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 M₂Q₄(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.