Lateral Force Images Research Articles

Chemical gradients on graphene via direct mechanochemical cleavage of atoms from chemically functionalized graphene surfaces.

Manipulating the surface chemistry of graphene is critical to many applications that are achievable by chemical functionalization. Specifically, tailoring the spatial distribution of functional groups offers more opportunities to explore functionality using continuous changes in surface energy. To this end, careful consideration is required to demonstrate the chemical gradient on graphene surfaces, and it is necessary to develop a technique to pattern the spatial distribution of functional groups. Here, we demonstrate the tailoring of a chemical gradient through direct mechanochemical cleavage of atoms from chemically functionalized graphene surfaces via an atomic force microscope. Additionally, we define the surface characteristics of the fabricated sample by using lateral force microscopy revealing the materials' intrinsic properties at the nanoscale. Furthermore, we perform the cleaning process of the obtained lateral force images by using a machine learning method of truncated singular value decomposition. This work provides a useful technique for many applications utilizing continuous changes in the surface energy of graphene.

Simultaneous acquisition of current and lateral force signals during AFM for characterising the piezoelectric and triboelectric effects of ZnO nanorods

Atomic force microscopy (AFM) is central to investigating the piezoelectric potentials of one-dimensional nanomaterials. The AFM probe is used to deflect individual piezoelectric nanorods and to measure the resultant current. However, the torsion data of AFM probes have not been exploited to elucidate the relationship between the applied mechanical force and resultant current. In this study, the effect of the size of ZnO nanorods on the efficiency of conversion of the applied mechanical force into current was investigated by simultaneously acquiring the conductive AFM and lateral force microscopy signals. The conversion efficiency was calculated based on linear regression analysis of the scatter plot of the data. This method is suitable for determining the conversion efficiencies of all types of freestanding piezoelectric nanomaterials grown under different conditions. A pixel-wise comparison of the current and lateral force images elucidated the mechanism of current generation from dense arrays of ZnO nanorods. The current signals generated from the ZnO nanorods by the AFM probe originated from the piezoelectric and triboelectric effects. The current signals contributed by the triboelectric effect were alleviated by using an AFM probe with a smaller spring constant and reducing the normal force.

Formation of zabuyelite, Li2CO3, on dolomite and kutnohorite {10.4} surfaces from supersaturated and undersaturated solutions: Growth behavior and anomalous friction at the nanoscale

Cleaved dolomite (CaMg(CO3)2) crystals were immersed in supersaturated, saturated, and undersaturated solutions with respect to zabuyelite (Li2CO3), and the growth process was characterized using atomic force microscopy (AFM) and scanning electron microscopy (SEM). In all cases, overgrowths formed by 2D-nucleation could be clearly distinguished in the lateral force images recorded by AFM. With highly supersaturated solutions, AFM imaging was hindered by the formation of 3D crystal aggregates. When saturated solutions were used, the overgrowths reproduce the topography of the substrates, indicating that the lattice of the overgrown phase undergoes a compressive strain. Furthermore, we performed a nanotribological characterization by sliding a nanotip over both the 2D-overgrowths and dolomite substrates. Thus, friction coefficients, µ, could be easily quantified. As a result, a µ = 0.08 on the 2D-overgrowths was measured, which is one order of magnitude lower than the value of µ = 0.89 on the dolomite substrate. This friction reduction in the sliding of a nanotip on the overgrowth as compared to the friction observed on the dolomite substrate, together with the fact that atomic-scale stick-slip is observed on dolomite but not on the 2D overgrowths, point towards an anomalously weak interaction of Li2CO3 overgrowths with the AFM probe.

Laser surface graphitization to control friction of diamond-like carbon coatings

To study the role of laser surface graphitization in the friction behavior of laser-patterned diamond-like carbon (DLC) films, we apply the scanning probe microscopy (SPM) in the lateral force mode (LFM) which allows to obtain simultaneously the lateral force and topography images and to determine local friction levels in laser-irradiated and original surface areas. Based on this approach in the paper, we report on (1) laser surface microstructuring of hydrogenated a-C:H and hydrogen-free ta-C films in the regime of surface graphitization using UV laser pulses of 20-ns duration and (2) correlation between the structure and friction properties of the laser-patterned DLC surface on micro/nanoscale using SPM/LFM technique. The SPM/LFM data obtained for the surface relief gratings of graphitized microstructures have evidenced lower friction forces in the laser-graphitized regions. For the hydrogenated DLC films, the reversible frictional behavior of the laser-graphitized micropatterns is found to take place during LFM imaging at different temperatures (20 and 120 °C) in ambient air. It is revealed that the lateral force distribution in the laser-graphitized areas is shifted to higher friction levels (relative to that of the unirradiated surface) at temperature 120 °C and returned back to the lower friction during the sample cooling to 20 °C, thus confirming an influence of adsorbed water layers on the nanofriction properties of laser-graphitized micropatterns on the film surface.

Chemical force microscopy using functionalized ZnO whisker probe tips.

We have developed an atomic force microscope-based technique utilizing a zinc oxide whisker crystal as a probe tip. This technique was used for measuring interactions between the chemically modified tip and oxidized silicon substrates as a function of solution pH. Surfaces terminating in amine and methyl functional groups were prepared by covalently modifying the silicon substrate with self-assembled monolayers of (3-amino-propyl)triethoxysilane and n-butyltrichlorosilane, respectively. Patterned samples prepared by microcontact printing consisted of amine-terminated square regions surrounded by a methyl-terminated background. The contrast in lateral force images of the patterned samples obtained with amine-functionalized tips was seen to strongly depend on solution pH. High friction was observed between the attractive and strongly interacting functional groups in the methyl-terminated regions at low pH values and the amine-terminated regions at intermediate pH values. Low friction was observed between repulsive and weakly interacting functional groups. In addition, interacting force measurements on approach showed long-range attractive forces between the amine-terminated surfaces at intermediate pH values. Adhesive force measurements showed a dependency on the state of ionization of the amine groups, which was controlled by varying the pH. The pKa value estimated from the measurements was shifted to a lower pH than that of primary amine groups in solution. These results show that functionalized ZnO whisker probe tips have great potential for chemically sensitive imaging.

Effect of Humidity and Hydrophobicity on the Tribological Properties of Self-Assembled Monolayers

The tribological properties of two distinctive alkanethiol SAMs, 16-mercaptohexadecanoic acid (MHA) and 1-octadecanethiol (ODT), on gold substrates in various humidity conditions were examined by lateral force microscopy (LFM). The results suggest that hydrophobic ODT SAM is insensitive to humidity. The difference of lateral force signal is within ±10% regardless of humidity. The lateral force signal of hydrophilic MHA SAMs has a significant decrease in signal in humid environments. The influence of bulk water was also investigated by LFM. By imaging under water, the capillary force is eliminated on ODT SAMs, which leads to a lower lateral force. However, the lateral force image was reversed on MHA SAMs, which suggested that hydrophobic forces dominated in water.

Shear Modulus Property Characterization of Nanorods

We demonstrate an innovative technique for the direct measurement on the shear modulus of an individual nanorod. This measurement is based on atomic force microscopy (AFM) and microfabrication techniques. A nanorod is first aligned along the edge of a small trench in a silicon substrate, and then one end of the nanorod is fixed on the substrate. When an AFM tip scans over the nanorod in contact mode, the nanorod will be twisted by the comprehensive action from the force of the AFM tip, confinement from the trench edge and the fixing end. The shear deformation and the corresponding force that caused the deformation can be retrieved from topography and lateral force image, respectively. By small-angle approximation, the shear modulus of the ZnO NR, which has a radius of 166 nm and a length of 4 μm, is measured to be 8.1 ± 1.9 GPa. This method can be applied directly to characterize the shear modulus of any nanowire/nanorod that possesses a polygon cross section.

Surface Morphology Studies on PBI Membrane Materials of High Temperature for Proton Exchange Membrane Fuel Cells

Compare with the conventional proton exchange membrane fuel cells (PEMFCs), high temperature proton exchange membrane fuel cells (HT-PEMFCs) have more advantages such as higher CO tolerance of catalyst, easier water management and higher catalyst activity. As the core component of the HT-PEMFC, proton exchange membrane should have excellent flexibility , thermal stability and high proton conductivity at high operation temperature and anhydrous environments. By atomic force microscope (AFM) technology, the surface topography image and lateral force image of the untreated and treated polybenzimidazole (PBI) membrane are investigated.

Probing atomic-scale friction on reconstructed surfaces of single-crystal semiconductors

Friction force microscopy (FFM) investigations have been performed on reconstructed (001) surfaces of InSb and Ge in an ultrahigh vacuum. On the $c(8\ifmmode\times\else\texttimes\fi{}2)$ reconstruction of InSb(001) atomic resolution is achieved under superlubric conditions, and the features observed in the lateral force images are precisely reproduced by numerical simulations, taking into account possible decorations of the probing tip. On the simultaneously acquired $(1\ifmmode\times\else\texttimes\fi{}3)$ reconstruction a significant disorder of the surface atoms is observed. If the loading force increases, friction becomes much larger on this reconstruction compared to the $c(8\ifmmode\times\else\texttimes\fi{}2)$ one. In FFM images acquired on the Ge(001)$(2\ifmmode\times\else\texttimes\fi{}1)$ characteristic substructures are resolved within the unit cells. In such a case, a strong dependence of the friction pattern on the scan direction is observed.

Identification of graphene crystallographic orientation by atomic force microscopy

The direct determination of the crystallographic orientation of graphene sheets was performed using lattice resolution atomic force microscopy images. A graphene sample, micromechanically exfoliated onto a SiO2 substrate showing well defined crystal edges, was imaged in lateral force mode. The lateral force images reveal the periodicity of the graphene hexagonal structure allowing the visualization of the lattice symmetries and determination of the crystal orientation. Crystal edges predominantly formed by zigzag or armchair directions were identified. The nature of the edges was confirmed by Raman spectroscopy.

Evaluation of fibrinogen self-assembly: role of its αC region.

Exposure of cryptic, functional sites on fibrinogen upon its adsorption to hydrophobic surfaces of biomaterials has been linked to an inflammatory response and fibrosis. Such adsorption also induces ordered fibrinogen aggregation which is poorly understood. To investigate hydrophobic surface-induced fibrinogen aggregation. Contact and lateral force scanning probe microscopy, yielding topography, image dimensions and fiber elastic modulus measurements were used along with transmission and scanning electron microscopy. Fibrinogen aggregation was induced under non-enzymatic conditions by adsorption on a trioctyl-surface monolayer (trioctylmethylamine) grafted onto silica clay plates. A more than one molecule thick coating was generated by adsorption on the plate from 100 to 200 μg mL⁻¹ fibrinogen solutions, and three-dimensional networks formed from 4 mg mL⁻¹ fibrinogen incubated with uncoated or fibrinogen-coated plates. Fibrils appeared laterally assembled into branching and overlapping fibers whose heights from the surface ranged from approximately 3 to 740 nm. The elastic modulus of fibrinogen fibers was 1.55 MPa. No fibrils formed when fibrinogen lacking αC-domains was used as a coating or was incubated with intact fibrinogen-coated plates, or when the latter plates were sequentially incubated with anti-Aα529-539 mAb and intact fibrinogen. When an anti-Aα241-476 mAb was used instead, fine, long fibers formed. Similarly, sequential incubations of fibrinogen-coated plates with recombinant αC-domain (Aα392-610 fragment) or αC-connector (Aα221-372 fragment) and fibrinogen resulted in distinctly fine fiber networks. Adsorption-induced fibrinogen self-assembly is initiated by a more than one molecule-thick surface layer and eventuates in three-dimensional networks whose formation requires fibrinogen with intact αC-domains.

Quantitative Friction-Force Measurements by Longitudinal Atomic Force Microscope Imaging

Since the first lateral force measurements by atomic force microscopy, one of the main obstacles to quantitative friction-force measurements has been the difficulty in measuring the torsional response of the probes. The influence of friction on images acquired in the usual longitudinal scanning direction has also long been recognized. However, in part due to its less favorable geometry, the longitudinal mode is not typically exploited for friction-force measurements. We show here that quantitative frictional-force measurements are possible in longitudinal imaging and provide several advantages over lateral-force imaging: for instance, topology and frictional effects are coupled in a well-defined way, and there is no need to estimate the torsional spring constant. More importantly, following frictional-force measurements by longitudinal imaging with traditional lateral-force imaging allows a convenient calibration that does not require additional equipment, cantilever preparation, or special samples.

Domain Structures on PS-PMMA Mixture Films

We studied the domain structures in immiscible polystyrene poly(methyl methacrylate) (PS/PMMA) lms by using atomic force microscopy, lateral force microscopy and force spectroscopy. As previously known, circular PS bumps are formed on the PMMA layer after annealing, which was con rmed by the topographic image. We found that lateral force image showed substructures on the PS bumps. The sub-structures (or domains) have circular shapes on the centers of the PS bumps. Because the lateral forces on the domains have similar values on the PMMA layer, the domains are thought to have the chemical composition of PMMA. This estimate is supported by the force spectroscopy result, which shows that the pull-o force between the domain and a cantilever tip also has values similar to that for the PMMA layer.

Transversal mode and thermal analysis of an InP laser diode by near-field scanning probe microscopies

Optical mode and thermal analysis of an indium phosphide (InP) laser diode using photon scanning tunneling microscopy (PSTM) combined with atomic force microscopy (AFM) techniques are performed for industrial development. Different laser diodes emitting at λ=0.98 μm and λ=1.55 μm differentiated by the InP areas doped n or p are characterized. AFM/PSTM analysis provides space localization of the optical modes, identification of doping agents, analysis of the modal structure of the emitting laser diode active area, measurement of losses, and control of the morphology of optical components. The results are corroborated by lateral force imaging.

Nano‐scale shear mode testing of the adhesion of nanoparticles to a surface‐support

AbstractUsing atomic force microscopy (AFM/SFM), the strength of the adhesion between nanoparticles (NPs) and a flat gold surface was studied. By scanning in contact mode, a lateral force was applied to the NPs, depending on the cantilever stiffness and the setpoint value of the topography feedback. In this way, a threshold force for NP detachment was determined. The sample under investigation consisted of arrays of tungsten NPs deposited on top of a flat gold surface. The NPs were generated using electron beam deposition (EBD) and the precursor gas W(CO)6. Owing to the array‐wise arrangement of the NPs, any change in their position was comparatively easy to detect by deviation from the array points. The beam dwell time was employed as a parameter to control the NP size. The force threshold value for particle detachment was found to increase with the beam dwell time. Loose NPs moved by the scanning tip displayed increased lateral forces. These appeared in the lateral force images as continuous tracks. After expansion of the scan area such tracks were seen to mostly originate from the edges of the previous scans where the NPs had piled up. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

411 集束イオンビーム加工によるナノパターンの創製とその応用(T03-3 マイクロナノ理工学:nmからmmまでのテクスチャリングの創成と機能(3),大会テーマセッション,21世紀地球環境革命の機械工学:人・マイクロナノ・エネルギー・環境)

This paper describes the fabrication methods nano-scale patterns and their applications in tribology studies. Focused ion beam (FIB) was used to create two types of asperity patterns. One is the periodic asperity array. The other is the single asperity. In each pattern, the asperity has a hemispherical peak. Three kinds of experiments were carried out using the nano-patterns. First, the pull-off force measured on each asperity pattern was proportional to the curvature radius of asperity peak. Second, a periodic asperity array was used calibrate the output from lateral force microscopy. Image calculated from topography data corresponded well with the lateral force image due to torsion of the cantilever. Finally, periodic asperity pattern was used to form a flat part on the cantilever probe by friction-induced wear.

Deciphering Structural Domains of Alkanethiol Self-Assembled Configurations by Friction Force Microscopy

High resolution and high sensitivity friction force microscopy (FFM) is used to distinguish between different crystallographic domains of standing up molecular configurations of self-assembled alkanethiols partially covering Au(111) surfaces. We propose two suitable methods to decipher structural domains of the same configuration depending on the two-dimensional (2D) symmetry of the organic adlayer. For the hexagonal (radical3xradical3)R30 degrees where no differences among equivalent domains are expected in lattice-resolved scanning force imaging, different molecular domains however can be observed in lateral force images because of the friction asymmetry caused by domains presenting different relative orientations between the molecular tilt direction and the tip scanning direction. Since no lateral packing anisotropy is expected in this close-packed configuration, no friction anisotropy however is observed. Conversely, because of its rectangular space group symmetry, lattice resolved stick-slip imaging is enough to solve between the existing domains for the (2xradical3) rectangular configuration.

Frictional force detection from lateral force microscopic image using a Si grating

We present a simple method to calibrate frictional force from lateral force and topographic images using a commercial scanning probe microscope. A standard sample of Si grating and commercial Si cantilever were used in this study. We have measured the frictional force between the Si tip and functionalized multi-wall carbon nanotube bundles using our calibration method.

Structural and Compositional Mapping of a Phase-Separated Langmuir−Blodgett Monolayer by Atomic Force Microscopy

The structure and composition of a phase-separated arachidic acid (C19H39COOH) (AA) and perfluorotetradecanoic acid (C13F27COOH) (PA) Langmuir-Blodgett monolayer film was characterized by several different types of atomic force microscopic measurements. At the liquid-air interface, surface pressure-area isotherms show that mixtures of the two acids follow the additivity rule expected from ideal mixtures. Topographic images of the deposited monolayer indicate that the surfactants are oriented normal to the substrate surface, and that the acids undergo phase separation to form a series of discontinuous, hexagonal domains separated by a continuous domain. A combination of lateral force (friction) imaging and adhesion force measurements show that the discontinuous domains are enriched in AA, whereas the surrounding continuous domain is a mixture of both AA and PA. This was further verified by selective, in situ dissolution of AA by n-hexadecane, followed by high-resolution topographical imaging of the discontinuous domains.

Atomic-scale topographic and friction force imaging and cantilever dynamics in friction force microscopy

Friction force microscopy (FFM) is commonly used for micro-, nano-, and atomic-scale topographic and friction (lateral) force imaging of surfaces. The experimental-obtained topographic and friction force images are closely correlated to the cantilever dynamics since in FFM, the normal and lateral forces between the cantilever tip and sample surface are measured from the cantilever flexural and twist angles. To understand the cantilever dynamics under tip-surface interaction and its effects on the measured topographic and friction force maps, efficacious models that can accurately simulate the cantilever behavior in operating conditions of FFM are essential. In this paper, a three-dimensional (3D) finite element (FE) beam model is employed to simulate the atomic-scale topographic and friction force profiling process in FFM. The tip-sample interaction forces are modeled as the interatomic forces between the tip and sample surface. It is identified that the topographic and lateral force maps obtained in FFM experiments are the combined results of the real spatial distributions of 3D tip-sample interatomic forces and cantilever dynamics. The experimental-obtained hexagonal (full atomic structure) and trigonal (atomic resolution of every other atom) topographic images of graphite surfaces are reproduced in simulations with different combinations of cantilever geometries, applied normal loads, and scan directions. Based on the simulated results, the methods to realize the observation of the full atomic structure of graphite are discussed.