**2. Experimental techniques**

#### **2.1. Fabrication of multi-branched nanostructures**

**Figure 3a**–**c** shows the fabrication protocol of typical six-branched nanostructures with 2D and 3D layout (without and with perforated metal (PM) topologies). A combination of electron beam lithography (EBL, Raith 150-Two), and reactive ion etching (RIE, SENTECH) has been successfully employed for the uniform production of 3D and 3D PM nano-architectures. As highlighted in **Figure 3a** planar and 3D nanostructures share a common EBL step. A 250 nm polymethyl-methacrylate (PMMA, 950 kDa) layer was spin coated at 3000 rpm for 60 s onto a p-type c-Si (100) wafer. The substrate has been heated at 180 °C for 9 min to get a homogenous PMMA film. After e-beam exposure (electron energy 30 keV and beam current 130 pA), the substrate has been developed in a 3:1 mixture of isopropanol and methyl isobutyl ketone at 4 °C for 3 min. For 2D structures, 3 nm Ti and 18 nm Au were deposited, and the unexposed PMMA removed by ultrasonically assisted lift-off in acetone. In the case of 3D multi-branched nanostar (MBNS) structures (**Figure 3b**), a 20 nm chromium layer was deposited on top of 3 nm Ti and 18 nm Au to act as an etch mask. The excess metal was removed using an ultrasonically assisted lift-off process. Thereafter, substrates were reactiveion etched (with an etch rate of ≈100 nm min−1) in an atmosphere of SF<sup>6</sup> (30 Standard Cubic Centimeters per Minute—SCCM) and C<sup>4</sup> F8 (32 SCCM) at 1 mTorr, where temperature, power and etching time were held at 4°C, 18 W and 25 s, respectively. After the RIE procedure, the chromium layer was removed by wet-etching in a ceric ammonium nitrate-based mixture (Sigma-Aldrich). 3D PM structures (**Figure 3c**) were obtained by depositing 20 nm of chromium after developing the PMMA. Excess chromium has been removed in acetone liftoff, leaving behind multi-branched shape chromium patterns on silicon, serving as etching masks. RIE was employed to produce the underlying pedestals. Subsequently, the chromium mask was removed leaving behind silicon stars on silicon posts. 3 nm Ti and 18 nm Au were evaporated at a deposition rate of 0.3 Å/s in order to form the Au MBNS as well as the perforated film on the underlying substrate.

#### **2.2. Numerical calculations**

Finite integration technique FIT (computer simulation technology-microwave, CST-MW) was used to calculate the near-field properties of the MBNS structures while rigorous coupled wave analysis (Synopsys' Optical Solutions, RSoft) was employed to calculate the far-field

**Figure 3.** (a–c) Schematic illustration of the protocol, for the fabrication procedure of six-branched nanostructure with different topologies. The corresponding SEM images for six-branched nanostructures are shown below. Reprinted with

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

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

17

The spectral response of the MB nanostructures under normal incidence irradiation was measured using an Olympus IX-73 research microscope. A 100 W halogen lamp (Olympus)

response of the nanostructures.

permission from reference [6]. Copyright 2017 John Wiley and Sons.

**2.3. Optical characterization**

Engineering 3D Multi-Branched Nanostructures for Ultra-Sensing Applications http://dx.doi.org/10.5772/intechopen.74066 17

**Figure 3.** (a–c) Schematic illustration of the protocol, for the fabrication procedure of six-branched nanostructure with different topologies. The corresponding SEM images for six-branched nanostructures are shown below. Reprinted with permission from reference [6]. Copyright 2017 John Wiley and Sons.

wave analysis (Synopsys' Optical Solutions, RSoft) was employed to calculate the far-field response of the nanostructures.

#### **2.3. Optical characterization**

SERS signals and homogeneity of the structures, alternative fabrication methods are needed. In this context, lithographic techniques can overcome these limitations, thus providing a feasible strategy towards the realization of uniformly patterned nanostructures over large areas. In the present chapter, fabrication and characterization of three-dimensional (3D) multibranched plasmonic architectures realized by means of electron-beam lithography (EBL) and reactive ion etching (RIE) techniques will be investigated. Numerical calculations, Raman and optical characterization will be used for demonstrating outstanding performances in analyte

**Figure 3a**–**c** shows the fabrication protocol of typical six-branched nanostructures with 2D and 3D layout (without and with perforated metal (PM) topologies). A combination of electron beam lithography (EBL, Raith 150-Two), and reactive ion etching (RIE, SENTECH) has been successfully employed for the uniform production of 3D and 3D PM nano-architectures. As highlighted in **Figure 3a** planar and 3D nanostructures share a common EBL step. A 250 nm polymethyl-methacrylate (PMMA, 950 kDa) layer was spin coated at 3000 rpm for 60 s onto a p-type c-Si (100) wafer. The substrate has been heated at 180 °C for 9 min to get a homogenous PMMA film. After e-beam exposure (electron energy 30 keV and beam current 130 pA), the substrate has been developed in a 3:1 mixture of isopropanol and methyl isobutyl ketone at 4 °C for 3 min. For 2D structures, 3 nm Ti and 18 nm Au were deposited, and the unexposed PMMA removed by ultrasonically assisted lift-off in acetone. In the case of 3D multi-branched nanostar (MBNS) structures (**Figure 3b**), a 20 nm chromium layer was deposited on top of 3 nm Ti and 18 nm Au to act as an etch mask. The excess metal was removed using an ultrasonically assisted lift-off process. Thereafter, substrates were reactive-

(30 Standard Cubic

(32 SCCM) at 1 mTorr, where temperature, power

ion etched (with an etch rate of ≈100 nm min−1) in an atmosphere of SF<sup>6</sup>

F8

and etching time were held at 4°C, 18 W and 25 s, respectively. After the RIE procedure, the chromium layer was removed by wet-etching in a ceric ammonium nitrate-based mixture (Sigma-Aldrich). 3D PM structures (**Figure 3c**) were obtained by depositing 20 nm of chromium after developing the PMMA. Excess chromium has been removed in acetone liftoff, leaving behind multi-branched shape chromium patterns on silicon, serving as etching masks. RIE was employed to produce the underlying pedestals. Subsequently, the chromium mask was removed leaving behind silicon stars on silicon posts. 3 nm Ti and 18 nm Au were evaporated at a deposition rate of 0.3 Å/s in order to form the Au MBNS as well as the perfo-

Finite integration technique FIT (computer simulation technology-microwave, CST-MW) was used to calculate the near-field properties of the MBNS structures while rigorous coupled

detection at ultra-low concentrations.

16 Raman Spectroscopy

**2. Experimental techniques**

Centimeters per Minute—SCCM) and C<sup>4</sup>

rated film on the underlying substrate.

**2.2. Numerical calculations**

**2.1. Fabrication of multi-branched nanostructures**

The spectral response of the MB nanostructures under normal incidence irradiation was measured using an Olympus IX-73 research microscope. A 100 W halogen lamp (Olympus) 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, respectively).

#### **2.4. Surface-enhanced Raman spectroscopy**

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 in this study, were purchased from Sigma-Aldrich.
