Filling in the blanks
Microcontact printing enables researchers to create orderly, microscopic arrays of molecules on a surface. The technique, which is cheaper than traditional methods of making patterns on this scale, could someday be used to produce everything from microelectronics to biosensors. Now, scientists have enhanced the technique, so that patterns can be laid more precisely than they currently can.
Much as a rubber stamp transfers ink to a sheet of paper, microcontact printing transfers molecular “inks” onto surfaces such as gold and copper. Sulfur-containing molecules called alkanethiols and gold are one widely used combination for the technique.
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Researchers can create a simple electronic circuit by starting with a gold surface on a silicon backing and then stamping a pattern of alkanethiols onto the gold. An etching process strips the gold from the silicon in those areas that aren’t covered with the alkanethiols. The resulting pattern of silicon and gold defines the circuit.
Sometimes, however, the alkanethiols bleed past their boundaries when stamped, notes Paul S. Weiss, a chemist and physicist at Pennsylvania State University in State College. In the case of the electronic circuit, this lack of precision can disrupt the conductive properties of the surface, deteriorating the circuit’s function.
Weiss and his team found that they could stanch the bleeding by first dipping the gold substrate into a solution of adamantanethiolate, a compound that contains carbon arranged in 10-atom cages. The adamantanethiolate binds weakly to the gold, but it gets knocked off the gold surface by the patterned alkanethiols. Wherever the stamp hasn’t inked the surface, however, the adamantanethiolate stays put, keeping the edges of the pattern sharp for subsequent processing steps. The researchers describe the technique in the September Nano Letters.