Structural Characterization of Self-Assembled Multifunctional Binary Nanoparticle Superlattices

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Abstract
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 superlattices1 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.