We study the range of orientational order of a single layer of cylindrical block copolymer microdomains annealed on several types of substrates. The orientational persistence length or nematic correlation length (ξ) is evaluated using recently developed imaging and analysis methods to measure the grain size of the block copolymer microdomains. We show that the substrate can lower ξ for block copolymers with a majority component that interacts strongly with the substrate, but this can be mitigated by attaching a buffer layer of polystyrene brushes to the substrate. In addition, we show that, for a block copolymer where the block that strongly interacts with the substrate is the minority component, the microdomain correlation length does not increase when substrates are treated with this buffer layer. We suggest that in this case the brushes do not increase ξ not only because of the lower volume fraction of the strongly interacting component but also because there are block copolymer wetting layers at the free and substrate interfaces that decouple the microdomains from the substrate in a similar manner as the polystyrene brushes.

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We have investigated the evolution of structural and optical properties of GaAsN nanostructures synthesized by N ion implantation into epitaxial GaAs, followed by rapid thermal annealing. Transmission electron microscopy and x-ray diffraction indicate the formation of nanometer-sized crystallites with lattice parameters close to those of pure zincblende GaN. The average crystallite size increases with annealing temperature while the size distribution is self-similar and the volume fraction remains constant, suggesting a coarsening process governed by Ostwald ripening. These GaAsN nanostructures exhibit significant photoluminescence in the near infrared range. The apparent lowering of the fundamental band gap is likely due to the incorporation of a small amount of As in GaN.

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