Portland, Ore. -- Carbon nanotubes have shown promise as a possible medium for post-CMOS electronics, and some research projects have used them as conductors, resistors and semiconductors in novel approaches to transistor and circuit design. Although the tubes are versatile electronic components, integrating them can be a problem. Now researchers at Columbia University's Nanoscience and Engineering Center have come up with a promising approach by blending carbon nanotubes with organic molecules that naturally bind to carbon.
"We have achieved something of a milestone by being able to wire up an ultrasmall transistor where we married carbon nanotubes with organic molecules," said Shalom Wind, a senior research scientist at Columbia University. "We are taking the best properties of both and optimizing them so that we can combine them into a single switch." In the experimental device, the high carrier mobility of the carbon nanotubes harnessed the chemical reactivity of an application-specific organic molecule.
Wind performed the work with Columbia chemistry professor Colin Nuckolls and a number of their colleagues.
Wind joined Columbia in 2003 after a stint at IBM Corp.'s T.J. Watson Research Center, where he helped build the first FET that used a nanotube as the transistor's channel. That architecture might solve problems for CMOS technology by offering a reliable conduction region in very small transistors. At today's feature sizes, the channel of a transistor is only a few atoms thick, which results in quirky behavior due to quantum effects. It is also difficult to confine electrons to such a small conduction channel. The nanotubes offer a uniform conductor that helps to smooth out electron behavior in the channel.
Like IBM's original design, the Columbia University molecular transistors started out using a carbon nanotube as the channel of the transistor. The novel twist was to cut the nanotube and insert an organic molecule with some desired behavior. Now the ends of the nanotube function as the source and drain of the transistor, and the single organic molecule operates as the gate.
"Molecular transistors represent the ultimate in scaling, so we need to understand them," Wind said.
The team reported successfully inserting several types of organic molecules and gave detailed test results for an application-specific organic molecule that changed its conductivity in response to pH, enabling that molecular transistor to act as a pH sensor.
"We know that nanotubes make fantastic conductors and semiconductors--they are one-dimensional, they are nanoscale and they are made of carbon, making the chemical bonds to organic molecules easy," said Wind.
The researchers first positioned a carbon nanotube on a silicon substrate with metal contacts at each end. The tube was then covered with PMMA, a mask material used in lithography. Using high-resolution electron-beam equipment, they cut a 10-nanometer-long window in the PMMA. Oxygen plasma was then applied to vaporize the section of nanotube exposed in the window. The resulting gap is so small that it cannot be seen with an electron microscope. Even an atomic-force microscope has problems resolving the gap.
Chemically, the oxygen plasma creates a particular molecular end structure on both sides of the gap consisting of attached carboxylic-acid molecules that narrow the gap to about 2 nm. These molecules bind readily to a wide variety of organic molecules, which makes the technique flexible in terms of different molecular operations.
This latest success is part of a program at the Nanoscience Institute to explore single-molecule behavior as a means of building electronic circuits. The program hopes to discover some fundamental principles that can be used to predict the behavior of small molecular configurations under an applied electric field. *