Two-dimensional Na₂Cl and CaCl crystals on graphitic surfaces

It is well-known that the valence states of sodium and calcium ions under ambient conditions are +1 and +2, respectively, and the corresponding crystals with these ions are insulating and cannot be ferromagnetic. In this paper, we review the recent discovery of two-dimensional Na₂Cl and CaCl crystals on reduced graphene oxide membranes under ambient conditions. Due to the abnormal ratio of the cations and anions in the crystal (unconventional stoichiometries), the two-dimensional CaCl crystals display metallicity, room-temperature ferromagnetism, and coexistence of piezoelectricity and metallicity under ambient conditions. Soft X-ray absorption spectroscopy performed with synchrotron radiation shows that the Ca ions are not divalent, indicating that they are in a new state. These crystals with unconventional stoichiometries have practical potential applications in designing novel transistors and magnetic devices with sizes down to the atomic scale. If we could apply these findings to biological systems, we may obtain new insights into the understanding of biomagnetism, including the origin of magnetoreception and magnetogenetics.