An example of the reflective color display developed by researchers at Linköping University. Photo: Thor Balkhed.
An example of the reflective color display developed by researchers at Linköping University. Photo: Thor Balkhed.

Researchers at Linköping University in Sweden have developed a method that may lead to new types of displays based on structural colors. Their discovery, reported in a paper in Advanced Materials, opens the way to cheap and energy-efficient color displays and electronic labels.

Colors are usually created by pigments, which absorb light at certain wavelengths and scatter light at other wavelengths. That's why leaves, for example, are green and tomatoes are red. But colors can be created in other ways, with some materials appearing colored due to their structure.

Structural colors can arise when light is internally reflected inside a material on a scale of nanometers; this is usually referred to as interference effects. An example found in nature are peacock feathers, which are fundamentally brown but acquire their characteristic blue-green sheen from small structural features.

Researchers at Linköping University, led by Magnus Jonsson, associate professor in the Laboratory of Organic Electronics, have now developed a new and simple method to create structural colors for use with reflective color displays. This new method may allow the manufacture of thin and lightweight displays with high energy-efficiency for a broad range of applications.

Reflective color displays differ from the color displays found in everyday devices such as mobile phones and computers, which consist of small light-emitting diodes of red, green and blue positioned close to each other so that together they create white light. The color of each light-emitting diode depends on the molecules from which it is built, or, in other words, its pigment. However, it is relatively expensive to manufacture light-emitting diodes, and the global use of emissive displays consumes a lot of energy.

Reflective displays are therefore being explored for applications such as the tablet computers used as e-readers and electronic labels. Reflective displays form images by controlling how incident light from their surroundings is reflected, which means they do not need their own source of illumination. But most reflective displays are intrinsically monochrome, and attempts to create color versions have proved rather complicated and have sometimes given poor results.

"We have developed a simple method to produce structural color images with electrically conducting plastics, or conducting polymers," says Shangzhi Chen, a newly promoted doctor at the Laboratory of Organic Electronics at Linköping University and principal author of the paper. "The polymer is applied at nanoscale thicknesses onto a mirror by a technique known as vapor-phase polymerization, after the substrate has been illuminated with UV light. The stronger the UV illumination, the thicker the polymer film, and this allows us to control the structural colors that appear at different locations on the substrate."

This method can produce all the colors in the visible spectrum. Furthermore, the colors can be adjusted by electrochemically varying the redox state of the polymer. This function has been popular for monochrome reflective displays, and the new study shows that the same materials can provide dynamic images in color by using optical interference effects combined with spatial control of nanoscale thicknesses.

"We receive increasing amounts of information via digital displays, and if we can contribute to more people gaining access to information through cheap and energy-efficient displays, that would be a major benefit. But much research remains to be done, and new projects are already under way," says Jonsson.

This story is adapted from material from Linköping University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.