For a research project, I am trying to synthesize lead (II) sulfide quantum dots using the method of Li et al.. I am injecting sulfur dissolved in 1-octadecene into a mixture of $\ce{PbO}$, oleic acid, oleylamine, and 1-octadecene at $210^{\circ}\ce{C}$, waiting 5 minutes, and quenching the reaction in an ice bath. The oleic acid : $\ce{PbO}$ molar ratio is 10 : 1 which, I think (roughly extrapolating from the data presented in the paper), should result in uniform sized quantum dots with absorption peak of about 900 nm. As described in the paper, the solution I'm working with turns black almost instantly, suggesting nanocrystal nucleation.

The problem is, when I characterize them with UV-Vis-NIR (after precipitation, centrifugation, and dispersion in hexane), instead of getting a clear absorption peak, I get a weird wavelength-vs-absorbance graph that shoots up abruptly at 300 nm and then gradually curves downward, with no indication of a peak anywhere near 900 nm. Why is this, and what are possible solutions?

I think it might have something to do with how I'm extrapolating the trend for oleic acid : $\ce{PbO}$ ratio, or perhaps how I'm precipitating/centrifuging/redissolving the QDs (though I'm not sure how this would affect the size or spectra). Or maybe some other factor I haven't thought of that affects the nucleation rate. Any ideas?

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    $\begingroup$ Could you include an image of the spectrum? Perhaps give us more details about your UV-Vis setup? What kind of cuvette are you using? $\endgroup$ – buckminst Jun 17 '15 at 17:01

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