By Mitch Zimmer
The worldwide demand for ever smaller semiconductor materials, the very stuff that makes up the backbone of transistors, has made it possible to pack the computing power of a decades old desktop computer into a modern smartphone. The continued success of these innovations into high-tech applications has now led to the worldwide effort to produce semiconductors at the nanoscale level. Such small semiconductors – one nanometer is about one 50,000th of the width of a strand of hair – can practically be built molecule by molecule. The contributions of John Corrigan in the Chemistry Department to exploring ways to develop this area of science has earned him this year’s Florence Bucke Prize. “Our focus is on establishing new reagents or new routes into the controlled assemblies of these nanometer sized architectures,” says Corrigan. The applications for these materials now extend beyond transistors to sensors and energy conversion.
When electrical current passes through household semiconductor devices, as found in computer chips, there is enough space within the framework of the solid itself for electrons to move relatively freely. The mechanism is different for the nanoscale where the energy, the wavelength and the spabe where electrons travel have influence. Spaces are so small within the structure that quantum mechanics play a factor in that the wavelength of photons generated by the electrons are determined by the physical distance within the molecule, even slight differences in the size and shape of the semiconductor can change its electronic properties. “We usually look at two or three types of elements bound to each other to make a semiconductor material. When you make them at the nanometer scale then you have the so-called size quantization effects.”
The process of building these materials starts by controlling assemblies made up of several dozen components in a piece of semiconductor “then we go to several hundred or sometimes several thousand of these atoms in a semiconductor core and that’s the number you look at when you’re looking at particles that are one to a few nanometers in size, “says Corrigan. The size of these quantized particles “affects the energies of those systems and, importantly as a chemist, you try to make sure every particle is the same.... We prepare them with new reagents or with a new property in a controlled way.” If these particles end up having a collection of different sizes and shapes, “then you lose those special electronic features.”
Once the finished particles are consistent Corrigan says that there is another procedure that has to be completed. “Part of our program is making these materials in such a way in that the individual particles assemble into a single crystal...that allows you to do a precise characterization so you have a complete three dimensional image of this nanometer sized piece of your material, that’s really what we focus on for a lot of our projects”
It turns out that the electronic structure of these nanoscale semiconductors governs their optical properties. Depending on the size and make up of the material, different characteristics can be displayed. These materials are very luminescent and can be induced to emit light at an energy that is related to the size of the particle. Other formulations can result in a photon of a different colour being emitted which may be used in producing quantum dots in the fluorescent imaging of biological systems. Their small size also results in the efficient absorbance properties; they can uptake light to generate electrons for applications in photovoltaic solar cells.