Rapid progress reported in emerging field of molecular electronics

The emerging field of molecular electronics — using nanoscale molecules as key components in computers and other electronic devices — is in excellent health and has a bright future, conclude UCLA, Caltech and University of California, Santa Barbara, chemists who assess the field in the Dec. 17 issue of the journal Science. “Molecular electronics is in its infancy, and its adolescence and adulthood will be very exciting as we push toward the promise of molecular electronics: smaller, more versatile and more efficient,” said Amar Flood, a UCLA researcher in Fraser Stoddart’s supramolecular chemistry group, and lead author of the Science paper. “The combination of active molecules with electronic circuitry is opening up exciting new areas of science,” Flood said. “It is too early to predict precisely what will come from this marriage, but we expect that the unique properties of molecules, including sight, taste and smell, may be put to very good effect by marrying them with silicon.”

The first applications are likely to involve hybrid devices that combine molecular electronics with existing technologies, such as silicon, said Stoddart, director of the California NanoSystems Institute (CNSI), who holds UCLA’s Fred Kavli Chair in NanoSystems Sciences. Molecular electronic components are already working, say Stoddart, Flood and co?authors James R. Heath, who is Elizabeth W. Gilloon Professor of Chemistry at Caltech and a member of CNSI’s scientific board; and David Steuerman, a CNSI postdoctoral fellow in physics at University of California, Santa Barbara. For example, logic gates, memory circuits, rectifiers, sensors and many other fundamental components have been demonstrated to work. Progress toward incorporating molecules as the active components in electronic circuitry has advanced rapidly over the past five years. Heath describes the progress as “real and rapid.”

“We have published 64-bit random access memory circuits using bistable rotaxane molecules as the memory elements, and we are in the process of fabricating a 16-kilobit memory circuit at a density of devices that far exceeds current technology,” Heath said. “On a Moore’s Law graph, our memory circuit is at a density of Intel-like circuits that will be manufactured decades from now.” “Dreams I was having less than a decade ago are becoming a reality in our labs,” said Stoddart, whose areas of expertise include nanoelectronics, mechanically interlocked molecules, molecular machines, molecular nanotechnology, molecular self-assembly processes and molecular recognition, among many other fields of chemistry. “Although many classes of molecules can be used for molecular electronics, only a small percentage of these have been assessed so far,” Flood said.

Over the past decade, scientists around the world have taken a few model molecular systems, including bistable catenanes and rotaxanes, and have addressed many of the fundamental scientific principles related to harnessing their potential in electronic circuits. The research summarized in the Science paper describes experiments in which the UCLA/Caltech team has used its bistable catenanes and rotaxanes in many different environments. For example, they use the bistable molecules in environments where chemists are comfortable, such as the solution phase, and in environments where engineers are comfortable: electronic circuits.

Heath said, “We can now correlate quantitatively the properties of bistable catenanes and rotaxanes from the solution phase, where they are easy to interrogate, to a device, where they are much more difficult to interrogate. Ultimately, we would like to have control over device properties through molecular synthesis. This paper in Science highlights the fact that we are beginning to achieve this goal.”

The UCLA/Caltech team has verified that bistable catenanes and rotaxanes work as molecular switches that can be turned on and off when they are attached to surfaces and when they are buried in polymer blends with the consistency of a rubber tire. “When we apply a positive voltage, they turn on, and when we apply a negative voltage pulse, they switch off instantly,” Stoddart said. “We have verified that the same mechanism works in a device, in solution and in two other environments. In addition, we have measured how fast the bistable molecules switch in different environments. We can slow down the switching on the order of 10,000 times on going from solution to device. What takes 10 minutes in a device takes one-tenth of a second in solution. This type of control allows us to store bits of memory using these molecules.”

The role of environments on the molecules’ switching speeds is elaborated on in the final issue in 2004 of Chemistry – A European Journal (volume 10, page 6,558).

The UCLA/Caltech team also can produce the colors red, green and blue within a single molecule. The red-to-green color changes are highlighted in pictures published in Angewandte Chemie International Edition earlier this month (volume 43, page 6,486). If Stoddart’s molecular switches are incorporated into a future generation of computers, there is also the prospect of using the same molecular switches as the basis for the displays in these and in other new technologies. The UCLA/Caltech team is working with multiple kinds of molecular switches, each with unique characteristics.