Nanopipettes and Nanoparticles Enable Detection of Single DNA Molecules
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of Central Florida, and their co-workers. As reported in the February issue of the Journal of the American Ceramic Society (p. 466, DOI: 10.1111/j.1551-2916.2005.00069.x), these nanostructures with rectangular cross sections and triangular tips result from high-temperature pyrolysis of polyureasilazane in the presence of an FeCl2 catalyst and N2 vapor. A powder precursor formed by curing the polyureasilazane and ball-milling it with the catalyst material was annealed at 1450ºC, yielding nanobelts 50–100 nm thick, 400–1000 nm wide, and ranging in length from hundreds of microns to several millimeters. Energy-dispersive spectrum analysis confirmed that the nanobelts consisted only of Si and N. High-resolution transmission electron microscopy revealed that the belts were perfectly crystalline α-Si3N4 with both [011] and [100] growth directions. Investigation of the structure using x-ray diffraction verified the presence of α-Si 3 N 4 , and a broad underlying diffraction peak was interpreted to come from an amorphous precursor that remained under the nanobelts. The researchers suggest a solid–liquid– gas–solid growth mechanism for the nanobelts. As the amorphous Si-C-N precursor is heated with the Fe catalyst, a SiFe-C liquid eutectic forms and becomes supersaturated with Si and C. The supersaturated liquid reacts with N2 vapor to form stable Si3N4 precipitates, which then grow into the nanowires. To account for the rectangular cross sections, the researchers propose that after the Si3N4 nucleates within the liquid droplets, the precipitate grows anisotropically due to the nature of the nitride crystal structure. Once the base of the triangular precipitate is the same dimension as the eutectic drop, the width ceases to increase but growth continues along the axial direction. Since other studies have shown that round nanowires can be grown in similar materials systems but at lower temperatures, Yang and co-workers are currently examining a hypothesis that the lower solubility of Si in the eutectic phase at lower temperatures results in smaller droplets, which in turn limits anisotropic growth of the nitride precipitates. AMANDA GIERMANN
Doped PMMA Used For 3D Multilayered Optical Memory As reported in the April 1 issue of Optics Letters (p. 774), H. Jiu and co-workers at the University of Science and Technology of China use a femtosecondpulsed infrared laser to sequentially create eight layers of voids, with diameters less than a micrometer, within a block of 332
doped poly(methyl methacrylate) (PMMA). Their result joins a body of research that uses pulsed lasers to controllably form voids within dielectric and polymer materials. This activity has been prompted mainly by the allure of applications in microfabrication and high-density optical storage. As a dopant, the researchers chose an easily synthesized rare-earth complex, Sm(DBM)3Phen, which is highly soluble in PMMA. Compared with pure PMMA, the doped polymer has a lower optical memory threshold, which is due to the strong absorption of DBM (dibenzoylmethane) at
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