“PVDF (polyvinylidene fluoride) is a beautiful plastic,” says team member Mengyuan Li. “It can withstand high temperatures, is chemically inert and reasonably cheap. The only problem is that it is difficult to make smooth ferroelectric PVDF – as required for microelectronics applications – but we have now found that processing at high temperatures can solve this problem. Subsequent application of a short electrical pulse to the plastic induces the ferroelectric polar delta-phase in the material.”

Normally, PVDF is made at room temperature and this material is relatively rough and non-ferroelectric because it crystallizes in a non-polar phase. “At high temperatures, however, PVDF turns into a lovely smooth film,” explained team leader Dago de Leeuw, “and, as an added bonus, becomes ferroelectric after applying a short electrical pulse.”

“PVDF actually has four different crystal phases and we have succeeded in forming the delta phase”, he added. This phase was predicted to exist in the 1980s, but had never been seen in a laboratory experiment until now.

Non-volatile memories

The team has already made non-volatile plastic memories from the d-PVDF films – a ferroelectric field-effect transistor and a dual-gate ferroelectric transistor. “These devices can be programmed at voltages exceeding the coercive field of the material and read out in a non-destructive way at lower biases,” Li told nanotechweb.org.

“Although such plastic memories already exist, they are made of a speciality copolymer of PVDF with trifluoroethylene”, explained de Leeuw. “That material is difficult to make, very expensive and also loses its ferroelectric properties at temperatures just above 80 °C. When that happens, you lose all stored data.” On the contrary, films of d-PVDF conserve any stored information up to temperatures of 170 °C and are thus ideal for memory applications.

“The d-PVDF films could be used in a wide range of plastic electronics applications, from smart food packaging (that keeps track of the use-by date) to wearable health monitors”, said de Leeuw. “d-PVDF is also suitable for roll-to-roll processing of ferroelectric microelectronic devices.”

Because the operating voltage of a ferroelectric memory unit based on d-PVDF can be dramatically lowered by decreasing the film’s thickness, the researchers are now busy trying to fabricate thinner, equally smooth films from the polymer. These could make for efficient functional memories that can be programmed and read out at voltages below 5 V.

The current work is published in Nature Materials.