Dec 15, 2008
Plastic memories on the horizon
Researchers in Belgium have made high-density arrays of ferroelectric polymer nanostructures using a simple nano-embossing technique. The arrays have densities of more than 33 Gbits/inch, a result that promises soft plastic memories compatible with all-organic electronics.
Ferroelectric materials have a permanent dipole moment, like their ferromagnetic counterparts. However, in ferroelectrics, the dipole moment is electric and not magnetic and so can be oriented using electric fields rather than magnetic ones. This opens up a host of novel device applications because it allows electrically digital information to be stored in ferroelectric thin films.
A few plastics are ferroelectric. These materials are attractive because plastics are so easy to process. However, their microstructures are rather disordered, which means that large electric fields are needed to switch the orientation of the dipole moments of single crystals. Typically, a few tens of volts are required for films just 100 nm thick and, to make things worse, heterogeneous behaviour at such small size scales prevents researchers from making high-density storage devices.
Alain Jonas, Zhijun Hu and colleagues at the Louvain Catholic University have now succeeded in simultaneously creating a nanopatterned film of a ferroelectric polymer and decreasing the voltage needed to switch the orientation of the permanent electric dipole on the polymer crystals down to a few volts. The result now means that the structures can safely be made on the nanoscale.
"The voltage range needed now is just 2–5 volts, which is compatible with standard CMOS technology," Jonas told nanotechweb.org. "We also demonstrate writing information at the 100 nm scale, which paves the way to using plastics as high-density storage media in conventional electronics. This is economically and environmentally much less demanding than current alternate technologies."
The work also brings us a step closer to all-plastic electronics that could be integrated into flexible transparent films or even fabrics, he adds.
The fabrication process to make the nanostructures is very simple. The researchers basically shape a polymer film by pressing mould-bearing nanofeatures into it – a technique known as nanoimprint lithography (NIL). While NIL has essentially been used to shape amorphous plastic so far, Jonas and colleagues have employed the technique to shape a semi-crystalline polymer and control its crystallisation. "In essence, we spin-coat a film, heat it, press the mould, anneal and then remove the mould," explained Jonas. "In all, the entire process takes just a couple of minutes and is easily scalable."
Better ferroelectric properties
And that's not all. Because the polymer crystallises in the tiny nanocavities of the mould, the resulting morphology is of a much higher quality than if the mould were not used. This results in much better ferroelectric properties, which in turn means that the voltage needed to store information is significantly decreased.
The researchers say that the arrays produced could be used as storage media for low-power, non-volatile digital storage, such as that found in mobile electronics (phones, MP3 and cameras, for example). The structures could also be integrated into flexible and transparent all-organic electronic devices. "We are currently exploring some of these applications, which obviously require further research before being demonstrated in real commercial devices," said Jonas. A patent protecting the technology is also pending.
The team is now trying to find partners to set up a larger research consortium that would explore different potential commercial applications. "We also plan to explore more basic issues related to crystallisation in nano-confinement and to the scaling down of ferroelectric polymer crystals," revealed Jonas. "We are very excited by the range of possibilities afforded by our discovery."
The work was published in Nature Materials.
About the author
Belle Dumé is contributing editor at nanotechweb.org