**1. Introduction**

Nanoplasmonics is an emerging area of scientific research with a variety of applications in spectroscopy, metamaterials engineering, biosensing, lasing, photocatalysis, nonlinear and quantum optics [1–8]. This discipline deals with the study of collective oscillations of conduction electrons at metal-dielectric interfaces, which can be resonantly excited upon external irradiation. Coupling of electromagnetic (EM) fields to the free-electron motion leads to enhanced optical near fields, confined in subwavelength regions and localized in close proximity of metallic nanoparticles (localized surface plasmon resonances—LSPRs), which are very sensitive to the local dielectric environment [9, 10]. Within this context, nanostructures endowed with sharp tips/edges and sub-10 nm interparticle separation (IPS) are ideal candidates for nanoscale manipulation of optical energy, promoting nanofocusing of EM radiation into hot-spots [11–15]. In recent years, a great variety of nanostructure geometries, for instance, nanospheres, nanocubes, nanocones, nanoantennas, nanoaggregates and nanostars, have been fabricated by bottom-up and top-down approaches [16–24], in order to engineer their plasmonic resonances and increase the field enhancement and hot-spot densities. Control over nanostructure morphology shows promising applications in bio/chemical sensing using the synergistic combination of LSPR and surface-enhanced Raman spectroscopy (SERS) as detection paradigm [9, 25–26].

surface influence the surrounding dielectric medium, thus resulting in an effective refractive index change and subsequent resonance shift. Although this technique is very powerful for molecule detection, it lacks analyte specificity, and it suffers from reduced efficiency at very

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

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

On the contrary, SERS enables the detection of biological and chemical analytes with high specificity and sensitivity even at ultra-low concentrations, exploiting the electromagnetic enhancement offered by plasmonic nanostructures [3, 27–32]. The huge enhancement typical of SERS substrates is mainly based on chemical and electromagnetic phenomena. In chemical enhancement [33], the charge transfer between electronic energy levels of the metal and the adsorbed molecules increases the Raman scattering cross-section up to a factor of 10<sup>2</sup>

The electromagnetic contribution, instead, is crucially depending on the near-field intensity associated with the nanostructure plasmonic activity. In SERS detection, the intrinsically

observed in [34, 35]), due to the interaction between the adsorbed molecules and the electromagnetic near-fields. In this respect, nanostructures featuring closely spaced gaps and/or sharp protrusions are of great interest in detecting analytes at ultra-low concentration [8, 36, 37]. However, the tiny availability of molecules at the single/few entities limit can dramatically reduce the effectiveness of a plasmon-based sensitive device. It is therefore imperative for overcoming this practical limitation to design and engineer nanostructure architectures with sufficiently high hot-spot densities. **Figure 2** shows the local EM field mapping on a multi-branched (MB) nanoparticle using electron energy loss spectroscopy: strong near-field intensities are clearly promoted and well-confined by the sharp apexes. In view of that, nanostructures with different layouts, multiple branches and single-digit IPS will be studied in the

Various nanofabrication methods, applying both bottom-up and top-down approaches, have been used to fabricate SERS-based platforms for sensing applications [38–46]. Among them, colloidal techniques provide a wide variety of nanoparticles with sharp protrusions, although they are still suffering for poor control over uniformity and arrangement [35]. The positioning of the analyte molecule in the vicinity of the hot-spot is critical for improving the SERS enhancement factor and hence the detection limit. Due to the lack of reproducible

**Figure 2.** (a) Scanning transmission electron microscopy dark-field image of an individual gold nanostar. (b and c) Electron energy loss spectroscopy intensity mapping and the calculated intensity map of the plasmon resonances around a nanostar apex, respectively. Reprinted with permission from [35]. Copyright (2009) American Chemical Society.

weak Raman signals can be enhanced by many orders of magnitude (a factor of 10<sup>8</sup>

.

15

has been

small concentrations.

present work.

As shown in **Figure 1**, the interaction of the incident light with a plasmonic nanostructure can promote resonant oscillation of the free electron cloud, which, for particles smaller than the exciting wavelength, can give rise to standing waves, i.e., to LSPRs. Noble metals, such as gold, silver and copper, are the best candidates for supporting plasmon activity, due to their low electron losses, high carrier densities and high field amplitudes on the particle surface. The nanostructure morphology (size, shape and arrangement) together with the surrounding dielectric environment plays a key role in the excitation of plasmonic resonances [10]. In LSPR-based sensing devices, the analytes adsorbed on the nanostructure

**Figure 1.** Schematic representation of the localized surface plasmon resonance on a metal particle.

surface influence the surrounding dielectric medium, thus resulting in an effective refractive index change and subsequent resonance shift. Although this technique is very powerful for molecule detection, it lacks analyte specificity, and it suffers from reduced efficiency at very small concentrations.

**1. Introduction**

14 Raman Spectroscopy

detection paradigm [9, 25–26].

Nanoplasmonics is an emerging area of scientific research with a variety of applications in spectroscopy, metamaterials engineering, biosensing, lasing, photocatalysis, nonlinear and quantum optics [1–8]. This discipline deals with the study of collective oscillations of conduction electrons at metal-dielectric interfaces, which can be resonantly excited upon external irradiation. Coupling of electromagnetic (EM) fields to the free-electron motion leads to enhanced optical near fields, confined in subwavelength regions and localized in close proximity of metallic nanoparticles (localized surface plasmon resonances—LSPRs), which are very sensitive to the local dielectric environment [9, 10]. Within this context, nanostructures endowed with sharp tips/edges and sub-10 nm interparticle separation (IPS) are ideal candidates for nanoscale manipulation of optical energy, promoting nanofocusing of EM radiation into hot-spots [11–15]. In recent years, a great variety of nanostructure geometries, for instance, nanospheres, nanocubes, nanocones, nanoantennas, nanoaggregates and nanostars, have been fabricated by bottom-up and top-down approaches [16–24], in order to engineer their plasmonic resonances and increase the field enhancement and hot-spot densities. Control over nanostructure morphology shows promising applications in bio/chemical sensing using the synergistic combination of LSPR and surface-enhanced Raman spectroscopy (SERS) as

As shown in **Figure 1**, the interaction of the incident light with a plasmonic nanostructure can promote resonant oscillation of the free electron cloud, which, for particles smaller than the exciting wavelength, can give rise to standing waves, i.e., to LSPRs. Noble metals, such as gold, silver and copper, are the best candidates for supporting plasmon activity, due to their low electron losses, high carrier densities and high field amplitudes on the particle surface. The nanostructure morphology (size, shape and arrangement) together with the surrounding dielectric environment plays a key role in the excitation of plasmonic resonances [10]. In LSPR-based sensing devices, the analytes adsorbed on the nanostructure

**Figure 1.** Schematic representation of the localized surface plasmon resonance on a metal particle.

On the contrary, SERS enables the detection of biological and chemical analytes with high specificity and sensitivity even at ultra-low concentrations, exploiting the electromagnetic enhancement offered by plasmonic nanostructures [3, 27–32]. The huge enhancement typical of SERS substrates is mainly based on chemical and electromagnetic phenomena. In chemical enhancement [33], the charge transfer between electronic energy levels of the metal and the adsorbed molecules increases the Raman scattering cross-section up to a factor of 10<sup>2</sup> . The electromagnetic contribution, instead, is crucially depending on the near-field intensity associated with the nanostructure plasmonic activity. In SERS detection, the intrinsically weak Raman signals can be enhanced by many orders of magnitude (a factor of 10<sup>8</sup> has been observed in [34, 35]), due to the interaction between the adsorbed molecules and the electromagnetic near-fields. In this respect, nanostructures featuring closely spaced gaps and/or sharp protrusions are of great interest in detecting analytes at ultra-low concentration [8, 36, 37]. However, the tiny availability of molecules at the single/few entities limit can dramatically reduce the effectiveness of a plasmon-based sensitive device. It is therefore imperative for overcoming this practical limitation to design and engineer nanostructure architectures with sufficiently high hot-spot densities. **Figure 2** shows the local EM field mapping on a multi-branched (MB) nanoparticle using electron energy loss spectroscopy: strong near-field intensities are clearly promoted and well-confined by the sharp apexes. In view of that, nanostructures with different layouts, multiple branches and single-digit IPS will be studied in the present work.

Various nanofabrication methods, applying both bottom-up and top-down approaches, have been used to fabricate SERS-based platforms for sensing applications [38–46]. Among them, colloidal techniques provide a wide variety of nanoparticles with sharp protrusions, although they are still suffering for poor control over uniformity and arrangement [35]. The positioning of the analyte molecule in the vicinity of the hot-spot is critical for improving the SERS enhancement factor and hence the detection limit. Due to the lack of reproducible

**Figure 2.** (a) Scanning transmission electron microscopy dark-field image of an individual gold nanostar. (b and c) Electron energy loss spectroscopy intensity mapping and the calculated intensity map of the plasmon resonances around a nanostar apex, respectively. Reprinted with permission from [35]. Copyright (2009) American Chemical Society.

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 detection at ultra-low concentrations.
