The important properties of polymers and nanocrystals both depend upon the precise control of three-dimensional structure. Living polymerization has transformed polymer chemistryproviding absolute control over molecular weights, yielding monodisperse chains, and enabling the production of copolymers with specifically tailored properties. Despite the apparent analogies between polymerization of organic monomers and nanocrystal growth from inorganic monomers, living growth approaches to nanocrystals have been slower to develop. Living nanocrystal growth methods promise to provide exquisite control over core size, size dispersion, doped composition, and core/shell structure. As a result, they have the potential to advance the development of predictive structure/property relationships and afford a finer level of structural control during nanomaterial synthesis. In this perspective, we outline the essential attributes of living nanocrystal syntheses and discuss prerequisites required to discover and develop reactions with these types of mechanisms. Examples from the literature are reviewed that share some attributes of living growth methods (e.g., seeded growth methods) in an attempt to identify existing approaches that might meet the living growth prerequisites. We describe recent findings from our laboratory on metal oxide nanocrystal synthesis that exhibit all the key attributes of living growth. We demonstrate the potential of this method for enhanced structural and compositional control in nanocrystal growth through examples involving efficient dopant incorporation into a metal oxide framework, precise control of the radial distribution of dopant atoms, and the production of core/shell metal oxide nanocrystals. Finally, we outline exciting future prospects for discovery and development of living growth systems and point out important research avenues critical for development of the field.

Most studies on iron oxide nanocrystals (NCs) suggest that the magnetic properties depend strongly on size for diameters below 10 nm, but there is less agreement about how the structure of the NC surface influences magnetic properties. Because the magnetic properties of iron oxide NCs hold promise for applications from cancer detection and therapeutics to environmental remediation, it is imperative to understand how size influences those properties. In most cases, the effective magnetic size is significantly lower than the measured physical size, a finding attributed to spin canting or disorder at the NC surface. A complicating factor is that the reaction conditions used to produce samples influence their magnetic properties. Thus, we employed a continuous growth method involving layer-by-layer addition of precursor to produce single-crystalline, spherical cores with subnanometer precision over a range of sizes under the same reaction conditions. Analysis of the NCs by small-angle X-ray scattering, transmission electron microscopy, and powder X-ray diffraction showed that the NCs possess the spinel structure (primarily maghemite) and are crystalline, defect-free, and uniform in size. The saturation magnetization values for a series of eight distinct diameters between 4 and 10 nm increase smoothly with increasing size, from 55 to 78 Am 2 /kg. Magnetic sizes of the NCs determined by fitting magnetization curves to the Langevin function are nearly identical to the physical sizes, suggesting low levels of strain-producing defects and a very thin nonmagnetic surface layer on the NCs. The results suggest that syntheses that permit slower growth at reduced temperatures through a single reaction mechanism can enhance, and offer fine control over, magnetic properties.

Iron oxide nanocrystals have the potential for use in a wide variety of applications if we can finely control and tune the diverse structural attributes that lead to specific, desired properties. At the high temperatures utilized for thermal decomposition based syntheses, commonly used Fe(III) alkylcarboxylate precursors are inadvertently reduced and produce wustite (FeO), which is paramagnetic, as opposed to the desired ferrimagnetic spinel phases of magnetite (Fe 3 O 4 ) and maghemite (γ-Fe 2 O 3 ). To circumvent this issue, we carried out syntheses at lower temperatures (∼230 °C) using an esterification-mediated approach. Under these conditions, formation of the FeO phase can be avoided. However, we found that the precursor oxidation state and ligation had a surprisingly strong influence on the morphologies of the resulting nanocrystals. To investigate the cause of these morphological effects, we carried out analogous nanocrystal syntheses with a series of precursors. The use of Fe(III) oleate precursors yielded highly crystalline, largely twin-free nanocrystals; however, small amounts of acetylacetonate ligation yielded nanocrystals with morphologies characteristic of twin defects. During synthesis at 230 °C, the Fe(III) oleate precursor is partially reduced, providing sufficient quantities of Fe(II) that are needed to grow the Fe 3 O 4 nanocrystals (wherein one-third of the iron atoms are in the Fe(II) state) without twinning. Our investigations suggest that the acetylacetonate ligands prevent reduction of Fe(III) to Fe(II), leading to twinned structures during synthesis. Harnessing this insight, we identified conditions to predictably and continuously grow octahedral, spinel nanocrystals as well as conditions to synthesize highly twinned nanocrystals. These findings also help explain observations in the thermal decomposition synthesis literature which suggest that iron oxide nanocrystals produced from Fe(acac) 3 are less prone to FeO contamination in comparison to those produced from Fe(III) alkylcarboxylates.

The flux of molecular or atomic species to the crystal surface has a significant influence on its growth. For nanocrystals, little is known about the influence of the growth processes on morphology despite the fact that their properties depend critically on atomic or nanoscale variations in composition and structure. A continuous growth method was used to carefully control the addition of a precursor to examine the influence of flux on the growth of indium oxide nanocrystals. Detailed analysis of the morphologies by high-resolution transmission electron microscopy showed that growth occurs by monomer addition to nanocrystal surfaces and not through particle coalescence events. High flux causes the growth of single-crystal, branched morphologies, whereas low flux results in uniform, faceted morphologies. As the reaction temperature increases, the growth process becomes less sensitive to the flux; at higher temperatures, branched growth does not occur until significantly higher fluxes are reached. A model is proposed that explains how surface diffusion of reactive species plays a key role in determining the growth and morphology of nanocrystals. At high fluxes, these species are more prone to nucleate new islands on the surface of the growing nanocrystals, leading to single-crystal, yet branched, morphologies. At lower fluxes, surface diffusion is efficient, resulting in growth at step edges that produce faceted nanocrystals. The studies suggest that flux is an important consideration in commonly employed nanocrystal synthesis methods. The proposed model can be used to establish conditions for controlling crystalline quality and morphology of nanocrystals in future syntheses.

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