**5. Future prospects and challenges**

In this chapter, we have reviewed the principles of STM-based TERS and discussed how molecular binding and halogen overlayer influence the intensity of the Raman signals. These two effects contribute to CE, which is system specific, that is, its magnitude depends on the metal-molecule system and experimental conditions of sample preparation. Further studies on the effects of molecular orientation on signal enhancement under more well-defined conditions, such as those provided by UHV environment, can bring us more knowledge on the mechanism of TERS.

Combined studies using cryogenic, polarized UHV-TERS and nanolithographically fabricated model nanostructures, supported by the state-of-the-art calculations to determine the Raman polarizability tensor components of a molecule-metal can lead to the formulation of TERS surface selection rules [46, 63, 64]. Home built STM-TERS systems in the Duyene and 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 shown in **Figure 9**.

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

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

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

**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

In this chapter, we have reviewed the principles of STM-based TERS and discussed how molecular binding and halogen overlayer influence the intensity of the Raman signals. These two effects contribute to CE, which is system specific, that is, its magnitude depends on the metal-molecule system and experimental conditions of sample preparation. Further studies on the effects of molecular orientation on signal enhancement under more well-defined conditions, such as those provided by UHV environment, can bring us more knowledge on the mechanism of TERS.

Combined studies using cryogenic, polarized UHV-TERS and nanolithographically fabricated model nanostructures, supported by the state-of-the-art calculations to determine the Raman polarizability tensor components of a molecule-metal can lead to the formulation of TERS surface selection rules [46, 63, 64]. Home built STM-TERS systems in the Duyene and

Cl2

species [55].

species. A highest intensity gain was observed for *m/z* = 465 corresponding to Au2

structure and (f) Raman spectra. Adapted from Rzeznicka *et al*. Copyright@Elsevier B.V [55].

**5. Future prospects and challenges**

82 Raman Spectroscopy

**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.

to 15 cm−1. Improvement in the optical density of the filters would allow for detection of Raman signal from organic molecules that have weak Raman scattering cross-sections.

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In the field of lithium-ion batteries, growth of conductive metal-organic interfaces with small contact resistances and catalytic functions is very attractive but remains very challenging. New experimental setups based on the TERS idea could allow for the study of interfacial processes during battery operation. A challenge in this case is a strong fluorescence signal from various battery components such as organic electrolytes, additives, binders, and so on. In this respect, systems based on near-infrared excitation would offer elimination of the fluorescence signal. Another advantage of this approach based on hyper Raman phenomena is that Raman signals originate from the small focal volume, which allow for distinguishing the interface signals from the signals originating from the bulk.

In summary, the demand for chemical analysis with nanoscale resolution makes SPM-based TERS attractive in many fields of science and engineering. We expect that, in the next 10 years, we will witness further developments in this technique and obtain more system-specific information, which will expand our knowledge of surface chemistry and the interactions of molecules with light in confined fields. Understanding system-specific chemical enhancement will advance the field of molecular plasmonics, which is an emerging field of science exploiting the molecule-plasmon interactions to harness light at the nanoscale for nanophotonic devices.
