Emmy Noether-Program

Programmable self-assembly of functional 2D/3D nanoparticle lattices
© André Gröschel

The controlled self-assembly of nanoparticle lattices in the range of 50-500 nm remains challenging. There is a lack of nano-sized building blocks with low size-dispersity and high resolution of directional interaction patterns for the long-range ordering of nanoparticle lattices. This Emmy Noether project seeks to bring together concepts of block copolymer synthesis, supramolecular self-assembly, and supracolloidal cluster formation to create open nanoparticle lattices with well-defined symmetry. The ultimate goal is to self-assemble a diamond nanoparticle lattice with promising photonic properties.
To approach these aims, we first synthesize ABC triblock terpolymers with suitable compositions, which directly translate into the number of surface beads or patches. These patches are incompatible with the core and arrange in a geometric order. This order controls directionality of aggregation, potentially leading to nanoparticle lattices with symmetries reflecting those of the patch geometry.

Self-assembly of ABC triblock terpolymer to patchy nanoparticles and crystallization into a diamond lattice.
© André Gröschel
  • Natural photonic crystal of about 5 cm length worth 30 000$ (Cairns, Australia)
    © André Gröschel

    Photonic Crystals

    Photonic crystals are found in a myriad of examples in nature (sea shells, bird feathers, Pollia fruit), where the most instructive example might be opals. Opals are natural gem stones of silica nanoparticles with diameter of 200-400 nm displaying bright and vivid play of colours (valuable specimens are highly transparent; see photo of a gem stone from Cairns, Australia). The alternating refractive index between air and silica causes modulation of visible light and reflection of specific colours. Since structural colours are a result of the architecture they never fade unlike e.g. photo-bleaching of organic dyes (physical colour vs. chemical colour). Block copolymers have recently demonstrated the ability to generate structural colours through ordered morphologies (usually lamella morphologies). In order to reach lattice parameters of 200-400 nm either very large molecular weights are required to fill the space or special voluminous polymer architectures (e.g. polymer brushes). To compensate the relatively narrow window of refractive indices of polymers (typically n = 1.3-1.7), inorganic materials may be mineralized in selected domains to enhance the optical contrast difference (brighter play of colours).

  • Photonic micelle crystals. Photograph flask with photonic crystals and two individual crystal types at varying angles. Fluorescence confocal microscopy image shows fcc lattice and cryo-TEM micelles in hexagonal packing
    © André Gröschel

    Photonic Liquids

    Block copolymer micelles with super-stretched corona and with very narrow size-distribution qualify as building blocks for tuneable and reversible formation of photonic fluids and photonic crystals. The block copolymer micelles first show slight bluish hues in concentrated gels (4wt.-%) with interparticle distances of around 150 nm as defined by electrosteric repulsion from the highly charged and stretched corona. Upon dilution (< 2wt.%), particle concentration is reduced and free-flowing fluids now consist of block copolymer micelles with an average interparticle distance of 150−300 nm accompanied by a colour change to green and red. The increased particle mobility allows for structural rearrangements and the formation of millimetre-sized free-floating photonic crystals with classical FCC lattice in the photonic fluid. We discuss the generic properties of photonic crystals with special emphasis on surprisingly narrow reflected wavelengths with full width at half-maximum as small as 1 nm. We expect this concept to open a facile way for self-assembled tuneable micellar photonic structures. This work was done in collaboration with the Molecular Materials group of the Aalto University (Finland) and the Bio-inspired Photonics group in Cambridge (UK).