Nanoscale polar whirlpools emerge in atom-thin semiconductors

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 polarisation, 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 polarisation behaves within this pattern and directly reveal a network of merons and antimerons.

Equal lead author Dr Emily Vu, a former Monash University PhD student, said the research provided experimental evidence that these unusual topological structures exist in twisted two-dimensional semiconductors.

“Here we utilise angle-resolved, high-resolution vector PFM to spatially resolve polarisation components and topological polar nanostructures in marginally twisted bilayer WSe₂ and provide experimental evidence for the existence of topologically non-trivial meron/antimeron structures,” Vu said.

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 postdoc at Deakin University.

The experimental results were backed by theoretical modelling and large-scale simulations, which reproduced the circulating polarisation and confirmed the characteristics expected of merons and antimerons.

Co-lead author Assistant Professor Daniel Bennett, formerly a post doc 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 polarisation and winding. That’s the clearest evidence yet of real merons and antimerons in a twisted semiconductor,” Bennett said.

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 polarisation patterns are being produced by twist, strain or a combination of the two.

Associate Professor Mark Edmonds, from the School of Physics and Astronomy, is the study’s last author. The international study involved researchers from 18 institutions, including Monash University, Harvard University, Nanyang Technological University, NIMS Japan, Penn State University, Flinders University, Swinburne University of Technology and the ARC Centre of Excellence in Future Low-Energy Electronics Technologies (FLEET).

Image credit: iStock.com/Nutjaree Yomjun