This electron-microscopy-derived composite image shows excitons in green. The moiré unit cell outlined in the lower right of the exciton map is about 8nm in size. Image: Sandhya Susarla and Peter Ercius/Berkeley Lab.Excitons are attracting interest as possible quantum bits (qubits) in tomorrow’s quantum computers, and are also central to optoelectronics and energy-harvesting processes. However, these charge-neutral quasiparticles, which exist in semiconductors and other materials, are notoriously difficult to confine and manipulate.
Now, for the first time, researchers have created and directly observed highly localized excitons confined in simple stacks of atomically thin, two-dimensional (2D) materials. This work, reported in a paper in Science, confirms theoretical predictions and opens new avenues for controlling excitons with custom-built materials.
“The idea that you can localize excitons on specific lattice sites by simply stacking these 2D materials is exciting because it has a variety of applications, from designer optoelectronic devices to materials for quantum information science,” said Archana Raja, co-lead of the project and a staff scientist at Lawrence Berkeley National Laboratory (Berkeley Lab)’s Molecular Foundry, whose group led the device fabrication and optical spectroscopy characterization.
The team fabricated devices by stacking layers of the 2D materials tungsten disulfide (WS2) and tungsten diselenide (WSe2). A small mismatch in the spacing of the atoms in these two materials gave rise to a moiré superlattice, a larger periodic pattern that arises from the overlap of two smaller patterns with similar but not identical spacing of elements. Using state-of-the-art electron microscopy tools, the researchers collected structural and spectroscopic data on the devices, combining information from hundreds of measurements to determine the probable locations of excitons.
“We used basically all the most advanced capabilities on our most advanced microscope to do this experiment,” said Peter Ercius, who led the imaging work at the Molecular Foundry’s National Center for Electron Microscopy. “We were pushing the boundaries of everything we can do, from making the sample to analyzing the sample to doing the theory.”
Theoretical calculations, led by Steven Louie, a faculty senior scientist at Berkeley Lab and a distinguished professor of physics at the University of California (UC) Berkeley, revealed that large atomic reconstructions take place in the stacked materials. These atomic reconstructions modulate the electronic structure to form a periodic array of ‘traps’ where excitons become localized. Discovery of this direct relationship between structural changes and the localization of excitons overturns prior understanding of these systems, and establishes a new approach to designing optoelectronic materials.
The researchers now intend to explore approaches for tuning the moiré lattice on demand and making the phenomenon more robust to material disorder.
This story is adapted from material from Lawrence Berkeley National Laboratory, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.