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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.

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.

Electrical and optical characterization of DNA molecules as a function of concentration in aqueous solutionWe have studied electric properties of carrier-injected deoxyribonucleic acid ͑DNA͒ molecules. First, a current (I CA ) through a single DNA molecule was measured by the two-probe dc method with varying a distance between a cathode and an anode (d CA ). The I CA -d CA curve showed that the current rapidly decreased with increasing d CA (I CA Շ0.1 nA for d CA տ6 nm) according to a hopping model. Next, we measured electric properties of DNA injected carriers by two methods; a field effect transistor ͑FET͒ 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 (d DS )ϳ20 nm. When a voltage was applied to the gate, the source-drain current (I DS ) 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 d DS ϳ20 nm. In order to flow a current through DNA over a distance ϳ100 m, we synthesized the DNA-acceptor cross-linked derivatives ͑DACD͒. In the structure of DACD, DNA molecules, which were attached acceptor molecules at guanine sites specifically, were cross-linked by linker molecules. We can modulate the carrier concentration in DACD with changing a guanine-cytosine pair content (p GC ) in a DNA double strand. We measured the current-voltage curves of DACD for various p GC . The conductivity of DACD increased nonlinearly with an increase in p GC . We explained this behavior using a percolation model, so that a two-dimensional conductive network would form in DACD.