Takeshi Fukuma

True atomic resolution in liquid by frequency-modulation atomic force microscopy

The increasing attention directed towards nanobiological science requires high-resolution imaging tools for the liquid environment. We have been successful in recording molecular-resolution images of polydiacetylene in water with the frequency-modulation atomic force microscopy (FM-AFM). With the oscillation amplitude of a force-sensing cantilever reduced to 0.20 nm, we were able to overcome the large frequency noise due to the low Q-factor of cantilever resonance in water. We have obtained vertical and lateral resolutions of 10 pm and 250 pm, respectively. This method enables nondestructive imaging of soft biological samples with a load force on the order of 1 pN.

Development of low noise cantilever deflection sensor for multienvironment frequency-modulation atomic force microscopy

We have developed a low noise cantilever deflection sensor with a deflection noise density of 17fm∕Hz by optimizing the parameters used in optical beam deflection (OBD) method. Using this sensor, we have developed a multienvironment frequency-modulation atomic force microscope (FM-AFM) that can achieve true molecular resolution in various environments such as in moderate vacuum, air, and liquid. The low noise characteristic of the deflection sensor makes it possible to obtain a maximum frequency sensitivity limited by the thermal Brownian motion of the cantilever in every environment. In this paper, the major noise sources in OBD method are discussed in both theoretical and experimental aspects. The excellent noise performance of the deflection sensor is demonstrated in deflection and frequency measurements. True molecular-resolution FM-AFM images of a polydiacetylene single crystal taken in vacuum, air, and water are presented.

Development of liquid-environment frequency modulation atomic force microscope with low noise deflection sensor for cantilevers of various dimensions

We have developed a liquid-environment frequency modulation atomic force microscope (FM-AFM) with a low noise deflection sensor for a wide range of cantilevers with different dimensions. A simple yet accurate equation describing the theoretical limit of the optical beam deflection method in air and liquid is presented. Based on the equation, we have designed a low noise deflection sensor. Replaceable microscope objective lenses are utilized for providing a high magnification optical view (resolution: <3μm) as well as for focusing a laser beam (laser spot size: ∼10μm). Even for a broad range of cantilevers with lengths from 35to125μm, the sensor provides deflection noise densities of less than 11fm∕Hz in air and 16fm∕Hz in water. In particular, a cantilever with a length of 50μm gives the minimum deflection noise density of 5.7fm∕Hz in air and 7.3fm∕Hz in water. True atomic resolution of the developed FM-AFM is demonstrated by imaging mica in water.

Nanoscale Mechanical Characterisation of Amyloid Fibrils Discovered in a Natural Adhesive

Using the atomic force microscope, we have investigated the nanoscale mechanical response of the attachment adhesive of the terrestrial alga Prasiola linearis (Prasiolales, Chlorophyta). We were able to locate and extend highly ordered mechanical structures directly from the natural adhesive matrix of the living plant. The in vivo mechanical response of the structured biopolymer often displayed the repetitive sawtooth force-extension characteristics of a material exhibiting high mechanical strength at the molecular level. Mechanical and histological evidence leads us to propose a mechanism for mechanical strength in our sample based on amyloid fibrils. These proteinaceous, pleated β-sheet complexes are usually associated with neurodegenerative diseases. However, we now conclude that the amyloid protein quaternary structures detected in our material should be considered as a possible generic mechanism for mechanical strength in natural adhesives.

Wideband low-noise optical beam deflection sensor with photothermal excitation for liquid-environment atomic force microscopy

I developed a wideband low-noise optical beam deflection sensor with a photothermal cantilever excitation system for liquid-environment atomic force microscopy. The developed sensor has a 10 MHz bandwidth and 4.7 fm/sq.rt.Hz deflection noise density in water. The theoretically limited noise performance (i.e., the noise level limited only by the photodiode shot noise) has been achieved in liquid for the first time. Owing to the wide bandwidth and the replaceable focus lens design, the sensor is applicable to cantilevers with various dimensions. The deflection noise densities of less than 7.8 fm/sq.rt.Hz have been achieved in water for cantilevers with lengths from 35 to 125 microm. The ideal amplitude and phase versus frequency curves without distortion are obtained with the developed photothermal excitation system. The excitation system is applicable to relatively stiff cantilevers (>20 N/m) in liquid, making it possible to obtain true atomic-resolution images in liquid. True atomic-resolution imaging of mica in water is demonstrated using the developed deflection sensor and the photothermal excitation system.

High resonance frequency force microscope scanner using inertia balance support

We have developed the atomic force microscope scanner with the high resonance frequency of 540kHz in the z axis using a piezosupport mechanism “inertia balance support.” In the method, a cubic piezoactuator is supported at the four sides perpendicular to the extension axis, by which the resonance frequency of the scanner remains as high as that of the actuator in the free vibration. The scanner allows driving at low voltage ±15V for the practical z scan range of 330nm. We demonstrate the applicability of the scanner to the true-atomic-resolution imaging of mica in liquid.

True-molecular resolution imaging by frequency modulation atomic force microscopy in various environments

In this study, we discuss the relationship between Q factor of the cantilever in various environments and frequency noise in frequency modulation atomic force microscopy (FM-AFM). We first present true-molecular resolution FM-AFM images of alkanethiol self-assembled monolayers taken in a moderate vacuum environment (vacuum pressure: 6 Pa) and in air (cantilever Q factor: 390) using FM-AFM with a low noise cantilever deflection sensor. The results reveal that the minimum Q factor to obtain true-molecular resolution in FM-AFM can be less than a few hundred.

Water distribution at solid/liquid interfaces visualized by frequency modulation atomic force microscopy

Abstract Interfacial phenomena at solid/water interfaces play an important role in a wide range of industrial technologies and biological processes. However, it has been a great challenge to directly probe the molecular-scale behavior of water at solid/water interfaces. Recently, there have been tremendous advancements in frequency modulation atomic force microscopy (FM-AFM), enabling its operation in liquids with atomic resolution. The high spatial and force resolutions of FM-AFM have enabled the visualization of one-dimensional (1D) profiles of the hydration force, two-dimensional (2D) images of hydration layers and three-dimensional (3D) images of the water distribution at solid/water interfaces. Here I present an overview of the recent advances in FM-AFM instrumentation and its applications to the study of solid/water interfaces.

Phase modulation atomic force microscope with true atomic resolution

We have developed a dynamic force microscope (DFM) working in a novel operation mode which is referred to as phase modulation atomic force microscopy (PM-AFM). PM-AFM utilizes a fixed-frequency excitation signal to drive a cantilever, which ensures stable imaging even with occasional tip crash and adhesion to the surface. The tip-sample interaction force is detected as a change of the phase difference between the cantilever deflection and excitation signals and hence the time response is not influenced by the Q factor of the cantilever. These features make PM-AFM more suitable for high-speed imaging than existing DFM techniques such as amplitude modulation and frequency modulation atomic force microscopies. Here we present the basic principle of PM-AFM and the theoretical limit of its performance. The design of the developed PM-AFM is described and its theoretically limited noise performance is demonstrated. Finally, we demonstrate the true atomic resolution imaging capability of the developed PM-AFM by i...

Nanometer-Scale Characterization of Ferroelectric Polymer Thin Films by Variable-Temperature Atomic Force Microscopy

The morphological changes of ferroelectric polymer thin films during the annealing process were directly imaged by variable-temperature atomic force microscopy (AFM). The growing process of the crystallites underlying the surface amorphous layer was observed as well as the morphological changes using intermittent-contact AFM. We found that fine structures appeared in the rod like grains at a temperature corresponding to the Curie point of the bulk copolymer in the cooling process. In addition, local piezoelectric response was mapped in order to investigate the relationship between the structural change and the ferroelectric phase transition. The local polarization prepared at room temperature disappeared after the film was heated above 120°C, which agreed with both the Curie temperature and the temperature of the observed structural change.