Chemistry Reference
In-Depth Information
Gao et al.
47
reported ''nanopropeller'' arrays of ZnO synthesized on a Al
2
O
3
substrate by a two-step high temperature solid-vapor deposition process
where ZnO and SnO
2
powders were reduced by graphite in a heated tube
furnace (ZnO(s)
þ
C(s)
Zn(v)
þ
CO(v)). Initially, ZnO backbone nanowires
were grown through the VLS process aided by Sn as a catalyst at a lower
temperature (600-700 1C) and then Sn vapors condensed on the surface of
backbone nanowires at a higher temperature (700-800 1C) from which
branch nanowires were grown with a six-fold symmetry (Figure 8.9(c)). Lao
et al.
48
also synthesized various types of ZnO hierarchical structures such as
ZnO ''nanonails'' on nanowires/nanobelts in a similar process to Gao
(Figure 8.9(d)).
This method has an advantage in that the oven or vacuum chamber need
not be opened for the sequential catalyst deposition, hence preventing oxi-
dation of previously grown nanostructures or deposited catalysts.
-
d
n
3
r
4
n
g
|
4
8.4.4 Combination of VLS/Thermal Evaporation Method and
Hydrothermal Growth
VLS (catalyst-mediated) and thermal evaporation (no catalyst) growth are
widely used methods for 1-D structure material synthesis, VLS for group
IV
49-51
and III-V
52,53
semiconductors and thermal evaporation for metal
oxides such as WO
3
,
54
Fe
2
O
3
55
and ZnO.
56
Hydrothermal growth, as men-
tioned earlier, is a facile route for nanowire synthesis over a large area at a
low cost and utilizing relatively simple equipment. To create hierarchical
nanostructures, typically the backbone stems are built by VLS or a thermal
evaporation process, and subsequently, branch nanowires are grown from
seed materials attached on the surface of the backbone via a hydrothermal
process.
This approach is particularly useful for the hetero-junction hierarchical
structures composed of two or more different materials. Hence, various
combinations of materials have been reported recently. More details and
examples of this process will be discussed in section 8.4.7.
.
8.4.5 Defect-driven Growth
Bierman et al.
57
and Zhu et al.
58
have reported a new nanowire synthesis
mechanism that results in a PbS and PbSe pine tree structure featuring
branches that are spirally arranged around the trunk nanowires as depicted
in Figure 8.10. Long trunk nanowires are generated via fast screw dis-
location-driven growth where the axial screw dislocations provide self-
perpetuating steps enabling catalyst-free 1-D crystal growth,
57,59
while
branches are formed by slower VLS epitaxial growth. The helically rotating
branches are a consequence of the strain of the axial dislocation called the
Eshelby twist
60
developed in the trunk nanowire. To achieve synergy between
the dislocation-driven and VLS mechanisms for hierarchical nanostructure
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