Kung-Hsuan Lin1,Redhwan Moqbel1,2,3,Yih-Ren Chang4,Zi-Yi Li1,2,Sheng-Hsun Kung1,Hao-Yu Cheng1,2,3,Chi-Cheng Lee5,Kosuke Nagashio4
Institute of Physics Academia Sinica1,National Taiwan University2,Taiwan International Graduate Program3,The University of Tokyo4,Tamkang University5
Kung-Hsuan Lin1,Redhwan Moqbel1,2,3,Yih-Ren Chang4,Zi-Yi Li1,2,Sheng-Hsun Kung1,Hao-Yu Cheng1,2,3,Chi-Cheng Lee5,Kosuke Nagashio4
Institute of Physics Academia Sinica1,National Taiwan University2,Taiwan International Graduate Program3,The University of Tokyo4,Tamkang University5
Two-dimensional (2D) multiferroic materials, which contain two or more intrinsic ferroic orders hold great promise for different optoelectronic applications<sup>[1]</sup>. Recently, monolayer orthorhombic structures of group IV metal monochalcogenides (such as GeS, GeSe, SnS, and SnSe) have been predicted to be 2D multiferroic materials, which possess both in-plane ferroelectric and ferroelastic orders with low domain wall energy and low migration barrier<sup>[1]</sup>. However, it is still challenging to fabricate monolayer or few layer of group IV metal monochalcogenides either from exfoliation or vapor deposition methods [2]. For most bulk crystals, macroscopic ferroelectricity or multiferroic properties disappear because each layers are stacked in an antiferroelectric manner. In this work, we fabricated unique SnS few layer single crystal flakes in which the layers were stacked in an order to keep in-plane polarizations of each layer the same direction. This opens the gate to experimentally manipulate multiferroic phases in these ferroelectric SnS few layer flakes. For measuring the ferroic phases and crystal orientations, it could be conveniently achieved by a non-contact technique of optical second harmonic generation (SHG). We utilized SHG microscopy to measure the polarization-resolved SHG from numerous SnS flakes prepared by physical vapor deposition on mica substrates. The angular-resolved patterns dramatically changed under excitation wavelength between 800 and 1000 nm due to the frequency dependence on SHG susceptibilities. By using first-principles methods within the density functional theory, we calculated the frequency-dependent SHG susceptibilities in the AA-stacking SnS and AC-stacking SnS. The effect of layer dependence on the band structures and SHG susceptibilities was also theoretically investigated. The variation trend of calculated SHG polar patterns as a function of frequency agrees well with that of the experimental results.<br/><br/><b>References</b><br/><br/>[1] H. Wang, X. Qian, <i>2D Mater.</i> <b>2017</b>, <i>4</i>, 015042.<br/>[2] Z. Hu, Y. Ding, X. Hu, W. Zhou, X. Yu, S. Zhang, <i>Nanotechnology</i> <b>2019</b>, <i>30</i>, 252001.