Science 348, 1226–1230 (2015)

The possibility to tune the optical properties of colloidal nanocrystals by controlling their size is key for the versatile use of these emitters in optoelectronic applications. In standard protocols for the synthesis of metal chalcogenide nanocrystals, metal and chalcogen precursors convert into crystalline nuclei that grow until the desired size is reached; at this stage the reaction is interrupted, leaving part of the precursors unreacted. Mark Hendricks and colleagues now show that the dimensions of the nanocrystals can be controlled more efficiently by tuning the conversion reactivity of the chalcogen precursors and running the reaction to full conversion of the reagents. They synthesized thiourea precursors that react with lead oleate with reaction rates spanning more than three orders of magnitude, and showed that a high reactivity increases the amount of nuclei produced in solution; the higher the number of nuclei, the smaller the nanocrystals at complete conversion. The size and size dispersion are not affected by the overall volume of solvent, suggesting that this high-yield protocol can be effectively implemented on large scales.