Ferroelectricity in Niobium Oxide Dihalides NbOX<sub>2</sub> (X = Cl, I): A Macroscopic- to Microscopic-Scale Study
Chaofei Liu, Xiuying Zhang, Xinyun Wang, Ziying Wang, Ibrahim Abdelwahab, Ivan Verzhbitskiy, Yan Shao, Goki Eda, Wanxin Sun, Lei Shen, Kian Ping Loh
Abstract
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 NbOX 2 (X = Cl, I) across different spatial scales. The in-plane ferroelectricity in NbOX 2 was investigated using transport and piezoresponse force microscopy (PFM) measurements, where it was observed that NbOCl 2 has a stronger ferroelectric order than NbOI 2 . A high local field, exerted by both PFM and scanning tunneling microscopy (STM) tips, was found to induce 1D collinear ferroelectric strips in NbOCl 2 . STM imaging reveals the unreconstructed atomic structures of NbOX 2 surfaces, and scanning tunneling spectroscopy was used to probe the electronic states induced at defect (vacancy) sites.