Regioselective Growth Mechanism of Single Semiconductor Tips on CdS Nanorods

Complex, anisotropic nanocrystals made from two or more components are extremely interesting functional materials that can drive directional, light-activated processes like charge separation and photocatalysis. However, while some synthetic protocols exist, little is known about the reaction mechanism for regioselective, heterogeneous nucleation of a second semiconductor material onto nanocrystal seeds. This paper presents the mechanism that leads to growth of a single tip at one end of CdS nanorods with yields between 50 and 80%. It is shown that the growth of only one tip is a result of tight control of the available, nucleating monomer in the reaction solution by working at a large chalcogenide excess. Conditions that facilitate this reaction pathway are characterized by a kinetic barrier to homogeneous growth. These match those for the formation of metastable magic-size clusters. Through this boundary condition, it can be understood why the formation of telluride tips is favored in comparison to selenides and sulfides, for which the regimes for cluster formation and nucleation on surfaces do not overlap.