New material changes shape when exposed to light

Scientists have identified a crystalline material that changes shape in response to light, and could help develop novel light-activated devices.
Perovskite crystals have received a lot of attention for their efficiency at converting sunlight into electricity.
Now, scientists at King Abdullah University of Science and Technology (KAUST) in Saudi Arabia show that the potential uses of the crystals extend far beyond the light- harvesting layer of solar panels.
Photostriction is the property of certain materials to undergo a change in internal strain, and therefore shape, with exposure to light.
KAUST electrical engineer Jr-Hau He and his colleagues have looked for photostriction in a new family of materials, the perovskites.

The researchers tested a perovskite called MAPbBr3 and revealed it had strong and robust photostriction behaviour.

To extensively test the material's photostriction capabilities, the team used Raman spectroscopy, which probes the molecular vibrations within the structure.
When bathed in light, photostriction alters the internal strain in the material, which then shifts the internal pattern of vibrations.
By measuring the shift in the Raman signal when the material was placed under mechanical pressure, the team could calibrate the technique and so use it to quantify the effect of photostriction.
"We will use this material to fabricate next-generation optoelectronic devices, including wireless remote switchable devices and other light-controlled applications," said Tzu- Chiao Wei, a member of the team.

"We demonstrated that in situ Raman spectroscopy with confocal microscopy is a powerful characterisation tool for conveniently measuring intrinsic photo-induced lattice deformation," Wei said.
"The same approach could be applied to measure photostriction in other materials," he said.
The perovskite material proved to have a significant photostriction coefficient of 1.25 per cent.
The researchers also showed that the perovskite's photostriction was partly due to the photovoltaic effect - the phenomenon at the heart of most solar cell operation.
The spontaneous generation of positive and negative charges when the perovskite is bathed in light polarises the material, which induces a movement in the ions the material is made from.

The robust and stable photostriction of perovskite makes it useful for a range of possible devices, said Wei.