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MOLECULAR ELECTRONICS
MOLECULE-BASED CIRCUITRY REVISITED Faked results implausible, but true molecular devices still may be possible
The idea that a single molecule or just a small handful of molecules can function as the active element in electronic devices gained support over the past few years as researchers in various institutions reported incremental advances in molecule-based circuitry. So when Jan Hendrik Schön and coworkers at Lucent Technologies' Bell Labs reported that they had succeeded in preparing functioning field-effect transistors (FETs) based on self-assembled monolayers (SAMs) of organic molecules, excited researchers in several labs tried to reproduce Schön's results. But none was successful. Then last year, an investigatory committee concluded that Schön fabricated much of the data presented to support his claims, and he was fired (C&EN, Sept. 30, 2002, page 9).
As shock waves from the research-fraud announcement reverberated through the molecular electronics community, many scientists continued to wonder whether it still might be possible to observe the electrical-transport properties reported in the Bell Labs papers using the type of SAMFETs described in those publications. Now, groups of scientists working independently at IBM's T. J. Watson Research Center in Yorktown Heights, N.Y., and at Dutch universities report that, based on numerous experiments, their results contradict Schön's claims. The team in the Netherlands includes Delft University of Technology research associate Jeong-O Lee and physics professor Cees Dekker and their coworkers at Delft, Eindhoven University of Technology, and the University of Twente. That group prepared SAMFETs using five phenylene-based p-conjugated molecules: 1,4-benzenedithiol, 1,4-phenylenediisocyanide, and 4,49-biphenyldithiol--molecules that were mentioned in the Bell Labs papers--and 1,3-benzenedithiol and a related benzonitrile (Nano Lett., published online Jan. 1, http://dx. doi.org/10.1021/nl025882+). In total, the Dutch group reports, more than 1,000 devices were fabricated. Yet despite careful preparation and characterization, most failed owing to short-circuiting. Of those that worked, only two FETs made from 1,3-benzenedithiol exhibited an appreciable gate effect, a key electrical property. But the magnitude of the effect was negligible compared with the claims made by Schön. IBM staff scientists Cherie R. Kagan, Richard Martel, and their coworkers investigated SAMFETs prepared from biphenyldithiols and related molecules. They conclude that, although the devices described in the Bell Labs papers cannot function as claimed, the chapter on molecular FETs remains open. Based on an analysis of electrostatics and charge tunneling, the group says that by adhering to certain geometrical constraints on the lengths and thicknesses of FET components (for example, a minimum molecular length of 2.5 to 3 nm), it may still be possible to prepare functioning FETs from monolayers of organic molecules (Nano Lett., published online Dec. 19, http:// dx. doi.org/10.1021/nl0259075). |
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In an effort to set the record straight, researchers in the U.S. and the Netherlands have investigated the feasibility of constructing molecular electronics devices that were reported to have been built and tested in a series of research papers later judged to be fraudulent. The new studies conclude that microscopic structures of the type described in the original reports do not function as claimed and cannot function in that way owing to fundamental limitations and other problems.
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