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.
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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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