May 2000

Molecular Computing

Continued from page 1

By David Rotman

The Recipe

In theory, at least, assembling a molecular electronic device is straightforward. In the version of the recipe favored by the HP/UCLA collaboration, the scientists first make a single monolayer of the right organic molecules in a chemical apparatus called a Langmuir trough; they then dip a silicon substrate covered by a pattern of metal electrodes into the trough. If the chemistry is just right, the molecules will bind to the metal electrodes, neatly orienting themselves. A second set of electrodes is then deposited on the molecules; the result is a monolayer of the organic molecules sandwiched between metal electrodes.

The challenge is that most organic molecules are not electrical conductors at all-never mind having the electronic properties that let them work as an effective switch. What is needed to make the system function electronically are specially tailored molecules that turn on and off repeatedly in a reliable and detectable way (the properties that have made silicon so successful). Coming up with molecules able to do the trick is the domain of chemistry wizards like Rice's Tour and UCLA's James Heath and Fraser Stoddart.

Their wizardry began paying off in a big way last fall. First, the HP/UCLA group published a paper describing what is in effect a molecular fuse-a one-time switch based on a complex, dumbbell-shaped organic molecule called rotaxane; the scientists have subsequently made reversible switches. They also showed how the device could perform simple logic and memory functions. Within months the Yale/Rice collaboration rivaled that feat by describing the synthesis of other organic molecules that act as electronic devices.

Despite the differences in molecular particulars, the two research groups are taking advantage of the same quantum effects that could eventually set fundamental limits on silicon semiconductors. The molecules separating the two electrodes would normally block the flow of current. In the nanoworld of individual molecules, however, electrons can "tunnel" through a barrier that, according to classical physics, should block their path. By manipulating a voltage placed across the electrodes, the scientists can adjust the tunneling rate and thus turn the current on or off.

Reed has already started thinking of ways to use molecular devices in combination with conventional silicon. One type of quantum logic gate that Reed has recently built would, for example, do the same specialized function as seven much larger silicon transistors, significantly reducing the size and power consumption of an integrated circuit. And while fabricating conventional transistors requires complex and expensive processing, the molecular device can be "glued on" to the circuit, says Reed.

Molecules could also provide ultra-cheap electronic memory with some attractive properties. The most common type of semiconductor memory is called DRAM, for dynamic random access memory. (This is the short-term memory your computer relies on when it's running a program.) The problem with DRAM is that the stored information evaporates when the power is shut off-it's "volatile." That's the reason you have to boot up Windows every time you turn your computer on, moving the program from your hard drive to the DRAM chips. But an experimental molecular device Reed made last fall holds data for more than 10 minutes after the power is shut off. "Suppose we can get that up to several years," says Reed. "It would essentially be nonvolatile memory. Imagine how many times you wouldn't have to boot up Windows."