Ferroelectricity in Niobium Oxide Dihalides NbOX₂ (X = Cl, I): A Macroscopic- to Microscopic-Scale Study

2D materials with ferroelectric and piezoelectric properties are of interest for energy harvesting, memory storage and electromechanical systems. Here, we present a systematic study of the ferroelectric properties in NbOX₂ (X = Cl, I) across different spatial scales. The in-plane ferroelectricity in NbOX₂ was investigated using transport and piezoresponse force microscopy (PFM) measurements, where it was observed that NbOCl₂ has a stronger ferroelectric order than NbOI₂. A high local field, exerted by both PFM and scanning tunneling microscopy (STM) tips, was found to induce 1D collinear ferroelectric strips in NbOCl₂. STM imaging reveals the unreconstructed atomic structures of NbOX₂ surfaces, and scanning tunneling spectroscopy was used to probe the electronic states induced at defect (vacancy) sites.