August 14, 2005 -- Researchers at UCSD and Clemson Universityhave discovered that specially synthesized carbon nanotube structuresexhibit electronic properties that are improved over conventionaltransistors used in computers. In a paper published* in the Septemberissue of Nature Materials and released online on August 14, UCSDMechanical and Aerospace Engineering professors Prabhakar Bandaru andSungho Jin, graduate student Chiara Daraio, and Clemson physicistApparao M. Rao reported that Y-shaped nanotubes behave as electronicswitches similar to conventional MOS (metal oxide semiconductor)transistors, the workhorses of modern microprocessors, digital memory,and application-specific integrated circuits.
“This is the firsttime that a transistor-like structure has been fabricated using abranched carbon nanotube,” said Bandaru. “This discovery represents anew way of thinking about nano-electronic devices, and I think peopleinterested in creating functionality at the nanoscale will be inspiredto explore the ramifications of these Y-junction elements in greaterdetail.”
The stunning increase in the speed and power efficiencyof electronics over the past two decades was primarily due to thesteady shrinkage in size of conventional transistors. Chip makers havereduced the minimum feature size of transistors to about 100nanometers, and that dimension is expected to shrink by the end of thisdecade. However, industry experts predict that fundamentaltechnological and financial limits will prevent the makers ofconventional MOS transistors to reduce their size much further. TheY-shaped nanotubes discussed in the Nature Materials paper are only afew tens of nanometers thick and can be made as thin as a fewnanometers.
“The small size and dramatic switching behavior ofthese nanotubes makes them candidates for a new class of transistor,”said Bandaru.
The new transistors were initially grown asstraight nanotube elements. Titanium-modified iron catalyst particlesadded to the synthesis mixture were then attached to the straightnanotubes, nucleating additional growth, which continued like branchesgrowing from a tree trunk. Consequently, the nascent nanotubes assumeda Y-shape with the catalyst particle gradually becoming absorbed at thejunction of the stem and two branches.
When electrical contactsare attached to the nanotube structures, electrons travel into one armof the Y, hop onto the catalyst particle, and then hop to the other armand flow outward. Experiments conducted in Bandaru’s lab at UCSD’sJacobs School of Engineering showed that the movement of electronsthrough the Y-junction can be finely controlled, or gated, by applyinga voltage to the stem. Bandaru hypothesized that positive chargeapplied to the stem enhances the flow of electrons through the twoarms, producing a strong “on” signal. Then, when the polarity of thecharge is reversed, the movement of electrons through the armsessentially stops, creating an “off” signal. Such binary logic is thebasis of nearly all transistors.
“Among electrical deviceengineers, this phenomenon is called gating,” said Bandaru. He said thephenomenon effectively makes Y-shaped nanotubes the smallest ready-madetransistor yet, with rapid switching speeds and possible three-waygating capability. In earlier attempts to make carbon nanotube-basedtransistors, separate gates were added rather than built in.
“Wethink this discovery extends the paradigm of nanotechnology beyond justmaking things small,” said Bandaru. “We can synthesize functionality atthe nanoscale, in this case to include the three elements of a circuit– the gate, source, and drain – and we don’t have to go to the troubleof making them separately and assembling them.”
The researchersplan to experiment with various other catalyst particles in order totailor the three-way gating properties of the Y-junctions. “If we caneasily fabricate, manipulate, and assemble these nano-devices on alarge scale they could become the basis of a new kind of transistor andnanotechnology,” said Bandaru.
* "Novel electrical switching behaviour and logic in carbon nanotubeY-junctions," P.R. Bandaru, C. Daraio, S. Jin and A.M. Rao, NatureMaterials, September 2005
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