The langmuir-blodgett approach to making colloidal photonic crystals from silica spheres.

The area of colloidal photonic crystal research has attracted enormous attention in recent years as a result of the potential of such materials to provide the means of fabricating new or improved photonic devices. As an area where chemistry still predominates over engineering the field is still in its infancy in terms of finding real applications being limited by ease of fabrication, reproducibility and 'quality'- for example the extent to which ordered structures may be prepared over large areas. It is our contention that the Langmuir-Blodgett assembly method when applied to colloidal particles of silica and perhaps other materials, offers a way of overcoming these issues. To this end the assembly of silica and other particles into colloidal photonic crystals using the Langmuir-Blodgett (LB) method is described and some of the numerous papers on this topic, which have been published, are reviewed. It is shown that the layer-by-layer control of photonic crystal growth afforded by the LB method allows for the fabrication of a range of novel, layered photonic crystals that may not be easily assembled using any other approach. Some of the more interesting of these structures, including so-called heterostructured photonic crystals comprising of layers of spheres having different diameters are presented and their optical properties described. Finally, we offer our comments as to future applications of this interesting technology.

Langmuir-Blodgett assembly of colloidal photonic crystals using silica particles prepared without the use of surfactant molecules.

This short communication reports the observation that in contrast to most previously reported procedures, it is possible to prepare 3D photonic crystal structures from silica particles that have not been deliberately treated with surfactant molecules, using the Langmuir-Blodgett method. We find that colloidal particles prepared simply via the Stöber method with diameters in the range 180-360 nm and dispersed in ethanol, may be effectively floated at the air/water interface and compressed into close packed layers prior to depositing the layers on a substrate. We also find, by comparing structures made with both particles treated with the surfactants 3-(trimethoxysilyl) propyl methacrylate or (3-aminopropyl)triethoxysilane and particles which have not been treated with any surfactant species, that the position of the Bragg peak and the reflectivity of the sample does not appear to be influenced by the presence of the surfactant molecules.

Langmuir-Blodgett films of preformed polymers containing biphenyl groups.

Polymers containing liquid crystal groups have been studied previously as waveguides, and they have been deposited as spacer materials with various chromophores to form alternating films However, only a few members of this group of materials have been studied in any detail, and very little structural information has been obtained so far. Therefore, a more detailed examination of these materials as mono- and multilayers was undertaken. A new group of materials including the same mesogenic group, polymeric sulfones, was also studied. The polymers gave steep isotherms with high collapse pressures, indicating good packing of the monolayers, and could be deposited to form thick multilayers. X-ray diffraction showed that an ordered multilayer was formed, and the effects of the polymers chemical nature on the structure of the LB films are discussed. It appears that the dominant factor in monolayer structure is the nature of the polymer backbone rather than that of the liquid crystal side chains, which play a secondary role.