Hirotaka Hosoi

A pH sensor based on electric properties of nanotubes on a glass substrate

We fabricated a pH-sensitive device on a glass substrate based on properties of carbon nanotubes. Nanotubes were immobilized specifically on chemically modified areas on a substrate followed by deposition of metallic source and drain electrodes on the area. Some nanotubes connected the source and drain electrodes. A top gate electrode was fabricated on an insulating layer of silane coupling agent on the nanotube. The device showed properties of ann-type field effect transistor when a potential was applied to the nanotube from the top gate electrode. Before fabrication of the insulating layer, the device showed that thep-type field effect transistor and the current through the source and drain electrodes depend on the buffer pH. The current increases with decreasing pH of the CNT solution. This device, which can detect pH, is applicable for use as a biosensor through modification of the CNT surface.

Magnetic imaging with scanning probe microscopy

We review our research on the application of scanning tunnelling microscopy (STM) and non-contact atomic force microscopy (NC-AFM) for magnetic imaging in high spatial resolution even down to the atomic scale. In the first part, we propose a new experimental scheme of spin-polarized STM (SP-STM) with a GaAs spin probe to obtain a large contribution of spin-polarized electrons in the tunnelling current. This is yielded by injecting the spin-polarized photo-excited electrons in an optically pumped GaAs tip into the spin-polarized empty states near the Fermi level of a bcc-Fe(001) surface. According to the bandgap energy of GaAs and the surface state of the sample observed at 0.4 eV above the Fermi level, the spin-dependant electron injection can be achieved by applying a sample bias voltage of −1 V. The tunnel current in the positive bias region depends on the helicity of the circular polarized pumping light, and is modified when the applied magnetic field is reversed. Mapping the current asymmetry provides a spin-dependent SP-STM image. In the second part, we describe the progress towards spin imaging with NC-AFM. The spin imaging can be achieved by detecting short-range magnetic interaction such as exchange interaction between a ferromagnetic tip and a magnetic sample. We demonstrate the capabilities of NC-AFM by imaging the spin structure of an antiferromagnetic NiO(001) surface on the atomic scale. The cross-sectional line profiles of the atomically resolved images obtained using several ferromagnetic tips (Fe, Ni) were analysed by adding the atomic corrugation amplitude on the basis of the periodicity of the image. The results of the analysis show that the difference of the neighbouring maxima depends on the crystal direction. On the other hand, no significant indication of the directional dependency can be seen on the images obtained by using a non-magnetic Si tip. The directional dependency coincides with the antiferromagnetic spin alignment of the NiO(001) surface.

Investigations on the topographical asymmetry of non-contact atomic force microscopy images of NiO(001) surface observed with a ferromagnetic tip

We investigated atomically resolved NC-AFM images of NiO(001) surfaces obtained with ferromagnetic Fe- and Ni-coated tips and bare Si tips. The cross-sections of atomically resolved images were analysed by adding the atomic corrugation amplitude on the basis of the periodicity of the image. The topographical asymmetry is defined as an index representing the difference of the adjacent maxima. We compare the topographical asymmetry calculated from the images obtained with ferromagnetic metal- and non-coated Si tips and discuss the possible origin of the asymmetry of NC-AFM images.

Direct observation of dynamic force propagation between focal adhesions of cells on microposts by atomic force microscopy

We investigated dynamic force propagation between focal adhesions of fibroblast cells cultured on polydimethylsiloxane micropost substrates, by atomic force microscopy. Live cells were mechanically modulated by the atomic force microscopy probe bound to cell apical surfaces at 0.01–0.5 Hz, while microposts served as a force sensor at basal surfaces. We observed that cells exhibited rheological behavior at the apical surface but had no apparent out-of-phase response at the basal surface, indicating that the dynamic force propagating through cytoskeletal filaments behaves in an elastic manner. Moreover, the direction of the propagated force was observed to be intimately associated with the prestress.

Noncontact Atomic Force Microscopy and its Related Topics

Scanning probe microscopy (SPM) methods such as scanning tunneling microscopy (STM) and noncontact atomic force microscopy (NC-AFM) are the basic technologies for nanotechnology and also for future bottom-up processes. In Sect. 23.1, the principles of AFM such as its operating modes and the NC-AFM frequency-modulation method are fully explained. Then, in Sect. 23.2, applications of NC-AFM to semiconductors, which make clear its potential in terms of spatial resolution and function, are introduced. Next, in Sect. 23.3, applications of NC-AFM to insulators such as alkali halides, fluorides and transition-metal oxides are introduced. Lastly, in Sect. 23.4, applications of NC-AFM to molecules such as carboxylate (RCOO – ) with R = H, CH3, C(CH3)3 and CF3 are introduced. Thus, NC-AFM can observe atoms and molecules on various kinds of surfaces such as semiconductors, insulators and metal oxides with atomic or molecular resolution. These sections are essential to understand the state of the art and future possibilities for NC-AFM, which is the second generation of atom/molecule technology.

Tip–sample distance dependency of non-contact atomic force microscopy images on a GaAs(110) surface

Tip–sample interactions are at the origin of atomic resolution in non-contact atomic force microscopy (NC-AFM) but also make it difficult to understand the meaning of atomic features observed as NC-AFM images. (110) surfaces of III–V semiconductors such as GaAs or InP have relaxed (1 × 1) surfaces with both anions and cations of different dangling bond states. Since these atoms are located at the surface where an atom attached to the tip apex can reach to interact with them and show significant relaxation via tip–sample interaction, NC-AFM images of the surface are expected to strongly depend upon the atomic species or the electronic state of the topmost atom at the tip apex. We have taken a number of NC-AFM images of GaAs(110) surfaces with a Si tip by means of room-temperature ultra-high vacuum NC-AFM and categorized them into two types of image which have different tip–sample distance dependencies. In comparison with a theoretical prediction of tip–surface interaction on the GaAs(110) surface, we have found that one of these images is due to a Si atom attached to the apex which reveals the tip–sample interaction with As and Ga surface atoms as individual peaks in the NC-AFM image.

Non-Contact Atomic Force Microscopy Observation on GaAs(110) Surface with Tip-Induced Relaxation

We investigate the tip-sample dependence of atomically resolved non-contact atomic force microscopy (NC-AFM) images of a GaAs(110) surface taken with a tip that can resolve the tip-sample interaction originating from the dangling bonds of Ga atoms and the valence charge distribution around As atoms. Comparing the NC-AFM images taken with various tip-sample distances with a theoretical investigation of tip-sample interactions on the surface, the tip-sample interaction near the As atoms and Ga atoms are experimentally distinguished, and it is suggested that observed NC-AFM images reflect the tip induced surface relaxation.

Atomically Resolved Imaging of a NiO(001) Surface

Metal oxides are of great technological importance because of the variety of applications in which they are involved. One example is their use in catalysis. The surface structure of metal oxides has thus been the subject of much investigation. However, the low conductivity inherent in metal oxides makes it difficult to measure surface structure using electron beams or by STM. NC-AFM provides atomically resolved images of semiconducting or conducting surfaces and even insulators. NC-AFM enables the direct surface imaging of metal oxides on an atomic scale. The forces detected by NC-AFM originate from several kinds of interaction between the surface and the tip, including in some cases magnetic interactions. Theoretical studies predict that shortrange magnetic interactions such as the exchange interaction enable NC-AFM to image magnetic moments on an atomic scale.

Observation of Mixed Fatty Acid Monolayer at the Air-Water Interface Using Phase Contrast Microscopy

We observed stearic and lignoceric acids, and their mixed fatty acid monolayers at the air-water interface in situ using phase contrast microscopy. It was found that these monolayers exhibit quite different cohesion processes depending on the surface pressure. For stearic acid, the domains of the monolayer on the subphase have smooth surfaces and change their shapes to cover free areas with increasing surface pressure. In contrast, the domains of lignoceric acid divide into a number of smaller domains to cover free areas as surface pressure increased. We also found that the domains of lignoceric acid monolayers have internal substructures (i.e., triangular and inhomogeneous). For the mixed fatty acid monolayers, we clearly observed that some domains have the same internal substructures as those of single component monolayers. These facts demonstrate that a phase contrast microscope is a powerful tool for investigating the cohesion process of fatty acid monolayers at the air-water interface without the need for any probe material.

Tip-induced relaxation and amplitude of cantilever vibration observed on GaAs(110) surface

Based on topographic images of the GaAs(110) surface obtained by non-contact atomic force microscopy (NC-AFM) with different tip–sample distances, we discuss the tip–sample distance dependence of the cantilever vibration amplitude and the tip-induced surface Ga atom relaxation. In the case of a tip which reveals only As atoms as one kind of protrusion in the NC-AFM image, the damping of the cantilever vibration amplitude is small, and the frequency shift decreases gradually with decreasing tip–sample distance. On the other type of tip with which the bright and darker protrusions corresponding to both As and Ga atoms are observed, a large damping of cantilever vibration amplitude is measured with decreasing tip–sample distance. The frequency shift curve measured with this tip has a singular point. From this frequency shift curve, we conclude that the force acting between this type of tip and the sample surface is a hysteretic force. In this tip case, tip-induced surface relaxation of the topmost Ga atoms occurs. There is a relationship between the damping of the cantilever vibration amplitude and the tip-induced surface atom relaxation; that is, the energy dissipation due to the relaxation becomes remarkable.