Tiny but Mighty: Layered Bismuth Oxide Exhibits Ferroelectricity at 1 Nanometer Scale

A study in Science led by Linxing Zhang from the University of Science and Technology Beijing (expriment) and Yue-Wen Fang (theory) from the Centro de Física de Materiales (CSIC-UPV/EHU) demonstrates the robust ferroelectricity in a 1 nm bismuth oxide, opening the door to even smaller electronics.
Tiny but Mighty: Layered Bismuth Oxide Exhibits Ferroelectricity at 1 Nanometer Scale

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In the study appearing in Science, Dr. Yuewen Fang, a computational physicist at the Centro de Física de Materiales (CFM, CSIC-UPV/EHU), and Dr. Linxing Zhang at the University of Science and Technology Beijing have collaborated to develop a new ferroelectric material. Ferroelectrics are functional materials with a century-long scientific history that display a spontaneous electric polarization that can be reversed by an external electric field. They are widely used in both industry and everyday life due to their excellent piezoelectric and thermoelectric properties. The ferroelectric industry is currently worth around $7 billion and continues to grow.

A model of subnanometer ultrathin memory devices based on ferroelctrics
Model of future memory devices.

The researchers designed a new ferroelectric material that is the smallest ever created, with a thickness of just 1 nanometer. The material, which consists of a layered structure of bismuth oxide stabilized by samarium bonding, exhibits a remanent polarization of up to 17 microcoulombs per square centimetre at room temperature, making it suitable for a wide range of applications.

The crystal structure prediction was primarily led by CFM (CSIC-UPV/EHU) in collaboration with Tel Aviv University. The researchers combined unbiased high-throughput first-principles structure screening with crystal structure prediction guided by transmission electron microscopy (TEM) images to determine the proposed crystal structures. The theoretically proposed crystal structure agrees with the cross-sectional high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images observed in different directions, demonstrating the accuracy of the crystal structure prediction.

The researchers found that the observed ferroelectricity in the new material is driven by the lone-pair electrons of Bi ions. These electrons reside in one site of Bi ions asymmetrically, which breaks the inversion space symmetry and induces the ferroelectricity in this new material. In addition, the doping of samarium into the sample is crucial for improving the thermodynamic stability of the polar structure, especially at the nanometer scale.

The researchers successfully prepared a Bi6O9 thin film using a cost-effective chemical solution deposition method. The thin films range in thickness from 1 to 4.56 nm and exhibit relatively large remanent polarizations, ranging from 17 to 50 microcoulombs per square centimetre. This makes the layered bismuth oxide among the most excellent ferroelectric materials at both the nanometer and subnanometer scales. The ultrathin ferroelectric films are highly suitable for future nano-electronic devices, particularly in the areas of field-effect-transistors, low-power logic, and non-volatile memories. The structure design of these films has enormous potential for manufacturing atomic-scale electronic devices.

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