Lattice-Expanded C 60 Achieves T c of 117 K

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RESEARCH/RESEARCHERS Semiconducting Behavior in DNA Device Detected by Triple-Probe Deoxyribonucleic acid (DNA) is considered by some to be the ideal building block for nanoelectronic circuits, and, consequently, the electronic behavior of these molecules has attracted significant interest. It has been reported that the electrical characteristics of DNA can vary from those of an ohmic conductor to those of a widebandgap semiconductor. To better understand the carrier transport in DNA, a team of researchers at Fujixerox Co.’s Corporate Research Center in Kanagawa-ken, Japan, have developed a probing system for measuring the electrical properties of DNA molecules and other nanoscale samples. As reported in the October 8 issue of Applied Physics Letters, H. Watanabe and coworkers have measured the electrical properties of a three-terminal single-molecule DNA device with a triple-probe atomic force microscope (T-AFM). A single DNA molecule was connected to the T-AFM with three carbon nanotube (CNT) electrodes, corresponding to a source, drain, and gate terminal. In addition to demonstrating the measurement capabilities of their T-AFM, the researchers showed that a salmon sperm DNA double strand was semiconducting. The T-AFM system is comprised of a nanotweezers setup and a conventional AFM system equipped with a conductive, 8-nm diameter multiwalled CNT (MWCNT) probe. The nanotweezers, two 8-nm-diameter MWCNTs connected to a 100-nm-diameter glass needle, were used to position submicron DNA samples with 1–2-nm accuracy. A double strand of salmon sperm DNA was deposited on a SiO 2 /Si(100) substrate, and a singlewalled CNT (SWCNT) was deposited through the T-AFM adjacent to the DNA molecule to serve as the gate terminal. A three-dimensional piezo-actuator was used to position the nanotweezers such that the two CNT probes grabbed the salmon sperm DNA molecule and served as the source and drain terminals. To avoid the problems associated with electrostatic attraction of the probes both to each other and the substrate, the researchers employed a method whereby the CNT probes were continuously vibrated. When the sample was in the proper location, the vibrations were stopped and the nanotweezer probes were allowed to attain a fixed closed position and grab the sample. The MWCNT AFM probe was then used to apply a bias voltage to the SWCNT gate terminal. The current-voltage (I-V) curves, which were measured at a source-to-drain distance of 25 nm in room temperature dry

MRS BULLETIN/NOVEMBER 2001

nitrogen, were nonlinear, indicating semiconducting behavior. Furthermore, the voltage gap decreased with increasing gate bias voltage, indicating a conductance rise and demonstrating the switching characteristics of the gate-biased DNA molecule. “While additional studies to further clarify carrier transport mechanisms in DNA are ongoing,” said Watanabe, “we anticipate that DNA will play a major role in the future of advanced biosensors and molecular electronics.” STEFFEN K. KALDOR

Lattice-Expanded C60 Achieves Tc of 117 K Scientists

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