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Conventional
methods of forming films composed of organic-capped nanocrystals,
such as by solvent evaporation in dry casting and spin coating,
usually result in films that are very rough over macroscopic distances,
such as a mm. A new method of forming smooth films of nanocrystals
over (at least) ~1 cm was developed by Professor Irving P. Herman
and colleagues at the Columbia MRSEC. This involves application
of a dc, uniform electric field across opposing electrodes in
a solution of nanocrystals. A small fraction of the nanocrystals
in solution are charged and move to the electrodes under the influence
of the field. Smooth, robust films of CdSe nanocrystals, capped
by TOPO, with tunable thickness, such as 0.7 micrometers, were
formed using this electrophoretic method. The properties of the
individual nanocrystals are retained in these electrophoretic
films, and these films are more resistant to chemical attack than
other films. By patterning the electrodes, spatially selective
deposition of the CdSe nanocrystal films are formed, as seen in
the accompanying SEM (scanning electron micrograph). The nanocrystal
film has grown on the gold electrodes (gray regions) but not on
the silicon dioxide (black) or on gold films not connected to
the electrode (light gray). So far, patterns with dimensions as
narrow as 1 micrometer have been selectively deposited. This method
has been used to deposit smooth films of other nanoparticles also,
such as maghemite (gamma-Fe2O3) nanocrystals,
and of mixtures of different nanoparticles. |
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