Friction and energy dissipation mechanisms in adsorbed molecules ...

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Advances in Physics 203Downloaded by [North Carolina State University] at 09:24 22 April 2013in contact-mode images is now attributed to the effect of atomic lattice stick–slip motion, whichis ever-present in AFM experimental studies [264] and intrinsically reflects the stiffness of thedevice. It also represents a major departure from the viscous friction law (Equation (2)) thatgoverns the films adsorbed in the unconfined geometries discussed so far. Static friction andstick–slip phenomenon are ubiquitous for the AFM geometry, and in the vast majority of cases,one stick–slip event is observed per unit cell of the substrate. This is true even in cases where theatomic cell contains more than one species. Friction levels and stick–slip phenomena thus exhibitvery strong dependencies on the direction that the tip moves with respect to the crystalline axesof the substrate (Figure 30). At the macroscopic scale, macroscopic contacts between solids, ingeneral, obey Amontons law, Equation (1), and exhibit friction that is independent of velocity.For AFM contact sizes in the nanometer range, friction has been reported to increase, decrease,or be independent of sliding velocity. A logarithmic increase of friction with scanning velocitywas reported, for example, [265], for polymer layers grafted on silica over a range of velocitiesand with four different probes. A thermally activated Eyring model was employed to analyzethe data, and estimates of the interfacial shear stresses and energetic barriers to sliding wereobtained. While some of the approximations that were used in the analysis lead to large variationsin calculated contact areas, the fits for the shear stresses and energy barriers were consistentbetween the different probes. Shear stresses were found to lie mostly between 200 and 300 MPafor bare silica and between 400 and 600 MPa for the grafted layers, with barrier heights in therange 1–2 × 10 −19 J for both systems.AFM studies of diamond, graphite, and amorphous carbon were reported to exhibit no dependenceon velocity [266] so long as the scanning velocity was much lower than a characteristic slipvelocity of the tip. Consequently, no velocity dependence of friction was observed for scanningspeeds of up to about 10 μm s −1 . A logarithmic dependence was observed for higher velocitiesconsistent with the result of Bouhacina et al. A different trend was reported by Bennewitz et al.Figure 30. (color online) Friction anisotropy in decagonal quasicrystal [416]. Friction levels in periodicdirections are eight times higher than in aperiodic directions. Reprinted figure with permission from J.Y.Park et al., Physical Review B 74, p. 024203, 2006 [416]. Copyright (2006) by the American PhysicalSociety.
204 J. KrimDownloaded by [North Carolina State University] at 09:24 22 April 2013on Cu(111) [267] and Gnecco et al. on NaCl(100) [268], who observed a logarithmic dependenceof friction on scanning velocity for velocities less than about 1 μms −1 . A thermally activated slipmodel and a modified Tomlinson model, respectively, were used to explain the trends observed inthe data sets. A very similar model was employed by Overney and coworkers [269] to explain thelogarithmic dependence of friction for n-hexadecane and octamethylcycloterasiloxane (OMCTS)physisorbed on silicon oxide wafers. In this case, the authors reported that interfacial liquidstructuring reduced the sliding friction and that the molecular topology ultimately dictated thelubricating properties of the boundary layer. Cannara et al. [36] meanwhile invoked the sametheory employed for phononic friction in sliding adsorbed layers to interpret AFM data recordedon surfaces of hydrogen and deuterium isotopes [37], and reinterpreted the result in terms of thespacing of the hydrogen or deuterium and not necessarily the difference in phonon frequency.Overall, it is becoming clear that the velocity and temperature dependence is system-dependent[76], and there is a competition between thermal, adhesive, phononic, and electronic effects whichultimately determines the velocity dependence. This topic is discussed in more detail in Sections 3and 4.With respect to adsorbed films, in particular, Buzio et al. have reported a detailed study of theform of the friction laws that govern lubricatedAFM contacts, particularly, those lubricated by slidingadsorbed layers of physisorbed OMCTS and squalane, which are common materials for studyby alternative techniques, and may be comparable to QCM studies of physisorbed layers [270]. Thestudies were performed in controlled conditions, and the friction laws for nanoasperities sliding onatomically flat substrates under controlled atmosphere and liquid environment, respectively, wereinvestigated. The group reported a power-law relation between the friction force and normal loadfor measurements performed in dry air, and a linear relationship, that is, Amontons’ law (Equation(1)) , for junctions fully immersed in the model lubricants studied (OMCTS and squalane). Theyobserved that the lubricated contacts display a significant reduction in friction levels, with liquidandsubstrate-specific friction coefficients. They compared the results to MD simulations andfound evidence that load-bearing boundary layers at the asperity junction entrance may cause theappearance of Amontons’ law and impart atomic-scale character to the sliding process. Finally,they suggest a first attempt at the general criteria that may lead to nanoscale lubrication due to thepresence of mobile adsorbed boundary layers.The presence of mobile adlayers has been observed to produce Amontons’ law behavior inAFM data. Mobile adlayers have also been implicated as giving rise to the atomic-scale originsof static friction [271]. As such, since mobile adlayers are present on virtually all macroscopicsurfaces, their importance to date may have been underestimated in macroscopic studies.2.5.3. STM–QCM and related geometriesA QCM surface electrode can easily reach speeds of cm s −1 to 1–2 m s −1 , and its amplitudeof oscillation can be measured directly by means of STM [63]. The product of the oscillationamplitude with the angular frequency of the QCM yields the maximum velocity amplitude ofthe electrode–tip contact, and high-speed sliding friction and high strain rate studies between asingle nanoasperity contact can be performed by lowering a tip into sliding contact with the QCMelectrode [272].STM–QCM has been employed to study diffusion in adsorbed monolayers, by examining thequality of the images when the oscillator is moving and stationary, and has been employed to probewhether the asperity contact regime is liquid- or solid-like, depending on the sign of the frequencyshift when the tip makes contact with the surface [273]. The technique has a great potential forfuture studies of contact melting and/or the lubricating properties of mobile adsorbed layers inhigh-speed asperity contacts.
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256 J. KrimTable 3. QCM slip time d
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258 J. KrimTable 5. QCM slip time d
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260 J. KrimTable 6. Slip time data
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282 J. Krimand friction forYBCO (T
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