PL emission peak of CdS QDs are usually below 550 nm...What are the ligands that you have used? what is the QY? What is the first maximum in the absorbance spectra?

If your CdS nanoparticles are uncapped then it is showing surface related defect PL emission. As the particles are uncapped it is surface have much defects which create intermediate energy states in between valence band and conduction band. So, energy gap is reduced or conversely wavelength will increase (red shifted PL emission). Hence you will get emission around 550-800 nm

I did not do the absorbance test. My Qds is grown in polymer and are capped.

Can you tell me the particle size of your synthesized CdS particles and absoption maxima in UV-Visible spectrum??

particle size is less than 50nm, I did not do UV-Vis

Precisely tell me the range.. ie in between 40-50nm or 10- 40 nm or something like that ???

If it is in the range of 20-30 nm then it is very much obvious.. as your particle size is very big hence you are getting red shifted PL emission . with increase in particles size also the energy band gap of semiconductor NPs decreases hence the show corresponding red emission. Go through some standard papers of '' tunning of PL emission with change in particle size of CdS nanoparticles''

The red shift spoken by Manish is just the fact that the material is behaving more like the bulk material which should be only a few tens of nm in wavelength. The shift you are observing suggests 2 things. The cap and/or the polymer reacts with the nanoparticules and that will modify the composition of the nanoparticules. The polymer you are using is straining the particule and thus changes the bandgap.

Dear Philippe, Can you give me some references? if it is possible send me via e-mail : [email protected]

I don't know the literature for CDS particles but look at http://arxiv.org/pdf/1010.1555.pdf

and http://arxiv.org/pdf/1010.1553.pdf where authors show the effect of strain on Germanium nanowires