Interplay between Auger and ionization processes in nanocrystal quantum dots

We study the interplay between Auger effects and ionization processes in the limit of strong electronic confinement in core/shell CdSe/ZnS semiconductor nanocrystal quantum dots. Spectrally resolved fluorescence decay measurements reveal a monotonic increase of the photoluminescence decay rate on excitation density. Our results suggest that Auger recombination accelerates ionization processes that lead to the occupation of dark, nonemissive nanocrystal states. A model is proposed in the quantized Auger regime describing these experimental observations and providing an estimate of the Auger assisted ionization rates.     

Room-temperature exciton storage in elongated semiconductor nanocrystals

The excited state of colloidal nanoheterostructures consisting of a spherical CdSe nanocrystal with an epitaxially attached CdS rod can be perturbed effectively by electric fields. Field-induced fluorescence quenching coincides with a conversion of the excited state species from the bright exciton to a metastable trapped state (dark exciton) characterized by a power-law luminescence decay. The conversion is reversible so that up to 10% of quenched excitons recombine radiatively post turn-off of a 1 micro s field pulse, increasing the delayed luminescence by a factor of 80. Excitons can be stored for up to 10(5) times the natural lifetime, opening up applications in optical memory elements.     

Colloidal synthesis and self-assembly of CoPt(3) nanocrystals

Reduction of platinum acetylacetonate and thermodecomposition of cobalt carbonyl in the presence of 1-adamantanecarboxylic acid were employed in different coordinating mixtures to produce monodisperse, highly crystalline CoPt(3) nanoparticles. The mean particle size can be varied from 1.5 to 7.2 nm by controlling the reaction conditions and the type of coordinating mixture. As-synthesized CoPt(3) particles represent single crystal domains and have chemically disordered face-centered cubic (fcc) structure. Nearly spherical CoPt(3) nanocrystals were found to assemble into two- (2D) and three-dimensional (3D) structures. An AB(5) type superlattice is observed by TEM after mixing two nanoparticle samples with different mean sizes. Slow precipitation led to the formation of facetted colloidal crystals with sizes up to 20 microm.     

Structural characterization of self-assembled multifunctional binary nanoparticle superlattices

Nanocrystals of different size and functionality (e.g., noble metals, semiconductors, oxides, magnetic alloys) can be induced to self-assemble into ordered binary superlattices (also known as opals or colloidal crystals), retaining the size tunable properties of their constituents. We have built a variety of binary superlattices from monodisperse PbS, PbSe, CoPt3, Fe2O3, Au, Ag, and Pd nanocrystals, mixing and matching these nanoscale building blocks to yield multifunctional nanocomposites (metamaterials). Superlattices with AB, AB2, AB3, AB4, AB5, AB6, and AB13 stoichiometry with cubic, hexagonal, tetragonal, and orthorhombic symmetries have been identified. Assemblies with the same stoichiometry can be produced in several polymorphous forms by tailoring the particle size and deposition conditions. We have identified arrays isostructural with NaCl, CuAu, AlB2, MgZn2, MgNi2, Cu3Au, Fe4C, CaCu5, CaB6, NaZn13, and cub-AB13 compounds emphasizing the parallels between nanoparticle assembly and atomic scale crystal growth and providing confidence that many more structures will follow. Recently, we have demonstrated that electrical charges on sterically stabilized nanoparticles in addition to such parameters as particle size ratio and their concentrations can provide the formation of a much broader pallet of binary nanoparticle superlattices as compared with the limited number of possible superlattices formed by hard noninteracting spheres. In this contribution, we demonstrate a large variety of different binary superlattices, provide their detailed structural characterization, and discuss the role of energetic and kinetic factors in the cocrystallization process. We found that Coulomb, van der Waals, charge-dipole, dipole-dipole, and other interactions can contribute equally to cocrystallization, allowing superlattice formation to be dependent on a number of tunable parameters. We present binary superlattices as a new class of materials with a potentially unlimited library of constituents over a wide range of tunable structures.     

Alkyl chains of surface ligands affect polytypism of cdse nanocrystals and play an important role in the synthesis of anisotropic nanoheterostructures

We show that the length of the alkyl chain of surface ligands can shift the equilibrium between the wurtzite and zinc blende polytypes of CdSe nanocrystals. In-situ wide-angle X-ray scattering measurements reveal that short-chain (e.g., propyl) phosphonic acids stabilize CdSe nanocrystals with the zinc blende phase whereas octadecylphosphonic acid stabilize nanocrystals with the wurtzite phase. We also demonstrate how this effect can be used to improve the shape selectivity in the synthesis of anisotropic CdSe/CdS and ZnSe/CdS nanoheterostructures.     

Nanocrystal superlattices with thermally degradable hybrid inorganic-organic capping ligands

Colloidal metallic and semiconductor nanocrystals (NCs) functionalized with metal chalcogenide complexes (MCCs) have shown a promise for designing materials that combine high carrier mobility with the electronic structure of strongly quantum-confined solids. Here we report a simple and general methodology for switching the repulsive forces responsible for colloidal stabilization of MCC-capped NCs from long-range electrostatic to short-range steric through the formation of tight ionic pairs with cationic surfactants. This noncovalent surface modification remarkably improved the ability of MCC-capped NCs to self-assemble into long-range ordered superlattices. These NCs are highly soluble in nonpolar solvents and compatible with various technologically relevant organic molecules and polymers. The hybrid inorganic-organic coating can be thermally decomposed at significantly lower temperatures compared to those required for removal of conventional organic ligands.     

Structural defects in periodic and quasicrystalline binary nanocrystal superlattices

Binary nanocrystal superlattices (BNSLs) emerge as an important class of man-made materials where components and functionalities can be added, tuned, or combined in a predictable manner. These amazingly complex structures spontaneously self-assemble from colloidal solutions containing binary mixtures of functional (semiconducting, magnetic, plasmonic, etc.) nanocrystals. Further developments of the BNSL-based materials require a deep understanding and control over BNSL formation and structural perfection. Like any solid, BNSL can contain different kinds of structural defects. It is well-known that defects can have a tremendous effect on the material's behavior. Defect engineering is used to modify and improve many of the mechanical, electrical, magnetic, and optical properties of conventional solids. In this work, we provide the first systematic analysis of structural defects in various BNSL structures. We used BNSLs as a platform for studying structural defects in both periodic (crystalline) and aperiodic (quasicrystalline) lattices, as well as for direct imaging of the interfaces between crystalline and quasicrystalline domains. Such direct observation of local imperfections in complex multicomponent lattices provides a unique insight into the fundamental aspects of crystal formation.