TRAPPED HOLE DIFFUSION IN NANOCRYSTALLINE SYSTEMS
Abstract
Carrier trapping plays a vital role in the performance of many semiconductors for photo(electro)chemistry, including water oxidation and photocatalysis in semiconductor nanocrystals (NCs). In nanocrystalline materials, such as CdSe and CdS, photoexcited holes trap rapidly and efficiently to the particle surfaces. Our currently AFOSR-funded work shows that these holes form small polarons, trapped to chalcogen atoms on the surface. Contrary to the conventional picture that trapped holes are static, we found that they are mobile. At room temperature, trapped holes undergo random walk diffusion through a sequence of incoherent hops along the particle surface. The diffusion coefficient is small, and the low hopping rates are a bottleneck for hole transfer (HT) to surface-adsorbed hole acceptors, including oxidation catalysts. Trapped hole motion has profound implications for oxidation chemistry in NCs and semiconductors more generally. For example, if the oxidation reaction is diffusion-limited, increasing the hole transfer rate to a catalyst will have only a modest impact on catalytic throughput because hole diffusion is rate-limiting. To achieve control of the hole-hopping rate, we must answer outstanding fundamental questions about the complex interface between a semiconducting NC and its solvent. In the next funding period, we propose to control the trapped-hole hopping rate in CdS and CdSe NCs, study hole-hopping in a variety of particle geometries, and connect trapped-hole diffusion to light-driven oxidation chemistry toward light-driven oxidation catalysis. The proposed work will combine experimental synthetic and spectroscopic work from the Dukovic group with theoretical and computational modeling led by co-PI Joel Eaves. The specific aims of this proposal are the following: 1. Use transient absorption (TA) spectroscopy to probe diffusion-limited recombination in CdS and CdSe non-uniform nanorods (NNRs) as a function of solvent, surface-capping ligand, and temperature. Microscopically detailed simulations and theory will deconvolve the impact of these parameters on the trap-to-trap electronic coupling and the coupling to critical nuclear motions that control the hole hopping rates—beyond the reorganization energy. 2. Design and implement surface modifications to enhance the hole hopping rates in NNRs. Measurements of TA spectra at different temperatures and theoretical analysis will allow us to extract hole hopping rates achieved with these surface modifications and understand their microscopic origins. 3. Using a combination of TA experiments and kinetic Monte Carlo simulations, explore the interplay between trapped-hole motion and HT in oxidation photochemistry of particles functionalized with hole acceptors. 4. Using a combination of experiments and simulations, probe trapped-hole diffusion dynamics for generic NC compositions, shapes, and effective dimensions. Significance: The work proposed here by a tight-knit collaborative team will elucidate some of the crucial aspects of hole dynamics in semiconductor nanoparticles. The knowledge gained will open a path toward light-driven oxidation catalysis, including water oxidation necessary for solar fuel generation. This work aligns strongly with the Molecular Dynamics and Theoretical Chemistry program as it fruitfully combines theory and experiment to advance fundamental knowledge and improve predictive capabilities for catalysis, efficient energy storage, and energy transfer in nanoscale/molecular systems. The work therefore addresses the program areas of Dynamics of Energy Transfer and Transport and Catalytic Reactivity and Mechanisms. The proposed research is also responsive to the call for the theory and computation on molecular reaction dynamics and the structure and properties of nanoparticles and interfaces.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Apr 20, 2023
- Source ID
- FA95502210347
Entities
People
- Gordana Dukovic
Organizations
- Air Force Office of Scientific Research
- Regents of the University of Colorado
- United States Air Force