ZnO NWs have been synthesized on Si(100) substrates by
vapor phase transport in an oxidizing ambient. Since the
unintentional formation of SiO2 in the early stages of the
growth prevents ZnO nucleation, a Zn thin (~100 Å) film is
deposited by radio frequency magnetron sputtering prior to the
process on a Si substrate previously cleaned with a
HF(10%wt)/ethanol buffer solution. For the sake of
comparison, Si(100) substrates without seed Zn layer have also
been used during the studies. Graphite is demonstrated to be
crucial for the reduction of the ZnO and the nucleation on the
substrate surface; thus, graphite powder is added to the crucible
in amounts one to four times larger than the amount of ZnO
(total mass=0.5 g). Both powders are conveniently mixed up to
assure that ZnO particles remain well surrounded by graphite
dust. Source and substrate are loaded into a Tempress furnace
and placed towards the middle part of the furnace keeping an
approximate distance of 10 cm between them. The process is
performed at temperatures between 750 and 950 °C under a
constant flow of Ar/O2 (1:1) gases (200-sccm total flux).
Synthesis process lasts about 30 min, leaving the sample in the
system overnight under an Ar flow for cooling-down.