Despite the availability of elaborate varieties of nanoparticles, their assembly into regular superstructures and photonic materials remains challenging. Here we show how flexible films of stacked polymer nanoparticles can be directly assembled in a roll-to-roll process using a bending-induced oscillatory shear technique. For sub-micron spherical nanoparticles, this gives elastomeric photonic crystals termed polymer opals showing extremely strong tunable structural colour. With oscillatory strain amplitudes of 300%, crystallization initiates at the wall and develops quickly across the bulk within only five oscillations. The resulting structure of random hexagonal close-packed layers is improved by shearing bidirectionally, alternating between two in-plane directions. Our theoretical framework indicates how the reduction in shear viscosity with increasing order of each layer accounts for these results, even when diffusion is totally absent. This general principle of shear ordering in viscoelastic media opens the way to manufacturable photonic materials, and forms a generic tool for ordering nanoparticles.
Finlayson, C. E., Spahn, P., Snoswell, D. R. E., Yates, G., Kontogeorgos, A., Haines, A. I., Hellmann, G. P., Baumberg, J. J. (2011). 3D Bulk Ordering in Macroscopic Solid Opaline Films by Edge-Induced Rotational Shearing. Advanced Materials, 23 (13), pp. 1540-1544.A breakthrough in the field of large area photonic structures is reported, based on permanent ordering of solid polymeric films of sub-micrometer spheres by edge rotational-shearing. The resulting high-quality polymer opal thin-films exhibit strikingly intense structural color, as confirmed by combining a number of spectroscopic approaches. This induced self-assembly on macroscopic length scales represents a step-change away from current surface lithographies, presenting new routes for assembling solid ordered photonic materials. Despite previous reports of shear-ordering in sedimentary colloids in solution, no precedents exist for the application of such techniques to these granular solvent-free systems, which allow formation of permanent composite structures in the solid-state.Peer reviewe
Electrically tuned photonic crystals are produced by applying fields across shear-assembled elastomeric polymer opal thin films. At increasing voltages, the polymer opal films stretch biaxially under Maxwell stress, deforming the nanostructure and producing marked color changes. This quadratic electro-optic tuning of the photonic bandgap is repeatable over many cycles, switches within 100 ms, and bridges the gap between electro-active materials and photonic crystals.
Hyperspectral goniometry reveals anisotropic scattering which dominates the visual appearance of self-assembled polymer opals. The technique allows reconstruction of the reciprocal-space of nanostructures, and indicates that chain defects formed during shear-ordering are responsible for the anisotropy in these samples. Enhanced scattering with improving order is shown to arise from increased effective refractive index contrast, while broadband background scatter is suppressed by absorptive dopants.
We report how the strength of resonant Bragg reflection from polymeric photonic crystals (polymer opals) varies linearly with the refractive-index contrast, ?n, in contrast to the quadratic buildup of Fresnel reflections scaling as (?n)2. This occurs due to the interplay of disorder and periodicity, in agreement with a simple 1-dimensional periodic model. Goniometry experiments show that opal films exhibit ?cones? of resonantly scattered light, which extend to ?20? angular deviation from the specular direction. The intensity of the scattering cones varies super-linearly with ?n. Such medium contrast photonic crystals are of significant interest for understanding structural colors exhibited in nature, by structures with inherent disorder.Peer reviewe
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