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Nanotubes to Go

USC Electrochemist's Technique Promises Far Greater Yield and Flexibility for Next-Generation Computing and Sensing Devices

March 01, 2006 —

Nanotubes in line: the golden threads are cylinders of carbon with walls one atom thick growing on a sapphire  crystal surface. The black bar is 1.25 nanometers, or 0.00000125 millimeters long.
A  year ago, USC engineer Chongwu Zhou described a new way to "grow" orderly arrays of carbon nanotubes on sapphire. A new publication this year describes a way to roll these arrays up off the sapphire into neat parcels of working transistors ready to insert into large-scale integrated electronic systems.
Zhou, of the USC Viterbi School of Engineering's department of electrical engineering's says the new approach "has great potential for high density large scale integrated systems based on carbon nanotubes for both micro and flexible electronics publication." His description is published in Nano Letters Vol. 6 No 1.

Nanotubes — ultrasmall  hollow cylinders with walls just one carbon atom thick — have attracted wide attention since their first synthesis a decade and a half ago, as investigators realized they could potentially function in integrated circuits like devices made from carbon's  periodic table first cousin silicon.

Exploitation of this possibility has been held back because previously nanotubes have had to be grown on silicon substrates, which have electrochemical  properties that interfere with those of the nanotubes. "Parasitic capacitance" is the term used to describe the effect.

Instant transistors: when the array of gold-titanium gates go down on top of the  aligned nanotubes , the result is a n array of transistors
Additionally, earlier methods required "registration" — elaborate,  difficult, expensive, and not very successful efforts to make sure the nanotubes grew in useful configurations, or removal of those not where they should be.

Zhou's earlier breakthrough work established that nanotubes naturally grew in useful configurations, regular, side-by-side  on some surfaces of sapphire crystals — on two of the four planes of the crystal (see diagram 1)

The new work builds on those findings, demonstrating the making of nanotubes into transistors right on top of the sapphire surface. The method is the well-established photolithography process, creating an array of source-drain pairs, which are dropped down on top of the nanotubes perpendicular to the parallel nanotubes.

"This … process delivered a high yield of potential devices," reported Zhou in the paper. "Electrical measurements of the source/drain electrode pairs revealed that 98% of the devices were connected by carbon nanotubes and exhibited significant conduction."

Even better, "we found that there was no conduction or cross-talking between adjacent devices. This can be understood easily because almost all the nanotubes were aligned in the vertical direction [see picture] … hence no conductive path could be found between the devices in one row."

Transistor arrays on sapphire are striking as a demonstration, but the crystal is not the ideal substrate. Better instead is an insulator, a plastic surface that doesn't conduct at all, like the plastics some silicon devices are built upon.

But now Zhou believes nanotubes on insulator (NOI) architectures are possible as well, and that may be the most far-reaching implication of the paper.  Zhou found that, once the nanotubes were

Roll-off: after depositing a film of insulator on the sapphire, embedded nanotubes can taken off.
deposited on the sapphire, his team could deposit a thin layer of plastic insulator on top of them.

Zhou subsequently peeled this film off the sapphire -- and the nanotubes came up with the film. The Titanium-gold gate arrays then drop right on top of the insulator-embedded nanotubes — and Zhou's paper describes other circuitry that can be deposited on top of that.

Zhou's team  included Xiaoolei Liu and Song Han. The research was supported by two separate NSF grants, including a CAREER award; plus SRC award,  and funds from the DARPA  MolAps Program.