**2.5. Analyte preparation**

A stock solution of p-MA at 1 × 10−3 M concentration was prepared by dissolving the solid analyte into ethanol. Afterward, 1 × 10−6 M to 1 × 10−15 M solutions were prepared by further dilution. Molecules were deposited onto the substrate by chemisorption process. The samples were dipped for 20 min and then rinsed in ethanol in order to remove the excess molecules that were not covalently bounded to the metallic surface. Finally, the substrates were dried with nitrogen gas.

for all the samples. A low E-field enhancement (E/E<sup>0</sup>

, where E and E<sup>0</sup>

Engineering 3D Multi-Branched Nanostructures for Ultra-Sensing Applications

http://dx.doi.org/10.5772/intechopen.74066

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incident electric fields) is observed for the MB nanostructures directly laid on the bulk Si substrate (for h = 0 nm), owing to the strong overlapping of the local fields within the high refractive index Si material. For h = 60 nm, a 6× improvement of local E-field enhancement compared to the 2D structure is observed due to the reduction of the overlap between local E-fields and Si substrate (**Figure 4b**). At h = 150 nm (**Figure 4c**), the local E-field enhancement is 15× that of the corresponding nanostructure with "planar" geometry. Thereafter, a slight reduction is observed with a further increase of h. The role of IPS on E-field enhancement is investigated (**Figure 4e**) with nanostructures of fixed L, h and different IPS ranging from 2 to 250 nm. For IPS of around 2 nm, an E-field enhancement of 85 is observed, and it decreases exponentially with increasing interparticle distances. The large E-fields at low IPS are due to the strong interaction of the LSPRs supported by the nanostructures, thus resulting in the strong localization of intense E-fields (hot-spots). **Figure 4f** shows the SEM images of the nanostructures with 6–200 nm IPS, top-down, respectively. In order to evaluate the effect of height and IPS experimentally, SERS measurements were performed with p-MA molecules chemisorbed from a solution at 10 μM concentration, **Figure 5**. The incident laser wavelength, power and acquisition time were set to 830 nm, 1.4 mW and 10 s, respectively. The incident light polarization was kept parallel to the IPS axis. **Figure 5a** shows the SERS spectrum of p-MA molecules on five-branched nanostructure dimers with 150 nm height and 6 nm IPS. Prominent modes of p-MA were clearly visible: strong bands

**Figure 4.** (a) Schematic representation of five-branched 3D PM nanostructure dimer with 150 nm structure size and 6 nm IPS. (b and c) E-field distribution of the nanostructures at h = 60 and 150 nm, respectively. The excitation source is set to 830 nm. Calculated E-field enhancement with respect to nanostructure height (d), and as a function of IPS (e). (f) Normalincidence SEM images of five-branched 3D PM nanostructure dimer with IPS varying from 6 to 200 nm (top to down,

respectively). Reprinted with permission from reference [36]. Copyright 2014 John Wiley and Sons.

are the local and the
