Twinning and phase control in template-directed ZnS and (Cd,Zn)S nanocrystals

We report on the nucleation and growth of ZnS and (Cd_xZn _1-x)S nanocrystals on polydiacetylene Langmuir films. It was found that the 3-5 nm nanocrystals form ordered linear arrays aligned at a constant 27 angle with respect to the conjugated direction of the polydiacetylene film, as derived from the optimal alignment between the two phases. ZnS nanocrystals were found to nucleate specifically from the zinc blende (001) face. Because of closely matched interfacial relations, twinning defects were induced on the {111} planes. These nanometer-sized twin crystals exhibit extra {111} electron diffraction reflections due to elongation of reciprocal space spots, despite their off axis orientation. The composition of solid solution (Cd,Zn)S nanocrystals depends on the Zn²⁺\Cd²⁺ ratio in the aqueous subphase. Their structure is affected by the template mismatch both by twinning, as is the case for ZnS, for which continuous compositional shift is observed, and by phase shift to hexagonal wurtzite, with a pure CdS composition. The nanocrystals exhibited a continuous energy-gap shift, reflecting the Zn/Cd ratio in the solid solution. We demonstrate control over the nanocrystals' crystal structure, defect structure, orientation, and composition, providing a potentially effective tool for band-gap engineering in organic-inorganic hybrid assemblies.