PolyU researchers make breakthrough discovery in structure and synthesis of 2D ferroelectrics, advancing technological development in microelectronics, artificial intelligence and quantum information

With their spontaneous electrical polarisations switchable by an external electric field, ferroelectrics have wide-ranging applications in transistors, memory, neuromorphic devices and more. In particular, two-dimensional (2D) ferroelectrics produced at the nanometre scale have emerged as superior materials for ultra-thin devices.

A research team led by Prof. Jiong ZHAO, Associate Professor of the Department of Applied Physics of The Hong Kong Polytechnic University (PolyU), has conducted research on the structure and potential of 2D van der Waals materials, and has unveiled a pioneering approach for large-scale synthesis of 2D ferroelectrics. Their findings significantly boost technological advancements in microelectronics, artificial intelligence and quantum information, and will subsequently foster the development of diverse applications including high-density memory devices, energy conversion systems, sensing technologies and catalysis technologies.

Compared to conventional materials, 2D ferroelectrics exhibit rapid carrier mobility, enabling swift data transfer, storage and computation. The notably reduced size of these materials also leads to considerably lower energy consumption. Moreover, their extreme thinness makes them exceptionally transparent and flexible, rendering them ideal for devices requiring these properties. Among the discovered 2D ferroelectrics, Indium Selenide (In2Se3) stands out as the most promising due to the co-presence of paraelectric, ferroelectric and antiferroelectric phases within its 2D quintuple layers. However, large-scale synthesis of 2D In2Se3 films with the desired phase is still lacking, while the stability for each phase also remains unclear.

To overcome the challenges, the research team utilised the transmission electron microscopy (TEM) technique to directly observe and analyse the ferroelectric domains, domain walls and other crucial features at the atomic level within the materials. They found that 2D In2Se3 films with pure phase can be synthesised separately by first controlling the Se/In ratios in the precursors when growing 2D In2Se3 films through chemical vapour deposition and then transferring the as-grown films onto flexible or uneven substrates. After repeated experiments, they successfully implemented phase-controllable synthesis and achieved precise structural control deemed unattainable previously. The findings have been published in the international journal Nature Nanotechnology.

The research team has also sought to explore novel 2D van der Waals materials and their potential. Using TEM, they have revealed a general plastic deformation mode in metal monochalcogenides, such as InSe, which contributes to the ultra-high plasticity of materials made by 2D metal monochalcogenides. The findings exhibit great potential for producing a high-performance plastic inorganic semiconductor and facilitate development of soft and flexible electronic materials, advanced additive manufacturing for semiconductors as well as solid-state lubricants. The research has been published in the journal in Nature Materials.

In a recent study, Dr Zhao’s team has additionally uncovered the in-plane polar vortex in 2D materials with twisted bilayers with the help of the advanced four-dimensional scanning transmission electron microscopy (4D-STEM). They also demonstrated the relation between the twist angle in the bilayers and their vortex patterns and polar structures, as well as the potential to manipulate the polar vortices and polar field distributions through an external electric field or interlayer and twisting. The discovery not only provides valuable perspectives on the complex behaviour of polar structures in twisted 2D bilayers but also paves the way for tuning emergent quantum properties at the atomic scale and creating promising 2D materials. The research has been published in Science.

PolyU research facilities, specifically the recently inaugurated Atomic Transmission Electron Microscopy Laboratory (AEML) under the University Research Facility in Materials Characterisation and Device Fabrication (UMF), have been crucial in facilitating these research endeavours. The Laboratory enables atomic resolution observations, which helped the team to directly reveal the critical mechanisms essential for synthesis and applications.

The research also greatly benefited from the contributions of the research team of Prof. Daniel LAU Shu Ping, Chair Professor of Nanomaterials and Head of the PolyU Department of Applied Physics; Prof. Ming YANG, Assistant Professor of the PolyU Department of Applied Physics; and Prof. LY Thuc Hue, Associate Professor of the Department of Chemistry of City University of Hong Kong.

Prof. Jiong Zhao said, “These scientific discoveries are set to usher in a paradigm shift in microelectronics and integrated circuits, while also driving the development of flexible, durable and efficient new-generation electronic devices. They will further open up promising prospects for various applications, such as new computation-in-memory devices with enhanced computation capacity and speed, and with no need for the data transfer between computation and memory units required in current computing chips. These advancements herald a new technological era where society is faster-moving, and is more energy-efficient and adaptable to change.

With his outstanding research achievements, Prof. Zhao has been awarded the Excellent Young Scientists Fund by the National Natural Science Foundation. His research projects have also received support from the Collaborative Research Fund of the Research Grants Council and the Innovation and Technology Fund of the Innovation and Technology Commission.

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