Quantitative evaluation of size selective precipitation of Mn-doped ZnS quantum dots by size distributions calculated from UV/Vis absorbance spectra

We demonstrate the quantitative evaluation of the sharp classification of manganese-doped zinc sulfide (ZnS:Mn) quantum dots by size selective precipitation. The particles were characterized by the direct conversion of absorbance spectra to particle size distributions (PSDs) and high-resolution transmission electron micrographs (HRTEM). Gradual addition of a poor solvent (2-propanol) to the aqueous colloid led to the flocculation of larger particles. Though the starting suspension after synthesis had an already narrow PSD between 1.5 and 3.2 nm, different particle size fractions were subsequently isolated by the careful adjustment of the good solvent/poor solvent ratio. Moreover, due to the fact that for the analysis of the classification results the size distributions were available, an in-depth understanding of the quality of the distinct classification steps could be achieved. From the PSDs of the feed, as well as the coarse and the fine fractions with their corresponding yields determined after each classification step, an optimum after the first addition of poor solvent was identified with a maximal separation sharpness κ as high as 0.75. Only by the quantitative evaluation of classification results leading to an in-depth understanding of the relevant driving forces, a future transfer of this lab scale post-processing to larger quantities will be possible.