Hierarchical self-assembly of suspended branched colloidal nanocrystals into superlattice structures

Abstract

Self-assembly of molecular units into complex and functional superstructures is ubiquitous in biology. The number of superstructures realized by self-assembly of man-made nanoscale units is also growing. However, assemblies of colloidal inorganic nanocrystals [1][2][3] are still at an elementary level, not only because of the simplicity of the shape of the nanocrystal building blocks and their interactions, but also because of the poor control over these parameters in the fabrication of more elaborate nanocrystals. Here, we show how monodisperse colloidal octapod-shaped nanocrystals self-assemble, in a suitable solution environment, on two sequential levels. First, linear chains of interlocked octapods are formed, and subsequently the chains spontaneously self-assemble into threedimensional superstructures. Remarkably, all the instructions for the hierarchical self-assembly are encoded in the octapod shape. The mechanical strength of these superstructures is improved by welding the constituent nanocrystals together.The organization of colloidal nanocrystals into ordered structures is a necessary step towards the fabrication of artificial solids and new devices. Superstructures can be built either by self-assembly directly in solution, or on a substrate following solvent evaporation or de-wetting [4][5][6] . A variety of forces can be involved in their formation: van der Waals (vdW) attractions between the particles, steric repulsions between the hydrophobic tails of the surfactants (often coating the nanocrystal surface), capillary forces during solvent evaporation, attractive depletion forces, Coulomb forces between surface charges or electric dipoles, and magnetic forces 1,3,5,[7][8][9][10][11][12] . The assembly of many ordered threedimensional (3D) superstructures, for example, simple, binary, or ternary assemblies of spherical nanoparticles [13][14][15][16][17] , and smectic-like multilayers of hexagonally packed nanorods 18 , as well as liquid crystalline phases, is found to be solely driven by entropy [19][20][21] . More elaborate assemblies could be achieved from such simple building blocks by encoding information for the self-assembly in the surface pattern of the nanoparticles, for instance by DNA functionalization to modify the strength and directionality of particle-particle interactions [22][23][24] . Furthermore, bifunctional linkers and key-lock molecular pairs have been employed to align nanorods in chain-like structures 25 . Directional electric and/or solvophobic interactions were further employed to drive the organization of spherical nanoparticles into lattices 26 . Finally, templating has been successfully applied to create hierarchical superstructures using principally spherical particles 27,28 limits to the quality and reproducibility of such assemblies and to their maximum attainable size.Branched nanocrystals such as tetrapod or octapod-shaped colloidal nanoparticles have recently emerged as promising materials for photovoltaics and electronics 27,28,30,31 , and questions have been rai...

Ordered Two-Dimensional Superstructures of Colloidal Octapod-Shaped Nanocrystals on Flat Substrates

“…32 Our group recently reported an experimental and simulation study of the hierarchical self-assembly of anisotropic branched colloidal nanocrystals, so-called octapods, into 3D superstructures in the liquid bulk phase. 20 In this Letter, we extend our findings to the formation of monolayers consisting of octapods, which were obtained by a deposition−evaporation procedure. In the experiments, we obtained monolayers in the micrometer size range in which the octapods with a pod lengthto-diameter ratio of L/D ≈ 4.8 arranged into a square-lattice crystal (SC).…”

Lyotropic Smectic B Phase Formed in Suspensions of Charged Colloidal Platelets

“…3−5 Subsequently, these studies were extended to colloids with anisotropic shapes, such as rods and plates, 6,7 and also to a variety of colloidal interactions. 8 Attraction (depletion, 9−12 van der Waals, 13,14 or Coulomb 13,15 ) and/or repulsion (steric 16 or Coulomb 17 ) were applied to govern the colloidal self-assembly process. Additionally, recognition mechanisms based on particles with complementary shapes 18 or on Watson−Crick attraction between DNA strands 19−22 have led to an extended control over the self-organization process.…”

Excited-State Dynamics in Colloidal Semiconductor Nanocrystals

“…Colloidal NCs can also selfassemble into three-dimensionally ordered colloidal superparticles [131]. The geometry and properties of these superstructures can be tailored by the size, shape, composition and surface chemistry of the NC or hetero-NC building blocks [129][130][131][132][133][134][135][136][137][138]. In particular, surface ligands have been shown to have a dramatic impact on the directionality of the self-organization process [135,[139][140][141][142][143], leading in some cases to atomically aligned NC superlattices [135,139,143].…”