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.

Automated summary

A systematic study of the ferroelectric properties in 2D materials NbOX2 (X = Cl, I) reveals that NbOCl2 exhibits stronger ferroelectricity than NbOI2. The presence of 1D collinear ferroelectric strips is observed in NbOCl2. Scanning tunneling microscopy (STM) imaging provides insights into the unreconstructed atomic structures of NbOX2 surfaces, while scanning tunneling spectroscopy probes the electronic states induced at defect (vacancy) sites.