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: Monash University-led researchers have directly imaged tiny swirling structures inside an atomically thin semiconductor, opening new possibilities for future low-energy electronic technologies. Published in Science Advances, the study reveals structures known as merons and antimerons, nanoscale "whirlpools" of electrical polarization, in twisted layers of the semiconductor tungsten diselenide (WSe₂).
The researchers stacked two atom-thin layers of the material and twisted them by just 0.1 degrees, creating a repeating nanoscale pattern known as a moiré superlattice. Using high-resolution imaging, they were able to map how electrical polarization behaves within this pattern and directly reveal a network of merons and antimerons. Emily Vu, a former Monash University Ph.D. student, said the research provided experimental evidence that these unusual topological structures exist in twisted two-dimensional semiconductors.
"Here we use angle-resolved, high-resolution vector PFM to spatially resolve polarization components and topological polar nanostructures in marginally twisted bilayer WSe₂ and provide experimental evidence for the existence of topologically nontrivial meron/antimeron structures," said Vu. The researchers were also able to distinguish between the effects of twisting the material and strain, an important distinction when trying to understand and ultimately control the structures. "By constructing vector maps we were able to differentiate between twist and strain and quantify the contributions of each in a moiré superlattice," said Vu, now a postdoctoral researcher at Deakin University.
The experimental results were backed by theoretical modeling and large-scale simulations, which reproduced the circulating polarization and confirmed the characteristics expected of merons and antimerons. Co-lead author Assistant Professor Daniel Bennett, formerly a postdoc at Harvard and now at Nanyang Technological University, Singapore, said combining experiment and theory provided particularly strong evidence for the discovery. "With differing interpretations across recent studies, we needed experiment and theory together.
Our PFM measurements across different samples and twist angles, backed by DFT and a full moiré-scale molecular dynamics model, reveal the same circulating polarization and winding. That's the clearest evidence yet of real merons and antimerons in a twisted semiconductor," said Bennett. Topological polar structures have previously been observed mainly in much thicker oxide materials.
In this study, the meron–antimeron network emerges in a semiconductor bilayer only a few atoms thick. This creates new possibilities for developing ultra-thin, low-energy electronic devices in which these nanoscale states could potentially be manipulated using electric fields, strain or engineered substrates. The technique also gives researchers a new way to investigate twisted two-dimensional materials and determine whether their polarization patterns are being produced by twist, strain or a combination of the two.
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