Abstract
Starfish-like TiO 2 :Sn/SiO 2 :Sn core-shell nanocables with SiO 2 :Sn nanowires were synthesized on a silicon substrate by thermal evaporation with addition of hydrogen gas at 1000°C. In this synthesis, SnO 2 , silica, and Ti powder were used as source materials. The hydrogen gas served to reduce the SnO 2 to SnO x (i.e., SnO) vapors and to reduce the silica to SiO x (i.e., SiO) vapors. These vapors, together with Ti vapor, were incorporated into Au liquid droplets (catalyst heads) and seeded the growth of nanostructures (nanocables and nanowires). FESEM images indicated that each catalyst head functioned as a growth site, such that the nanostructures grew in starfish-like forms via a vapor-liquid-solid (VLS) mechanism. HRTEM images revealed that each nanocable was composed of a single-crystalline rutile TiO 2 :Sn core (about 10-15 nm in diameter) with an amorphous SiO 2 :Sn shell (about 50-60 nm in thickness). The amorphous SiO 2 :Sn nanowires also grew densely on the Si substrate from the catalyst heads because of the high concentration of SiO x vapor. Cathodoluminescence spectra revealed four emission peaks at 373 nm (3.32 eV), 455 nm (2.72 eV), 534 nm (2.42 eV), and 604 nm (2.05 eV) in the as-synthesized products. The TiO 2 :Sn cores and nanocrystals exhibited a quantum confinement effect, as revealed by Raman spectroscopy. The field-emission properties of the as-synthesized products provided a low turn-on field, which was estimated at ∼3.1 V/μm at a current density of 1 μA/cm 2 . © 2008 American Chemical Society.