Influence of the Inner‐Shell Architecture on Quantum Yield and Blinking Dynamics in Core/Multishell Quantum Dots

Abstract

Choosing the composition of the shell for QDs is not trivial, since both the band-edge energy offset and interfacial lattice mismatch play roles in influencing the final optical properties. One way to balance these competing effects is by forming multi-shells and/or gradient-alloyed shells. However, this introduces multiple interfaces and their relative effects on quantum yield and blinking are not yet fully understood. Here we undertake a systematic, comparative study of adding inner shells of single composition vs gradient-alloyed shells of cadmium/zinc chalogenides onto CdSe cores, and then capping with a thin ZnS outer shell to form various core/multi-shell configurations. We show that the inner shell architecture between the CdSe core and the outer ZnS shell plays a significant role in both quantum yield and blinking dynamics but that these effects are not correlated – a high ensemble quantum yield doesn’t necessarily equate to reduced blinking. Two mathematical models have been proposed to describe the blinking dynamics – the more common power-law model and a more recent multi-exponential model. By binning the same data with 1 ms and 20 ms resolution, we show that the on-times can be better described by the multi-exponential model while the off-times can be better described by the power-law model. We discuss physical mechanisms that might explain this behavior and how it can be affected by the inner shell architecture.

Shell-Dependent Photoluminescence Studies Provide Mechanistic Insights into the Off–Grey–On Transitions of Blinking Quantum Dots

“…However, the correlation of the amplitude of this fast component with on-state intensity and its anticorrelation with average on dwell time, at least for QDs with 5 ML of CdS shell, seems to be in contradiction with this assignment. It is worth noting at this point that the relative amplitudes of the fluorescence lifetime components in the ensemble and single molecule experiments did not agree with each other, similar to our previous reports, which may be due to a dark fraction not observable in single QD measurements. ,− We had also previously postulated the existence of slow-blinking and fast-blinking events in core/multishell QDs …”

“…CdSe core samples were synthesized by modification of the literature methods. , Briefly, 0.04 M cadmium (Cd) precursor was prepared by degassing under vacuum and then heating a mixture of 0.02565 g of CdO, 0.4452 g of OA, and 2 g of ODE to 200 °C under argon flow until the solution became clear. The temperature was then reduced to 50 °C, and then 1.5092 g of ODA and 0.5026 g of TOPO wre added.…”

“…Such control can be difficult due to the delocalized charge carriers entering trap states that can lead to fast nonradiative energy decay in the form of heat before more productive pathways such as radiative recombination (photonics) or charge separation (electronics) can occur. These trap states are strongly influenced by factors such as QD architecture, , structural defects, surface ligands, , and the external environment. − One way to better control radiative recombination or charge separation is to add one or more shells onto a core, and core/shell nanomaterials are more commonly used than core-only materials in most applications. Using type I core/shell materials, both the electron and hole are confined to the core by the higher energy of the conduction and valence band states of the shell, which increases photoluminescence (PL) quantum yield (QY).…”

Rationally designed synthesis of bright AgInS2/ZnS quantum dots with emission control

“…1(c), when compared with traditional II-VI QDs. For example, the absorption and emission spectra of monodisperse hydrophobic thick shell CdSe/CdS QD in hexane with a QY close to 1 show a FWHM of the first sharp excitonic absorption maximum of about 37 meV and a FWHM of the emission band of 65 meV [64][65][66][67]. The spectral widths of the emission bands were obtained assuming a symmetric shape of the emission spectra that were only recorded up to 790 nm.…”

CuInS2-Doped ZnS Quantum Dots Obtained via Non-Injection Cation Exchange Show Reduced but Heterogeneous Blinking and Provide Insights into Their Structure–Optical Property Relationships

“…At e m t on / ). 23,28,30,55−57 More recent studies have questioned the power law model and instead proposed that a multiexponential model is more suitable, 31,58 particularly as far as the on dwell times are concerned ( = ∑ τ − P A e i i t on / i ). Our group recently compared the reduced χ 2 values when fitting CdSe-based core/shell QDs to both models and found that on dwell times do fit better to a multiexponential model than an inverse power law although it was not possible to distinguish between the models for the off dwell times.…”