Lateral Force Microscopy Measurements Research Articles

Lateral Force Microscopy Reveals the Energy Barrier of a Molecular Switch.

Copper phthalocyanine (CuPc) is a small molecule often used in organic light emitting diodes where it is deposited on a conducting electrode. Previous scanning tunneling microscopy (STM) studies of CuPc on Cu(111) have shown that inelastic tunneling events can cause CuPc to switch between a ground state and two symmetrically equivalent metastable states in which the molecule is rotated. We investigated CuPc on Cu(111) and Ag(111) with STM and lateral force microscopy (LFM). Even without inelastic events, the presence of the tip can induce rotations and upon closer approach, causes the rotated states to be favored. Combining STM measurements at various temperatures and LFM measurements, we show that the long-range attraction of the tip changes the potential energy landscape of this molecular switch. We can also determine the geometry of the rotated and ground states. We compare our observations of CuPc on Cu(111) to CuPc on Ag(111). On Ag(111), CuPc appears flat and does not rotate. Stronger bonding typically involves shorter bond lengths, larger shifts of energy levels, and structural stability. Although the binding of CuPc to Cu(111) is stronger than that on Ag(111), the nonplanar geometry of CuPc on Cu(111) is accompanied by two metastable states which are not present on the Ag(111) surface.

Graphene Nanoribbons with Atomically Sharp Edges Produced by AFM Induced Self-Folding.

The ability to create graphene nanoribbons with atomically sharp edges is important for various graphene applications because these edges significantly influence the overall electronic properties and support unique magnetic edge states. The discovery of graphene self-folding induced by traveling wave excitation through atomic force microscope scanning under a normal force of less than 15 nN is reported. Most remarkably, the crystallographic direction of self-folding may be either along a chosen direction defined by the scan line or along the zigzag or armchair direction in the presence of a pre-existing crack in the vicinity. The crystalline direction of the atomically sharp edge is confirmed via careful lateral force microscopy measurements. Multilayer nanoribbons with lateral dimensions of a few tens of nanometers are realized on the same graphene sheet with different folding types (e.g., z-type or double parallel). Molecular dynamics simulations reveal the folding dynamics and suggest a monotonic increase of the folded area with the applied normal force. This method may be extended to other 2D van der Waals materials and lead to nanostructures that exhibit novel edge properties without the chemical instability that typically hinders applications of etched or patterned graphene nanostructures.

Femtosecond laser microstructuring of diamond-like nanocomposite films

We report on femtosecond laser surface modification and microstructuring of diamond-like nanocomposite (DLN) films (a-C:H,Si:O), and investigation of the frictional properties of laser-micropatterned DLN films on the nano, micro and macroscale. DLN films of 2–3μm thickness were irradiated using a femtosecond laser (wavelength 1030nm, pulse duration 320fs, pulse repetition rate 101kHz) to produce periodic linear micropatterns over the areas of 40mm2. Laser irradiation was performed at low fluences (below the single-pulse ablation threshold) corresponding to the conditions of surface graphitization and incipient ablation developing during the multipulse irradiation. Frictional properties of laser-micropatterned DLN films were studied using (i) lateral force microscopy (LFM) and (ii) ball-on-flat tribometer under linear reciprocating sliding against 100Cr6 steel and Si3N4 balls. The LFM measurements revealed significant changes in the friction behavior of the laser-patterned films during transition from nano to microscale, demonstrating much lower friction forces within laser-graphitized strips than on the original film. Such microfriction behavior was attributed to (i) higher hydrophobicity of laser-graphitized nanostructured surface and (ii) strong influence of capillary forces of adsorbed water layers on friction under the ‘nano’ loads. Macroscopic friction properties of the fs-laser-patterned DLN films were shown to depend on the friction pair (DLN vs steel ball, DLN vs Si3N4 ball), both at the initial stage of sliding and during prolonged sliding tests.

Effects of UV laser micropatterning on frictional performance of diamond-like nanocomposite films

We report on UV laser modification and micropatterning of diamond-like nanocomposite (DLN) films (a-C:H,Si:O) with nanosecond pulses and effects of laser surface microstructuring on the frictional performance of DLN films on the nano- and macroscale. A technique of direct laser interference patterning was applied to produce arrays of periodic linear microstructures on the DLN films. The UV laser irradiation was performed at low fluences corresponding to the regime of surface graphitization and incipient ablation. At the initial stage of the thin film modification, the laser-induced spallation and graphitization in the surface layers were found to strongly influence the nanoscale topography and mechanical properties of the DLN surface. Frictional properties of the laser-patterned DLN films were studied using (1) atomic force microscopy in lateral force mode and (2) a ball-on-flat tribometer under linear reciprocating sliding against a 100Cr6 steel ball. The lateral force microscopy measurements revealed that the laser-irradiated regions were characterized by increased friction forces due to microspallation effects and enhanced surface roughness, correlating with tribotests at the initial stage of sliding. During prolonged sliding in ambient air, both the original and laser-patterned DLN surfaces exhibited low-friction performance at the friction coefficient of 0.07–0.08.

The effect of cross-linking density and dangling chains on surface molecular mobility of network polyurethanes

Chemically cross-linked polyurethanes (PUs) were synthesized with two different cross-linking densities and dangling chain concentrations to understand the difference between the molecular mobility of the bulk and free surface based on dynamic viscoelastic properties, lateral force microscopy (LFM) and scanning viscoelasticity microscopy (SVM) measurements. The PUs were synthesized from poly(oxypropylene) triol (PPT; Mn = 3000) and tolylene-2,4-diisocyanate (TDI) with a formulation ratio (= [NCO]/[OH]) of 0.85 and 1.20. The former PU has a lightly cross-linked structure with dangling chains, and the latter one has a densely cross-linked structure with allophanate groups and without dangling chains. The α-relaxation process, associated with glass transition of the PPT chains for the bulk and surface, was observed for both PUs in dynamic viscoelastic properties and lateral force microscopy measurements, respectively. The temperature region, at which the α-relaxation process was observed, for the surface was lower and broader in comparison with bulk and the trend was more obvious for the PU with dangling chains. These results can be explained well with the existence of dangling chains and their segregation at the top surface because of lower conformational entropy.

Activated physical properties at air-polymer interface

The surface molecular motion of monodisperse polystyrene (PS) films was examined using scanning viscoelasticity microscopy (SVM) in conjunction with lateral force microscopy (LFM). The dynamic storage modulus,E′, and loss tangent, tanσ, at a PS film surface with number-average molecular weights,Mn, smaller than 30 k were found to be smaller and larger than those for the bulk sample, even at room temperature, meaning that the PS surface is in a glass-rubber transition or fully rubbery sate at this temperature when theMn is small. In order to quantitatively elucidate the dynamics of the molecular motion at the PS surface, SVM and LFM measurements were performed at various temperatures. The glass transition temperature,Tg, at the surface was found to be markedly lower than the bulkTg, and this discrepancy between the surface and bulk became larger with decreasingMn. Such an intensive activation of the thermal molecular motion at the PS surfaces can be explained in terms of an excess free volume in the vicinity of the film surface induced by the preferential segregation of the chain end groups.

Synthesis and In Situ Atomic Force Microscopy Characterization of Temperature-Responsive Hydrogels Based on Poly(2-(dimethylamino)ethyl methacrylate) Prepared by Atom Transfer Radical Polymerization

Well-defined copolymers of 2-(dimethylamino)ethyl methacrylate (DMAEMA) and benzophenone methacrylate (BPMA) with different compositions were synthesized via atom transfer radical polymerization. The molecular weights of these copolymers were Mn approximately 30 000 g/mol, while the BPMA content varied from 2.5 to 10 mol %. The copolymers with a low content of BPMA (2.5 and 5 mol %) exhibited a sharp thermal transition at 33-36 degrees C in aqueous solution. A hydrogel was immobilized and patterned on a silicon wafer via UV treatment of the spin-coated polymer layer using a photomask technique. The thermoresponsive behavior of the patterned polymer gel was quantitatively investigated by variable temperature in situ contact mode atomic force microscopy, which revealed the presence of two lower critical solution temperature regions. One region was between 25 and 30 degrees C, corresponding to the topmost layer of the hydrogel film, and the other region, around 40 degrees C, corresponded to the bulk of the hydrogel. Concurrent lateral force microscopy measurements revealed that, just above the transition temperature, the bulk region exhibited enhanced friction.

Molecular motions of different scales at thin polystyrene film surface by lateral force microscopy

Lateral force microscopy (LFM) was used to probe the molecular motions at thin polystyrene film surface. The effect of the applied load on the LFM measurements was investigated by presenting both the LFM results and the surface morphology after several scans over the same area. Depending on the loads, the scanning can be nonperturbative (without alternating the surface morphology) or perturbative (patterning the surface). Temperature-dependent LFM measurements were conducted in order to determine the apparent transitions at the surface. Perturbative scans under high loads (e.g., 150 nN) witnessed that the apparent transitions shifted to low temperatures with an increasing scan rate, while the transitions behaved oppositely under lower loads (1, 10, and 20 nN). The heating effect is suggested to account for the behavior under high loads. According to our results from nonperturbative LFM, the apparent glass transition temperature (T(g)s) is more than 10 K lower than the bulk value. Moreover, rate-dependent LFM measurements were performed under 1 nN in order to detect the surface molecular motions. Time-temperature superposition yields a master curve exhibiting three apparent relaxation peaks. The molecular motions at the surface are discussed on the context of the coupling model.

Reproducible lateral force microscopy measurements for quantitative comparisons of the frictional and chemical properties of nanostructures

Atomic force microscopy (AFM) is a widely used technique for characterizing the topography and frictional properties of nanostructures. Inherent misalignments between the AFM cantilever and the feedback hardware lead to crosstalk between topography data and lateral force microscopy (LFM) data. Because the degree of crosstalk depends on the positioning of the cantilever, LFM and topography data of the same structure can vary from one experiment to the next. For nanostructures with large LFM contrast, errors as large as 50% in topography and LFM can be observed. This paper describes an empirical strategy for correcting this alignment error. The technique is used to characterize the frictional properties of scanning probe-induced oxide nanostructures and the hydrogen-terminated Si(1 1 1) surfaces on which they are patterned. Reproducible differences in the frictional properties of the oxide nanostructures patterned on HF-treated and NH 4F-treated Si(1 1 1) surfaces are observed and attributed to the mixed-hydride versus monohydride termination of each surface. The observed frictional contrast is consistent with known differences in surface reactivity and demonstrates how LFM measurements can provide insight into the frictional and chemical properties of nanostructures

Surface morphology of immiscible polymer blend using scanning force microscopy

Scanning force microscopy was applied for the study of the surface morphology and mechanical properties of (poly(methyl acrylate)/poly(methyl methacrylate) 95/5 by wt) (PMA/PMMA) blend film. X-ray photoelectron spectroscopy (XPS), lateral force microscopy (LFM), and scanning viscoelasticity microscopy (SVM) measurements of the phase-separated PMA/PMMA blend film were carried out to evaluate the effect of surface roughness on the surface composition. It was revealed that the surface composition of the blend film measured by XPS was much higher than that by LFM or SVM. The remarkable difference of the surface composition between XPS and LFM measurements was explained by the surface roughness of the blend film because LFM and SVM only reflect the surface physical properties of the material, regardless of the surface roughness.

Force Calibration in Lateral Force Microscopy

An analysis of the contact mechanics and the forces of interaction in lateral force microscopy measurements is presented. This analysis allows for a new method of interpretation of the frictional forces, the lateral contact stiffness, and the contact shear strength. The technique was developed for the interpretation of frictional data obtained with colloidal probes, although results are presented which illustrate its ability to interpret measurements recorded with both colloidal probes and standard atomic force microscopy tips. The technique is found to compensate for the variations in the contact geometry, giving repeatable results for probes of different sizes. A critical review of other techniques which have been employed to interpret the frictional force in lateral force microscopy is also presented.

Surface glass transition temperatures of monodisperse polystyrene films by scanning force microscopy

Surface molecular motion of monodisperse polystyrene (PS) films was examined by scanning viscoelasticity microscopy (SVM) in conjunction with lateral force microscopy (LFM). The dynamic storage modulus, ƴ, and loss tangent, tan d , at a PS film surface with a smaller number-average molecular weight, Mn, than 40k were found to be smaller and larger than those for the bulk sample even at room temperature, meaning that the PS surface is in a glass–rubber transition state or a fully rubbery one at this temperature if the Mn, is small. In order to elucidate quantitatively how vigorous the molecular motion at the PS surface is, SVM and LFM measurements were made at various temperatures. The glass transition temperature, Tg, at the surface wasdiscerned to be markedly lower than its bulk Tg, and the discrepancy of Tg between surface and bulk becomes larger with the decreasing Mn. Such an intensive activation of thermal molecular motion at the PS surfaces can be explained in terms of an excess free volume in the vicinit of the film surfaceinduced by the preferential segregation of chain end groups.

Specific detection of interactions between uncharged surfaces indifferent solvents: high-resolution imaging by chemical forcemicroscopy*

The detection of hydroxyl-groups by chemical force microscopy is investigated in twocontrasting solvents: water and hexadecane. Gold-coated cantilevers were chemically modifiedin a defined way by self-assembly monolayers (SAMs) formed by mercapto-undecanol. Thesamples employed consisted of ultraflat gold surfaces terminated by hydroxyl- and methyl-groups.Both surface terminations were present on the sample surface in a well-defined patternobtained by micro-contact printing (µCP). High-resolution lateral force microscopy (LFM)was conducted in both solvents. These LFM measurements were complemented bymeasurements of the adhesion forces on the same substrates. In contrast to water, the empolyment of hexadecane,it enables specific detection of hydroxyl-groups and clear interpretation of the friction contrast.

Ti 3SiC 2—A layered ceramic exhibiting ultra-low friction

Currently known solid lubricants will sometimes exhibit ultra-low coefficients of friction, ( μ<5×10 −3) under carefully controlled conditions, but μ is rarely lower than 0.05 in a realistic service environment. The layered carbide-derived ceramic, Ti 3SiC 2, has been shown recently to have many exceptional electrical and mechanical properties. We report lateral force microscopy measurements which show μ<(2–5)×10 −3. The ceramic has planes of easy cleavage as demonstrated by FeSEM and AFM images and previous studies of its mechanical behaviour. It is on such cleavage faces that the lowest values of μ were recorded (sometimes below the limit of resolution). The ultra-low friction persists after exposure to ambient air for several months.

Microtribological behavior of Au(001) studied by AFM/FFM

Fundamental microtribological properties such as the interaction force between surfaces, friction, and wear are studied by performing atomic and lateral force microscopy measurements on Au(001) under ultrahigh vacuum conditions. The system tip/sample surface is treated as a model-like micromechanical system in sliding motion. The adhesion force appears to be a substantial contribution to the interaction force between tip and sample surface. The wear of the Au(001) surface is estimated and its impact is discussed, identifying elastic deformation as the main dissipative contribution to the friction process. Combining the results of the friction study with those of the adhesion force measurements, the friction behavior of the microsystem is favored to be explained by a sliding single-asperity contact.

Effect of Water on Lateral Force Microscopy in Air

Lateral force microscopy (LFM) images on polymeric and inorganic substrates demonstrate how water is transported to and from a substrate by the capillary formed by the LFM tip and sample. Although it is known that water qualitatively affects the atomic force microscopy and LFM measurements, herein we report the condition dependent range of sizes of the water menisci formed in the capillary during such measurements. The effect of water in air can change the force of friction by a factor ≥2, with the magnitude of this effect dependent on the chemical nature of the sample and tip dynamics. These results have important implications for any analytical measurement of friction by LFM in air.

The aging of silicate glass surfaces in humid air

Slicate glass surfaces were observed over several weeks in normally humid laboratory air by atomic force microscopy. Auger spectroscopy and lateral force microscopy measurements were also made. Samples in the study include; vitreous quartz, bulk insulation glass, bulk and fibrous textile e-glass, and bulk container glass. Various features developed on these surfaces are consistent with patterns seen in crystal growth from supersaturated solution. Auger spectroscopy reveals the presence of chemically bound carbon and supports the hypothesis that these crystals are alkali and alkali earth carbonate species. The presence of these crystals is expected to affect the bonding of a coating to silicate surfaces.

Scanning force microscopy characterization of thin lipid films on a substrate

The surface morphology and frictional property of self-assembled domains of ultrathin organic films adsorbed on a solid substrate have been studied by a commercially available scanning probe microscopy with an operation mode of simultaneous atomic force microscopy and lateral force microscopy measurements. Two lipid film systems have been studied; one is a film system formed from synthetic dialkyl lipid, N, N-dioctadecylsuccinamic acid, and the other is films formed from biological phospholipids such as dipalmitoylphosphatidylcholine (DPPC), phosphatidylcholine, and phosphatidylethanolamine. The characteristic self-assembled domain structures as well as the local friction behaviors of the lipid films are presented. A preliminary result is also shown which suggests the hydrophobic alkyl tail groups appeared on multilamellar surfaces of DPPC film can be distinguished from the other hydrophilic polar headgroups.