Banks have long considered placing silicon transistors on currency for security purposes, but the technology was too chunky and intensive for paper bills. Now, tiny low-power organic transistors developed by German scientists could make it possible to really follow the money.
"Up to this day, no country has a bank note that contains active electronic features," said Hagen Klauk, head of the Max Planck Institute of Solid State Research's organic electronics research group. "We've taken the very first baby step, and that is to demonstrate that it is possible in principle to put organic transistors on the surface of a bank note."
Klauk, working with colleagues in Stuttgart and a group of Japanese scientists, started out making organic thin-film transistors that could be fabricated at room temperature without toxic solvents. Manufactured with minute amounts of aluminum and gold using a special dry process, each flexible 250-nanometer-thick array only needs three volts to operate.
The materials scientists didn't initially have bank notes in mind, but then they began to work on attaching the transistors to unconventional surfaces.
Paper is more challenging than other surfaces like plastic because it's not smooth. Adhering silicon membranes to paper usually required a chemical process that would have been too damaging for currency.
"With all the optimization we've done over the past years, our transistors are a little bit more qualified to work well on rough surfaces," Klauk said. Their films are thin enough to stick to an uneven surface. The scientists published their work in the journal Advanced Materials, showing the transistors on a U.S. dollar, Swiss franc, euro, and Japanese yen.
These low-voltage transistors could one day provide added security or tracking by transmitting information wirelessly to a scanner.
For now, however, an electronic system is still conceptual. Klauk said it will be up to other scientists, including circuit designers and mechanical engineers, to create a fully functional remotely controlled electronic circuit on a bank note.
"You'd never think you could make a transistor on a surface like that," Alberto Salleo, an assistant professor in materials science and engineering at Stanford University, told Discovery News. "As far as I can tell, Hagen is the only one to do this."
The researchers proposed depositing materials on the surface of a bank note, but Salleo imagines a scenario where they'd be placed inside the paper. "You could have a bank note with active electronics on it right out of the mint," he said.
"Some people actually call it 'electronics on anything,'" Klauk said. "So choosing a bank note was just one more unconventional surface that seemed useful to demonstrate the possibility, the opportunity."