I have measure the CdSe-ZnS quantum dots（QDs）fluorescent lifetime by using TCSPC instrument. And we got the three mechanism（lifetime）for this QDs fluorescence. （like the attached graph, all the graph belong to the NTHU university Qlab）
But in fundamental theory, there are just（1）Band-edge emission（2）Defect emission（3）Activator emission for semiconductors, and this type of QDs were belong to the band-edge emission type, which means I should just get one lifetime value. But I got the three?
How can I explain to this problem?
The reason I choose the tail-fit method is because its range is ns scale and my pulse is ps, so that is a big difference for my source and my sample.
And I use 3-exponential function to describe the QDs dynamic mechanism are attributed from some reference using the same technique to fit.（like the attached link）
Or you can provide me some advice, that would be great.
Actually I am thinking how about Gaussian or Lorentz function to describe the QDs dynamic mechanism（like the attached graph）because the QDs was not a uniform objects like you said.
http://pubs.acs.org/doi/abs/10.1021/jp3092013
http://pubs.acs.org/doi/abs/10.1021/am303149r
Quantum dots (QDs) generally show multi-exponential decay dynamics (even for core/shell systems). Ideally multi-exponential decays should correspond to multiple different sites (or excited state populations) decaying. Since QD system is very heterogenous (size distribution, electron/hole wavefunctions leaking to inorganic/organic shell etc.), multi-exponential decay dynamics is often observed. There is no consensus (that I know of) for the origin of the multi-exponential behaviour of QDs. I would suggest you to fit these decay profiles with multi-exponential decay dynamics and use intensity weighted average lifetimes for your analysis.
You could also try fitting your decay curves to stretched -exponential functions, where beta parameter accounts for breadth of distribution of different excited state populations decaying. For a single emitting excited state (or mono-exponential decay), beta should be 1.
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