Direct Observation of Higher Multiexciton Formation and Annihilation in CdSe Quantum Dots

Most experiments on multiexcitons (MX) in quantum dots focused on the biexciton (XX), which is now well-understood. In contrast, there is little understanding of higher MX in quantum dots as a result of their difficulty to observe. Here, we apply time-resolved photoluminescence (t-PL) spectroscopy with 3 ps time resolution, sufficient to directly resolve previously unobserved spectral dynamics of a higher MX in CdSe quantum dots. These experiments resolve the controversy of the sequence of MX emissions, revealing that the higher channels sequentially populate the lower channels. There is a strong dependence of MX recombination kinetics upon a higher MX state, following a universal volume scaling law for Auger recombination for larger dots. Smaller dots show deviations for higher MX. In addition to triexcitons (3X), these experiments reveal MX up to the tetraexciton (4X). These experiments provide a direct observation of MX formation and annihilation in quantum dots. The impact of this observation is a step toward designing quantum dots to exploit higher MX processes.