Binary colloidal crystals

Abstract

Binary crystals are crystals composed of two types of particles having different properties like size, mass density, charge etc. In this thesis several new approaches to make binary crystals of colloidal particles that differ in size, material and charge are reported We found a variety of crystal structures, amongst which five not seen before with colloids (LS3, LS6-, two LS8- and NiAs-type), and show that the general feeling that growing binary colloidal crystals is an arduous task needs to be revised. Before presenting our methods and the crystals obtained, we report a characterization of the spectroscopic properties of fluorescently labeled PMMA spheres used in the fabrication of the binary crystals. Absorption, excitation and emission spectra of several fluorophores before and after their incorporation in the particles are measured and the photostability of the dyes in the colloids under continuous laser illumination is investigated. The first approach, presented in the thesis, for making binary colloidal crystals utilized a combination of the method of controlled drying of a suspension on a substrate with a layer-by-layer procedure. We created large areas of binary structures with different stoichiometries (LS, LS2 and LS3, the last one never seen before). Binary crystals of particles having different composition were made as well and by selectively removing one of the two components in these crystals a non-closed packed crystal of the other species was obtained. Another method described in the thesis utilizes an external electric field to grow binary crystals of NaCl-type from silica particles. By changing the strength of the electric field we were able to control the strength of the dipolar interactions between the particles from low to high compared to kBT and to obtain NaCl-type crystals, with different mechanisms underlying their formation in the two cases. Next, binary crystals made from oppositely charged particles are described. By controlling the amount and the sign of the charge on the particles we obtained colloidal analogues of the ionic crystals with a variety of structures, whose stoichiometry is not dictated by charge neutrality. Binary crystals not only from oppositely charged particles with the same material but also from mixtures of silica and PMMA spheres having large density differences are created and also binary structures from mixtures of charged and neutral species are obtained. It is shown that an external electric field can be used to manipulate the crystals formed and to induce a phase transition to lane-like structures. For binary mixtures of oppositely charged particles with a size ratio of 0.31 three new binary structures are found experimentally: LS6bcc (analogue to the fullerene compounds A6C60), LS8fcc and LS8hcp, the last two with no atomic analogues and never seen before. Computer simulations confirmed the stability of these structures and a novel interactive simulation method predicted a variety of new structures from oppositely charged particles. Some preliminary observations concerning the crystallization mechanism of the LS8-type binary crystals and the coexistence of binary structures with a large difference in their stoichiometries are presented as well.