Nano tool kits go beyond CMOS — sideways

Nov. 7, 2006 — Researchers working on nanowires and compound semiconductors for next generation electronics reported at SEMI NanoForum in San Jose last week that they’re getting practical results by turning their focus to relatively simpler applications, such as batteries and lighting.

MIT professor Angela Belcher reported that improved control of the phage display process to make nanowires has created a new toolkit for an expanding range of potential new applications less complex than advanced circuit assembly, from flagging semiconductor defects to assembling alloys for thin film battery anodes. And compound semiconductor researchers said nanoscale surface engineering is starting to enable economical low-energy white LED lighting.

“This isn’t your usual beyond-CMOS stuff,” noted Robert Doering, senior fellow and technology strategy manager at Texas Instruments, and one of the organizers of the trade group’s meeting on nano manufacturing issues. “It’s more like beyond CMOS — sideways.”

Belcher reported her lab at MIT, in conjunction with colleagues Paula Hammond and Yet-Ming Chiang, has developed viruses that stick to dislocations in silicon, for easily locating defects in germanium on silicon wafers. They’ve also gotten good results, at least under laboratory conditions, with viruses that attach to fatiguing sections of alloys in aircraft engine parts, to quickly flag potential failure areas. And taking the biological process with its expanded repertoire of inorganic materials back to the medical world, they’re developing a detector that marks the different stages of colon cancer.

Belcher noted that evolving viruses to selectively stick to just about any desired material is now very easy to do. “We have high school students do it, or visiting physicists, and we teach it in our undergraduate labs,” she said. They pick a group of likely viruses from libraries developed by the drug industry, expose the viruses to the desired material to find ones that stick, wash off all those that don’t, feed the few that do into bacteria to grow a new generation, and repeat the processes. It typically takes about five rounds to get an organism that attaches stably to the desired metal, insulator, or semiconductor. Add a fluorescent tag, spray a solution of the viruses on a surface that might containing a material of interest, and they’ll mark it if it’s there.

The MIT group has reported steady progress in the trickier work of getting the viruses to coat themselves with particular materials from solution by cloning back into their DNA to change receptors on their surface. Initially the viruses just attached the nanoparticles to their head, or at scattered spots along their 880nm length. Making too many changes to their coating DNA created unstable organisms that fell apart.

But researchers have now gotten better at making bigger changes, creating viruses that can not only coat themselves with an orderly arrangement of material all along their rod-shaped bodies, but can now do so with two different materials at once. “New genetic engineering in the last year now allows alloys,” said Belcher. The group is working on the synthesis of three materials at once, such as InGaN for solar cells, but that’s proving more difficult.

Designing a virus that can coat itself with both cobalt (as Co3O4) and gold from solution enables researchers to make a densely packed monolayer of crystalline nanowires for a thin film battery anode that has 2X to 2.5X higher energy density than current electrode materials.

It’s made by a simple low-cost process of dipping a polymeric electrolyte film into several solutions. Currently the thin-film electrolyte and anode are attached to a conventional cathode, while researchers work on developing viruses that can grow the trickier three-material cathode. Still, Belcher projected the current proof of concept work could lead to a working thin film battery prototype in two years or so. “I like starting with a simple system, where things don’t have to be perfectly aligned,” she said. “We’re working now on a problem we think we can solve.”

Improving nano-control of semiconductor deposition and doping processes, and nano-engineering of surfaces, could make high efficiency LED lighting economic in about two years, potentially saving billions in energy costs and carbon dioxide emissions, argued Steven DenBaars, of the University of California Santa Barbara. Progress in atomic-level control of deposition, parts per billion control of doping, and nano structuring of the device surface, have brought the efficiency of white gallium nitride (GaN) LEDs up to 130 lumens per watt in recent months, to surpass fluorescent bulbs as the most efficient light source.

That means a 7W LED can put out the same amount of light as a conventional 60W light bulb, or as a 15W compact fluorescent. The LED bulb still costs around $60, but DenBaars pointed out it’s dropped from more than $100 last year, and figured it’s likely to get down to $20 in about two years, shrinking the payback period to about a year.

Making a layer of micro cones across the GaN device surface lets more light out much more effectively with less scattering, as does a mega cone of zinc oxide, or an imprinted diffraction grating. “The ability to fabricate at the nano-level gives a great increase in light output,” said DenBaars, noting 2X to 3X improvements from such surface treatments.

The current generation of white LEDs are mostly used in cell phone backlights, but the new low power, high brightness versions are showing up in off-grid streetlights in Japan, powered by solar cells and battery packs, and a hundred will reportedly line the main entry road for the Beijing Olympics. The low power lights are also likely to be particularly useful in hybrid cars, and are slated to be used on the 2007 Toyota Camry.

Yang Yang of UCLA argued that organic LEDs could likely reach the same levels of high efficiency with similar nano surface engineering. Currently his lab is producing light at 20 lumens per watt efficiency from a simple polymer coating between two electrodes. But noted Yang, “It’s just coated on, and most of the light is trapped in the glass. Substrate engineering should improve it significantly.” He pointed out that the OLED is a planar, not a point source, and its production is very cheap and very high yield.



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