Formation of Unconventional Stoichiometric Na-Cl Magic-Number Nanoclusters and 2D Assembly on Ir(111)

Sodium chlorides in non-1:1 stoichiometry are counterintuitive but recently their existence has been found under the high pressure condition or in the confined space between graphene sheets. Here the direct observation of the formation of Na₃ Cl nanoclusters, a stable magic-number structure, is reported on an Ir(111) surface using scanning tunneling microscopy and noncontact atomic force microscopy. The stability of Na₃ Cl nanoclusters in the free and adsorbed state is corroborated by density functional theory calculations. It is also found that a density of nanoclusters together with Cl adatoms may further aggregate and self-assemble into a Na₃ Cl₄ monolayer, forming a novel metastable phase of NaCl(111) with a honeycomb lattice. Further calculations suggest that charge transfer between the polar nanoclusters and the metal substrate stabilizes NaCl of non-1:1 stoichiometry. The work exhibits the possibility of exploring unconventional ionic crystals on the surface with atomically precise control of structure and composition.