Unraveling excitonic dynamics of solution-processed quantum well stacks
Author(s)
Advisor
Demir, Hilmi VolkanDate
2015-08Publisher
Bilkent University
Language
English
Type
ThesisItem Usage Stats
163
views
views
79
downloads
downloads
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.
Keywords
Colloidal quantum wellsSemiconductor nanoplatelets
Stacking, exciton dynamics
Förster resonance energy transfer