After conducting PL lifetime measurements on nanocrystals of different sizes (3 and 8 nm), I observe that the smaller particles have a longer lifetime than the bigger ones (especially the faster component: 3 vs 8 ns). It is unexpected, since a larger exciton band gap and a higher trap density should lead to faster recombination. Am I missing a mechanism here which could explain this phenomenon?
Hi, I don't know what nanocrystals you look at, but often the very small ones are perfect single crystals while the bigger ones tend to have more defects in them, such as stacking faults, twins etc, which can act as non-radiative recombination centres.
Dear Dr. Walther,
Thank you for your suggestion. I am dealing with CsPbBr3 perovskite nanocrystals. Here, however, only the fast component of the bi-exponential fitting seems to be slower. This component has been reported to be related to exciton spontaneous recombination, while the slower component (around 35 ns) could be attributed to trapping/detrapping mechanisms. I just wonder if it is physically possible for direct excitonic transition to be slower in smaller crystals.
Hi Victor,
There are several path of recombination: one is non-radiative and the other is radiative one (PL). The samples show bi-exponential decay, that means the farster component is attributed to the non-radiative one due to defects.
Provskite is quite complex crystal structure, so I guess the defects are mostly point defects. As Thomas suggested, a small particle has less chance to contain a defect.
And, I guess that there is another possibility. Different size of particles were produced from different processes. Hence, there is a possibility that the crystal qualities are different between these two samples. What do you think?
Agreed with Yoshiki . Overall PL decay in nanocrystalls is a complex mechanism. Before trapping exicton should migrate to the trap , so exciton migration rate also can be size dependable. In addition , did you check how PL decay depends on the excitation intensity ? - exciton annihilation could be involved as well making decay curve more complex than bi-exponential function, so try to measure PL decay at minimum excitation intensity to avoid annihilation effect
Dear Victor
The PL intensity is proportionnal to the Oscillator strength (OS) of exciton , i.e the Coulombic bounded electron-hole pair . The OS is proportionnal of recover integral of electron hole wave function . OS increases with decreasing quantum dot or nanoparticle size. OS is inversely proportionnal to the PL nanocrytals lifetime !
Dear Yoshiki,
Thank you for your suggestions. I personally cannot understand how any component from the PL decay could be originating from "non-radiative" recombination. If it is "non-radiative", by definition, it doesn't participate to any PL intensity. Right?
Dear Igor,
Thank you for your input. The signal is already not so strong, and I am afraid that it will become very noisy at minimal intensity. But I will give it a try.
Thank you
Dear S. Jaziri,
If I understand, taking into account only the OS, smaller crystals > higher OS > faster recombination.
Thank you all for your comments. I would be grateful if you could suggest any good literature I can use to fill my knowledge gap on the topic. Sincerely
Hi Victor,
do you excite in the barriers, or resonantly in the QDs? What is the temperature of the sample?
Dear Vesselin,
Our system is quite limited (no temperature control, no intensity control, fixed excitation sources). The QDs have an excitonic band gap of 2.55 eV and I use a 3.4 eV (365 nm) source.
do you use room temperature, or the sample is cooled to liquid nitrogen/helium temperature?
As I mentioned in my previous post, we don't have (yet) any control on the temperature. So all measurements are carried at room temperature. I know that makes it difficult to deconvolute the useful signal, but that's what we have at the moment.
just an idea :
In the small QD the energy levels are deeper. So it is more difficult for the carriers to escape out of the dots and recombine via non-radiative recombination centers in the barriers. In lager QDs more carriers escape and this way are lost for the PL process, because in the barriers they find non-radiative recombination centers. Thus, the PL in larger QDs drops faster.
I got exactly opposite results but not in perovskite When the size is small, it is faster than the big sizes.
Why do think higher density of trap lead to fast recombination?
I think due to the polar charged trap interact with neutral exciton with Coulomb force which makes longer PL decay time. because of disturbance of direct recombination .
And, physically, and in perfect environment. You thought is right.
However,
the quantum dot which shows very high surface to volume ratio could exhibit high density of surface trap(surface defect due to dangling bonding), which result in formation of depletion width that have bended energy band. Therefore, relatively, the non-radiative recombination process could increase, which leads to be longer PL decay time.
Dear Victor!!
Could you provide me some reference for these measurements?
thanks
The PL is proportional to the exciton confined in the nanocrystals and proportional inversely with lifetime.
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