Crystal changes shape in ultraviolet light

By Kimberly Patch, Technology Research News

A crystal is usually a very stable chemical structure that withstands many stresses that would completely transform liquids and gases.

A group of researchers from Kyushu University in Japan, however, has found that diarylethene crystal will change from colorless to blue when exposed to ultraviolet light, then lose its color again in the presence of visible light. The color change points to structural changes in the crystal that could eventually be harnessed to drive microscopic devices.

The researchers discovered the phenomenon accidentally, then figured out exactly how the color change happened using x-rays and an atomic force microscope. The crystal literally rearranges its chemical bonds as it absorbs ultraviolet photons. "X-ray crystallographic analysis of the crystals revealed that the crystals reversibly change [their] volume by the photo irradiation," said Masahiro Irie, a chemistry professor at Kyushu University.

The researchers then looked at the surface of the crystals with an atomic force microscope, and found that the chemical bond rearrangement translates to tiny steps on the surface of the crystal.

The ultraviolet light literally shrinks each molecule by a tiny amount, and the shrinkage causes the steps. When the shrinkage reaches 600 molecular layers deep, those 600 layers are reduced by the width of one molecule, resulting in a one nanometer step on the surface of the crystal, according to the researchers. The reactions can take place as deep as 500 microns, or about 500,000 molecular layers, to create larger steps, according to the researchers.

These steps on the surface of the crystal changes the way it reflects light, making it appear blue. The visible light reverses the molecular bond changes, erasing the steps, and making the crystal clear again.

The researchers ultimately plan to develop the material into a photomechanical device that directly converts photon energy to mechanical work, he said. The crystals could eventually be used as nanoscale actuators because they change thickness but require no moving parts to do so, said Irie. "We hope to use them for nanomechanical systems [that function using] noncontact photon energy."

The crystals could be used in devices in about five years, said Irie.

Irie's research colleagues were Seiya Kobatake and Masashi Horichi. They published the research in the March 2, 2001 issue of the journal Science. The research was funded by the Japan Science and Technology Corporation (JST) and by the Japanese Ministry of Education, Culture, Sports, Science and Technology.

Timeline:   5years
Funding:   Corporate, Government
TRN Categories:  Semiconductors and Materials
Story Type:   News
Related Elements:  Technical paper, "Reversible Surface Morphology Changes of a Photochromic Diarylethene Single Crystal by Photoirradiation," Science, March 2, 2001.


April 18, 2001

Page One

Defects boost disc capacity

Alternative quantum bits go natural

Light powers molecular piston

Bumps could make better biochips

Crystal changes shape in ultraviolet light


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