© 1999 Robert A. Freitas Jr. All Rights Reserved.

Robert A. Freitas Jr., Nanomedicine, Volume I: Basic Capabilities, Landes Bioscience, Georgetown, TX, 1999

10.2.2.1 Molecular Wires

Before one can seriously discuss molecular electronic devices embedded in single molecules, it must be asked whether a small single molecule can be made to conduct electrical current at all. A series of difficult and sensitive experiments^{1765,1767,1800} and theoretical investigations^1758-1761 during the 1990s answered this question affirmatively. Single molecule conduction was demonstrated by Tour^1766,1767 using a conducting molecule characterized by repeating structures -- in this case, a series of benzene-like rings connected by acetylene linkages -- each part of which is linked to the next by bonds including many p-electrons above and below the plane of the structure. These electrons in the p orbitals¹⁸¹⁸ conjugate with each other, or interact, to form a single large orbital throughout the length of the wire to permit mobile electrons to flow.^1778,1831 Thiol (-SH) functional groups at either end adsorb well to gold surfaces and act as "alligator clips" for attaching molecular electronic units to metal substrates.^{1819,1764,1867-1870} Tour's polyphenylene wires can carry ~nanoampere currents. Such molecular wires also have the desirable property that they can be made quite long, if necessary, because they can be lengthened systematically using chemosynthetic methods.^1766,1820 Another experiment using a single benzene ring attached between two gold electrodes through thiol groups across a 0.846-nm gap found a singlering threshold resistance of ~20 megohm and a capacitance on the order of ~0.1 e² / kT ~ 10^-19 farad.^{1811,1812,1837}

Fullerene carbon nanotubes (Section 2.3.2) or "buckytubes"¹⁸²¹ are hollow cylindrical tubes, essentially rolled-up sheets of graphite, measuring >~1 nm in diameter that (in many chiral and compositional forms; Section 10.2.2.4) can conduct electricity. For instance, both individual 10-20 nm diameter carbon nanotubes^1822,1844 and atomic wires pulled from an unraveling carbon nanotube⁶⁴³ can conduct currents of ~1-10 microamps (vs. nanoamps via tunneling). Maximum nanotube current density is ~10¹⁰-10¹¹ amp/m² over a 0.2-6.0 volt range,^1844,1857 superior to the ~10⁹ amp/m² typically achieved in superconductors. Nanotube resistivity measures ~8-20 ohm-m for straight tubes, ~38-49 ohm-m for slightly curved (5-30°) tubes, and >100 ohm-m for highly curved (65-80°) tubes.¹⁸⁴⁴ Interestingly, electronic transport in carbon nanotubes is ballistic (e.g., like electron waveguides, permitting only a few propagating modes¹³⁰⁸), with all waste heat being dissipated in the leads to the nanotube element and none in the nanotube itself.¹⁸⁵⁷ Buckytubes are stiff enough to serve as supports for molecular circuit elements, and if filled with conducting metal can create one of the structurally strongest nanowires that is chemically possible. (See also Section 10.2.2.4.)

Other 0.6-3 nm molecular wires,^{1767,1801,1802,1864} 3-20 nm semiconductor wires,¹⁸⁴³ and nanoscale metallic wires^{1740,1862,1863} have been studied as well.

Last updated on 23 February 2003