Searching the Web for "zinc oxide nanostructures" produces thousands of hits -- most of them associated with the name of one scientist: Zhong Lin Wang, a Regent's professor at the Georgia Institute of Technology. Wang, a physicist who joined Georgia Tech in 1995, has almost single handedly launched a new field of research that takes advantage of the unique properties of zinc oxide nanostructures.
Wang is perhaps best known for nanogenerators that harvest mechanical energy from the environment, taking advantage of the piezoelectric properties of zinc oxide nanowires to produce electrical current. Starting with output that could barely be measured in 2006, his research team has steadily improved the devices until today arrays of connected nanogenerators can produce as much as 30 volts.
More recently, he has used the piezoelectric properties of the nanostructures to control charge transport in electronic devices, a technology known as piezotronics, which provides an alternative to traditional CMOS technology. He has also coined the term "piezo-phototronics" to describe techniques for controlling electro-optical processes in devices such as light-emitting diodes.
By leaving a gap in the PowerPoint slide he uses to describe his family of zinc oxide nanostructures, Wang suggests there's more ahead.
Wang received a 2011 Materials Research Society Medal Nov. 30th at the organization's fall meeting in Boston. The medal's commendation notes his "seminal contributions in the discovery, controlled synthesis, and fundamental understanding of zinc oxide nanowires and nanobelts, and the design and fabrication of novel, nanowire-based nanosensors, piezotronic devices and nanogenerators for energy harvesting."
Wang joined Georgia Tech in 1995, after earning his Ph.D. at Arizona State University and working for Oak Ridge National Laboratory and the National Institute of Standards and Technology (NIST). His first interest was electron microscopy, where he helped other Georgia Tech researchers see the world of the very small.
He got his first major international attention from research on carbon nanotubes -- tiny structures of interlocked carbon atoms that helped create a new research area in the mid-1990s. Collaboration with Georgia Tech physicist Walt de Heer -- now known for his work with epitaxial graphene -- produced a 1996 paper on nanotube properties that included a microscope image of a carbon particle on the end of a nanotube. Analyzing the nanotube's vibration allowed the researchers to determine the approximate mass of the particle, and their device become known worldwide as a "nanobalance."
But Wang soon realized that the popularity of carbon nanotubes made that research field a crowded one. In search of a research area with more opportunity, he returned to his undergraduate roots in oxide materials -- zinc oxide in particular.
"Zinc oxide has a lot of advantages because of its semiconducting, piezoelectric, optical and other properties for sensors, transducers, energy applications and other uses," he noted. "I wanted to have a material that I could develop, to study the material in great detail, and to own the key intellectual property."
He began work on the material in 1999 and quickly produced significant results, including the development of "nanobelts" reported in the journal Science in 2001. Details of the structures and their synthesis attracted other researchers, and the paper has now been cited more than 3,500 times.
The nanobelt paper was followed by years of work investigating the properties and synthesis of zinc oxide structures. Perhaps the most significant advance was the ability to grow aligned arrays of zinc oxide nanowires, knowledge that led directly to the development of nanogenerators. Reported in April 2006 in the journal Science, nanogenerators drew international attention to Georgia Tech and rapidly led to a series of improvements that opened up new ways of powering nanometer-scale devices for building self-powered nanotechnology. The generators now produce enough power to operate conventional electronic components such as LED displays.
In the last few years, Wang has branched out into new forms of electronic devices, including piezotronic logic gates and memory, as well as light-emitting diodes enhanced with the piezo-phototronic effect. Multiple devices have been combined into self-powered sensing systems that not only detect harmful materials, but also alert authorities wirelessly. He has also built systems that combine different kinds of power harvesting, such as nanogenerators and photovoltaic cells -- and developed a hybrid cell for the first time.
"When nobody else is paying attention to a technology area, that is when you can be a pioneer," he said. "By the time most people have begun to pay attention to it, you have already made significant progress."
In his 16-year career at Georgia Tech, Wang has produced 28 patent applications, along with another dozen invention disclosures. He has formed a startup company to commercialize the technology, and published more than 20 articles in Science and high-profile Nature journals. Overall, he estimates his research team has produced more than 700 publications that have been cited 45,000 times with an h index of 103.
"When we began this work, we could see only dots -- no picture," he added. "Today we are able to see the big picture of what can be done with these nanostructures."
Though the MRS Medal recognizes Wang's research accomplishments, he's also proud of his role as teacher and mentor to students -- hundreds of them since 1995. Wang leads a large research group composed of post-doctoral fellows, graduate students and undergraduate students. From his laboratory have come seven graduates who now hold assistant professor positions at prestigious U.S. institutions -- and more than 50 working at universities in China or Taiwan.
"As a professor, I view my job as having two parts: being an outstanding scientist and an outstanding educator," he said. "Our most important products are the students. If they can go on to be faculty members at the most prestigious universities in the world, we have done our job."
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Writer: John Toon