Abstract
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.
Highlights
Despite the availability of elaborate varieties of nanoparticles, their assembly into regular superstructures and photonic materials remains challenging
Various transient ordered structures such as strings, sliding layers and crystallites have been generated in colloidal suspensions[19], while crystals have been induced by oscillatory shear in colloidal glasses and granular systems near the jamming point[20,21], showing its potential for dense particulate assembly
Instead of blending PS spheres and PEA matrix, polymer opals (POs) are fabricated from PS-PMMA-PEA core-shell a Colloidal b
Summary
PO film fabrication and the mechanism of BIOS. Instead of blending PS spheres and PEA matrix (as they cannot be suitably mixed), POs are fabricated from PS-PMMA-PEA core-shell a Colloidal b. The only evidence of additional bandgaps is seen at high angles along the yy direction in U-BIOS samples[44,45], suggesting the presence of (200)-type planes from more threedimensional (3D)-ordered fcc crystals (Supplementary Fig. 7). Reconstructing cross-sections at different depths for different samples (Fig. 6b, Supplementary Figs 11–19, Supplementary Movie 4) and extracting the sphere positions allows the effective refractive index neff(z) distribution with depth to be calculated Both the main spatial frequency, which gives the average layer spacing over the area (F 1⁄4 dhÀkl1) and the refractive contrast Dneff (Fig. 6c,d, Supplementary Fig. 20) reveal details of the BIOS mechanism. Diffraction rings from the amorphous structures are seen, but these steadily organize into hexagonal patterns of diffraction spots with increasing BIOS passes, tracking the crystallization process and formation of hcp planes.
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