Browsing by Subject "Förster resonance energy transfer"

Now showing 1 - 3 of 3

Open Access
FRET-LEDs involving colloidal quantum dot nanophosphors
(Webcom Communications, 2010) Nizamoğlu, S.; Sari, E.; Baek, J. H.; Lee, I. H.; Sun, X. W.; Demir, Hilmi Volkan
Semiconductor nanocrystal quantum dots (NQD) with their narrow and tuneable emission are promising candidates to serve as color convertors integrated on light-emitting diodes (LEDs). The use of nonradiative energy transfer, also known as Förster-type resonance energy transfer (FRET), in such NQD nanophosphors provides additional benefits for color-conversion in solid state lighting. In this paper we discuss these NQD-integrated FRET-LEDs for lighting applications.
Open Access
Stacking in colloidal nanoplatelets: tuning excitonic properties
(American Chemical Society, 2014) Guzelturk, B.; Erdem, O.; Olutas M.; Kelestemur Y.; Demir, Hilmi Volkan
Colloidal semiconductor quantum wells, also commonly known as nanoplatelets (NPLs), have arisen among the most promising materials for light generation and harvesting applications. Recently, NPLs have been found to assemble in stacks. However, their emerging characteristics essential to these applications have not been previously controlled or understood. In this report, we systematically investigate and present excitonic properties of controlled column-like NPL assemblies. Here, by a controlled gradual process, we show that stacking in colloidal quantum wells substantially increases exciton transfer and trapping. As NPLs form into stacks, surprisingly we find an order of magnitude decrease in their photoluminescence quantum yield, while the transient fluorescence decay is considerably accelerated. These observations are corroborated by ultraefficient Forster resonance energy transfer (FRET) in the stacked NPLs, in which exciton migration is estimated to be in the ultralong range (>100 nm). Homo-FRET (i.e., FRET among the same emitters) is found to be ultraefficient, reaching levels as high as 99.9% at room temperature owing to the close-packed collinear orientation of the NPLs along with their large extinction coefficient and small Stokes shift, resulting in a large Forster radius of similar to 13.5 nm. Consequently, the strong and long-range homo-FRET boosts exciton trapping in nonemissive NPLs, acting as exciton sink centers, quenching photoluminescence from the stacked NPLs due to rapid nonradiative recombination of the trapped excitons. The rate-equation-based model, which considers the exciton transfer and the radiative and nonradiative recombination within the stacks, shows an excellent match with the experimental data. These results show the critical significance of stacking control in NPL solids, which exhibit completely different signatures of homo-FRET as compared to that in colloidal nanocrystals due to the absence of inhomogeneous broadening.
Open Access
Unraveling excitonic dynamics of solution-processed quantum well stacks
(2015-08) Erdem, Onur
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.