Published on Science Advances (21 October 2022)

Author(s): Xiaodong Zheng, Wei Han, Ke Yang, Lok Wing Wong, Chi Shing Tsang, Ka Hei Lai, Fangyuan Zheng, Tiefeng Yang, Shu Ping Lau, Thuc Hue Ly, Ming Yang, Jiong Zhao

Abstract

Phase transitions in two-dimensional (2D) materials promise reversible modulation of material physical and chemical properties in a wide range of applications. 2D van der Waals layered In₂Se₃ with bistable out-of-plane ferroelectric (FE) α phase and antiferroelectric (AFE) β′ phase is particularly attractive for its electronic applications. However, reversible phase transition in 2D In₂Se₃ remains challenging. Here, we introduce two factors, dimension (thickness) and strain, which can effectively modulate the phases of 2D In₂Se₃. We achieve reversible AFE and out-of-plane FE phase transition in 2D In₂Se₃ by delicate strain control inside a transmission electron microscope. In addition, the polarizations in 2D FE In₂Se₃ can also be manipulated in situ at the nanometer-sized contacts, rendering remarkable memristive behavior. Our in situ transmission electron microscopy (TEM) work paves a previously unidentified way for manipulating the correlated FE phases and highlights the great potentials of 2D ferroelectrics for nanoelectromechanical and memory device applications.

Fig. 1. Phase evolutions and thickness dependence of 2D In₂Se₃. (A) Atomic scheme of the phase transitions in 2D In₂Se₃. (B) Phase diagram of α-In₂Se₃ and β′- In₂Se₃ on strain and layer number, obtained by DFT calculations. (C) Polarized-light OM image at the polarizer azimuth angle φ = 70° presenting the typical gradient morphology of β′-In₂Se₃. (D) HAADF image of the edge of the β′-In₂Se₃. (E) The SAED patterns corresponding to the 2L-5L In₂Se₃ respectively. The superspots (red circles) of In₂Se₃ disappear for layer number less than four. (F) Atomic HAADF image and corresponding image simulation results of few-layer β′-In₂Se₃. (G) HAADF image and corresponding image simulation results of 1L α-In₂Se₃. (H) Intensity line profile corresponding to the line in (G). Atomic differential phase contrast image of (I) β′-In₂Se₃ phase and (J) α-In₂Se₃, respectively. The yellow arrows indicate the intrinsics polarization directions in AFE β′-In₂Se₃. Scale bars, 50 μm (C), 2 μm (D), 2 1/nm in (E), 1 nm (F and G), and 2 nm (I and J), respectively.

Read more: https://www.science.org/doi/10.1126/sciadv.abo0773