Pedro Ludwig Hernandez Martinez

Observation of Biexcitons in Nanocrystal Solids in the Presence of Photocharging

In nanocrystal quantum dots (NQDs), generating multiexcitons offers an enabling tool for enhancing NQD-based devices. However, the photocharging effect makes understanding multiexciton kinetics in NQD solids fundamentally challenging, which is critically important for solid-state devices. To date, this lack of understanding and the spectral-temporal aspects of the multiexciton recombination still remain unresolved in solid NQD ensembles, which is mainly due to the confusion with recombination of carriers in charged NQDs. In this work, we reveal the spectral-temporal behavior of biexcitons (BXs) in the presence of photocharging using near-unity quantum yield CdSe/CdS NQDs exhibiting substantial suppression of Auger recombination. Here, recombinations of biexcitons and single excitons (Xs) are successfully resolved in the presence of trions in the ensemble measurements of time-correlated single-photon counting at variable excitation intensities and varying emission wavelengths. The spectral behaviors of BXs and Xs are obtained for three NQD samples with different core sizes, revealing the strength tunability of the X-X interaction energy in these NQDs. The extraction of spectrally resolved X, BX, and trion kinetics, which are otherwise spectrally unresolved, is enabled by our approach introducing integrated time-resolved fluorescence. The results are further experimentally verified by cross-checking excitation intensity and exposure time dependencies as well as the temporal evolutions of the photoluminescence spectra, all of which prove to be consistent. The BX and X energies are also confirmed by theoretical calculations. These findings fill an important gap in understanding the spectral dynamics of multiexcitons in such NQD solids under the influence of photocharging effects, paving the way to engineering of multiexciton kinetics in nanocrystal optoelectronics, including NQD-based lasing, photovoltaics, and photodetection.

Critical role of CdSe nanoplatelets in color-converting CdSe/ZnS nanocrystals for InGaN/GaN light-emitting diodes.

Here we report CdSe nanoplatelets that are incorporated into color-converting CdSe/ZnS nanocrystals for InGaN/GaN light-emitting diodes. The critical role of CdSe nanoplatelets as an exciton donor for the color conversion was experimentally investigated. The power conversion efficiency of the hybrid light-emitting diode was found to increase by 23% with the incorporation of the CdSe nanoplatelets. The performance enhancement is ascribed to efficient exciton transfer from the donor CdSe nanoplatelet quantum wells to the acceptor CdSe/ZnS nanocrystal quantum dots through Förster-type nonradiative resonance energy transfer.

Near-Unity Efficiency Energy Transfer from Colloidal Semiconductor Quantum Wells of CdSe/CdS Nanoplatelets to a Monolayer of MoS2

A hybrid structure of the quasi-2D colloidal semiconductor quantum wells assembled with a single layer of 2D transition metal dichalcogenides offers the possibility of highly strong dipole-to-dipole coupling, which may enable extraordinary levels of efficiency in Förster resonance energy transfer (FRET). Here, we show ultrahigh-efficiency FRET from the ensemble thin films of CdSe/CdS nanoplatelets (NPLs) to a MoS2 monolayer. From time-resolved fluorescence spectroscopy, we observed the suppression of the photoluminescence of the NPLs corresponding to the total rate of energy transfer from ∼0.4 to 268 ns-1. Using an Al2O3 separating layer between CdSe/CdS and MoS2 with thickness tuned from 5 to 1 nm, we found that FRET takes place 7- to 88-fold faster than the Auger recombination in CdSe-based NPLs. Our measurements reveal that the FRET rate scales down with d-2 for the donor of CdSe/CdS NPLs and the acceptor of the MoS2 monolayer, d being the center-to-center distance between this FRET pair. A full electromagnetic model explains the behavior of this d-2 system. This scaling arises from the delocalization of the dipole fields in the ensemble thin film of the NPLs and full distribution of the electric field across the layer of MoS2. This d-2 dependency results in an extraordinarily long Förster radius of ∼33 nm.

Förster-type Resonance Energy Transfer (FRET): Applications

In this chapter, we present several applications of Forster-type nonradiative energy transfer (FRET) related phenomena. In particular, we review light generation and light harvesting applications as well as bio-applications.

Förster-Type Nonradiative Energy Transfer Rates for Nanostructures with Various Dimensionalities

In this chapter, we derive the energy transfer rate for the cases of X → NP (nanoparticle), X → NW (nanowire), and X → QW (quantum well), where X is an NP, an NW, or a QW, and obtain simply expression for the long distance approximation. This chapter is reprinted (adapted) with permission from Ref. [1]. Copyright 2013 American Chemical Society. We need to recall the results in Chap. 5 from Understanding and Modeling Förster-type Resonance Energy Transfer (FRET) Vol. 1, where the Fermi’s Golden Rule is simplified into

Excitonic energy transfer dynamics in hybrid organic/inorganic nanocomposites at high loading levels

Temperature dependent exciton migration in the hybrid nanocomposites of conjugated polymers chemically integrated with quantum dots is studied at high loading levels. The underlying interplay between the exciton transfer and diffusion is revealed.