Colloidal semiconductor nanocrystals (NCs) are versatile nanomaterials, whose properties are determined by their size, shape, composition, and compositional profile (i.e., single component, gradient or homogeneous alloy, doped, heterostructured). Heterostructured semiconductor NCs (hetero-NCs) are particularly attractive, since they consist of two (or more) different materials joined together by heterointerfaces. This allows the spatial localization of photogenerated charge carriers to be manipulated by controlling the band offsets between the materials that are combined at the heterointerface, which has a dramatic impact on several properties (viz., quantum yields, stability, photoluminescence wavelength, reabsorption cross-sections, radiative lifetimes, exciton-phonon coupling strength, Auger recombination, etc.). These characteristics have turned colloidal semiconductor NCs and Hetero-NCs into promising materials for a myriad of applications, motivating extensive research into their synthesis.

In this talk, we will address a synthesis approach that has been attracting increasing attention in recent years, and has established itself as a resourceful route to size-, shape- and composition-tailored NCs and Hetero-NCs that would otherwise not be attainable: Cation Exchange. Very often the anionic sublattice is conserved, leading to topotactic reactions that allow the product nanocrystals to inherit the size, shape, crystal structure and anionic heteroarchitecture of the parent nanocrystals, thereby circumventing thermodynamic and kinetic barriers. In combination with controlled interdiffusion, cation exchange allows the properties of colloidal semiconductor hetero-NCs to be post-synthetically tailored under constant size, shape and total composition, by tuning the elemental distribution profile from a core/shell geometry with a sharp heterointerface to a homogenous alloy NC, via gradient alloy NCs of increasing homogeneity. To illustrate these strategies, we will discuss recent work by our group in which post-synthetic cation exchange and interdiffusion are exploited to achieve the targeted synthesis of NCs and hetero-NCs that cannot be obtained by direct synthesis protocols (e.g. PbSe/CdSe core/shell QDs and heterodimers; ultranarrow (Zn,Cd)Te/CdSe heteronanowires; (Zn,Cd)Se core/shell and alloy QDs, wurtzite CuInE₂ NCs, nanorods and multipods, CuInSe₂/CuInS₂ dot core/rod shell nanorods and concentric core/shell QDs, etc.).