Yuji Miyato

Dispersion of carbon nanotubes by photo- and thermal-responsive polymers containing azobenzene unit in the backbone

Photo- and thermal-responsive polymers containing azobenzene units in the main chain have been utilized as removable dispersants for single-walled carbon nanotubes (SWNTs) in organic solvents. Intermolecular interactions between SWNTs and the polymers are reversibly controllable by tuning the trans-cis composition.

Local Surface Potential Measurements of Carbon Nanotube FETs by Kelvin Probe Force Microscopy

We fabricated carbon nanotube FETs (CN-FETs) by dielectrophoresis method. Two types of CN-FETs with either Au or Ti electrodes were prepared for the study of electrical junctions between metal electrodes and nanotubes. Local surface potential of the CN-FETs in working condition was mapped by Kelvin probe force microscopy (KFM). A large surface potential drop occurred at the interface between the Ti electrodes and the bundle of SWNTs, and this potential drop was changed by the applied gate voltage. This result suggests that the Schottky barrier at the drain edge is modulated by the gate bias.

Multi-Probe Atomic Force Microscopy Using Piezoelectric Cantilevers

We developed a multi-probe atomic force microscopy (AFM) system using piezoelectric thin film (PZT) cantilevers. The use of self-sensing cantilevers with integrated deflection sensors as probes markedly reduced complexity in the ordinary AFM setup. Address-patterned samples having microfabricated x–y coordinate patterns, fabricated by electron beam lithography, were developed as well. These samples allow us to evaluate the relative distance between the probes by the comparison of the images obtained. Although the minimum distance between these probes was 126 µm using the original cantilevers, it was reduced to 9.2 µm by using the PZT cantilevers modified by a focused ion beam. Furthermore, we found that the interaction forces between the cantilevers were detected by determining the change in the amplitude of each cantilever.

Surface potential investigation on single wall carbon nanotubes by Kelvin probe force microscopy and atomic force microscope potentiometry

Surface potentials of single wall carbon nanotubes (SWNTs) connecting two metallic electrodes have been investigated by both Kelvin probe force microscopy (KFM) and atomic force microscope potentiometry (AFMP). By comparing the surface potential measurements obtained by both methods, we also studied the major factors affecting the potential measurements of the SWNTs, such as the surroundings, stray electric fields, and the effect of the AFM tip size, which can be larger than the SWNT diameter. In this study, we used KFM based on non-contact AFM and AFMP using the point-by-point contact mode in which the AFM tip worked as a voltage probe.

Multi-Probe Atomic Force Microscopy with Optical Beam Deflection Method

We developed a multi-probe atomic force microscope (AFM) having two AFM cantilevers independently controlled using the optical beam deflection method. We succeeded in simultaneously obtaining images with two independent probes by frequency modulation (FM) detection method. To evaluate the distance between the AFM tips of the cantilevers, we used a new the address-patterned sample, which was also developed for this study. The images obtained show that the distance between the probes was 2 µm. The development of the multi-probe AFM opens a wide variety of applications in the present nanoscience and engineering field.

Piezoresistive properties of carbon nanotubes under radial force investigated by atomic force microscopy

We investigated the piezoresistive properties of single-wall carbon nanotubes (SWCNTs) under the tip-induced force in the radial direction using atomic force microscopy. We found that the conductance of the bundled SWCNTs was modulated by the applied radial force. The polarity and amount of the conductance change were different on every bundle and even dependent on the location where the force was applied. These phenomena were explained by the modulation of the band structures of the SWCNTs, which was caused by the deformation at the critical pressure.

Resistive switching effects in single metallic tunneling junction with nanometer-scale gap

We fabricated a single tunneling junction with a nanometer-scale gap between Pt electrodes. We found that the gap distance became smaller after a current sweep, which was presumably caused by the migration of the Pt atoms at the anode. The junction showed a reproducible negative differential resistance characteristic after reduction in the gap. The junction also showed resistive switching characteristics with a resistance ratio of over 100 by applying voltage of different waveforms. The tunneling area and gap distance for on/off-state were quantitatively estimated by fitting the measured characteristics to the simple model as 100 nm2 and 0.8/1.2 nm, respectively.

Local potential profiling of operating carbon nanotube transistor using frequency-modulation high-frequency electrostatic force microscopy

The local potential measurement of an operating carbon nanotube transistor provides information on the defects in the nanotube and the interfacial potential barriers. While Kelvin-probe force microscopy is a powerful technique to measure the local surface potential, its accuracy is often degraded by the charges trapped on the surrounding insulator surface. Here, we introduce an alternative method to measure the local potential profile along the nanotube being less affected by those charges. We identified the location of a defect and detected the variation in the potential profile for different gate bias voltages, which were not detected by the conventional method.

Position Control and Electrical Characterization of Single-Walled Carbon Nanotubes Debundled by Density Gradient Ultracentrifugation

We fabricated field-effect transistors of isolated single-walled carbon nanotubes (SWNTs) that were debundled by density gradient ultracentrifugation (DGU). A solution of SWNTs in heavy water was mixed with iodixanol and then centrifuged. After DGU, the layer in which the SWNTs with a chirality of (6,5) were enriched was extracted and diluted with ultrapure water. The debundled SWNTs were bridged by dielectrophoresis between pairs of Pd electrodes. We also characterized the electrical properties of the SWNTs. Some SWNTs showed semiconductor characteristics, whereas others showed metallic behavior.