Two-dimensional CaCl2 flakes with piezoelectric properties on highly oriented pyrolytic graphite at ambient conditions

Two-dimensional inorganic salt flakes have attracted increasing attention due to their unique properties and broad prospects for both experimental and industrial applications. However, fabricating these flakes under ambient conditions remains a significant challenge. Here, we present a novel approach for the facile preparation of thin, two-dimensional calcium chloride (CaCl₂) flakes on highly oriented pyrolytic graphite (HOPG) surfaces through solution-based treatment under ambient conditions. The ultrathin CaCl₂ flakes exhibited thicknesses ranging from 1.0 to 6.0 nm, with a predominant thickness of ∼1.5 nm. We further systematically explored the impact of various fabrication parameters, including ambient humidity, salt solution processing time, and pure water wetting duration, on the formation of distinct nanostructures. The ultrathin CaCl₂ flakes exhibit excellent stability under various environmental and vacuum conditions at different temperatures. Notably, piezoresponse force microscopy (PFM) measurements reveal that ultrathin CaCl₂ flakes possess piezoelectric properties. Owing to the poor mechanical and adhesive properties of 1.5 nm thick flakes, 4.0 nm thick flakes with enhanced stability were selected for reliable piezoresponse studies, yielding a piezoelectric coefficient of 21.4 p.m./V, exceeding that of most two-dimensional semiconductors. Furthermore, we show the broad applicability of this method by demonstrating that other halides, such as MgCl₂ and CuCl₂, can also form similar ultrathin flakes. We attribute this phenomenon to the strong cation-π interaction between the cation and the aromatic rings of graphite, which drives the affinity between the cation and the HOPG surface, promoting the formation of ultrathin sheets. Our study provides a straightforward and innovative strategy for the fabrication of ultrathin halide salt flakes, offering valuable insights into the formation mechanism of carbon-based surface nanostructures, and opens new avenues for potential applications of these flakes in coatings, sensors, capacitors, and battery electrodes.