Two nanotechnology researchers whose work is sponsored by the U.S. Office of Naval Research (ONR) are among Scientific American´s SA-50, a list of 50 American technology and policy leaders for 2006. A third ONR-funded nanotechnology investigator has earned a 2006 TR35 Young Innovator award from MIT´s Technology Review. Each year, this publication´s editors honor 35 researchers under the age of 35 whose work they deem "most exciting."
Carbon in Two Dimensions
Philip Kim of Columbia University, one of the Scientific American 50 awardees, led one of two independent research groups (the other was led by Andre Geim of the University of Manchester) that confirmed experimentally the electronic behavior of the novel material graphene. Graphene, a form of carbon, is essentially a single atomic layer of graphite in which the carbon atoms are arranged in a flat sheet of interconnected hexagons, in much the same fashion as "chicken wire" or hexagonal floor tile. Carbon nanotubes, tiny cylinders that are finding application in a variety of fields, are a tubular form of graphene.
It was thought previously that a stable two-dimensional material such as graphene could not exist in nature, according to Chagaan Baatar, an ONR program officer who specializes in nanoelectronics research. In 2004, Kim and Geim discovered that they could lift very thin flakes from ordinary graphite that were, indeed, sheets of graphene.
Kim and Geim´s groups later showed that the electrons in graphene have zero "effective mass." That is, regardless of how much energy the electrons have, they travel through the graphene at a constant speed of 800 kilometers per second (km/s). This constant speed is described by Einstein´s relativistic laws of motion, which were first used to show that light of any color (energy) travels through a vacuum at a constant 300,000 km/s. Graphene´s unusual behavior could one day form the basis for some radically novel type of electronic devices and sensors.
Prabhakar R. Bandaru, another of Scientific American´s top 50, led researchers from the University of California, San Diego, and Clemson University (South Carolina) in demonstrating a new type of transistor made from carbon nanotubes. Previous versions of nanotube transistors used a metal gate to control the current passing through the nanotube. Bandaru´s group produced Y-shaped nanotube transistors in which one branch of the Y serves as the gate electrode. Varying the voltage across this gate controls the current flowing through the other two branches. Eliminating the metal gate simplifies the manufacturing process, and the Y-shaped transistors are much smaller and faster than existing silicon microelectronic devices.
The transistors were formed from straight carbon nanotubes, to which titanium-modified iron catalyst particles were added. This caused the ends of the growing nanotubes to split, forming two branches, with the catalyst particles remaining in the junction of the "stem" and the two branches. Electrical contacts attached to the nanotubes cause a current to flow along one branch of the Y until it reaches the catalyst particle. The current then flows out of the nanotube along the other branch. By applying a positive charge to the stem, the current flow can be greatly enhanced, producing an "on" signal, equivalent to a 1 in binary code. Reversing the polarity of the charge stops the current flow, the equivalent of a binary code 0.
Tiny Dip Pen Makes Tiny Dots
William King of the Georgia Institute of Technology, the TR35 honoree, collaborated with Lloyd Whittman of the Naval Research Laboratory to come up with a "soldering iron" that "writes" tiny dots of polymer onto a substrate. (King has recently accepted a faculty position at the University of Illinois at Urbana–Champaign.) The soldering iron is actually a silicon atomic force microscopy (AFM) cantilever with a 20-nm tip. The temperature of the AFM tip cycles back and forth between 1200 °C and room temperature several millions of times per second to deposit polymer drops less than 80 nm across. King refers to his technique as "thermal dip-pen nanolithography."
Thermal dip-pen nanolithography is less time consuming than conventional dip-pen nanolithography, and it offers much greater control over the orientation of the polymer molecules. Researchers at the U.S. Naval Research Laboratory and Georgia Tech are using the thermal technique to control the thickness and molecular orientation of polymer droplets at the level of a single layer of molecules.
This year, King´s group used the heated AFM tips to grow clusters of carbon nanotubes and monitor the growth of the nanotubes under different conditions in real time.