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COMMENTARY | FOCUS
A brief history of molecular electronics
Mark Ratner
The eld of molecular electronics has been around for more than 40years, 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.
The concept of electrons moving through single molecules comes in two dierent guises. The rst is
electron transfer, which involves a charge moving from one end of the molecule to the other1. The second, which is closely related but quite distinct, is molecular charge transport and involves current passing through a single molecule that is strung between electrodes2,3. The two are related because they both attempt to answer the same fundamental question: how do electrons move through molecules4,5?
Understanding the movement of electrons to and through a single molecule is central to the eld of molecular electronics, but presents a signicant experimental and theoretical challenge.
The principal problem concerns the statistical uctuations present in single-molecule spectroscopy data6. In general, the uctuations expected from n observations scale as 1/n. For bulk measurements,n is very large and the uctuations are generally unimportant. However, forsingle molecules, they can be of the same order as the property being measured,and become comparable to the charge transport signal itself (Fig.1). Despite these difficulties, molecular electronics has made considerable progress in recent years and a variety of important mechanistic insights have been obtained, which could have implications for the development ofdevices.
The early days
The eld of molecular electronics canbe traced back to studies conductedby Hans Kuhn and colleagues in the 1970s. In particular, Kuhn and Bernhard Mann reported, in 1971, conductivity measurements through monolayersof cadmium salts of fatty acids7.
These measurements showed that the
conductivity decreased exponentially with layer thickness, therefore revealing electron tunnelling through the organic monolayer.
In 1974, Arieh Aviram and I published the rst theoretical discussion of transport through a single molecule8. On reection now, there are some striking features about this work. First, we suggested a very ad hoc scheme for the actual calculation. (This was in fact the beginning of many slightly awed theoretical approaches, which were nally successfully united through the development of the non-equilibrium Greens function approach in the early...