Recently, researchers at Beijing Academy of Quantum Information Sciences (BAQIS) realized SnTe/PbTe monolayer lateral heterostructures and discovered both clockwise or counterclockwise vortex-oriented quadrant configurations of ferroelectric domains for the first time. This work, published online in Advanced Materials on July 3rd, 2021, could bring the application of two-dimensional (2D) in-plane ferroelectric heterostructures in nanoscale quantum devices one step closer. This work is in cooperation with Max Planck Institute of Microstructure Physics, Halle, Germany, and Department of Physics, University of Arkansas, USA.

After more than a decade of research, 2D materials continue to exhibit superior performances in novel heterostructures and devices. Their weak inter-layer coupling allows the relatively straightforward fabrication of vertical heterostructures by mechanical stacking of 2D materials. “However, the creation of lateral heterostructures (LHSs), is much less explored as it requires more complex growth and doping techniques,” said Dr. Kai Chang, Principal Investigator of Team of Low-Dimensional Quantum Materials at BAQIS, who is the first author and one of the corresponding authors of this work.

Five years ago, for the first time, Dr. Chang fabricated high-quality thin films of orthorhombic SnTe down to the limit of a single van der Waals monolayer with stable in-plane spontaneous polarization, and discovered a huge enhancement of ferroelectric transition temperature in comparison of its bulk counterpart (from ~100 K to 270 K). Since then, the study of in-plane ferroelectric materials has rapidly developed. However, experimental studies of LHSs containing 2D ferroelectric materials—and the concomitant understanding of their interfacial tuning effects—are still rare. In this paper, Dr. Chang and colleagues took a step forward by the molecular beam epitaxial (MBE) growth and low-temperature scanning tunneling microscope (STM) characterization of the LHSs between two distinct group-IV monochalcogenide monolayers—an in-plane polarized ferroelectric SnTe ML, and a paraelectric PbTe ML. They revealed interesting vortex-oriented domain quadrant configurations (see the figure below) and elaborated the mechanism behind this phenomenon.

Crystalline structure and quadrant vortex domains in SnTe/PbTe ML LHSs. (a) Schematic of the structure of the SnTe/PbTe LHS plates. (b) Lattice structure of the LHS. (c) STM topography images of an LHS plate. The arrows indicate the direction of polarization in each ferroelectric domain. d) Spontaneously acquired differencial conductance image displaying different moiré patterns in each domain.

“In order to minimize electrostatic and elastic energies, the SnTe monolayer at the perimeter of these nanoplates develop in-plane vortex-oriented quadrant domains whose polarizations point inwards,” said the authors. The electrostatic energy is determined by the difference of work functions between SnTe and PbTe monolayers, which was characterized by the measurement of Gundlach oscillation, a phenomenon of electron tunneling through the triangular barrier between the STM tip and the sample at a relatively high bias voltage.

The novel vortex-oriented domains in 2D material lateral heterostructures hereby demonstrated highlights the possibilities of engineering the polarization state of 2D ferroelectrics with an in-plane polarization, and opens unforeseen opportunities for use of these novel materials.

Work at BAQIS was supported by National Natural Science Foundation of China (Grant No. 12074038).

URL: https://onlinelibrary.wiley.com/doi/10.1002/adma.202102267