Exploration on two-dimensional layered polar crystals

Crystal symmetries and their breaking fundamentally determine the crystal topology and lead to many important physical properties in the emerging 2D materials. For example, the inversion symmetry breaking allows the presence of valley degree of freedom in 2D transition metal dichalcogenide, which is the foundation for potential valleytronics. In this talk, I will focus on mirror symmetry and its breaking, which can also shape 2D layered materials substantially. Our experimental investigation and engineering of 2D polar crystals and associated optoelectronic properties will be covered, including the creation of first monolayer polar crystal and the discovery of ferroelectricity with dipole locking in 2D layered materials. Significant vertical dipoles were observed in both materials by second harmonic generation (SHG) and piezoforce microscopy (PFM). In addition, a 3-nm-thick In2Se3 shows electrically switchable polarizations with a very high ferroelectric transition temperature Tc up to 700 K. These findings are applicable to Rashba spin physics, electromechanical sensors and memory devices at molecular level. More importantly, the large tunability of these 2D materials, regarding external mechanical, electrical stimuli, or interfacial proximity, make potential symmetry engineering more accessible than ever to achieve new functionalities.