Are there different peaks of undiluted and diluted CdSe QDs, Size of CdSe QDs depends temperature and time dependent. Peak varies with these conditions. With dilution there will be change in a.u. of PL spectra whearse wavelength is same. You may confirm with these things. What is your QDs dispersion material (Solution)?

I’m not familiar with the CdS quantum dots fluorescence, so cannot comment precisely. I did low-temperature fluorescence analysis of photosynthetic complexes. However, there some general remarks about fluorescence. Indeed, in some cases dilution may affect peak position. At high concentration of fluorescing particles, they, perhaps, can interact resulting in overlapping of their electronic orbitals. In turn, this leads to change in their (orbitals’) energetic levels seen as fluorescence peak shift (and, perhaps, also amplitude). Solution conditions are of importance as well: pH, pK, etc, because the fluorescing particles interact with other solution molecules, which also may influence (under particular concentrations) energetic properties of the particles and thus the emission peak position(s)/amplitude. Also, temperature is very important as mentioned above by Dr. Rabindra Kumar. More information about CdS quantum dots fluorescence should be available in literature or protocols if this approach is standard.

Its an aqueous dispersion of CdS quantum dot.

Depending on the cuvette size and the light beam geometry, an effect called "fluorescence re-absorption" may occur. Which means that in the short-wavelength range of the fluorescence spectrum the intensities are lower than expected and, consequently, the maximum is shifted to longer wavelengths. The effect is stronger in less diluted solutions.

Commenting Dr. Friedrich Menges’ post: This is the case only if the emission peak is a sum of two (or more) peaks with very close maxima (separated by 1-2 nm). Indeed, at high concentration reabsorption may occur, so as some amount of fluorescence light emerging from the shorter wave-length component is (re-)absorbed by the longer wave-length component, which, in turn, emits with higher amplitude. As emission maxima of these two components are too close (far only by 1-2 nm) (and, also, they must be of comparable amplitudes), the maximum of the summarized peak may be red-shifted (by approx. 1-2 nm, too). Actually, reabsorption takes place for components that have wider range (up to 40-50 nm) between their emission maxima. However, no maxima shift is observed for such peaks just because they are pretty distinct (non-overlapping).

By all means, beam path (general absorption depending, for instance, by curette size) and light scattering are not the reasons for the emission peak shifts.

Mr. Ghajesh I attached here some links regarding aqueous dispersion of CdSe QDs. It may be helpful to you.

http://pubs.acs.org/doi/pdf/10.1021/ja017002j

http://www.ias.ac.in/matersci/bmsoct2010/543.pdf

http://pubs.acs.org/doi/abs/10.1021/jp0105488

And one more note: Hopefully, we all here talk about fluorescence emission spectra – not about fluorescence excitation spectra :)

P.S. the best help is from Dr. Rabindra Kumar.. lol

regarding dr. Shpilyov's comment:

obviously, the strength of the emission reabsorption effect depends on the cuvette size on beam geometry, the longer the pathlength travelled by the emitted light, the more it can be reabsorbed.

Second, a maximum shift can also occur for broad emission bands. Have a look at two examples here:

http://www.rsc.org/ej/CP/2009/b912178a/b912178a-f4.gif

http://www.nature.com/nprot/journal/v8/n8/fig_tab/nprot.2013.087_F9.html

By the way Mr. Ghajesh which procedure you followed and what are the PL spectra of Undiluted and diluted CdSe QDs? You have UV-visible spectra with your QDs?

Mr. Rabindra Kumar , I have been working with Cadmium sulfide and not cadmium selenide quantum dots. since i did not have selenium with me , i went ahead with Cadmium sulfide which needs only common lab chemicals. But the problem is that I need the absorption maxima to be in visible region. All the papers that I have read gives only Cadmium sulfide Qd whose abs max is around 380 nm but I want it to be around 532 nm for my next step of studies.

I have attached the paper that I followed.

Sorry, I do not understand anything about quantum dots. With organic molecules re-absorption is the most likely explanation. At high concentrations also bi-molecular effects (e.g. excimer-or exciplex formation) can occur. pH-effects could easily be studied by varying the acidity of the solution, but this is trivial. Could it be that aggregates are formed in higher concentrations?

kind regards

Walter Kloepffer

You may change sulfur precursor if possible. It may be hinder the result which you want. You will try to check purity of chemicals which you used for CdS QDs. Hope it will help you out.

I suspect is due to devesse of self andorption

There is no sufficient information. Kubelka Munk sieve effect or wavelenght dependent scattering or self absorption all depend on heterogeneity and concentration. Fluorescence intensity should be calibrated in quanta. All alternatives need different checks

Regarding Dr. Friedrich Menges' comment:

Yes, I agree. But such situation is for fluorescence emission (band) emerging upon excitation at very close wavelength (the absorption (excitation) and emission bands are overlapping). I'm experienced in chlorophyll and phycobilin (tetrapyrrols) fluorescence. They have (main) absorption maxima in blue (about 400 nm) and red (about 620-690 nm). So, the common procedure is to record fluorescence emission spectra in the red range (620-750 nm) upon excitation in blue (at about 400 nm). In such a case, absorption at blue wavelength itself doesn't affect positions of the fluorescence emission recorded in red region, i.e., not the reason for shift of the emission maxima.

Hi Ghajesh,

First you cannot get CdS absorption at 532 nm since that is lower than the bulk band gap. You need to change the material to CdSe or a mixture of CdSeS.

The change in emission needs to be understood more quantitatively upon dilution. Its quite possible that at very low concentrations the organic ligands that passivate the surface get ripped off. If you could tell the nature of the shift and its reversibility, it might be possible to interpret the data.