**3.1. Effect of geometry, IPS and polarization on SERS enhancement**

The effect of 2D and 3D geometry on electric field and SERS enhancements was initially studied using five-branched nanostructure dimers with sub-10 nm IPS as a test-bench. **Figure 4a** shows the schematic presentation of the five-branched 3D PM dimer structure, where L = 150 nm, h = 150 nm, B<sup>w</sup> = 50 nm and PD = 40 nm denote star size, height, branch width and Si pillar diameter, respectively. The effect of nanostructure height on E-field enhancement is shown in **Figure 4b**–**d**, where the structure size and IPS were kept constant Engineering 3D Multi-Branched Nanostructures for Ultra-Sensing Applications http://dx.doi.org/10.5772/intechopen.74066 19

with a broadband illumination source in the visible and near-infrared spectral range has been used. The linearly polarized light was obtained by a Glan-Taylor polarizer. Reflection spectra were measured with a 50× objective of numerical aperture 0.5. The collected light has been acquired through a spectrometer with a Peltier-cooled charge-coupled-device from Ocean Optics (QE65000 and NIRquest512 for visible and near-infrared measurements,

SERS spectra were recorded with a Renishaw inVia micro-Raman spectrometer equipped with laser excitations at 830, 785, 633 and 532 nm, and a thermo-electrically cooled chargecoupled device (CCD) as detector. A 150× LEICA HCX PL APO objective (numerical aperture 0.95) was used. The diameter of the laser spot was around 680 nm, 800 nm, 1 μm and 1.07 μm for excitation wavelengths at 532, 633, 785 and 830 nm, respectively. The first order silicon peak at 520 cm−1 was used to calibrate the instrument; all the spectra were recorded at room temperature in the backscattering geometry. Wire 3.0 software was used to correct the baseline with a third-order polynomial fit. The probe molecules, p-aminothiophenol (p-MA) used

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

Raman efficiency and SERS signal intensity have been systematically investigated in different plasmonic platforms, elucidating the effect of substrate, IPS, polarization, metal composition, number of branches and geometrical arrangement, on EM near-field localization and

The effect of 2D and 3D geometry on electric field and SERS enhancements was initially studied using five-branched nanostructure dimers with sub-10 nm IPS as a test-bench. **Figure 4a** shows the schematic presentation of the five-branched 3D PM dimer structure, where L = 150 nm, h = 150 nm, B<sup>w</sup> = 50 nm and PD = 40 nm denote star size, height, branch width and Si pillar diameter, respectively. The effect of nanostructure height on E-field enhancement is shown in **Figure 4b**–**d**, where the structure size and IPS were kept constant

**3. Results and discussion: engineering 3D MBNS structures**

**3.1. Effect of geometry, IPS and polarization on SERS enhancement**

respectively).

18 Raman Spectroscopy

**2.5. Analyte preparation**

with nitrogen gas.

enhancement.

**2.4. Surface-enhanced Raman spectroscopy**

in this study, were purchased from Sigma-Aldrich.

**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.

for all the samples. A low E-field enhancement (E/E<sup>0</sup> , where E and E<sup>0</sup> are the local and the 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

is shown in **Figure 5d**. An exponential increment in the SERS intensity (around 50×) has been observed upon reduction of the IPS from 200 nm to 6 nm. The experimental findings, i.e., SERS signal dependence on nanostructure height and interparticle separation, are in good

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

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

21

To evaluate the impact of the metal layer composition on SERS signal enhancement, fivebranched nanostructure dimers in a ring structure with Au, Ag and AgAu metal layers were investigated. The schematic representation of the AgAu architecture is presented in **Figure 6a**, while SEM images of the corresponding structure are reported in **Figure 6b**–**d**. The near-field distribution of Au and AgAu nanostructures has been summarized in **Figure 7a**–**d**. Due to

**Figure 6.** (a) Schematic representation of five-branched 3D PM nanostructure dimer. (b) Normal-incidence SEM image of the nanostructures over a large area. (c and d) Top and tilted view images of individual nanostructures. Reprinted with

permission from Ref. [8]. Copyright 2014 American Chemical Society.

agreement with the numerical calculations reported in **Figure 4d** and **e**.

**3.2. Effect of bimetal layer, and recycling of SERS substrates**

**Figure 5.** (a and b) Typical SERS spectra of p-MA molecules (chemisorbed at 10 μM concentration) on five-branched 3D PM nanostructure dimer with 6 nm IPS. The excited polarization is set parallel (a) and perpendicular (b) to IPS axis. The corresponding calculated surface-charge distribution is represented by direction arrows, shown in the inset. (c) Experimental SERS signal intensity at 1077 cm−1 as a function of nanostructure height, where the IPS is fixed at 6 nm. The inset shows the SERS spectra of five-branched nanostructures at h = 0 (blue spectrum) and 150 nm (red spectrum). (d) Variation of the SERS signal intensity with respect to IPS with constant h at 150 nm. Reprinted with permission from Ref. [36]. Copyright 2014 John Wiley and Sons.

are centered at 1077 and 1590 cm−1 while low-intense bands correspond to 1140, 1179, 1390 and 1438 cm−1 [6, 8]. Polarization-dependent SERS signal enhancement of MB nanostructures is shown in **Figure 5a** and **b**, where the polarization was set parallel and perpendicular to IPS axis, respectively. Insets represent the calculated surface-charge distributions in correspondence of the IPS region. A dipolar-like distribution of surface charges was observed when the incident light is parallel to the IPS axis (inset of **Figure 5a**). The in-phase dipole moments generate an intense E-field (hot-spot) in the IPS region due to the strong coupling of the sub-10 nm gapped nanostructures. In the case of perpendicular polarization (inset of **Figure 5b**), the effective dipoles are aligned across the interparticle nanocavity leaving a low-intensity local E-field in the IPS region. The corresponding SERS spectra (**Figure 5a** and **b**) discriminate the polarization-induced SERS signal intensities with a factor of 15. The C-S stretching mode located at 1077 cm−1 was used to calculate the SERS enhancement factor. **Figure 5c** displays the SERS intensity dependence on nanostructure height of the specific strong band located at 1077 cm−1, showing an exponential-like growth. The inset confirms the high signal-to-noise (SNR) ratio of the 3D PM structures compared to the planar case, for h = 0 (blue spectrum) and h = 150 nm (red spectrum), respectively. The impact of IPS on SERS signal enhancement is shown in **Figure 5d**. An exponential increment in the SERS intensity (around 50×) has been observed upon reduction of the IPS from 200 nm to 6 nm. The experimental findings, i.e., SERS signal dependence on nanostructure height and interparticle separation, are in good agreement with the numerical calculations reported in **Figure 4d** and **e**.
