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

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

10.2.2.2 Electromechanical Molecular Switching Devices

In 1998, the best-studied category of molecular electronics device is probably the electromechanical molecular switch. This class of switch employs electrically or mechanically applied forces to turn a current on and off, either by changing the conformation of the molecule¹⁷⁶⁸ or by moving a switching molecule or group of atoms in the manner of a gate.^1805,1806 These switches are promising because they could be laid down in a dense network on a solid substrate, or in some cases in three-dimensional arrays; unfortunately, no integrated CPU-scale designs have yet been attempted, although plans for a half-adder¹⁷⁶⁹ and a molecular shift register¹⁷⁵⁰ have been published. Some examples:

A. Electromechanical Amplifier -- Joachim and Gimzewski¹⁷⁶⁸ measured conductance through a single buckyball held between an STM tip and a conducting substrate. By pressing down harder on the STM tip, the buckyball deforms, tuning the conduction off-resonance and reducing current ~50%. Although the STM tip could be replaced with a small in-situ piezoelectric gate, the ideal actuator would be a molecular-scale electromechanical actuator akin to a logic rod; the major fundamental limit to the speed of this switch is the natural mechanical vibrational resonance frequencies of a buckyball, ~10¹³ Hz.

B. Linear Atomic Relay -- Researchers at Hitachi Corporation¹⁸²³ simulated a two-state electronic switch wherein a mobile atom that is not firmly attached to a substrate moves back and forth between two terminals (Figure 10.5). If the switching atom is in place, the device conducts electricity; if the switching atom is displaced from the two wires, the resulting gap greatly reduces the current that can flow through the atom wire. A small negative charge placed on the third atom wire "gate" near the switching atom moves the switching atom out of its place in the wire; the switching atom is pulled back into place by a second "reset" gate after each use of the switch. Actual experiments approximating this design created a bistable atom switch using a xenon switching atom moving back and forth between an STM tip and a substrate,^1807,1808 demonstrating that the movement of a single atom can be the basis of a nanometer-scale switch. In a more mature device, relays could be ~10 nm² in size with a speed limited only by the intrinsic vibrational frequency of atoms, typically ~100 THz, but it seems likely that atom relays could only operate at very low temperatures because not much energy would be required to evaporate a switching atom off the substrate and out of the plane of the atom wires, destroying the switch.¹⁷⁴⁷

C. Rotational Molecular Relay -- A more reliable two-state device based on atom movement uses the rotation of a molecular group to affect an electric current. The switching atom is part of a rotating group, or "rotamer,"¹⁸²⁴ which itself is part of a larger molecule, perhaps affixed to the same surface as the atom wires. Figure 10.6 is a conceptual diagram illustrating this approach,¹⁷⁴⁷ wherein the electric field of a nearby gate forces the switching atom to rotate in or out of the atom wire. When the switching atom is "in," wire conductance is high and the switch is "on"; when the switching atom is "out," a second group takes its place, hindering current flow and turning the switch "off." A third large group may provide resistance to thermal free rotation. Alternatively, hydrogen bonding might provide enough rotational resistance to stop the rotamer in the conducting position, but not so much that reversing the gate voltage would be insufficient to turn the rotamer. Controlled rotational state switching of a Pt-adsorbed oxygen molecule by STM using 0.15-volt 20-microsec pulses was demonstrated in 1998.¹⁸⁷⁴

While a rotating switch with a methyl-like rotamer group has three distinct switch positions, a hinging switch that rotates back and forth between two distinct states might be preferable. Cyclohexane, a simple example of this kind of molecule, can bend into two different forms, commonly known at the "boat" and "chair" conformations.^1809,1810 As shown schematically in Figure 10.7, a voltage on a nearby gate might force the cyclohexane switch into one of its two conformations, affecting the conductivity of a nearby atom wire. The cyclohexane-type molecule attaches to a molecular framework while the remaining ring carbons are replaced by groups tailored to use steric repulsions or van der Waals attractions to reduce undesired switching caused by thermal energy. Switch speed is limited by molecular rotational and torsional frequencies, typically ~1-1000 GHz, which is slower than the atom relay but more reliable. Also in contrast to the atom relay, molecular relays could be packed in three dimensions, possibly up to ~1 nm³ densities.

D. Molecular Shuttle Switch -- A rotaxane "shuttle switch" has been synthesized by F. Stoddart's group¹⁸⁰⁵ that consists of one ring-shaped molecule that encircles and slides along a shaft-like chain molecule (Figure 10.8). Two large terminal groups at either end of the shaft prevent the shuttle ring from slipping off the shaft. The shaft contains two other functional groups, a biphenol group and a benzidine group, that serve as natural stations between which the shuttle moves. The shuttle molecule contains four positively charged functional groups that cause it to be attracted to shaft sites with extra negative change, so the shuttle spends 84% of its time at the benzidine station, which is a better electron donor than the biphenol station. Removing an electron from the benzidine station forces the shuttle to switch to the other station; switching may be controlled chemically or electrochemically.¹⁸⁰⁵ A charged gate could be added at one or both ends to force the shuttle to move between stations, with switch state probed by arranging for the ring to complete an electric circuit in one of its two positions.¹⁷⁴⁷ Switching speed is slow, limited by the speed of electron transfer to and from the benzidine station and the sluggish motion of the ring which is ~10⁶ times heavier than an electron, but three-dimensional packing densities of ~0.01 nm³ appear plausible. Other rotaxane shuttle systems have been studied.^{1845,1846,2487,2522,2529,2530} In 1999, the first chemically-assembled rotaxane-based logic gate was demonstrated,³⁵⁴¹ although the gates could be opened only irreversibly.

Last updated on 23 February 2003