Abstract
Owing to the chemical instability of ZnO nanowires and to their large surface area exhibiting a high density of traps, the use of a protective, passivating shell plays a major role in the performances of many devices. We show that a conformal TiO2 shell can be grown on the ZnO nanowires by a double-step process using atomic layer deposition and postdeposition thermal treatment in air. An in situ analysis of the crystallization process of the amorphous TiOx shell grown by atomic layer deposition is achieved in detail by temperature-dependent Raman scattering and X-ray diffraction measurements, indicating that the crystallization starts around 280 °C in air and that only the anatase phase is formed. It is further revealed from transmission electron microscopy-based experiments that the TiO2 shell with the anatase phase is epitaxially grown on the ZnO nanowires with a range of orientation relationships. Interestingly, the epitaxial TiO2 shell has a beneficial passivating effect and its crystallization resulting from the postdeposition thermal treatment at 350 °C in air leads to the redistribution of the hydrogen-related defects in the center of the ZnO nanowires, as shown by steady-state and time-resolved photoluminescence measurements. These findings demonstrate that the atomic layer deposition technique coupled with a postdeposition thermal treatment is a powerful tool to form ZnO/TiO2 core−shell nanowire heterostructures with high structural and optical quality. They open the way for developing the present double-step processin other nanostructured material systems to master their related surface and interface properties.