Ultrafast Dynamics and Ultrasensitive Single-Particle Intermittency in Small-Sized Toxic Metal Free InP-Based Core/Alloy-Shell/Shell Quantum Dots: Excitation Wavelength Dependency Toward Variation of PLQY

Abstract

Observation of strong deviation of photoluminescence excitation curve from absorption curve especially at lower wavelength range, below 450 nm in case of small-sized toxic metal free InP-based corealloy-shell quantum dots, hints toward interesting exciton dynamics. Photoluminescence quantum yield (PLQY) has been observed to be dependent on the excitation wavelength. Monitoring the bleach dynamics employing femtosecond ultrafast pump−probe technique, it could be shown that the rise time increases with decrease in pump excitation wavelength from 100 fs for 550 nm excitation to 220 fs for 430 nm pump/excitation. Therefore, exciton cooling takes a longer time for lower wavelength excitation, and thus the exciton becomes more prone to get trapped. About twofold enhancement in magnitude of normalized bleach signal at the bandedge (∼0.1 (for 430 nm excitation) to ∼0.2 (for 550 nm excitation)) following exciton relaxation has been observed. Thus, in comparison to lower wavelength excitation, for near band-edge pump/ excitation there is higher probability of radiative exciton recombination, therefore increasing PLQY. Hot exciton trapping dynamics has been noted to be occurring at a time scale ∼750 fs. From ultrasensitive single-particle measurement, magnitudes of power-law exponent for both ON and OFF events remain similar to each other, and the magnitude remains unaltered for different excitation wavelengths (405−568 nm). As long as 100 s ON event could be observed, making this InP-based nontoxic quantum dot (QD) quite suitable for single-particle tracking, etc. Interestingly, ON-event truncation time has been found to increase from 6 to 16 s, and OFF-event truncation time has been found to decrease from 11 to 5.5 s; thereby, exciton detrapping rate/trapping rate increases from 0.5 to nearly 3 on going from 405 to 568 nm excitation. Thus, as the trapping gets suppressed and detrapping gets enhanced, PLQY gets enhanced. The extent of relative decrease of PLQY value with increase in excitation energy above band-edge has been observed to be much more pronounced in CdSe-based CAS QD than InP-based CAS QD.

Extent of Shallow/Deep Trap States beyond the Conduction Band Minimum in Defect-Tolerant CsPbBr3 Perovskite Quantum Dot: Control over the Degree of Charge Carrier Recombination

“…The magnitude of m ON has been found to be ∼1.0 for all λ ex (Table 2), and these values are significantly less in comparison to CdSe (∼1.2) and InP (∼1.2) based CAS QDs. 28,29 The magnitude of m OFF for CsPbBr 3 PQDs (∼1.5−1.7) is however significantly higher in comparison to that of reported optically robust CdSe (∼1.3− 1.4) and InP (∼1.1−1.3) based CAS QDs. 28,29 Thus, CsPbBr 3 PQDs exhibiting higher probability of ON events and lower probability of OFF events are optically superior in comparison to CdSe-and InP-based CAS QDs at the single-particle level.…”

“…28,29 The magnitude of m OFF for CsPbBr 3 PQDs (∼1.5−1.7) is however significantly higher in comparison to that of reported optically robust CdSe (∼1.3− 1.4) and InP (∼1.1−1.3) based CAS QDs. 28,29 Thus, CsPbBr 3 PQDs exhibiting higher probability of ON events and lower probability of OFF events are optically superior in comparison to CdSe-and InP-based CAS QDs at the single-particle level. These observations provide direct evidence of optically inactive mid-gap trap/defect states for PQDs, unlike CdSe/InP QDs.…”

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.…”

Near-Ergodic CsPbBr3 Perovskite Nanocrystal with Minimal Statistical Aging

“…Comparatively lower magnitude of the COV for p values of OFF event distributions (Table ) further confirms that the extent of variation in OFF → ON switching processes is lesser than that of ON → OFF switching processes . ON/OFF event durations are correlated with trapping and detrapping rates. ,,− Relatively more disperse ON event distribution as compared to OFF event distribution suggests that trapping rates are broadly distributed in comparison to detrapping rates. This is indeed another very important piece of information which remains masked in the regular PDD analysis of event durations.…”

“…Blinking dynamics in QDs/PNCs has in general been analyzed employing the histogramming-thresholding (HT) technique, and the probability density distribution (PDD) of ON and OFF event durations has been fitted with different equations (see Supporting Information, p S7). − However, cumulative HT analyses of PL blinking dynamical analyses yield only ensemble-averaged information, which is not quite useful to probe ergodicity or statistical aging of the PNC.…”