Unraveling excitonic dynamics of solution-processed quantum well stacks

Abstract

Colloidal semiconductor quantum wells, also commonly known as nanoplatelets (NPLs), are a new class of atomically at nanocrystals that are quasi twodimensional in lateral size with vertical thickness control in atomic precision. These NPLs exhibit highly favorable properties including spectrally narrow photoluminescence (PL) emission, giant oscillator strength transition and negligible inhomogeneous broadening in their emission linewidth at room temperature. Also, as a unique property, NPLs may self-assemble themselves in extremely long chains, making one-dimensional stacks. The resulting excitonic properties of these NPLs are modified to a great extent in such stacked formation. In this thesis, we systematically study the excitonic dynamics of these solution-processed NPLs in stacks and uncover the modification in their excitonic processes as a result of stacking. We have showed that, with increased degree of controlled stacking in NPL dispersions, the PL intensity of the NPL ensemble can be reduced and their PL lifetime is decreased. We also investigated temperature-dependent time-resolved and steady-state emission properties of the nonstacked and completely stacked NPL films, and found that there are major differences between their temperature-dependent excitonic dynamics. While the PL intensity of the nonstacked NPLs increases with decreasing temperature, this behaviour is very limited in stacked NPLs. To account for these observations, we consider F orster resonance energy transfer (FRET) between neighboring NPLs in a stack accompanied with charge trapping sites. We hypothesize that fast FRET within a NPL stack leads increased charge trapping, thereby quenching the PL intensity and reducing the PL lifetime. For a better understanding of the modification in the excitonic properties of NPL stacks, we developed two different models, both of which consider homo-FRET between the NPLs along with occasional charge trapping. The first model is based on the rate equations of the exciton population decay in stacks. The rate equations constructed for each different stack were solved to successfully estimate the PL lifetime of the stacked ensembles. In the second one, excitonic transitions in a stack are modeled as a Markov chain. Using the transition probability matrices for the NPL stacks, we estimate the PL lifetime and quantum yield of the stacked ensembles. Both models were shown to explain well the experimental results and estimate the observed changes in the excitonic behaviour when the NPLs are stacked. The findings of this thesis work indicate that it is essential to account for the effect of NPL stacking to understand their resulting time resolved and steady-state emission properties.