Stacking in Colloidal Nanoplatelets: Tuning Excitonic Properties

Abstract

Cataloged from PDF version of article.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

Record High External Quantum Efficiency of 19.2% Achieved in Light‐Emitting Diodes of Colloidal Quantum Wells Enabled by Hot‐Injection Shell Growth

“…Thanks to their strong quantum confinement solely in the vertical direction, 2D CQWs possess many unique thickness‐dependent optical characteristics including ultranarrow emission with suppressed inhomogeneous broadening, giant oscillator strengths, extraordinarily large linear and nonlinear absorption cross‐sections, and molar extinction coefficients . All these superior properties render CQWs to offer great potential for optoelectronic applications including solar energy harvesting, lasing, and light‐emitting diodes (LEDs) …”

“…Therefore, it is reasonable that low AR based CQW‐LEDs exhibits the highest EQE among these three devices. On the other hand, the stacking in CQW films is detrimental to the efficiency through fast nonradiative exciton transfer giving defected sub‐populations, which act as exciton sinks, undermining the efficiency of LEDs . As HAADF TEM images clearly show in Figure S6 (Supporting Information), negligible stacking is observed in low AR CQWs while stacking exists in high AR CQWs, suppressing the efficiency reduction in low AR devices .…”

Platelet‐in‐Box Colloidal Quantum Wells: CdSe/CdS@CdS Core/Crown@Shell Heteronanoplatelets

“…For these stacked NPLs, strongly quenched photoluminescence has been reported arising from ultraeffi cient exciton transport assisted hole trapping. [ 13 ] Up to now, to enhance stability and PL-QY of NPLs, core/ crown (laterally grown shell) [14][15][16][17][18] and core@shell (vertically grown shell) [ 19,20 ] architectures have been synthesized and studied intensively. Among different varieties of core/crown (C/C) NPLs, CdSe/CdS C/C NPLs have been one of the most widely studied material systems.…”

Three‐Dimensional Self Assembly of Semiconducting Colloidal Nanocrystals: From Fundamental Forces to Collective Optical Properties

“…Addition of an antisolvent such as ethanol accelerates the stacking drastically. This provides a way to control the stacking by purposefully adding small amounts of antisolvent to a stable dispersion …”

“…This phenomenon has recently been shown to be the origin of the decrease in quantum yield of stacked NPLs. Upon addition of an antisolvent, Guzelturk et al . evidenced this loss of fluorescence and showed that it was coupled to a significant acceleration of the transient fluorescence decay.…”