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二维铁电材料因其自发的铁电极化和天然的纳米厚度，突破了传统铁电材料中的尺寸效应，在微纳尺度的铁电功能器件中具有广泛的应用前景。非共价的层间范德华力使得二维材料能够较为容易地滑移和旋转，从而产生了一系列新奇的物理现象，为其铁电性质提供了新的调控维度。
来自西安交通大学的邓俊楷教授团队与澳大利亚墨尔本大学的刘哲教授，通过第一性原理计算，在具有皱褶结构的双层IV-VI族二维材料MX（M=Ge,Sn; X=S, Se）中，发现了层间滑移诱发的面内可逆铁电-反铁电相变现象。深入分析发现，通常看似较弱的层间范德华力可以引起MX面内晶格发生畸变，在层间滑移过程中导致上层MX的面内共价键发生重构，导致极化方向翻转，从而实现铁电和反铁电相之间的相互转变。此外，基于该物理现象，提出了利用层间滑移诱发铁电-反铁电相变的纳米发电机模型。由于滑移过程中，能够产生高达40 μC/cm2的铁电极化变化，同时借助二维范德华材料的超润滑特性，理论上可以产生巨大电信号输出。计算表明其理论功率输出高于二维材料压电效应约四个数量级，表明了IV-VI族二维层状材料，在作为纳米尺度力电能量转换器件方面具有巨大的潜力。
该文近期发表于npj Computational Materials 8: 47 (2022)，英文标题与摘要如下，点击左下角“阅读原文”可以自由获取论文PDF。
Van der Waals force-induced intralayer ferroelectric-to-antiferroelectric transition via interlayer sliding in bilayer group-IV monochalcogenides
Bo Xu, Junkai Deng, Xiangdong Ding, Jun Sun & Jefferson Zhe Liu
Two-dimensional materials with ferroelectric properties break the size effect of conventional ferroelectric materials and unlock unprecedented potentials of ferroelectric-related application at small length scales. Using first-principles calculations, a sliding-induced ferroelectric-to-antiferroelectric behavior in bilayer group-IV monochalcogenides (MX, with M = Ge, Sn and X = S, Se) is discovered. Upon this mechanism, the top layer exhibits a reversible intralayer ferroelectric switching, leading to a reversible transition between the ferroelectric and antiferroelectric states in the bilayer MXs. Further results show that the interlayer van der Waals interaction, which is usually considered to be weak, can actually generate an in-plane lattice distortion and thus cause the breaking/forming of intralayer covalent bonds in the top layer, leading to the observed anomalous phenomenon. This unique property has advantages for energy harvesting over existing piezoelectric and triboelectric nanogenerators. The interlayer sliding-induced big polarization change (40 μC cm−2) and ultrahigh polarization changing rate generate an open-circuit voltage two orders of magnitude higher than that of MoS2-based nanogenerators. The theoretical prediction of power output for this bilayer MXs at a moderate sliding speed 1 m s−1 is four orders of magnitude higher than the MoS2 nanogenerator, indicating great potentials in energy harvesting applications.
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