Subwavelength optical resonant cavity-induced enhancement of the near-band-edge emission from ZnO-core/SnO₂-shell nanorods

ZnO-core/SnO₂-shell nanorods were fabricated by a two-step process: thermal evaporation of ZnO powders and atomic layer deposition of SnO₂. Transmission electron microscopy and X-ray diffraction revealed the cores and shells of the as-prepared core-shell nanorods to be single crystal wurtzite-type ZnO and polycrystalline rutile-type tetragonal SnO ₂, respectively. Photoluminescence (PL) measurements showed that the intensity of near-band edge (NBE) emission of ZnO nanorods was enhanced significantly by the SnO₂ coating. The maximum intensity of NBE emission of the ZnO-core/SnO₂-shell nanorods obtained with a shell layer thickness of 15 nm was ∼25 times higher than that of the ZnO nanorods. The enhancement of the NBE emission might be due to the combination of the following sources: the giant oscillator strength effect due to subwavelength optical resonant cavity formation in the nanorods, the quantum confinement of photogenerated carriers inside the ZnO cores, the suppression of visible emission and nonradiative recombination due to the formation of a depletion region in the ZnO cores, and the suppression of carrier capture by surface states. In particular, the exceptionally high NBE emission intensity for a specific shell layer thickness of 15 nm was attributed mainly to subwavelength optical resonant cavity formation.