Molecular layer acts like chameleon

May 1, 2001
NAGOYA—Researchers at the Nagoya Institute of Technology have developed a structural color-forming system that exhibits chameleonlike change of color when influenced by external stimuli.

THIN-FILM INTERFERENCE

Incorporating news from O plus E magazine, Tokyo

NAGOYA—Researchers at the Nagoya Institute of Technology have developed a structural color-forming system that exhibits chameleonlike change of color when influenced by external stimuli. Color-formation mechanisms can be separated into two main categories: changes in chemical structure, and changes in mechanical structure. Color formation as a result of chemical structure occurs when light hits an object and causes an electron transition. In contrast, color formation due to mechanical structure is based on the thickness and refractive properties of a layer of material. The new method is of the latter type and involves creating a unimolecular film of a proteinlike material on a silicon substrate (see figure).

Click here to enlarge image

Rodlike polypeptide molecules layered in parallel sheets create a thin-film reflective interference filter (left). The molecules can also be oriented perpendicular to the substrate (right). In the latter case, an external stimulus such as an electric or magnetic field, a temperature change, or irradiation by light changes the orientation of the molecules, altering the color of the filter.

The layer consists of rodlike polypeptide molecules that are aligned in the parallel direction. The experiment revealed that, when 40 to 160 layers of this material are stacked on top of one another, the color of the material changes depending on the number of layers. The group has succeeded in creating a structural color-forming wafer that can change color on the basis of just light interference, without the use of dyes or pigments.

In a further step, a stimulus response group can be added to the rodlike polypeptide and the molecules aligned in the perpendicular direction. Because the rodlike molecules are 15 to 20 nm long, a film of this thickness forms on the substrate. When external stimuli such as an electric field, magnetic field, temperature change, or irradiation is applied, the length and angle of the molecules change; the thickness of the film changes accordingly, leading to a color change. For the experiment, azobenzene was used as the stimulus response group. Azobenzene undergoes optical isomerization under the influence of ultraviolet light. Irradiating the azobenzene-added molecules changed the structure and refractive properties of the material.

Plans are under way to create materials that change color under the influence of electric fields and temperature gradients. The results of this research can be applied to ultrathin displays, as well as to novel types of reflective displays and sensors.

Courtesy O plus E magazine, Tokyo

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