Multistate Blinking and Scaling of Recombination Rates in Individual Silica-Coated CdSe/CdS Nanocrystals

Abstract

Nonradiative Auger recombination is the primary exciton loss mechanism in colloidal nanocrystals and an impediment for prospective optoelectronic applications. Recent development of new core/shell nanocrystals with suppressed Auger recombination rates has opened the possibility for studying multicarrier states using time-resolved photoluminescence (PL) spectroscopy. An important aspect not addressed in previous works is the scaling of radiative and nonradiative decay rates with the increasing number and type of excitons in individual nanocrystals. Here we conduct extensive single-dot PL spectroscopy of emissive states in PL blinking trajectories of giant silica-coated CdSe/CdS nanocrystals. At low fluences, we observe the appearance of neutral and charged exciton (trion) states. Both negative and positive trions show strongly suppressed Auger recombination rates resulting in PL quantum yields close to 50%. At higher excitation powers, we observe consecutive emergence of lower efficiency states, indicative of higher order excitons. We employ a scaling model for Auger and radiative decay rates and attribute these states to doubly charged excitons, biexcitons, and a triexciton. Simultaneous analysis of the second-order correlation statistics proves that the biexciton Auger recombination channel can be represented in terms of the superposition of independent recombination channels of trions. Analysis of the PL emission of the triexciton state suggests nonstatistical scaling, likely due to the involvement of the transitions between different symmetries. Finally, measurements at high excitation fluence of nanocrystals with low trion quantum yields does not reveal any higher order excitonic states, corroborating the validity of the scaling model and confirming Auger-related mechanisms responsible for blinking behavior in such core/shell nanocrystals.