of the AFM tip with alkanethiol molecules on the humidity dependence of Au(111) friction has also been investigated.     

On the electrical properties of dislocations in ZnS using electric force microscopy

A combination of electric force microscopy (EFM) and noncontact atomic force microscopy (AFM) was used to study microscratching-induced dislocations in sphaleritic ZnS single crystals. Dislocation bands predominantly consisting of either anion-type (S) or cation-type (Zn) dislocations were induced by scratching along either [111] or [111] on a (110) surface. A significant difference of local distortions in electrical potential between the S(g) and Zn(g) dislocation bands was observed from the EFM images. Electric charges of these dislocations were determined quantitatively and the results were compared with theoretical models.     

Frictional effects in atomic force microscopy of Langmuir-Blodgett films

Frictional effects in atomic force microscopy (AFM) of Langmuir-Blodgett films of 1, 2-dipalmitoyl-snglycero-phosphoglycerol were examined. Height measurements of the Langmuir layers are strongly influenced by the orientation of the cantilevers used in AFM relative to the sample. A simple model is used to describe the frictional effects and to calculate the real height of the monolayers.