Sensors - Hierarchical Nanostructures for Energy Devices - page 248

nanostructures. Hierarchical metal oxide nanostructures were prepared by
multi-step growth. The condensed power source material in a vessel is va-
porized at the elevated temperature usually inside a quartz tube. The lowered
pressure facilitates the sublimation of the solid source to vapor. The vapor is
transported by carrier gas, usually inert argon gas, and condenses under
certain conditions such as temperature, pressure, atmosphere, substrate,
etc. 6 The detailed mechanisms are reviewed and summarized in reference. 5
If a mixture of several elements is used as the source material, doped ran-
dom hierarchical nanostructures might be produced. 8

d n 3 r 4 n g | 4

11.3.1.2 Hydrothermal and Solvothermal Self-assembly
The hydrothermal/solvothermal method involves chemical reactions to
prepare crystalline nano building blocks of oxides. The synthesis is con-
ducted in a liquid. Usually, water or other solvents are used, so that the
preparation method is termed as a hydrothermal/solvothermal growth pro-
cess. This method is advantageous in that it can produce good quality
crystalline metal oxide nanostructures while maintaining control over the
composition, size and shape distribution. It is also known that crystalline
nano building blocks form incident aggregates which serve as nuclei for
further radial growth under appropriate conditions. Various hierarchical
metal nanostructures were produced using this method as shown in
Table 11.1.

11.3.2 Hierarchical Metal Nanostructures
Hierarchical metal nanostructures are widely used in SERS applications as
shown in Figure 11.1. There are many reports that show that the enhance-
ment factor increases greatly with the degree of hierarchy of the metal
nanostructures resulting in higher resolution sensors. Metal nanostructures
for SERS applications were mostly prepared by a top-down approach. This
method is very advantageous and is more controllable, but is limited to a
very narrow region and the fabrication cost is very high. Since a top-down
approach is beyond of the scope of this topic, readers are kindly encouraged
to refer to other literature for the method. Due to a lack in the capacity of the
top-down approach, bottom-up growth or a combination of the two ap-
proaches was widely used to prepare SERS active platforms that will be used
for molecular sensing applications.

.

11.3.2.1 Bottom-up Approach
A bottom-up approach is widely used to fabricate nanostructures using
chemical or physical forces. It is dicult to fabricate a structure with a size
less than 50 nm using top-down approaches, so the bottom-up approach has
been noticed as the alternative fabrication method. As shown in
Figure 11.1(e), Zhang et al. 9 fabricated core-shell Ag wires for high sensitivity