Highly efficient sequestration of aqueous lead on nanostructured calcite substrates.

Following defocused ion beam sputtering, large area highly corrugated and faceted nanoripples are formed on calcite (10.4) faces in a self-organized fashion. High resolution atomic force microscopy (AFM) imaging reveals that calcite ripples are defined by facets with highly kinked (11.0) and (21¯.12) terminations.In situAFM imaging during the exposure of such modified calcite surfaces to PbCl2aqueous solution reveals that the nanostructured calcite surface promotes the uptake of Pb. In addition, we observed the progressive smoothing of the highly reactive calcite facet terminations and the formation of Pb-bearing precipitates elongated in registry with the underlying nanopattern. By SEM-EDS analysis we quantified a remarkable 500% increase of the Pb uptake rate, up to 0.5 atomic weight % per hour, on the nanorippled calcite in comparison to its freshly cleaved (10.4) surfaces. These results suggest that nanostructurated calcite surfaces can be used for developing future systems for lead sequestration from polluted waters.

Dynamics of Sliding Friction between Laser-Induced Periodic Surface Structures (LIPSS) on Stainless Steel and PMMA Microspheres.

In this work, we investigated the sliding friction measured between poly(methyl methacrylate) (PMMA) colloidal probes with two different diameters D (1.5 and 15 µm) and laser-induced periodic surface structures (LIPSS) on stainless steel with periodicities Λ of 0.42 and 0.9 µm, when the probes are elastically driven along two directions, perpendicular and parallel to the LIPSS. The time evolution of the friction shows the characteristic features of a reverse stick-slip mechanism recently reported on periodic gratings. The morphologies of colloidal probes and modified steel surfaces are geometrically convoluted in the atomic force microscopy (AFM) topographies simultaneously recorded with the friction measurements. The LIPSS periodicity is only revealed with smaller probes (D = 1.5 µm) and when Λ takes the largest value of 0.9 µm. The average value of the friction force is found to be proportional to the normal load, with a coefficient of friction µ varying between 0.23 and 0.54. The values of µ are rather independent of the direction of motion, and they reach their maximum when the small probe is scanned on the LIPSS with the larger periodicity. The friction is also found to decrease with increasing velocity in all cases, which is attributed to the corresponding decrease of the viscoelastic contact time. These results can be used to model the sliding contacts formed by a set of spherical asperities of different sizes driven on a rough solid surface.

Moiré-Tile Manipulation-Induced Friction Switch of Graphene on a Platinum Surface.

Friction control and technological advancement are intimately intertwined. Concomitantly, two-dimensional materials occupy a unique position for realizing quasi-frictionless contacts. However, the question arises of how to tune superlubric sliding. Drawing inspiration from twistronics, we propose to control superlubricity via moiré patterning. Friction force microscopy and molecular dynamics simulations unequivocally demonstrate a transition from a superlubric to dissipative sliding regime for different twist angles of graphene moirés on a Pt(111) surface triggered by the normal force. This follows from a novel mechanism at superlattice level where, beyond a critical load, moiré tiles are manipulated in a highly dissipative shear process connected to the twist angle. Importantly, the atomic detail of the dissipation associated with the moiré tile manipulation─i.e., enduring forced registry beyond a critical normal load─allows the bridging of disparate sliding regimes in a reversible manner, thus paving the road for a subtly intrinsic control of superlubricity.

Flexible Superlubricity Unveiled in Sidewinding Motion of Individual Polymeric Chains.

A combination of low temperature atomic force microcopy and molecular dynamic simulations is used to demonstrate that soft designer molecules realize a sidewinding motion when dragged over a gold surface. Exploiting their longitudinal flexibility, pyrenylene chains are indeed able to lower diffusion energy barriers via on-surface directional locking and molecular strain. The resulting ultralow friction reaches values on the order of tens of pN reported so far only for rigid chains sliding on an incommensurate surface. Therefore, we demonstrate how molecular flexibility can be harnessed to realize complex nanomotion while retaining a superlubric character. This is in contrast with the paradigm guiding the design of most superlubric nanocontacts (mismatched rigid contacting surfaces).

Graphene Confers Ultralow Friction on Nanogear Cogs.

Friction-induced energy dissipation impedes the performance of nanomechanical devices. Nevertheless, the application of graphene is known to modulate frictional dissipation by inducing local strain. This work reports on the nanomechanics of graphene conformed on different textured silicon surfaces that mimic the cogs of a nanoscale gear. The variation in the pitch lengths regulates the strain induced in capped graphene revealed by scanning probe techniques, Raman spectroscopy, and molecular dynamics simulation. The atomistic visualization elucidates asymmetric straining of CC bonds over the corrugated architecture resulting in distinct friction dissipation with respect to the groove axis. Experimental results are reported for strain-dependent solid lubrication which can be regulated by the corrugation and leads to ultralow frictional forces. The results are applicable for graphene covered corrugated structures with movable components such as nanoelectromechanical systems, nanoscale gears, and robotics.

Numerical study of pattern formation in compliant surfaces scraped by a rigid tip.

The emergence of surface patterns on the surfaces of compliant materials subject to plowing wear is a complex problem which can be quantitatively characterized, e.g., on polymer surfaces scraped by an atomic force microscope (AFM) tip. Here we explore the applicability of a phenomenological model recently introduced to describe this phenomenon. Based on the competition between the viscoplastic indentation and the elastic shear stress caused by the tip, the model is able to reproduce the wavy features (ripples) observed when the tip is scanned along a series of parallel lines. For low values of the driving velocity v and the spacing b between scan lines, the existence of dotted areas formed by variously oriented pit alignments is observed. Moreover, coexistence of rippled with dotted domains is also observed at suitable parameter values. The formation process of the ripples is also described in detail. The amplitude, period, and orientation of these features are estimated numerically for different values of v and b parameters. We have also revisited the formation of the wavy patterns formed when a single line is scanned, and derived an equation which correctly describes their period and depth, and the static friction as well. This equation is not applicable when several lines are scanned one after the other and the ripples emerge as result of a cooperative process which involves the scanning of several lines.

Plowing-Induced Structuring of Compliant Surfaces.

The structures produced by a sharp tip scraping a compliant surface are modeled in the illustrative case of scan patterns formed by a series of parallel lines. This is made possible by a modified version of the Prandtl model for stick-slip friction, with an interaction energy landscape replicating the morphology of the evolving surface. As a result, a ripple motif emerges with a tilt angle increasing linearly with the distance between the scan lines, except for the region close to the left boundary of the scanned area, where the ripples are oriented at 90°. This region can penetrate considerably to the right, forming a complex branched pattern. These predictions are substantiated by atomic force microscopy nanolithography experiments on polystyrene surfaces at room temperature. A simple and robust theoretical protocol for reproducing early-stage wear processes (potentially going beyond single contacts) is thus made available.

Conformations and cryo-force spectroscopy of spray-deposited single-strand DNA on gold.

Cryo-electron microscopy can determine the structure of biological matter in vitrified liquids. However, structure alone is insufficient to understand the function of native and engineered biomolecules. So far, their mechanical properties have mainly been probed at room temperature using tens of pico-newton forces with a resolution limited by thermal fluctuations. Here we combine force spectroscopy and computer simulations in cryogenic conditions to quantify adhesion and intra-molecular properties of spray-deposited single-strand DNA oligomers on Au(111). Sub-nanometer resolution images reveal folding conformations confirmed by simulations. Lifting shows a decay of the measured stiffness with sharp dips every 0.2-0.3 nm associated with the sequential peeling and detachment of single nucleotides. A stiffness of 30-35 N m-1 per stretched repeat unit is deduced in the nano-newton range. This combined study suggests how to better control cryo-force spectroscopy of adsorbed heterogeneous (bio)polymer and to potentially enable single-base recognition in DNA strands only few nanometers long.

Nanoconfinement and Sansetsukon-like Nanocrawling Govern Fibrinogen Dynamics and Self-Assembly on Nanostructured Polymeric Surfaces.

Surface nanostructures are increasingly more employed for controlled protein assembly on functional nanodevices, in nanobiotechnology, and in nanobiomaterials. However, the mechanism and dynamics of how nanostructures induce order in the adsorbed protein assemblies are still enigmatic. Here, we use single-molecule mapping by accumulated probe trajectories and complementary atomic force microscopy to shed light on the dynamic of in situ assembly of human plasma fibrinogen (HPF) adsorbed on nanostructured polybutene-1 (PB-1) and nanostructured polyethylene (PE) surfaces. We found a distinct lateral heterogeneity of HPF-polymer nanostructure interface (surface occupancy, residence time, and diffusion coefficient) that allow identifying the interplay between protein topographical nanoconfinement, protein diffusion mechanism, and ordered protein self-assembly. The protein diffusion analysis revealed high-diffusion polarization without correlation to the anisotropic friction characteristic of the polymer surfaces. This suggests that HPF molecules confined on the nanosized PB-1 needle crystals and PE shish-kebab crystals, respectively, undergo partial detachment and diffuse via a Sansetsukon-like nanocrawling mechanism. This mechanism is based on the intrinsic flexibility of HPF in the coiled-coil regions. We conclude that nanostructured surfaces that encourage this characteristic surface mobility are more likely to lead to the formation of ordered protein assemblies and may be useful for advanced nanobiomaterials.

Estimation of interaction energy and contact stiffness in atomic-scale sliding on a model sodium chloride surface in ethanol.

Friction force microscopy (FFM) in aqueous environments has recently proven to be a very effective method for lattice-resolution imaging of crystal surfaces. Here we demonstrate the use of ethanol for similar measurements on water-soluble materials. Lattice resolved frictional stick-slip traces of a cleaved NaCl(100) surface submerged in ethanol are compared with previous obtained FFM results in ultrahigh vacuum (UHV). We use the Prandtl-Tomlinson framework to estimate the amplitude of the corrugation potential and the contact stiffness. The surface potential amplitude scales with the applied normal loads are in good agreement with data obtained for NaCl measured under UHV conditions, but demonstrates deviations from the ideal periodic potential given by the Prandtl-Tomlinson model. An additional finding is that the use of ethanol allows us to explore higher load ranges without detectable evidence of surface wear. The contact stiffness does not vary significantly with the normal load up to 38 nN, while above it a sudden increase by almost one order of magnitude was observed. Comparing this to previous results suggests that considerable atom rearrangements may occur in the contact region, although the (100) surface structure is preserved by ethanol-assisted diffusion of Na and Cl ions.

Subnanometer Resolution and Enhanced Friction Contrast at the Surface of Perylene Diimide PDI8-CN2 Thin Films in Ambient Conditions.

We report high-resolution surface morphology and friction force maps of polycrystalline organic thin films derived by deposition of the n-type perylene diimide semiconductor PDI8-CN2. We show that the in-plane molecular arrangement into ordered, cofacial slip-stacked rows results in a largely anisotropic surface structure, with a characteristic sawtooth corrugation of a few Ångstroms wavelength and height. Load-controlled experiments reveal different types of friction contrast between the alternating sloped and stepped regions, with transitions from atomic-scale dissipative stick-slip to smooth sliding with ultralow friction within the surface unit cell. Notably, such a rich phenomenology is captured under ambient conditions. We demonstrate that friction contrast is well reproduced by numerical simulations assuming a reduced corrugation of the tip-molecule potential nearby the step edges. We propose that the side alkyl chains pack into a compact low-surface-energy overlayer, and friction modulation reflects periodic heterogeneity of chains bending properties and subsurface anchoring to the perylene cores.

Stick-Slip Motion of ssDNA over Graphene.

We have performed molecular dynamics simulations of nanomanipulation experiments on short single-stranded DNA chains elastically driven on a graphene surface. After a brief transient, reproducible stick-slip cycles are observed on chains made by 10 units of thymine, cytosine, adenine, and guanine. The cycles have the periodicity of the graphene substrate, and take place via an intermediate stage, appearing as a dip in the sawtooth variations of lateral force recorded while the chains are manipulated. Guanine presents remarkable differences from the other bases, since a lower number of nucleotides are prone to stick to the substrate in this case. Nevertheless, the magnitudes of static friction and lateral stiffness are similar for all chains (30 pN and 0.7 N/m per adsorbed nucleotide respectively).

Time Strengthening of Crystal Nanocontacts.

We demonstrate how an exponentially saturating increase of the contact area between a nanoasperity and a crystal surface, occurring on time scales governed by the Arrhenius equation, is consistent with measurements of the static friction and lateral contact stiffness on a model alkali-halide surface at different temperatures in ultrahigh vacuum. The "contact ageing" effect is attributed to atomic attrition and is eventually broken by thermally activated slip of the nanoasperity on the surface. The combination of the two effects also leads to regions of strengthening and weakening in the velocity dependence of the friction, which are well-reproduced by an extended version of the Prandtl-Tomlinson model.

A molecular-scale portrait of domain imaging in organic surfaces.

Progress in the general understanding of structure-property relationships in organic devices requires experimental tools capable of imaging structural details, such as molecular packing or domain attributes, on ultra-thin films. An operation mode of scanning force microscopy, related to friction force microscopy (FFM) and known as transverse shear microscopy (TSM), has demonstrated the ability to reveal the orientation of crystalline domains in organic surfaces with nanometer resolution. In spite of these promising results, numerous questions remain about the physical origin of the TSM domain imaging mechanism. Taking as a benchmark a PTCDI-C8 sub-monolayer, we demonstrate experimentally and theoretically that such a mechanism is the same atomic scale stick-slip ruling FFM leading to the angular dependence of both signals. Lattice-resolved images acquired on top of differently oriented PTCDI-C8 molecular domains are crucial to permit azimuthal sampling, without the need for sample rotation. The simulations reveal that, though the surface crystallography is the direct cause of the FFM and TSM signals, the manifestation of anisotropy will largely depend on the amplitude of the surface potential corrugation as well as on the temperature. This work provides a novel nanoscale strategy for the quantitative analysis of organic thin films based on their nanotribological response.

Atomic-Scale Sliding Friction on Graphene in Water.

The sliding of a sharp nanotip on graphene completely immersed in water is investigated by molecular dynamics (MD) and atomic force microscopy. MD simulations predict that the atomic-scale stick-slip is almost identical to that found in ultrahigh vacuum. Furthermore, they show that water plays a purely stochastic role in sliding (solid-to-solid) friction. These observations are substantiated by friction measurements on graphene grown on Cu and Ni, where, oppositely of the operation in air, lattice resolution is readily achieved. Our results promote friction force microscopy in water as a robust alternative to ultra-high-vacuum measurements.

Sub-nanometer resolution of an organic semiconductor crystal surface using friction force microscopy in water.

Organic semiconductors (OSC) are attracting much interest for (opto)electronic applications, such as photovoltaics, LEDs, sensors or solid state lasers. In particular, crystals formed by small π-conjugated molecules have shown to be suitable for constructing OSC devices. However, the (opto)electronic properties are complex since they depend strongly on both the mutual orientation of molecules as well as the perfection of bulk crystal surfaces. Hence, there is an urgent need to control nano-topographic OSC features in real space. Here we show that friction force microscopy in water is a very suitable technique to image the free surface morphology of an OSC single crystal (TDDCS) with sub-nanometer resolution. We demonstrate the power of the method by direct correlation to the structural information extracted from combined single crystal (SC-) and specular (s-) XRD studies, which allows us to identify the pinning centers encountered in the stick-slip motion of the probing tip with the topmost methyl groups on the TDDCS surface.

Superlubricity of graphene nanoribbons on gold surfaces.

The state of vanishing friction known as superlubricity has important applications for energy saving and increasing the lifetime of devices. Superlubricity, as detected with atomic force microscopy, appears when sliding large graphite flakes or gold nanoclusters across surfaces, for example. However, the origin of the behavior is poorly understood because of the lack of a controllable nanocontact. We demonstrated the superlubricity of graphene nanoribbons when sliding on gold with a joint experimental and computational approach. The atomically well-defined contact allows us to trace the origin of superlubricity, unraveling the role played by ribbon size and elasticity, as well as by surface reconstruction. Our results pave the way to the scale-up of superlubricity and thus to the realization of frictionless coatings.

Single-Molecule Tribology: Force Microscopy Manipulation of a Porphyrin Derivative on a Copper Surface.

The low-temperature mechanical response of a single porphyrin molecule attached to the apex of an atomic force microscope (AFM) tip during vertical and lateral manipulations is studied. We find that approach-retraction cycles as well as surface scanning with the terminated tip result in atomic-scale friction patterns induced by the internal reorientations of the molecule. With a joint experimental and computational effort, we identify the dicyanophenyl side groups of the molecule interacting with the surface as the dominant factor determining the observed frictional behavior. To this end, we developed a generalized Prandtl-Tomlinson model parametrized using density functional theory calculations that includes the internal degrees of freedom of the side group with respect to the core and its interactions with the underlying surface. We demonstrate that the friction pattern results from the variations of the bond length and bond angles between the dicyanophenyl side group and the porphyrin backbone as well as those of the CN group facing the surface during the lateral and vertical motion of the AFM tip.

Controlled Suppression of Wear on the Nanoscale by Ultrasonic Vibrations.

Wear on the nanoscale, as evidenced by the formation of periodic ripples on a model polystyrene thin film while a sharp tip is sliding on it with a normal force in the µN range, is shown to be suppressed by the application of ultrasonic vibrations of amplitude Aexc. An accurate calibration of the transducer excitation amplitude is achieved by a home-built setup based on a laser Doppler vibrometer. The corrugation of the typical ripple pattern that is formed in the absence of vibrations is reduced when the excitation frequency matches the contact resonance of the system and Aexc progressively increases. Above a critical value of Aexc, the ripples completely disappear, while the friction levels off at a finite value determined by the normal force and the vibration amplitude. This value can be significantly smaller than the value of the macroscopic friction coefficient. In addition to the control of wear in general, this opens up the possibility of controlled nanolithography with improved accuracy.