Shape-shifting organic crystals

Researchers at New York University Abu Dhabi have unlocked new possibilities for organic (carbon-based) crystals by making one type with an ability to expand and contract on demand. They say that the large changes achieved by their material could increase the potential for using organic materials in robotics and lightweight soft electronic applications.

Many research teams have previously achieved a degree of success in generating organic crystals that can transform energy into movement, or vice-versa, but until now they have achieved no more than 12% changes in size. According to the authors of this new research, most of the existing systems work only at prohibitively high temperatures and would not be cost-effective for practical applications.

Their new material can achieve what is called a linear stroke (linear size change) of 51%. This gives it the potential to compete with existing mechanical switches within a temperature range between 30 and 0°C.

The work uses single crystals derived from guanidinium nitrate (GN). Temperature changes in the convenient range around room temperate can induce the ions of the crystal to undergo the internal changes in orientation that alter the length of the crystal overall. The team explored a variety of GN derivatives and report full details of one that shows what they describe as “extraordinary expansion on heating.”

The researchers were also able to uncover details of how the change in shape is derived from the ferroelectric properties of the ions in the crystal. These cause the ions to undergo a rapid and reversible phase transition, changing the overall crystal shape due to a rearrangement of hydrogen bonds. Understanding these chemical details could help the utilisation of this material and in designing other similar materials in future.

“This work demonstrates the untapped potential of ionic organic crystals for applications such as light-weight capacitors, dielectrics, ferroelectric tunnel junctions and thermistors,” the authors conclude.

The ability to make lightweight and soft devices may be especially suitable for those that interact with the body, such as medical prosthetics and biometric monitoring applications.

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