Why Does the Photoluminescence Efficiency Depend on Excitation Energy in Case of a Quantum Dot? A Case Study of CdSe-Based Core/Alloy Shell/Shell Quantum Dots Employing Ultrafast Pump–Probe Spectroscopy and Single Particle Spectroscopy

Core/alloy-shell/shell quantum dots (CASS QDs) have been shown to exhibit excitation energy dependent PL efficiency. The magnitude of the normalized transient population has been shown to increase more than 5-fold with decreasing the excitation energy. For high energy excitation cooling of the exciton (predominantly electron) to band edge is much slower (rise time of ∼526 fs) in comparison to low energy excitation (rise time of <∼100 fs). Time constant associated with the excitation energy dependent dynamics of the hot electron trapping is ∼1 ps. Time constant related to excitation energy independent hot hole dynamics is ∼35 ps. Truncation time obtained from single particle investigation has been shown to increase four folds (20 to 80 s) and the magnitude of additional exponential time constant responsible for hole trapping has been shown to increase nearly two folds with decrease in excitation energy. Thus, by employing ensemble level ultrafast dynamics and single particle PL blinking dynamics, it could be shown that the extent of hot electron trapping decreases, and the extent of hot hole trapping increases, as the excitation energy is lowered. Thus, the extent of the nonradiative Auger process decreases, thereby leading to enhanced PL efficiency for lower energy excitation.