terization and electron diffraction of individual nanostructures
indicate that both the base and wires are single-crystalline
and the wires grow along the [0001] direction (Figure 2B,C).
This unique parallel growth of densely packed nanowires
from a thin single-crystalline base can be explained in two
different ways. (i) Individual nanowires self-assemble in
solution and form bundles, or (ii) the thin platelike base forms
first and acts as a seed for epitaxial nanowire growth. The
reaction was stopped at an early growth to further examine
the mechanism. SEM images show nanorods growing from
flat, hexagonal crystallites while maintaining the same
orientation and symmetry (Figure 3). This suggests an
epitaxial growth mechanism (growth mechanism ii), where
a two-step growth process leads to a single-crystalline bundle.
The high optical quality of the as-grown wires was verified
with absorption and PL studies (Figure 4). The absorption
spectrum of a 2-propanol solution of nanowires shows a
sharp peak at 3.35 eV corresponding to the band gap of bulk
ZnO. The emission profile shows a marked band edge at
3.2 eV. It should be noted that the emission does not exhibit
any solvent quenching effects and the band-gap emission is
remarkably intense for a solution-phase sample.45 Absorption
and PL were acquired with a continuous-wave laser source
(HeCd, 325 nm) on individual wires cast onto a clean silicon
substrate. Low-temperature PL reveals a blue shift of 90 meV
in the band-edge emission from room temperature to 5 K
(Figure 4B)