Colloidal chemistry has revolutionized the synthesis of inorganic nanomaterials. The field has evolved tremendously, both in the fundamental understanding of nucleation, growth and surface chemistry of nanocrystals and in the ability to provide a toolset for the preparation of functional materials with precisely engineered size, shape, and structure. However, the current methodology has several limitations that need to be carefully explored and addressed.

The lack of atomic precision in the synthesis of functional nanomaterials restricts the ability to harness all the power of this broad and diverse class of materials. Real nanomaterials always vary in size to a certain extent. This variation introduces inhomogeneous broadening of the absorption and emission spectra and reduces charge carrier mobilities in nanocrystal films. The polydispersity originates from a weak size dependence of the free energy related to the addition or removal of individual atoms to/from a nanoscale object. In this case, size distribution can only be controlled by kinetic factors and it would be difficult to further improve the homogeneity of kinetically controlled reaction products. We discuss a paradigm-shifting approach for the colloidal synthesis of nanomaterials with minimal, ideally no, size distribution. The goal is to establish means to thermodynamically control nanomaterials synthesis using a sequence of two complementary self-limiting surface reactions. This concept is inspired by the success of gas-phase atomic layer deposition (ALD) widely used in microelectronics and other fields. Our studies show that the ALD concept can be implemented in solution and, when applied to colloidal nanomaterials, enables layer-by-layer growth of crystalline lattices with true atomic precision.

The other general limitation of traditional colloidal chemistry is related to the thermal stability of organic solvents at high temperatures required for some hard-to-crystallize materials. Very few organic substances remain liquid above 400°C, and solvent or ligands decomposition becomes a serious problem at even higher temperatures. The use of inorganic salts as solvents eliminates this issue and brings new opportunities. As an example, colloidal synthesis of GaAs nanocrystals was unsuccessful for over 20 years. We found that GaAs NCs synthesized in organic solvents generally have a high concentration of vacancies and antisite defects. Annealing of the colloid in a molten salt at 500°C eliminated these structural defects without NC sintering. We envision multiple exciting opportunities for colloidal chemistry in molten salts.