For Mark Reed, the future of molecular electronics has just arrived. A self-described “device guy,” Reed, who heads Yale University’s electrical engineering department, prides himself on having a distinctly practical bent. Ask him about the possibility of one day using molecules to replace silicon in computers that are billions of times faster than today’s PCs or that fit on the head of a pin, and he grimaces. “I don’t know how to do that. I don’t think anyone does,” he says dismissively.
But that doesn’t dim the excitement that Reed, a leading researcher in molecular electronics, is feeling. Using molecules synthesized by Rice University chemist James Tour, Reed has fabricated electronic memories and a simple logic element made up of molecules that function as tiny, individual switches. The devices, which rely on small organic molecules tailored by the Rice chemists to have just the right electronic properties, are crude laboratory experiments. But they work-the molecules acting as a component in ultrasmall electronic devices able to turn current “on” and “off.” What’s more, these early prototypes have already shown hints of performing memory and logic tricks not possible with silicon semiconductors.
Most impressive, says Reed, is that the molecular devices are astonishingly easy-and potentially cheap-to make. You simply dip a silicon wafer lined with metal electrodes into a beaker filled with the right chemicals and give the molecules a few minutes to form on the electrodes. If you’re clever enough with the chemistry, it’s possible to coax the molecules to spontaneously orient themselves on the electrodes. “It works beautifully-and it works every time,” says Reed.
It may work every time, but there’s considerable controversy about what these chemical reactions will ever amount to. While true believers envision a world in which microscopic molecular computers made at low cost perform remarkable calculations, skeptics think the field has lost sight of the real world of engineering limits. Meanwhile, “device guys” like Reed think the future-in the form of workable prototypes that can be integrated with conventional silicon technology-is now.
The core advantage of molecular computing is the potential to pack vastly more circuitry onto a microchip than silicon will ever be capable of-and to do it cheaply. Semiconductor-makers can now cram about 28 million transistors on a chip by shrinking the smallest features of the transistors down to about 180 nanometers (billionths of a meter). Using conventional chip-making methods, however, the smaller you make a feature, the more expensive and difficult the process becomes. Many semiconductor experts doubt commercial fabrication methods can economically make silicon transistors much smaller than 100 nanometers. And even if chip-makers could figure out a reasonable way to etch them onto a chip, ultrasmall silicon components probably wouldn’t work: At transistor dimensions of around 50 nanometers, the electrons begin to obey odd quantum laws, wandering where they’re not supposed to be.
Molecules, on the other hand, are only a few nanometers in size, making possible chips containing billions-even trillions-of switches and components. In initial experiments, scientists have sandwiched a large number of molecules between metal electrodes. The devices work, however, because each molecule operates as a switch. If it were possible to wire a small number of molecules together as individual electronic components to form circuits, the result would change everything in computer design. Molecular memories could have a million times the storage density of today’s best semiconductor chips, making it possible to store the experiences of a lifetime in a gadget the size of a wristwatch. Supercomputers could be small enough and cheap enough to incorporate into clothing. Worries that computing technology will soon hit a wall would disappear.
Those applications are decades off-if they ever materialize. Still, Reed argues, some uses for molecular electronics could soon be feasible. Ultrasmall, cheap molecular devices could sit side-by-side with silicon, reducing the number of transistors and the power required by the circuit. “This is something you could use today, something you could sell in Radio Shack,” says Reed. “This has a chance to totally change the economics of silicon.”
To make that a reality, Reed, Tour and chemists from Pennsylvania State University have co-founded a startup called Molecular Electronics. The group declines to say what the initial products will be, but Tour says having “a working system in a couple years doesn’t seem unrealistic.”
Until very recently, that prediction would have seemed far-fetched. But in the last year, the field has taken a leap from theory to the realm of the practical. Like their competitors at Yale and Rice, a West Coast collaboration of chemists and computer scientists from Hewlett-Packard and the University of California, Los Angeles, have recently characterized molecules capable of acting as electronic switches and memory (see past issue: “Computing After Silicon,” TR September/October 1999). R. Stanley Williams, who heads the effort at HP, says his team expects to build a prototype of a logic circuit that integrates a small number of nanoscale molecular devices within 18 months. “We have the switches and wires-the components to actually make true nanocircuitry,” says Williams.