Tokyo, Japan – One of the simplest and most beautiful naturally occurring patterns can be observed when light is shined through a pair of slightly misaligned periodic structures. This phenomenon, known as the moiré effect, is not only pretty to look at, but also has important consequences for the properties of materials.
In an article published in ACS Nano, a team led by researchers from the Institute of Industrial Science, The University of Tokyo, announced the discovery of a previously unseen moiré pattern: a series of periodic one-dimensional bands in tungsten ditelluride bilayers.
In nanomaterials, moiré patterns depend on the relative angle between two layers of atoms; by adjusting the angle between the lattices, different patterns can be realized. Typically, this twist angle is small – only a few degrees – since the characteristic size of the pattern decreases with increasing twist angle. However, when the researchers experimented with larger twist angles, something unexpected happened.
"The resulting pattern is a series of parallel stripes," says Yijin Zhang, one of the corresponding authors of the study. "Typical interference patterns look like two-dimensional arrays of bright spots. These one-dimensional bands are completely distinct from all previously known patterns."
This phenomenon can partly be explained by the choice of material. Tungsten ditelluride has a very unconventional crystal structure, consisting of distorted quadrilaterals rather than an ordered honeycomb-like lattice.
"A more disordered lattice means fewer constraints on the twist angle," explains Tomoki Machida, senior author. "By choosing to study this material, we are free to explore the patterns that emerge when the angle is increased significantly."
Through theoretical modeling and transmission electron microscopy experiments, the team was able to confirm that the one-dimensional bands occur precisely at twist angles of 61.767º and 58.264º. Perturbing the angle even by a tenth of a degree causes the interference pattern to revert to the traditional bright spots.
"Moiré patterns govern the optoelectronic properties of materials, so this discovery opens the door for engineering materials with uniquely anisotropic properties," says Zhang. "For example, it may soon be possible to tune nanomaterials to conduct heat or electricity in a particular direction."
The researchers hypothesize that other materials also possess similar one-dimensional patterns at large twist angles and are currently searching for them, as well as devising ways to apply their discovery to the study of one-dimensional phenomena. Regardless of what they find, more interesting interference patterns are almost certain to follow.
