Frequency Modulation Atomic Force Microscopy Research Articles

Frequency modulation atomic force microscopy (FM-AFM) observation for the superlow friction interface of diamond-like carbon lubricated with an environmentally friendly ester-based oil

Frequency modulation atomic force microscopy (FM-AFM) observation for the superlow friction interface of diamond-like carbon lubricated with an environmentally friendly ester-based oil

Molecular-scale structures of the surface and hydration shell of bioinert mixed-charged self-assembled monolayers investigated by frequency modulation atomic force microscopy.

We studied the surface structure and hydration structure of a bioinert mix-charged self-assembled monolayer (MC-SAM) comprised of sulfonic acid (SA)- and trimethylamine (TMA)-terminated thiols in liquid by frequency modulation atomic force microscopy (FM-AFM) at a molecular-scale. The TMA end groups showed a highly-ordered rectangular arrangement on a gold substrate in phosphate buffer saline (PBS). Highly structured water was observed at the interface of the MC-SAM and PBS, whereas a less structured hydration structure was observed on bioactive SAMs such as those with OH– and COO– terminal groups. Differences in surface and interface structures between the bioactive and bioinert SAMs suggest that the highly structured water at the bipolar MC-SAM surface works as a physical barrier to prevent adsorption or adhesion of protein and cells. Our results led to the idea that the hydration structure is an important factor in the determination of interactions between SAMs and biomolecules.

Measurement of Adsorbed Gas Molecules on Graphite Surface by Using Frequency Modulation Atomic Force Microscopy in Liquid

Measurement of Adsorbed Gas Molecules on Graphite Surface by Using Frequency Modulation Atomic Force Microscopy in Liquid

Atomic-resolution three-dimensional hydration structures on a heterogeneously charged surface

Local hydration structures at the solid–liquid interface around boundary edges on heterostructures are key to an atomic-level understanding of various physical, chemical and biological processes. Recently, we succeeded in visualising atomic-scale three-dimensional hydration structures by using ultra-low noise frequency-modulation atomic force microscopy. However, the time-consuming three-dimensional-map measurements on uneven heterogeneous surfaces have not been achieved due to experimental difficulties, to the best of our knowledge. Here, we report the local hydration structures formed on a heterogeneously charged phyllosilicate surface using a recently established fast and nondestructive acquisition protocol. We discover intermediate regions formed at step edges of the charged surface. By combining with molecular dynamics simulations, we reveal that the distinct structural hydrations are hard to observe in these regions, unlike the charged surface regions, possibly due to the depletion of ions at the edges. Our methodology and findings could be crucial for the exploration of further functionalities.

Rhombic-Shaped Nanostructures and Mechanical Properties of 2D DNA Origami Constructed with Different Crossover/Nick Designs.

DNA origami methods enable the fabrication of various nanostructures and nanodevices, but their effective use depends on an understanding of their structural and mechanical properties and the effects of basic structural features. Frequency-modulation atomic force microscopy is introduced to directly characterize, in aqueous solution, the crossover regions of sets of 2D DNA origami based on different crossover/nick designs. Rhombic-shaped nanostructures formed under the influence of flexible crossovers placed between DNA helices are observed in DNA origami incorporating crossovers every 3, 4, or 6 DNA turns. The bending rigidity of crossovers is determined to be only one-third of that of the DNA helix, based on interhelical electrostatic forces reported elsewhere, and the measured pitches of the 3-turn crossover design rhombic-shaped nanostructures undergoing negligible bending. To evaluate the robustness of their structural integrity, they are intentionally and simultaneously stressed using force-controlled atomic force microscopy. DNA crossovers are verified to have a stabilizing effect on the structural robustness, while the nicks have an opposite effect. The structural and mechanical properties of DNA origami and the effects of crossovers and nicks revealed in this paper can provide information essential for the design of versatile DNA origami structures that exhibit specified and desirable properties.

Dynamics of anchored oscillating nanomenisci

We present a self-contained study of the dynamics of oscillating nanomenisci anchored on nanometric topographical defects around a cylindrical nanofiber -- radius below 100 nm. Using frequency-modulation atomic force microscopy (FM-AFM), we show that the friction coefficient surges as the contact angle is decreased. We propose a theoretical model within the lubrification approximation that reproduces the experimental data and provides a comprehensive description of the dynamics of the nanomeniscus. The dissipation pattern in the vicinity of the contact line and the anchoring properties are discussed as a function of liquid and surface properties in addition to the sollicitation conditions.

Apical surface supracellular mechanical properties in polarized epithelium using noninvasive acoustic force spectroscopy

Maintenance of epithelial tissue integrity requires coordination between cell–cell adherens junctions, tight junctions (TJ), and the perijunctional actomyosin cytoskeleton. Here we addressed the hypothesis that alterations in TJ structure and remodeling of the actomyosin cytoskeleton modify epithelial mechanics. Current methods to measure supracellular mechanical properties disrupt intact monolayers, therefore, we developed a novel method using noncontact acoustic frequency-modulation atomic force microscopy (FM-AFM) and tested it on MDCK polarized monolayers. Our results show that double knockdown (dKD) of ZO-1/ZO-2 elevates the apical epithelial tension and effective viscosity. Interestingly, epithelial tension is more sensitive to inhibition of myosin II ATPase activity than to inhibition of ROCK activity, but viscosity is highly sensitive to both. Additionally, we showed epithelial intercellular pulling forces at tricellular junctions and adhesion forces in dKD cells are elevated with an increase in contractility. In conclusion, FM-AFM enables the physiological and quantitative investigation of mechanics in intact epithelium.

Self-assembled monolayers of sulfonate-terminated alkanethiols investigated by frequency modulation atomic force microscopy in liquid

A molecular-scale understanding of self-assembled monolayers (SAMs) of sulfonate-terminated alkanethiols is crucial for interfacial studies of functionalized SAMs and their various applications. However, such an understanding has been difficult to achieve because of the lack of direct information on these molecular-scale structures in real space. In this study, we investigated the structures of sulfonate SAMs of sodium 11-mercapto-1-undecanesulfonate (MUS) by frequency modulation atomic force microscopy (FM-AFM) in liquid. The subnanometer-resolution FM-AFM images showed that the single-component MUS SAM prepared in pure water had random surface structures. In contrast, the MUS SAM prepared in a water–ethanol mixed solvent showed periodic striped structures with a flat-lying conformation. The results suggest a significant solvent effect on molecular-scale structures of long-chain sulfonate SAMs. In addition, we investigated the molecular-scale structures of mixed SAMs of MUS and 11-mercapto-1-undecanol (MUO) with alkane chains of the same length. The FM-AFM images of the mixed SAMs showed clear phase separation between MUS SAM and MUO SAM domains. In the MUO SAM domains, the incorporated MUS molecules appeared as protrusions. The results obtained in this study provide direct structural information on long-chain sulfonate and mixed SAMs.

Cross-Sectional Imaging of Boundary Lubrication Layer Formed by Fatty Acid by Means of Frequency-Modulation Atomic Force Microscopy.

To observe in situ the adsorption of fatty acid onto metal surfaces, cross-sectional images of the adsorption layer were acquired by frequency-modulation atomic force microscopy (FM-AFM). Hexadecane and palmitic acid were used as the base oil and typical fatty acid, respectively. A Cu-coated silicon wafer was prepared as the target substrate. The solvation structure formed by hexadecane molecules at the interface between the Cu substrate and the hexadecane was observed, and the layer pitch was found to be about 0.6 nm, which corresponds to the height of hexadecane molecules. This demonstrates that hexadecane molecules physically adsorbed onto the surface due to van der Waals forces with lying orientation because hexadecane is a nonpolar hydrocarbon. When hexadecane with palmitic acid was put on the Cu substrate instead of pure hexadecane, an adsorption layer of palmitic acid was observed at the interface. The layer pitch was about 2.5-2.8 nm, which matches the chain length of palmitic acid molecules well. This indicates that the original adsorption layer was monolayer or single bilayer in the local area. In addition, a cross-sectional image captured 1 h after observation started to reveal that the adsorbed additive layer gradually grew up to be thicker than about 20 nm due to an external stimulus, such as cantilever oscillation. This is the first report of in situ observation of an adsorbed layer by FM-AFM in the tribology field and demonstrates that FM-AFM is useful for clarifying the actual boundary lubrication mechanism.

Nanoscale characterisation of TiO2(110) annealed in air

Abstract Rutile titanium dioxide (TiO2) (110) surfaces annealed in air were examined by frequency modulation atomic force microscopy (FM-AFM), X-ray photoelectron spectroscopy (XPS) and low-energy electron diffraction (LEED) techniques. The terraces separated by monatomic steps showed an atomically irregular appearance in the obtained FM-AFM images, while the surface produced a (1 × 1) LEED pattern that indicated atomic ordering of the top few surface layers. Subsequent annealing in atmospheric-pressure O2 produced several ten-nanometre-sized pits with irregular shapes on the terraces. Molecule-sized spots were recognised on the subsequently O2-annealed surface. Half-order spots along the 〈001〉 directions emerged in the LEED pattern. We propose the rutile substrate covered with hydroxylated amorphous TiO2 of monatomic thickness as the structure of the air-annealed surface. The amorphous TiO2 was dehydroxylated by subsequent O2 annealing and was transformed into the thermodynamically stable rutile structure. The molecule-sized spots were attributed to hydroxyl (OH) and hydroperoxyl (OOH) groups that were produced on the area with the (1 × 1) structure. A part of the OH and OOH groups were arranged in a (2 × 1) structure and produced the half-order spots in the LEED pattern.

Interface structure between tetraglyme and graphite.

Clarification of the details of the interface structure between liquids and solids is crucial for understanding the fundamental processes of physical functions. Herein, we investigate the structure of the interface between tetraglyme and graphite and propose a model for the interface structure based on the observation of frequency-modulation atomic force microscopy in liquids. The ordering and distorted adsorption of tetraglyme on graphite were observed. It is found that tetraglyme stably adsorbs on graphite. Density functional theory calculations supported the adsorption structure. In the liquid phase, there is a layered structure of the molecular distribution with an average distance of 0.60 nm between layers.

Researchers observe detailed structure of a model interface for a rechargeable battery

Japanese scientists use frequency-modulation atomic force microscopy to study the tetraglyme-graphite interface.

Focus on microscopy, imaging, and nanotechnology

Focus on microscopy, imaging, and nanotechnology

Localization of cesium on montmorillonite surface investigated by frequency modulation atomic force microscopy

Cation exchange of clay mineral is typically analyzed without microscopic study of the clay surfaces. In order to reveal the distribution of exchangeable cations at the clay surface, we performed in situ atomic-scale observations of the surface changes in Na-rich montmorillonite due to exchange with Cs cations using frequency modulation atomic force microscopy (FM-AFM). Lines of protrusion were observed on the surface in aqueous CsCl solution. The amount of Cs of the montmorillonite particles analyzed by energy dispersive X-ray spectrometry was consistent with the ratio of the number of linear protrusions to all protrusions in the FM-AFM images. The results showed that the protrusions represent adsorbed Cs cations. The images indicated that Cs cations at the surface were immobile, and their occupancy remained constant at 10% of the cation sites at the surface with different immersion times in the CsCl solution. This suggests that the mobility and the number of Cs cations at the surface are controlled by the permanent charge of montmorillonite; however, the Cs distribution at the surface is independent of the charge distribution of the inner silicate layer. Our atomic-scale observations demonstrate that surface cations are distributed in different ways in montmorillonite and mica.

Low noise current preamplifier for qPlus sensor deflection signal detection in atomic force microscopy at room and low temperatures.

The resolution of frequency modulation atomic force microscopy is limited by instrumental noise. When using a qPlus sensor, the deflection detector noise is the dominant noise contribution. It can be reduced by improving the preamplifier used to amplify the sensor deflection signal. We present a simple single-stage differential preamplifier which outperforms previous designs known to us by at least a factor of two in the deflection noise density. We show specific versions of this preamplifier to use in ambient conditions, in ultra-high vacuum at room temperature, and at 4.2 K. Furthermore, we compare the thermal peak analysis and the frequency shift noise density method as a means to determine the deflection noise density. We note that this preamplifier can also be used for any current-generating sensors such as other piezoelectric sensors and photodiodes, but, in this paper, we restrict our analysis to qPlus sensors.

Dissolution Processes at Step Edges of Calcite in Water Investigated by High-Speed Frequency Modulation Atomic Force Microscopy and Simulation.

The microscopic understanding of the crystal growth and dissolution processes have been greatly advanced by the direct imaging of nanoscale step flows by atomic force microscopy (AFM), optical interferometry, and X-ray microscopy. However, one of the most fundamental events that govern their kinetics, namely, atomistic events at the step edges, have not been well understood. In this study, we have developed high-speed frequency modulation AFM (FM-AFM) and enabled true atomic-resolution imaging in liquid at ∼1 s/frame, which is ∼50 times faster than the conventional FM-AFM. With the developed AFM, we have directly imaged subnanometer-scale surface structures around the moving step edges of calcite during its dissolution in water. The obtained images reveal that the transition region with typical width of a few nanometers is formed along the step edges. Building upon insight in previous studies, our simulations suggest that the transition region is most likely to be a Ca(OH)2 monolayer formed as an intermediate state in the dissolution process. On the basis of this finding, we improve our understanding of the atomistic dissolution model of calcite in water. These results open up a wide range of future applications of the high-speed FM-AFM to the studies on various dynamic processes at solid-liquid interfaces with true atomic resolution.

Structural change of nonionic surfactant self-assembling at electrochemically controlled HOPG/electrolyte interface

In this study, we have observed structural change of self-assembling nonionic pentaethylene glycol monododecyl ether (C12E5) surfactant formed on a potential-controlled highly oriented pyrolytic graphite (HOPG) surface by electrochemical frequency modulation atomic force microscopy (EC-FM-AFM). At a negative and a positive potential side with respect to the point of zero charge (pzc), a periodic hemicylindrical structure is formed on the whole surface, while a disordered single-layered thin film is formed at a potential around the pzc. This demonstrates that the electrochemical tuning of the electrode potential enables precise control of the nonionic self-assembly structure. We also found that the self-assembly structure is expanded by Na+ ion contained in C12E5 solution. This can be reasonably explained by selective cation binding of a C12E5 molecule like a crown ether. In addition, we estimated a response time of phase transition between the two phases by electrochemical impedance measurement.

Quasi-stabilized hydration layers on muscovite mica under a thin water film grown from humid air

The interfaces between solids and water films in air play fundamental roles in physicochemical phenomena, biological functions, and nano-fabrication. Though the properties of the interfaces have been considered to be irrelevant to the water film thickness, we found distinctive mechanical features of the interface between a cleaved muscovite mica surface and a thin water film grown in humid air, dissimilar to those in bulk water, using frequency-modulation atomic force microscopy. The thin water film grew with quasi-stabilized hydration networks of water molecules, tightly bound each other at the interface, to a thickness of ~2 nm at near-saturating humidity. Consequently, defective structures of the hydration networks persisted vertically through the hydration layers at the interface, and K+ ions on the cleaved surface remained without dissolution into the water film. The results provide atomistic insights into thin water films in regard to epitaxial-like growth from vapour and the motion of water molecules and ions therein.

Force-gradient sensitive Kelvin probe force microscopy by dissipative electrostatic force modulation

We report a Kelvin probe force microscopy (KPFM) implementation using the dissipation signal of a frequency modulation atomic force microscopy that is capable of detecting the gradient of electrostatic force rather than electrostatic force. It features a simple implementation and faster scanning as it requires no low frequency modulation. We show that applying a coherent ac voltage with two times the cantilever oscillation frequency induces the dissipation signal proportional to the electrostatic force gradient which depends on the effective dc bias voltage including the contact potential difference. We demonstrate the KPFM images of a MoS2 flake taken with the present method are in quantitative agreement with those taken with the frequency modulated Kelvin probe force microscopy technique.

New Information on the Hydrophobic Interaction Revealed by Frequency Modulation AFM.

Using ultrahigh resolution atomic force microscopy (AFM) operated in frequency modulation mode, we extend existing measurements of the force acting between hydrophobic surfaces immersed in water in three essential ways. (1) The measurement range, which was previously limited to distances longer than 2-3 nm, is extended to cover all distances, down to contact. The measurements disclose that the long-range attraction observed also by conventional techniques, turns at distances shorter than 1-2 nm into pronounced repulsion. (2) Simultaneous measurements of the dissipative component of the tip-surface interaction reveal an anomalously large dissipation commencing abruptly at the point where attraction begins. The dissipation is more than 2 orders of magnitude larger than expected from bulk water viscosity or from similar measurements between hydrophilic surfaces. (3) The short-range repulsion is oscillatory, indicating molecular ordering of the medium as the hydrophobic surfaces approach each other. The oscillation period, ∼0.5 nm, is larger than the ∼0.3 nm period observed with hydrophilic surfaces. Their range, ∼1.5 nm, is longer as well. These observations are consistent with a conspicuous change in the properties of the surrounding medium, taking place simultaneously with the onset of attraction as the two surfaces approach each other.