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Results ■ Scientific Highlights ■ Self-organization of nanoparticles on device-scale areas

Self-organization of nanoparticles on device-scale areas

Organisation of spherical nanoparticles in three-dimensional superstructures is well advanced and many beautiful examples exist for binary, ternary and quasicrystalline superlattices. The formation of superstructures of shape-controlled nanoparticles is still at a very elementary level. However, our Department has developed various assembly techniques for nanorods, which exploit for example depletion forces and the formation of liquid crystalline phases at high volume fractions of nanoparticles.

These approaches yield various types of ordered assemblies, such as multilayers of rods vertically aligned with respect to the substrate, or multilayers of nanorods that are preferentially laying with their long axis parallel to the substrate. In one approach, by tuning depletion attraction forces between hydrophobic colloidal nanorods of semiconductors, dispersed in an organic solvent, we showed that these could be assembled into 2D monolayers of close-packed hexagonally ordered arrays directly in solution. Once formed, these layers could be fished onto a substrate, and sheets of vertically standing rods were fabricated, with no additional external bias applied. Alternatively, the assemblies could be isolated and re-dispersed in polar solvents, yielding suspensions of micrometer-sized sheets which could be chemically treated directly in solution. Depletion attraction forces were also effective in the shape-selective separation of nanorods from binary mixtures of rods and spheres.

In another approach, we have reported the formation of multilayers of vertically aligned CdSe/CdS nanorods on a cm2 scale on a variety of substrates. We demonstrated that this assembly strategy is independent on the type of substrate, which is indicative of a pre-organization of the rods already in the solution phase. Moreover, we monitored how the degree of order varies with annealing temperature of the film, and we showed that the degree of order is progressively lost upon increasing the temperature, up to the melting of the nanocrystals.

The reported procedures have the potential to enable powerful and cost effective fabrication approaches to materials and devices based on self-organized anisotropic nanoparticles.

Relevant papers of the Nanochemistry Department on this highlight:

  1. D. Baranov et al., “Assembly of Colloidal Semiconductor Nanorods in Solution by Depletion Attraction”, Nano Lett., 2010, 10, 743-749
  2. M. Zanella et al., “Assembly of shape Controlled Nanocrystals by Depletion Attraction”, Chem. Comm., 2011, 47, 203-205
  3. M. Zanella et al., “Self-Assembled Multilayers of Vertically Aligned Semiconductor Nanorods on Device-Scale Areas”, Adv. Mater., 2011, 23(19), 2205-2209

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