Materials Science and Technology of Polymers at the University of Twente

Polymer Colloidal Crystals: Synthesis and Template-Assisted Fabrication with Controlled Structure and Orientation.

The method of “Electrophoretic deposition of charged polymer colloids on patterned substrates” was developed, where patterned surfaces predetermine the location of colloidal particles on the surface and an electric field acts as a driving force in the colloidal crystallization process. This method allows an independent control of colloidal crystal structure, orientation and crystal thickness. The technique was successfully applied to grow single and binary colloidal monolayers, colloidal crystals with FCC crystal structures and (111), (100) and (110) plane orientations, BCC colloidal crystals grown from a (100) crystal plane, non-close-packed colloidal multilayers with a HCP crystal sequence and binary colloidal crystals with NaCl and AlB2 analogs. Planar defects were easily introduced and controlled by the set of deposition parameters. A brief introduction to the topics that are relevant for this thesis is presented in Chapter 1. Attention was given to various colloidal crystallization methods, such as colloidal crystallization on flat, chemically and lithographically modified substrates as well as under applied external fields. Special emphasis was placed on reviewing the current status in the area of colloidal crystallization on topologically patterned substrates. Crystallization of oppositely charged colloidal particles was studied using “Molecular Dynamics” computations in Chapter 2. The stability of colloidal crystals with various crystal structures was examined. It was found that stable colloidal crystal structures resemble those found among the inorganic materials. The range of sphere size ratios for stable colloidal structures was determined. Colloidal crystals with the zinc blende inorganic analog were found to be stable at size ratio 0.2, with the sodium chloride analog – at size ratio 0.4 and with the cesium chloride analog – at values of size ratios higher than 0.6. The presence of a surface charge was necessary to keep the structure together, but the size of this charge was not important for stable structures, although a large surface charge assists in breaking down unstable crystal structures. Seeded emulsifier-free emulsion polymerization of styrene monomer allowed the synthesis of monodispersed colloidal particles with controlled surface charge densities and a colloidal size in the sub-micrometer range. Chapter 3 and Chapter 4 described the synthesis and characterization of negatively and positively charged colloidal particles, respectively. Colloidal size and size monodispersity were controlled in the first polymerization stage (core particles) and were characterized by HR-SEM measurements. Surface charge densities were controlled via polymerization of styrene monomer and ionic co-monomer around core particles in the second stage (core-shell particles). Surface charge densities were determined by conductometric (for negatively charged particles) and potentiometric (for positively charged particles) titrations. The influence of co-monomer type, initiator and polymerization conditions on colloid size, size monodispersity and surface charge density were discussed. A new co-monomer was found that yields high surface charge densities in a seeded polymerization process without forming polyelectrolyte residues in the colloidal suspension. In Chapter 5, a lithographic method for electrode surface patterning was developed. The method comprised the transfer of a photoresist pattern in a dielectric SiO2 layer, applying a lift-off process. Hexagonal, square and rectangular types of periodic patterns in photoresist were generated by laser interference lithography (LIL). LIL allowed us to generate periodic patterned surfaces with pattern periodicities between 300 nm and 900 nm. Optimal process parameters for photoresist patterning and SiO2 layer etching were found. The method of electrophoretic deposition of colloidal particles onto patterned electrode substrates was introduced in Chapter 6. Deposition on flat and patterned electrode surfaces was compared. Deposition on flat electrode surfaces led to the formation of disordered colloidal monolayers and crystals, while patterned surfaces pre-determined the location of colloidal particles on electrode surfaces by physical confinement. Deposition parameters such as electric field strength, colloid concentration, surface charge density, withdrawal speed of electrodes etc. were shown to play an important role in colloidal crystal growth. It was found that colloidal particles with low surface charge densities allowed a better control over colloidal crystallization. An optimal deposition voltage of 3.5 V was found for colloidal crystal growth. At higher deposition voltages, more disorder in colloidal crystals occured and at lower voltages the deposition process was ineffective. The withdrawal speed of electrodes from the colloidal suspension plays an important role in determining the thickness and growth of colloidal crystals with non-close-packed structures. For different crystal structures, the withdrawal speed was optimized for the systems considered. Specifically, the formation of colloidal monolayers (two-dimensional crystals) was discussed in this chapter. The growth of colloidal crystals with FCC and BCC structures was discussed in Chapter 7. The electrophoretic method, presented in Chapter 6, was applied. Colloidal crystals were deposited on electrode surfaces with various symmetries in order to induce colloidal crystal growth from different crystal planes. Colloidal crystals with FCC crystal structure grown in [111], [100] and [110] crystal directions and BCC in the [100] direction were presented. The influence of topologically patterned electrode surfaces on colloidal crystal growth was a main topic of this chapter. It was found, that growth of FCC (111) and (100) could be easily controlled at low withdrawal speeds, while growth of FCC (110) colloidal crystals was limited to two colloidal layers. Thick colloidal crystals grown in the [110] crystal direction finally possessed small polycrystalline regions with (111), (100) and (110) crystal planes. Disorder and polycrystallinity were caused by the geometrical factor of this plane, which is not close-packed in one of the main (perpendicular) directions. Growth of high quality close-packed FCC colloidal crystals was determined by the precise match of crystal lattice constant and electrode pattern periodicity. Fabrication of colloidal crystals with a BCC structure from the (100) crystal plane was also discussed in this chapter. Optimal deposition parameters were found for the systems investigated, allowing us to grow up to seven layers of this crystal structure. The layer-by-layer electrophoretic deposition method of colloidal particles on patterned electrode surfaces was introduced in Chapter 8. Applying this method, binary colloidal monolayers with LS, LS2, LS3, LS4 and LS5 stoichiometries were formed on hexagonally and square patterned electrode surfaces. Non-close-packed colloidal crystals with A-B-A sequence (an analog of the HCP crystal structure) were successfully fabricated. FCC colloidal crystal growth in the direction perpendicular to the (110) crystal plane was substantially improved. Plane defects using colloidal particles of different size were easily introduced. Finally, bimodal colloidal crystals with NaCl and AlB2 analogs were fabricated by layer-by-layer deposition of colloidal particles either of the same charge or opposite charge, respectively.