Aug 22, 2007
Nanoink on target to hit flexible price points
"A critical step towards all-printed electronics" is how scientists are describing the fabrication of high-resolution organic transistors on flexible polymer substrates. Key to the group's success is the use of laser-sintered, inkjet-printed metal nanoparticles – a technique that came to light back in 2004.
Today, researchers from the University of California, Berkeley, US, and ETH Zurich, Switzerland, think that they are close to hitting the required price points and expect to roll out a prototype large area display within the next four years. Other applications set to benefit include ultra-low cost RFID tags, thin-film photovoltaics and flexible sensors.
At the heart of the printing process is a nanoink of gold particles (diameter 1–3 nm) stabilized by a self-assembling monolayer (SAM). "The size of the nanoparticles and the fluidic properties of the carrier solvent are some of the most important considerations," Seung Ko told nanotechweb.org. "The size-dependent melting temperature depression is critical to our work and can be observed only for particles smaller than 10 nm." Bulk gold starts to melt above 1063 °C, whereas the melting point of a 2 nm diameter gold nanoparticle is just 150 °C.
The team uses an argon ion laser beam (wavelength 514.5 nm) to selectively sinter the original printed pattern, reducing the line-width from an inkjet limited resolution of around 50 µm to less than 10 µm. According to the researchers, laser-sintered gold lines show much greater uniformity and higher resolution (down to 1–2 µm) than inkjet printed patterns that have simply been treated with a heater. Surplus nanoink is easily washed away using an inorganic solvent.
"We use a simple filtering process to reclaim any unsintered gold nanoparticles," explained Ko. "The SAM protected nanoparticles are very stable and can be re-applied to reduce manufacturing costs without affecting the quality of the device."
Ko and his colleagues have shown that their plastic compatible process suits the fabrication of organic field effect transistors (OFETs), a building block for more complex devices. To evaluate the sintered electronics, the team compared its OFET with a device fabricated using conventional lithography.
"We have yet to run the tests over the long term (years), but the laser-sintered devices were as good as the lithographically processed electronics over a period of several months," said Ko. "Currently, we are performing fatigue testing under mechanical cyclic loading – the performance of our device has not degraded after 65,000 cycles."
The researchers presented their work in Nanotechnology.
About the author
James Tyrrell is editor of nanotechweb.org.