Nanowire transformation by size-dependent cation exchange reactions.

The unique properties of nanostructured materials enable their transformation into complex, kinetically controlled morphologies that cannot be obtained during their growth. Solution-phase cation-exchange reactions can transform sub-10 nm nanocrystals/nanorods into varying compositions and superlattice structures; however, because of their small size, cation-exchange reaction rates are extremely fast, which limits control over the transformed products and possibilities for obtaining new morphologies. Nanowires are routinely synthesized via gas-phase reactions with 5-200 nm diameters, and their large aspect ratios allow them to be electrically addressed individually. To realize their full potential, it is desirable to develop techniques that can transform nanowires into tunable but precisely controlled morphologies, especially in the gas-phase, to be compatible with nanowire growth schemes. We report transformation of single-crystalline cadmium sulfide nanowires into composition-controlled Zn(x)Cd((1-x))S nanowires, core-shell heterostructures, metal-semiconductor superlattices (Zn-Zn(x)Cd((1-x))S), single-crystalline ZnS nanotubes, and eventually metallic Zn nanowires by utilizing size-dependent cation-exchange reaction along with temperature and gas-phase reactant delivery control. This versatile synthetic ability to transform nanowires offers new opportunities to study size-dependent phenomena at the nanoscale and tune their chemical/physical properties to design reconfigurable circuits.