Researchers led by Monash University have managed to directly observe tiny swirl-shaped structures inside a semiconductor only a few atomic layers thick, a discovery that could open new possibilities for future low-energy electronic technologies.
The study, published in the journal Science Advances, identified structures known as merons and antimerons — “vortices” at the nanometer scale of electric polarization — in twisted layers of tungsten diselenide (WSe2).
To create these structures, the researchers stacked two atomic layers of the material and twisted them by only 0.1 degrees, giving rise to a nanometer-scale periodic pattern known as a moiré superlattice. Using high-resolution imaging techniques, they mapped the behavior of the electric polarization in this pattern and directly revealed a network of merons and antimerons.
The study’s principal author, Emily Vu, a former PhD student at Monash University and currently a postdoctoral researcher at Deakin University, states that the work provides experimental evidence for the existence of these topological structures in twisted two-dimensional semiconductors.
“Here we use high-resolution angle-dependent vector PFM to spatially resolve the components of polarization and the topological polar nano-structures in slightly twisted WSe₂ bilayer, providing experimental evidence for the existence of nontrivially topological meron/antimeron structures,” says Emily Vu.
The researchers were also able to distinguish the effects caused by twisting from those arising from mechanical strain in the material, a difference deemed important for understanding and, in the future, controlling these structures.
“By constructing vector maps, we were able to differentiate twisting from strain and quantify the contribution of each in a moiré superlattice,” she adds.
The experimental results were complemented by theoretical models and large-scale simulations, which reproduced the circulating polarization and confirmed the expected characteristics of merons and antimerons.
Daniel Bennett, co-lead author of the study, a former postdoctoral researcher at Harvard and currently an assistant professor at Nanyang Technological University, in Singapore, emphasizes the importance of combining experimental observations and theoretical models.
“With differing interpretations in recent studies, we needed to bring together experience and theory. Our PFM measurements on different samples and twist angles, supported by DFT and by a full moiré-scale molecular dynamics model, reveal the same circulating polarization and the same winding. This is the clearest evidence to date of the real existence of merons and antimerons in twisted semiconductors,” he says.
Topological polar structures had already been observed primarily in much thicker oxide materials. In this case, however, the network of merons and antimerons arises in a bilayer of a semiconductor with only a few atomic layers.
The discovery could contribute to the development of ultrathin, low-power electronic devices in which these nanometer-scale states could potentially be manipulated through electric fields, mechanical strain, or substrates designed for that purpose.
The technique used also offers a new way to study twisted two-dimensional materials and determine whether the observed polarization patterns result from twisting, from mechanical strain, or from a combination of both.
The study’s last author was Mark Edmonds, associate professor at Monash University’s School of Physics and Astronomy, and it 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).
