Organizing quantum dots

by Alexandra George

Michael Bockstaller receives DOE-BES grant to study the parameters that cause nanoparticles, like quantum dots, to assemble into organized structures when polymer chains are attached to the particle’s surface.

Image display quality and technology is rapidly advancing. You can watch movies at home with the same or even better quality than at the movie theater. While the difference from even a few years ago is big, one reason for the improvement is very tiny: nanoparticles.

Illustration of red and green dots separating and organizing — **Source:** Michael Bockstaller

Illustration of phase separation that drives particles to autonomously organize into “color-pure domains.”

Many electronic devices are beginning to use quantum dots, or nanoparticles from a certain type of semiconductor, to make displays brighter. The quantum dots are placed in a random mixture on a film on the back of the display where they convert blue LED light into other colors. But not all of the absorbed light is converted in this way; some of it is converted into and absorbed as heat by neighboring dots of a different color. On big displays, the film heats up and eventually breaks down the device.

Materials Science and Engineering (MSE) Professor Michael Bockstaller and his team are working on ways that these quantum dots can self-assemble into organized patterns. When the dots are surrounded by particles of the same color, it minimizes heat absorption and increases efficiency. The team has already proved that they are able to control these nanoparticles by attaching a polymer chain to them. Now, they plan to study exactly what about the polymer causes the particles to move.

“If you take a set of marbles that have different colors and mix them together, they will randomly mix. There is nothing going on that will tell white marbles to go on one side and black marbles to go the other side,” said Bockstaller. “Similarly, the nanoparticles don't have any driving force to separate themselves in two. We proved that polymer chains attached to the surface of these nanoparticles can drive this separation and self-assembly process.”

There’s motivation to help further develop devices that currently are being limited by physical barriers.
Michael Bockstaller, Professor, Materials Science and Engineering, Carnegie Mellon University

Bockstaller recently received a Department of Energy Basic Energy Sciences (DOE-BES) grant to take this work to the next step and determine what parameters enable the polymer chains to drive the nanoparticles to self-assemble in the most efficient way. The grant is worth $1.1 million over three years, with Carnegie Mellon receiving $750,000, and their collaborators at the University of Houston receiving $350,000.

For their method to work for commercial applications, they need to use the least amount of polymer to drive nanoparticle movement. This means they need to understand the conditions under which polymers interact strongly with each other and what conditions favor or disfavor interactions. The grant will enable them to study these questions by providing resources, including access to national laboratories with neutron scattering capabilities.

Microscope slides — **Source:** Michael Bockstaller

A close-up view of nanoparticle phase separation.

“To really learn about what goes on with the polymer chains, we need to make these particles invisible in some magic way,” said Bockstaller. “It turns out that in neutrons, this magic does exist. Neutrons don’t bounce off of electrons in atoms; they bounce off the nucleus. So you can create situations where even though there are many particles next to the polymer chains, only the polymer chains will deflect neutrons.”