Nanoscale Periodic Modulations on Sodium Chloride Induced by Surface Charges

The sodium chloride surface is one of the most common platforms for the study of catalysts, thin film growth, and atmospheric aerosols. Here we report a nanoscale periodic modulation pattern on the surface of a cleaved NaCl single crystal, revealed by non-contact atomic force microscopy with a tuning fork sensor. The surface pattern shows two orthogonal domains, extending over the entire cleavage surface. The spatial modulations exhibit a characteristic period of 5.4 nm, commensurate with the atomic rows of the NaCl surface. The modulations are robust in vacuum, not affected by the tip-induced electric field or gentle annealing ($<$300 \r{ }C); however, they are eliminated after exposure to water and an atomically flat surface can be recovered by subsequent thermal annealing after water exposure. A strong electrostatic charging is revealed on the cleavage surface and the modulations appear to reflect a surface structural reconstruction facilitated by surface charges.