Before digging too far in this study, I looked for it in the literature and I could not find anything. The case is simple: the first exciton energy of my quantum dots in solution blue shifts when I decrease the concentration (of the measured sample). I am suspecting a type of solvatochromism, but I would be happy if someone could tell me if they already saw such a thing.
Does the width of the absorbance band (the HWHM on the longer wavelength side) change also with dilution? And how about the fluorescence peak energy and width of fluorescence emission band? Do these change with dilution?
Are the quantum dots capped? If there is surfactant/ligand with conjugated electronic system present in the system, this could shift the absorption maximum of the quantum dots. Diluting the quantum dot system like this could remove some of the capping from the quantum dots and shift the absorption back. I have seen this happen, although in my case, it was blue shift on dilution, because the ligand shifted the absorption of quantum dots red.
Hi Vladimir,
Yes we have the same phenomenon, I am sorry I meant blue shift not red shift. And yes it's related to the ligands. Would you have any recommendation of articles describing why ligands red shift the absorption? Especially when it is still in solution. I know that red shift is observed after being cast on a solid substrate. Thank you for your comment.
It has to do with electronic interaction. If your ligand couples electronically to the quantum dot, then electron transfer from quantum dot to the ligand (and vice versa) is possible and that will red shift the gap in both. Extension of the electronic system stabilizes LUMO more than it does HOMO. I haven't really looked for papers about this, it was just a peripheral thing in my project, so I can't really recommend any.
Hi Victor,
Look into papers by Emily Weiss. Here is a good place to start: http://sites.northwestern.edu/weiss-lab/quantum-dot-ligand-strong-coupling/
Thank you for this suggestion, this paper ends up being highly relevant and gives an extensive theoritical idea of the mechanisms in cause. Very appreciated.
Hi,
I believe it is due to the quantum confinement size effect: Decreasing the nanocrytal concentration means decreasing the number of particles in sol. Less number of particle in sol yields smaller size distribution of nanocrystals. The smaller the nano crystals, the larger the band gap due to the quantum size effect (and viceversa). The phenomena is related to the exciton radius, bohr radius and wave function of the electron.
Do you observe a continues shift in absorption peak by decreasing the concentration or only a sudden shift in a very low concentration?
How low are your low concentrated NP samples? Usually, nanoparticles loose their capping ligands in very low concentrations about 10^(-8) and their absorption spectra changes drastically.
Also, do you see the other way effect as well? I mean have you tried to spin down a low concentrated sample and redispersed it in a smaller volume of the solvent to see if you observe any red shift? If that is the case, it will be so interesting. I have some other idea to bring in then. :)
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