Anisotropic Colloids: Synthesis and Phase Behavior of Eccentric, Dimer and String-like Colloids

The research described in this thesis focuses on synthesis and phase behavior of anisotropic colloids prepared through different synthetic strategies. Namely, eccentric core-shell particles, dimers, string-like particles and core-shell particles are the systems investigated throughout this work. The synthesis routes are described for these colloidal systems and their physical/chemical properties are extensively characterized. Furthermore, potential applications of these colloidal systems in fields such as size-selective catalysis, colloidal crystallization, optical tweezing are investigated

A general approach for monodisperse colloidal perovskites, Chemistry of Materials

We describe a novel general method for synthesizing monodisperse colloidal perovskite particles at room temperature by postsynthesis addition of metal hydroxides to amorphous titania colloids. In previous work, we used titania particles to synthesize homogenously mixed silica-titania composite particles by addition of a silica precursor to the titania particles (Demiro¨ rs et al. Chem. Mater., 2009, 21, 979). Here we show that the principle is general and can be used to prepare different titania-based composite beads. We demonstrate the synthesis of colloidal perovskites like BaTiO3, SrTiO3, CaTiO3 and Ba0.5Sr0.5TiO3, which have importance for the electronics industry for their dielectric properties. Mixed perovskites like BaxSr1-xTiO3 bring tunability in structure and properties by changing the ratio of metal cations. We also show fabrication of metal core, BaTiO3 shell particles, which could find applications in percolative capacitors, and luminescence properties of Pr3+ doped colloidal perovskites

A general method to coat colloidal particles with titiana

We describe a general one-pot method for coating colloidal particles with amorphous titania. Various colloidal particles such as silica particles, large silver colloids, gibbsite platelets, and polystyrene spheres were successfully coated with a titania shell. Although there are several ways of coating different particles with titania in the literature, each of these methods is applicable to only one type of material. The present method is especially useful for giving the opportunity to cover many types of colloidal particles with titania and forgoes the use of a coupling agent or a precoating step. We can produce particles with a smooth titania layer of tunable thickness. The monodispersity, which improves during particle growth, and the high refractive index of titania make these particles potential candidates for photonic crystal applications. We also describe various ways of fabricating hollow titania shells, which have been intensively studied in the literature for their applications in electronics, catalysis, separations, and diagnostics. Note that our method initially produces amorphous shells on the particles, but these can be easily turned into crystalline titania by a calcination step. We also find that the growth of titania is a surface-reaction-limited process

Synthesis of eccentric titania-silica core-shell and composite particles

We describe a novel method to synthesize colloidal particles with an eccentric core-shell structure. Titania-silica core-shell particles were synthesized by silica coating of porous titania particles under Sto¨ber (Sto¨ber et al. J. Colloid Interface Sci. 1968, 26, 62) conditions. We can control access of silica to the pores in the titania, allowing us to produce either core-shell or composite particles. Calcination of the core-shell particles gives unique eccentric core-shell structures, as a result of extensive shrinkage of the highly porous titania core with respect to the silica shell. However, when the titania particles are silica treated prior to drying they result in composite titania-silica spheres, where two materials are mixed uniformly. These spheres are interesting for catalysis, (switchable) photonic crystal applications, optical tweezing, and new titiana based materials. We demonstrate photocatalytic activity of the eccentric spheres where the silica layer acts as a size selective membrane

Phase behavior and structure of a new colloidal model system of bowl-shaped particles

We study the phase behavior of bowl-shaped (nano)particles using confocal microscopy and computer simulations. Experimentally, we find the formation of a wormlike fluid phase in which the bowl-shaped particles have a strong tendency to stack on top of each other. However, using free energy calculations in computer simulations, we show that the wormlike phase is out-ofequilibrium and that the columnar phase is thermodynamically stable for sufficiently deep bowls and high densities. In addition, we employ a novel technique based on simulated annealing to predict the crystal structures for shallow bowls. We find four exotic new crystal structures and we determine their region of stability using free energy calculations. We discuss the implications of our results for the development of materials consisting of molecular mesogens or nanoparticles

Directed self-assembly of colloidal dumbbells with an electric field

We demonstrate the assembly of colloidal particles with the shape of diatomic molecules (“dumbbells”) into crystals that we study with confocal microscopy. The literature on the preparation of nonspherical colloidal particles has grown steadily. Assembly of these particles into regular three-dimensional crystalline lattices, however, is rarely, if ever, achieved and has not yet been studied quantitatively in 3D real space. We find that, by application of an electric field, such particles assemble quite readily. By varying the particle aspect ratio, range of interactions, and electric field strength, we find several different crystal structures of which three have never before been observed. Moreover, the electric field can be used to switch between different structures and manipulate/switch the photonic properties. Moreover, our work sheds light on fundamental questions related to the self-assembly of nonspherical particles

Switching plastic crystals of colloidal rods with electric fields

When a crystal melts into a liquid both long-ranged positional and orientational order are lost, and long-time translational and rotational self-diffusion appear. Sometimes, these properties do not change at once, but in stages, allowing states of matter such as liquid crystals or plastic crystals with unique combinations of properties. Plastic crystals/glasses are char- acterized by long-ranged positional order/frozen-in-disorder but short-ranged orientational order, which is dynamic. Here we show by quantitative three-dimensional studies that charged rod-like colloidal particles form three-dimensional plastic crystals and glasses if their repulsions extend significantly beyond their length. These plastic phases can be reversibly switched to full crystals by an electric field. These new phases provide insight into the role of rotations in phase behaviour and could be useful for photonic applications

Switching plastic crystals of colloidal rods with electric fields

When a crystal melts into a liquid both long-ranged positional and orientational order are lost, and long-time translational and rotational self-diffusion appear. Sometimes, these properties do not change at once, but in stages, allowing states of matter such as liquid crystals or plastic crystals with unique combinations of properties. Plastic crystals/glasses are char- acterized by long-ranged positional order/frozen-in-disorder but short-ranged orientational order, which is dynamic. Here we show by quantitative three-dimensional studies that charged rod-like colloidal particles form three-dimensional plastic crystals and glasses if their repulsions extend significantly beyond their length. These plastic phases can be reversibly switched to full crystals by an electric field. These new phases provide insight into the role of rotations in phase behaviour and could be useful for photonic applications

Seeded Growth of Titania Colloids with Refractive Index Tunability and Fluorophore-Free Luminescence

Titania is an important material in modern materials science, chemistry, and physics because of its special catalytic, electric, and optical properties. Here, we describe a novel method to synthesize colloidal particles with a crystalline titania, anatase core and an amorphous titania-shell structure. We demonstrate seeded growth of titania onto titania particles with accurate particle size tunability. The monodispersity is improved to such an extent so that colloidal crystallization of the grown microspheres becomes feasible. Furthermore, seeded growth provides separate manipulation of the core and shell. We tuned the refractive index of the amorphous shell between 1.55 and 2.3. In addition, the particles show luminescence when trace amounts of aminopropyl-triethoxysilane are incorporated into the titania matrix and are calcined at 450 C. Our novel colloids may be useful for optical materials and technologies such as photonic crystals and optical trapping

Nanonewton optical force trap employing anti-reflection coated, high-refractive-index titania microspheres

Optical tweezers are exquisite position and force transducers and are widely used for high-resolution measurements in fields as varied as physics, biology and materials science1, 2, 3. Typically, small dielectric particles are trapped in a tightly focused laser and are often used as handles for sensitive force measurements. Improvement to the technique has largely focused on improving the instrument and shaping the light beam1, 4, and there has been little work exploring the benefit of customizing the trapped object5. Here, we describe how anti-reflection coated, high-refractive-index core–shell particles composed of titania enable single-beam optical trapping with an optical force greater than a nanonewton. The increased force range broadens the scope of feasible optical trapping experiments and will pave the way towards more efficient light-powered miniature machines, tools and applications