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26 September 2003, Marc S. Lavine, Science Editor's Choice
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About: Printing nanoparticles from the liquid and gas phases using nanoxerography.

Photocopies are created in a three-step process. First, light reflected from the white parts of the original is focused onto a charged printing "drum," where it locally dissipates charge. Next, positively charged toner particles are attracted to the remaining charged areas that were dark in the original. Finally, paper is run over the drum to pick up the toner.
Barry et al. use similar steps to create a nanoxerographic process that can pattern very small carbon and metal particles with a resolution about 1000 times better than that of a typical photocopier. A silicon wafer was coated with either silicon dioxide or poly(methylmethacrylate) as the electret, which corresponds to the charged drum in the photocopier. A pattern was transferred from a stamped or lithographed master by bringing the two together and applying an external voltage. In a liquid-phase process, the charged electret and an aggregate of carbon or iron particles were both placed in a solvent and sonicated. Within a few seconds, the aggregate was broken up and the nanometer-sized particles assembled onto the electret. A gas-phase process was also tested in which particles were generated in a furnace and directed by an electric field toward the charged electret. The process is partially limited by the resolution of the master, but 100- and 200-nanometer features sizes were achieved for the gas- and liquid-phase processes, respectively.

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