In this project, funded by the Macromolecular, Supramolecular and Nanochemistry program of the Chemistry Division, Prof. Tianquan Lian and a team of graduate and undergraduate students at Emory University investigate two emerging classes of nanomaterials. One type of material (an inorganic class of compounds called perovskites) consists of extremely small spherical particles (called quantum dots), where the size of the particle determines how it absorbs and emits light. The other material is a very thin plate, which can be viewed as a structure with only two dimensions (called nanoplatelets), made from various semiconductor compounds. The flow of energy in these materials, in discrete amounts (excitons) can be followed and controlled. These new materials have potential applications in solar energy conversion and nanoscale lasers. Developing efficient and cost-effective solar energy conversion materials is one of the most important scientific challenges of the 21st century. The proposed work will lead to a fundamental understanding of the properties of these materials, facilitating their further development and many potential applications.
Bulk perovskite materials have achieved impressive performance in multiple applications, including highly efficient solar cells (with certified power conversion efficiencies of > 20%) and low threshold lasing. Colloidal perovskite quantum dots offer the possibility to further tune, improve and optimize the properties of these materials through the quantum confinement effect. Colloidal two-dimensional cadmium chalcogenide nanoplatelets, are an emerging novel class of semiconductor quantum well materials with atomically precise thickness. The uniform energy over lateral dimensions of 10s to 100 nm in nanoplateletes suggests the possibility of novel exciton properties, such as the giant oscillator strength transitions effect. These unique properties, differing significantly from zero-dimensional quantum dots and one-dimenisonal nanorods, suggest many novel applications in optoelectronic devices and in solar energy conversion. The proposed studies are aimed at a fundamental understanding of exciton and charge carrier dynamics in these novel materials, which is essential to their development, rational design and improvement. The specific aims of the research plan are: 1) To study how the electron and hole transfer rates from perovskite QDs to molecular acceptors depend on the QD size and anion identity, testing theoretical models for charge transfer from QDs; 2) To examine how the interfacial electron transfer rates from perovskite QDs to oxide nanocrystalline thin films depend on the QD (size and anion) and oxide (TiO2, SnO2 and ZnO), testing current theoretical models of interfacial ET at QD/oxide interfaces; 3) To measure 2D exciton Bohr's radius and exciton center of mass coherent delocalization length of CdX(X=S, Se, Te) NPLs as a function of layer thickness, materials, solvent dielectric constant and temperature, testing the applicability of 2D hydrogenic model for describing excitons in NPLs and the extent of the giant oscillator strength transition effect; 4) To directly image the spatial distribution of excitons in CdSe core-only, type I CdSe/CdS core/crown and type II CdTe/CdSe core/crown NPLs using electron acceptor (TiO2) modified AFM tips, examining how band alignment in NPL heterostructures affect exciton spatial distribution. In addition to the training of graduate students and undergraduate students, the proposed work will lead to the development of new course materials on clean energy at the undergraduate level and broadened participation of under-represented minority students.
