Observation of single-element ferroelectricity in a two-dimensional bismuth layer

Ferroelectrics are intriguing due to their non-volatile spontaneous polarization, which has applications in data storage, electric sensors, and even photovoltaic cells and neuromorphic devices. Since polarization arise exclusively from ordered anions and cations, making ferroelectric materials compound in nature. Although some theoretical works have attempted to expand and enhance our comprehension of ferroelectricity by challenging the conventional boundary of ionic polarization in elementary substances, experimental advancement continue to be impeded by the complex characteristics of single-element materials at low dimensions.
In this work, we employed the scanning tunnelling microscopy (STM) and non-contact atomic force microscopy (nc-AFM) to investigate the black-phosphorus-like Bi (BP-Bi) monolayer. Different from the centrosymmetric black phosphorus, the BP-Bi monolayer exhibits a buckled atomic arrangement between its two sublattice due to weak and anisotropic sp orbital hybridization, thus supporting the noncentrosymmetry in BP-Bi. Our electronic state measurements and kelvin probe force microscopy (KPFM) also reveal electron redistribution between the two sublattices, suggesting an in-plane polarization. Most importantly, by applying an in-plane electric field using STM, we successfully demonstrate the polarization switching at both microscopic and macroscopic scales. This experimental discovery unveils the emergent single-element ferroelectricity, which has the potential to expand our understanding of ferroelectric mechanism and facilitate the development of ferroelectric devices in the future.