  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.
