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.

Perovskite quantum dots (PQDs) are known to be defect-tolerant, possessing a clean band gap with optically inactive benign defect states. However, we show that there exist significant deep trap states beyond the conduction band minimum, although the extent of shallow trap states is observed to be minimal. The extent of deep trap states beyond the conduction band minimum seems to be significant in PQDs; however, the extent is less than that of even optically robust CdSe-and InP-based core/alloy-shell QDs. In-depth analyses based on ultrafast transient absorption and ultrasensitive single-particle spectroscopic investigations decode the underlying degree of charge carrier recombination in CsPbBr 3 PQDs, which is quite important for energy applications.

There is no literature report of simultaneously achieving near-unity PLQY (ensemble level) and highly suppressed blinking (ultrasensitive single-particle spectroscopy (SPS) level) in a toxic-metal-free QD. In this Letter we report accomplishing near-unity PLQY (96%) and highly suppressed blinking (>80% ON fraction) in a toxic-metal-free CuInS 2 / ZnSeS Core/Alloy-Shell (CAS) QD. In addition, (i) gigantic enhancement of PLQY (from 15% (Core) to 96% (CAS QD)), (ii) ultrahigh stability over 1 year without significant reduction of PLQY at the ensemble level, (iii) high magnitude (nearly 3 times) of electron detrapping/trapping rate, and (iv) very long ON duration (∼2 min) without blinking at the SPS level enable this ultrasmall (∼3.3 nm) CAS QD to be quite suitable for single-particle tracking/bioimaging. A model explaining all these excellent optical properties has been provided. This ultrabright CAS QD has been successfully utilized toward fabrication of lowcost microcontroller-based stable and bright yellow and white QD-LEDs.

Contrary to the belief of the chemistry community, intense charge transfer is observed between meta-oriented donor–acceptor moieties in ultrasmall single-benzenic fluorophores. Red emission in any solvent has so far been reported with molecular fluorophores having the lowest molecular weight (MW) of 252.5 Da; however, we have achieved the same with a meta-fluorophore having an MW of only 203.1 Da. Red emission in a nonpolar solvent has so far been reported with a fluorophore having a minimum MW of 464.5 Da, but we have achieved the same with a meta-fluorophore having an MW of only 255.3 Da. Intense Stokes (260 nm) and solvatochromic (160 nm) shifts, high magnitudes of fluorescence quantum yield (>0.4), and excited-state lifetime (>16 ns) have been obtained in these single-benzenic meta-fluorophores, and these values are comparatively much higher in comparison to corresponding o-/p-fluorophores. These extraordinary meta-fluorophores have been employed to measure subcellular nanopolarity in live stem cells toward cost-effective white fluorescent ink and white light-emitting diodes (LEDs) in solid state.

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