NANOWIRES AND NANOTUBES Harvard chemists use surface patterning methods and four semiconducting nanowires (top) to connect the elements of a NOR logic gate (as indicated by the circuit symbol). Delft researchers fabricate transistors (bottom) using carbon nanotubes and metal electrodes.

Physical constraints make it unlikely that circuits based on lithographically patterned semiconductor chips can be made much smaller than they are presently. So researchers around the globe have been searching for alternatives to today's approach to microcircuitry fabrication.

A promising strategy calls for using components with molecular dimensions to construct circuits. That route has led investigators in recent months to use nanowires, carbon nanotubes, and even molecules to build increasingly sophisticated electronic devices such as transistors and logic circuits (referred to as logic gates).

But now researchers have advanced the field even further by improving transistor design and performance and by developing new methods for preparing electronic components. The new techniques have resulted in transistors that produce large signal amplification (gain) and have raised the level of complexity of nanoscale computing devices.

In one study, chemistry professor Charles M. Lieber and coworkers at Harvard University used semiconductor nanowires to build field-effect transistors (FETs) and have performed basic digital computations using logic devices built from multiple nanowire FET logic gates [Science, 294, 1313 (2001)].

At Delft University of Technology in the Netherlands, a team of scientists led by physics professor Cees Dekker has demonstrated multiple-transistor logic circuitry based on carbon nanotube FETs [Science, 294, 1317 (2001)].

Chemical synthesis and microfluidic patterning methods lie at the heart of the Harvard group's technique. The team uses metal catalysts to prepare p-type (positive-charge-carrying) silicon nanowires and n-type gallium nitride nanowires. They arrange the wires into circuits from solution by flowing suspensions of nanowires onto a surface patterned with narrow channels.

Lieber notes that the nanowire building blocks are prepared using methods that provide tight control over the wires' physical and chemical properties--for example, the thickness of a critical insulating oxide layer. This high level of control is essential for building nanoelectronic devices whose performance is predictable and reproducible, he stresses.

Meanwhile, carbon nanotubes are being used by a number of research groups as key elements in electronic circuits. But in most cases reported so far, the circuit design has prevented nanotube FETs from being controlled (switched on or off) individually--limiting the range of potential uses for such circuits.

Dekker and coworkers devised a way around the "all or none" problem by fabricating an aluminum wire that serves as one of the FET electrodes (the gate). By exposing the nanotube assembly to air, the Delft team builds up a thin, insulating oxide layer on the aluminum that enables multiple nanotube transistors to function independently and be integrated on a single chip.

Commenting on the work in the same issue of Science, Greg Y. Tseng and James C. Ellenbogen of Mitre Corp., McLean, Va., note that the present studies are "the first to advance molecular-scale electronics fully from the single-device level to the circuit level."

[Previous Story] [Next Story]

Chemical & Engineering News
Copyright © 2001 American Chemical Society