**1. Introduction**

Raman spectroscopy is an analytical technique based on inelastic scattering of light. The light scattered by a molecule shows a wavenumber shift with respect to the excitation line. This effect was first described by C. V. Raman in 1928 and quickly became used as a powerful method for identifying molecules through their vibrational motions [1]. However, until the 1970s, Raman spectroscopy did not receive much attention of scientists working in the field of surface analysis. This is because intrinsic Raman scattering cross-sections of molecules is small, on the order of 10−32 cm<sup>2</sup> sr−1 per molecule [2]. The observation of Raman signal enhancement of

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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).

The TERS technique eliminates the need for rough or nanostructured surfaces, allowing for investigation of the details of molecular adsorption under well-defined conditions. The biggest advantage of TERS over SERS is its capability to acquire Raman signals and nanoscale images

Nanoscale Insights into Enhanced Raman Spectroscopy http://dx.doi.org/10.5772/intechopen.72284 69

In this chapter, readers will be introduced to the experimental and theoretical aspects of TERS based on a scanning-tunneling microscopy (STM-TERS). Subsequently, the results of TERS studies of molecules adsorbed on flat metal surfaces are summarized with an emphasis on the molecular orientation and surface selection rules. Later, the effect of chlorine activation of metal surfaces on the adsorption of organic molecules and halogen overlayer-templated growth of surface-grown metal-organic layered structures are described. At the end of the chapter, future prospects and challenges of TERS in studies of molecular adsorption on metal

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

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 pro-

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

incident photons being scattered [27]. Theoretical reports

modified gold surface, using STM tip [29]. Ultimate

low temperature, ultra-high vacuum conditions and ultra-low noise electronics.

−108

of the molecule adsorbed on a solid substrate simultaneously.

surfaces are discussed.

**2. Tip-enhanced Raman spectroscopy**

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

cess with only one in every 10<sup>6</sup>

of malachite green adsorbed on CN−

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 nature of the Raman polarizability, scales as *E4* , where *E* is the intensity of the electromagnetic 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.

CE describes various effects affecting the Raman polarizability αR with respect to the *α*<sup>0</sup> *<sup>R</sup>* of a 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 by suitable electronic structure calculations of the molecule-metal system.

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. The TERS technique eliminates the need for rough or nanostructured surfaces, allowing for investigation of the details of molecular adsorption under well-defined conditions. The biggest advantage of TERS over SERS is its capability to acquire Raman signals and nanoscale images of the molecule adsorbed on a solid substrate simultaneously.

In this chapter, readers will be introduced to the experimental and theoretical aspects of TERS based on a scanning-tunneling microscopy (STM-TERS). Subsequently, the results of TERS studies of molecules adsorbed on flat metal surfaces are summarized with an emphasis on the molecular orientation and surface selection rules. Later, the effect of chlorine activation of metal surfaces on the adsorption of organic molecules and halogen overlayer-templated growth of surface-grown metal-organic layered structures are described. At the end of the chapter, future prospects and challenges of TERS in studies of molecular adsorption on metal surfaces are discussed.
