Radio-frequency identification (RFID) tags have made paying toll fees and public transit fares a breeze. But the tags, which are made of silicon, are still too expensive to replace ubiquitous barcodes to similarly speed up grocery store checkout lines by remotely scanning a product while it’s still in the basket.
Cheap plastic RFID tags could soon change that. Researchers in Sunchon, South Korea, have printed RFID circuits on plastic films using a combination of industrial methods: roll-to-roll printing, ink-jet printing, and silicone rubber-stamping. They use inks containing various materials–silver, carbon nanotubes, and a nanoparticle-polymer hybrid–to deposit the circuit’s components, such as capacitors and transistors, layer by layer.
Gyoujin Cho, a professor of printed electronics engineering at Sunchon National University, who led the work, estimates that the tags cost three cents apiece. To replace barcodes, RFID tags will need to cost a penny or less. But Cho says this should be achievable if all the layers on a tag can be deposited with a roll-to-roll process. A version of the current prototype that is capable of holding useful amounts of data should be on the market later this year, he says.
The new RFID tags will be the first product to use printed transistors made from carbon nanotubes. Researchers have been developing nanotube inks for a decade, but the only nanotube electronic product on the market so far is a film for display electrodes. Rick Jansen at carbon nanotube ink maker SouthWest NanoTechnologies says that good quality nanotube inks that are uniform and viscous enough to print have been costly to produce.
Making transistors using nanotube ink is also hard because mixtures are typically two-thirds semiconducting and one-third metallic, and the metallic component makes the mixture conducting overall. Cho and researchers at Paru Corporation in Sunchon have patented a simple process to make nanotube inks semiconducting. They coat the metallic tubes in the solution with a polymer. “You shake them with certain polymers and wrap them up and you just leave them in,” says Rice University chemistry professor James Tour, who was also involved in the new work.
The resulting transistors are large and don’t perform on par with silicon devices. But, says Tour, “RFID tags are a perfect application for them because you only need a handful of bits.”
Making transistor arrays that control the pixels in a flexible display with nanotube ink would be more challenging. “With displays you need better transistors,” he says. “We can print small transistors with carbon nanotube inks, but printing a large number of them with good alignment is hard.” Nevertheless, Cho says, the Korean team is working on making display control circuits with their nanotube transistors.
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.