Two years ago that critical gap in the technology of nanotechnology may
have been solved when Georgia Tech’s Zhong Lin Wang published an
article in Science describing the creation of piezoelectric
nanogenerators that offered the potential of converting mechanical,
vibrational, or hydraulic energy from the environment into electricity for
powering nanodevices. Wang’s paper (Z. L. Wang, J.H. Song,
Science, 312: 242-6, 2006), became a fixture in the
Chemistry Top Ten, racking up well over 200 citations in just two and a
half years and continuing Wang’s remarkable run of influential
research in the field of nanotechnology.
Wang, 47, obtained his bachelor’s of science degree in 1982 at the
Northwest Telecommunication Engineering Institute (now Xidian University)
in Xi’an, China in 1982. He then came to the U.S. and received his
doctorate in physics from Arizona State University in 1987. For the next
eight years, Wang lived a peripatetic research existence, working at SUNY
Stony Brook, at the famous Cavendish Lab at the University of Cambridge, at
Oak Ridge National Laboratory, and the National Institute of Standards and
Technology. In 1995, Wang joined Georgia Tech, where he’s now a
distinguished professor in the College of Engineering.
It wasn’t until the late 1990s that
you started working on nanotechnology. What were you working on until
then, and what prompted the switch?
After I graduated from Arizona State in 1987, I worked for a long time
on the fundamental physics of electron-solid interactions, mostly on
elastic and inelastic scattering theory. I was also involved in
surface-image analysis of reflected electrons, a topic on which I wrote
a book in 1996. I also worked for a long time on high-temperature
superconductors. I decided to switch to nanotechnology because I
realized the limitation of the theory I was working on—that what
we'd developed was not going to be particularly high-impact, and I
needed to find a new area that could fully utilize all my strengths and
where I might have a significant impact. That got me full-time into
nanotechnology. Because I had worked as a student with transmission
electron microscopy, always looking at atoms and surfaces at very small
scales, turning to nanotechnology was a natural choice.
How did you first approach the field? What did
you work on first?
When I first started in 1995, people were just beginning to work on the
self-assembly of nanoparticles—metallic particles of gold,
silver, etc., so I worked on that. I worked on shape control of
self-assembled nanoparticles and on carbon nanotubes, and I developed a
method called in situ nano-measurement, which means that I put
these nanotubes on a specimen holder inside a transmission electron
microscope and could then directly image the nanotube itself while
measuring its various physical properties. A lot of researchers doing
measurements at the time couldn’t actually see what they were
measuring, and so could only explain it in terms of the assumptions of
their models. With my technique I could directly determine the
structure and directly measure the physical properties. There was a
one-to-one correspondence between structure and property. This work was
published in Science and has since evolved into an entire
field of research—in situ nano-mechanics. My major
effort on that was between 1997 and 2000, although we’re still
doing it in my lab, but it's now only a small part of our efforts.
What happened in 2000 to change the direction
of your research so radically?
I'd been working on carbon nanotubes for three or four years when I
started wondering how much they’d ever be used for electronics,
considering the difficulty people have controlling the chirality of
nanotubes. Nanotubes can be metallic or they can be semiconductors, and
this can depend on the chirality, and usually it would be random as to
which way they go. Even today people have difficulty with that. So I
thought, why not start using oxides? Ultimately, most of my work
focused on zinc oxide.
What is it about oxides and particularly zinc
oxide that makes it such an ideal compound for these nanoscale
structures and devices?
Highly Cited Papers by
Zhong Lin Wang and
Published Since 1996
(Ranked by total
Z.W. Pan, Z.R. Dai, Z.L. Wang,
"Nanobelts of semiconducting
291(5510): 1947-9, 2001.
S. Frank, et al., "Carbon
nanotube quantum resistors,"
T.S. Ahmadi, et al.,
"Shape-controlled synthesis of
colloidal platinum nanoparticles,"
P. Poncharal, et al.,
"Electrostatic deflections and
electromechanical resonances of
Whetten, et al.,
"Nanocrystal gold molecules,"
Adv. Materials, 8(5):
Oxides have very interesting properties. First, they have
well-controlled structures. An oxide is a semiconductor, it’s
piezoelectric—meaning that it has the capacity to convert a
mechanical signal to an electric signal and vice versa—and it has
a well-controlled morphology, orientation, and structure. So that makes
a lot of applications really easy to do. Zinc oxide, which we’ve
focused on now for eight years, has some particularly useful
properties: it’s an optically transparent, wide band gap
semiconductor. A second advantage is that it's semiconducting and
piezoelectric, so you can use it to transmit energy. And the third
advantage is that you can make nanostructures from it at temperatures
as low as 50° to 80° Celsius. You can grow this material in
chemical beakers on any shape of substrate. And it's biologically
compatible—it’s bio-safe. There are no environmental side
effects. It’s a green material. You put all this together, and
it's got all the properties you need to do a lot of very, very creative
If they're so useful, why wasn’t anyone
else working on oxides?
Everyone was working on carbon nanotubes. You have to have the courage
to shift your focus to something completely new. We left carbon
nanotubes completely when we started working on this. In retrospect, we
obviously made the right decision, but there was no way to know it for
sure at the time.
How did you come on the idea for
nanogenerators and self-powered nanodevices?
We were working on oxide nanobelts, on nanowire growth, trying to
understand the fundamental science. Then, in 2004, I was thinking about
what’s missing in all this. We grow all kinds of
nanostructures—everybody's making nanomaterials, nanodevices,
these extremely small devices, but how do we power them? Where’s
the extremely small power source for these devices? Maybe we should
figure out how to build self-powered nanodevices. So how can we harvest
energy from the environment to power these devices? Maybe we should not
only make the device but provide the energy. Over the long term, the
device alone is not enough. What we need is self-powered
nanotechnology. That was my vision.
Okay, that’s the vision. How do you go
about making it a reality?
You start by looking at what can be converted into energy. What’s
the advantage of nanotechnology? Small size, small power consumption.
What sources of energy are available to us? There's solar energy, but
to convert solar energy you need a solar panel, and a lot of
applications are inside biological systems, or indoors, or other places
where solar isn’t an option. So how about mechanical energy? A
lot of things generate mechanical energy. When we talk about sonic
waves, those are mechanical energy. Walking. My foot step is mechanical
energy. A heart beat is mechanical energy. Muscle stretch is mechanical
energy. Air flow is mechanical energy. Traffic noise. Your air
conditioner blowing. Curtain movements. All are mechanical energy. Can
we convert this energy to electricity? That was my idea. So the first
question was: is this possible? In 2005, I asked my students, if we use
an atomic force microscope, AFM, to bend a single nanowire, can we
convert the bending into electric power? We know zinc oxide has this
piezoelectric effect. So can we demonstrate this using an AFM and a
single zinc-oxide nanowire?
It worked, obviously.
That’s right. We published this in Science in 2006
(312: 242-6, 2006). We used an AFM to push a nanowire and got a
voltage output of 3 to 12 millivolts from a single wire. That’s
not a lot, but then how big is this wire? Thirty nanometers in
diameter, 2 microns in length. This is only the first step. The next
step toward making this useful was to make millions, billions of
nanowires convert energy. And then we wanted to get rid of the AFM in
order to apply this technology in vivo. We wanted to use
ultrasonic waves or any indirect way to provide the mechanical energy.
A year of effort solved those problems, and we published our next big
paper in 2007 in Science. [Note: as mentioned above, currently
#3 in the Chemistry Top Ten.] We used a zigzag electrode to replace the
AFM tip; we now have multiple v-shaped electrodes, making a zigzag
shape. And that makes hundreds and thousands of nanowires generate
electricity, so we’ve immediately established that this
technology can be scaled up.
Can you describe for us the state of the
technology, circa 2008?
We followed that work with an ultrasound wave-driven nanogenerator. We
used fabric, and we grow these nanowires on fibers, textile fibers, and
when one fiber brushes against another, we convert body-movement energy
to electricity. That's targeted at a lower frequency, a couple of
hertz. We got tremendous publicity for this work, which we published in
Nature (Y. Qin, et al., Nature, 451(7180):
What has to be done to translate this to
applications and a working technology?
What we have to do now is raise the voltage that we can generate with
this nanogenerator. If we can reach fractional volts, like half a volt,
it can become really useful. We also want to integrate them into
three-dimensional nanogenerators: one layer is not enough. So how about
10 or 20? That way we can raise the voltage and current. We can also
integrate this into biological systems—try to use muscles to
generate electricity, to use blood flow, sonic waves, noises, wind.
How close are you now to the half a volt
We started at 10 millivolts, .001 volts. We’ve increased it by a
factor of 100, so we’re now at about .1 volts. Our goal is .3 or
.4 volts. If we can get there we can use it for biologic sensors, such
as cancer-detection sensors, blood-sugar measurement sensors, and other
kinds of chemical sensors.
You’ve mentioned the piezoelectric
properties of oxides several times, and what you call
"piezotronics" is a major focus of your research. What is
piezotronics and what role does it play or will it play in
This is another field I’ve now created. In my work on bending
nanowires with an AFM, one side of the wire is
stretched—tensile—and this tensile surface has a positive
piezoelectric potential. The other side is compressed and has a
negative piezoelectric potential, so across the wire there's a
potential drop. This can serve as a gate voltage to gate current
through a transistor. So now I can build piezoelectric field-effect
transistors, a piezoelectric diode. Instead of using a classic
semiconductor p-n junction to make a diode, we can now use a
piezoelectric junction. These devices use force or pressure to trigger
a transistor or a diode. Now you have a transistor or a diode that is
very sensitive to mechanical force deformation, to strain. It’s a
new electronic component. What do you use it for? You use the coupled
piezoelectric and semiconductor properties. Traditionally these are two
different fields. Semiconductors and piezeoelectric devices are two
different things. I’ve now combined them into one device, using
zinc-oxide nanowires to achieve this objective. We can now make these
devices to measure strains—strains in muscles, in buildings, in
structures. I call it piezotronics.
In five years' time, what real applications
would you expect to see emerging from your research?
In five years I want to be able to provide a nanogenerator that can
harness energy and power tiny devices—little units that can
operate themselves, harvest energy from the environment, drive
themselves, and operate wirelessly, remotely, sustainably. Devices you
can use for whatever applications you desire. That should be possible.
I also hope, in five years, to furnish little biological sensors that
will harvest energy from muscle movement and power themselves. In five
years, I should be able to provide tiny transducers, sensors, based on
piezotronics that can measure pressure and force. Actually, I can
probably do that in two years. In the long run, I want to make
piezotronics truly into a field of its own. I want to make
nanogenerators fully useful for a lot of people and a lot of
Related Information: read a
Fast Breaking Paper, and two Fast
Moving Front commentaries from Zhong Lin Wang
Keywords: Zhong Lin Wang, Georgia Institute of Technology, Georgia
Tech, nanotechnology, nanowires, piezotronics, nanogenerator,