Physicists at the National University of Singapore (NUS) have discovered a new type of ferroelectricity in a single-element bismuth monolayer that can generate regular and reversible dipole moments for future applications such as nonvolatile storage and electronic sensors.
The phenomenon of certain materials exhibiting spontaneous electric polarisation that can be reversed by applying an external electric field is known as ferroelectricity, and the ferroelectric materials are characterised by the absence of a symmetry centre in their crystal structure.
Ferroelectric materials have sparked extensive research interest due to their potential applications in data storage. Also, their piezoelectric, thermoelectric, and nonlinear optical capabilities have been intensively researched in fields like renewable energy, micro-electro-mechanical systems, and optical devices.
Two-dimensional (2D) ferroelectric materials have emerged as a new challenge in the field of neuromorphic synaptic devices in recent years, owing to their low dimensionality. Yet, due to the scarcity of accessible materials, the development of 2D ferroelectric materials remains hampered.
Ferroelectricity is most typically found in compounds with many constituent elements, where electron gain and loss between the constituents encourage the development of positive and negative ions in the crystal. Regular atom distortion or charge ordering between sublattices causes the central symmetry to be broken, facilitating the creation of ferroelectric polarisation.
A research team led by Professor Andrew Wee of NUS’s Department of Physics recently discovered the single-element ferroelectric state in 2D black phosphorus-like bismuth (BP-Bi), upending the previously described standard concept of ferroelectricity.
Single-element ionicity, single-element in-plane polarisation, and single-element ferroelectricity were all experimentally proven in the bismuth monolayer for the first time. This discovery challenges the notion that ionic polarisation arises solely in compounds containing cations and anions and thus broadens the possibility of future ferroelectricity research.
This work is being done in partnership with Professor Lan Chen of the Chinese Academy of Sciences’ Institute of Physics and Professor Yunhao Lu of Zhejiang University’s School of Physics.
In comparison to magnetism, ferroelectricity is advantageous since it may be controlled solely by an electric field. As a result, it is more appropriate for inclusion in integrated circuit devices. Several investigations have discovered that by linking ferroelectricity with these qualities, it is feasible to influence other material attributes.
The bending degree of the atomic structure in BP-Bi governs the ferroelectric polarisation while also controlling the basic band structure. As a result, the electrical structure and ferroelectric polarisation become interlocked. By ferroelectric distortion, this novel type of ferroelectricity offers a promising technique to control the electronic structure of materials by an external electric field.
Lead author Dr Jian Gou said that additional studies have also demonstrated that BP-Bi shows topologically nontrivial states at a particular buckling height, providing a potential prospect for tuning topological states through an electric field.
In actuality, the fundamental optical and electrical properties of materials are significantly influenced by the polarisation features. The study of the fundamental physical characteristics of simple substances gains a fresh perspective with the discovery of single-element ferroelectric polarisation.
The researchers 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. This would challenge the conventional wisdom that ionic polarisation only occurs in compounds.
