The excitation-dependent feature of nitrogen and sulfur doped carbon dots was discussed from 270 nm to 500 nm. With increasing of the excitation from 270 nm to 300 nm, the maximum luminescence peak will have a blue-shift, however, keeping the increase of the excitation, there are red-shift of maximum peak.
Dear Abhishek,
A few things
a) First, do record/include the excitation peak in your spectra and normalize all spectra to that excitation wavelength. Difference caused by increase/decrease of your excitation band causes changes in the recorded spectra.
b) Your sample is polydisperse, which is why particles with different sizes and physiochemical properties influence their electronic structure causing either red/blue shift of the spectra.
c) I feel that you have two kinds of chemical species: i) particles with less QY, slightly polydisperse, fluorescing between 270-300 nm, and ii) particles with relatively higher QY, more polydisperse, fluorescing at higher wavelengths.
It might be essential to understand the nature of these species (in case I am right). Hence, you might record the PL again (including excitation peak) with higher integration times, and maybe 4 nm spacing, for the region 270-300 nm. It is possible that you might observe an isosbestic point, ~@430 nm.
Also the following idea could be far-fetched but you might change the molar ratios of Sulpher vs. Nitrogen doping (0-1) and observe how the PL change. It might be possible that samples with dominating S or N content in the corresponding CQDs are contributing the blue shift at lower wavelength.
I could be wrong, but keep trying! Do let me know the cause, once you find it....Best ;)
@ Saunak Das, Thanks a bunch for the insights. It is really helpful. I will remeasure the PL spectrum (with the added details) and get back to you with the results.
Agree with Saunak Das that the excitation dependence emission spectrum of carbon dot is due to presence of poly-disperse Carbon dots which acts as different fluorophores giving different emission spectrum. Article Metamorphosis of Ruthenium Doped Carbon Dots: In Search for ...
Article On the Molecular Origin of Photoluminescence of Non-Blinking...
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