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

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 core-alloy-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 band-edge (∼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.