Chemist Charles Lieber and co-workers at Harvard University created simple logic circuits incorporating up to six transistors by crisscrossing nanometer-wide wires of silicon and gallium-nitride, each junction of which forms a transistor. This technique works by catalyzing the growth of the crystal wires from solutions of each material with the assistance of a laser. "It's the first case where [some]one's actually used an assembly strategy to create logic devices," Lieber explains. He believes nanowires could pave the way for molecular electronics because they are more predictable than other techniques and can be built without conventional lithography. "The approach that we're taking is one that can be directly scaled to the next level of complexity," he says.
Taking a somewhat more conventional approach, physicist Adrian Bachtold and colleagues at Delft University of Technology in the Netherlands carved aluminum strips from a layer of the metal and deposited carbon nanotubes on top. They then attached strips of gold to both ends of each nanotube, creating a transistor, and linked up to three such devices in various ways to make circuits that would execute simple logical functions: flipping a signal from off to on or vice versa, turning two off signals into an on, storing a unit of information or creating an oscillating signal.
Taking yet another tack, physicist Jan Hendrik Schn, with help from other researchers at Bell Laboratories, has refined a technique he recently described for making transistors out of a layer of small carbon molecules. Diluting these transistor molecules with insulating carbon chains, Schn found that just one was enough to turn a signal on or off, making a rudimentary circuit element.
In a commentary accompanying the first two reports, nanotechnologists Greg Tseng and James Ellenbogen of the MITRE Corporation in McLean, Va., explain that the two groups "are the first to advance molecular-scale electronics fully from the single-device level to the circuit level." However, they add, scientists still have to reduce the complete circuit to molecule size. They will then need to figure out how to assemble up to a trillion parts quickly, cheaply and precisely onto a chip the size of a fingernail. But the fact that several groups have assembled basic circuits from molecule-scale parts, they write, "is an indicator of how far molecular electronics and nanotechnology have come and is very encouraging for the future."