vania State University, University Park, where it was hydrolyzed to the thiol and incorporated into a self-assembled monolayer film by chemistry professor David L. Allara. The end result was a Place a single organic molecule be- film consisting of n-dodecanethiolate tween two electrodes and show it is (DT) and the molecular wire coadsorbed electrically conductive. The concept to a gold substrate at the sulfur atoms. may be simple, but the execution has The DT molecules in the film far outnumber the molecular wires and thus eluded scientists—until now. A team of chemists has just performed serve to support and isolate individual this experiment, which is basic to the de- molecular wires in a sea of relatively invelopment of molecule-based electronics sulating molecules. [Science, 271,1705 (1996)]. The molecular wire was designed to Long, conjugated molecules that con- be longer than the DT molecule, so each duct electricity—"molecular wires"— wire protrudes about 7 Â above the film are envisioned as the components that surface. This allowed another team will interconnect molecular devices on member—associate professor of chemisthe nanometer scale. Many candidates try Paul S. Weiss of Penn State—to for molecular wires have been pre- probe individual molecular wires. First, Weiss and coworkers used the tip pared. But no one previously could of a scanning tunneling microscope (STM) to locate and STM tip images 'molecular wire' image the molecular wires jutting out STM tip of the film. Then they used an instrument developed in STM tip trajectory ,.-.. Weiss's lab—a tunable microwavefrequency alternating-current STM— to probe the wire's electrical properties. In their setup, the gold substrate and the STM tip are the S S S5 S S s s s s two electrodes. The I I 1 ι ι ι ι STM tip transfers Gold electrons to and from the organic The tip of a scanning tunneling microscope (STM) can molecules, allowlocate and image individual conducting conjugated ing the molecules' molecules ("molecular wires") that are inserted into a relative conductiviself-assembled monolayer film of alkanethiolate mole ties to be assessed. cules because the conjugated molecules extend about 7 A above the surface of the film. Weiss's measurements indicate that each molecular wire has a higher conductivity than the aldemonstrate that a single molecular wire is conductive—in part because of kanethiolates surrounding it, although the difficulty of hooking up a single quantitative measurements haven't yet molecule to probes connected to mac- been made. roscopic measuring instruments. Even so, the work is "pretty darn imThe experiment that succeeded began pressive," says Chad A. Mirkin, an aswith synthesis of the molecular wire—a sociate professor of chemistry at Northrelatively short phenylene-ethynylene western University, Evanston, 111. molecule with a thioacetate end group. It "These guys are doing things that we was prepared in the laboratory of James only dreamed of five or six years ago. M. Tour, professor of chemistry at the It's really fantastic." University of South Carolina, Columbia. Nevertheless, researchers working toThe compound was taken to Pennsyl- ward the goal of molecular electronics
Single molecular wire shown to be conductive
still have a long way to go. Actual devices will rely on nanoelectrodes, not STM tips. Chemical strategies will have to be developed to get molecular wires to span the gap between two nanoelectrodes. And "that's a whole different ball game," Mirkin says. Ron Dagani
Monsanto takes steps to enhance stock value With its common stock hovering at historic highs, Monsanto has outlined several steps to further enhance the value of its shares, including a possible stock split, new incentive and stock option plans for employees, and a stock repurchase program. Investors apparently approve. On March 14, the day plans were announced, Monsanto shares closed at $1495/e, up $45/e. At press time, the high last week was $158. Shares started 1996 at $122Vi. Monsanto will ask stockholders at its annual meeting next month to authorize issuance of 650 million additional common stock shares, enabling it to contemplate a possible stock split of 4-for-l or 5-for-l. It is already authorized to issue 200 million common shares and now has about 118 million outstanding. The stock was last split, 2-for-l, in June 1990. New compensation incentive programs for senior management include a premium-price stock option that can only be exercised when certain stockprice targets are reached during a set period of years. Another plan will allow senior management to buy substantial numbers of shares by taking a loan from Monsanto. A portion of the loan will be forgiven if the shares are held long term. Another portion of the loan will be forgiven if Monsanto meets financial targets putting it in the top half or quarter of Standard & Poor's 400 Industrials list through 2000. Monsanto also will resume its stock repurchase program, which previously was authorized to buy back up to 8 million shares. In addition, the company will introduce a stock option plan for about 27,000 employees who previously were not eligible for such options. Monsanto employs about 28,500 workers worldwide; only about 1,200 were previously eligible for stock options. George Peaff MARCH 25,1996 C&EN 7
