Enhancement of the near-band edge emission from ZnO nanowires by SnO2 coating

초록

The ZnO-core/SnO2-shell nanowires were fabricated by a two-step process: the thermal evaporation of ZnO powders and the atomic layer deposition of SnO2. Both the transmission electron microscopy and X–ray diffraction analyses revealed that the cores and shells of the as-prepared core-shell nanowires were single crystal wurtzite type ZnO and polycrystalline SnO2, respectively. Photoluminescence (PL) measurement showed that the near-band edge (NBE) emission of the ZnO nanowires was significantly enhanced in intensity by coating them with SnO2. The NBE emission of the ZnO-core/SnO2-shell nanowires with the shell layer thickness of 15 nm was ~25 times higher in intensity than that of the ZnO nanowires. The NBE emission enhancement may be attributed to the quantum confinement of the photo-generated carriers inside the ZnO cores due to the larger energy band gap of the neighboring SnO2 shells, the suppression of visible emission and nonradiative recombination due to the formation of a depletion region in the ZnO cores, the suppression of carrier capture by surface states due to the passivation of the ZnO cores by the SnO2 shells, and the giant oscillator strength effect due to the optical resonant cavity formation in the nanowires. In particular, the exceptionally high NBE emission intensity for the shell layer thickness of 15 nm may be mainly attributed to the optical resonant cavity formation. In addition, low-temperature PL measurements showed that the NBE emission of the ZnO nanorods was blue-shifted by ~0.054 eV by SnO2 coating, which was caused by the surface state passivation effect.