Now, the Tempe team reports "a reliable method for chemically bonding metal contacts to either end of an isolated molecule" and measuring the current-voltage characteristics of the resulting circuit [Science, 294, 571 (2001)].

The researchers--Arizona State chemistry professor Devens Gust, physics professor Stuart M. Lindsay, and their coworkers--begin by forming a monolayer of octanethiol molecules on a gold surface, which serves as one electrode. Using a solvent technique, they then remove some of these molecules, replacing them with 1,8-octanedithiol molecules, which are capable of chemically bonding to gold at both ends.

By exposing the monolayer to a suspension of gold nanoparticles that are less than 2 nm in diameter, the researchers tether nanoparticles to the free ends of the octanedithiol molecules scattered throughout the monolayer. A gold-coated conducting tip of an atomic force microscope is then used to locate and contact individual nanoparticles bonded to the octanedithiol "wires," thus forming an electrical circuit. The octanethiol molecules, which are bound only to the bottom electrode, serve as molecular insulators, isolating the dithiol wires from one another.

The Arizona researchers made current-voltage measurements on more than 4,000 nanoparticles, producing only five distinct families of curves. These curves correspond to integer multiples of a fundamental curve, which they believe represents conduction through a single dithiol molecule attached to both gold contacts. The other curves correspond to conduction through two or several dithiol molecules bridging the two contacts.

Based on measurements on more than 1,000 single molecules, the researchers peg the resistance of an octanedithiol molecule at about 900 megohms. When they tried to measure the resistance of octanethiol monolayers (in which chemical bonding to both ends of the molecules is not possible), they obtained resistance values that were at least four orders of magnitude higher. These measurements also were less reproducible and had a different voltage dependence, they note, "demonstrating that the measurement of intrinsic molecular properties requires chemically bonded contacts."

In an accompanying commentary, K. W. Hipps, a professor of chemistry and materials science at Washington State University says the work "finally gives us a [reliable] tool to begin in earnest the study of single-molecule devices."

MOLECULAR CIRCUIT The conductivity of an isolated 1,8-octanedithiol molecule can be measured by chemically bonding one of its sulfur atoms (orange) to a gold base electrode and the other sulfur atom to a gold nanoparticle that is in contact with the gold tip of a conducting atomic force microscope.

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