Nanomedicine, Volume I: Basic Capabilities
© 1999 Robert A. Freitas Jr. All Rights Reserved.
Robert A. Freitas Jr., Nanomedicine, Volume I: Basic Capabilities, Landes Bioscience, Georgetown, TX, 1999
10.2.2.4 Other Molecular Electronic Devices
Other classes of molecular electronic devices have been studied, including picosecond photoactive or photochromic molecular switching devices1777-1783,1835,1836 using light, and electrochemical molecular devices1803,1805 using electrochemical reactions, to change the shape, orientation, or electron configuration of a molecule in order to switch a current. However, photoactive devices in a dense network would require an NSOM-like emitter (Section 4.8.4) to switch individually, since optical photon pathways are not easily confined at length scales very much below optical wavelengths of ~500-1000 nm; electrochemical molecular devices might require immersion in solvent to operate.1747
Fullerenes exhibit a wide range of quantized conductivities that may be useful in nanoelectronic computers.1821,2120 For example, chemisorption alters properties: 3 alkali dopant atoms per C60 buckyball gives high conductivity, while none or 6 dopant atoms gives an insulator.1826 Buckytube conductivity also varies with mechanical stress and torsion,1844,1872 tube diameter, and other geometrical parameters.1852-1854,1821 Depending upon the direction of the nanotube chiral vector, carbon nanotubes may be either metallic or semiconducting.1847,1848 Suitably-sized, doped, flexed, stressed, or chiralized fullerenes could be nested to make insulated wires1829 or joined to make nanoelectronic computer components1821,1858-1861,1876-1878 including resistors with quantized "staircase" resistance of nh/2e2, n = 1, 2, ... (e is electronic charge, h is Planck's constant),1857 two-terminal devices such as diodes,1838,1873 three- or four-terminal devices such as heterojunction,1828,1849,1858 TUBEFET,1875 or thin-film1827 transistors, and gates that introduce electric fields.641,1850 A single-molecule transistor using semiconducting buckytubes has been demonstrated experimentally at room temperature.2276
Carbon nanotubes have been cut to length, size-sorted by field flow fractionation, their open ends derivatized with thiol groups and then tethered to 10-nm gold particle "junction boxes" which AFM images show can connect together at least two separate tubes.1525,2713 If the tethering process can be spatially controlled using DNA complementarity (Section 10.2.3) or by other means, then fullerene nanoelectronic circuit elements could be assembled into three-dimensional CPU-like structures. P. Collins1873 expects an all-carbon 8-bit adder with nanometer dimensions by the year 2002, and J.C. Ellenbogen of MITRE Corp. has designed a molecular half-adder that measures ~10 nm x 10 nm in size.2275 This research area was extremely active in 1998.
Fullerenes may also allow compact memory devices. For example, it has long been speculated that a ~1 nm buckyball encapsulated in a ~1 nm diameter nanotube segment may glide freely back and forth, trapped weakly in each end cap by van der Waals forces. An external voltage could nudge a charged C60 molecule to one end of the other, creating a two-state "buckyshuttle RAM" storage device.1308,2849 Such a device could store one bit of information in a ~10 nm3 volume, a storage density of ~108 bits/micron3; the C60 thermal velocity of ~100 m/sec at 310 K inside a ~10 nm memory element gives a typical read/write time of ~0.1 nanosec. In early 1999, Kwon and colleagues2951 reported that finely dispersed 4-6 nm diamond powder thermally annealed by heating in a graphite crucible under argon at 1800°C for 1 hour produced oblong multiwall carbon nanostructures which in some cases could move around inside each other, like C60 trapped within C480. Kwon noted that there are potential energy minima at either end, so a charged buckyball (e.g., an enclosed ion) could be shuttled with an applied electric field, which could be used for memory storage. Computational simulation of writing a bit showed that the interior buckyball neatly shuttled to the correct end, then gently bounced to rest in ~0.01 nanosec (i.e., ~100 GHz operation) dissipating only ~10 kT in the C480. Kwon proposed a high density memory board using aligned buckyshuttles in a hexagonal lattice with addressing wires above and below, not unlike a ferrite core memory; the memory would likely be nonvolatile at room temperature, with mass production based on self-assembly.
Last updated on 24 February 2003