Creating Functional Nanocrystal-Molecule Interfaces for Spin-triplet Energy Transfer

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The Macromolecular, Supramolecular, and Nanochemistry Program in the Chemistry Division supports Professor Sean T. Roberts at the University of Texas at Austin to study light-triggered processes that occur in materials. Light provides the energy that fuels plant growth via photosynthesis, is used in electronics for wireless communication, and even forms the basis for noninvasive medical treatments, such as photodynamic therapy used to treat cancer. However, light transmits its energy in a peculiar manner, via particles known as photons. Different colors of light contain photons with different amounts of energy, and it is important that photon energies be matched to their intended applications. Photons with too little energy are unable to meet their intended task while photons with too much energy waste energy and can cause damage to components. This research project develops materials that reshape light’s energy content by combining pairs of photons into a single high-energy one or converting individual photons into multiple lower-energy photons. Once formed, these materials can serve as energy transfer agents improving solar energy harvesting and quantum computation. In addition to working with students closely on the research projects, Professor Roberts also founded and participates in the GReen Energy At Texas (GREAT) program, which was formed between the University of Texas at Austin and Austin Community College. GREAT aims to increase the number of community college student that earn degrees through targeted mentorship and research experiences.Semiconductor quantum dots chemically functionalized with organic dye molecules offer an ideal interface for photon conversion. Quantum dots energized by light absorption can pass their energy to molecules at their surface, promoting them into spin-triplet exciton states. If formed in high quantity, pairs of spin-triplet excitons can undergo triplet fusion, a process that combines their energy to form a single high-energy spin-singlet state that can radiate light. Likewise, photoexciting the dye molecules can fuel triplet fusion’s inverse process, singlet fission, which creates pairs of triplet excitons that can each pass to the quantum dot to drive infrared light emission. These schemes require a key step, the transfer of a spin-triplet exciton across the quantum dot:molecule interface, yet an understanding of how the structure of these interfaces impacts the rate and efficiency of spin-triplet exciton transfer is elusive. To address this need, Professor Roberts at the University of Texas at Austin directs a team that employs controlled chemical synthesis, 2D NMR and femtosecond time-resolved spectroscopies, and electronic structure calculations. The research team investigates how the chemical structure of quantum dot:molecule interfaces formed by two complementary quantum dot materials of PbS and Si impact spin-triplet exciton transfer. This project contributes to the creation of new hybrid organic:inorganic junctions for light-driven production of energy and fuels as well as materials that use spin entanglement for quantum computation and information storage.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.