(262d) Modeling Exciton Dynamics and Low-Frequency Vibrations in Quantum Dot Assemblies

Authors: 
Colloidal quantum dots (QDs) are a promising material platform for solution-processable optoelectronic devices such as solar cells, light-emitting diodes, thermoelectric modules, and flexible electronics. Central to the operation of these devices is the formation, transport, and conversion of energy carriers at the nanoscale, such as in the form of excitons and phonons, for transferring electronic and vibrational energies, respectively. 

In the first part of the talk, I will present a computational study of exciton transport in QD solids comprised of polydisperse dots that gives rise to energetic disorder. To gain insight into the relationship between energetic disorder and the spatiotemporal dynamics of excitons, we performed kinetic Monte Carlo (KMC) simulations. We find that a model in which excitons hop between energetically (and spatially) disordered QDs with transition rates determined using Förster theory is capable of reproducing the experimental trends in both exciton displacement and transient energies. Moreover, we use KMC method to investigate the role of energetic disorder in seemingly subdiffusive behavior of excitons that lead to time-varying diffusivity, which we observe in time-resolved optical microscopy experiments.

In the second part of the talk, I will show how the characteristics of collective vibrational modes or low-frequency phonon modes depend on mechanical and physical properties of QDs, such as QD size, ligand identity, and core-shell structure. Here we propose an improvement over the traditional continuum elastic theories for describing confined acoustic phonons in colloidal systems by including the mass-loading effect of the ligands. Calculated l=0 phonon energies from our model agree very well with those probed by low-frequency Raman spectroscopy for various CdSe QD samples. Lastly, we demonstrate how our model can help infer elastic properties, such as sound velocities and elastic moduli, of the surrounding ligand materials that may be difficult or impossible to directly probe otherwise.