Covalent Organic-Inorganic Layered 2D CdCl2(n-hexylamine)2 and Not Cd2S2(n-hexylamine)

Abstract

The organic ammonium cations (A+) and the inorganic [PbX4]2- (X: Cl, Br, I) anions bind to each other through electrostatic interactions forming the layered two-dimensional (2D) A2PbX4 hybrid perovskites. So, they dissociate in water. In difference, the 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 reported hybrid II-VI compound Cd2S2(n-hexylamine), but surprisingly it does not form. Instead, the obtained product, referred 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 Cd2S2(n-hexylamine) quantum well structure. The obtained layered compound CdCl2(n-hexylamine)2 crystallizes in P21/c space group. Each Cd2+ are coordination with 4 equatorial Cl- and two axial n-hexylamines forming distorted octahedra that propagates in 2D forming the layered structure. Note that the organic and inorganic components in CdCl2(n-hexylamine)2 are covalently (coordinate bond) bound 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.