**2.1. TERS technique: a historical overview**

molecules adsorbed on roughened metal surfaces was a corner stone in the development of a family of enhanced vibrational spectroscopies with the surface-enhanced Raman spectroscopy (SERS) being the most widely practiced [3–5]. In SERS, Raman signals are amplified by placing a molecule in the vicinity of metal nanostructures. A large number of papers have been published over a period of 40 years on the origin of Raman signal enhancement on SERS active substrates [6–8]. Consequently, the overall enhancement of the signal has been attributed to two main mechanisms: electromagnetic enhancement (EME) and chemical enhancement (CE).

EME is explained by the enhancement of the electromagnetic field at the molecule's position by excitation of the localized surface plasmon resonances of metallic nanostructures [9, 10]. Basic EME is now well understood and believed to be the major contribution to the enhancement of Raman signals on SERS active substrates. In the process, both the incoming and outgoing field is amplified. For a single molecule, basic EME, excluding polarization effects and tensorial

field. Polarization effects influence signal intensity but in a more complex way, depending on the symmetry of the vibrational mode. Moskovits, in his 1982 work, described Raman surface selection rules, that is, modification of band intensities for a molecule adsorbed on a flat metal surface [11]. Experimental justification of polarization effects requires Raman data obtained on flat metal surfaces and a nanoscale characterization of the environment in which a molecule is present. With the development of tip-enhanced Raman spectroscopy (TERS), we can now define the nanoscale environment of a molecule and correlate it directly with the Raman signal. It is expected that polarization studies in TERS settings, supported by calculations of the Raman polarizability tensor components of a molecule, will bring further insights into

mechanism of Raman signal enhancement and surface selection rules.

by suitable electronic structure calculations of the molecule-metal system.

CE describes various effects affecting the Raman polarizability αR with respect to the *α*<sup>0</sup>

free molecule [12]. Among them is the formation of a metal-molecule complex, anion and field gradient effects [8, 13–16]. CE can be separated into a resonant and nonresonant mechanism. The resonant mechanism is discussed based on the strength of a metal-molecule coupling. In the weak coupling regime, the molecular electronic states may shift and broaden upon their interaction with the metal, and enhancement of the Raman scattering can be achieved by the resonance Raman effect, in which the incident beam is in resonance with the electronic excitation of a molecule. In the strong coupling regime (chemisorption), new electronic states appear, so-called charge-transfer (CT) states that act as intermediate states in Raman scattering [6, 17, 18]. The enhancement of the signal is due to resonance of the incident beam with an excitation from the metal to the molecule or *vice versa*. The nonresonant effect, called the static chemical enhancement, involves various effects, such as the binding geometry of the adsorbate, the influence of co-adsorbed anions and the presence of non-zero static electric fields [14–16, 19, 20]. It has been difficult to investigate how these effects influence the strength of the Raman signals as the signals measured in SERS are averaged over the diffraction-limited spot. A deeper understanding of CE requires Raman studies with nanoscale resolution aided

Combining Raman spectroscopy with scanning probe microscopy into so-called TERS has made it possible to investigate CE and EME in detail with nanoscale resolution [21, 22]. TERS is an offspring of SERS, in which a "hot spot" is created between a metallic tip and a flat surface.

, where *E* is the intensity of the electromagnetic

*<sup>R</sup>* of a

nature of the Raman polarizability, scales as *E4*

68 Raman Spectroscopy

Nanoscale vibrational spectroscopy has been a longstanding dream of scientists working in various fields. In the early 1980s, a scanning tunneling microscope was invented which brought us the capability to explore surfaces with unprecedented subnanometer scale resolution [23]. Later on, images of molecules adsorbed on metal and semiconductor surfaces were obtained [24]. STM is based on a tunneling current between a conductive surface and a sharp metallic tip. In the basic mode of operation, STM does not yield vital chemical information, and formation of surface complexes cannot be confirmed. Consequently, inelastic electron tunneling spectroscopy (IETS) was developed to obtain chemical information from single molecules adsorbed on metal surfaces [25]. Despite its ultimate resolution and sensitivity, IETS has not evolved into a common nanoscale vibrational spectroscopic method. This is because IETS is a very challenging technique requiring low temperature, ultra-high vacuum conditions and ultra-low noise electronics.

From the development of the field of plasmonics and the demonstration of large electromagnetic field enhancement in SERS experiments, the idea of nanoscale vibrational spectroscopy on surfaces using Raman scattering has been brought to life [26]. Raman spectroscopy has many advantages as it is based on the optical response of the system and can be applied under ambient conditions and in water. However, Raman scattering is intrinsically a very weak process with only one in every 10<sup>6</sup> −108 incident photons being scattered [27]. Theoretical reports in the late 1990s showed that the electric field at the metallic tip end is dramatically enhanced under certain polarization direction [28]. This report inspired scientists working in the field of optical microscopy and surface science to utilize the enhancement effect to develop a highresolution molecular spectroscopic technique. The Kawata group in Japan and the Zenobi group in Switzerland simultaneously reported TER spectra from multilayers of organic molecules deposited on a glass surface using a metalized cantilever probe of an atomic force microscope (AFM) [21, 22]. Later, Pettinger *et al.* reported TER spectra from the monolayer of malachite green adsorbed on CN− modified gold surface, using STM tip [29]. Ultimate sensitivity and resolution of a single molecule were demonstrated in 2008 with TERS optics incorporated into ultra-high vacuum environment (UHV-TERS) [30]. Over the last decade, TERS has been adapted as an analytical tool in chemistry, biology and materials science [31].

a dichroic mirror and passing the incident light through the transparent side of the opaque

Nanoscale Insights into Enhanced Raman Spectroscopy http://dx.doi.org/10.5772/intechopen.72284 71

Side illumination (**Figure 1b**) is the most commonly used setup in TERS built on STM. In this geometry, the incoming laser light as well as the outgoing scatter signal is collected by a long working distance microscope objective placed at an angle in the range of 45–70° relative to the tip axis that maximizes the electric light component along the tip axis. The advantage of this geometry is an easy integration with commercial STM setups. It can be used both for transparent and nontransparent samples. However, only conductive samples can be studied. The disadvantage of the side illumination geometry is relatively low collection efficiency in comparison to the bottom light illumination geometry. This is because objective lenses with a low numerical aperture are employed due to space restriction around the STM tip. Another disadvantage of the side illumination geometry is an asymmetric illumination of the tip, which leads to larger spot size and higher far-field background. In addition to tip resonances, so-called gap mode resonances can be excited in the side illumination geometry [36, 37]. They play a big role in the enhancement of the signal when the distance of the tip to the surface is below 2 nm [38]. Excitation of the gap-mode resonances improves sensitivity and resolution of the TERS tech-

sample [35]. The bottom illumination is well suited for biological samples.

nique. The side illumination geometry is widely used in the field of surface science.

**2.3. Understanding TERS**

ated between the apex of a tip and a flat metal substrate.

Top illumination (**Figure 1c**) is the second most commonly used geometry in the STM-TERS community. This geometry makes focusing of the incident beam on the STM tip easier. The tip is placed at an angle to the surface with the microscope objective placed normal to the surface. Although some of the scattered light is shadowed by the tip, this geometry offers still higher excitation and collection efficiency than the side-illumination geometry does. Another advantage is a reduction in the far-field background. Both opaque and transparent samples can be studied. This geometry is ideally suited for investigating nanoscale phenomena on silicon or graphene samples. A geometry utilizing a parabolic mirror (**Figure 1d**) was first demonstrated by researchers working with an STM unit operating in UHV conditions [30]. It was developed in order to increase the collection efficiency by allowing collection of light from all directions. In this geometry, a hole is made in a parabolic mirror to accommodate the STM tip. STM-TERS setups with a parabolic mirror work in a reflection mode, allowing both opaque and transparent samples to be studied. Compared to the side and top illumination geometries, the parabolic mirror geometry offers high collection efficiency. Two major disadvantages of this geometry are: it is difficult to integrate into commercial STM units and optical alignment is difficult. A small angular mismatch results in defocusing of the laser spot and loss of the signal. Thus, this geometry has not been widely used.

The origin of Raman signal enhancement in TERS is similar to that in SERS, which is due to EME and CE. In contrast to SERS, in which Raman signals are obtained from multiple hot spots across the surface, the signal in TERS originates from one central hot spot, which is cre-

Various effects, such as electrostatic lightening rod effect, excitation of localized surface plasmon polaritons (SPPs) on the tip and antenna resonances, contribute to EM field enhancement [39]. The lightening rod effect is independent of the excitation wavelength. On the other hand,
