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«Nanoparticle-tuned structural color from polymer opals Otto L. J. Pursiainen1, Jeremy J. Baumberg1*, Holger Winkler2, Benjamin Viel3, Peter ...»

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Nanoparticle-tuned structural color from

polymer opals

Otto L. J. Pursiainen1, Jeremy J. Baumberg1*, Holger Winkler2, Benjamin Viel3,

Peter Spahn3,and Tilmann Ruhl3

School of Physics and Astronomy, University of Southampton, Highfield, Southampton, SO17 1BJ, United Kingdom

Merck KGaA, Frankfurter Strasse 250, D-64291 Darmstadt, Germany

Deutsches Kunststoff-Institut (DKI), Schlossgartenstrasse 6, D-64289 Darmstadt, Germany


Abstract: The production of high-quality low-defect single-domain flexible polymer opals which possess fundamental photonic bandgaps tuneable across the visible and near-infrared regions is demonstrated in an industrially-scalable process. Incorporating sub-50nm nanoparticles into the interstices of the fcc lattice dramatically changes the perceived color without affecting the lattice quality. Contrary to iridescence based on Bragg diffraction, color generation arises through spectrally-resonant scattering inside the 3D photonic crystal. Viewing angles widen beyond 40º removing the strong dependence of the perceived color on the position of light sources, greatly enhancing the color appearance. This opens up a range of decorative, sensing, security and photonic applications, and suggests an origin for structural colors in Nature.

©2007 Optical Society of America OCIS codes: (999.9999) Photonic crystals, (330.1690) Color, (160.4760) Optical properties, (290.5850) Scattering, particles, (160.5470) Polymers References and links

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1. Introduction Structural color in nature arises mostly from optical interference of multiple light paths reflected inside periodically-textured materials. Natural opals consisting of stacked silica spheres acquire their characteristic iridescence due to Bragg interference from the lattice planes [1-6]. Similarly butterfly wings can change their color with viewing angle due to the grating-like fine structure on their surfaces [6-9].

Photonic crystals have attracted great interest since initial proposals suggested the inhibition of light emission in periodic structures [10-11]. Formation of complete photonic band gaps requires large refractive index contrast between the different components. Recently, there has been great interest in fabricating artificial opals or inverse opal structures [12-16] as their structural color can replace toxic and carcinogenic dyes. However, the problem with these structures is their poor lattice quality due to cracks and poly-crystallinity, and their rigidity, which limits practical structural color applications.

Here we present here an alternative but industrially-scalable approach that produces low contrast flexible opals from polymers for structural color applications. In contrast to previous methods for producing flexible opals [17-19], large-scale structural assembly is directed by the shear-flow alignment of core-shell nano-spheres in a polymer melt. This process additionally allows the localized doping of these opals by incorporation of nanoparticles, which is difficult to achieve in other fabrication procedures. Such nanoparticle doping allows the color hue and directionality to be tailored, despite the low relative refractive index contrast between the polymeric components. These crystals exhibit color features not explained by any conventional Bragg diffraction scheme, which is the normal mechanism invoked for the structural colors of butterfly wings, iridescent beetles or natural opals. Instead, they rely on a #82645 - $15.00 USD Received 1 May 2007; revised 4 Jul 2007; accepted 9 Jul 2007; published 17 Jul 2007 (C) 2007 OSA 23 July 2007 / Vol. 15, No. 15 / OPTICS EXPRESS 9554 new color generation phenomenon arising from resonant scattering events that take place inside the structured environment of the photonic crystal.

Our ability to compression mould these flexible films, and to create emissive quantumdot-doped polymer opals, anticipates their use in a range of decorative, sensing, security and photonic applications. Our results also suggest a re-examination of structural color in nature which we believe may exploit the same phenomena.

2. Opal fabrication Our fabrication technique exploits polymer nano-sphere interactions in the viscous melt when lateral shear forces are applied, either by transverse pressure, or through extrusion [20]. These sub-micron particle precursors have a core-shell architecture consisting of a hard crosslinked polystyrene (PS) sphere coated with a thin polymethyl-methacrylate (PMMA) inter-layer which anchors the outer shell composed of soft polyethylacrylate (PEA). During shear flow [Fig. 1(a)], these core particles self-assemble into an fcc-lattice with the soft PEA shell filling the interstitial voids [20]. The fundamental optical response arising from the Bragg reflections (or incomplete photonic bandgaps) can be directly tuned across the visible and near-infrared spectral regions by varying the precursor nano-sphere size (around 200-350nm), and hence the resulting lattice parameter. The thickness of these thin opals can be straightforwardly varied between 100-300 µm. One of the most attractive features is the flexibility and the malleability of these structures, with color highlights produced by the bending or stretching modification of the (111) plane spacing [21].

–  –  –

Fig. 1. (a). Compression shear-assembly of polymer opal. (b-c) Optical images under natural lighting of 10cm diameter polymer opal films, which are (b) undoped (S3) and (c) doped (S4) with 0.05% by weight carbon nanoparticles.

Critical for the work here, is the ability to introduce additional sub-50nm nanoparticle dopants into the interstices of the photonic crystal lattice without disrupting the lattice quality.

This is achieved by introducing the nanoparticle dopants into the precursor mix resulting in their homogeneous distribution within the interstitial PEA voids. A detailed description of the manufacturing can be found in Ref. [20]. The nanoparticle dopant we concentrate on in the present paper is carbon, however we have also successfully incorporated gold and hematite nanoparticles as well as CdSe colloidal quantum dots to produce emissive films. Results here are taken on 15cm-diameter polymer opal sheets produced by polymer melt compression, which we have shown previously are representative of the process, and can be well controlled.

Samples studied in this work vary in precursor sphere size and different carbon black nanoparticle concentration (Table 1).

The key result we concentrate on here is that by introducing only 0.05% (by weight) of carbon nanoparticles into the lattice structure, the visual appearance of the elastomeric opals changes quite remarkably from milky white to intense green [Figs. 1(b)-1(c)]. These pictures are taken under natural lighting conditions and show the weak and highly-orientationdependent Bragg diffraction gives way to a new color mechanism which is relatively insensitive to viewing angle. Detailed measurements of the angular optical response below strongly indicate that this effect is due to resonant scattering inside the photonically-structured environment and is critically dependent on the use of low-, not high-, refractive-index-contrast photonic crystals.

–  –  –

We confirm that the introduction of the nanoparticle doping at such low concentrations does not affect the structural quality of the opaline films, using a number of techniques. A large number of cross-sections examined by transmission electron microscopy (TEM) in every case show excellent lattice ordering in the hundreds of layers that light penetrates from the top of the films. Both with and without doping, this lattice uniformity is preserved [Figs. 2(a)b)], with up to 30% loading of each interstice in the lattice with a carbon nanoparticle. Note that shear in the TEM images is produced in the microtoming of the relatively soft films. This excellent surface-ordering quality can also be verified from the six-fold surface diffraction pattern observed across the entire sample [Fig. 2(b) inset], taken with a 244nm UV laser normally incident on sample S2 with the diffracted UV light observed on a fluorescent white card. The penetration of UV light is on the order of a few hundred nanometers in these polymers (due to UV absorption in the polymer mix) and hence only surface ordering can be probed. Further confirmation of the single domain nature of the opal fabrication is seen [Fig.

2(c)] when comparing the Bragg diffraction from the poly-crystals of typical high-quality sedimented opal (left) with that from the mono-domain composing the opaline film (right, note that the gradation in color is because the flexible opal is intentionally bent).

These structural colors can be readily incorporated into applications, for instance in compression moulding onto an automobile model [Fig. 2(d)] producing highlighted color changes on the edges. We have produced 100m lengths of such extruded polymer opals showing the capability for industrial scalability.

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