The development of nanoscale electronic devices–devices whose components have dimensions of a few nanometers–would lead, for example, to the ability to pack immense computing power into a very small space. (A nanometer (abbreviated nm), or 10^-9 meter, is 10 Angstrom units; about three gold atoms could be placed side-by-side within 1 nm.)

An advance in this direction was made by Ron P. Andres, Cliff Kubiak, and coworkers of Purdue University, who found a way to deposit an ordered, two-dimensional array of gold clusters, each having a diameter of 3.7 nm, on a solid support such as MoS₂ (1). The clusters were stabilized by organic linkers, which interconnected gold atoms in different separate clusters. The view is that the linkers form covalent gold-sulfur bonds.

These cluster arrays show, at room temperature, the typical characteristics of correlated single-electron tunneling (SET), that is, a stairstep current-voltage profile. In other words, as an increasingly higher voltage is applied, the current through these arrays does not increase continuously, but rises in quantized steps; a current vs. voltage plot shows a “stairstep” pattern. This is what would be expected if the metal clusters can be thought of as conducting “islands” isolated from each other by highly resistive wires (the molecular linkers). The linkers provide a barrier for the transfer of electrons from one cluster to the next. In a similar case, the authors estimate the resistance of each linker at about 43 million ohms!

If you are familiar with bonding in organic molecules, you know that the electrons of conjugated double and triple bonds (double and triple bonds separated by one formal single bond) are highly delocalized and can move throughout the molecular framework. Consequently, the high resistivity of the linkers might be somewhat surprising. The authors liken the linkers to wires connected to conductors by really poor “alligator clips.” The gold-sulfur-carbon linkage is the molecular “alligator clip” that is evidently the source of the greatest electrical resistance. The resistance barrier provided by these linkers is essential to the SET behavior, which, in turn, results from the quantum properties of the electron.

This work represents an important advance in the general area of nanotechnology. Nanotechnology encompasses not only electronic devices, but also gears, camshafts, and motors engineered on a nanometer scale. The smallest feature on the Intel Pentium computer chip is currently on the order of 350 nm. The semiconductor industry has targeted the development of devices with feature sizes of about 70 nm by the year 2010.

Acknowledgement to Professor Cliff Kubiak of Purdue for providing some helpful explanations and materials.

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