Non-classical crystal growth of inorganic and organic materials

Investigation of the early stage morphologies of some organic and inorganic systems has established that their crystal growth does not always follow the classical route established 120 years ago. The classical theory assumes that a highly symmetric crystal develops from the nucleation and repeated attachment of atoms, molecules or ions to a single nucleus. In instances where surfactants or polymers are introduced, the crystal growth is often found, however, to follow a 'reversed crystal growth' route. This non-classical growth route is initiated by organic molecules adsorbed onto the surface of the nanocrystallites in the early stages of growth. In the reversed route, these nanocrystallites form large disordered aggregates followed by surface crystallisation to create a thin, highly crystalline, dense shell and, finally, an extension of the crystallisation from the surface to the core. Although systems following a non-classical crystal growth route have been known for many years, their formation mechanisms have a much shorter history. Discovery of the reversed crystal growth route in several hollow, core-shell and twin crystal type morphologies not only assists in the designing of materials for industrial applications but also helps to understand the interactions between organic and inorganic components. This review highlights recent research relating to organic and inorganic systems whose growth follows the reversed crystal growth route.