Noriaki Oyabu

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.

True Atomic-Resolution Imaging of (101̅4) Calcite in Aqueous Solution by Frequency Modulation Atomic Force Microscopy

Calcite (CaCO3) is one of the most abundant minerals on earth and plays an important role in a wide range of different fields including, for example, biomineralization and environmental geochemistry. Consequently, surface processes and reactions such as dissolution and growth as well as (macro)molecule adsorption are of greatest interest for both applied as well as fundamental research. An in-depth understanding of these processes requires knowledge about the detailed surface structure in its natural state which is quite often a liquid environment. We have studied the most stable cleavage plane of calcite under liquid conditions using frequency modulation atomic force microscopy. Using this technique, we achieved true atomic-resolution imaging, demonstrating the high-resolution capability of frequency modulation atomic force microscopy in liquids. We could reproduce contrast features reported before using contact mode atomic force microscopy, originating from the protruding oxygen atom of the carbonate groups. Besides this contrast, however, our results, indeed, indicate that we obtain more detailed structural information, revealing the calcium sublattice of the (1014) cleavage plane.

Drift-compensated data acquisition performed at room temperature with frequency modulation atomic force microscopy

The authors have performed distortionless atom imaging and force mapping experiments, under a large thermal drift condition at room temperature (RT), using frequency modulation atomic force microscopy (FM-AFM) that had been done previously only at low temperature. In the authors’ experimental scheme, three-dimensional position feedback with atom tracking detects the thermal drift velocity that is constant for a period of time at RT. The detected velocity is then used as the model for implementing the feedforward in order to compensate for the thermal drift. This technique can be expected to be used for precise positioning of the tip-sample in atom manipulation experiments using the FM-AFM at RT.

Lateral manipulation of single atoms at semiconductor surfaces using atomic force microscopy

Experimental results on the lateral manipulation of single atoms at semiconductor surfaces using non-contact atomic force microscopy (NC-AFM) are presented. These experiments prove that deposited adsorbates on top of a surface, as well as intrinsic adatoms of semiconductor surfaces, are suitable for being manipulated using the short-range interaction force acting between the outermost atoms of a semiconductor tip and the atoms at the surface. The analysis of the data from some of the experiments presented here indicates a pulling process of the tip on the manipulated atoms. The atom-by-atom creation, at room temperature, of patterns composed by a few inherent atoms of a heterogeneous surface is also presented.

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.

Atomic-Resolution Imaging of Graphite–Water Interface by Frequency Modulation Atomic Force Microscopy

Atomic-resolution images of a graphite (0001) surface in water were successfully obtained by frequency modulation atomic force microscopy. Atomic scale features with a periodicity of 0.25 nm were resolved with an interaction force of less than 100 pN using a stiff cantilever and a very small oscillation amplitude of 0.11 nm (0.21 nm peak-to-peak). Furthermore, structured-water layers on a hydrophobic graphite surface were visualized by two-dimensional frequency shift mapping. The results were compared with a molecular-scale hydration structure at an interface between a hydrophilic mica surface and water.

High-Resolution Frequency-Modulation Atomic Force Microscopy in Liquids Using Electrostatic Excitation Method

We developed a novel method to drive the cantilever oscillation for frequency modulation atomic force microscopy (FM-AFM) in liquid environments using electrostatic excitation. The cantilever with a gold backside coating was vibrated by applying an oscillating bias voltage between the cantilever backside and an optically transparent electrode used as a liquid cell window. The frequency spectrum of the oscillation shows a simple resonance curve without spurious peaks. The method does not require electrical conductivity of samples at all. In fact, both muscovite mica and potassium chloride surfaces in aqueous solutions were successfully imaged on an atomic scale.

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.

Practical aspects of Kelvin-probe force microscopy at solid/liquid interfaces in various liquid media

The distributions of surface charges or surface potentials on biological molecules and electrodes are directly related to various biological functions and ionic adsorptions, respectively. Electrostatic force microscopy and Kelvin-probe force microscopy (KFM) are useful scanning probe techniques that can map local surface charges and potentials. Here, we report the measurement and analysis of the electrostatic and capacitive forces on the cantilever tip induced by application of an alternating voltage in order to discuss the feasibility of measuring the surface charge or potential distribution at solid/liquid interfaces in various liquid media. The results presented here suggest that a nanometer-scale surface charge or potential measurement by the conventional voltage modulation techniques is only possible under ambient conditions and in a non-polar medium and is difficult in an aqueous solution. Practically, the electrostatic force versus dc voltage curve in water does not include the minimum, which is us...

Analysis of capacitive force acting on a cantilever tip at solid/liquid interfaces

Dielectric properties of biomolecules or biomembranes are directly related to their structures and biological activities. Capacitance force microscopy based on the cantilever deflection detection is a useful scanning probe technique that can map local dielectric constant. Here we report measurements and analysis of the capacitive force acting on a cantilever tip at solid/liquid interfaces induced by application of an alternating voltage to explore the feasibility of the measurements of local dielectric constant by the voltage modulation technique in aqueous solutions. The results presented here suggest that the local dielectric constant measurements by the conventional voltage modulation technique are basically possible even in polar liquid media. However, the cantilever deflection is not only induced by the electrostatic force, but also by the surface stress, which does not include the local dielectric information. Moreover, since the voltage applied between the tip and sample are divided by the electric...