Colloidal Crystal Wires

Abstract

Colloidal crystals have been extensively applied in many areas of science and technology. They can be used as amodel of crystal order and phase transition processes of atoms and molecules. As the size of colloidal particles, of customary use, are in the micrometer and sub-micrometer regimes, they produce strong diffraction effects in the optical region. This effect has been, and is still being, extensively applied to several fields of research such as photonic crystals and sensors. Diffraction effects are dependent on the type of crystal order, that is, on the point group symmetry by which colloids are ordered. Hard-sphere colloidal particles order in closedpacked assemblies with preference for the face-centered cubic (fcc) symmetry. However, the crystallization process of colloidal suspensions in confined geometries is very different from that in bulk systems. So far, most of the work in this area has focused on colloidal crystal thin films, which corresponds to one-dimensional confinement, where different types of close-packed and non-closed-packed facets appear. In the case of two-dimensional confinement, colloidal crystal wires (CCW) are obtained. The important parameter to be considered here is the ratio between the confinement size and particle diameter, which we call the confinement ratio D. There are two limiting cases depending on the value. When D! 1, particles arrange themselves in a wide range of close-packed structures depending of the geometric shape of the template. For example, squarelike colloidal fibers were obtained with lithographically prepared open channels templates, as well as with square-shaped optical fibers. Also, a colloidal crystal capillary column has recently been reported. New colloidal crystal arrangements appear when the patterned substrates have a lateral dimension similar to the particle size, that is, when D 1. Then, non-close-packed structures are favored and, in some cases, helical ordering is induced. But, most of all above cases do not show a pure 2D confinement, because they concern particle ordering into open patterned templates. Spheres distribute themselves in wirelike manner as has been modelled by Ericksson and Picket et al., through a hard-sphere packingmodel, and then shown for samples with beads in the millimeter range. These authors have demonstrated that, depending on the factor, particles pack themselves in a manifold variety of pattern configurations. Owing to their potential applications, colloidal crystals fibres have raised great interest. Also, they can be used as a model for the formation of chiral molecules as liquid crystals or DNA. Following these seminal works, some groups have reported experimental results on the arrangement of spherical particles into cylindrical pores in a broad length scale ranging from 100 micrometers up to the nanometer range. Recently, another approach that concerns the infiltration of particles into porous membranes made of alumina or silicon has been reported. Free-standing CCW were obtained. Although in most cases random packing geometries were observed, some fibers show helical twist, a fingerprint of chirality. However, to the best of our knowledge, no systematic experimental study of the influence of the parameter on the particle order has been published. Herein, we report on the infiltration of polystyrene (PS) spheres inside the pores of a siliconmembrane with a similar procedure to that employed by Wiley et al. We have performed a careful study of the particle arrangement for values between 1 and 3. As we have employed chargestabilized colloidal particles, the hard sphere model can not account for the experimental results. However, it will be used as a guide to understand particle arrangements. Figure 1 shows a scheme of the method to produce CCWs (see Experimental for more details). Sulphate-stabilized PS spheres were infiltrated into the silicon membrane. To cover the parameter values described above, a combination of PS spheres with different diameters (1, 1.2, and 1.5mm) and high-quality porous silicon membranes with a pore diameter between 2 and 4mm were used. The silicon membrane was manufactured by means of an electrochemical etching procedure. Such a C O M M U N IC A IO N