Taishi Shigematsu

Dual-probe scanning tunneling microscope: Measuring a carbon nanotube ring transistor

We have constructed a dual-probe scanning tunneling microscope (D-STM). We used multiwall carbon nanotubes [(NT), diameter: ∼10 nm] as STM probes. The D-STM allows us to elucidate the electric property of a sample with a spatial resolution of ∼1 nm. Using this system, we have measured the current–voltage curves of a single NT ring as a transistor. The curves show the possibility of nanometer-scale electronic circuits composed of NT devices.

Single molecule DNA device measured with triple-probe atomic force microscope

We have measured the electric properties of a three-terminal single molecule DNA device with a triple-probe atomic force microscope (T-AFM). The T-AFM permits us to connect a single DNA molecule with carbon nanotube (CNT) electrodes as source, drain, and gate terminals. As the gate bias voltage is increased, the voltage gap region decreased in the current–voltage (I–V) curves. Furthermore, we can observe the clear steps in the I–V curve at crossing the DNA molecule and the CNT-gate electrode with gate biased.

Transport properties of carrier-injected DNA

We have studied electric properties of carrier-injected deoxyribonucleic acid (DNA) molecules. First, a current (ICA) through a single DNA molecule was measured by the two-probe dc method with varying a distance between a cathode and an anode (dCA). The ICA–dCA curve showed that the current rapidly decreased with increasing dCA (ICA≲0.1 nA for dCA≳6 nm) according to a hopping model. Next, we measured electric properties of DNA injected carriers by two methods; a field effect transistor (FET) arrangement and a chemical doping. In the FET arrangement, we set three electrodes on a single DNA molecule as source, drain, and gate electrodes with a source–drain distance (dDS)∼20 nm. When a voltage was applied to the gate, the source–drain current (IDS) could be detected to be 0.5–2 nA. This showed that charge injection with the FET arrangement would yield a carrier transportation through DNA at least dDS∼20 nm. In order to flow a current through DNA over a distance ∼100 μm, we synthesized the DNA-acceptor cross-...

An advanced electric probing system: Measuring DNA derivatives

We have developed an advanced electric probing system, which has two probes, with the spatial resolution of ∼1 nm and the detection limit of <1 pA in order to measure electric properties of nanometer-scale samples. This system consists of a conventional AFM system and a piezoactuator system. In electric measurements of samples, two probes must be connected to the sample with keeping an electric isolation between two probes. For a connection of probes with a nanometer-scale sample, the radiuses of curvature of the probes should be smaller than the sample size. Thus, we used carbon nanotube as one of two probes, so that we could measure current–voltage (I–V) curves of the nanometer-scale samples. We have applied our system to measuring I–V curves of a lambda phage DNA (λ-DNA) bundle. The curves showed that the current through the λ-DNA was less than ∼1 pA. In order to increase conductance of DNA molecule with chemical doping, we synthesized DNA-acceptor cross-linked derivatives (DACD). We measured I–V curve...

Triple-probe Atomic Force Microscope: Measuring a carbon nanotube/DNA MIS-FET

We have constructed an advanced electric probing system, which is a triple-probe atomic force microscope (T-AFM). The T-AFM consists of “Nanotweezers” and an AFM with a carbon nanotube probe. Using this system, we fabricated a single-walled carbon nanotubes (SWNTs)/deoxyribonucleic acid (DNA) three-terminal device and measured the current-voltage ( I-V ) curves of this device. In this three-terminal device, DNA strands were entangled with the SWNT bundle, and behaved as a gate-insulator-layer. This three-terminal device worked as a metal-insulator-semiconductor field effect transistor (MIS-FET) with depletion switching behavior.

Dual-probe scanning tunneling microscope and a carbon nanotube ring transistor

For measuring molecular device, we developed a dual-probe scanning tunneling microscope (D-STM) composed of two STM systems in which a carbon nanotube (NT) was used for STM tip. Using D-STM, we fabricated a NT ring device. The NT ring device showed a switching behavior with applying gate bias. Furthermore, in STM imaging for various gate biases, we could observe directly hole injection into the NT ring.

Energy gap of a molecularly doped polymer measured by a new method of electron spectroscopy: Measurement of charge transfer force

We have proposed a new method of electron spectroscopy which is measuring charge transfer (CT) force by atomic force microscopy. We have applied this new method to a typical molecularly doped polymer system [N,N′-diphenyl-N,N′-bis(3-methylphenyl)-[1,1′-biphenyl]-4,4′ diamine (TPD) and polycarbonate] to elucidate the electronic structure of the system. CT force spectra obtained show that the energy gap (Eg) continuously decreases with increasing the TPD concentration. We discuss the change in Eg from two different points of view: homogeneous and inhomogeneous distributions of TPD.