Synthetic Metals 139 (2003) 667–670 The early stages of the self-assembly process of polystyrene beads for photonic applications P. Nozar a,∗ , C. Dionigi a , A. Migliori b , G. Calestani c , L. Cademartiri a,c a ISMN-Bologna Section, CNR, Via Gobetti 101, I-40129 Bologna, Italy b IMM-Bologna Section, CNR, Via Gobetti 101, I-40129 Bologna, Italy c Department of Chemistry, University of Parma, Viale delle Science, I-43100 Parma, Italy Abstract Results of a colloidal crystal growth by vertical deposition of nanometric polystyrene beads on a glass substrate under oscillatory shear are reported in this paper. This technique of deposition enabled us to study an early stage of the growth of films as a function of the bead concentration in the water suspension. The surprising analogy between single layer growth and the bulk colloidal crystal behavior has been found. Indeed, the sequence of transitions from low density disordered (LDD) phase to intermediate density ordered phase and then to high density ordered phase with increasing bead concentration has been observed. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Self-assembly process; Polystyrene beads; Photonic applications 1. Introduction In recent years, the systems composed of layers of self- assembled nanometric beads become important for differ- ent applications, e.g. as patterned arrays in soft lithographic techniques [1], microlenses [2] and as templates for inverse opal structures for sensors [3], catalysts and optical appli- cations [4]. Several different methods have been published for the preparation of templates in the form of films. Two of the most promising methods are a forced crystallization through physical confinement and hydrodynamic flow [5] and the vertical deposition from the suspension of beads by lowering the meniscus of the suspension [6]. It is the last mentioned method which exhibits important advantages over other methods as, e.g. the relative simplicity, low cost and rather good control of the thickness of the deposited film and its uniformity. The common disadvantage of all methods used up to now for film deposition is the presence of relatively high number of different structural defects as, e.g. vacancies, dislocations, stacking faults, grain boundaries and inclusions of grains with different orientations. For this reason we have combined the method of vertical deposition with the method of oscillatory shear [7], which has given ∗ Corresponding address. Tel.: +39-051-639-8503; fax: +39-051-639-8539. E-mail address: p.nozar@ism.bo.cnr.it (P. Nozar). good results for defect-free growth of bulk samples. The good quality of our films has enabled us to study the early stages of the growth of film layers. 2. Experimental The syntheses of charged polystyrene (PS) beads have been performed by emulsifier-free polymerisation of styrene in water using potassium persulfate (K 2 S 2 O 8 , 99% Aldrich, negatively charged beads) as initiators. A fresh batch of styrene has been prepared by removing the polymerisation inhibitor (tert-butyl alcohol) with an inhibitor remover col- umn under nitrogen atmosphere. Two hundred milliliters of freshly boiled deionised water has been mixed with 6 ml of styrene (Aldrich, 99%) in a 500 ml three neck round-bottom flask under mechanical stirring. The reaction started at 75 ◦ C, when 1.9×10 -3 mole of the initiator was added. The stirring rate has been fixed at 500 rpm. The temperature has been kept constant during all the synthesis. The reaction has been stopped after 24 h. For the separation of the reaction byprod- ucts, unreacted reagents and polydispersed beads from the suspension a multiple centrifugations of the suspension fol- lowed by re-dispersion of the wet sediment in bidistilled water has been used. The deposition process used is the combination of the method of the colloidal crystal growth under oscillatory 0379-6779/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0379-6779(03)00286-8