Overview of Nanoelectronic Devices <s> E. Electromechanical Molecular Electronic Devices <s> The molecular motions in solid neopentane were investigated by the continuous-wave and the pulsed N.M.R. methods from 4·2 K to its melting point. A single broad minimum of the spin-lattice relaxation time T 1 at 100 K is caused by reorientation of the methyl groups and of the entire molecule. An attempt was made to interpret the T 1 data in the low-temperature phase by a detailed treatment, that is the triangular structure of methyl protons was taken into the calculation of intermethyl dipole interactions. The jumping rate (angular frequency) of methyl reorientation about its C 3 symmetry axis is ω 3 0=1·65×1012 exp (−8·0 kJ mol−1/RT) s−1 and the overall molecular reorientation about its C 2 or C 3 symmetry axis occurs in an isotropic (ω 2=ω 3=2·49×1O15 exp (−18·5 kJ mol−1/RT) s−1) or slightly anisotropic manner (ω 2≠ω 3). The potential barrier hindering the rotation of methyl groups is not a simple sinusoidal function and the tunnelling frequency of the methyl group is of the same magnitude as the local ... <s> BIB001 </s> Overview of Nanoelectronic Devices <s> E. Electromechanical Molecular Electronic Devices <s> THE scanning tunnelling microscope1 (STM) has been employed in recent years in attempts to develop atomic-scale electronic devices, both by examining device-like characteristics in preexisting structures2,3 and by creating new structures by the precise manipulation of atoms and molecules with the STM tip4–6. Here we report the operation of a bistable switch that derives its function from the motion of a single atom. A xenon atom is moved reversibly between stable positions on each of two stationary conducting 'leads', corresponding to the STM tip and a nickel surface. The state of the switch is set (that is, the xenon atom is moved to the desired location) by the application of a voltage pulse of the appropriate sign across the leads. The state of the switch is identified by measuring the conductance across the leads. This switch is a prototype of a new class of potentially very small electronic devices which we will call atom switches. <s> BIB002 </s> Overview of Nanoelectronic Devices <s> E. Electromechanical Molecular Electronic Devices <s> Self replicating systems have long been seen as an economical method of exploring space. For example, the costs involved in the exploration of the galaxy using self replicating probes would be almost exclusively the design and initial manufacturing costs. Subsequent manufacturing costs would drop dramatically. <s> BIB003 </s> Overview of Nanoelectronic Devices <s> E. Electromechanical Molecular Electronic Devices <s> PHYSICAL PRINCIPLES. Classical Magnitudes and Scaling Laws. Potential Energy Surfaces. Molecular Dynamics. Positional Uncertainty. Transitions, Errors, and Damage. Energy Dissipation. Mechanosynthesis. COMPONENTS AND SYSTEMS. Nanoscale Structural Components. Mobile Interfaces and Moving Parts. Intermediate Subsystems. Nanomechanical Computational Systems. Molecular Sorting, Processing, and Assembly. Molecular Manufacturing Systems. IMPLEMENTATION STRATEGIES. Macromolecular Engineering. Paths to Molecular Manufacturing. Appendices. Afterword. Symbols, Units, and Constants. Glossary. References. Index. <s> BIB004 </s> Overview of Nanoelectronic Devices <s> E. Electromechanical Molecular Electronic Devices <s> Nanometer scale electronics present a challenge for the computer architect. These quantum devices have small gain and are difficult to interconnect. I have analyzed current device capabilities and explored two general design requirements for the design of computers: error correction and long range connections. These two principles follow when Turing machines are implemented as integrated circuits. I consider the roles of electromigration through thin wires, circuit layout, and error rates for devices with small gain. The analysis brings into sharp focus the future of nanocomputers and suggests solutions to some of its difficulties. It gives a theoretical model for a nanocomputer, separating the roles of devices and algorithms. Within the model one can implement a stochastic computer, which operates despite quantum device limitations. <s> BIB005 </s> Overview of Nanoelectronic Devices <s> E. Electromechanical Molecular Electronic Devices <s> We describe the first single-molecule electromechanical amplifier, the active element of which is one fullerene molecule (C60) with a junction length and width of ∼ 1 nm. Forces in the nano-Newton range are generated by a metallic tip pressing on the molecule in its adsorbed state. This working nanodevice is based on the modulation of virtual resonance tunneling through the molecule by electromechanical deformation of the C60 cage. <s> BIB006
Electromechanical molecular switching devices are not so closely analogous to microelectronic transistors as are the molecular devices we have considered so far. They are controlled by deforming or reorienting a molecule rather than shifting around electrons. The input may even be mechanical rather than electrical. However, just like all those other switches, they can turn on or off a current between two wires, which makes them interesting for nanocomputing. 1) Single-Molecule Electromechanical Amplifier: It is already possible to make such switches composed of only one or a few molecules. In very recently reported results, Joachim and Gimzewski have been able to measure conductance through a single buckyball held between an STM tip and a conducting substrate. By pressing down harder on the STM tip they deformed the buckyball and tuned conduction onto and off of resonance, producing a 50% reduction of current off resonance. The deformation was reversible, a measure of the strength and resilience of the carbon fullerenes. Clearly it would be impractical for computers to use an STM to operate each switch, but Joachim and Gimzewski recommend replacing the STM tip with a small in-situ piezoelectric gate or other electromechanical actuator. The only fundamental limit to the speed of such a device would be the vibrational frequency of a buckyball-over 10 THz ( Hz), though the prototype misses this goal by 12 orders of magnitude BIB006 . 2) Atom Relay: A team of researchers at the Hitachi Corporation in Japan has simulated a two-state electronic switch of atomic dimensions  . The concept for this proposed device, termed an "atom relay," has some simi- larities to the molecular shuttle switch. In the atom relay, a mobile atom that is not firmly attached to a substrate would move back and forth between two terminals. The atom relay would be made from carefully patterned lines of atoms on a substrate. The Hitachi simulations showed that such a line, or "atom wire," can conduct a small electric current. As shown in Fig. 9 , two atom wires connected by a mobile switching atom form the relay. If the switching atom is in place, the whole device can conduct electricity. However, if the switching atom is displaced from the two wires, the resulting gap dramatically reduces the amount of current that can flow through the atom wire. A third atom wire that passes near the switching atom is termed the "gate" of the atom relay in analogy to the gate of a field effect transistor. Placing a small negative charge on the gate wire 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. In an actual experiment that approximates this design, Eigler et al. created a bistable atom switch with the aid of an STM. In their switch, a xenon atom transfers back and forth between the tip of an STM and a substrate BIB002 ,  . The location of this switching atom greatly affects the tunneling current that flows from the STM tip to the Fig. 9 might be refined by attaching the switching atom to a rotating molecular group. The orientation of the rotating group is to be controlled by a nearby gate molecule, to which a voltage can be applied for that purpose. (a) (b) Fig. 11 . Refined molecular switch, Type 2. Switching atom might be attached to a "rotamer" that permits the atom to (a) be swung into position to turn the switch "on" by filling the gap in the atom wire, or (b) the switching atom is swung up out of the wire to turn "off" the current in the wire. Orientation of the rotamer is to be controlled by adjusting the polarity of the charge on the gate molecule, shown at the very top of both (a) and (b). surface. While the operation of the switch fabricated by Eigler's group is different from that of the theoretical atom relay, these experiments have shown that the movement of a single atom can be the basis of a nanometer-scale switch. However, the designs for logic gates using atom relays could be limited to a two-dimensional plane. The Hitachi group did not demonstrate how two separate atom wires could cross. Without crossing wires, only a subset of all possible logic functions can be implemented with atom relays BIB005 . On the positive side, individual relays would have the advantage of being extremely small, on the order of 10 square nm. The speed of the relays would be limited only by the intrinsic vibrational frequency of atoms (approximately 10 cycles per second), which is several orders of magnitude faster than present-day semiconductor transistors. Energy requirements, while not reported by the authors, would be rather low, resulting mostly from frictional forces between a single atom and the substrate. On the other hand, not much energy would be required to evaporate a switching atom off the substrate and out of the plane of the atom wires, thereby destroying the switch. For this reason, it seems likely that atom relays could only work at very low temperatures. While switching based on atom movement has the advantages of high speed and low power dissipation, incorporating this mechanism into a more reliable device would improve its chances for practical applications. 3) Refined Molecular Relay: A more reliable two-state device based on atom movement might use the rotation of a molecular group to affect an electric current. We suggest that the atom relay discussed in the last section might be refined and made more reliable by attaching the switching atom to a rotating group, or "rotamer." This rotamer would be part of a larger molecule, perhaps affixed to the same surface as the atom wires. See Fig. 10 for a conceptual diagram of this arrangement (based upon a methyl-like group) and Takeda et al. BIB001 for a discussion of rotamers. The electric field of a nearby gate would force the switching atom to rotate in or out of the atom wire. When the switching atom is in the atom wire, the conductance of the atom wire is high-i.e., the switch is "on." When the switching atom is rotated out of the wire, a second group takes its place. This replacement group hinders the flow of current through the atom wire, turning the switch "off." A large third group on the rotamer could prevent it from rotating freely due to thermal energy. Alternatively, hydrogen bonding might provide a resistance to rotation just adequate to stop the rotamer in the conducting position, but not so much that reversing the gate voltage would be insufficient to turn the rotamer. Use of such a rotamer to effect atom switching would prevent the evaporation of the mobile switching atom, alleviating one of the principal weaknesses of the atom relay discussed in the previous section. The refined molecular relay would operate somewhat similarly to the shuttle switch described below in Section IV-E4. The rotamer in the refined relay would likely be faster but also more sensitive to energetic perturbations than the molecular shuttle, because the rotamer would be lighter and would have a much smaller range of motion between switching positions. The molecular relays and the shuttle switch are a kind of hybrid between electronic switches and the molecular-scale mechanical devices suggested by Drexler BIB004 , Merkle BIB003 , and others. One of the disadvantages of a rotating switch based upon a methyl-like rotamer group is that there are three different switch positions associated with the three groups attached to the rotamer. A more suitable molecule might be one that moves back and forth between only two distinct states. Cyclohexane, a simple example of this type of molecule, can bend into two different forms, commonly known as the "boat" and "chair" conformations  ,  . As shown in Fig. 11 , a voltage on a nearby gate might force the cyclohexane switch into one of its two configurations, affecting the conductivity of a nearby atom wire. The cyclohexanetype molecule could link to a molecular framework while  . Ring-like "bead" molecule slides along wire-like chain molecule to perform switching function as described in the text. the remaining ring carbons would be replaced by groups tailored to use steric repulsions or chemical attractions to reduce undesired switching caused by thermal energy, while also sterically protecting the conducting atom from chemical attack. These two designs should operate at speeds governed by molecular rotation, which typically occurs at frequencies in the vicinity of billions of cycles per second, or GHz. This is slower than the atom switch, but the operation could be much more reliable. In contrast to the atom relay, these refined relays could be packed in three dimensions, attaining much higher packing density. This should not cause overheating since the energy dissipated in their operation ought to be very low, primarily arising from breaking weak van der Waals attractions and/or hydrogen bonds.