A set of fluorescent microscopic images of a (PEA)2PbI4 crystal taken at the same lamp power

A crystal glows different colours depending on how much it is compressed (lower right, at atmospheric pressure; upper left, at roughly 35,000 times atmospheric pressure). Credit: Liu Sheng

Optics and photonics

‘Chameleon’ crystals change colour under pressure

Layered materials called perovskites give in to pressure by glowing red, orange and yellow.

A crystal that glows green under normal circumstances shines in a wide range of colours when put under pressure.

The class of crystals called perovskites hold promise for use in nanometre-scale lasers and other light-emitting devices. Qihua Xiong at Nanyang Technological University in Singapore and his colleagues studied a 2D perovskite made of alternating layers of organic molecules and a compound called lead iodide. The researchers bathed the perovskite in light, which excited some of the material’s electrons and allowed them to escape their original atoms. Eventually the electrons rejoined atoms and, as a result, released their excess energy as photons.

This process caused the crystal to glow green when it was at atmospheric pressures. But as the team increased the pressure on the crystal, the electrons escaped their atoms at lower energies. That meant that when the electrons rejoined atoms, they released lower-energy, redder photons.

At 3.5 gigapascals, which is roughly 35,000 times atmospheric pressure, the crystal glowed red. When the pressure was reduced, the crystal reverted to its original green. At intermediate pressures, it glowed orange and yellow.