August 26, 2009: Researchers from the U. of Maryland and the National Institute of Standards and Technology (NIST) say they’ve come up with a way to overcome a principle obstacle in creating molecular switches: sandwich organic molecules between silicon and metal.
The general concept of molecular switches isn’t new, but a key problem is the fragility and susceptibility of organic molecules to semiconductor manufacturing process steps, particularly the high temperatures of metal deposition for attaching to electrical contacts.
To address this, they placed a nonstick surface down before depositing the metal (in this case, gold), which cooled to an ultrasmooth surface, and on top of that overlaid a plastic substrate. The nonstick layer beneath allowed removal of the laminated gold “as easy as peeling off plastic wrap,” NIST claims in a statement. The final step is to attach organic molecules to the gold, and then flip the entire assembly onto a silicon substrate, sandwiching the organic molecules in between.
The scientists admit their solution, dubbed “flip-chip lamination,” has been tried before, but a new “nanotransfer printing” machine makes it now possible to “press the three layers together so the organic molecules contact both the silicon and gold, but without smashing or otherwise degrading them,” says Coll Bau, NIST materials scientists and paper author.
The flip-chip lamination method creates an ultra-smooth gold surface (top), which allows the organic molecules to form a thin yet even layer between the gold and silicon. Gold surfaces created by other methods are substantially rougher (bottom), and would result in many of the molecular switches either being smashed or not contacting the silicon. (Credit: Coll Bau, NIST)
More analysis, from their paper published in the Journal of the American Chemical Society:
After molecular junction formation, the monolayers were characterized with p-polarized backside reflection absorption infrared spectroscopy (pb-RAIRS) and electrical current-voltage measurements. The monolayer quality remains largely unchanged after lamination to the Si(111) surface, with the exception of changes in the COOH and Si-O vibrations indicating chemical bonding. Both vibrational and electrical data indicate that electrical contact to the monolayer is formed while preserving the integrity of the molecules without metal filaments.
Potential application is seen in biosensors due to the interaction of organics and electronics; Bau also suggests the process could be “a fabrication paradigm” for nanomanufacturing, e.g. making molecular junctions with dense monolayers chemically bonded to metals and silicon electrodes.