Complex Dielectric Media
An alternative way to grow a disorder distribution of monodisperse spheres is what we call alloy colloidal crystals. To do that, binary colloidal suspensions of PMMA and PS spheres were ordered by vertical deposition. Once the alloy colloidal crystal is synthesized, we apply a chemical selective etching of one of the two kind of spheres which composed the crystal. In particular, we use Cyclohexane to selectively dissolve PS. Varying the diameters ratio between both kinds of spheres, different disordered systems can be grown.
An optical and morphological study of disorder in the resulting mixed PS-PMMA samples has already been reported, but only in very polydisperse systems. Our aim is to have monodisperse disordered thin films. For that purpose, PS spheres were selectively dissolved with Cyclohexane leaving PMMA intact. Two kinds of disordered systems can be obtained. If the spheres diameters are initially different, the resulting system is a random and disordered thin film composed by PMMA spheres. On the contrary, if the spheres diameters are initially equal, a different kind of disordered system is obtained. SEM figure shows an alloy colloidal crystal initially composed by PS spheres and PMMA spheres after PS etching. The resulting system is a vacancy doped colloidal photonic crystal, where the amount of vacancies is determined by the initial relative concentration of PS-PMMA spheres in the initial colloidal suspension and their positions are random. These samples have typically the dimensions of a colloidal photonic crystal grown by this method, that is, few microns thick and few centimetres wide. It is worth mentioning that the homogeneity of the resultant structure after etching which might be useful to control disorder by, for example, adding very small amounts of dopants (vacancies) one could build up systems where random or unwanted defects could be tested in a controlled manner.
Photonic crystals with different density of vacancies