二维MX铁电材料面内畴特性的光学二次谐波探测及极化调制研究

Two-dimentional (2D) atomic-thick group-IV monochalcogenides MX(M=Ge,Sn;X=S,Se) have potential applications in many devices such as memories with high density and low power consumption because they possess the stable in-plane spontaneous polarization. Ferroelectric domain is the foundation of the various physics phenomena in ferroelectric materials and determines the properties of ferroelectric devices. Unfortunately, the in-plane domain structure and its switching process in 2D MX ferroelectrics have not been well known to us. The optical second harmonic generation (SHG)is a non-contact, non-destructive and high-sensitive probing technique for the investigation of ferroelectric order. From this point, a SHG technique will be developed to characterize the in-plane domain structures of 2D MX ferroelectrics. By analyzing the effect of the applied external electric field and crystal microstructures on the SHG signals, we are going to analyze the in-plane domain structure and mechanism of domain formation in 2D MX ferroelectrics. Then, a high time-resolved SHG detection method will be developed to observe the in-plane domain switching process in 2D MX ferroelectrics. On the other hand, we will study the in-plane spontaneous polarization in 2D MX ferroelectrics by first-principles calculation. We will try to uncover the mechanism of in-plane domain switching in 2D MX ferroelectrics by the abovementioned experimental and theoretical studies. Furthermore, the effect of elastic strain on the in-plane spontaneous polarization and the corresponding mechanism will be studied. These studies will accelerate the application of 2D MX ferroelectrics in ferroelectric memories and other ferroelectric devices.

二维IV族单硫属化物MX(M=Ge,Sn;X=S,Se)在原子级厚度具有稳定的面内自发极化，是一类非常有应用前景的二维铁电材料，可应用于高密度低功耗存储等领域。铁电畴是决定铁电材料及器件性能的基础，但目前对二维MX铁电材料的面内畴结构及畴翻转特性缺乏足够的认识。光学二次谐波(SHG)探测因其非接触、非破坏、灵敏度高等优点，在微弱极化信号探测方面独具优势。本申请项目拟建立二维MX铁电材料面内畴结构的SHG探测方法，分析外加电压和材料微观结构对SHG信号的影响，研究面内畴结构特性及产生机理；在此基础上，建立超高时间分辨SHG探测方法实现对二维MX铁电材料面内畴翻转动力学过程的探测，并结合第一性原理计算，研究面内畴翻转特性及机制。此外，利用上述方法进一步研究应变对面内畴结构和畴翻转动力学过程的影响，探讨应变对面内极化的调制机理。为二维MX铁电器件的开发和应用提供指导和依据。

本项目紧紧围绕二维硫族铁电材料，分三个层次进行了研究。首先，利用气相沉积法和机械剥离法制备了SnS、SnSe、α-In2Se3、CuInP2S6等二维硫族铁电材料，并对所制备二维硫族铁电材料的微观结构进行了表征，探究了不同生长参数对所制备材料质量的影响，分析了材料的生长机制，并实现了二维硫族铁电材料的可控性生长。同时，发展了二维硫族铁电材料畴结构的光学二次谐波表征方法，并搭建了相应的光学系统，通过改变入射光偏振角度和样品移动距离等参数，研究了样品产生光学二次谐波信号的相关变化，实现了二维硫族铁电材料铁电畴结构成像扫描和探测研究；另外，采用基于几何相位的Berry Phase方法，并结合第一性原理计算，通过改变晶格常数计算二维硫族铁电材料拉/压应变下和不同应变下的自发极化特性，并分析了二维硫族铁电材料极化与应变的关联。本项目研究将为设计和表征前景广阔的二维铁电材料提供了实验指导和依据。