Using room-temperature (RT) site-specific force spectroscopy F(Z), we developed a nondestructive single-atom chemical identification method at RT, which is independent of tip, topography (height difference) and chemical coordination, but dependent on atom species, with atomic resolution. We demonstrated atom-by-atom chemical identification of intermixed individual atoms on a Pb/Sn/Si(111)-(√3×√3) alloy surface. Further, we developed a method for RT site-specific force curve measurements F(Z) to apply to RT force mapping F(X, Z). From RT site-specific frequency-shift maps Δf(X, Z) of a Si(111)-(7×7) surface, we deduced normal force maps FZ(X, Z), then potential maps U(X, Z), and finally lateral force map FX(X, Z), successively. Thus, we obtained not only potential maps, but also lateral force maps of the Si(111)-(7×7) surface under noncontact region at RT on an atomic scale.
Atomic force microscopy (AFM) using frequency modulation (FM) detection has been widely used for the atomic scale investigations of various materials. However, high-resolution imaging in liquids by FM-AFM is severely hindered by the extreme reduction of the Q-factor due to the hydrodynamic interaction between the cantilever and the liquid. Recently the use of the small amplitude mode and the large noise reduction in the cantilever deflection sensor brought a great progress in FM-AFM imaging in liquids. In this article the details of the problems in FM-AFM imaging in liquids as well as the general directions of the improvements are described. The present status of the high-resolution FM-AFM imaging is also presented. Furthermore, future prospects and possible applications of FM-AFM in liquids are discussed.
Force sensing in nano-scale can be realized by using nanoparticles of “elastico-luminescence” (a new inorganic material which emits light in response to externally applied mechanical stress in elastic region). By applying the material as a coating on the surface of subject, a technology for two-dimensional visualization of stress distribution has been innovated. In order to characterize the stress-induced luminescence from a single particle, we have developed an evaluation equipment utilizing an atomic force microscope (AFM) and photon counting system, and have succeeded in capturing the stress-induced luminescence from a single nanoparticle. The luminescence intensity increases with the increasing force on the nanoparticle, demonstrating that elastico-luminescence nanoparticle can act as force/strain sensor in nanometer region. A force sensor with a diameter of 20 nm has been developed.
The aim of this study is to measure spatial distribution of local elasticity in living cell surfaces. A scanning probe microscope (SPM) has been developed as a powerful tool for obtaining high resolution topographic images of biological samples. The SPM can also be used to evaluate mechanical properties because its probe is physically in contact with the samples during measurement. To obtain cellular stiffness with SPM, we have developed two methods: a force modulation mode and a force mapping mode. It was revealed that spatial and temporal distributions of cellular stiffness originate in cytoskeletal distribution, mode of cellular migration, and intracellular contractile force.
A novel surface spectroscopic method referred to itself as noncontact atomic force spectroscopy (nc-AFS) is presented, which is based on noncontact atomic force microscopy (nc-AFM) and scanning tunneling spectroscopy (STS) of the family of scanning probe microscopy (SPM). The interaction force and current are measured with sweeping bias voltage between a tip and a sample at a close tip-sample separation, and analyzed in terms of surface spectroscopy. The spectra obtained by the nc-AFS indicate that the resonance states, i. e., covalent bonding, between a Si tip and a Si sample can be formed by tuning the bias voltage, corresponding to the relative shift of energy levels of tip states and sample states. Moreover, the nc-AFS combined with current measurement exhibits potential of evaluating the collapse of tunneling barrier and analyzing the correlation between force interaction and electron conductance between two pieces of condensed matter in proximity.
We report here surface potential and differential electrostatic force images of DNAs, proteins, and gold nanoparticles on insulating substrates. The frequency-shift mode was essential for these experiments to satisfy enough sensitivity to use indirect modulation and to avoid unexpected charge injection into insulating substrate surface caused by tip-sample contact. It is obvious that the surface potential of insulating substrate is indefinable and influenced by treatment process and conditions. However, the potential difference between adsorbates and substrate surface is meaningful, reflecting charge or dipoles of adsorbates. We demonstrated that electrostatic force microscopy provides characteristic contrast inversion between DNA and transcription complex images reflecting the difference of electric polarizability of these molecules. These findings indicate that the electrostatic properties of individual biological molecules can be imaged on an insulator substrate during the retaining complex formation.
We report the studies of redox-active ferrocene (Fc) and tetrathiafulvalene (TTF) derivative islands embedded in n-decanethiol self-assembled monolayers on Au(111) using electrochemical scanning tunneling microscopy (EC-STM). We performed EC-STM measurements of both islands of varied sizes at various potentials in 0.05 M HClO4 solution. While the apparent height of Fc islands depended on the tip and sample potentials, that of TTF islands did not depend on the potentials except in the case of very small islands (less than ten molecules). Furthermore, the larger the size of TTF islands, the higher the apparent height in the STM images. This is quite contrary to the case of Fc islands where the apparent height did not depend on the island size. We rationalize these behaviors in terms of the differences in intermolecular interaction and effective electron transfer between redox-active moieties.
Humans obtain a tactile feeling when they rub a surface of an object. The feeling is associated with some properties of the object, therefore they obtain some information about it by tactile sensation. For example, humans memorize the tactile feeling of five kinds of sand particles with different mean diameters of 0.24−0.66 mm, and discriminate them precisely to some extent by using the slight differences in their tactile feelings. An artificial system has been constructed for obtaining signals induced by friction with sand particles between sliding surfaces. The signals are found to contain the information about particle size, and the system discriminates the five kinds of sand particles comparably with humans.
The electronic structures of many materials can be well described by the one-particle picture within the band theory. A quantitative evaluation of band gaps in semiconductors and insulators, however, needs special care because the subtle balance between the exchange and correlation effects must be properly taken into account. This article reviews theoretical approaches for calculating band gaps of materials based on 1) many-body perturbation theory, 2) density-functional theory, and 3) wavefunction-based theory, and the advantages and of each theory disadvantages are discussed.