ELECTRICAL RECTIFICATION BY A MOLECULE : THE ADVENT OF UNIMOLECULAR ELECTRONIC DEVICES

In 1959, the late Richard P. Feynman proposed, in his usual witty way, that there was “plenty of room at the bottom”, i.e., that atomic and molecular dimensions had not yet been exploited in information storage.1 In electronic technology, what was initially called “microminiaturization” did provide fantastic economies of scale, cost, and speed: the integrated circuits (IC) introduced by Noyce and Kilby were the beginning of this trend. It was observed that the scale of ICs or “computer chips” has halved, at first every 2 years, then every 18 months;2 this brought a concomitant increase in computing speed (“VAX on a chip”, then “Cray on a chip”) and an astonishing decrease in unit cost. However, there is trouble ahead. Circuit designers talk about “design rules”, the closest distance between adjacent electronic components in the IC. These design rules define the clock cycle, which is the time required to travel between the furthest components on the chip: shorter cycles mean faster computing. These design rules have now crept down to about 180 nm commercially. If photolithography is used, the design rules are limited, by Rayleigh’s criterion,3 to about one-half the wavelength of light used. Capacitative coupling between components and heat dissipation are perennial headaches. Three-dimensional integration (rather than planar integration) has remained an elusive goal. To achieve better performance, i.e., going to design rules of 100 nm or below, requires abandoning UV radiation and resorting to X-ray or electron beam lithography, with much higher error rates. At 50 nm, an even more drastic limit sets in: one can no longer “dope” Si uniformly. Present projections are that this 50 nm “silicon wall” will be reached by the year 2005.4 The idea of using molecules as electronic devices has gained attention and respectability in the past quarter century. By chemical insertion of electron-donating or electron-withdrawing groups, molecules can become oneelectron donors (D) or one-electron acceptors (A). To work properly, the oxidation or reduction of these molecules must be chemically reversible. In group IV chemistry (today, group 14: Si, Ge), one dopes a crystal of Ge or Si with dilute concentrations of interstitial or substitutional electron-rich elements (group V, or 15: N, P, As, etc.) to achieve an “n-doped” material. To make a “p-doped” crystal, one dopes with group III (or 13: Al, Ga, In, etc.). Thus, “D” corresponds to “n”, and “A” corresponds to “p”. By accosting a micrometer-thick film of organic D molecules to a micrometer-thick film of an organic A molecules, one gets a microscopic DA rectifier (one-way conductor) of electrical current, equivalent to an inorganic pn rectifier.5 In the 1960s, and particularly in the early 1970s, organic charge-transfer crystals and conducting polymers yielded organic equivalents of inorganic electronic systems: semiconductors, metals, superconductors, batteries, etc.6 But this wave of “me-too-ism” did not create a new technology: the organic systems did not perform better, or less expensively, than their inorganic counterparts. The two niche areas that survived are liquid crystal displays and (maybe) light-emitting diodes based on conducting polymers. In the early 1980s, sparked by three scientific conferences organized by the late Forrest L. Carter, the idea of “molecular electronics”, that is, electronic devices consisting solely of molecules, gained large-scale interest.7-9 Aficionados of biological processes started talking about “biomolecular electronics”. The term “molecular electronics” was extended to all electronic properties of polymers, crystals, etc.swhat we might call “large-scale molecular electronics”. This field, as outlined above, has not fared well in the marketplace. A persistent view has been that unimolecular, or “oligomolecular”,10,11 or “molecular-scale” 12 electronics have a very bright future, just as the new millennium begins. Molecules, with their 1-3 nm sizes, should step in where inorganic chemistry finally fails. Thus, unimolecular electronics will come to the rescue: they will finally find a central role in electronic technology.