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We report the catalyst-free synthesis of ZnO nanobranches on Si nanowires using metalorganic chemical vapor deposition. The formation of single-crystalline ZnO nanobranches on Si nanowire backbones has been confirmed by lattice resolved transmission electron microscopy. Depending on the growth parameters, especially the growth temperature, the morphology and size of the ZnO nanobranches evolved from nanothorn-shaped (at 350 °C) to nanoneedle-shaped structures (at 500 °C). When the growth temperature was further increased to 800 °C, thin ZnO nanowire branches grew out of the Si nanowire backbones coated with thin ZnO shells, whereas no ZnO branch was formed on bare Si nanowires due to limited nucleation. The growth behavior was further exploited to fabricate ZnO/Si nanowire networks by growing the ZnO nanowires selectively on laterally aligned Si–ZnO core-shell nanowire arrays. In addition, cathodoluminescent properties of ZnO nanobranches on Si nanowire backbones are discussed with respect to position and size.

I. INTRODUCTION

One-dimensional (1D) semiconductor nanocrystals are of interest because they can efficiently transport electrons and optical excitations.1,2 The ability to control the structure,2–5 morphology,6–8 and composition9–14 of these building blocks may allow engineers to tailor their electrical and optical properties. In particular, branched nanocrystals allow for the assembly of three-dimensionally interconnected networks,15,16 which can be exploited as active components in a wide range of device applications. Recently, self-organized dendritic nanowire structures have been synthesized by a thermal vapor transportation and condensation approach,17–19 but this method results in limited control over composition and structure. A multistep metal catalyzed vapor-liquid-solid (VLS) technique has also been introduced to grow branched and hyperbranched nanowire structures.20–22 However, the strategy for the sequential deposition of metal catalysts on nanowire backbones to grow nanowire branches is not well established, even though this is crucial to controlling the position and density of the branched nanowires. At the same time, the introduction of a large quantity of metal catalyst on the backbones could result in the incorporation of metal impurities into both backbones and

branches, which may significantly affect the electrical and optical properties of semiconductor nanowires. Catalyst-free synthesis of various nanostructures provides the growth of high quality 1D ZnO nanostructures on various substrates,23–26 but the synthesis of branched nanostructures on the 1D nanowire backbones is not well established. Here, we report on the synthesis of ZnO/Si branched nanowire heterostructures (NWHSs) by growing ZnO nanobranches on Si nanowire backbones with the use of catalyst-free metalorganic chemical vapor deposition (MOCVD). By adjusting the growth conditions, coupled with the existence of a ZnO shell layer on core Si nanowire backbone, the size (diameter, length) and density of the ZnO nanobranches hav