Meccano on the Nanoscale—A Blueprint for Making Some of the World's Tiniest Machines

Molecular compounds—comprised of mechanically interlocked components—such as rotaxanes and catenanes can be designed to display readily controllable internal movements of one component with respect to the other. Since the weak noncovalent bonding interactions that contribute to the template-directed synthesis of such compounds live on between the components thereafter, they can be activated such that the components move in either a linear fashion (rotaxanes) or a rotary manner (catenanes). These molecules can be activated by switching the recognition elements off and on between components chemically, electrically, or optically, such that they perform motions reminiscent of the moving parts in macroscopic machines. This review will highlight how the emergence of the mechanical bond in chemistry during the last two decades has brought with it a real prospect of integrating a bottom-up approach, based on molecular design and micro- and nanofabrication, to construct molecular electronic devices that store information at very high densities using minimal power. Although most of the research reported in this review on switchable catenanes and rotaxanes has been carried out in the context of solution-phase mechanical processes, recent results demonstrate that relative mechanical movements between the components in interlocked molecules can be stimulated (a) chemically in Langmuir and Langmuir–Blodgett films, (b) electrochemically as self-assembled monolayers on gold, and (c) electronically within the settings of solid-state devices. Not only has reversible, electronically driven switching been observed in devices incorporating a bistable [2]catenane, but a crosspoint random access memory circuit has been fabricated using an amphiphilic, bistable [2]rotaxane. The experiments provide strong evidence that switchable catenanes and rotaxanes operate mechanically in a soft-matter environment and can withstand simple device-processing steps. Studies on single-walled carbon nanotubes used as one of the electrodes in molecular switch tunnel junctions have revealed that interfacial chemical interactions involving electrodes containing carbon, silicon, and oxygen are good choices when carrying out molecular electronics on the class of rotaxane- and catenane-based molecules reported in this review. This conclusion is supported by differential conductance measurements (at 4 K) made with single-molecule transistors using the break-junction method. It transpires that the electronic transport properties in such devices are more sensitive to the chemical nature of the molecule–electrode contacts than the details of the molecules' electronic structure away from the contacts. This result has profound implications for molecular electronics and highlights the importance of also considering the molecules and the electrodes as an integrated system. It all adds up to an integrated systems-oriented approach to nanotechnology that finds its inspiration in the transfer of concepts like molecular recognition from the life sciences into materials science and provides a model for how, in principle, to transfer elements of traditional chemistry to technology platforms that are being developed on the nanoscale. Before there can be any serious prospect of a technology, there has to be some good, sound science in the making. Molecular electronics is very much in its infancy and, as such, it can be expected to give rise to a great deal of intellectually stimulating science before it stands half a chance of becoming a viable companion to silicon-based technology.