Passive RFID tags, which are used to track objects, are made of two main parts: a silicon integrated circuit and an antenna that's typically made of solid copper or printable silver ink. The antenna coil captures AC power from the reader's radio frequency signal, and the AC power is converted into DC power at a rectifier circuit. Another circuit uses this power to generate the signals that are transmitted back to the reader, conveying the information stored on the tag.

Cho and his colleagues start by using a roll-to-roll process to deposit the antenna coils, a bottom electrode layer of silver ink, and a subsequent insulating layer, a barium titanate nanoparticle-polymer hybrid ink. Next, they put down layers of carbon nanotube inks using an ink-jet printer to make the circuit's transistors. Finally, they use a silicone rubber stamp to print the capacitors and diodes needed to make the RFID tag's rectifier circuit. They use an ink of cobalt-doped zinc oxide nanowires to make the semiconducting layer in the diode, and aluminum paste for the top electrodes. The researchers outline their process in the March issue of the journal IEEE Transactions on Electron Devices.

The finished tag is three times the size of a standard barcode, and it stores just one bit of information, a 1 or a 0, so it can only give a yes or no response to the reader. Cho says that a 64-bit tag should be available on the market next year. The final goal is a 96-bit tag to replace barcodes.

"The real impact would be if they can compete in price," says Pulickel Ajayan, a mechanical engineering and materials science professor at Rice who wasn't involved in the work. "That's one of the reasons why nanotubes might come into play. It's a roll-to-roll process, which makes it feasible to get into the market."

Improving the resolution and accuracy of the roll-to-roll printer should give smaller tags that carry more information, Cho says. But they also need to improve the circuit so it emits higher power signals. The reader only works up to 10 centimeters away right now--not yet enough to work at a checkout line.