Unraveling Nanoscale Energy Transport in Quantum Dots Superlattices with Time-Resolved Ultafast Stimulated Emission Depletion Microscop (Trusted)
Quantum dot (QD) solids are promising materials for future light harvesting devices and general energy transport applications. Further progress towards a QD-based electronics technology is dependent on the development of controllable methods for electronic-functionalization of QDs in the solid state. In an analogous fashion to conventional electronics, atomic doping is one possibility to exquisitely tune the electronic character of a solid QD assembly, via the introduction of midgap states, without perturbing an individual nanocrystal’s shape, size, or interparticle distance to adjacent QDs. The extent to which dopants modulate electronic energy transport (e.g., excitons) is widely unexplored, but is an important insight for determining how best to tailor atomic doping for specific functions like LEDs or solar cells. Towards better understanding the role of dopants in exciton transport, we use an ultrafast transformation of STED microscopy (time-resolved ultrafast STED, or TRUSTED) to resolve exciton diffusion in tellurium doped CdSe/CdS QD superlattices. In this talk, I will show our preliminary results using TRUSTED on both doped and undoped CdSe/CdS QDSLs to compare the extent exciton diffusivities are influenced by the presence of Te-dopants. Using Forster resonance energy transfer (FRET) theory as a simple model for exciton transport in QDSLs, one would predict Te-doping would decrease the overall rate of exciton “hopping” by virtue of a decrease in the spectral overlap of donor and acceptor pairs, and indeed, we measure lower diffusivities in Te-doped QDSLs than in undoped QDSLs. Interestingly, both doped and undoped QDSLs exhibit exciton diffusivities that are far higher that FRET can predict. I will show our attempts to reconcile the difference between experiment and theory with complementary measurements and simulations. Our work will provide a deeper understanding on the role dopants play in transport functionality as well as generally the extent to which theories like FRET breakdown in the context of solid-state QD assemblies.
Rongfeng is a postdoctoral researcher in the Ginsberg group at UC Berkeley, where he uses an ultrafast transformation of STED microscopy to investigate nanoscale energy transport in light harvesting materials. He received his Ph.D. in Chemistry from Stanford University, where he studied aqueous hydrogen bond network dynamics with ultrafast 2D IR spectroscopy in the Fayer group.