Volume 3, Issue 20 April/May 2006
Nanowired
Peidong Yang is an associate professor of chemistry. (courtesy Berkeley Lab)
The future of tomorrow's electronic devices, from microchips to biosensors to solar cells, may be a tiny wire that's 4,000 times thinner than a human hair. UC Berkeley chemist Peidong Yang has pioneered methods to grow these nanowires from the bottom-up. At such a small scale, unusual physical properties emerge that could lead to solar cell paint, labs-on-a-chip that analyze single cells to detect disease, and new computer processors thousands of times faster than today's speediest PCs.
"We're attacking three fundamental issues," Yang says. "Can we make these building blocks of nanodevices? Can we identify and harness useful physical properties in them? And can we integrate them in parallel? Individual devices are fundamentally interesting. But more importantly, we need massive numbers of them to work together as one system."
Cross-sectional scanning electron micrograph image of vertically-grown silicon nanowires off of a silicon substrate. (courtesy the researchers)
Yang and his colleagues have had a string of successes on all fronts. Just this month, the researchers reported that they had designed a brand new kind of nanowire transistor. Transistors are the basic building block of computer circuits. The more transistors that can be packed on a silicon chip, the faster that chip's processing power. Transistors usually are planar–they're fabricated horizontally onto the surface of a chip. While transistors made from nanowires are not new, Yang's innovation is to change the design to three-dimensional, dramatically increasing how densely they can be packed into the same area. The device sprouts vertically from the surface. The other components of the transistor, responsible for controlling the flow of the electricity, surround the vertical wire.
"The transistor occupies just the space taken up by the footprint of the wire," says Yang, who is also a chemist with Lawrence Berkeley National Laboratory's Materials Sciences Division.
Already, the researchers have demonstrated that their first-generation Vertically Integrated Nanowire Field Effect Transistors (VINFETs) are comparable in performance to other nanowire FETs. And with just a 5 nanometer footprint, they're far tinier than the transistors that traditional semiconductor fabrication techniques can crank out. The next step, Yang says, is to stack the devices so that each wire can function as multiple transistors or devices, further increasing performance.
Cross-sectional scanning electron micrograph image of a VINFET device. The false color is added just for clarity. (courtesy the researchers)
Last year, Yang devised another very different kind of transistor. Traditional transistors are essentially valves that control the flow of electricity to perform calculations. But what if, instead of voltages, a transistor could manipulate the flow of biological molecules like proteins and DNA? Yang and Arun Majumdar, professor of mechanical engineering, developed the world's first nanofluidic transistor. Fabricated from hollow nanotubes or glass tubes, this nanofluidic transistor might someday detect cancer in a drop of blood much smaller than the period at the end of this sentence.
The researchers demonstrated that minute voltages could control the flow of ions through the nanoscale plumbing system. In the future, the same technique might be used to shuttle proteins or pieces of DNA from a biological sample through the tubes in a lab-on-a-chip. Yang is currently developing a technique to conduct optical sensing within the nanofluidic channels so that the whole lab is self-contained in one device. Several years ago, Yang's group built the world's smallest ultraviolet laser. Now, he's researching the use of nanowires as sub-wavelength optical waveguides, channels that can steer laser light to the sample for analysis. The light would then bounce off the biological molecules trapped in the microfluidic channel.
A prototype nanofluidic transistor. (courtesy the researchers)
"By detecting how a molecule inside the microfluidic channel interacts with the light carried by sub-wavelength nanowire waveguides, we can determine its chemical signature and identify it," Yang says.
The VINFET and nanofluidic transistor are only two of the myriad devices enabled by Yang's nanowires. He and his colleagues are exploring how the nanowires may be used in longer-lasting batteries and ultra-sensitive detectors for poison in drinking water. Recently, he's demonstrated that sunlight-absorbing nanowires can harvest solar energy.
The nanowire wave guide has been demonstrated guiding red, green, and blue light. (courtesy the researchers)
"To me, the most important thing is that we're not just making nanoscale devices that are simply scaled down versions of traditional devices," Yang says. " All of these devices have new physics or chemistry inside of them that will enhance their performance."