Sensors - Hierarchical Nanostructures for Energy Devices - page 263

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Figure 11.9 Hierarchical nanostructure of Si/ZnO nano-trees decorated with Ag
nanoparticles for SERS applications. (a) SEM image (b) SERS spectra
of R6G molecule with a concentration of 1 10 5 M. a-c represent
spectra from Si/ZnO nano-trees, the 2D Ag nanoparticle substrate, and
Si/ZnO nano-trees decorated with Ag nanoparticles (adapted from ref.
12 with permission).

followed by direct growth of AgNP using a galvanic redox reaction. It was
demonstrated that the silicon nanowire array decorated with Ag nano-
particles had an enhancement factor of 10 8 -10 10 . Cheng et al. 12 prepared a
higher degree hierarchical nanostructure of Ag decorated Si/ZnO nano-trees
as shown in Figure 11.9(a) using a combination of top-down lithography for
a Si nanopillar on a wafer scale and bottom-up hydrothermal growth of ZnO
nano-trees followed by photochemical deposition of Ag nanoparticles. This
hierarchical nanostructure was implemented for the detection of Rhodamin
6G (R6G) leading to the SERS spectra shown in Figure 11.9(b). The enhanced
Raman signal was observed for the Ag-decorated Si/ZnO nano-tree structure
(curve c) compared to control samples of a Si/ZnO nano-tree without Ag
nanoparticles (curve a) and sputtered 2D Ag nanoparticles on a smooth
surface (curve b). Furthermore, an enhanced SERS signal was found even for
a1 10 9 M concentration of R6G, which proved the excellence of the
hierarchical nanostructures. The Si/ZnO nano-tree gives an ample surface
area

.

on which decorated Ag nanoparticles provide tremendous

nanoscale gaps.
A highly porous Cu nanotube was prepared by Wu et al. 48 though elec-
trodeposition of metals on electrospun fiber templates and subsequent wet
etching as shown in Figure 11.10(a) and (b). The evenly distributed nano-
scale pores on the sidewall played the role of 'hot spot' where SERS signals
are much amplified. The target molecule used in this study was crystal violet
(CV) solution. As a result, the hierarchical Cu porous nanotube had an en-
hanced SERS signal compared to a porous Cu substrate and smooth nano-
tubes as shown in Figure 11.10(c). This SERS platform could detect CV
concentrations as low as 10 6 M.
In addition, the surface roughness of a metallic periodic structure is also
important in SERS platforms. The surface roughness could be changed by