15


mode that is being probed. The transferred energy is the “shift” that we refer to in this terminology. The energy of the incident light is systematically subtracted out. In other words, we set the laser excitation equal to zero cm-1


of Raman shift. Stokes transitions


have positive Raman shifts and Anti-Stokes transitions have negative Raman shifts. Raman spectra obtained using different laser sources (e.g., 532 nm and 785 nm) can be compared directly to each other and the characteristic peaks for any given vibration are expected at the same Raman shift values [1,3].


The Raman scattering selection rule


The energy of a molecule is described by its vibrational states. Depending on the number of atoms and their geometry, the molecule can move within its degrees of freedom. Possible movements include translation, rotation, and various stretching and bending modes. The concept of molecular vibrations is well-known to chemists from the interpretation of infrared (IR) spectra. A subset of the possible molecular vibrations gives rise to strong Raman scattering and thus a peak in the Raman spectrum. The basic selection rule for Raman scattering is that it results from vibrations that change the polarisability of the molecule. The polarisability of a molecule describes how easily the electron cloud around the molecule can be distorted by an electric fi eld [1,2].


The magnitude of this change in polarisability is also important as larger changes in polarisability result in more intense Raman scattering. The electron density distribution of a molecule will react to an external electric fi eld with redistribution of the electron density and the formation of an induced electric dipole moment.


Lattice vibrations in crystalline materials


The topic of lattice vibrations requires some special attention because of its relevance to most electrochemical applications. Until this point, the discussion of vibrations has been in the context of molecules with covalent bonds. However, crystalline materials like salts and metal oxides also give vibrational spectra. A common example of this is crystalline quartz which gives well-defi ned Raman peaks, while fused quartz gives broad bands in the Raman spectrum. Raman scattering from crystalline solids occurs when the incident radiation induces vibrations throughout the lattice. The quantised vibrations of the atoms in a crystal lattice travel and propagate waves. They are called phonons. Lattice vibrations can follow the propagation direction of the incident light - these are called longitudinal modes (denoted by the letter L). Perpendicular vibrations are called transverse modes (denoted by the letter T) [4]. Figure 2 illustrates these two types of vibrations using ordered spheres in a one-dimensional lattice. Longitudinal vibrations give rise to compression and rarefaction (increased spacing) of the distance between the spheres.


Figure 3. Stained glass artwork in the the Rose window of the Sainte-Chapelle cathedral in Paris (Credit: Didier B (2006), licensed by Creative Commons).


Signal enhancement strategies


As noted in a previous section, Raman scattering interactions are relatively infrequent compared with other vibrational processes. This makes the signal weak and introduces some challenges for the instrumentation design and the researcher. The two most obvious ways of working with an inherently weak signal are to either concentrate the sample or to increase the sampling duration. Both approaches result in a better signal- to-noise ratio in the spectrum. However, when combining Raman spectroscopy with electrochemical systems, neither of these approaches is ideal.


With electrochemical systems there is an additional consideration - many of the processes are interfacial processes, meaning that they are spatially limited to the surface of the working electrode [6]. Thus, there is a need for good spatial resolution for the sampling area for the Raman signal. Also, the fact that the target molecules are limited to the area near the electrode surface means that the sample size is generally small. This introduces a particularly strong need to enhance the Raman signal in EC- Raman applications. Thankfully, enhancement strategies are available, and they are well-suited to electrochemical applications, so these challenges are routinely overcome by researchers.


Surface-enhanced Raman scattering (SERS)


Surface-enhanced Raman scattering (SERS) refers to a phenomenon that can be exploited for Raman signal enhancement and is often used in hyphenated EC-Raman experiments. SERS makes use of the same equipment, in terms of the light source and spectrometer, as a traditional Raman measurement. However, for SERS there is an important substrate that interacts with the incident light and the target molecule. This is the “surface” part of the SERS acronym. Generally, SERS is performed on a roughened or nanostructured gold, silver, or copper substrate; the reasons for this are explained below. For hyphenated EC-Raman applications, the use of SERS is a natural progression. The gold and silver materials that make appropriate SERS substrates are already very well-characterised for use as electrodes, so it is relatively easy to design EC-Raman experiments with these materials as the working electrode. The rough surface of the SERS substrate is also benefi cial for the electrochemical part of the experiment as it will enhance the EC signal through the increased surface area [6,7].


Figure 2. Lattice vibrations illustrated using ordered spheres in a one-dimensional lattice.


The appearance of Raman peaks or bands from lattice vibrations depends on the crystallinity and purity of the materials. The Raman spectrum for a perfectly ordered single crystal usually has few sharp peaks corresponding to the phonons described above. Because these materials are highly symmetrical, the number of unique vibrations is low. In the case of polycrystalline materials, the peaks are broadened. The presence of impurities in the lattice adds additional peaks. Solids with low crystallinity, like the fused quartz mentioned earlier, have broad bands in the Raman spectrum. Amorphous materials have no long range order. Waves cannot translate and propagate through such materials. Strictly speaking, there are no phonons in this case. However, there is a collection of areas of short-range order where vibrations occur, and there are also chemical bond interactions that give rise to scattering. Thus, amorphous materials like glass have peaks and bands in the Raman spectrum [2]. Raman spectroscopy can be used to study glass with different additives and dopants. In fact, Raman spectroscopy


The Raman scattering signal enhancement observed during SERS is understood to originate from two processes. There is an electromagnetic enhancement process that is dominant, and a chemical process that contributes to the enhancement in a minor way [8]. In this description we will focus only on electromagnetic enhancement as it is the most important. Figure 4a helps to illustrate the electromagnetic part of the SERS effect. The example starts with a surface covered with spherical gold nanoparticles. Please note that the same principles could be applied to roughened surfaces or nanoparticles of various shapes. If an electric fi eld (such as light from the laser source) is applied to the nanoparticle it will induce the formation of a dipole, shown in Figure 4a as the displacement of the electron cloud.


When the induced fi eld is removed, the dipole relaxes back to the original state. The free electrons in the metal nanoparticle have an inherent oscillation frequency that depends on intrinsic properties of the material (i.e., its dielectric constant) as well as the geometry (size and shape) of the particle or the local environment (in the case of a roughened surface) [2,9]. When the frequency of the incident light matches the inherent oscillation frequency of the material, a surface plasmon will form. For certain materials like metal nanostructures, the surface plasmon can be highly localised (Localised Surface Plasmon Resonance, LSPR) [10]. In the case of silver and gold nanoparticles,


is often used in art history and art conservation studies. For example, it was used to study the stained glass of the Sainte-Chapelle cathedral in Paris (Figure 3) to distinguish between glass panels from the original construction in the 15th century and panels added during later restoration projects [5].


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