© 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 devices^{1777-1783,1835,1836} using light, and electrochemical molecular devices^1803,1805 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 components^{1821,1858-1861,1876-1878} 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