By Josh Perry, Editor
[email protected]

Researchers from the U.S. Department of Energy (DoE) Lawrence Berkeley National Laboratory (Berkeley, Calif.) developed a method for stacking two-dimensional layers of tungsten disulfide and tungsten diselenide into superlattices that turned the semiconductors into quantum material.

The twist angle formed between atomically thin layers of tungsten disulfide and tungsten diselenide acts as a “tuning knob,” transforming these semiconductors into an exotic quantum material. (Berkeley Lab)

According to a report from Berkeley Lab, the research team created a moiré superlattice, which is a repeating pattern of well-aligned 2-D materials. The team found that at the twist angle between the layers, a “turning knob” was formed that turned the 2-D semiconductor into a quantum material.

This breakthrough was discovered while studying the absorption of light by the 2-D device. Instead of finding one peak to signal the energy of excitons in the material, but instead of just one peak there were three.

Using a transmission electron microscope, the researchers visualized the tungsten disulfide/tungsten diselenide layers to see how they were aligned and saw that a superlattice had formed.

“The researchers next plan to measure how this new quantum system could be applied to optoelectronics, which relates to the use of light in electronics; valleytronics, a field that could extend the limits of Moore’s law by miniaturizing electronic components; and superconductivity, which would allow electrons to flow in devices with virtually no resistance,” the article added.

The research was recently published in Nature. The abstract read:

“Moiré superlattices enable the generation of new quantum phenomena in two-dimensional heterostructures, in which the interactions between the atomically thin layers qualitatively change the electronic band structure of the superlattice. For example, mini-Dirac points, tunable Mott insulator states and the Hofstadter butterfly pattern can emerge in different types of graphene/boron nitride moiré superlattices, whereas correlated insulating states and superconductivity have been reported in twisted bilayer graphene moiré superlattices.

“In addition to their pronounced effects on single-particle states, moiré superlattices have recently been predicted to host excited states such as moiré exciton bands.

“Here we report the observation of moiré superlattice exciton states in tungsten diselenide/tungsten disulfide (WSe₂/WS₂) heterostructures in which the layers are closely aligned. These moiré exciton states manifest as multiple emergent peaks around the original WSe₂ A exciton resonance in the absorption spectra, and they exhibit gate dependences that are distinct from that of the A exciton in WSe₂monolayers and in WSe₂/WS₂ heterostructures with large twist angles.

“These phenomena can be described by a theoretical model in which the periodic moiré potential is much stronger than the exciton kinetic energy and generates multiple flat exciton minibands. The moiré exciton bands provide an attractive platform from which to explore and control excited states of matter, such as topological excitons and a correlated exciton Hubbard model, in transition-metal dichalcogenides.”