van der Waals Epitaxial Growth of One-Unit-Cell-Thick Ferroelectric CuCrS₂ Nanosheets.

Abstract

Ferroelectric two-dimensional (2D) materials with a high transition temperature are highly desirable for new physics and next-generation memory electronics. However, the long-range polar order of ferroelectrics will barely persist when the thickness reaches the nanoscale. In this work, we synthesized 2D CuCrS₂ nanosheets with thicknesses down to one unit cell via van der Waals epitaxy in a chemical vapor deposition system. A combination of transmission electron microscopy, second-harmonic generation, and Raman spectroscopy measurements confirms the <i>R</i>3<i>m</i> space group and noncentrosymmetric structure. Switchable ferroelectric domains and obvious ferroelectric hysteresis loops were created and visualized by piezoresponse force microscopy. Theoretical calculation helps us understand the mechanism of ferroelectric switching in CuCrS₂ nanosheets. Finally, we fabricated a ferroelectric memory device that achieves an on/off ratio of ∼10² and remains stable after 2000 s, indicating its applicability in novel nanoelectronics. Overall, 2D CuCrS₂ nanosheets exhibit excellent ferroelectric properties at the nanoscale, showing great promise for next-generation devices.