An avid amateur astronomer during her childhood in Vukovar, Croatia, Silvija Gradečak, associate professor in materials science and engineering, was not content observing the physical world only from a distance: “I discovered what I really liked about science were experiments, and having the ability to make something with my hands,” she says.
Today, handling the smallest elements in nature, Gradečak is generating large-scale results that may transform energy production, storage, and lighting. Her enthusiasm for both basic and applied research will help to power MIT.nano, the Institute’s $350 million nanoscale laboratory now under construction. Gradečak looks forward to working “with people from different backgrounds, advanced nanofabrication tools, and the seamless integration of the technologies needed to work on these problems.”
At the Swiss Federal Institute of Technology, where Gradečak pursued her doctorate, an electron microscope revealed a new terrain ripe for exploration and manipulation. “I saw individual atoms for the first time, and came to realize that having the ability to arrange them on the nanoscale is a powerful tool,” she says. “There were so many new problems available to work on. All kinds of possibilities emerge when you have the capability to develop materials with unique structure and properties not found in nature.”
Teasing out these properties becomes possible when examining materials at the nanoscale (a nanometer is one-billionth of a meter, and nanoscale materials run one to 100 nanometers in size). During graduate school, Gradečak zeroed in on gallium nitride, GaN, a synthetic compound used by the semiconductor industry that turned out to feature some extraordinary optical properties: If the composition of GaN is altered at the nanoscale, the compound can produce light ranging from the ultraviolet to the infrared.
As a young researcher investigating nanoscale defects in GaN that changed the compound’s behavior, Gradečak “opened up a new world,” she says. “We all have to find a niche, our passion, and learning that I could design materials, tune their properties and emissions — this ability was amazing to me.”
Gradečak was especially fascinated by the wealth of potential optical and electrical applications for these nanoscale materials. GaN and similar semiconducting compounds are capable not just of emitting light at a range of wavelengths, but of conducting electricity and heat more efficiently, too.
Gradečak set about harnessing the power of nanoscale compounds. She developed a unique repertoire of laboratory methods that involve manipulating compounds in their vapor phase in a growth chamber. Inside, atoms take root on substrates in particular configurations based on Gradečak’s desired outcomes.
In one venture, Gradečak created nanowires, slender, solid fibers composed of nanoscale semiconductor materials that can be grown on varied surfaces such as silicon or flexible polymers. Of infinitesimal diameter, these nanowires are essentially one-dimensional objects, and because they can be millions of times longer than they are wide, they are ideally suited for transmitting energy in the form of electricity, heat, and light.
One signature application to emerge from this nanowire research is a new and different kind of light-emitting diode (LED). Gradečak’s device more closely approximates sunlight’s red and green wavelengths than current LED technologies. In addition, instead of utilizing expensive materials such as sapphire as a growth medium, as is the typical practice of current manufacturers, Gradečak’s nanowire-based LEDs can be grown on abundant, inexpensive substrates, including flexible plastics. Her invention may prove much more economical for home and industry consumers.
Another key development from Gradečak’s lab is a solar cell made from zinc oxide nanowires embedded with tiny quantum dots — nanocrystals made from a semiconductor material that are so small they essentially have no dimension. While the device does not yet convert solar energy to electricity as efficiently as today’s silicon-based solar cells, Gradečak notes, “Our devices are transparent and flexible, and in just a few years, we’ve improved efficiency of our cell by two orders of magnitude; this is an amazing accomplishment.”