**2.3. Understanding TERS**

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 sharp metallic tip is a central part of the TERS setup and its quality (size and shape) defines the spatial resolution of TERS and, to some extent, the magnitude of the enhanced field. Owing to its importance, several papers have discussed various tip fabrication methods [32–34]. Tips are usually made of gold or silver as these metals have plasmon resonances in the visible region of the electromagnetic spectrum. Electrochemical etching is a common method to prepare the tips. In a typical procedure, Au tips are prepared by direct current electrochemical etching in a 50:50 (v:v) mixture of concentrated HCl and ethanol. Au wire (a tip after etching) serves as the anode and a gold or platinum ring acts as the cathode. The end of the Au wire is submerged 1–2 mm into the solution at the center of the gold ring. A voltage in the range of 1.7–2.5 V is applied between the cathode and the anode. The etching reaction proceeds until the electrochemical current drops to zero. Typically tips with a tip-apex size in the range of 20–50 nm are obtained in this way.

TERS requires integration of STM/AFM with the optical components used in Raman spectroscopy. Various optical geometries have been adapted in the past 15 years to work with a variety of different samples [34]. They include a bottom-, side-, top- and a parabolic mirror illumination depicted in **Figure 1**. Each geometry has its own advantages and disadvantages,

The bottom illumination geometry (**Figure 1a**), in which the incident laser light is fed through the microscope objective placed at the bottom and the rear side of the sample, is not practiced among STM-based TERS users as it requires transparent samples. This geometry is commonly used in AFM-based TERS, in which an inverted optical microscope platform is used. The advantage of this geometry is a very high collection efficiency of scattered light as oilimmersion objectives with a high numerical aperture (NA) can be used. Recently, the Deckert group has modified the bottom illumination geometry to work with opaque samples by using

**Figure 1.** Four common optical geometries used in TERS (a) bottom illumination, (b) side illumination,(c) top illumination,

**2.2. Experimental aspects of STM-TERS**

*2.2.1. Tip fabrication and related issues*

70 Raman Spectroscopy

*2.2.2. Optical geometries*

which are summarized hereafter.

(d) parabolic mirror illumination.

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

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, excitation of SPPs is wavelength dependent with maximum field enhancement achieved when the laser energy coincides with the localized surface plasmon resonances of the tip. Finitedifference time domain (FDTD) calculations show that the magnitude of the enhancement due to plasmon excitation depends on the laser light polarization, tip radius and dielectric properties of the surrounding medium [38]. In-plane light polarization (*p*-polarization, parallel to the tip axis) gives much higher enhancement than the out-of-plane polarization (*s*-polarization) does. The maximum enhancement is predicted for tips with the apex radius of 15–20 nm [38]. Tips made of silver provide higher enhancement when visible light excitation is used. **Figure 2** shows all possible effects contributing to the enhancement of the signal in TERS.

Although simultaneous observation of Raman and infrared (IR) vibrational modes are exclusive to each other in the case of centrosymmetric molecules, some TER spectra published in the literature show the presence of IR active or silent modes [44]. Polbutko explained appearance of these lines due to strong quadrupole light-molecule interactions arising from strongly

Nanoscale Insights into Enhanced Raman Spectroscopy http://dx.doi.org/10.5772/intechopen.72284 73

The Dong group has recently demonstrated sub-nanometer resolution in TERS experiments [46]. As subnanometer resolution is difficult to understand in terms of the classical electromagnetic theory, these results have inspired theoreticians to work on proposing new mechanisms that could explain the results. Duan and Luo have proposed involvement of nonlinear optical processes [47]. Creation of an "atomic-scale hot spot" has also been proposed [48]. In addition, multiple elastic scattering of light between molecular dipoles adsorbed on the surface has been

The ultimate goal to understand TERS from molecules adsorbed on metal surfaces is to understand how relative intensities of Raman lines depend on the molecular orientation and polarization direction of the excitation light. These studies are still challenging as there are many parameters, such as molecular binding geometry, Raman tensors and direction of local field polarization, that have to be determined. It is still not possible to formulate rules similar to the IR metal surfaceselection rule (SSR). The IR SSR states that, for a molecule adsorbed on a metal surface, vibrational modes having a dipole moment perpendicular to the surface are the most enhanced [50]. Moskovits described the concept of SSR in SERS experiments for flat metal surfaces [11]. The author's work in 1982 gave theoretical grounds for local field polarization. Recently, Ru *et al.* have experimentally validated Moskovits theory by studying polarization and incident angle dependences of the SERS signals [51]. Similar studies using TERS can offer more insights into the Raman

inhomogeneous electromagnetic fields, which exist near rough metal surface [45].

proposed to explain the improved signal intensity and TERS spatial resolution [49].

SSR as the Raman signal can be directly related to the molecules present under the tip.

**3. Enhancement of Raman signals due to chemisorption**

dine (4,4′-BiPy) and 4,4′-bipyridine *N,N′*-dioxide (4,4′-BiPyO2

**3.1. Experimental details**

In order to comprehend enhancement mechanisms in detail, more sophisticated experiments and theoretical analysis are required. Emerging TERS studies under UHV and on well-defined systems can deliver more results, which should eventually bring us closer to understanding origin of signal enhancement and contribute to the development of TERS as a reliable analytical tool.

This section summarizes our studies on molecular adsorption and orientation of 4,4′-bipyri-

thin films deposited on muscovite mica substrates using STM-TERS supported by calculated Raman tensor polarizability components. The enhancement of the Raman signals is attributed to the formation of a chemisorbed overlayer with a standing up molecular configuration [52].

A TERS setup with the side-illumination geometry was used in the experiments described here. The setup consists of a commercial STM unit (Nanoscope E, Veeco Instruments Inc., USA), a spectrograph (SP-2150i, Roper Scientific, GmbH) and optical components. The STM

) in monolayers formed on gold

When the tip-metal surface distance is smaller than 2 nm, additional EM field enhancement is observed. At this distance, LSP of the tip and a metal interact with each other to form hybridized modes, called gap modes [37]. The enhancement due to excitation of the gap mode resonances depends strictly on the tip-metal surface separation [40, 41]. The gap modes are efficiently excited when *D/R* < 1, where *D* is the distance of the particle from the surface and *R* is the radii of the tip apex. The enhancement of the scattered light intensity is found to be as high as 1012 for a 20 nm radius gold tip and tip-substrate separation of 1 nm [38]. Such small tip-substrate separations are easily controlled by the tunneling feedback function of the STM.

CE due to chemisorption, formation of a surface-complex and anion surface modification can be studied with excellent resolution using STM-TERS. These studies began in the field of surface science. A resonance enhancement of 10<sup>6</sup> has been reported by Pettinger *et al.* for a malachite green molecule adsorbed on an Au(111) surface [42]. Ren *et al.* have shown that Raman signal can be obtained from monolayers of non-resonant molecules with weak Raman cross-sections [43]. Observed frequency shifts between Au and Pt surfaces indicate that TERS is sensitive enough to identify molecular orientation and revealed details of molecule-surface interaction.

**Figure 2.** Possible CE and EME effects contributing to Raman signal enhancement in TERS.

Although simultaneous observation of Raman and infrared (IR) vibrational modes are exclusive to each other in the case of centrosymmetric molecules, some TER spectra published in the literature show the presence of IR active or silent modes [44]. Polbutko explained appearance of these lines due to strong quadrupole light-molecule interactions arising from strongly inhomogeneous electromagnetic fields, which exist near rough metal surface [45].

The Dong group has recently demonstrated sub-nanometer resolution in TERS experiments [46]. As subnanometer resolution is difficult to understand in terms of the classical electromagnetic theory, these results have inspired theoreticians to work on proposing new mechanisms that could explain the results. Duan and Luo have proposed involvement of nonlinear optical processes [47]. Creation of an "atomic-scale hot spot" has also been proposed [48]. In addition, multiple elastic scattering of light between molecular dipoles adsorbed on the surface has been proposed to explain the improved signal intensity and TERS spatial resolution [49].

The ultimate goal to understand TERS from molecules adsorbed on metal surfaces is to understand how relative intensities of Raman lines depend on the molecular orientation and polarization direction of the excitation light. These studies are still challenging as there are many parameters, such as molecular binding geometry, Raman tensors and direction of local field polarization, that have to be determined. It is still not possible to formulate rules similar to the IR metal surfaceselection rule (SSR). The IR SSR states that, for a molecule adsorbed on a metal surface, vibrational modes having a dipole moment perpendicular to the surface are the most enhanced [50]. Moskovits described the concept of SSR in SERS experiments for flat metal surfaces [11]. The author's work in 1982 gave theoretical grounds for local field polarization. Recently, Ru *et al.* have experimentally validated Moskovits theory by studying polarization and incident angle dependences of the SERS signals [51]. Similar studies using TERS can offer more insights into the Raman SSR as the Raman signal can be directly related to the molecules present under the tip.

In order to comprehend enhancement mechanisms in detail, more sophisticated experiments and theoretical analysis are required. Emerging TERS studies under UHV and on well-defined systems can deliver more results, which should eventually bring us closer to understanding origin of signal enhancement and contribute to the development of TERS as a reliable analytical tool.
