Charge-tunnelling and self-trapping: common origins for blinking, grey-state emission and photoluminescence enhancement in semiconductor quantum dots

Abstract

Understanding instabilities in the photoluminescence (PL) from light emitting materials is crucial to optimizing their performance for different applications. Semiconductor quantum dots (QDs) offer bright, size tunable emission, properties that are now being exploited in a broad range of developing technologies from displays and solar cells to biomaging and optical storage. However, instabilities such as photoluminescence intermittency, enhancement and bleaching of emission in these materials can be detrimental to their utility. Here, we report dielectric dependent blinking, intensity-"spikes" and low-level, "grey"-state emission, as well as PL enhancement in ZnS capped CdSe QDs; observations that we found consistent with a charge-tunnelling and self-trapping (CTST) description of exciton-dynamics on the QD-host system. In particular, modulation of PL in grey-states and PL enhancement are found to have a common origin in the equilibrium between exciton charge carrier core and surface-states within the CTST framework. Parameterized in terms of size and electrostatic properties of the QD and its nanoenvironment, the CTST offers predictive insight into exciton-dynamics in these nanomaterials.

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

“…In a final analysis of TPL cut-off times for on-time PDDs, we found the increasing truncation rate observed experimentally to be strongly dependent on biexciton formation within the CTST mechanism for SX + -state quenching ( Figure S4 in SI). 37 Here, xx approximates an x decreases with a similar dependence on QD-core diameter, approximately (2 ) −2.8 , in the presence of a pyridyl-functionalised porphyrin surfactant molecule. 51 In this case, the molecule acted as a charge-carrier trap and probe of exciton-wavefunction leakage at the QD-surface, with the probability density of electron-wavefunction overlap at the probe displaying the same QD-core size dependency as the PL-quenching rate.…”

“…To restrict modification of the CTST-model to a single change in definition of the QD-core radius, we maintain the hole-trap radius defining the localization of h + at the QD-surface at h = 0.3, as per previous studies. 37 However, it should be recognized that the cut-off rate constant defined by Equation 1 will be sensitive to exact values of the h + -trap size and the dielectric constant at the QD-host interface. For example, increasing the dielectric constant at the QD-surface to that of the glass substrate, s = 3.8, requires a 1/3 reduction in the trap radius to obtained cut-off times within 50% of those obtained for s = 2 and h = 0.3 nm (Table 1), although the trend to shorter times with increasing shell-thickness remains consistent.…”

“…The dependence is analogous to the exponential growth in h + / h − with shell-thickness that we find in CTST ( Supplementary Table S1). 37 Finally, we note that the modified-CTST model presented here, does not formerly Chemical were used without further purification.…”

“…Focus was maintained using a motorized focus drive (Prior Scientific, PS3H122, UK) and an automated macro (ImageJ, USA).Image processing and analysis. Image-stacks of QD movies were processed (ImageJ, USA) to extract single-QD PL intensity trajectories PL-on and off-time using standard procedures 37. A threshold was set equal at 2 above the dark-state mean, typically corresponding to the minima in the intensity histogram of bright and dark states.…”

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

“…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 . In the following we will term for simplicity all external conditions “interfaces.” It is tempting to relate the broad distribution of blinking events to disordered interfaces …”