Our fabrication of ZnO nanostructures was based on the thermal evaporation
process of Zn powder. The synthesis process was carried out in a horizontal quartz
tube inserted in a furnace. Catalyst free Si (1 1 1) plates were used as substrates,
which were ultrasonically cleaned in alcohol for 10 min previously. Zn powder and
O2 gas were employed as source materials. The temperature of the furnace was
raised to 973 K, the inset quartz tube with the Zn powder and Si (1 1 1) substrates
were loaded in. Then the quartz tube was enclosed and pumped. At the same time,
the mixture of O2 (99.99%, 5 sccm) and Ar (99.99%, 20 sccm) was introduced into the
system. During the whole growth process, the system was kept at the pressure of
10−2 Pa, and maintained for 1 h. After evaporation, the quartz tube was drawn out
from the furnace and cooled down to room temperature.