What caused the secondary peak in our photoluminescence experiment?

Septim
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Greetings everyone,

We have performed a photoluminescence experiment and plotted the resultant spectra for two samples of different concentration after correcting for the response of the CCD camera and got the results that is attached to this post. My question is that in the sample with higher concentration the peak is at the same wavelength; however, we have an additional secondary peak at around 400 nm. We were not able to explain this and I would be happy to know why it was observed. Both of the samples were CdS quantum dot structures in toluen and the samples were excited with a 325 nm 50 mW continuous wave operating Kimmon UV laser.

Thanks in advance
 

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It may be QD1 and QD2 with different size dispersions.
 

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