Crystals step up to a new surface

You can buy sunglasses that become darker when you go outside. Similarly, some crystals reversibly change color from, say, clear to blue when exposed to certain wavelengths of light.

These are nice tricks. But researchers in Japan have now found that light can trigger a more profound reaction in certain crystals–a change in shape. The crystals are the first that scientists have gotten to reversibly change shape just by shining the right light on them, says Masahiro Irie of Kyushu University.

Because of this novelty, such crystals could eventually serve as photo-driven actuators in nanoscale machines, Irie’s team reports in the March 2 Science.

The researchers developed diarylethene crystals that change from transparent to blue under ultraviolet (UV) light and then revert to clear when irradiated with visible light. In addition, the studies with an atomic force microscope revealed that as the crystal turns blue, it simultaneously forms minuscule steps on its surface, says Irie. Each step has a height that’s a multiple of about 1 nanometer, the height of one molecular layer in the crystal.

When UV light hits the crystal, both the length and thickness of constituent molecules shrink, causing both the color and surface changes in the crystal, says Irie. Future nanoscale machines could be made with such crystals, Irie suggests. For example, the crystals might be used to make tiny tweezers whose action is controlled with light.

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