Coherent Magnetic Resonance of Nanocrystal Quantum‐Dot Luminescence as a Window to Blinking Mechanisms

Abstract

Blinking of colloidal nanocrystal quantum dots, random intermittency in the stream of photons emitted by single particles, has long commanded the curiosity of researchers. Why does the particle suddenly shut off, and what are the pathways to quench emission? Single-particle microscopy is not the only way to approach these fundamental questions on the interaction of light and matter: time-domain sub-ensemble spectroscopies can also yield relevant information on microscopic electronic processes. We illustrate recent advances in pulsed optically detected magnetic resonance and highlight the conceptual relevance to unravelling mechanisms controlling intermittency on the single-particle level. Magnetic resonance reveals two distinct luminescence quenching channels, which appear to be related to those previously surmised from single-particle studies: a trapped charge-separated state in which the exciton is quenched by dissociation and the particle remains neutral; and a charged state of the particle in which spin-dependent Auger recombination quenches luminescence.

Localization and Dynamics of Long‐Lived Excitations in Colloidal Semiconductor Nanocrystals with Dual Quantum Confinement

“…The remaining emission arises from the long‐lived excitations as discussed above. This long‐lived emission, which can also be isolated accurately using time‐resolved magnetic resonance techniques,32, 46 is the focal point of these datasets. The top panels (Figure 3 a and Figure 3 b) report the time‐resolved emission spectra of each system on a logarithmic false‐color scale.…”

Theory of exciton-polaron complexes in pulsed electrically detected magnetic resonance

“…These are expected to include triplet-exciton polaron quenching or the dynamics of a bipolaron stabilized by a counter-ion in organic systems 26 . Other three-particle processes also include spindependent Auger recombination in nanocrystals 27 , trion states in silicon 28 , and electrons interacting with a common nuclear bath 29 . In this work we develop a general formalism for describing the spin-dependent response of three-particle complexes in pulsed magnetic resonance, thus providing the theoretical foundations for the quantitative characterization of the time and energy scales present in these systems.…”

“…These are expected to include triplet-exciton polaron quenching or the dynamics of a bipolaron stabilized by a counter-ion in organic systems 26 . Other three-particle processes also include spindependent Auger recombination in nanocrystals 27 , trion states in silicon 28 , and electrons interacting with a common nuclear bath 29 .…”

Photon correlations probe the quantized nature of light emission from optoelectronic materials

“…27 In these materials, the distribution of "on" and "off" times in the luminescence is often found to follow a power-law distribution 28 rather than the exponential functionality associated with a well-defined lifetime of a single dark excited state. 27,29,30 A broad range of experimental techniques spanning singleparticle Stark spectroscopy, 31,32 single-particle spectroelectrochemistry, 33 and optically detected magnetic resonance spectroscopy 34 have been able to associate this intermittency with charge trapping on the surface of the nanocrystals. Interestingly, in a surprising analogy to triplet quenching in molecules, it was shown that the power-law exponents of luminescence blinking of nanocrystals, and hence of the intensity autocorrelation function, can be modified by exposure to air, presumably because of an oxidation reaction facilitated by the ambient moisture.…”