Discovery of ferroelectricity in basic substances

(Nanowerk News) Physicists from the National University of Singapore (NUS) have discovered a new form of ferroelectricity in single-element bismuth monolayers that can generate regular and reversible dipole moments for future applications of non-volatile memory and electronic sensors.

Ferroelectricity refers to the phenomenon that certain materials exhibit a spontaneous electric polarization which can be reversed by applying an external electric field. Ferroelectric materials are characterized by a crystal structure that has no center of symmetry.

Due to their potential applications for data storage, ferroelectric materials have attracted extensive research attention. Moreover, their piezoelectric, thermoelectric and nonlinear optical properties have been extensively studied in research fields such as renewable energy, micro-electro-mechanical systems and optical devices. In recent years, two-dimensional (2D) ferroelectric materials have emerged as a new competitor in the field of neuromorphic synapse devices, demonstrating the advantages of low dimensions. However, the development of 2D ferroelectric materials is still limited due to the small number of available materials.

Ferroelectricity generally occurs in compounds consisting of several constituent elements, where the gain and loss of electrons between constituents promotes the formation of positive and negative ions in the crystal. The distortion of ordinary atoms or the ordering of charges between the sub-lattices causes a break in the symmetry of the center, thereby promoting the formation of a ferroelectric polarization.

Recently, a research team led by Professor Andrew WEE from the Department of Physics, NUS, made the groundbreaking discovery of single-element ferroelectric states in bismuth such as 2D black phosphorus (BP-Bi), overturning the aforementioned traditional understanding of ferroelectricity.

Using optimized scanning tunneling microscopy (STM) and non-contact atomic force microscopy (nc-AFM) techniques, the researchers made detailed observations of the centrosymmetry breaking in the atomic structure and charge transfer between sub-lattices in BP-Bi.

For the first time, single-element ionicity, polarization in single-element planes, and single-element ferroelectricity were all demonstrated in bismuth monolayers experimentally. This discovery changes the concept that ion polarization exists only in compounds with cations and anions, and broadens the scope of future ferroelectric development. This work was carried out in collaboration with Professor Lan CHEN of the Institute of Physics, Chinese Academy of Sciences and Professor Yunhao LU of the School of Physics, Zhejiang University.

The findings of this novel ferroelectric material are published in Natural (“Two Dimensional Ferroelectricity in a Bismuth Monolayer Coating”). Figure (A) shows a ball-and-stick model of black phosphorus-like bismuth (BP-Bi) with top view (top panel) and side view (bottom panel). In contrast to black phosphorous, BP-Bi has a non-zero bending Dh. Figures (B) and (C) show typical images of non-contact atomic force microscopy (nc-AFM) and Kelvin probe microscopy (KPFM) maps of the same locations. Figure (D) represents the domain wall movement detected by nc-AFM while performing polarization switching. The top and bottom are the images obtained after forward and backward bias respectively. Figure (E) is the current tip-height dependence during bias sweep. (Image: NUS)

The researchers prepared high-quality BP-Bi on the Van der Waals graphite surface so that the monolayer BP-Bi was intact and flat enough for measurement. Taking advantage of the high spatial resolution of nc-AFM, the bending atomic configuration (Dh≠0) of BP-Bi, as well as the redistribution of charges between the two sublattices was determined by AFM imaging and Kelvin probe microscopy (KPFM) measurements. Thereafter, the dipole arrangement in the regular plane was confirmed in the BP-Bi monolayer.

In comparison, single-layer phosphors (phosphorene) have no buckling in any of the sub-layers – therefore they are centrosymmetric and non-polarized. Then, the BP-Bi polarization switching is realized using the in-plane electric field generated by the STM tip, which is the basis for writing to non-volatile memory devices.

Ferroelectricity relative to magnetism is advantageous for its manipulation only with electric fields. This makes them more suitable for being contained within integrated circuit devices. Many studies have found that it is possible to manipulate other properties of materials by combining ferroelectricity with these properties.

In BP-Bi, the degree of bending of the atomic structure determines the ferroelectric polarization and, at the same time, controls the baseband structure. This results in an interlock between the electronic structure and the ferroelectric polarization. This new type of ferroelectric offers a promising way to modulate the electronic structure of materials with an external electric field via ferroelectric distortion.

Dr Jian GOU, lead author for the research paper, said, “Other studies have also shown that BP-Bi exhibits a nontrivial topological state at a certain buckling height, indicating a potential opportunity to tune the topological state via an electric field.”

In fact, polarization characteristics have a critical impact on the fundamental optical and electrical properties of materials. The discovery of single-element ferroelectric polarization adds a new perspective for studying the basic physical properties of elementary substances.

Prof Wee said, “As well as overturning the common sense notion that ionic polarization only exists in compounds, we believe that single-element ferroelectricity in BP-Bi will introduce new perspectives to the study and design of new ferroelectric materials, and inspire new physics of elemental materials in the future. .”