Molecular-scale tribology of amorphous carbon coatings: effects of film thickness,adhesion, and long-range interactions

Classical molecular dynamics simulations have been conducted to investigate the atomic-scale friction and wear when hydrogen-terminated diamond (111) counterfaces are in sliding contact with diamond (111) surfaces coated with amorphous, hydrogen-free carbon films. Two films, with approximately the same ratio of sp(3)-to-sp(2) carbon, but different thicknesses, have been examined. Both systems give a similar average friction in the load range examined. Above a critical load, a series of tribochemical reactions occur resulting in a significant restructuring of the film. This restructuring is analogous to the "run-in" observed in macroscopic friction experiments and reduces the friction. The contribution of adhesion between the probe (counterface) and the sample to friction was examined by varying the saturation of the counterface. Decreasing the degree of counterface saturation, by reducing the hydrogen termination, increases the friction. Finally, the contribution of long-range interactions to friction was examined by using two potential energy functions that differ only in their long-range forces to examine friction in the same system.     

Nanotribology of octadecyltrichlorosilane monolayers and silicon: self-mated versus unmated interfaces and local packing density effects

We use atomic force microscopy (AFM) to determine the frictional properties of nanoscale single-asperity contacts involving octadecyltrichlorosilane (OTS) monolayers and silicon. Quantitative AFM measurements in the wearless regime are performed using both uncoated and OTS-coated silicon AFM tips in contact with both uncoated and OTS-coated silicon surfaces, providing four pairs of either self-mated or unmated interfaces. Striking differences in the frictional responses of the four pairs of interfaces are found. First, lower friction occurs with OTS present on either the tip or substrate, and friction is yet lower when OTS is present on both. Second, the shape of the friction versus load plot strongly depends on whether the substrate is coated with OTS, regardless of whether the tip is coated. Uncoated substrates exhibit the common sublinear dependence, consistent with friction being directly proportional to the area of contact. However, coated substrates exhibit an unusual superlinear dependence. These results can be explained qualitatively by invoking molecular plowing as a significant contribution to the frictional behavior of OTS. Direct in situ comparison of two intrinsic OTS structural phases on the substrate is also performed. We observe frictional contrast for different local molecular packing densities of the otherwise identical molecules. The phase with lower packing density exhibits higher friction, in agreement with related previous work, but decisively observed here in single, continuous images involving the same molecules. Lateral stiffness measurements show no distinction between the two OTS structural phases, demonstrating that the difference in friction is not due to divergent stiffnesses of the two phases. Therefore, the packing density directly affects the interface's intrinsic resistance to friction, that is, the interfacial shear strength.     

On scale dependence in friction: transition from intimate to monolayer-lubricated contact

Scale dependence in friction is studied in the present paper using the newly developed mesoscale friction tester (MFT). A transition in frictional shear strength from several hundreds of MPa to several tens of MPa was observed over a very limited range of contact radii (20-30 nm) in both ambient and dry environments. Thus, a single apparatus has been able to establish these two limits which are consistent with the values previously obtained from friction experiments using atomic force microscopy (AFM) and the surface force apparatus (SFA), respectively. Consequently, it is hypothesized here that a shear strength in the hundreds of MPa results from intimate contact (solid-solid) and a shear strength in the tens of MPa results from a monolayer-lubricated contact. Furthermore, both the probe size and the normal pressure govern the interfacial conditions in the contact zone and it is these conditions, rather than the nominal environment, which in turn determine the resulting shear strengths. A continuum analysis based on the Lifshitz theory for van der Waals interactions is used to explain the quantized shear strengths which were obtained from our experiments and previous AFM and SFA friction experiments. This quantized friction behavior [J.N. Israelachvili, P.M. McGuiggan, A.M. Homola, Science 240 (1988) 189] results from the discrete separation due to the different interfacial conditions that can arise between two sliding surfaces. The consistency between the analysis and the experimental results shows that this analysis is applicable for nonwear friction with single asperity contact.     

Quantitative lateral force microscopy study of the dolomite (104)-water interface

The friction and lateral stiffness of the contact between an atomic force microscopy (AFM) probe tip and an atomically flat dolomite (104) surface were investigated in contact with two aqueous solutions that were in equilibrium and supersaturated with respect to dolomite, respectively. The two aqueous solutions yielded negligible differences in friction at the native dolomite-water interface. However, the growth of a Ca-rich film from the supersaturated solution, revealed by X-ray reflectivity measurements, altered the probe-dolomite contact region sufficiently to observe distinct friction forces on the native dolomite and the film-covered surface regions. Quantitative friction-load relationships demonstrated three physically distinct load regimes for applied loads up to 200 nN. Similar friction forces were observed on both surfaces below 50 nN load and above 100 nN load. The friction forces on the two surfaces diverged at intermediate loads. Quantitative measurements of dynamic friction forces at low load were consistent with the estimated energy necessary to dehydrate the surface ions, whereas differences in mechanical properties of the Ca-rich film and dolomite surfaces were evidently important above 50 nN load. Attempts to fit the quantitative stiffness-load data using a Hertzian contact mechanical model based on bulk material properties yielded physically unrealistic fitting coefficients, suggesting that the interfacial contact region must be explicitly considered in describing the static and dynamic contact mechanics of this and similar systems.     

Semiconductor surface-induced 1,3-hydrogen shift: the role of covalent vs zwitterionic character

X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) are used to compare the reaction of 1,2-cyclohexanedione (1,2-CHD) with Si(001) and diamond(001) surface dimers under ultra-high-vacuum conditions. 1,2-CHD is known to undergo a keto-enol tautomerization, with the monoenol being the primary equilibrium species in the solid and gas phases. XPS and FTIR data demonstrate that 1,2-CHD reacts with diamond(001) through the OH group of the monoenol, resulting in only one O atom being bonded to the surface. In contrast, XPS and FTIR data suggest that both oxygen atoms in the 1,2-CHD molecule bond via Si-O-C linkages to the Si(001) surface dimer, and that the molecule undergoes an intramolecular 1,3-H shift. While the Si(001) and diamond(001) surfaces are both comprised of surface dimers, the diamond(001) dimer is symmetric, with little charge separation, whereas the Si(001) dimer is tilted and exhibits zwitterionic character. The different reaction products that are observed when clean Si(001) and diamond(001) surfaces are exposed to 1,2-CHD demonstrate the importance of charge separation in promoting a 1,3-H shift and provide new mechanistic insights that may be applicable to a variety of organic reactions.     

Friction and nanowear of polystyrene against hydrophobic and hydrophilic substrates

The ambition of this study is to analyze the role of interfacial interactions in friction and nanowear of polystyrene, by comparing friction against hydrophobic wafers (methyl‐terminated) and hydrophilic wafers (hydroxyl‐terminated) as a function of sliding velocity and normal force. Friction experiments are performed with a translation tribometer and nanowear investigation is achieved by using atomic force microscopy (AFM) analysis of the wafer surfaces after friction. Experimental results show that the friction coefficients measured on hydrophilic surfaces are always larger than those obtained with hydrophobic surfaces, indicating a relationship between friction and interfacial interactions. Elsewhere, AFM analysis shows that polystyrene transfer appears for a higher normal force in the case of hydrophobic substrates compared to hydrophilic one. However, the corresponding tangential (or friction) force necessary to detect transfer is quite similar for both types of substrates, indicating that the initial wear of polystyrene occurs for a similar threshold interfacial shear. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2449–2454, 2006     

Mechanisms of single-walled carbon nanotube probe-sample multistability in tapping mode AFM imaging

When using single-walled carbon nanotube (SWNT) probes to create AFM images of SWNT samples in tapping mode, elastic deformations of the probe and sample result in a decrease in the apparent width of the sample. Here we show that there are two major mechanisms for this effect, smooth gliding and snapping, and compare their dynamics to the case when a conventional silicon tip is used to image a bare silicon surface. Using atomistic and continuum simulations, we analyze in detail the shape of the tip-sample interaction potential for three model cases and show that in the absence of adhesion and friction forces, more than two discrete, physically meaningful solutions of the oscillation amplitude are possible when snapping occurs (in contrast to the existence of one attractive and one repulsive solution for conventional silicon AFM tips). We present experimental results indicating that a continuum of amplitude solutions is possible when using SWNT tips and explain this phenomenon with dynamic simulations that explicitly include tip-sample adhesion and friction forces. We also provide simulation results of SWNT tips imaging Si(111)-CH3 surface step edges and Au nanocrystals, which indicate that SWNT probe multistability may be a general phenomenon, not limited to SWNT samples.     

Ab initio calculation of the adhesion and ideal shear strength of planar diamond interfaces with different atomic structure and hydrogen coverage

We propose a method to calculate the ideal shear strength τ of two surfaces in contact by ab initio calculations. This quantity and the work of adhesion γ are the interfacial parameters usually derived from tip-based friction force measurements. We consider diamond interfaces and quantitatively evaluate the effects of surface orientation and passivation. We find that in the case of fully passivated interfaces, γ is not affected by the orientation and the alignment of the surfaces in contact. On the contrary, τ does show a dependence on the atomic-scale roughness of the interface. The surface termination has a major impact on the tribological properties of diamond. The presence of dangling bonds, even at concentrations low enough to prevent the formation of interfacial C-C bonds, causes an increase in the resistance to sliding by 2 orders of magnitude with respect to the fully hydrogenated case. We discuss our findings in relation to experimental observations.     

Influence of solvent environment and tip chemistry on the contact mechanics of tip-sample interactions in friction force microscopy of self-assembled monolayers of mercaptoundecanoic Acid and dodecanethiol

Friction force microscopy measurements have been made for self-assembled monolayers of mercaptoundecanoic acid (C10COOH) and dodecanethiol (C11CH3) in different liquid media. In perfluorodecalin, the friction-load relationship was nonlinear and consistent with adhesion-controlled sliding. The effective range of the attractive force was controlled by using AFM tips functionalized with alkanethiols (chemical force microscopy). Like pairs of interacting molecules yielded data that were characterized by the Johnson-Kendall-Roberts model of contact mechanics, whereas the interaction between dissimilar pairs of molecules fitted the behavior predicted by the Derjaguin-Muller-Toporov model. In ethanol, the adhesive force was much smaller, and sliding was not adhesion-controlled. Under this condition of low adhesion, the friction force varied linearly with the applied load.     

Roughing titanium quantum wire on patterned monohydride diamond (001) surface

The authors have performed the roughing of titanium (Ti) quantum wires forming on a hydrogen-terminated diamond (001)-2x1 surface patterned with an ordered bare strip array and demonstrated that well-ordered Ti quantum wires are achieved only if the growth conditions (temperature and flux) have optimal values via kinetic Monte Carlo simulations. Considering that a scanning tunneling microscope is capable of selectively desorbing H from diamond (001)-2x1-H surface, they proposed a viable and easy approach to fabricate "ideal quantum wires" on the patterned hydrogen-terminated diamond (001) surface. The physical origin of the Ti quantum wire formation was pursued.     

Surface friction of thermoresponsive poly(N-isopropylacrylamide) gels in water

In this paper, we report the experimental results of surface friction between thermoresponsive poly(N-isopropylacrylamide) gels in water. The static friction force was found to depend on the waiting period prior to slider movement after contact between gel surfaces, which was a result of two relaxation mechanisms: the stress decay process due to macroscopic deformation under a normal load and the microscopic conformational change in the real contact area of polymer networks. The sliding velocity and the normal load dependence of the kinetic friction force were extensively measured. The results suggested that the following two mechanisms depended on the sliding velocity: the friction force generated by direct contact of the solid-like behavior and the viscous resistance of the liquid-like behavior. The strong temperature dependence of kinetic friction was observed, which was a result of a change in the balance between hydrophobic and hydrophilic interactions. The experimental results are discussed in terms of the multi-asperity contacts between the swollen gel/gel interfaces (solid friction, depending on the waiting period) and the viscous resistance and lubricating effect between the gel/water interfaces (fluid friction, depending on the sliding velocity).     

Equilibrium properties of Ag clusters on Pt(111)

Molecular dynamics simulations (MD) and Embedded Atom Method (EAM) potentials are employed to investigate the properties of small Ag clusters deposited on the (111) surface of Pt. The computation of the dissociation energies reveals the most stable structures up to 19 atoms, while MD simulations allow to extract the dynamical diffusion barrier for adatom, dimer and trimer. We obtain dissociation energies in the range of 0.3 eV for the trimer and 0.45 eV for the heptamer and a migration energy as low as 0.05 eV for the adatom.     

Responsive organometallic polymer grafts: electrochemical switching of surface properties and current mediation behavior

Quantitative adherence and friction measurements between atomic force microscopy (AFM) tips and reversibly oxidized and reduced poly(ferrocenyl dimethylsilane) (PFDMS) molecular layers grafted to Au are reported. Poly(ferrocenylsilanes) (PFSs) such as PFDMS owe their redox responsiveness to the presence of ferrocene units, bridged by substituted silicon units, in the main chain. Polymers were obtained by anionic polymerization, which allowed us to copolymerize sulfur containing end groups that facilitated grafting to Au surfaces. Electrochemical atomic force microscopy (ECAFM) was used to study adherence and friction as a function of the oxidation state of the polymer. Measurements of interfacial friction as a function of applied load on the nanoscale using Si(3)N(4) AFM tips revealed a reversible increase of the friction coefficient and adherence strength of the PFDMS layers with increasing oxidation state in NaClO(4) electrolytes. The variation of the electrolyte salts (NaClO(4) or NaNO(3)) allowed an assessment of surface counterion adsorption effects. Issues related to the interpretation of observed friction and adherence changes such as electrolyte anion-ferrocenium ion pair effects, and electrostatic forces due to tip surface charges are discussed. Unidirectional current flow was detected in cyclic voltammograms of the PFDMS layers in NaClO(4). This electrode rectification behavior could in principle be utilized for applications in thin film devices based on PFS films.     

Chemical force titrations of functionalized Si(111) surfaces

Chemical force titrations-plots of the adhesive force between an atomic force microscope tip and sample as a function of pH-were acquired on alkyl monolayer-derivatized Si(111) surfaces. Gold-coated AFM tips modified with thioalkanoic acid self-assembled monolayers (SAM) were employed. Alkyl monolayer-derivatized Si(111) surfaces terminated with methyl, carboxyl, and amine groups were produced via hydrosilylation reactions between 1-alkene reagents and H-terminated silicon. The functionalized surfaces were characterized using standard surface science techniques (AFM, FTIR, and XPS). Titration of the methyl-terminated surface using the modified (carboxyl-terminated) atomic force microscope tip resulted in a small pH-independent hydrophobic interaction. Titration of the amine-terminated surface using the same tip resulted in the determination of a surface pKa of 5.8 for the amine from the pH value from the maximum in the force titration curve. A pK(1/2) of 4.3 was determined for the carboxyl-terminated Si(111) in a similar way. These results will be discussed in relation to the modified Si(111) surface chemistry and organic layer structure, as well as with respect to existing results on Au surfaces modified with SAMs bearing the same functional groups.     

Evaluation of nanotribological behavior of amorphous polystyrene: The macromolecular weight effect

Tribological testing of polymers is of prime importance in many industrial applications. Silicon nitride AFM tips have been used to mimic the contact between amorphous polystyrene surfaces and a hard asperity, which is useful in understanding of how a multitude of asperities behave in a macroscopic contact. In this study, the adhesion force and the friction force of four PS molecular weights were measured and the average contact pressure was calculated by using the JKR contact theory. The nanotribological behavior of polystyrene showed a dependence on macromolecular weight with varying applied normal force and sliding velocity. The study indicates that the length of polymer chains noticeably influences the tribological behavior of amorphous polystyrenes. Mechanisms governing such behavior differences were ascribed to energy dissipating modes.     

Different adsorption structures of pyridine on Si(001) and Ge(001) surfaces

The adsorption and reaction of pyridine on the Si(001) and Ge(001) surfaces are investigated by first-principles density-functional calculations within the generalized gradient approximation. On both surfaces the N atom of pyridine initially reacts with the down atom of the dimer, forming a single bond between the N atom and the down atom. On Ge(001) such an adsorption configuration is most favorable, but on Si(001) a further reaction with a neighboring dimer occurs, resulting in formation of a bridge-type configuration. Especially we find that on Ge(001) the bridge-type configuration is less stable than the gas phase. Our results provide an explanation for a subtle difference in the adsorption structures of pyridine on Si(001) and Ge(001), which was observed from recent scanning tunneling microscopy experiments.     

Catalytic probe lithography: catalyst-functionalized scanning probes as nanopens for nanofabrication on self-assembled monolayers

This article describes the use of scanning catalytic probe lithography for nanofabrication of patterns on self-assembled monolayers (SAMs) of reactive adsorbates. Catalytic writing was carried out by scanning over bis(omega-tert-butyldimethyl-siloxyundecyl)disulfide SAMs using 2-mercapto-5-benzimidazole sulfonic acid-functionalized gold-coated AFM tips. The acidic tips induced local hydrolysis of the silyl ether moieties in the contacted areas, and thus patterned surfaces were created. Diffusion effects arising from the use of an ink were excluded in these type of experiments, and therefore structures with well-defined shapes and sizes were produced. The smallest lines drawn by this technique were about 25 nm wide, corresponding to the actual contact area of the tip. Lateral force microscopy studies performed on different SAMs helped to clarify the nature and cause of the friction contrasts observed by AFM. Dendritic wedges with thiol functions inserted into the catalytically written areas, thus enhancing the height contrast. The created patterns open possibilities to build 3D nanostructures.     

Development and validation of an integrated computational approach for the study of ionic species in solution by means of effective two-body potentials. The case of Zn2+, Ni2+, and Co2+ in aqueous solutions

In this paper we have developed an effective computational procedure for the structural and dynamical investigation of ions in aqueous solutions. Quantum mechanical potential energy surfaces for the interaction of a transition metal ion with a water molecule have been calculated taking into account the effect of bulk solvent by the polarizable continuum model (PCM). The effective ion-water interactions have been fitted by suitable analytical potentials, and have been utilized in molecular dynamics (MD) simulations to obtain structural and dynamical properties of the ionic aqueous solutions. This procedure has been successfully applied to the Co2+-H2O open-shell system and, for the first time, Co-oxygen and Co-hydrogen pair potential functions have been determined and employed in MD simulations. The reliability of the whole procedure has been assessed by applying it also to the Zn2+ and Ni2+ aqueous solutions, and the structural and dynamical properties of the three systems have been calculated by means of MD simulations and have been found to be in very good agreement with experimental results. The structural parameters of the first solvation shells issuing from the MD simulations provide an effective complement to extended X-ray absorption fine structure (EXAFS) experiments.     

Properties of hybridized DNA arrays on single-crystalline undoped and boron-doped (100) diamonds studied by atomic force microscopy in electrolytes

Properties of hybridized deoxyribonucleic acid (DNA) arrays on single-crystalline undoped and boron-doped diamonds are studied at the microscopic level by atomic force microscopy (AFM) in buffered electrolytic solutions. DNA is linked to diamond via aminodecene molecules (TFAAD) that are attached to undoped diamonds by a photochemical reaction and via nitrophenyl-diazonium molecules attached electrochemically to boron-doped diamonds. Both H-terminated and oxidized diamond surfaces are used in this process. On H-terminated surfaces, AFM measurements detect compact DNA layers. By analyzing phase and height contrast in AFM, a DNA layer height of 76 A is determined on the photochemically functionalized diamonds and a DNA layer height of up to 92 A is determined on the electrochemically functionalized diamonds. Based on the layer thickness, the DNA chains are tilted under the angle of 31 degrees . The morphology of the DNA layers exhibits long-range (30-50 nm) undulations of 20 A in height and a nanoroughness of 8 A. Using Hertz's model for calculating the contact area of the AFM tip on a DNA layer and a geometrical model of DNA arrangement on diamond yields the DNA density on diamonds of 6 x 10(12) cm(-2) on both photochemically and electrochemically functionalized diamonds. The structure of these dense DNA layers is not significantly influenced by variations in buffer salinity of 1-300 mM NaCl. DNA molecules can be removed from the diamond surface by contact-mode AFM with forces >or= 45 nN and >or= 76 nN on photochemically and electrochemically functionalized diamonds, respectively, indicating that DNA is bonded covalently and stronger on diamond than on gold substrates. The DNA arrangement and bonding strength are similar on oxidized diamond surfaces when using an electrochemical process. On oxidized surfaces after photochemical processing, DNA is bonded noncovalently as deduced from the removal force < 6 nN. The presence of hybridized DNA as well as the selective removal of DNA by AFM scanning are corroborated by fluorescence microscopy.     

On the adhesion between fine particles and nanocontacts: an atomic force microscope study

In this study we measured the adhesion forces between atomic force microscope (AFM) tips or particles attached to AFM cantilevers and different solid samples. Smooth and homogeneous surfaces such as mica, silicon wafers, or highly oriented pyrolytic graphite, and more rough and heterogeneous surfaces such as iron particles or patterns of TiO2 nanoparticles on silicon were used. In the first part, we addressed the well-known issue that AFM adhesion experiments show wide distributions of adhesion forces rather than a single value. Our experiments show that variations in adhesion forces comprise fast (i.e., from one force curve to the next) random fluctuations and slower fluctuations, which occur over tens or hundreds of consecutive measurements. Slow fluctuations are not likely to be the result of variations in external factors such as lateral position, temperature, humidity, and so forth because those were kept constant. Even if two solid bodies are brought into contact under precisely the same conditions (same place, load, direction, etc.) the result of such a measurement will often not be the same as that of the previous contact. The measurement itself will induce structural changes in the contact region, which can change the value for the next adhesion force measurement. In the second part, we studied the influence of humidity on the adhesion of nanocontacts. Humidity was adjusted relatively fast to minimize tip wear during one experiment. For hydrophobic surfaces, no signification change in adhesion force with humidity was observed. Adhesion force versus humidity curves recorded with hydrophilic surfaces either showed a maximum or continuously increased. We demonstrate that the results can be interpreted with simple continuum theory of the meniscus force. The meniscus force is calculated based on a model that includes surface roughness and takes into account different AFM tip (or particle) shapes by a two-sphere model. Experimental and theoretical results show that the precise contact geometry has a critical influence on the humidity dependence of the adhesion force. Changes in tip geometry on the sub-10-nm length scale can completely change adhesion force versus humidity curves. Our model can also explain the differences between earlier AFM studies, where different dependencies of the adhesion force on humidity were observed.