Sizing Up Excitons in Core–Shell Quantum Dots via Shell-Dependent Photoluminescence Blinking

Abstract

Semiconductor nanocrystals or quantum dots (QDs) are now widely used across solar cell, display, and bioimaging technologies. While advances in multishell, alloyed, and multinary core-shell QD structures have led to improved light-harvesting and photoluminescence (PL) properties of these nanomaterials, the effects that QD-capping have on the exciton dynamics that govern PL instabilities such as blinking in single-QDs is not well understood. We report experimental measurements of shell-size-dependent absorption and PL intermittency in CdSe-CdS QDs that are consistent with a modified charge-tunnelling, self-trapping (CTST) description of the exciton dynamics in these nanocrystals. By introducing an effective, core-exciton size, which accounts for delocalization of charge carriers across the QD core and shell, we show that the CTST models both the shell-depth-dependent red-shift of the QD band gap and changes in the on/off-state switching statistics that we observe in single-QD PL intensity trajectories. Further analysis of CdSe-ZnS QDs, shows how differences in shell structure and integrity affect the QD band gap and PL blinking within the CTST framework.

Measurement of ligand coverage on cadmium selenide nanocrystals and its influence on dielectric dependent photoluminescence intermittency

“…Notably, this may be further complicated by atmospheric effects, where it has been found that moisture in the air may passivate surface traps and alter PI blinking statistics 25 , although this was not found to impact the former dielectric dependence studies on polymer substrates performed under ambient air, where even a low concentration of water molecules might be expected to dominate the local dielectric constant ðε H 2 O ¼ 80Þ 13,14,16 . Osborne and Fisher further investigated the correlations between PI and the dielectric properties of the QD surround via a recently published charge-tunnelling and selftrapping (CTST) model 15,16 . In the CTST scheme ( Fig.…”

“…Early work by Cichos and colleagues identified a linear correlation between the residence time of a QD in a dark state and reaction field factor, (1 − 1/ε m ) where ε m is the dielectric constant of the substrate or embedding medium 13,14 . Further investigations by Osborne and Fisher led to the introduction of perturbations to the host-medium dielectric constant to account for coverage of the QD surface by stabilising ligands 15,16 . In this case, an effective dielectric constant was employed to define the self-trap energies for exciton chargecarriers in the vicinity of the QD interface, in the quantitative modelling of PI using a recently advanced charge-tunnelling and self-trapping (CTST) model 15 .…”

“…Further investigations by Osborne and Fisher led to the introduction of perturbations to the host-medium dielectric constant to account for coverage of the QD surface by stabilising ligands 15,16 . In this case, an effective dielectric constant was employed to define the self-trap energies for exciton chargecarriers in the vicinity of the QD interface, in the quantitative modelling of PI using a recently advanced charge-tunnelling and self-trapping (CTST) model 15 . To better understand the interplay between PI dynamics in QDs and the dielectric properties of the QD interface, we employ high resolution magic angle spinning (HR-MAS) NMR and XPS to measure the ligand surface population and molecular dynamics (MD) simulation to gauge ligand surface coverage and its contribution to the local QD nanoenvironment.…”

Correlation of Intermittency of Quantum Dot Photoluminescence Intensity, Decay Time, and Energy

“…It is evident that the matrix, especially its polarity, has considerable impact on blinking dynamics of QDs . However, the existence of C‐type blinking is more sensitive to the type of capping ligands, though the time scale of blinking is again controlled by solvent properties.…”

“…It is thus much longer as compared to quantum mechanically based intrinsic dynamics, such as radiative and non‐radiative decay processes which are typically in the range of nanoseconds. It should be emphasized that we here exclude those phenomena from the discussion which are related to “intrinsic” blinking processes such as emission from, e.g., long lived (optically forbidden) triplet states in case of organic molecules for which the blinking phenomena are often referred to as “photon bunching.” It is obvious from many experiments that blinking is related to and even controlled by the interface of the respective quantum object or its (embedding) environment . The large time scale reflects a broad distribution of interactions with a generalized interface constituted by the surface of the quantum object itself including ligands and/or the supporting substrate or embedding matrix .…”