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Separating The Core And Shell Of CdSe Quantum Dots

LEDs, solar cells and sensors could benefit from a new technology to study the interface between the core and shell of quantum dots composed of CdTe based materials
The J. William Fulbright College of Arts & Sciences at the University of Arkansas, has received a $650,000 award from the National Science Foundation.

Colin Heyes, an assistant professor in the department of chemistry and biochemistry in the institute was instrumental in receiving the award.

The Faculty Early Career Development Program award was given to further his investigation of the interfaces between the core and shell of colloidal quantum dots.

Colloidal quantum dots are microscopic semiconductor crystals that are grown in solution.

Adding a shell to the core quantum dot provides a way to control the functionality of these crystals, which can be used to emit light for biomedical imaging, LEDs and spectroscopy or photocurrents for solar cells and chemical sensors.

The research will help scientists better understand the relationship between the structure of the quantum dot and its functionality.

“All of these modern applications rely on the same fundamental electronic processes within quantum dots," Heyes says. “Our work will provide a better understanding of how to control these crystals to eventually build brighter, faster, longer-lasting and more efficient products."

Heyes studies the interfacial chemistry between the core, shell and ligands of colloidal quantum dots. Ligands sit on the shell surface and “hold" the colloidal quantum dots in solution; they also provide a chemical connection to the “outside world" so that quantum dots can attach to biological cells, solar cells or act as chemical sensors.

There is a lack of fundamental understanding about the structural properties of the core-shell and shell-ligand interfaces.

Scientists can observe the boundary between the core and shell materials using powerful electron microscopes, but they do not yet understand how the nature of the structural mismatches between the two materials affects their optical and electrical properties.

These mismatches create “holes" or “trap states" that result in losing control of excitons, which are electrons that have been energetically excited. The inability to control excitons result in energy lost as heat rather than converted into useful energy, such as light or electrical currents.

The NSF grant will expand Heyes’ investigation of how the optical and electrical properties of quantum dots are related to the core-shell and shell-ligand interfaces at the single quantum dot level.

Understanding single quantum dots is necessary to advance miniaturised optoelectronics and single molecule fluorescence applications. His research team has produced preliminary data demonstrating that as these interfaces are systematically varied, the optical properties of single quantum dots can be tuned.

“We hypothesise that understanding the relationship between the structures of the core-shell or shell-ligand interfaces and the trap states will allow us to more precisely control these excitons that underlie the optical and electrical properties," Heyes says.

Heyes’ team will focus specifically on understanding how the trap states are formed and how they contribute to the optical and electronic properties with the eventual goal of avoiding their formation altogether.

The grant will support Heyes’ research in this area for the next five years and will encourage and promote the participation of graduate, undergraduate and minority students. As part of the grant, a two-week, hands-on workshop will be held each summer on the U of A campus.

Students from the university and from institutions in Arkansas and Oklahoma will perform research experiments in Heyes’ lab to promote and foster their interest in chemistry and nanomaterial science for eventual careers in the fields of science, technology, engineering and maths.

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