My work is focused on the synthesis, purification, processing, thermodynamic study, and post-synthetic modification of colloidal semiconductor nanocrystals. Colloidal semiconductor nanocrystals, which are also known as quantum dots, are nanomaterials which are generally few nanometers in size and are capped with organic ligands for better colloidal stability. These materials have been identified as a prospective candidate for future optoelectronic devices. For example, quantum dots composed of lead halide perovskites (e.g. CsPbX3) have gained considerable attention in the past decade due to their excellent optical and electronic properties. These properties motivate their potential applications in various electronic devices such as solar cells, light emitting diodes, scintillators, and photo/radiation detectors. However, despite recent developments, the problems of purification, long term instability, ion migration and lead toxicity remain major obstacles toward their commercialization. The ionic character of these quantum dots makes them very sensitive to polar solvents that are typically used for purification of conventional QDs.
During my PhD, I investigated the use of gel permeation chromatography as an alternative route for effective purification of CsPbBr3 quantum dots without exposure to polar solvents allowing us to achieve effectively purified nanocrystals with high photoluminescence quantum yield (high brightness). High brightness is desired for materials that are required for use in light emitting diodes. By exploiting the organic ligands on the surface of these nanocrystals, their physical and/or chemical properties such as stability can be improved. Hence, there has been ardent search for ligand exchange reactions that could stabilize these particles. Sometimes, contradictory results of these ligand exchange reactions have been reported. For example, dimethyldidodecyl ammonium bromide is one of the most widely studied ligands in efforts to stabilize CsPbBr3 quantum dots through surface modification. While some researchers have reported improved quantum yield (brightness), optoelectronic performance and stability through such ligand exchange, others have reported it to cause a phase transformation.
During my PhD journey, I explored the use of isothermal titration calorimetry to resolve literature conflicts on the impact of this ligand on the surface of halide perovskites nanocrystals. I studied the thermodynamics of reactions occurring when dimethyldidodecyl ammonium bromide ligand is introduced to these nanocrystals. Using isothermal titration calorimetry, I was able to resolve the different reactions occurring on the surface of these nanocrystals with their respective enthalpies, entropies, and free energy values. In order to further establish the viability of isothermal titration calorimetry as a promising technique to understanding the complexity of NC-surface ligand interactions, we, the Andrew Greytak group at USC, recently published a featured article in Chemical Communication detailing isothermal titration calorimetry as a viable technique for studying the thermodynamics of the ligand association and exchange at nanocrystals surfaces. Therein, we discussed the principles of operation of isothermal titration calorimetry and its promise in resolving the complexity of quantum dot surface reactions. We also provided an in-depth description of various thermodynamic models that can be used to interpret nanocrystal–ligand interactions as measured not only by isothermal titration calorimetry, but also by nuclear magnetic resonance, fluorescence quenching, and fluorescence anisotropy techniques.
It’s a tradition in our lab for the graduating doctor to open a bottle of champagne and see how far the cork can go. After that, the drink will be shared among the group members and the empty bottle will be signed by the new doctor and it is then kept in our advisor’s office as a souvenir. My advisor, Andrew B. Greytak has a bottle of champagne representing each member that has graduated in our lab. This is us during the opening of the champagne bottle.
In the last year of my PhD, I probed the fundamental understanding of the process and mechanism of anion exchange in perovskite nanocrystal solutions. This ion migration and exchange can be used to tune electronic properties such as band gap energy in order to adapt them to various electronic devices. In the same vein, this process (ion migration) can lead to phase separation in mixed halide perovskites-based electronics resulting in their instability. Our recent fundamental study of the process of this anion exchange will help in providing better-informed guidelines towards engineering perovskite quantum dots for different applications such as tandem cells and efficient devices for lighting and display technology.
Over the last four years of my PhD program, my research has varied widely from inorganic synthesis and thermodynamic studies to electronic applications and energy transfer studies in solid state materials. I have won several presentation awards at various conferences including ACS 65th anniversary of Division of Inorganic (DIC) Chemistry, NanoGe fall 2021 and NC Photochem to mention a few. Going forward, I will be pursuing a career as a process engineer at Intel.