Strong corellation between Peierls Distortion and SHG in vdW NbOX2 – Dr. Xujie Lü

Building a link between structure and property is the key to revealing novel physics and designing high-performance materials. Recently, a research team led by Dr. Xujie Lü from the Center for High Pressure Science and Technology Advanced Research (HPSTAR) and Prof. Fuqiang Huang from the Shanghai Institute of Ceramics of the Chinese Academy of Sciences reported the discovery of a relationship between Peierls distortion and SHG intensity through either high pressure or chemical substitution. The study is published in the latest issue of the Journal of the American Chemical Society.

Two-dimensional (2D) van der Waals (vdW) materials that possess extraordinary nonlinear optical (NLO) properties at the 2D limit are promising for next-generation nano-photonic applications. However, fundamental questions remain and need to be addressed to improve materials design principles for excellent NLO properties.

The structural distortion considerably affects the NLO response. However, we have yet to systematically understand the structure-property correlation due to the lack of suitable materials and diagnostic tools. The research team proposed a strategy to continuously regulate the local structural distortion to quantitatively reveal the potential relationship.

The researchers regulated the second-harmonic generation (SHG) response of NbOI₂, which possesses a unique distorted structure, by pressure. “By continuously regulating the Peierls distortion in NbOI₂ using external pressure, we have demonstrated a general relationship between the Peierls distortion and SHG intensity, which is a typical second-order NLO process. We have also shown that increasing Peierls distortion could enhance SHG response,” said Tonghuan Fu, a Ph.D. student at HPSTAR.

"Based on understanding the underlying mechanisms responsible for the enhanced SHG from high-pressure studies, we intentionally replaced the I⁻ anions in NbOI₂ with smaller Br⁻ to simulate the pressure effects,” said Dr. Kejun Bu. The results show that chemical substitution can also tune the SHG response by regulating the Peierls distortion. Particularly, the newly developed NbO(I_0.6Br_0.4)₂ exhibits an impressively strong SHG intensity, a record-high value among reported 2D vdW NLO materials.

Caption: Illustration of the analogous influence of physical pressure and chemical substitution on Peierls distortion and second-harmonic generation in NbOX₂ (X = I, Br).

二维范德华层状材料具有高的二阶非线性磁化率、结构可调控性以及宽松的相位匹配条件，在非线性光电子器件领域具有广阔的应用前景。深入理解二维范德华层状材料的结构-物性关系有助于揭示非线性极化的产生规律，为新型非线性光学响应二维材料的设计提供思路。北京高压科学研究中心（HPSTAR）吕旭杰团队联合中国科学院上海硅酸盐研究所黄富强团队等，利用压力调控结合多种原位测试技术和理论计算揭示了二维范德华层状材料NbOI₂中Peierls 畸变与二次谐波响应的构性关系。并以此为指导，通过化学方法合成了具有极高二次谐波响应的二维材料NbO(I_0.60Br_0.40)₂。相关成果以“Manipulating Peierls distortion in van der Waals NbOX₂ maximizes second-harmonic generation”为题近期发表于《美国化学会志》Journal of the American Chemical Society上。