Kenjiro Kimura

Visualizing water molecule distribution by atomic force microscopy

Hydration structures at biomolecular surfaces are essential for understanding the mechanisms of the various biofunctions and stability of biomolecules. Here, we demonstrate the measurement of local hydration structures using an atomic force microscopy system equipped with a low-noise deflection sensor. We applied this method to the analysis of the muscovite mica/water interface and succeeded in visualizing a hydration structure that is site-specific on a crystal. Furthermore, at the biomolecule/buffer solution interface, we found surface hydration layers that are more packed than those at the muscovite mica/water interface.

Visualization of hydration layers on muscovite mica in aqueous solution by frequency-modulation atomic force microscopy

A three-dimensional interaction force mapping experiment was carried out on a muscovite mica surface in an aqueous solution using a high-resolution and low-thermal drift frequency-modulation atomic force microscope. By collecting oscillatory frequency shift versus distance curves at the mica∕solution interface, complicated hydration structures on the mica surface were visualized. Reconstructed two-dimensional frequency shift maps showed dot-like or honeycomb-like patterns at different tip-sample distances with a separation of 0.2 nm with each other, which agree well to the water molecule density maps predicted by a statistical-mechanical theory. Moreover, site-specific force versus distance curves showed a good agreement with theoretically calculated site-specific force curves by a molecular dynamics simulation. It is found that the first and second hydration layers give honeycomb-like and dot-like patterns in the two-dimensional frequency shift images, respectively, corresponding to the lateral distribution function in each layer.

Water and 2-propanol structured on calcite (104) probed by frequency-modulation atomic force microscopy.

The structure of liquid water and 2-propanol on the (104) surface of calcite (CaCO3) was probed by frequency-modulation atomic force microscopy. The microscope tip scanned each liquid to record the tip-surface force perturbed by the liquid structure at the interface. In water, the force distribution on planes cross-sectional to the surface presents a 0.5-nm-thick checkerboard-like pattern matching the corrugated topography of the calcite surface. This provides evidence that the local water density was laterally and vertically modulated. With 2-propanol, a laterally uniform, vertically layered structure was found between the first laterally structured layer and the bulk liquid. These results are consistent with the density distributions of water and ethanol proposed in earlier X-ray and simulation studies.

Solution–TiO2 Interface Probed by Frequency-Modulation Atomic Force Microscopy

The topography and solvation structure of a solution–TiO2 interface were observed in the dark using highly sensitive, frequency-modulated atomic force microscopy (FM-AFM). The nucleation and growth of an ionic solute, KCl, in this study, were observed in constant frequency-shift topography. The force applied to the tip was determined as a function of tip–surface distance. Modulations were identified on some force curves and were found to be related to the site-specific density of water molecules.

Frequency-modulation atomic force microscopy at high cantilever resonance frequencies using the heterodyne optical beam deflection method

We have developed a frequency-modulation atomic force microscope (FM-AFM) with a wideband cantilever deflection sensor using the heterodyne optical beam deflection method. The method enhances the bandwidth of the deflection measurement up to the maximum frequency for the laser power modulation, which can be as high as gigahertz order. The phase and frequency of the cantilever vibration at 5.24MHz are detected with a deflection noise density of 100fm∕Hz. FM-AFM imaging is performed on a Au(111) surface with a high-frequency cantilever.

Two-dimensional dopant profiling by scanning capacitance force microscopy

Abstract We developed scanning capacitance force microscopy (SCFM) capable of mapping local differential capacitance (∂C/∂V), based on atomic force microscopy (AFM), by detecting an electrostatic force (ESF) between a tip and a sample. While an electric field alternating at an angular frequency (ω) is applied between the tip and the sample, an induced ESF oscillating at its third harmonic frequency (3ω), which contain information on ∂C/∂V is detected using a lock-in amplifier (LIA). In this paper, we showed some dynamic-mode SCFM results obtained on a Si test sample. Clear dopant contrasts were obtained by dynamic-mode SCFM operated in air. An apparent position of the p–n junction was moved when an applied d.c. bias voltage was changed. A dynamic-mode SCFM image obtained in a vacuum condition utilizing frequency modulation (FM) detection method also showed clear dopant contrast.

Minitips in Frequency-Modulation Atomic Force Microscopy at Liquid--Solid Interfaces

A frequency-modulation atomic force microscope was operated in liquid using sharpened and cone-shaped tips. The topography of mica and alkanethiol monolayers was obtained with subnanometer resolution, regardless of nominal tip radius, which was either 10 or 250 nm. Force–distance curves determined over a hexadecane–thiol interface showed force modulations caused by liquid layers structured at the interface. The amplitude of force modulation and the layer-to-layer distance were completely insensitive to the nominal tip radius. These results are evidence that minitips smaller than the nominal radius are present on the tip body and function as a force probe.

Dynamic force microscopy at high cantilever resonance frequencies using heterodyne optical beam deflection method

We have developed a dynamic force microscope (DFM) with a wideband cantilever deflection sensor using heterodyne optical beam deflection (HOBD) method. The bandwidth of HOBD method is limited only by the maximum frequency for laser power modulation, which can be as high as gigahertz order. This technique allows us to use high cantilever resonance frequencies for improving the sensitivity and time response of DFM. In this letter, basic principle and experimental setup of HOBD method are described. Deflection measurement of a cantilever vibration at about 7MHz is demonstrated. Using this cantilever, DFM imaging with a relatively fast scanning speed is performed.

FM-AFM imaging of a commercial polyethylene film immersed in n-dodecane

The subnanometer topography of a partially crystalline polyethylene film was observed in liquid n-dodecane using a frequency-modulation atomic force microscope. Locally ordered structures were found and assigned to a (100) facet of crystalline domains.

Improving sensitivity in electrostatic force detection utilizing cantilever with tailored resonance modes

In order to increase the sensitivity of potential or capacitance measurement in Kelvin probe force microscopy or scanning capacitance force microscopy, the frequency of alternating electric field applied between the tip and the sample is tuned close to the mechanical resonance frequencies of the cantilever. The authors have designed a cantilever with a tailored resonance modes suitable for these measurements. The ratio of the first and the second resonance frequencies is optimized by the finite element method. Furthermore, they modified a commercially available cantilever according to the calculation. The performance of the modified cantilever was demonstrated in the scanning capacitance force microscopy measurement.