Synthesis and Characterization of Monodisperse Nanocrystals and Close-Packed Nanocrystal Assemblies

Abstract

Solution phase syntheses and size-selective separation methods to prepare semiconductor and metal nanocrystals, tunable in size from ∼1 to 20 nm and monodisperse to ≤5%, are presented. Preparation of monodisperse samples enables systematic characterization of the structural, electronic, and optical properties of materials as they evolve from molecular to bulk in the nanometer size range. Sample uniformity makes it possible to manipulate nanocrystals into close-packed, glassy, and ordered nanocrystal assemblies (superlattices, colloidal crystals, supercrystals). Rigorous structural characterization is critical to understanding the electronic and optical properties of both nanocrystals and their assemblies. At inter-particle separations 5-100Å, dipole-dipole interactions lead to energy transfer between neighboring nanocrystals, and electronic tunneling between proximal nanocrystals gives rise to dark and photoconductivity. At separations <5Å, exchange interactions cause otherwise insulating assemblies to become semiconducting, metallic, or superconducting depending on nanocrystal composition. Tailoring the size and composition of the nanocrystals and the length and electronic structure of the matrix may tune the properties of nanocrystal solid-state materials. MURRAY KAGAN BAWENDI the electronic and optical properties of metals (1-4) and semiconductors (5-9) strongly depend on crystallite size in the nanometer size regime. Efforts to explore structures on the nanometer length scale unite the frontiers of materials chemistry, physics, and engineering. It is in the design and characterization of advanced materials that the importance of new interdisciplinary studies may be realized. Uncovering and mapping size-dependent materials properties requires synthetic routes to prepare homologous size series of monodisperse nanometer size crystals, known as nanocrystals (NCs). NC samples must be monodisperse in terms of size, shape, internal structure, and surface chemistry. A diverse set of structural probes is combined to characterize and develop consistent structural models of NC samples. Optical, electrical, and magnetic studies of well-defined NC samples reveal the unique size-dependent properties of materials in this intermediate, nanometer size regime between molecular species and bulk solid. When atoms or molecules organize into condensed systems, new collective phenomena develop. Cooperative interactions produce the physical properties we recognize as characteristic of bulk materials. Like atoms or molecules, but in the next level of hierarchy, NCs may also be used as the building blocks of condensed matter. Routes enabling controlled manipulation of NCs into the glassy and ordered states of matter lead to the preparation of close-packed NC solids. Assembling NCs into solids opens up the possibilities of fabricating new solidstate materials and devices with novel physical properties, as interactions between proximal NCs give rise to new collective phenomena. Building upon rigorous understanding of the physical proper...

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