**1. Introduction**

Over the past 20 years surface plasmons in metallic nanoparticles have attracted great attention due to their potential for many applications in the fields of sensing [1], light guiding [2] and

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

energy conversion [3]. Surface plasmons are collective electronic oscillations that can be excited by an external electric field. At the surface plasmon resonance wavelength of the nanoparticle, the excitation of the electronic oscillation results in high electric near-fields. In the far-field the resonance frequency is strongly scattered and absorbed, but less transmitted. The resonance frequency of a given nanoparticle depends strongly on parameters such as, for example, the shape, dimension, and composition [4, 5]. Therefore, besides the continuous progress on the study of plasmonic properties, also the development of suitable methods to synthesize plasmonic nanostructures with very well-controlled characteristics is of great importance both for basic research as well as for technological applications.

electron microscope (TEM) that allows to analyze surface plasmons at specific positions of the nanostructure, for example, by the tip or in a gap [14, 20–22], together with the very broad energy range from ~0.3 eV to several eV. Different from other high spatial resolution techniques such as cathodoluminescence in the TEM [23] or scanning near-field optical microscopy (SNOM) [8], EELS not only allows investigating bright modes that couple efficiently to

Plasmonic Modes in Au and AuAg Nanowires and Nanowire Dimers Studied by Electron Energy…

http://dx.doi.org/10.5772/intechopen.79189

41

Surface plasmon measurements with EELS are based on the interaction of the traveling electrons in the TEM with the electric field corresponding to the surface plasmons (excited by the electron itself) [25, 26]. The energy loss of the electrons is equivalent to the energy that is necessary to excite a given surface plasmon mode. For measuring this energy loss a magnetic prism in the TEM is used, deflecting electrons dependent on their energy loss [27]. Two dif-

For scanning transmission electron microscopy combined with electron energy loss spectroscopy (STEM-EELS), the electron beam is scanned along a defined path. At each position, a spectrum is created at the dispersive plane. The energy resolution of this technique is given by the natural energy width of the primary electrons that is decreased by a monochromator in the TEM to typically 0.1 eV. It is measured via the full width half maximum of the dominating zero loss peak of the spectrum. The high spatial resolution of the technique allows visualizing

For energy filtered transmission electron microscopy (EFTEM), a moveable slit is inserted at the position of the dispersive plane. Only electrons with a specific energy loss can pass the slit (typical slit width 0.2 eV) and deliver at the image plane a 2D image of the nanostructure. This image is created only by electrons with energy loss in the given range. In contrast to STEM-

For a more detailed introduction into the technique, the reader is referred to the following

In this chapter, we review our recent activities on the investigation of surface plasmon modes in metallic nanowires and nanowire dimers by STEM-EELS. In Section 2, the synthesis of various Au-based nanowires by electrodeposition in etched ion-track membranes is presented,

dealloying of Au0.4Ag0.6 nanowires [31]. Following, the visualization of multipole order surface plasmon modes in these nanowires by STEM-EELS and the study of their resonance energies as a function of, for example, nanowire dimensions and nanowire porosity, is discussed [14, 32]. Section 3 describes the synthesis of nanowire dimers by electrodeposition of segmented Au-rich/Ag-rich/Au-rich nanowires and the subsequent dissolution of Ag [33]. It continues presenting STEM-EELS measurements on these nanostructures, which allowed the investigation of surface plasmon hybridization in nanowires separated by gaps of less than 10 nm [14] as well as connected by small metallic bridges [34]. In such structures, new modes arise that depend strongly on gap or bridge sizes, respectively. Section 3 finishes with discussing mode coupling of heterodimers consisting of two wires with different length [35]. All experimental results are supported by representative finite element calculations. The chapter concludes

wires, as well as the fabrication of porous Au wires by subsequent

two dimensional EEL probability maps in the surrounding of the nanostructure.

EELS, the energy resolution is worse but the measurement technique is faster.

with a summary and final conclusions, both presented in Section 4.

light, but also dark modes [20, 24].

ferent measurement modes exist:

review articles [26, 28–30].

including Au and Au1−xAg<sup>x</sup>

### **1.1. Surface plasmons in nanowires**

Surface plasmons in nanowires can be excited in two different directions [6]. Longitudinal surface plasmons are oscillations that occur in the direction of the long nanowire axis, whereas transversal modes oscillate in the direction of the nanowire diameter. This makes nanowires especially interesting for both plasmonic basic research as well as applications. In particular, longitudinal modes attract great attention since by varying the length of the nanowires, the resonance frequency can be tuned accurately within a wide range; for Au and Ag wires from the visible to infrared frequencies [7–10]. In this frequency range, the specific spectroscopic finger print of many molecules is located. This can be used for molecular sensing by surface enhanced infrared spectroscopy (SEIRA) [1]. When the molecules are located close to a Au or Ag nanowire, the high electric fields generated by surface plasmons can strongly increase the molecular absorption signals, thus enhancing the sensitivity of the method. For Au nanowires, enhancement factors up to a factor of 105 were reported [1].

Surface plasmons in nanowires have been investigated for different kind of metals, as well as for alloy and segmented wires [11]. Compared to other metals, Au nanowires offer the advantages of being chemically stable and nontoxic. This is important because, since infrared light is less energetic that other radiation types, also in vivo applications of surface plasmons in these structures such as, for example, photothermal therapy are envisaged [12].

Another advantage of Au nanowires is the possibility of exciting multipolar modes in addition to the dipolar one (in analogy to standing waves in a resonator). Whereas in spherical particles these modes usually overlap, the geometry of the nanowires results in a clear energetic separation for several multipole orders [8, 13, 14].

Single Au nanowires are thus very interesting objects to obtain fundamental knowledge on the basics of surface plasmons. However, not only single nanowires but also complex systems consisting of more than one wire separated by small gaps of few nanometers or small metallic connections are attracting great interest [15–18]. In these kinds of structures coupling of the surface plasmons of the individual structures is possible, which results in many new modes and further enhanced electromagnetic fields [15, 19].

### **1.2. Electron energy loss spectroscopy**

Electron energy loss spectroscopy (EELS) is a powerful technique to visualize surface plasmon modes in nanostructures. It benefits from the very high spatial resolution of the transmission electron microscope (TEM) that allows to analyze surface plasmons at specific positions of the nanostructure, for example, by the tip or in a gap [14, 20–22], together with the very broad energy range from ~0.3 eV to several eV. Different from other high spatial resolution techniques such as cathodoluminescence in the TEM [23] or scanning near-field optical microscopy (SNOM) [8], EELS not only allows investigating bright modes that couple efficiently to light, but also dark modes [20, 24].

energy conversion [3]. Surface plasmons are collective electronic oscillations that can be excited by an external electric field. At the surface plasmon resonance wavelength of the nanoparticle, the excitation of the electronic oscillation results in high electric near-fields. In the far-field the resonance frequency is strongly scattered and absorbed, but less transmitted. The resonance frequency of a given nanoparticle depends strongly on parameters such as, for example, the shape, dimension, and composition [4, 5]. Therefore, besides the continuous progress on the study of plasmonic properties, also the development of suitable methods to synthesize plasmonic nanostructures with very well-controlled characteristics is of great importance both for basic research

Surface plasmons in nanowires can be excited in two different directions [6]. Longitudinal surface plasmons are oscillations that occur in the direction of the long nanowire axis, whereas transversal modes oscillate in the direction of the nanowire diameter. This makes nanowires especially interesting for both plasmonic basic research as well as applications. In particular, longitudinal modes attract great attention since by varying the length of the nanowires, the resonance frequency can be tuned accurately within a wide range; for Au and Ag wires from the visible to infrared frequencies [7–10]. In this frequency range, the specific spectroscopic finger print of many molecules is located. This can be used for molecular sensing by surface enhanced infrared spectroscopy (SEIRA) [1]. When the molecules are located close to a Au or Ag nanowire, the high electric fields generated by surface plasmons can strongly increase the molecular absorption signals, thus enhancing the sensitivity of the method. For Au nanow-

Surface plasmons in nanowires have been investigated for different kind of metals, as well as for alloy and segmented wires [11]. Compared to other metals, Au nanowires offer the advantages of being chemically stable and nontoxic. This is important because, since infrared light is less energetic that other radiation types, also in vivo applications of surface plasmons

Another advantage of Au nanowires is the possibility of exciting multipolar modes in addition to the dipolar one (in analogy to standing waves in a resonator). Whereas in spherical particles these modes usually overlap, the geometry of the nanowires results in a clear ener-

Single Au nanowires are thus very interesting objects to obtain fundamental knowledge on the basics of surface plasmons. However, not only single nanowires but also complex systems consisting of more than one wire separated by small gaps of few nanometers or small metallic connections are attracting great interest [15–18]. In these kinds of structures coupling of the surface plasmons of the individual structures is possible, which results in many new modes

Electron energy loss spectroscopy (EELS) is a powerful technique to visualize surface plasmon modes in nanostructures. It benefits from the very high spatial resolution of the transmission

in these structures such as, for example, photothermal therapy are envisaged [12].

were reported [1].

as well as for technological applications.

ires, enhancement factors up to a factor of 105

getic separation for several multipole orders [8, 13, 14].

and further enhanced electromagnetic fields [15, 19].

**1.2. Electron energy loss spectroscopy**

**1.1. Surface plasmons in nanowires**

40 Plasmonics

Surface plasmon measurements with EELS are based on the interaction of the traveling electrons in the TEM with the electric field corresponding to the surface plasmons (excited by the electron itself) [25, 26]. The energy loss of the electrons is equivalent to the energy that is necessary to excite a given surface plasmon mode. For measuring this energy loss a magnetic prism in the TEM is used, deflecting electrons dependent on their energy loss [27]. Two different measurement modes exist:

For scanning transmission electron microscopy combined with electron energy loss spectroscopy (STEM-EELS), the electron beam is scanned along a defined path. At each position, a spectrum is created at the dispersive plane. The energy resolution of this technique is given by the natural energy width of the primary electrons that is decreased by a monochromator in the TEM to typically 0.1 eV. It is measured via the full width half maximum of the dominating zero loss peak of the spectrum. The high spatial resolution of the technique allows visualizing two dimensional EEL probability maps in the surrounding of the nanostructure.

For energy filtered transmission electron microscopy (EFTEM), a moveable slit is inserted at the position of the dispersive plane. Only electrons with a specific energy loss can pass the slit (typical slit width 0.2 eV) and deliver at the image plane a 2D image of the nanostructure. This image is created only by electrons with energy loss in the given range. In contrast to STEM-EELS, the energy resolution is worse but the measurement technique is faster.

For a more detailed introduction into the technique, the reader is referred to the following review articles [26, 28–30].

In this chapter, we review our recent activities on the investigation of surface plasmon modes in metallic nanowires and nanowire dimers by STEM-EELS. In Section 2, the synthesis of various Au-based nanowires by electrodeposition in etched ion-track membranes is presented, including Au and Au1−xAg<sup>x</sup> wires, as well as the fabrication of porous Au wires by subsequent dealloying of Au0.4Ag0.6 nanowires [31]. Following, the visualization of multipole order surface plasmon modes in these nanowires by STEM-EELS and the study of their resonance energies as a function of, for example, nanowire dimensions and nanowire porosity, is discussed [14, 32]. Section 3 describes the synthesis of nanowire dimers by electrodeposition of segmented Au-rich/Ag-rich/Au-rich nanowires and the subsequent dissolution of Ag [33]. It continues presenting STEM-EELS measurements on these nanostructures, which allowed the investigation of surface plasmon hybridization in nanowires separated by gaps of less than 10 nm [14] as well as connected by small metallic bridges [34]. In such structures, new modes arise that depend strongly on gap or bridge sizes, respectively. Section 3 finishes with discussing mode coupling of heterodimers consisting of two wires with different length [35]. All experimental results are supported by representative finite element calculations. The chapter concludes with a summary and final conclusions, both presented in Section 4.
