Composition variation and formation of core/ shell heterostructures have allowed the extension and applicability of size-tunable optical properties of colloidal semiconductor nanocrystals. Anisotropic shapes such as nanorods (NRs) provide new and further refined properties but relatively simple and empowering approaches such as alloying and controlled heterostructure formation are much more challenging to apply to these materials. Here, we start from a well-defined CdS/CdSe barbell-shaped NR heterostructure and perform sequential cation exchanges first with Cu and then with Cd/Zn under relatively mild conditions to achieve alloying while preserving the structure. By using a mixture of Cd and Zn in the presence of a thiol, controllable amounts of alloying in both components could be achieved. Growth of a final ZnSe shell enhances the photoluminescence and allows the creation of a series of double-heterojunction NRs with emission spanning the visible spectral range.
Indium phosphide (InP) nanocrystals have emerged as a viable alternative to heavy metal-based colloidal quantum dots for optoelectronic applications. Traditionally, the presence of trace amounts of water during the synthesis of colloidal quantum dots is considered an undesired impurity because it prevents or slows down colloidal growth and alters the surface properties. Here, we report that fine-tuning the amount of trace water is the key for achieving sizefocused growth of monodisperse InP nanocrystals synthesized using aminophosphine precursors. Using solid-state and solution nuclear magnetic resonance, we investigated the role of trace amounts of water in surface oxidation and precursor conversion reactions. Molecular insights from UV−vis spectroscopy and NMR revealed a profound contrast between the growth rates of the nanocrystals upon the addition of water to the reaction system. We demonstrate that by addition of a specific amount of water, the reactivity of the phosphorous precursor can be tuned to enable a constant supply of monomer throughout the reaction. Under an optimal precursor conversion rate, a size-focused growth behavior that is rare for InP nanocrystals is observed, suggesting the presence of an artificial LaMer-like growth regime.
Anisotropic heterostructures of colloidal nanocrystals embed size-, shape-, and composition-dependent electronic structure within variable three-dimensional morphology, enabling intricate design of solution-processable materials with high performance and programmable functionality. The key to designing and synthesizing such complex materials lies in understanding the fundamental thermodynamic and kinetic factors that govern nanocrystal growth. In this review, nanorod heterostructures, the simplest of anisotropic nanocrystal heterostructures, are discussed with respect to their growth mechanisms. The effects of crystal structure, surface faceting/energies, lattice strain, ligand sterics, precursor reactivity, and reaction temperature on the growth of nanorod heterostructures through heteroepitaxy and cation exchange reactions are explored with currently known examples. Understanding the role of various thermodynamic and kinetic parameters enables the controlled synthesis of complex nanorod heterostructures that can exhibit unique tailored properties. Selected application prospects arising from such capabilities are then discussed.
Colloidal nanorod heterostructures of I–III–VI2 semiconductors have been synthesized in a solution starting from wurtzite-like CuGaS2 nanorods. Growth of CuInS2 or CuInSe2 on CuGaS2 nanorods results in interesting sawtooth structures with larger lattice strain leading to sharper, more pronounced teeth. A final inorganic shell of ZnSe or ZnS grown on the CuGaS2/CuInSe2 nanorod heterostructures enhances photoluminescence. Unusual brush-like structures arise with prolonged ZnSe growth. Time-resolved photoluminescence measurements on CuGaS2/CuInSe2/ZnS nanorod heterostructures reveal a lifetime approaching one microsecond, suggesting charge separation within the nanorods. The results shown here provide previously unknown nanorod heterostructures exhibiting interesting and potentially useful optical properties along with insights into heterostructure formation in colloidal I–III–VI2 nanorods.
Cu2-XS nanocrystals can serve as templates and intermediates in the synthesis of a wide range nanocrystals through seeded growth, cation exchange, and/or catalytic growth. This versatility can facilitate and accelerate...
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