COMMENTARY | FOCUS A brief history of molecular electronics Mark Ratner The field of molecular electronics has been around for more than 40 years, but only recently have some fundamental problems been overcome. It is now time for researchers to move beyond simple descriptions of charge transport and explore the numerous intrinsic features of molecules. he concept of electrons moving conductivity decreased exponentially with Conference by one of the inventors of through single molecules comes layer thickness, therefore revealing electron the STM how to account for the fact Tin two different guises. The first is tunnelling through the organic monolayer. that charge could actually move through electron transfer, which involves a charge In 1974, Arieh Aviram and I published fatty acids containing long, saturated moving from one end of the molecule to the first theoretical discussion of transport hydrocarbon chains. the other1. The second, which is closely through a single molecule8. On reflection The first significant work attempting related but quite distinct, is molecular now, there are some striking features about to measure single-molecule transport charge transport and involves current this work. First, we suggested a very ad came from Mark Reed’s group at Yale passing through a single molecule that is hoc scheme for the actual calculation. University, working in collaboration with strung between electrodes2,3. The two are (This was in fact the beginning of many James Tour’s group, then at the University related because they both attempt to answer slightly flawed theoretical approaches, of South Carolina10. Their papers in the the same fundamental question: how do which were finally successfully united late 1990s and early 2000s advanced our electrons move through molecules4,5? through the development of the non- understanding of how such measurements Understanding the movement of equilibrium Green’s function approach in could be made, and provided insights electrons to and through a single molecule the early twenty-first century2,5,9.) Second, about the transport properties of is central to the field of molecular we suggested that a single molecule could different molecules. The dominance of electronics, but presents a significant act as a device — here a molecular rectifier large fluctuations in the experimental experimental and theoretical challenge. — and that a single-molecule circuit with data, and therefore the need to treat The principal problem concerns the two electrodes could actually be made and molecular transport in a similar way to statistical fluctuations present in single- measured. At the time, this was probably single-molecule spectroscopy, remained molecule spectroscopy data6. In general, the somewhere between science fiction and unrecognized. The success of these early fluctuations expected from n observations state-of-the-art, but the most important measurements ignited broad interest scale as 1/√n. For bulk measurements, problem was how to attach electrodes to worldwide, and significant review articles n is very large and the fluctuations are a molecule. appeared in 200011 and in 20015. This was generally unimportant. However, for Later in the 1970s, several conferences the true beginning of molecular electronics. single molecules, they can be of the same in Washington DC were devoted to order as the property being measured, molecular transport, and some interesting Some major issues along the way and become comparable to the charge ideas and observations were presented, Single-molecule electronics requires the transport signal itself (Fig. 1). Despite including unusual transport behaviours combined effort of synthetic chemists, these difficulties, molecular electronics has and possible mechanisms for new devices. experimental physicists and physical made considerable progress in recent years These meetings were also important for chemists, and theoreticians, all of and a variety of important mechanistic raising wider interest in the topic. whom have faced and are facing several insights have been obtained, which could It was in the 1980s, however, that challenges. The synthetic problem has been have implications for the development perhaps the most important advances reasonably straightforward to address: once of devices. in molecular electronics occurred: the the core of a molecule is completed, it is development of the scanning tunnelling possible to attach a structural component The early days microscope (STM) and later the atomic at either end that will bond effectively to The field of molecular electronics can force microscope (AFM), both largely the electrodes. The preferred material for be traced back to studies conducted products of IBM laboratories in Zurich. It electrodes is gold or platinum, to avoid by Hans Kuhn and colleagues in the quickly became clear that these tools could oxidation and degradation. For gold 1970s. In particular, Kuhn and Bernhard be used to measure the conductance of electrodes, sulphur, amine or a number Mann reported, in 1971, conductivity single molecules, but at first, observations of lone-pair species are typically used measurements through monolayers of the ability of pure σ-bonded systems to to attach the molecule; for platinum of cadmium salts of fatty acids7. permit transport were puzzling. I recall, for electrodes, lone-pair species are those most These measurements showed that the example, being asked at a Gordon Research often used. 378 NATURE NANOTECHNOLOGY | VOL 8 | JUNE 2013 | www.nature.com/naturenanotechnology © 2013 Macmillan Publishers Limited. All rights reserved FOCUS | COMMENTARY The biggest problems faced by involves both closed-shell and odd-spin experimental aspects. This is in contrast experimentalists involve conductance molecular species; work on vibronic effects to intramolecular charge transfer, where measurements of single molecules, involving interaction between electronic femtosecond timescale measurements are which almost inevitably result in very and vibrational degrees of freedom; work common, and the attosecond scale is being large fluctuations in experimental data. on excitation of the molecular junction investigated. As the two phenomena are The dominant schemes for making using polarized light17; work on quantum closely related, surprises may yet occur good transport measurements are based interference and decoherence; work on in understanding the time dependence on either the electrochemical break molecular chirality17; work on molecular experimentally. Theoretically, the time junction12–14 or the mechanical break stretching and distortion; and work on dependence can be investigated using junction15. Both techniques produce the thermoelectric response in molecular several approximate schemes, ranging from extensive data sets, because measurements junctions18,19. Related fundamental Ehrenfest dynamics to more sophisticated can be made rapidly, and therefore work has focused on noise in molecular propagation techniques4. Using a density statistical theoretical analysis on the basis junctions and statistical analysis including matrix approach (as is usual in magnetic of the information in histograms and the full counting statistics20. resonance) is quite common for model distributions of data can be carried out, Recent studies on vibronic effects using investigations22, and by using density which allows mechanistic insight to be inelastic electron tunnelling spectroscopy functional methods, closure of the quantum obtained. Break junctions can also be gated, (IETS), for example, have shown striking Liouville equation for the molecular density either electrochemically or by an external aspects of the electronic–vibrational matrix can be attained — recent work in third electrode, but these experiments, coupling during charge transfer in single- this area is particularly promising because although important because they provide molecule junctions21. Depending on the of the interpretive advantages of density knowledge of energy levels, are not magnitude of the conductance through matrix methodology23. required for a fundamental understanding the relevant channels, IETS data can result of charge transport. in different plots of current versus voltage Some quantum coherence effects For theory, the question ‘how does that can be described by the interaction of Some elegant and striking measurements current move through molecules?’ is now vibrational excitations with the electronic have been completed on a number of answered using non-equilibrium Green’s motion corresponding to conductance. designed organic molecular systems. function techniques, a modification of This can be used in various ways, such Generally, the observations of the the original approaches to mesoscopic as deducing pathways for transport structure–function relationship between transport in solids developed amongst through intensities of IETS spectra, or the individual molecules and their others, by Rolf Landauer, Markus Büttiker, studying magnetic effects in electronic– conductances have been based on the Yigal Meir and Ned Wingreen2,3,9. The vibronic coupling. empirical rules of thumb just discussed. adaptation of non-equilibrium Green’s Another important aspect of charge Indeed, the Simmons analysis and the function techniques to molecular transport transport is linked to time-evolution of HOMO/LUMO structures are standard for problems has been undertaken by a the process. Standard circuit analysis explaining molecular transport even for number of laboratories,