Novel two-dimensional Ca-Cl crystals with metallicity, piezoelectric effect and room-temperature ferromagnetism

TL;DR

This paper reports the discovery of novel 2D Ca-Cl crystals on graphene with unusual properties like metallicity, room-temperature ferromagnetism, and piezoelectricity, opening new avenues for advanced electronic and magnetic applications.

Contribution

It experimentally demonstrates the formation of abnormal 2D Ca-Cl crystals with metallicity and ferromagnetism, and explains their formation via strong cation-pi interactions, suggesting potential for various applications.

Findings

2D Ca-Cl crystals exhibit metallicity and ferromagnetism at room temperature.

The formation is driven by strong cation-pi interactions with graphitic surfaces.

Potential applications include transistors, magnetic devices, and hydrogen storage.

Abstract

Recently we have reported the direct observation of two-dimensional (2D) Ca-Cl crystals on reduced graphene oxide (rGO) membranes, in which the calcium ions are only about monovalent (i.e. ~+1) and metallic rather than insulating properties are displayed by those CaCl crystals. Here, we report the experimental observation and demonstration of the formation of graphene-Ca-Cl heterojunction owing to the metallicity of 2D Ca-Cl crystals, unexpected piezoelectric effect, room-temperature ferromagnetism, as well as the distinct hydrogen storage and release capability of the Ca-Cl crystals in rGO membranes. Theoretical studies show that the formation of those abnormal crystals is attributed to the strong cation-pi interactions of the Ca2+ with the aromatic rings in the graphitic surfaces. Since strong cation-pi interactions also exist between other metal ions (such as Mg2+, Fe2+, Co2+, Cu2+, Cd2+, Cr2+ and Pb2+) and graphitic surfaces, similar 2D crystals with abnormal valence state of the metal cations and corresponding abnormal properties as well as novel applications are highly expected. Those findings further show the realistically potential applications of such abnormal CaCl material with unusual electronic properties in designing novel transistors and magnetic devices, hydrogen storage, catalyzer, high-performance conducting electrodes and sensors, with a size down to atomic scale.