Synthesis and characterization of monodisperse nanocrystals and close-packed nanocrystal assemblies

Solution phase syntheses and size-selective separation methods to prepare s emiconductor and metal nanocrystals, tunable in size from similar to 1 to 2 0 nn acid monodisperse to less than or equal to 5% are presented. Preparati on of monodisperse samples enables systematic characterization of the struc tural, electronic, and optical properties of materials as they evolve from molecular to bulk in the nanometer size range. Sample uniformity makes it p ossible to manipulate nanocrystals into close-packed, glassy, and ordered n anocrystal assemblies (superlattices, colloidal crystals, supercrystals). R igorous structural characterization is critical to understanding the electr onic and optical properties of both nanocrystals and their assemblies. At i nter-particle separations 5-100 Angstrom, dipole-dipole interactions lead t o energy transfer between neighboring nanocrystals, and electronic tunnelin g between proximal nanocrystals gives rise to dark and photoconductivity. A t separations <5 Angstrom, exchange interactions cause otherwise insulating assemblies to become semiconducting, metallic, or superconducting dependin g on nanocrystal composition. Tailoring the size and composition of the nan ocrystals and the length and electronic structure of the matrix may tune th e properties of nanocrystal solid-state materials.