Colloids and Surfaces A: Physicochem. Eng. Aspects 401 (2012) 38–47 Contents lists available at SciVerse ScienceDirect Colloids and Surfaces A: Physicochemical and Engineering Aspects jou rnal h om epa ge: www.elsevier.com/locate/colsurfa Structure and physical properties of colloidal crystals made of silica particles Sabine Portal-Marco a,∗ , M a Àngels Vallvé b , Oriol Arteaga a , Jordi Ignés-Mullol b , Carles Corbella a , Enric Bertran a a FEMAN Group, IN 2 UB, University of Barcelona, Martí Franquès, 1, E-08028 Barcelona, Spain b SOC & SAM Group, IN 2 UB, University of Barcelona, Martí Franquès, 1, E-08028 Barcelona, Spain a r t i c l e i n f o Article history: Received 24 December 2011 Received in revised form 6 March 2012 Accepted 7 March 2012 Available online 16 March 2012 Keywords: Colloidal crystals Size distribution 2D crystal structure Photonic band gap Optical anisotropy Wettability a b s t r a c t The relationship between the particles size distribution of colloidal crystals and the structure and physical properties of the resulting 2D colloidal crystals is presented in this work. The colloids were constituted of silica sub-micron spheres with different size distributions comprised between 150 and 520 nm, synthe- sized by the Stöber method and assembled in one monolayer through the Langmuir–Blodgett technique. Optical and wetting properties of the resulting crystals were studied by different techniques – UV–visible spectrometry, spectroscopic ellipsometry and contact angle measurements. They were related to the crys- tal structure which was analyzed by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The conservation of orientational and translational orders over a large area is found for crys- tals with narrow particle size distribution. Optical interference bands in the transmittance spectra are linked to the crystal periodicity and the presence of optical anisotropy is attributed to distortions in the film generated by the deposition process. Moreover, large particle size distribution results in the most hydrophobic crystals. Thus the physical properties of the colloidal crystals can be tailored or tuned by controlling the particle size distribution and the deposition parameters. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Photonic crystals (PhCs), which are defined by a refractive index periodicity in the sub-micrometer to micrometer length scale, have been broadly studied and developed in the last few years [1–3] and a peak in the number of publications was reached in 2008. Because of the easiness of production and the presence of either incomplete or full photonic gap, PhCs with a periodicity in 1 [4] or 3 dimen- sions [5,6] are the most common ones. Nevertheless 2D PhCs [7] encounter also a growing interest because they represent an inter- mediary state between 1 and 3D PhCs from a theoretical point of view. In addition, the so-called 2D PhCs are intrinsically related to the concept of “surface” and therefore, this topic deals with optical phenomena, but also with all kind of interactions as those treated in surface science in which the crystalline order plays a specific role, such as tribology (friction and wear), contact angle, surface chem- istry and filtering. Also, we can find 2D PhCs in nature with different optical effects – iridescent Morphos butterfly wings [8], feathers [9], and beetles – and different purposes (defense, camouflage, mime- sis, communication, alert, sun protection, etc.). They are imitated in nature-inspired devices [10] or biomimetic tools like structural ∗ Corresponding author. Present address: Department of Photonics and Tera- hertztechnology, Ruhr-University Bochum, 44780 Bochum, Germany. E-mail addresses: sabineportal@hotmail.com, Sabine.Portal@rub.de (S. Portal-Marco). colour inks [11], vapour sensors [12], self-cleaning surface such as lotus leaves [13] and anisotropic tribological coatings [14]. Artificial 2D PhCs are usually made by lithography [15] and self-assembly methods [16] and are used in many applications like lenses [17], large displays [18], 2D grating [19], polarizers [20], polarimeters [21], in photovoltaics [22] and as support for biosensors [23,24]. Ordered holes in high refractive index materials [15], colloidal crystals [25,26] also called crystal monolayers [27] or Self Assembled Monolayers (SAMs), and 2D colloidal gratings [28] constitute the various categories of PhC slabs, which consist in structures with 2D periodicity but with finite thickness [29]. In particular, SAMs made of particles with a narrow size distri- bution are fabricated by different techniques: Langmuir–Blodgett technique [30–32], dip-coating [33], vertical deposition [34,35] and spin-coating [27,28,36]. SAMs are characterized by their degree of structural arrangement which can be scaled from disordered to highly ordered one, from non-close-packed (NCP) [37] to ran- dom hexagonal close-packed one (RHCP) (mixture of fcc and hcp) [38] and finally from hexagonal non-close packed (HNCP) [39] to hexagonal close packed one (HCP). In this work, we studied the structural effects of 2D crystals of silica particles on their physical properties. First, the influence of the particles size distribution on the final arrangement was stud- ied. Mainly, three particle size distributions were synthesized and consisted of batches of bimodal polydisperse particles, multimodal polydisperse particles and unimodal monodisperse particles. Then samples representative of the three different batches were selected 0927-7757/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2012.03.007