Strong corellation between Peierls Distortion and SHG in vdW NbOX2

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. Their study, "Manipulating Peierls distortion in van der Waals NbOX₂ maximizes second--harmonic generation", 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.

More information: "Manipulating Peierls Distortion in van der Waals NbOX₂ Maximizes Second-Harmonic Generation" Fu et al., J. Am. Chem. Soc. 145, 16828–16834 (2023).