Stability of cubic tin sulphide nanocrystals: role of ammonium chloride surfactant headgroups.

New semiconducting metastable cubic phases have been recently discovered in the tin monosulfide and monoselenide systems. Surface energy calculations and experimental studies indicate that this cubic π-phase is stabilized by specific ligand adsorption on the surface. In this work, it is shown experimentally that the synthesis carried out using mixtures of oleylamine and oleylammonium chloride (OACl) surfactants results in the cubic phase, transforming the growth from orthorhombic to cubic nanoparticles with increasing OACl concentration up to a limiting point. Complementary ab initio calculations find that adsorbed ligands lower the surface energies for both the cubic phase and the orthorhombic phase, relative to the pristine surfaces. The decrease in the surface energy increases with ligand coverage. Stronger binding energies to the cubic phase suggest a higher coverage, and therefore preferential stabilization of this phase. Upon further increasing the coverage, the surface energy becomes negative, effectively destabilizing the particles in agreement with experimental observations. Bonding analysis shows that Cl bonds to Sn and replaces missing Sn-S bonds at the surface of the cubic structure. In the competing orthorhombic layered phase, Cl also bonds to a Sn atom but at the expense of one of the Sn-S bonds of this Sn atom. This observation can explain the trends of the surface energies. This combined experimental and computational analysis sheds light on the stabilization processes of these nano-materials and indicates the path to improve synthetic routes.