Dmytro V. Byelov

Self-assembly of Colloidal Cubes via Vertical Deposition

The vertical deposition technique for creating crystalline microstructures is applied for the first time to nonspherical colloids in the form of hollow silica cubes. Controlled deposition of the cubes results in large crystalline films with variable symmetry. The microstructures are characterized in detail with scanning electron microscopy and small-angle X-ray scattering. In single layers of cubes, distorted square to hexagonal ordered arrays are formed. For multilayered crystals, the intralayer ordering is predominantly hexagonal with a hollow site stacking, similar to that of the face centered cubic lattice for spheres. Additionally, a distorted square arrangement in the layers is also found to form under certain conditions. These crystalline films are promising for various applications such as photonic materials.

Phase behavior of colloidal silica rods

Recently, a novel colloidal hard-rod-like model system was developed which consists of silica rods [Kuijk et al., JACS, 2011, 133, 2346]. Here, we present a study of the phase behavior of these rods, for aspect ratios ranging from 3.7 to 8.0. By combining real-space confocal laser scanning microscopy with small angle X-ray scattering, a phase diagram depending on concentration and aspect ratio was constructed, which shows good qualitative agreement with the simulation results for the hard spherocylinder system. Besides the expected nematic and smectic liquid crystalline phases for the higher aspect ratios, we found a smectic-B phase at high densities for all systems. Additionally, real-space measurements on the single-particle level provided preliminary information on (liquid) crystal nucleation, defects and dynamics in the smectic phase.

Sol-gel transitions and liquid crystal phase transitions in concentrated aqueous suspensions of colloidal gibbsite platelets

In this paper, we present a comprehensive study of the sol-gel transitions and liquid crystal phase transitions in aqueous suspensions of positively charged colloidal gibbsite platelets at pH 4-5 over a wide range of particle concentrations (50-600 g/L) and salt concentrations (10(-4)-10(-1) M NaCl). A detailed sol-gel diagram was established by oscillatory rheological experiments. These demonstrate the presence of kinetically arrested states both at high and at low salt concentrations, enclosing a sol region. Birefringence and iridescence show that in the sol state nematic and hexagonal columnar liquid crystal phases are formed. The gel and liquid crystal structures are studied in further detail using small-angle X-ray scattering (SAXS) and cryo-focused ion beam/scanning electron microscopy (cryo-FIB-SEM). The gel formed at high salt concentration shows signatures of a sponge-like structure and does not display birefringence. In the sol region, by lowering the salt concentration and/or increasing the gibbsite concentration, the nematic phase gradually transforms from the discotic nematic (ND) into the columnar nematic (NC) with much stronger side-to-side interparticle correlations. Subsequently, this NC structure can be either transformed into the hexagonal columnar phase or arrested into a birefringent repulsive gel state with NC structure.

Uniaxial and biaxial liquid crystal phases in colloidal dispersions of board-like particles

Dispersions of board-like goethite (α-FeOOH) particles with short-range repulsive interaction form a versatile colloidal model system, showing a nematic, smectic A and columnar phase. In high magnetic fields a biaxial nematic phase is induced with the shortest dimension of the particles aligned along the field. Moreover, if particles have a shape almost exactly in between rod-like and plate-like they can spontaneously, without external magnetic field, form biaxial nematic and biaxial smectic A phases, which is in accordance with theoretical predictions. The macroscopic domains were oriented by a magnetic field and their structure was revealed by small-angle X-ray scattering. Our results suggest that biaxial phases can be readily obtained by a proper choice of the particle shape.

Double stacking faults in convectively assembled crystals of colloidal spheres.

Using microradian X-ray diffraction, we investigated the crystal structure of convectively assembled colloidal photonic crystals over macroscopic (0.5 mm) distances. Through adaptation of Wilsons theory for X-ray diffraction, we show that certain types of line defects that are often observed in scanning electron microscopy images of the surface of these crystals are actually planar defects at 70.5 degrees angles with the substrate. The defects consist of two parallel hexagonal close-packed planes in otherwise face-centered cubic crystals. Our measurements indicate that these stacking faults cause at least 10% of stacking disorder, which has to be reduced to fabricate high-quality colloidal photonic crystals.

Fabrication of artificial opals by electric-field-assisted vertical deposition.

We present a new technique for large-scale fabrication of colloidal crystals with controllable quality and thickness. The method is based on vertical deposition in the presence of a DC electric field normal to the conducting substrate. The crystal structure and quality are quantitatively characterized by microradian X-ray diffraction, scanning electron microscopy, and optical reflectometry. Attraction between the charged colloidal spheres and the substrate promotes growth of thicker crystalline films, while the best-quality crystals are formed in the presence of repulsion. Highly ordered thick crystalline layers with a small amount of stacking faults and a low mosaic spread can be obtained by optimizing the growth conditions.

Structures and Phase Behavior in Mixtures of Charged Colloidal Spheres and Platelets

The experimental phase diagram for aqueous mixtures of charged gibbsite platelets and silica spheres is presented. The platelets are 95 nm in diameter, and the diameter ratio between the spheres and the platelets is 0.18. Here the spheres are acting as depletants in the mixtures perturbing the phase behavior of the pure platelet suspensions. An important finding is that a large isotropic/columnar coexistence region has been identified in the phase diagram, which appeared already at low concentrations of the platelets. Microradian X-ray diffraction measurements revealed the structure of the liquid crystalline phases and the orientational order of platelets. An interesting observation is that in the columnar phase the silica spheres are located between the columnar stacks. All samples were in equilibrium because sedimentation did not affect the system because of the relatively small size of the colloidal particles and the charges present at their surfaces.

Long-range ordering in anodic alumina films: A microradian X-ray diffraction study

A quantitative analysis of long-range order in the self-organized porous structure of anodic alumina films has been performed on the basis of a microradian X-ray diffraction study. The structure is shown to possess orientational order over macroscopic distances larger than 1 mm. At the same time, the interpore positional order is only short-range and does not extend over more than � 10 interpore distances. These positional correlations are mostly lost gradually rather than at the domain boundaries, as suggested by the divergence of the peak width for the higher-order reflections. In the direction of the film growth the pores have a very long longitudinal self-correlation length of the order of tens of micrometres.

Self-assembly of colloidal hematite cubes: a microradian X-ray diffraction exploration of sedimentary crystals

The structure of spontaneously formed crystals in the sediments of colloidal hematite (α-Fe2O3) cubes is studied using microradian X-ray diffraction. The hematite cubes possess a superball shape and a permanent magnetic dipole moment, both influencing the self-assembly behaviour and inducing directionality in different ways. To control the inter-cube interactions, the cubes were dispersed in different solvents and/or coated with a silica layer and allowed to form sediments under the simultaneous effect of the gravitational field and an applied magnetic field. The microradian X-ray diffraction revealed that changing the double layer repulsion that opposes the magnetic and van der Waals attractions can induce short to long-range ordering in the fast sedimenting systems of the cubes. The presence of an external magnetic field during sedimentation induced the formation of a single crystal with long-range order that showed different symmetries, rectangular or hexagonal, close to the fluid–solid interface. The identified 3D crystal structure of both was found to have a body centred monoclinic lattice periodicity that rotates deeper in the sediment due to gravitational compression. This experimental structure differs from the expected crystal lattices determined by simulations and is most likely due to the effects of long-range repulsion and dipole–dipole interactions present here.

Structure of the repulsive gel/glass in suspensions of charged colloidal platelets

Rheological, optical and structural properties of colloidal suspensions of charge-stabilized gibbsite platelets across the sol–gel transition region are investigated. In this work we focus on samples with a low salt content (10−4 M). While at a gibbsite concentration of 300 g l−1, a nematic–columnar phase separation is observed, an arrested state with a nematic signature and highly elastic response has been observed for a concentration of 400 g l−1. A temporal evolution of the structure of the arrested state, which leads to stronger interparticle correlations, has been observed on a timescale of 20 months. The results suggest that the arrested state develops into a glass with a columnar nematic structure.