Epitaxial TiO2/SnO2 core-shell heterostructure by atomic layer deposition

Taking TiO2/SnO2 core-shell nanowires (NWs) as a model system, we systematically investigate the structure and the morphological evolution of this heterostructure synthesized by atomic layer deposition/epitaxy (ALD/ALE). All characterizations, by X-ray diffraction, high-resolution transmission electron microscopy, selected area electron diffraction and Raman spectra, reveal that single crystalline rutile TiO2 shells can be epitaxially grown on SnO2 NWs with an atomically sharp interface at low temperature (250 degrees C). The growth behavior of the TiO2 shells highly depends on the surface orientations and the geometrical shape of the core SnO2 NW cross-section. Atomically smooth surfaces are found for growth on the {110} surface. Rough surfaces develop on {100} surfaces due to (100) - (1 x 3) reconstruction, by introducing steps in the [010] direction as a continuation of {110} facets. Lattice mismatch induces superlattice structures in the TiO2 shell and misfit dislocations along the interface. Conformal epitaxial growth has been observed for SnO2 NW cores with an octagonal cross-section ({100} and {110} surfaces). However, for a rectangular core ({10 $$(1) over bar} and {010} surfaces), the shell also derives an octagonal shape from the epitaxial growth, which was explained by a proposed model based on ALD kinetics. The surface steps and defects induced by the lattice mismatch likely lead to improved photoluminescence (PL) performance for the yellow emission. Compared to the pure SnO2 NWs, the PL spectrum of the core-shell nanostructures exhibits a stronger emission peak, which suggests potential applications in optoelectronics.