Millimeter-Scale Assembly of CdSe Nanorods into Smectic Superstructures by Solvent Drying Kinetics

Molecular and materials self-assembly are important fundamental principles and strategies for nanofabrication and nanotechnology in general. Hierarchical organization of molecules, polymers, and particles can be induced by various interactions, such as hydrogen bonding and ionic interactions. Furthermore, capillary, electric, magnetic, and entropic forces can influence the assembly of solid-state materials over a range of length scales. Recently, colloidal nanocrystals have emerged as building blocks in nanotechnology research, particularly because of their unique sizeand shape-dependent physical properties. Significant progress in synthesis procedures has made it possible to prepare high-quality samples of controlled composition, size, and shape. Owing to their favorable optical and electronic properties, semiconducting nanocrystals are used in many applications, including devices (e.g., light emitting diodes, memory, and solar cells) and biomedical labeling. Large-scale ordered structures consisting of nanocrystals can have unique properties with broad-ranging applications, and many results on nanocrystal self-assembly have been reported. In the case of spherical nanocrystals large-area close-packed arrays in two and three dimensions have been produced and binary-superlattices composed of two nanocrystal types have recently been demonstrated. Ordered self-assembly of anisotropic nanocrystals, such as nanorods (NRs), is more complicated than that of spheres because of their reduced shape symmetry, though significant progress has been made. Isotropic and nematic liquidcrystalline phases have been observed by birefringence imaging of CdSe NRs in solution after they have been allowed to phase-segregate over days. In the solid state, CdSe NRs