With continuing progress in the chemical synthesis of colloidal semiconductor nanocrystals (NC), one property that remains elusive to the rational design is the ensemble photoluminescence (PL) line width. Given the growing demand for NC-based light-emitting materials, substantial research effort has been dedicated to this issue. Here, we demonstrate a postsynthetic strategy that allows reducing emission line widths of CdSe and CdS NCs to near single-particle levels while enhancing the PL quantum yield. The key idea behind the synthetic approach lies in employing a nonclassical coalescence growth mechanism, which leads to size focusing irrespective of the initial sample morphology. Numerical simulations accurately predict the observed particle size evolution, confirming the ability of coalescence growth to promote size focusing of semiconductor colloids. Ultimately, we expect that the demonstrated coalescence growth strategy could enable a rational control of nanocrystal size distributions and corresponding spectral line widths in many types of semiconductor NCs.

Electrically coupled quantum dots (QDs) can support unique optoelectronic properties arising from the superposition of single-particle excited states. Experimental methods for integrating colloidal QDs within the same nano-object, however, have remained elusive to the rational design. Here, we demonstrate a chemical strategy that allows for the assembling of colloidal QDs into coupled composites, where proximal interactions give rise to unique optoelectronic behavior. The assembly method employing “adhesive” surfactants was used to fabricate both homogeneous (e.g., CdS–CdS, PbS–PbS, CdSe–CdSe) and heterogeneous (e.g., PbS–CdS, CdS–CdSe) nanoparticle assemblies, exhibiting quasi-one-dimensional exciton fine structure. In addition, tunable mixing of single-particle exciton states was achieved for dimer-like assemblies of CdSe/CdS core–shell nanocrystals. The nanoparticle assembly mechanism was explained within the viscoelastic interaction theory adapted for molten-surface colloids. We expect that the present work will provide the synthetic and theoretical foundation needed for building assemblies of many inorganic nanocrystals.