Control of Colloidal Crystal Growth by external Fields
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0901-Rb10-08.1
Control of Colloidal Crystal Growth by external Fields
E. von Rhein, A. Bielawny and S. Greulich-Weber, University of Paderborn, Department of Physics, Warburger Str. 100, Paderborn, Germany
ABSTRACT Synthetic opals serve as templates for the most promising three-dimensional photonic crystals, so-called inverted opals. The time consuming growth of artificial opals by self-assembly of monodisperse sub-micron spheres made of silica or organic materials resulting in fcc and hcp colloidal crystals usually yields in limited crystal qualities. The large scale fabrication and thus their application in photonics suffer from the reproducible growth of defect-free extended bulk crystals. In this paper we propose a sonic wave supported fast colloidal crystal growth in a centrifugal field. White noise acoustic irradiation is thought to realize an artificial temperature allowing particle diffusion during crystal growth.
INTRODUCTION Since the introduction of the concept of photonic bandgaps [1, 2] much effort has been targeted at producing and characterizing 3D photonic crystals working in the optical wavelength range. The use of synthetic opals has been proposed as an extremely cheap and easy alternative [3] to sophisticated nano structuring technologies like for example lithography of semiconductors [4]. Spherical colloidal particles of different dielectric materials, e.g. silica or organic, are easy and at low cost producible and exhibit a strong tendency to self assemble spontaneously into an fcc crystal lattice [5]. Various procedures for the preparation of large scale defect-free colloidal crystals for photonic applications were repeatedly proposed in the past, however, it still remains a demanding task, especially in view of reproducibility. Sedimentation of spherical sub-micron particles in a protic solvent is the dominating method for growing colloidal bulk crystals [6], though, depending on the density ratios of colloidal particles and solvent, respectively, it could be a long lasting procedure. Best crystal qualities were obtained for particle sedimentation velocities of about 0.4 mm/h [7]. The sedimentation velocity not only depends on the density ratios, but also on the spheres diameters. This aggravates the situation of finding a universal fast procedure for a reproducible fabrication of high quality colloidal bulk crystals. Furthermore deposition on any type of substrate is preferable and no limitation to prestructured substrates should be required. In the following we propose a working crystal growth method suitable for almost all kinds of sphere sizes and materials. The method adopts principles of atomic crystal growth. Since for atomic crystal growth like e.g. after Czochralski temperature gradients are essential, we apply an artificial temperature during colloidal crystal growth. An artificial temperature is realized by application of sonic noise fields. Consequently the sedimentation velocity is no longer a critical parameter for high quality results, thus we were able to drastically enhanc
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