Like an architect who designs skyscrapers, Timothy Fisher spends a lot of time thinking about building upward. But for Fisher, an associate mechanical engineering professor at Purdue University, "building upward" requires only a tiny amount of height.
Working with Purdue engineering professor Timothy Sands, Fisher has developed a method for vertically growing individual carbon nanotubes. The technique promises to lead to nanoelectronic devices with circuits and components stacked in layers. "Vertical orientation can allow us, potentially, to gain the ability to fit more devices into a space by adding layers and not increasing the footprint size," says Fisher.
Fisher and Sands started with a thin film containing two layers of aluminum sandwiching an even thinner layer of iron. To form the film, they used electron-beam evaporation. They then anodized the film to create a porous surface topography with a precise array of tiny cylindrical cavities.
Next, the researchers flowed a hydrogen-methane mixture into the holes and applied microwave energy to break down the methane. This forced the iron layer, exposed by the holes, to act as a catalyst, prompting the carbon nanotubes to self assemble from the carbon originating in the methane. As expected, the tubes grew vertically out of the holes (see the figure).
"One of the major innovations was to put the catalyst inside the nano-porous material," says Fisher. "The fundamental rationale behind this approach was to try and grow, separately, individual nanotubes and put them where we wanted them."
The approach streamlines nanotube creation and positioning. "Most features that you want to create at the nanoscale require quite a bit of intricate processing, both preprocessing and post-processing," he says. "This is really a bulk, bottom-up, assembly approach, where at the end of the process you have distinct, individual nanotubes in pores that are ordered in such a way that you know where each tube is."
He notes that his work was the first to grow single-wall carbon nanotubes in pores. Besides potentially paving the way toward nanoscale circuit and component layering, the technique also is significant for its substrate independence, which promises to simplify manufacturing.
"In many cases, for carbon nanotube and other nanowire growth, you're growing on a bulk substrate that is essential to the process," Fisher says. "In this case, we're growing [nanotubes] in a very thin nanoporous layer that eventually could be transferred to another substrate."
The vertical orientation also lets designers make better use of nanotubes. " Having a tube standing up vertically gives you access to the entire circumference of the tube, and from an electrical device standpoint that can have a lot of benefits in terms of device speed in particular," Fisher says. "When we want to apply a gate dielectric and a gate electrode, for instance, we can do so much more easily than with a nanotube that's laying down on a flat substrate."
But don't expect to get your hands on vertical nanotube technology anytime soon. The research is still at an early stage, and nano-circuit manufacturing remains many years away. "The most probable [timeframe] would be five to 10 years. It will be worth the wait," Fisher promises.
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