Based on selective luminescence study, The as-synthesized carbon nano dots (CDs) show both excitation-dependent/independent emission wavelength at one time. how can we explain that multiple behavior?
The wavelength dependency or independency is mainly related to the presence of different sized particles which is also called polydispersity. Although it may be related with the surface adsorbed functional groups too. If you go for a colloidal synthesis of QDs certainly you will get different sized particles and hence a wavelength dependent behaviour, however, if you perform membrane filteration you will be left with a finite sized particles and there will be no wavelength dependency.
You may see the following papers'
https://pure.uva.nl/ws/files/25486422/Uniform_luminescent_carbon_nanodots.pdf
https://www.doc88.com/p-2897810464259.html
In line to Himanshu Sir's reply, I would add:
If emission behaviour of CDs corresponds to excitation dependent PL behaviour, then it is due to polydispersity in CDs having a distribution in band gaps. If you excite with higher wavelengths (lower energies), you become less and less able to excite the small CQDs with high band gap. As a result, you lose the emission at short wavelengths and the overall emission redshifts and vice-versa. Therefore, to omit this, purification of CDs is practiced with dialysis membrane to attain monodisperse particles which will enhance their fluorescence and owe to excitation independent emission characteristics.
Dr. Thakur,
Can you please provide me with any reference that support your comment. Thanks
presently, I am trying to understand the nature of excitation dependent or independent photo-luminescence in carbon dots. I guess the main reason should be the purity of carbon dots. If you can purify via thin-layer chromatography, you will obtain the carbon dots with excitation independent emission. If you only purify using dialysis or other sample methods, maybe you will get the dots with with excitation dependent emission.
I prepared polymeric microspheres that demonstrated such property. Multiple Band gaps are prior requirement that can be selectively excited and there should be no transfer of energy among them. For CDs and GCDS there are few articles published but the mechanism behind such emission is still unclear...
See this review article
Z. Gan, H. Xu, Y. Hao, Mechanism for excitation-dependent photoluminescence from graphene quantum dots and other graphene oxide derivates: consensus, debates and challenges, Nanoscale 8(15) (2016) 7794-7807.
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