Composition-driven tunable optical and electrical properties in van der Waals ferroelectric NbOI2− <i>x</i> Cl <i>x</i> alloys

Abstract

Layered niobium oxide dihalides NbOX2 (X = I, Cl), as a new family of van der Waals (vdW) ferroelectrics, have attracted extensive attention, but achieving nonvolatile modulation of their optical and electrical properties remains challenging, thereby limiting their integration into next-generation nanoelectronics and optoelectronics. Here, we report the controlled fabrication of highly crystalline NbOI2−xClx vdW alloys with composition-driven tunable optical and electrical properties via a chemical vapor transport method. Comprehensive experimental characterization combined with first-principles calculation shows that the crystal lattices, phonon modes, and band structures of NbOI2−xClx can be well tailored, which are distributed between NbOI2 and NbOCl2. Both the amplitude and polarization of the second harmonic generation optical signal in NbOI2−xClx exhibit pronounced compositional dependence, offering optical evidence for tunable in-plane ferroelectric characteristics. Moreover, field-effect transistors based on NbOI2−xClx display robust n-type semiconducting behavior, with threshold voltage and carrier mobility precisely modulated through adjustment of I/Cl molar ratio. Furthermore, 2D NbOI2−xClx photodetectors across all compositions exhibit exceptional gate-tunable current on/off ratio and strong polarization-sensitive photo-response. This study thus provides a new vdW ferroelectric material platform with tunable optical and electrical properties, paving the path for its implementation in modern nanophotonics and nanoelectronics.