Most of the reported carbon dot shows maximum absorbance at 270-310 nm in optical absorption spectra. So, actually it should exhibit highest PL intensity at excitation wavelength of 270-310 nm. However, it exhibits highest PL intensity at excitation wavelength of 350-380 nm. So, what is the reason behind it?
Could you please give an example of a paper with well described measurements of absorption and photoluminescence of carbon dots?
I am also pleased if you provide the reference you have already mentioned.
Hard to know without a description of your materials and methods. These publications (and references within) may give you the information you need.
"On the Excitation Wavelength Dependence of the Luminescence Yield of Colloidal CdSe Quantum Dots" Tonti et al, Nano Letters, 2483-2487, 4(12) 2004
"Engineering surface states of carbon dots to achieve controllable luminescence for solid-luminescent composites and sensitive Be21 detection" Xiaoming Li et al, Scientific Reports, 1-8, 4:4976 2014
Few of the example reports are,
X. Jia, et al. Nanoscale, 2012, 4, 5572–5575.
S. Sahu, et al. Chem. Commun., 2012, 48, 8835–8837.
Z. Zhang, et al. RSC Adv., 2012, 2, 8599-8601.
Both papers I referenced describe the role of surface traps. Excitation-independent luminescence is reported for surface passivated quantum dots.
It may well be applicable (I mean the answer of Paul). When the excitation wavelength gets shorter, the penetration depth of radiation often strongly decreases. If this also holds for carbon dots, photons with high energy get absorbed in a very thin layer near the surface of the particle, which is full of defects, so that the absorbed energy is dissipated and not converted to emission of photoluminescence.
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