Covalent organic-inorganic layered 2D CdCl2(n-hexylamine)2 and not Cd2S2(n-hexylamine)

Abstract

Organic ammonium cations (A+) and inorganic [PbX4]2− (X: Cl, Br, I) anions bind to each other through electrostatic interactions, forming layered two-dimensional (2D) A2PbX4 hybrid perovskites. Thus, they dissociate in water. In contrast, charge-neutral organic amines (L) can covalently bind to metal M (M: Zn, Cd), forming M2Q2(L) (Q: S, Se, Te) hybrid II–VI semiconductors. We attempted to explore the optoelectronic properties of such a reported hybrid II–VI compound, Cd2S2(n-hexylamine), but surprisingly it did not form. Instead, the obtained product, referred to here as product-1, is a mixture of a new layered halide compound CdCl2(n-hexylamine)2 and CdS nanocrystals (NCs). The quantum confinement in ∼3 nm CdS NCs shows interesting optoelectronic properties, which were initially misinterpreted as signatures of a Cd2S2(n-hexylamine) quantum well structure. The obtained layered compound CdCl2(n-hexylamine)2 crystallizes in the P21/c space group. Each Cd2+ is coordinated with 4 equatorial Cl− and two axial n-hexylamines, forming distorted octahedra that propagate in 2D, forming the layered structure. Note that the organic and inorganic components in CdCl2(n-hexylamine)2 are covalently bound (coordinate bonds), making the compound water-stable, unlike the electrostatically bound A2PbX4 perovskites. The covalent organic–inorganic bonding nature of the layered 2D hybrid halide compounds might be explored further for designing water-stable hybrid halide perovskite-like materials.