Research Article Received: 7 August 2011 Revised: 17 December 2011 Accepted: 18 December 2011 Published online in Wiley Online Library: (wileyonlinelibrary.com) DOI 10.1002/pi.4206 PTFE – PMMA core – shell colloidal particles as building blocks for self-assembled opals: synthesis, properties and optical response Diego Antonioli, a Simone Deregibus, a Guido Panzarasa, a Katia Sparnacci, a Michele Laus, a* Luca Berti, b Luca Frezza, b Matteo Gambini, b Luca Boarino, c Emanuele Enrico c and Davide Comoretto b* Abstract A seeded surfactant-free methyl methacrylate emulsion polymerization is employed to prepare core–shell particles with predetermined size and narrow size distribution. The particle size is determined by the ratio between methyl methacrylate (MMA, shell) and polytetrafluoroethylene (PTFE, core). Monodisperse particles in the 100–350 nm range are obtained. These particles are used to grow colloidal photonic crystals (opals) of very high optical quality, thus indirectly demonstrating the excellent control of microbead size distribution achieved by this preparation technique. The optical properties of the opals are investigated by means of reflectance and polarized-angle-resolved transmittance spectroscopies. These data provide a rough determination of the effective refractive index of the system, which is favorably compared with values obtained by simple effective medium models. c  2012 Society of Chemical Industry Supporting information may be found in the online version of this article. Keywords: polytetrafluoroethylene(PTFE); poly(methyl methacrylate) (PMMA); core – shell colloidal particles; photonic crystals; opals INTRODUCTION Nanostructured materials featuring a periodical modulation of the dielectric constant on the wavelength scale of visible light (photonic crystals, PhCs) are currently extensively investigated as light semiconductors. 1 A wide range of different applications can be recognized in photonics such as for example low threshold laser action, high bending angle waveguides, the superprism effect, sensors, optical switches and photovoltaic devices. 2–5 Among PhCs, a special position is held by those obtained by self-assembly of nanometer or submicrometer sized spheres. As natural opals consist of an ordered packing of monodisperse silica spheres, thus resulting in the appearance of the typical brilliant iridescence, these synthetic materials are also referred to as artificial opals. 6,7 When light interacts with such face-centered close-packed structures, strong diffraction effects from each sphere plane occur. Constructive and destructive interference of diffracted beams determine if light is allowed or forbidden respectively to propagate inside the structure. For photon energies leading to destructive interference (photonic band gap), light is backward diffracted (i.e. reflected), thus giving rise to the typical peak (Bragg peak) observed in the reflectance spectrum mainly responsible for the color of such structures. Opals do not present a complete photonic band gap, i.e. a gap for all crystallographic directions and light polarizations like inverse opals with suitable dielectric contrast, 8,9 but still retain a great interest for photonics since they are a simple and easily affordable system which can be successfully infiltrated with a variety of materials ranging from metal nanoparticles and quantum dots to fluorophores and organic semiconductors. 10–13 Indeed, applications of opals in different devices and technologies have been proposed despite their reduced dielectric contrast. 14–19 The spectral position of the photonic stop band and all other optical features are governed by the dielectric lattice spacing and by the refractive index of the composing materials. Consequently, it is possible to tune opal optical properties by using spheres featuring different diameters or refractive index. 20–23 To obtain opals of good optical quality, it is necessary to employ nanospheres 24 with dispersity lower than 5%. For this reason, the control of particle size distribution and composition is a critical issue in the field of colloidal PhCs and relative optical circuitry. 25 A diameter distribution slightly wider than 5% both increases the ∗ Correspondence to: Michele Laus, Dipartimento di Scienze dell’Ambiente e della Vita, Viale T. Michel 11, Universit` a del Piemonte Orientale ‘A. Avogadro’, 15121 Alessandria, Italy. E-mail: laus@mfn.unipmn.it Davide Comoretto, Dipartimento di Chimica e Chimica Industriale, Universit` a degli Studi di Genova, via Dodecaneso 31, I-16146 Genova, Italy. E-mail: davide.comoretto@unige.it a Dipartimento di Scienze dell’Ambiente e della Vita, Via G. Bellini 25 g, Universit` a del Piemonte Orientale ‘A. Avogadro’, INSTM, UdR Alessandria, 15100 Alessandria, Italy b Dipartimento di Chimica e Chimica Industriale, Universit` a degli Studi di Genova, via Dodecaneso 31, I-16146 Genova, Italy c NanoFacilityPiemonte,ElectromagnetismDivision,IstitutoNazionalediRicerca Metrologica, Strada delle Cacce 91, 10135 Torino, Italy Polym Int (2012) www.soci.org c  2012 Society of Chemical Industry