Equilibrium thermodynamics of semiconductor nanocrystal ligand and ion exchange via calorimetry

University Of South Carolina At Columbia, Columbia SC

Investigators

Abstract

Equilibrium thermodynamics of semiconductor nanocrystal ligand and ion exchange via calorimetry This grant supports Dr. Andrew Greytak of the University of South Carolina in the effort to develop an improved understanding of colloidal quantum dot chemistry through the use of isothermal titration calorimetry (ITC). Nanocrystal quantum dots (QDs) are soluble, nanometer-scale particles composed of semiconductor materials. QDs can have bright and size-tunable fluorescence, and have current applications in flat-panel displays and in bio-imaging. QDs are also of interest for solution-processable solar cells that can be scaled sustainably to meet worldwide energy needs. However, the performance of QD solar cells remains far below the theoretical limit. There is considerable variability in fluorescence performance among QD samples of similar size and composition, due to structural differences at the QD surface. It is important to connect emerging analytical methods and theoretical models to the design of stable QDs with desirable surface properties. This project takes a new approach to developing a fundamental understanding of QD structure and chemistry by measuring the heat that is emitted or absorbed when chemical transformations of QDs take place. The ITC approach is expected to be broadly applicable to semiconductor nanocrystal science. The technique is being used here to advance knowledge that could lead to improved QD solar cell performance. Dr. Greytak's research program at the University of South Carolina engages students at the high school, undergraduate, and graduate levels. He has demonstrated nationally-recognized leadership in chemistry education. He also serves as the head judge for chemistry at the South Carolina Midlands Regional Science Fair for high school students. Colloidal nanocrystals (NCs) are complex assemblies of a crystalline core and an interfacial layer that, given time, may exchange matter with the solution and other NCs. It is highly advantageous to have a set of metrics that allows different NC samples to be profiled, both so that they can be characterized with sufficient precision to predict synthetic and physical behaviors. It is also adventitious to have a sufficiently rich experimental description of the system to constrain models that begin to capture the complexity of NC-ligand and ligand-ligand interactions. In this program, Dr. Greytak is being supported to measure equilibrium constants, enthalpies, equivalencies, and interaction terms associated with representative ligand exchange and ion exchange reactions at NC quantum dot (QD) surfaces in anhydrous solvents. Purification techniques such as gel permeation chromatography are used to provide QDs in a well-defined initial state. The knowledge acquired from these types of measurements represents a fundamental advance in QD chemistry that hastens the discovery of higher-performing and more sophisticated QD assemblies and solution structures. In particular, the performance and material scope of QD-based photovoltaics can be expanded through improved understanding of ligand exchange and ion exchange reactions. This project also provides research training opportunities to graduate and undergraduate students, who are building expertise in nanocrystal preparative chemistry, calorimetry, and other analytical techniques that are necessary to advance the field of nanoscience into practical technologies. Dr. Greytak has been active in broadening research participation through the Scientific Advocate Network and the American Chemical Society's Project SEED program. He has also initiated an annual USC Sustainability Research and Practice Showcase that creates a focal point for research groups, graduate and undergraduate students, and staff to educate each other about opportunities for research participation, entrepreneurship, integrative learning, and outreach that advances regional and national competitiveness.