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Nanoscale Electrochemistry - a tool for the formation of structures of almost atomic dimensions. Nanostructuring of materials, i.e., its modification on a nanometer scale, is nowadays one of the key technologies for the fabrication of electronic as well as micromechanical devices. Combining this with electrochemical methods would provide a very convenient way of producing nanostructures.
Structures of different materials could be formed just by supplying these as ions dissolved in a liquid solution, i.e., an electrolyte. As published in the June 22 issue of Physical Review Letters, a group of scientists at the Fritz Haber Institute, Berlin, achieved the direct electrochemical deposition of nanometer clusters as well as the local etching of a substrate.
The authors employed an electrochemical scanning tunneling microscope (STM), where the needle-like probe of the STM and a gold sample are immersed into the electrolyte. Upon application of ultrashort voltage pulses between the STM tip and the gold surface, copper clusters containing only a couple of hundred atoms were formed on a gold surface. Similar to the processes occurring in a battery, electrons are transferred from the gold surface onto positively charged copper ions dissolved in the electrolyte. These neutralized metal atoms then stick to the surface forming a small pile of material. By reversing the polarity of the voltage pulses, the substrate itself was etched. Gold atoms lose electrons (they are oxidized), resulting in their dissolution in the electrolyte. This leads to the formation of holes on the surface, again of only nanometer dimension.
Such electrochemical modifications on the nanometer scale are in contrast to the expectations in conventional electrochemistry. There, the modifications usually occur over the entire electrodes mostly independent of the geometrical arrangement and the shape of the electrodes. Therefore, the size of modifications in conventional electrochemistry is limited to the micrometer rather than to the nanometer scale, even with very tiny electrodes.
The clue of the investigations described above is the application of very short voltage pulses (of the order of nanoseconds) and the use of a highly concentrated electrolyte. This conducts the chemical reaction far from thermodynamic equilibrium and, hence, confines it to the region around the very end of the needle-like probe. Besides the structuring of gold surfaces this method opens new avenues also for the modification and micromachining of semiconductors, the main material for modern eletronic devices. Even clusters of different materials could be deposited with this 'pulse electrochemical STM' simply by exchanging the electrolyte. This is one of the prerequisites for building a real electronic device.
Journal
Physical Review Letters