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Recently, the research group of the International Functional Materials Quantum Design Center and Physics Department of the Hefei Microscale National Physical Laboratory of the University of Science and Technology of China, University of Science and Technology of China, collaborated with colleagues in and out of the university, predicting that the first class has both in-plane and out-of-plane polarizations through theoretical calculations. Monolayer stable two-dimensional ferroelectric material. The research results are based on the Prediction of intrinsic two-dimensional ferroelectrics in In2Se3 and other III2-VI3 van der Waals materials and were published in the journal Nature Communications 8, 14956 (2017) on April 7. The first authors of the dissertation were doctoral students Ding Wenyi, Zhu Jianbao, and Wang Yu.
As a system with self-generating polarization and its polarization direction can be reversed by an external electric field, ferroelectric materials have a wide range of applications in information storage, field-effect devices, and inductive devices. Research on traditional ferroelectric materials has mainly focused on material systems represented by perovskite oxides. However, when such a ferroelectric material is formed into a thin film by a surface epitaxial growth technique, the ferroelectricity thereof will disappear at a certain critical thickness due to the effect of the depolarization field. Van Dehua's layered two-dimensional system is one of the hot topics in materials research in recent years. Since the first experiment succeeded in obtaining single-layer graphene in 2004, hundreds of new two-dimensional materials have been discovered and synthesized experimentally. They exhibit very rich physical and chemical properties, and further miniaturization of devices in the future. And flexibility provides new opportunities and material foundations. Surprisingly, in all known two-dimensional materials, ferroelectric materials with a single-layered structure and a ferroelectric perpendicular to the two-dimensional surface are not yet available. The reason for this is that there is a contradiction between the symmetry breaking required for forming the vertical polarization and the stability of the material. Therefore, finding a two-dimensional ferroelectric material with vertical polarization and monolayer stability in the van der Waals two-dimensional material system is a challenging scientific problem.
To address this challenge, the team used first-principles calculations to discover that the monolayer of In2Se3, a layered material that is already present in nature, is a stable two-dimensional ferroelectric material with both in-plane and out-of-plane polarization. For the structure of this material, previous experimental studies have shown that its room temperature phase has a layered structure similar to graphite, in which every five atomic layers form a stable two-dimensional unit through covalent bonds, and weak van der Waals between different units. The interactions are combined so that the material can be stripped into thin, even monolayered, two-dimensional films, but previous studies have been indeterminate about the stacking structure of atoms within a single 2D cell. The study first identified the most stable structure of a single 2D cell (as shown in the figure) and found that due to the asymmetry of its atomic layer distribution perpendicular to the 2D plane, it creates a plane perpendicular to the 2D plane. The polarization of self-generated electricity is external, and the direction of its polarization can be reversed by a smart multi-atomic coordinated motion. Further calculations show that the energy barriers needed to reverse the out-of-plane directional polarisation are similar to those of conventional perovskite ferroelectric materials, and that the corresponding energy barrier can be further reduced by applying a vertical external electric field to break the original. The balance between the two polarization directions of energy degeneracy drives the system to change to a certain polarization direction. In addition, since the single layer of the stable structure does not have a central inversion symmetry in the plane, it also has spontaneous in-plane direction of the ferroelectric polarization, and its polarization direction is correlated with the direction of out-of-plane polarization. This is expected to achieve cross-coupling control of the electric field and polarization direction. On the basis of this discovery, it is further predicted that compounds formed from the same elements as In2Se3 can form a similar layered structure and the ferroelectric phase will also become a stable ground state structure.
The discovery of such two-dimensional ferroelectric materials has effectively expanded the functionality of the two-dimensional material family, and in particular has provided new space for the regulation of the physical properties of multi-layer van der Waals two-dimensional heterojunction systems composed of a variety of two-dimensional materials. The study also demonstrated its regulatory ability by constructing a two-layer heterojunction composed of two-dimensional ferroelectric materials and other two-dimensional materials. For example, in the heterojunction composed of In2Se3 and WSe2, the inversion of the In2Se3 direction can be achieved by the external electric field, and the system can be transformed from semiconductivity to near-metallicity (as shown in the figure); in In2Se3 and graphene In the heterojunction, the inversion of the In2Se3 polarization direction can change the height of the Schottky barrier formed between the interfaces. More potential applications for this new type of material need further exploration and research.
This study was funded by the National Thousand Talents Program, the National Natural Resources Commission, the Ministry of Science and Technology, the Chinese Academy of Sciences and the Ministry of Education.
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