Spectator Exciton Effects on Nanocrystal Photophysics II: PbS

Abstract

The spectator exciton method follows stepwise changes in a nanocrystal’s absorption with the accumulated number of excitons. It involves comparison of pump–probe spectra in pristine samples with that from nanocrystals excited with a progressive number of cold excitons. Using this approach, 50% of hot electrons were shown to be blocked from cooling directly to the band edge in CdSe nanodots already excited with a single cold exciton due to electron spin orientation conflicts. Similar experiments which are reported here in quantum-confined PbS nanodots have not detected spin-related relaxation blockades. Instead, a progressive broadening of the 1Se1Sh absorption band with loading of cold excitons is manifest in the difference spectra, demonstrating the inadequacy of any single-probe wavelength to quantify band-edge state filling. Comparing data for single and double spectator states proves that the sub band gap induced absorption characteristic of hot exciton states in all quantum dots is not the low-energy half of biexciton shifting to the 1Se1Sh absorption band. Instead, it is part of a broad induced absorption characteristic of hot excitons regardless of the spectator count. Finally, the unique capability of spectator excitons (SXs) to detect contributions of stimulated emission to pump–probe signals is demonstrated in singly excited PbS nanocrystals. Contrary to recent literature reports of such contributions, none are observed. These outcomes call for revisions to the conventional interpretation of excited state spectroscopy in quantum confined nanocrystals.

Confinement and Exciton Binding Energy Effects on Hot Carrier Cooling in Lead Halide Perovskite Nanomaterials

“…A more likely explanation for the suppressed hot phonon bottleneck is, therefore, the enhanced influence of carrier–carrier scattering, involving the interactions between an electron and hole bound in the same exciton. ,, In other words, the presence of a cooling pathway mediated by carrier–carrier scattering increases the critical hot phonon density required to slow carrier cooling, which flattens the observed dependence of τ cool on n hot . Herz et al also recently highlighted the role of cold carriers in the hot carrier dynamics of Sn-based perovskites using pump–push–THz-probe spectroscopy, and Ruhman et al discuss the effect of cold spectator excitons in the intraband relaxation of PbS NCs.…”

“…47,48,80 In other words, the presence of a cooling pathway mediated by carrier−carrier scattering increases the critical hot phonon density required to slow carrier cooling, which flattens the observed dependence of τ cool on n hot . Herz et al 83 also recently highlighted the role of cold carriers in the hot carrier dynamics of Sn-based perovskites using pump−push− THz-probe spectroscopy, and Ruhman et al 78 discuss the effect of cold spectator excitons in the intraband relaxation of PbS NCs. To investigate whether the effects observed so far are specific to NPLs or generally applicable to 2D perovskites, where excitonic character is enhanced further, we extended our measurements to RP-type (PEA) 2 PbI 4 .…”

“…Pump–push–probe (PPP) spectroscopy has been introduced as a means of isolating intraband relaxation in systems ranging from inorganic NCs ,,− to molecular systems . We, ,, and others, − have recently employed PPP spectroscopy to complement existing TAS studies of bulk and nanocrystalline LHPs.…”

Design rules for obtaining narrow luminescence from semiconductors made in solution

“…The timescale for equilibration between the bright and dark exciton in PbS QDs is not settled. In contrast to CdSe, 106 spectator exciton experiments did not detect a spin-blockade for PbS, 112 and the authors noted that a rapid spin-flip hypothesis would reduce but not eliminate the blockade at the band edge. One possible explanation, not suggested by the authors, is that rapid intervalley scattering among the four valleys in PbS could provide a way around any spin blockade created by up to 3 spectator excitons (the experiments probed one and two spectator excitons).…”

“…However, Ruhman et al were unable to detect stimulated emission in recent spectrally resolved pump-probe and spectator exciton experiments on larger PbS nanocrystals. 112 This might be due to the combined effect of the relatively small stimulated emission signal (10% of maximum 2D signal), the ±5-10% error bars on the spectrally resolved pump-probe experiment, the obscuring effect of any sample heterogeneity in spectrally resolved pump-probe experiments, and extensive absorption-PL overlap noted for their sample (the Stokes' shift decreases as size increases, -the data of ref.…”