Catalyst-free synthesis and cathodoluminescent properties of ZnO nanobranches on Si nanowire backbones

Abstract

The ability to control the structure, morphology, and composition of semiconductor nanocrystals could provide the potential to design their electrical and optical properties. In particular, branched nanocrystals allow for the assembly of three-dimensionally interconnected nanowire networks, and can be exploited as active components in a wide range of device applications. Recently, multi-step metal-catalyzed vapor-liquid-solid (VLS) technique has been introduced to grow branched and hyperbranched nanowire structures. However, the strategy for sequential seeding of catalysts on the nanowire backbones to grow the nanowire branches is not well established even though this is very crucial to control the position and density of the nanowire branches. At the same time, the introduction of a large amount of metal catalysts on the backbones could result in the incorporation of the metal impurities into both backbones and branches, which might significantly affect on the electrical and optical properties of semiconductor nanowires. Here, we report the synthesis of ZnO/Si branched heterostructures by employing catalyst-free metalorganic vapor deposition. The formation of single-crystalline ZnO nanobranches on Si backbones with an abrupt heterointerface was conformed by transmission electron microscopy (TEM) (Fig. 1). By adjusting the growth conditions, the size and density of the ZnO nanobranches have been tuned. In particular, we found that very thin ZnO nanowire by the thickness of thin ZnO shell layers coated on Si nanowire backbones, which was CL spectra of the ZnO nanobranches exhibited the strong free exciton emissions at 3.23 eV for ZnO nanoneedles and 3.30 eV for very thin ZnO nanowires (Fig. 2), and the quantum confinement effect was introduced to explain the blue-shift in the emission peak for very thin ZnO nanowire branches.