New memory from plastic

Plastic electronics are hot. They’re inexpensive, flexible and can be used in food packaging or clothing. But although many of the components had already been realized, something was missing: an efficient memory element. The existing plastic memory was made from a very expensive specialized polymer, and lost all stored information when heated beyond 80 degrees Celsius.

Scientists from the Zernike Institute for Advanced Materials at the University of Groningen and the Max Planck Institute for Polymer Research have found a cool (or rather, hot) solution to both problems. Their work has just been published in Nature Materials .

The existing plastic memory is based on a polymer with ferroelectric properties. This material exhibits an electric polarization, comparable to the north and south poles of a magnet. The polarization of ferroelectric materials can be switched by the application of an electric field. The bistable polarization state (high or low, up or down, 0 or 1) can be used to store information.

The commodity polymer PVDF was recognized early on as a good candidate for the fabrication of plastic memory, because it is not only very inexpensive but is thermally and chemically extremely stable and can have ferroelectric properties. But making a functional electric switch from neat PVDF proved to be notoriously difficult.

‘There are two reasons for this’, says Professor of Physics Dago de Leeuw, one of the authors of the Nature Materials article. ‘First, it was very challenging to make a smooth, thin film from PVDF.’ The resultant films were rough, resembling microscopic sandpaper. ‘In addition, conventional processing yields non-ferroelectric thin films, because the PVDF crystallizes in a non-polar phase.’

University of Groningen PhD student Mengyuan Li was able to solve these problems. She used an alternative method to produce thin films from PVDF. ‘Controlling the processing conditions turned out to be the crucial step’, says Li. De Leeuw adds, ‘Usually you make this type of film at room temperature. PVDF, however, turns into a lovely smooth thin film at high temperatures.’ As a bonus, the films became ferroelectric after applying a short electrical pulse.

‘PVDF has four different crystal phases’, De Leeuw continues. With her work, Li has achieved the delta phase, which is both ferroelectric and stable at high temperatures. Delta-PVDF was predicted in the 1980s, but had never been experimentally proven in a thin film.

In the article, the researchers show that delta-PVDF can be used to make plastic memory. The film preserves the stored information up to temperatures of about 170 degrees Celsius. As a result, delta-PVDF is the ideal candidate for data storage in plastic electronics.

Plastic electronics has developed rapidly and is now on the verge of commercialization, promising new applications from smart food packaging that keeps track of the expiry date to wearable health monitors. All these applications require programmable non-volatile memory.

The University of Groningen has already made several breakthroughs in the field of plastic electronics. De Leeuw says, ‘Both the plastic ferroelectric transistor and the plastic ferroelectric diode were invented at ZIAM. We can now add usable plastic memory to that list.’ The eMbedded Organic Memory Arrays (MOMA) European project is presently upscaling this new technology.