CHEMISTRY 1000 Topics of Interest #4: Organic Electronics on Banknotes

Using Chemistry to Catch Counterfeits As better and better colour printing and copying technologies become more and more widely available, it becomes more important for banks and mints to implement anticounterfeiting technology to prevent people from simply printing their own money. Many different approaches can be taken, and banks/mints usually combine a wide variety: Watermarks (see image of 50 Euro banknote) Threads through the paper Foil strips Printing on plastic instead of paper Fluorescent inks (see image of Swiss banknote under normal light and under UV light) Optically variable inks (different colours at different angles to light; see below) Holograms

Using Chemistry to Catch Counterfeits Recent work by chemists in Germany and Japan looks to be useful in developing a way to track currency electronically by embedding an electronic circuit (which could be a unique tag) on currency. This would be very difficult for counterfeiters to replicate, and could potentially allow tracking of currency as well. Why isn’t this done already? Conventional electronics are simply too big to print on currency and the processing techniques to transfer a silicon membrane are too destructive. 1 Nature (2010) 468, U. Zschieschang et al., Adv. Mater. ; DOI: /adma some images from (Oct. 5, 2011)

Using Chemistry to Catch Counterfeits What’s different about this approach? Instead of a silicon membrane, an organic (i.e. carbon-based) thin film transistor is used. How does this work? A very thin layer of aluminium is exposed to an oxygen plasma which oxidizes the surface to aluminium oxide. A solution of organic phosphonic acid (an acid containing a carbon chain) in a gentle solvent is then washed over the surface and the acid groups react with the surface of the aluminium oxide, bonding to it. Each acid molecule can potentially react with three different oxygen atoms on the aluminium oxide surface, allowing for crosslinking. The resulting circuits are less than 250 nm thick and can be operated at low voltages (less than 3 V). 1 Nature (2010) 468, U. Zschieschang et al., Adv. Mater. ; DOI: /adma some images from (Oct. 5, 2011)