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

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

Ref. [36]. Copyright 2014 John Wiley and Sons.

20 Raman Spectroscopy

**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 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.

**Figure 7.** E-field distribution of five-branched 3D PM nanostructure dimer with Au (a) and AgAu (b) metal layers, in the x–y plane. (c and d) E-field distribution of five-branched 3D PM nanostructure dimer in a ring structure with AgAu metal layers. Reprinted with permission from reference [8]. Copyright 2014 American Chemical Society. (e) Typical SERS spectrum of p-MA molecule (at 10 μM concentration) on five-branched 3D PM nanostructure dimer in the ring structure. The exciting laser source, power and accumulation time were set to 830 nm, 1.4 mW and 15 s, respectively. The incident light polarization was set parallel to IPS axis. (f) SERS signal intensity at 1077 cm−1 as a function of metal layer compositions.

The SERS signal intensity of the C─S stretching band of p-MA measured after each regeneration cycle is reported in **Figure 8b**. A good correspondence between different recycling steps

**Figure 8.** (a) Schematic representation of the maskless recycling process. (b) SERS signal intensity of p-Ma at 1077 cm−1 vs. number of regeneration cycles and corresponding SEM images at each recycling step in the top panel. Reprinted with

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

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

23

So far, we have engineered the five-branched nanostructure dimers to improve the hot-spot intensity as a function of nanostructure height, IPS, incoming light polarization and metal composition. Nevertheless, at very low concentrations (down to the femtomolar scale), the number of molecules available per hot-spot is decreased, and thus it is necessary to endow the nanostructures with high hot-spot densities in order to improve their detection limit. In view of that, single plasmonic nanostructures with multiple branches (4–10) and sharp protrusions

The schematic representation of a typical eight-branched bimetallic AgAu nanostructure with 3D PM geometry and 200 nm IPS is shown in **Figure 9a**. Nanostructures with 4–10 branches, 20 nm Ag and 20 nm Au metal layers, 140 nm width, 150 nm high and tip radius of 10 nm were used to investigate the effect of hot-spot density on SERS signal enhancement. Normalincidence SEM image of eight-branched MB nanostructure is shown in **Figure 9b**, where the inset depicts a 54° tilted view. Besides, normal-incidence SEM images of individual MB structures with 4–10 branches are reported in the bottom panel of **Figure 9b**. A typical reflection spectrum of eight-branched 3D PM nanostructures is shown in **Figure 9c**, where multiple plasmonic resonances are clearly observable. To evaluate the effect of branch number on the LSPRs and hot-spot generation, far-field optical spectra and near-field distribution were calculated using

RSoft and CST-MW numerical approaches. Distribution of E-field enhancement (E/E<sup>0</sup>

branched 2D nanostructures at their respective LSPRs is shown in **Figure 10a**–**g**. The corresponding data points are shown in **Figure 10h**, along with their experimental counterparts. The incident polarization is set parallel to the x-axis. In the case of 4 branched nanostructures, two

) for 4–10

is clearly observed, showing an average SERS signal deviation below 10%.

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

**arrangement**

(tip radius < 10 nm) were investigated.

**3.3. Effect of branch number, perforated metal layer and overall nanostructure** 

the strong plasmon resonance of Ag, bimetallic AgAu configurations are endowed with a 4 times higher EM field enhancement. The effect of the nanoring on the MB dimer is shown in **Figure 7c** and **d**. The scattered light is reflected back towards the ring centre where the nanostructures are placed, hence increasing the EM field enhancement in the IPS region by a factor 1.65. **Figure 7e** shows the typical Raman spectrum of p-MA (10 μM concentration) on AgAu five-branched dimers in the ring structure with an IPS of 5 nm. The polarization of the incident light was fixed parallel to the IPS axis. Characteristic Raman bands of p-MA can be clearly observed in the acquired spectrum. The impact of metal layer composition on SERS signal enhancement is plotted for the band located at 1077 cm−1 (**Figure 7f**), using 20 nm IPS MB dimer structures with 20 nm Au, 20/20 nm Ag/Au and 20 nm Ag metal layers.

The 3D nanostructure configuration presented so far allows recycling, long-term stability and reutilization of the SERS substrate, thus reducing fabrication costs. **Figure 8a** shows the regeneration protocol for the present architecture. However, as clearly highlighted by the cleaning steps, the process is fully compatible with any kind of 3D plasmonic configuration. In 3D geometry, the nanostructure morphology is conserved by the underlying Si template, while the plasmon active layer is simply recovered by a mask-less wet etching process, followed by metal redeposition. The chemisorbed analyte molecules used for SERS measurements will be completely removed together with the pre-existent metal layer, thus allowing employment of different molecular species after recycling. In order to test the effectiveness of the regeneration process, we have recycled the AgAu nanostructures up to five times (**Figure 8b**) and investigated their SERS response. The corresponding SEM images are shown in the top panel.

**Figure 8.** (a) Schematic representation of the maskless recycling process. (b) SERS signal intensity of p-Ma at 1077 cm−1 vs. number of regeneration cycles and corresponding SEM images at each recycling step in the top panel. Reprinted with permission from Ref. [8]. Copyright 2014 American Chemical Society.

The SERS signal intensity of the C─S stretching band of p-MA measured after each regeneration cycle is reported in **Figure 8b**. A good correspondence between different recycling steps is clearly observed, showing an average SERS signal deviation below 10%.
