Ferroelectric materials have drawn much research interest due to their possible uses in data storage. Additionally, their piezoelectric, thermoelectric, and nonlinear optical characteristics have been thoroughly researched in fields like renewable energy, micro-electro-mechanical systems, and optical devices.
Due to their minimal dimensionality, two-dimensional (2D) ferroelectric materials have recently become a contender for neuromorphic synapse devices. The lack of readily accessible materials, however, continues to constrain the development of 2D ferroelectric materials.
Earlier this year, a study team led by Professor Andrew WEE from the Department of Physics at NUS made the ground-breaking discovery of the single-element ferroelectric state in the 2D material black phosphorus-like bismuth (BP-Bi), which challenged the previously mentioned conventional understanding of ferroelectricity.
The centrosymmetry-breaking on the atomic structure and charge transfer between sublattices in BP-Bi was closely examined by the researchers using optimized scanning tunneling microscopy (STM) and non-contact atomic force microscopy (nc-AFM) methods.
The single-element ionicity, single-element in-plane polarization, and single-element ferroelectricity were all experimentally proven for the first time in the bismuth monolayer. This finding challenges the idea that ionic polarisation only occurs in molecules containing cations and anions and broadens the potential applications of ferroelectricity in the future.
Scientists created the high-quality BP-Bi on the Van der Waals graphite surface to ensure that the monolayer BP-Bi is unbroken and sufficiently flat for the measurements. AFM imaging and Kelvin probe microscopy (KPFM) measurements were used to use the high spatial resolution of nc-AFM to identify the buckling atomic configuration (Dh0) of BP-Bi and the charge redistribution between the two sublattices.
The BP-Bi monolayer’s regular in-plane dipole configuration is subsequently confirmed. In contrast, single-layer phosphorous (phosphorene) is centrosymmetric and nonpolarized because there is no buckling in the sublayers. The in-plane electric field generated by the STM tip is then utilized to achieve polarisation switching of BP-Bi, which forms the foundation for writing on non-volatile memory devices.
Ferroelectricity has an advantage over magnetism because it can be controlled only by an electric field. Because of this, it is better suited to be included in integrated circuit devices. Many investigations have discovered that by combining ferroelectricity with these qualities, it is possible to control other material properties.
The ferroelectric polarization and fundamental band structure in BP-Bi are controlled by the degree of bending in the atomic structure. As a result, the ferroelectric polarization and electronic structure are interlocked. This novel form of ferroelectricity presents a promising approach to ferroelectric distortion, which can be used to alter the electronic structure of materials by an external electric field.
Dr. Jian GOU, the lead author for the research paper, said, “Other research has also shown that BP-Bi exhibits topologically nontrivial states at a specific buckling height, suggesting a potential opportunity for tuning topological states through an electric field.”
In actuality, the polarization features significantly influence the fundamental optical and electrical properties of materials. The study of the fundamental physical characteristics of simple substances gains a fresh perspective with the discovery of single-element ferroelectric polarization.
Prof Wee said, “Besides overturning the common-sense idea that ionic polarisation only exists in compounds, we believe that single-element ferroelectricity in BP-Bi would introduce a new perspective to the study and design of novel ferroelectric materials and inspire new physics of elemental materials in the future.”
- Gou J*; Bai H; Zhang X; Huang YL; Duan S; Ariando A; Yang SA; Chen L*; Lu Y*; Wee ATS*, “Two-Dimensional Ferroelectricity in a Single-Element Bismuth Monolayer” NATURE DOI: 10.1038/s41586-023-05848-5 Published: 2023.
- Gou J; Kong L; He X; Huang YL; Sun J; Meng S; Wu K; Chen L*; Wee ATS*, “The Effect of Moiré Superstructures on Topological Edge States in Twisted Bismuthene Homojunctions” SCIENCE ADVANCES Volume:6 Issue:23 Article Number: eaba2773 DOI: 10.1126/sciadv.aba2773 Published: 2020.