## 2 Graphene and Graphene-Related Materials
### Graphene and Related Materials, Characterization
#### 2.2.1. Spectroscopical Techniques  
Several spectroscopical techniques have been used to investigate the main features of graphene and related materials.  
Raman spectroscopy is a non-destructive technique that allows researchers to evaluate the lattice defectivity of the graphene plates' structure. The Raman spectrum is reported as  
Figure 1: Structures of pristine graphene (**a**) and its GO derivatives based on [PERSON] (**b**), Ruess (**c**), Scholz–Boehm (**d**), Nakajima–Matsuo (**e**), Lerf–Klinowski (**f**), and Szabo (**g**) models. Reproduced, adapted, and reprinted with permission from Ref. [48] under CC license.  
the intensity of the signal (arbitrary units) vs. Raman shift (cm\({}^{-1}\)). The fingerprint of sp2 carbon structures such as graphene is the presence of the D-band at around 1350 cm\({}^{-1}\), arising from the disorder-induced phonon mode (A1g-band); the G-band at around 1590 cm\({}^{-1}\), assigned to the Raman-allowed phonon mode (E2g-band) [61]; and the 2D band at circa 2700 cm\({}^{-1}\). The 2D band is at almost double the frequency of the D-band and originates from second-order Raman scattering process. The extent of the lattice defectivity can be evaluated by the ratio of the intensities of D-band over G-band (I\({}_{\text{D}}\)/I\({}_{\text{G}}\)) [62]. The higher the ratio, the greater the amount of lattice defects. The quality and number of graphene layers can be further investigated using Raman spectroscopy.  
UV-visible spectroscopy could also be a powerful tool to investigate graphene. Pristine graphene and single-layer graphene oxide (GO) show absorption at 262 and 230 nm respectively in the UV-visible spectrum. This is due to the \(\pi\)-\(\pi^{\star}\) transitions of aromatic C-C bonds. Nonetheless, graphene has less transparency than GO, which is attributed to the recovery of sp\({}^{2}\) carbons to restore electronic conjugation in reduced graphene after reduction [63]. Moreover, the transmittance of monolayer graphene is 97.1% at a wavelength of 550 nm, which is higher than that of stacked graphene, as reported by [PERSON] et al. [64]. With the increase in the number of graphene layers, the transmittance of graphene decreased from 94.3% of that of bilayer graphene to 83% of that of six-layer graphene at the same wavelength. Thus, the number of non-defective graphene layers can be determined using UV-visible spectroscopy.  
X-ray photoelectron spectroscopy is a surface-sensitive quantitative spectroscopic technique that measures the elemental composition, the chemical state, and the electronic state of the elements that exist within a material [65]. XPS is very useful to investigate graphene and GO because the technique shows what elements are present in the sample and how they are bonded, allowing structural investigation on the material surface [66; 67]. XPS technique can give crucial information about the functionalization of graphene and its related materials by the analysis of the C1s peak in the region from 284.1 [68] to 289.4 eV [69]. The decomposition of the C1s peak could be used to identify and quantify the relative ratio of carbon bond types. Furthermore, the presence of a \(\pi\)-\(\pi^{\star}\) component at 290.5 eV could be diagnostic to discriminate rGO/graphene from GO that does not show it [70].