This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: You may have once tried taking a close-up picture of a computer screen and noticed a wavy, rippling effect. This optical effect occurs whenever two fine, repeating grids overlap and slightly misalign, such as when the pixel grid of your camera's sensor overlaps with the pixel grid of the screen.
Photographers know this wavy effect as a moiré pattern and usually try to get rid of it. Researchers at the University of Twente, in collaboration with researchers from Utrecht, Brazil and Japan, created one on purpose. They stacked two sheets of molybdenum disulfide, a semiconductor crystal just three atoms thick, and rotated the top sheet by 2 degrees.
The resulting pattern changes how the material interacts with light and repeats every 9 nanometers. Ten thousand of those repeats fit across the width of a single human hair. The research, now published in Nature Physics, has possible applications in ultrasmall light sources and optical sensors.
When light hits a semiconductor, its energy can be absorbed to create pairs of particles called excitons. These pairs largely determine how the material absorbs and emits light. Scientists have long suspected that a moiré pattern can trap excitons at fixed positions, which would offer a way to control light in a programmable grid.
But until now, nobody had been able to see this behavior directly. Optical measurements average over thousands of repeats of the pattern, and sharper microscopy methods only worked at extremely low temperatures. "With this new method, we have demonstrated that this moiré pattern indeed acts as a nanoscale landscape that guides and traps excitons in specific locations at room temperature," says Pantelis Bampoulis, who led the research.
First author Laurens Westenberg scanned the stacked layers with an atomically sharp needle while light of a precisely tuned color illuminated them from below. At each point, the needle detected the tiny electric current generated by the light, allowing the team to map where light energy is absorbed. The resulting maps show where each type of exciton sits, with nanometer precision.
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