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Raman spectroscopy is a sensitive light scattering technique, complementary to FTIR, and provides a spectral fingerprint for samples.

When monochromatic radiation hits a molecule, most of the light will be elastically scattered (Rayleigh scattering). In this case, the radiation excites the molecule to a virtual energy state. The molecule then relaxes back down, almost instantly, to its original energy state by releasing a photon with equal energy to the incident photon.

Raman spectroscopy is concerned with the less common Stokes, and anti-Stokes, inelastic light scattering phenomenon. In this case, incoming electromagnetic radiation polarises the electron cloud of a molecule, distorting its shape, size, or orientation. The incident photon excites a virtual electron-hole pair, which is then scattered by a phonon mode.26 At room temperature, a molecule will normally be in its ground vibrational state. Thus when Raman scattering occurs and an incident photon promotes a molecule to a virtual energy state, a lower energy photon will be emitted when the lattice relaxes; this is Stokes scattering. For anti-Stokes scattering, the lattice must already be in an excited vibrational state on interaction with electromagnetic radiation. As a result, the lattice can relax from its virtual energy state to a lower energy vibrational state than before, emitting a blue-shifted photon in the process.

These three light scattering events – Rayleigh, Stokes and anti-Stokes – are illustrated in figure 2.19.

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Fig. 2.19 Jablonski diagram showing the energy state transitions for elastic and inelastic light scattering.

This change in the wavelength of the incident photon to give the emitted photon in Raman scattering is known as the Raman shift. As vibrational energy levels are highly specific, Raman shifts are characteristic of particular bonds, and can thus be used to provide structural information.

Only a small proportion of incident light will be Raman scattered (approximately 0.0001% of photons for conventional, non-resonant Raman scattering), thus the signal is often weak; large sample volumes, high intensity lasers, and sensitive detectors are required. If the signal is still not sufficient, or is obscured by fluorescent transitions, surface enhanced Raman spectroscopy (SERS) can be employed.

Raman can be a very useful analytical tool for graphene-based materials as the system of delocalised electrons is easily polarisable,27 providing a large photon scattering cross-section and thus strong Raman signals. Graphene has three optical phonon modes – two of which are in-plane, and one of which is out-of-plane28 – which give rise to the primary signals seen in Raman spectra of graphene-type materials.

Graphene-type materials will exhibit a dominant band at ̴1580 cm-1, known as the G band. The G band arises from a first order excitation process from the doubly degenerate sp2

1 0 2 3 4 Vibrational energy states Virtual energy states Rayleigh scattering Stokes Raman scattering anti-Stokes Raman scattering

2.3 Spectroscopic Studies 45 (a) G band (b) D and G’ bands stretching modes shown in figure 2.20 (a), the band is thus characteristic of graphitic materials.

Fig. 2.20 Schematic showing the optical phonon vibrations of graphene, with the sp2 C-C stretching modes which correspond to (a) the G band, and (b) the D and G’ bands in Raman.

The G’ (or 2D) band seen around 2700 cm-1 is also characteristic of sp2 systems, arising from a second-order excitation process:29 an electron near the K point (Dirac point) at the corner of the Brillouin zone is excited to the conduction band of graphene, and itself (or the corresponding hole) is further scattered, by the phonon vibration shown in figure 2.20 (b), to the inequivalent K’ point.30 In order to conserve energy and momentum, the electron (or hole) is scattered by a second phonon vibration, back to the K point where the electron-hole recombines, emitting a photon in the process. These G and G’ bands can be seen in the Raman spectrum of highly ordered pyrolytic graphite (HOPG), as shown in figure 2.21.

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Fig. 2.21 Example Raman spectra of as-produced GO, showing the D (1340 cm-1), G (1600 cm-1) and G’ (2700 cm-1) bands, and of HOPG, showing the sp2 characteristic G and G’ bands.

The shape, intensity and position of the G and G’ bands in graphitic samples reveal the extent of exfoliation in graphene-type materials: significant differences between the respective peaks of monolayer aGO and HOPG can be seen in figure 2.20.

The third band of interest for graphene-type materials is the D band which, as shown in the Raman spectrum of aGO in figure 2.21, occurs around 1340 cm-1. Although characteristic of graphitic materials, in pristine sp2 hybridised samples this band is Raman inactive. Like the G’ Raman scattering, an electron (or hole) is excited by an incident photon, and is further scattered by the phonon breathing mode shown in figure 2.20 (b). The D band is a one phonon process and thus requires an atomic defect which breaks the symmetry of the sp2 lattice to scatter the electron (or hole) back to the K point making the process Raman active.31 As a result the D band is known as the disorder band, with the relative areas of the D and G bands being used as a measure of defect density in a graphene-type structure.32 Although, it is worth noting that the D/G ratio of defective graphene-type materials does not tend to change significantly in response to further surface functionalisation – the D/G ratio is dominated by the structural defects (holes) of a sheet rather than the level of oxidation, for example, which has comparatively little effect on the D/G ratio.

Raman can be a very convenient method of characterisation: the analysis is quick, non- destructive and uses solid-state samples. Raman thus provides an alternative method of

D band ~ 1340 cm-1 G band~ 1600 cm-1 G’ band ~ 2700 cm-1

500 1000 1500 2000 2500 3000 3500

Raman Shift / cm-1

aGO HOPG

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structural characterisation for samples which are unsuitable for AFM and TEM studies due to poor dispersibility.