**4.1. Chlorine overlayer-templated growth of Au-4,4′-BiPy crystals on Au(111)**

**Figure 7a** shows a large area STM image of the Au surface obtained after immersion of Au/mica film into a 4,4′-BiPy solution, adjusted with 0.1 M HCl to pH 3, for 2 days at room temperature. A zoom into the flat part of the image shows a periodic overlayer structure, shown in **Figure 7b**. The overlayer consists of bright stripes having a width of ~7.5 Å. The width is close to the length of 4,4-BiPy, which measures ~ 7.1 Å. Each stripe shows contrast modulation with periodicity of

**Figure 7.** STM images after immersion of Au slide into 1 mM ethanolic solution of 4,4-BiPy, acidified to pH = 3 with HCl: (a) 100 × 100 nm image showing well-defined overlayer; (b) 15 × 15 nm zoomed image into (a) showing a striped structure; (c) 30 × 30 nm image showing rotational domains; (d) 31 × 31 nm image showing a p(√ \_\_ 3 × √ \_\_ 3)R30°-Cl overlayer structure; (e) zoomed 12 × 12 nm image of (d); (f) zoomed 4.7 × 4.7 nm image of (e) into the p(√ \_\_ 3 × √ \_\_ 3)R30°-Cl overlayer. Possible molecular models of the striped structure; (g) growing on top of chlorine overlayer and (h) growing on top of surface chloride. Adapted from Rzeznicka *et al.* Copyright@Elsevier B.V [55].

~ 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 arrangement is assigned to the *p*(√ \_\_ 3 × √ \_\_ 3)*R* 30° structure, which has been observed upon adsorption of 0.33 ML of Cl2 onto the Au(111) surface at room temperature [56]. The density functional 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 model, [4,4′-BiPyH2 ] 2+ bipyridine cations are assumed to have a "flat-on" or "edge-on" orientation. Molecules are aligned along the [10¯ 1] direction. The self-assembly is mainly driven by an electrostatic interaction between protonated bipyridine cations and chlorine anions. **Figure 7h** shows a model for the overlayer growth on top of a surface chloride.

follow the observed frequencies, except for the peak at 850 cm−1. The value of |(*α*ij

**4. Enhancement of Raman signals due to halogen overlayer-**

Raman spectroscopy were used to reveal details of a crystal growth [55].

**4.1. Chlorine overlayer-templated growth of Au-4,4′-BiPy crystals on Au(111)**

the Raman signal intensity supports the "edge-on" orientation.

**templated crystal growth**

80 Raman Spectroscopy

is zero for z//Z. The appearance of this 850 cm−1 band denies z//Z orientation. In conclusion,

This section describes particular surface chemistry leading to the growth of metal-organic surface crystals in the presence of halogen overlayer. The crystals were grown on an Au surface from ethanolic solutions of 4,4′-BiPy, in the presence of HCl. STM-TERS and ordinary

**Figure 7a** shows a large area STM image of the Au surface obtained after immersion of Au/mica film into a 4,4′-BiPy solution, adjusted with 0.1 M HCl to pH 3, for 2 days at room temperature. A zoom into the flat part of the image shows a periodic overlayer structure, shown in **Figure 7b**. The overlayer consists of bright stripes having a width of ~7.5 Å. The width is close to the length of 4,4-BiPy, which measures ~ 7.1 Å. Each stripe shows contrast modulation with periodicity of

**Figure 7.** STM images after immersion of Au slide into 1 mM ethanolic solution of 4,4-BiPy, acidified to pH = 3 with HCl: (a) 100 × 100 nm image showing well-defined overlayer; (b) 15 × 15 nm zoomed image into (a) showing a striped

Possible molecular models of the striped structure; (g) growing on top of chlorine overlayer and (h) growing on top of

\_\_ 3 × √ \_\_

\_\_ 3 × √ \_\_

3)R30°-Cl overlayer

3)R30°-Cl overlayer.

structure; (c) 30 × 30 nm image showing rotational domains; (d) 31 × 31 nm image showing a p(√

structure; (e) zoomed 12 × 12 nm image of (d); (f) zoomed 4.7 × 4.7 nm image of (e) into the p(√

surface chloride. Adapted from Rzeznicka *et al.* Copyright@Elsevier B.V [55].

)*E0* |(|*E0*


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

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 spectrum above 1000 cm−1 is consistent with the Raman spectrum of a solid BiPyH2 Cl2 [59]. The spectrum is similar to the TER spectrum but bands are more intense.

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 and 3450 cm−1. They were assigned to the N-H<sup>+</sup> …Cl− –4,4′-BiPy stretching, and free N-H stretching 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

the Wang group are making first steps in this direction. We have collaborated with the company Unisoku in Japan in the development of a commercial UHV-TERS and have shown its capability to obtain relatively strong Raman signals from organic molecules adsorbed on a metal surface. Cryogenic cooling has been found to resolve issues of spectral fluctuations, as

Nanoscale Insights into Enhanced Raman Spectroscopy http://dx.doi.org/10.5772/intechopen.72284 83

Fukumura *et al.* have proposed that single molecule sensitivity could be facilitated by employing vibrational excitation of molecules using inelastic scattering of tunneling electrons synchronized with the laser excitation to the excited states [65]. The technical challenge with this approach lies in the synchronization of the laser pulse with the electric pulse. The Duyene group has just started incorporating ultra-short laser pulses with UHV-TERS [66]. Apart from a purely academic interest, STM-TERS could contribute to understanding surface chemistry under ambient or solution conditions and aid in the development of large-scale metal protective organic layers. Moreover, metal leads are also important in electrical applications. It is a challenging task to minimize Ohmic losses for metal electrodes covered with thin organic films. As demonstrated in this chapter, halogen-modified surfaces could act as templates for the subsequent growth of

Studies using electrochemical STM-TERS (EC STM-TERS) could assist in the fabrication of conductive metal/organic molecule thin films by utilizing anion-overlayers as templates for formation of well-defined organic thin films, as demonstrated here. Such organic thin films are increasingly important in the field of sensing, molecular electronics and optoelectronics. A challenge in Raman spectroscopy of organic molecules adsorbed on metal surfaces is detection of low frequency Raman signals, which give information on the chemical state of the molecule and possible metal–organic surface complex formation. Utilizing ultra-narrowband notch filters and a pinhole in front of the spectrograph slit, we recently observed signals down

**Figure 9.** UHV-TER spectra of 1, 2-di-(4-pyridyl)-ethylene (BPE) at room (300 K) (left) and liquid nitrogen temperature

(78 K)(right). Adapted from http://www.unisoku.com/products.

metal-organic framework structures directly on the surfaces of metals.

shown in **Figure 9**.

**Figure 8.** STM images after prolonged immersion of an Au slide into 1 mM ethanolic solution of 4,4-BiPy, acidified to pH = 3 with HCl: (a) a 70 × 70 nm image showing a chain structure; (b) a 20 × 20 nm zoomed image of the chain structure; (c) a 14 × 14 nm zoomed image of the chain structure showing development of the next top layers; (d) a bright-field microscope image of the Au slide showing surface-grown large 3D crystals; (e) a possible molecular model of the chain structure and (f) Raman spectra. Adapted from Rzeznicka *et al*. Copyright@Elsevier B.V [55].

in dinuclear Au complexes containing Cl, and in the case of pyridine adsorption on Ag electrodes. In dimethylgold halides, Au-Au vibrations are found at ~74 cm−1 [62]. Thus, these two peaks were assigned to Au-Au and Au-Cl stretching vibrations, respectively. The assignment was supported by the results of secondary mass ion spectrometry (SIMS) which yields information on the surface species. A highest intensity gain was observed for *m/z* = 465 corresponding to Au2 Cl2 species [55].
