**3.1. Experimental details**

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

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.

all possible effects contributing to the enhancement of the signal in TERS.

72 Raman Spectroscopy

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

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 has a modified piezo scanner head which allows to install a high numerical aperture objective lens (Mitutoyo, LWD 100×, NA = 0.7, WD = 6 mm) in front of the STM tip. The lens is placed at an angle of 60° to the surface normal with the light polarization parallel to the tip axis (*p*-polarization). This objective lens is used to deliver the excitation laser beam as well as to collect the backscattered light from a tip-surface junction.

with *D2*

tensor, ⟨*k*|*α*̂

the frequency *ω<sup>k</sup>*

/∂ *Qk*

tives <sup>∂</sup> *<sup>α</sup>ij*

*ij*|0⟩, where *α*̂

⟨*k*|*α*̂

polarizability tensor components, *αij*

a way that the electric vector, *E0*

where *ħ* is the Planck constant divided by 2π, *ω<sup>k</sup>*

ability tensor. The Raman tensor elements, ⟨*k*|*α*̂

istic molecular Raman tensor, (*αij*) (*i*, *j* = *x*, *y*, *z*)

normal mode coordinate *Qk*

symmetry. Each vibrational mode has been ascribed to a given symmetry mode and

\_\_\_\_\_\_\_\_\_\_\_\_ \_\_\_\_\_ ℏ 2 *μk ω<sup>k</sup>*

and *μk*

were obtained by five-point numerical differentiation of the calculated polariz-

<sup>∂</sup> *<sup>α</sup>ij* \_\_\_\_ ∂ *Qk*

and the reduced mass *μk*

, were calculated by the facilities involved in Gaussian 09

//*Z*, that is, *Z* axis is perpendicular to the surface. The molecules

*ij*|0⟩, in Eq. (1) were derived in this way.

(1)

75

are the angular frequency and the

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

of each mode *k*. The

through the character-

, and the deriva-

*ij* (*i*, *j* = *x*, *y*, *z*) denotes the polarizability tensor in usual (electronic

The intensities of the Raman scattering were evaluated with the matrix elements of the Raman

off-resonant) conditions, and ∣0⟩ and ∣*k*⟩ are the vibrational ground state and the first excited state for the normal mode *k* (= 1, 2,…, 3*n*−6), respectively [54]. This matrix element is represented in the harmonic approximation with the polarizability derivative with respect to the

*ij*|0⟩ <sup>=</sup> <sup>√</sup>

reduced mass of the mode *k*, respectively. The molecule-fixed coordinates were defined with the principal axes of inertia, where the *z* axis is along the long molecular axis, and the *x* axis is nearly perpendicular to the rings. The principal axes of polarizability tensor coincide with the *x, y and z* axes, to give *αxx*, *αyy* and *αzz*. The vibrational analysis was performed to obtain

Subsequently, Raman scattering intensity was simulated. The intensity of the light scattered from the molecule is proportional to the square of the electric vector of the Raman scattered


In order to determine orientation of the molecule, the experimental scattering intensities were compared with the scattering intensities calculated for three representative molecular orientations. The direction of the incident radiation was described in the surface-fixed coordinate system (*X, Y*, *Z*). The polarization of an incident laser beam in our TERS experiment was adjusted in

can take various orientations having various molecular Euler angles, with respect to the Au substrate plane. Three representative configurations were considered, that is, when *x*//*Z*, *y*//*Z* and *z*//*Z*. The corresponding molecular orientation for each case is shown in **Figure 4**. For the molecule perpendicular to the surface ("end-on" configuration), the molecular *z* axis is parallel to the surface-fixed *Z* axis (*z*//*Z*), that is, perpendicular to the surface. The molecule with the "edgeon" configuration and the "face-on" configuration are denoted as *y*//*Z* and *x*//*Z*, respectively.

Adsorption of 4,4′-BiPy on the surface of an Au thin film proceeded in two stages. A first adsorption stage was observed after a short immersion time (3–5 h) of the Au film into a 1 mM ethanolic solution of 4,4′-BiPy. An image of the surface at this stage is shown in **Figure 5a**. No well-defined

Raman polarizability tensor components were calculated for each of the mode.

as follows:

*,* normal mode coordinate *Qk*

with varying molecular geometries displaced along the normal coordinate *Qk*

light, *E*sc, which is related to the electric vector of the incident light, *E*<sup>0</sup>

**3.3. Adsorption and molecular orientation of 4,4′-BiPy on Au(111)**

The optical pathway adapted in the study is shown in **Figure 3**. A red, He-Ne laser beam (632.8 nm, max. Output 30 mW, CVI Melles Griot, USA) with circular polarization was used for the excitation. The laser light was allowed to pass through a band-pass filter (Sigma Koki, Japan, bandwidth = 3 nm) and a polarizer. A transmitted light was reflected by a mirror, passed through a 45° dichroic beam splitter (RazorEdge, type U, Semrock), and reflected by two other mirrors before being focused on the tip-surface junction by the objective lens.

The backscattered radiation is collected by the same objective lens and reflected by two mirrors before falling on the dichroic beam splitter. The scattered signal passes through an ultra-steep longpass edge filter (RazorEdge, type E, Semrock), and is focused by a lens (diameter = 25 mm, focal length = 100 mm) onto the slit of the spectrograph. A back-illuminated, charge-coupled device (CCD) camera (Spec-10, Princeton Instruments) cooled by liquid nitrogen was used to acquire Raman spectra. The spectrograph was installed with 300 g/mm diffraction grating. The spectral resolution of the system was 10 cm−1. All experiments were carried out in ambient conditions with the incident laser power of 0.4 mW, giving power density of 8 × 107 W/m<sup>2</sup> in the focal region.
