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

Abstract

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.

Excitation Energy Dependence of Photoluminescence Quantum Yields in Semiconductor Nanomaterials with Varying Dimensionalities

“…The PL Eff ( E ) collected for other samples of CdSe QDs contained both minima and decreasing PL efficiencies with increasing excitation energy . These latter EED trends are consistent the results obtained for various QD systems ,,− and perovskite NPs. , …”

Instantaneous, room-temperature, open-air atmosphere, solution-phase synthesis of perovskite quantum dots through halide exchange employing non-metal based inexpensive HCl/HI: ensemble and single particle spectroscopy

“…Similar results have been reported for the CdSe and InP-based QDs. [41][42][43][44][45][46] As can be observed from Fig. 4(i), the ON-OFF events are anti-correlated.…”

Excitation Energy Dependence of Semiconductor Nanocrystal Emission Quantum Yields

“…Application of near-unity QY NCs to demonstrating condensed phase laser cooling, however, requires better understanding of how NC QYs are impacted by excitation conditions such as excitation intensity and/or excitation wavelength. The latter is of particular concern given prior studies, suggesting that varying NC excitation energies leads to different emission QYs. − Such excitation energy-dependent (EED) emission QYs were first demonstrated by Hoheisel et al who showed through photoluminescence excitation (PLE) measurements that CdSe NC emission QYs decreased monotonically when ensembles were excited to the blue of their band edges. This was rationalized as exciting particles into a continuum of states, distinct from confined NC states.…”