Ice Used As Resist for Patterning Nanostructures
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turned off. The researchers noted several possible improvements to the system, including optimization of the zeolite chemistry and the use of more sophisticated optical reflection measurement techniques. COLIN MCCORMICK
Highly Ordered Isoporous Membranes Fabricated from Nanocomposites Isoporous films, which have application in photonics, biotechnology, and biomedical devices, have previously been formed by the incorporation of inorganic nanoparticles into an organic polymer matrix. However, the properties of this class of nanocomposite materials are adversely affected by nanoparticle aggregation. In general, attachment of polymer chains to nanoparticles increases their dispersion and results in improved mechanical properties and thermal stability. Recently, D. Nystrom and colleagues from KTH, together with M. Whittaker from the University of Queensland, grafted polymers onto silica nanoparticles and used them to fabricate isoporous membranes. As reported in Macromolecular Rapid Communications 26 (p. 524; DOI: 10.1002/ marc.200400617), KTH researcher A. Hult and colleagues used atom-transfer radical polymerization to graft polystyrene (PS) onto functionalized silica nanoparticles. Infrared spectroscopy confirmed the PS grafts, which had a molecular weight of 23,400 g/mol. The researchers cast films on a glass substrate from a mixture of hybrid nanoparticles, CS2, and linear PS under humid conditions. A temperature decrease resulting from solvent evaporation caused water to condense onto the film surface. Stabilized water droplets were then formed when hybrid nanoparticles precipitated at the water–solvent interface. An opaque film formed when all solvent evaporated, with the final membrane containing 10 wt% silica. The researchers, using an optical microscope and atomic force microscopy to analyze the membrane structure, observed a hexagonal array of pores, that is, a honeycomb structure, with an average pore diameter of 2.5 μm, a nearly monodisperse pore size distribution, and a pore depth varying from 0.8–1.5 μm. By changing the humidity and the rate of air flow, the researchers were able to vary the pore size from 2 μm to 8 μm. Hult and co-researchers said that their method is applicable to a broad range of materials and that the large surface areas, open pore structure, and isoporous nature of the films make them very attractive for membrane applications. Hult said, “We can also make nanoMRS BULLETIN • VOLUME 30 • JULY 2005
composites in which copolymers are grafted to the silicon microstructure.” STEVEN TROHALAKI
Ice Used As Resist for Patterning Nanostructures G.M. King and researchers at Harvard University have recently demonstrated the use of frozen water as a resist for electronbeam lithography. They patterned chromium lines as narrow as 17 nm and lines
of local surface chemical transformations as narrow as 5 nm, as reported in the June 8 issue of NanoLetters (p. 1157; DOI: 10.1021/nl050405n). The researchers cooled a silicon substrate in a scanning electron microscope chamber to 128 K before leaking in
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