We have developed a method referred to as three-dimensional scanning force microscopy (3D-SFM) which enables us to visualize water distribution at a solid-liquid interface with an atomic-scale resolution in less than 1 min. The 3D-SFM image obtained at a mica-water interface visualizes 3D distributions of adsorbed water molecules above the center of hexagonal cavities and the laterally distributed hydration layer. The atomically resolved 3D-SFM image showing mirror symmetry suggests the existence of surface relaxation of the cleaved mica surface next to the aqueous environment. DOI: 10.1103/PhysRevLett.104.016101 PACS numbers: 68.37.Ps, 07.79.Lh Muscovite mica ( Fig. 1) is known as a prototype of clay minerals and hence has importance in fundamental research regarding clay swelling in geological science [1][2][3] and cloud seeding in ecological science [4,5]. In addition, owing to the ease of cleavage to present an atomically flat surface, a mica-water interface has been widely used as a model system to investigate nanofluidics in engineering and physics [6], lubrication in tribology, and molecular adsorption and diffusion in biology and chemistry. To date, the water distribution at a mica-water interface has been extensively studied by various techniques [7][8][9][10][11][12][13]. However, its atomistic model has not been established due to the difficulties in visualizing molecular-scale water distribution directly at a solid-liquid interface.Scanning force microscopy (SFM) is a nanoscale imaging technique which visualizes an ''isosurface'' of an interaction force acting between a sharp tip and a surface as a two-dimensional (2D) image [ Fig. 2(a)]. SFM has widely been used for imaging atomic-scale structures at solid-liquid, solid-air, and solid-vacuum interfaces. However, an interface inherently has a three-dimensional (3D) extent in subnanometer dimensions. Therefore, a 2D image obtained by SFM often fails to present important nature of interfacial phenomena. In particular, at a solidliquid interface, solvent molecules interacting with a surface often show 3D local distribution, which has not been fully accessible with conventional 2D-SFM. Here we propose a method referred to as 3D-SFM [ Fig. 2(b)], which enables us to visualize 3D distribution of water at a micawater interface in 53 sec with an atomic-scale resolution. With the obtained 3D-SFM image, we discuss the 3D distribution of adsorbed water molecules and hydration layers as well as the atomic-scale structure of cleaved mica surface next to an aqueous environment.Although the basic principles of 2D-and 3D-SFMs are applicable to various SFM operating modes, here we explain them in the case of frequency modulation (FM) detection mode [14], where the tip-sample interaction force is detected as a resonance frequency shift (Áf) of the vibrating cantilever. In 2D-SFM, the vertical tip position (z t ) is regulated to keep the Áf constant. With this tipsample distance regulation, a tip is laterally scanned in XY to present a 2D height image of ''Áf isosu...

The atomic force microscope (AFM) has a unique capability of allowing the high-resolution imaging of biological samples on substratum surfaces in physiological solutions. Recent technological progress of AFM in biological research has resulted in remarkable improvements in both the imaging rate and the tip force acting on the sample. These improvements have enabled the direct visualization of dynamic structural changes and dynamic interactions occurring in individual biological macromolecules, which is currently not possible with other techniques. Therefore, high-speed AFM is expected to have a revolutionary impact on biological sciences. In addition, the recently achieved atomic resolution in liquids will further expand the usefulness of AFM in biological research. In this article, we first describe the various capabilities required of AFM in biological sciences, which is followed by a detailed description of various devices and techniques developed for high-speed AFM and atomic-resolution in-liquid AFM. We then describe various imaging studies performed using our cutting-edge microscopes and their current capabilities as well as their limitations, and conclude by discussing the future prospects of AFM as an imaging tool in biological research.

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.

True atomic resolution of frequency-modulation atomic force microscopy in liquid is demonstrated. Hexagonal lattice of a cleaved ͑001͒ surface of muscovite mica is resolved in water. Nonperiodic structures such as defects and adsorbates are simultaneously imaged with the atomic-scale features of mica surface. The use of small oscillation amplitude ͑0.16-0.33 nm͒ of a force sensing cantilever allows us to obtain vertical and lateral resolutions of 2-6 and 300 pm, respectively, even with a low Q factor in water ͑Q = 20-30͒.