**3.2. Details of theoretical calculations used to derive molecular orientation**

The geometry of the molecules and fundamental vibrational frequencies were calculated using the Gaussian 09 package. Molecular structures in the ground state were optimized by the B3LYP exchange-functional of the density functional theory and 6-31G++(d,p) basis set [53]. The optimized geometries of both molecules (4,4′-BiPy and 4,4′-BiPyO2 ) are nonplanar

**Figure 3.** STM-TERS setup. The inset shows a SEM picture of the Au tip, etched at a bias of 2.4 V. Adapted from Rzeznicka *et al.* [52]. Copyright@Elsevier B.V.

with *D2* symmetry. Each vibrational mode has been ascribed to a given symmetry mode and Raman polarizability tensor components were calculated for each of the mode.

The intensities of the Raman scattering were evaluated with the matrix elements of the Raman tensor, ⟨*k*|*α*̂ *ij*|0⟩, where *α*̂ *ij* (*i*, *j* = *x*, *y*, *z*) denotes the polarizability tensor in usual (electronic 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 normal mode coordinate *Qk* as follows:

$$\left = \sqrt{\frac{\hbar}{2\,\mu\_{i}\,\omega\_{k}}}\frac{\partial a\_{\,\,\,i}}{\partial Q\_{i}}\tag{1}$$

where *ħ* is the Planck constant divided by 2π, *ω<sup>k</sup>* and *μk* are the angular frequency and the 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 the frequency *ω<sup>k</sup> ,* normal mode coordinate *Qk* and the reduced mass *μk* of each mode *k*. The polarizability tensor components, *αij* , were calculated by the facilities involved in Gaussian 09 with varying molecular geometries displaced along the normal coordinate *Qk* , and the derivatives <sup>∂</sup> *<sup>α</sup>ij* /∂ *Qk* were obtained by five-point numerical differentiation of the calculated polarizability tensor. The Raman tensor elements, ⟨*k*|*α*̂ *ij*|0⟩, in Eq. (1) were derived in this way. 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 light, *E*sc, which is related to the electric vector of the incident light, *E*<sup>0</sup> through the characteristic molecular Raman tensor, (*αij*) (*i*, *j* = *x*, *y*, *z*)

$$\left| E\_{sc} \right| \ll \left| \left( a\_{\Downarrow} \right) E\_{\text{o}} \right| \tag{2}$$

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 a way that the electric vector, *E0* //*Z*, that is, *Z* axis is perpendicular to the surface. The molecules 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.

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

**Figure 3.** STM-TERS setup. The inset shows a SEM picture of the Au tip, etched at a bias of 2.4 V. Adapted from Rzeznicka

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

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 geometry of the molecules and fundamental vibrational frequencies were calculated using the Gaussian 09 package. Molecular structures in the ground state were optimized by the B3LYP exchange-functional of the density functional theory and 6-31G++(d,p) basis set

in the focal region.

) are nonplanar

collect the backscattered light from a tip-surface junction.

74 Raman Spectroscopy

the incident laser power of 0.4 mW, giving power density of 8 × 107 W/m<sup>2</sup>

**3.2. Details of theoretical calculations used to derive molecular orientation**

[53]. The optimized geometries of both molecules (4,4′-BiPy and 4,4′-BiPyO2

*et al.* [52]. Copyright@Elsevier B.V.

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

the solution for 4 days is shown in **Figure 5b**, **c**. A homogeneous monolayer with pits having a depth of a single-gold-atom was observed. It looks similar to monolayers formed by alkanethiols on Au surface, indicating involvement of Au adatoms in the process of self-assembly. TER spectra for the short immersion time and the long immersion time are shown in **Figure 5d**. The Raman spectrum for the short immersion time has only few bands with very low intensity. The absence of low-frequency vibrational signals, which could be assigned to Au-N stretching band, indicated that molecules were only weakly adsorbed (physisorbed) on the surface. In the case of the long immersion time, intensities of the Raman signals were higher, and many vibrational bands, which were not observed in the case of the short immersion time, appeared. An intense Au-N stretching signal was detected at 185 cm−1, indicating that molecules were chemisorbed on the surface.

Vibrational frequencies for the two cases are summarized in **Table 1**. Each mode has been ascribed to a given symmetry mode, and Raman polarizability tensor components were calculated for each of the mode. The scattering intensities for the three possible molecular orienta-

**Results of calculations Modal assignment**

750 *m* 688 *B2* 13 0.25 0 0.25 pyridyl ring deformation 840 *s* 864 *B2* 18 0.26 0 0.26 γ(C–H) + γ(C–C) +

**|**(|*E0* **| = 1)/**

*x***//***Z y***//***Z z***//***Z*

γ(C–C)int + γ(C-N)

Nanoscale Insights into Enhanced Raman Spectroscopy http://dx.doi.org/10.5772/intechopen.72284 77

δ(C-N) + ν(C-N)

ν(C-C)int + ν(C-N)

ν(C-C)int + ν(C-N)

δ(C-N) + ν(C-C) + ν(C-N)

**atomic unit**

tions were calculated and used to aid in determining the molecular orientation.

**Figure 5 Figure 5 |(αij)***E0*

**Symmetry class**

**Frequency /cm−1**

**Experimental TERS peak position/cm−1**

> **Long immersion**

*s*-strong, *m*-medium, *w*-weak intensity. The symmetry index stands on *D2*

Adapted from Rzeznicka *et al.* [52]. Copyright@Elsevier B.V.

**Table 1.** Experimental and calculated Raman scattering data for 4,4′-BiPy.

**Short immersion**

First molecular orientation of 4,4′-BiPy in the case of long immersion time is discussed.

**Mode number** *k*

185 *s* — — — — — — ν(Au-N)

928 *m* 984 *B2* 22 0.01 0 0.01 γ(C-H) 1016 *w* 1016 *w* 1014 *A* 26 0.44 0.73 1.94 γ(C-H) + δ(C-C) +

1337 *m* 1337 *m* 1364 *B3* 38 0 0.32 0.32 δ(C-H) + ν(C-C) 1533 *s* 1492 *s* 1540 *A* 42 0.14 0.47 0.88 δ(C-H) + ν(C-C) +

1624 *w* 1624 *m* 1645 *A* 46 0.22 0.83 3.43 δ(C-H) + ν(C-C) +

 class for a free molecule. ν-stretching; δ-in-plane bending; γ-out-of-plane bending; ν(C-C)int denotes interring C-C vibration.

1092 *m* 1098 *A* 29 0.15 0.11 0.52 δ(C-H) + δ(C-C) +

1231 *m* 1276 *B2* 34 0.16 0 0.16 ν(C-C) + ν(C-N)

1543 *m* 1583 *B2* 43 0.07 0 0.07 δ(C-H) + ν(C-C) + ν(C-N)

**Figure 4.** Molecular coordinate system. The molecular axes are x, y and z, and the surface axes are X, Y and Z. The three cases of adsorption configuration discussed in the text are shown. *E* defines the electric vector of an incident radiation. Adapted from Rzeznicka *et al*. [52]. Copyright@Elsevier B.V.

overlayers were observed. The surface of Au looked very rough and dynamic. Imaging was very unstable due to apparent adsorbate-induced surface reconstruction. Surface reconstruction is associated with the ejection of gold atoms and their diffusion over the surface. Low-coordinated gold atoms are highly reactive, and they may form a complex with molecules in the solution and diffuse over the surface to stable adsorption sites. These transient species are seen in the image as whitish spots. A second stage of adsorption was observed upon a prolonged immersion time. In this stage, a well-defined overlayer was formed. An STM image of the surface immersed into

**Figure 5.** STM images of 4,4-BiPy adlayer formed on Au(111) after immersion of the film into a 1 mM ethanolic solution for (a) 3 hours, (b) 4 days and (c) zoom into (b). (d) TERS spectra corresponding to the layer shown in image a and b. A depth profile across the A-A' line is shown in image (b). Adapted from Rzeznicka *et al*. [52]. Copyright@Elsevier B.V.

the solution for 4 days is shown in **Figure 5b**, **c**. A homogeneous monolayer with pits having a depth of a single-gold-atom was observed. It looks similar to monolayers formed by alkanethiols on Au surface, indicating involvement of Au adatoms in the process of self-assembly. TER spectra for the short immersion time and the long immersion time are shown in **Figure 5d**. The Raman spectrum for the short immersion time has only few bands with very low intensity. The absence of low-frequency vibrational signals, which could be assigned to Au-N stretching band, indicated that molecules were only weakly adsorbed (physisorbed) on the surface. In the case of the long immersion time, intensities of the Raman signals were higher, and many vibrational bands, which were not observed in the case of the short immersion time, appeared. An intense Au-N stretching signal was detected at 185 cm−1, indicating that molecules were chemisorbed on the surface.

Vibrational frequencies for the two cases are summarized in **Table 1**. Each mode has been ascribed to a given symmetry mode, and Raman polarizability tensor components were calculated for each of the mode. The scattering intensities for the three possible molecular orientations were calculated and used to aid in determining the molecular orientation.


First molecular orientation of 4,4′-BiPy in the case of long immersion time is discussed.

*s*-strong, *m*-medium, *w*-weak intensity.

overlayers were observed. The surface of Au looked very rough and dynamic. Imaging was very unstable due to apparent adsorbate-induced surface reconstruction. Surface reconstruction is associated with the ejection of gold atoms and their diffusion over the surface. Low-coordinated gold atoms are highly reactive, and they may form a complex with molecules in the solution and diffuse over the surface to stable adsorption sites. These transient species are seen in the image as whitish spots. A second stage of adsorption was observed upon a prolonged immersion time. In this stage, a well-defined overlayer was formed. An STM image of the surface immersed into

**Figure 4.** Molecular coordinate system. The molecular axes are x, y and z, and the surface axes are X, Y and Z. The three cases of adsorption configuration discussed in the text are shown. *E* defines the electric vector of an incident radiation.

Adapted from Rzeznicka *et al*. [52]. Copyright@Elsevier B.V.

76 Raman Spectroscopy

**Figure 5.** STM images of 4,4-BiPy adlayer formed on Au(111) after immersion of the film into a 1 mM ethanolic solution for (a) 3 hours, (b) 4 days and (c) zoom into (b). (d) TERS spectra corresponding to the layer shown in image a and b. A depth profile across the A-A' line is shown in image (b). Adapted from Rzeznicka *et al*. [52]. Copyright@Elsevier B.V.

The symmetry index stands on *D2* class for a free molecule.

ν-stretching; δ-in-plane bending; γ-out-of-plane bending; ν(C-C)int denotes interring C-C vibration.

Adapted from Rzeznicka *et al.* [52]. Copyright@Elsevier B.V.

**Table 1.** Experimental and calculated Raman scattering data for 4,4′-BiPy.

As expected for the polarization direction perpendicular to the surface only vibrational modes with *A*, *B2* (*xz*) and *B3* (*yz*) symmetry are observed. For the observed vibrational modes, only the "end-on" orientation does not have null Raman intensity values suggesting that the "end-on" orientation is the most plausible. The values are equally distributed over all symmetry modes, which imply that the molecule is tilted in all three directions of Au(111) surface. A presence of the Au-N stretching peak is another strong evidence to support the "end-on" orientation 4,4′-BiPy. Henceforward, we concluded that the 4,4′-BiPy molecules, in the case of long immersion time are adsorbed in a standing-up but tilted orientation, with one of two nitrogen ends anchored to Au.

**3.4. Adsorption and molecular orientation of 4,4′-BiPyO<sup>2</sup>**

ethanolic solution of 4,4′-BiPyO2

region of the uncoordinated 4,4′-BiPyO2

substrate. Orientation of the 4,4′-BiPyO2

In **Table 2**, x//Z-values of |(*αij*)*E0*

**Experimental TERS peak position/cm−1**

**TERS Powder**

**Raman**

*s*-strong, *m*-medium, *w*-weak intensity. The symmetry index stands on *D2*

Adapted from Rzeznicka *et al.* [52].Copyright@Elsevier B.V.

**Table 2.** Experimental and calculated Raman scattering data for 4,4′-BiPyO2

culated x//Z-values are zero for the *B3*

marized in **Table 2**. TER spectrum for 4,4-BiPyO2

**Figure 6 |(αij**)*E0*

**Frequency/ cm−1**

BiPyO2

 **on Au(111)**

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

. A two-dimensional overlayer, consisting of parallel rows,

. The most intense band is at 1492 cm−1 followed by

is deduced in the same manner, as done for 4,4′-BiPy.


bands. On the other hand, the values for y//Z and z//Z

ν(C-C) + ν(C-C)int

ν(C-C) + ν(C-C)int + ν(C-N) + ν(N-O)

, shown in **Figure 6c** contains three bands in

with the Au

79

**Figure 6a** shows an STM image of the Au surface upon 30 min immersion into a neutral 1 mM

extending over a triangular terrace of the Au(111) surface was observed. A two-dimensional Fourier transform (2D–FFT) of the image, shown in **Figure 6b**, revealed the spacing between parallel rows to be 1.5 and 2.2 nm−1, respectively. The angle between stripes and the edges of the terrace was 30°. The overlayer is designated as (6 × 9) overlayer. **Figure 6c** shows TER signals from an Au thin film surface immersed for 6 h in a neutral 1 mM ethanolic solution of 4,4′-

peaks at 1563 and 1614 cm−1, similarly to 4,4′-BiPy. No Au-N or Au-O stretching bands were

Vibrational frequencies, their symmetry modes and calculated Raman intensities are sum-

A symmetry. Since neither Au-O nor Au-N vibrational modes were observed, it is more likely

**Results of calculations Modal assignment**


*x***//***Z y***//***Z z***//***Z*

.

**atomic unit**

found in TER spectrum, which indicated rather weak interaction of 4,4′-BiPyO2

that a molecule has its molecular long axis parallel to the Au surface.

**Symmetry class**

1190 *m* 1202 *m* 1210 *A* 37 0.10 0.63 3.55 δ(C-H)

1253 *w* 1266 *B3* 39 0 0.51 0.51 ν(C-C) + ν(C-N)

1492 *s* 1512 *m* 1499 *B3* 46 0 0.23 0.23 δ(C-H) + ν(C-C)

 class for a free molecule. ν-stretching; δ-in-plane bending; γ-out-of-plane bending; ν(C-C)int denotes interring C-C vibration.

**Mode number**  *k*

1326 *m* 1300 *m* 1319 *A* 40 0.23 0.51 7.16 δ(C-H) + δ(C-C) + δ(C-N) +

1563 *s* 1572 *B3* 50 0 0.39 0.39 δ(C-H) + ν(C-C) + ν(C-N) 1614 *m* 1617 *s* 1667 *A* 51 0.27 0.92 9.67 δ(C-H) + δ(C-C) + δ(C-N) +

850 *w* 852 *m* 858 *B1* 24 0.22 0.22 0 δ(C-H) + δ(C-C) + ν(C-N) + ν(N-O)


. The spectrum contains a peak at 850 cm−1, assigned to the in-plane ring vibrations and the N-O stretching vibrations, and a peak at 1190 cm−1, which draws its intensity mainly from the in-plane C-H bending vibrations. The position of these bands falls into the frequency

In the case of short immersion time, many of the vibrational peaks seen in the long immersion time spectrum were missing. There was no Au-N stretching signal, and the *B2* -symmetry vibrational modes were not observed which rejects possibility of the "face-on" configuration. The peak intensities coincide with the y//Z-values of |(*α*ij)*E0* |(|*E0* | = 1) in **Table 1**. For the missing signals, the calculated y//Z-value of |(*α*ij)*E0* |(|*E0* | = 1) is zero or nearly zero. Henceforward, we concluded the 4,4′-BiPy has y//Z orientation, that is, the "edge-on" orientation, without the N atoms bonded to the Au substrate.

Our analysis is based on Raman polarizability tensor components calculated for a free molecule and on the assumption that a local electric field is perpendicular to the surface. As in the case of long immersion, chemisorption may change polarizability of the bonds, and Raman tensor elements may be different than the tensor elements calculated for a free molecule.

**Figure 6.** (a) 100 × 70 nm constant current STM image of 4,4-BiPyO2 adlayer formed on Au(111) after immersion of the film in a neutral, 1 mM ethanolic solution for 6 h. (b) the two-dimensional Fourier-transform of the image. (c) TERS spectra of the overlayer. (d) Schematic representation of a (6 × 9) BiPyO2 adlayer. Adapted from Rzeznicka *et al.* [52]. Copyright@Elsevier B.V.

#### **3.4. Adsorption and molecular orientation of 4,4′-BiPyO<sup>2</sup> on Au(111)**

As expected for the polarization direction perpendicular to the surface only vibrational modes

"end-on" orientation does not have null Raman intensity values suggesting that the "end-on" orientation is the most plausible. The values are equally distributed over all symmetry modes, which imply that the molecule is tilted in all three directions of Au(111) surface. A presence of the Au-N stretching peak is another strong evidence to support the "end-on" orientation 4,4′-BiPy. Henceforward, we concluded that the 4,4′-BiPy molecules, in the case of long immersion time are adsorbed in a standing-up but tilted orientation, with one of two nitrogen ends anchored to Au. In the case of short immersion time, many of the vibrational peaks seen in the long immersion

tional modes were not observed which rejects possibility of the "face-on" configuration. The

we concluded the 4,4′-BiPy has y//Z orientation, that is, the "edge-on" orientation, without the

Our analysis is based on Raman polarizability tensor components calculated for a free molecule and on the assumption that a local electric field is perpendicular to the surface. As in the case of long immersion, chemisorption may change polarizability of the bonds, and Raman tensor elements may be different than the tensor elements calculated for a free molecule.

film in a neutral, 1 mM ethanolic solution for 6 h. (b) the two-dimensional Fourier-transform of the image. (c) TERS

)*E0* |(|*E0* )*E0* |(|*E0*

time spectrum were missing. There was no Au-N stretching signal, and the *B2*

peak intensities coincide with the y//Z-values of |(*α*ij

**Figure 6.** (a) 100 × 70 nm constant current STM image of 4,4-BiPyO2

Copyright@Elsevier B.V.

spectra of the overlayer. (d) Schematic representation of a (6 × 9) BiPyO2

signals, the calculated y//Z-value of |(*α*ij

N atoms bonded to the Au substrate.

(*yz*) symmetry are observed. For the observed vibrational modes, only the




adlayer formed on Au(111) after immersion of the

adlayer. Adapted from Rzeznicka *et al.* [52].

with *A*, *B2*

78 Raman Spectroscopy

(*xz*) and *B3*

**Figure 6a** shows an STM image of the Au surface upon 30 min immersion into a neutral 1 mM ethanolic solution of 4,4′-BiPyO2 . A two-dimensional overlayer, consisting of parallel rows, extending over a triangular terrace of the Au(111) surface was observed. A two-dimensional Fourier transform (2D–FFT) of the image, shown in **Figure 6b**, revealed the spacing between parallel rows to be 1.5 and 2.2 nm−1, respectively. The angle between stripes and the edges of the terrace was 30°. The overlayer is designated as (6 × 9) overlayer. **Figure 6c** shows TER signals from an Au thin film surface immersed for 6 h in a neutral 1 mM ethanolic solution of 4,4′- BiPyO2 . The spectrum contains a peak at 850 cm−1, assigned to the in-plane ring vibrations and the N-O stretching vibrations, and a peak at 1190 cm−1, which draws its intensity mainly from the in-plane C-H bending vibrations. The position of these bands falls into the frequency region of the uncoordinated 4,4′-BiPyO2 . The most intense band is at 1492 cm−1 followed by peaks at 1563 and 1614 cm−1, similarly to 4,4′-BiPy. No Au-N or Au-O stretching bands were found in TER spectrum, which indicated rather weak interaction of 4,4′-BiPyO2 with the Au substrate. Orientation of the 4,4′-BiPyO2 is deduced in the same manner, as done for 4,4′-BiPy.

Vibrational frequencies, their symmetry modes and calculated Raman intensities are summarized in **Table 2**. TER spectrum for 4,4-BiPyO2 , shown in **Figure 6c** contains three bands in A symmetry. Since neither Au-O nor Au-N vibrational modes were observed, it is more likely that a molecule has its molecular long axis parallel to the Au surface.



*s*-strong, *m*-medium, *w*-weak intensity.

The symmetry index stands on *D2* class for a free molecule.

ν-stretching; δ-in-plane bending; γ-out-of-plane bending; ν(C-C)int denotes interring C-C vibration. Adapted from Rzeznicka *et al.* [52].Copyright@Elsevier B.V.

**Table 2.** Experimental and calculated Raman scattering data for 4,4′-BiPyO2 . follow the observed frequencies, except for the peak at 850 cm−1. The value of |(*α*ij)*E0* |(|*E0* | = 1) is zero for z//Z. The appearance of this 850 cm−1 band denies z//Z orientation. In conclusion, the Raman signal intensity supports the "edge-on" orientation.

~ 3 Å, as indicated in the figure. The overlayer has few dark vacancies (DV). The depth of dark vacancies is in the range of 1.3–1.5 Å. The overlayer was observed to grow along the crystallographic directions of the underlying Au(111) surface as shown in **Figure 7c**. By careful alternation of the tunneling current and scanning speed, another structure originated from the underlying layer was detected, as shown in **Figure 7d** (notice the transition at the bottom of the image). In this underlying layer, individual atoms are found to be arranged in a rectangular lattice with a unit cell of *a* = 5 Å. **Figure 7(e**, **f)** shows large area and a zoom image of the lattice. This atomic

theory (DFT) calculations predicted that the fcc hollow site is the most stable adsorption site for chlorine in this overlayer [56]. Based on these facts, the most possible molecular model for the stripe phase observed after adsorption of 4,4′-BiPy onto the Au(111) surface in the presence of chlorine ions was proposed. The proposed overlayer structure is shown in **Figure 7g**. In this

electrostatic interaction between protonated bipyridine cations and chlorine anions. **Figure 7h**

**Figure 8a** shows an STM image of Au surface after prolonged immersion of Au/mica film into the acidic solution of 4,4′-BiPy. A new overlayer with a long-range order was observed as shown in **Figure 8b**. The overlayer consists of bright stripes with a periodicity of ~ 10 Å. A growth of the next top layers can be seen at the left side of **Figure 8c**. The top layer, is rotated in respect to the bottom layer, at an angle of 120°, indicating a three-dimensional growth with the Au(111) surface registry. The stripes of the top layer consist of bright protrusions with a height of 1.5–1.8 Å. A TER spectrum taken on this surface is shown in **Figure 8f**. In-plane vibrational modes are observed above 1000 cm−1. Six vibrational peaks are found in the spectrum: peaks at 1606, 1503, 1293, 1225, 1071 and 1017 cm−1. The observed vibrational frequencies correspond to protonated form of 4,4′-BiPy [57]. No out-of-plane modes are observed, suggesting the "edge-on" molecular orientation. Below 1000 cm−1, only a small peak at ~255 cm−1 is observed. Pettinger *et al.* assigned vibration at this frequency to the metal-halogen vibration

structure, which has been observed upon adsorp-

Nanoscale Insights into Enhanced Raman Spectroscopy http://dx.doi.org/10.5772/intechopen.72284 81

1] direction. The self-assembly is mainly driven by an

Cl2 [59].

–4,4′-BiPy stretching, and free N-H stretching

onto the Au(111) surface at room temperature [56]. The density functional

2+ bipyridine cations are assumed to have a "flat-on" or "edge-on" orienta-

\_\_ 3 × √ \_\_ 3)*R* 30°

shows a model for the overlayer growth on top of a surface chloride.

of a surface complex containing metal adatom, halogen ions and pyridine [58].

spectrum above 1000 cm−1 is consistent with the Raman spectrum of a solid BiPyH2

The spectrum is similar to the TER spectrum but bands are more intense.

and 3450 cm−1. They were assigned to the N-H<sup>+</sup>

An optical microscopic image of the sample after a prolonged immersion into the solution is shown in **Figure 8d**. Rectangular shaped, 3D islands of different sizes are found on the surface. Depression defects are always seen near the islands. We speculate that these defects act as a supply of Au adatoms that are further incorporated into the crystal. **Figure 8f** shows a Raman spectrum taken within the area of a large 3D island using a confocal Raman unit. The

In contrast to TER spectrum, out-of-plane modes are also observed suggesting that molecules with "flat-on" orientation are also present. A very weak Raman signals were also observed at 2460

…Cl−

vibrations, respectively [60, 61]. At very low frequencies, two strong peaks at ~88 and 116 cm−1 with the shoulder at 134 cm−1 were observed. Similarly, low-frequency Raman peaks are observed

arrangement is assigned to the *p*(√

]

tion. Molecules are aligned along the [10¯

tion of 0.33 ML of Cl2

model, [4,4′-BiPyH2
