# Advances in Graphene Field Effect Transistors (FETs) for Annine Neurotransmitter Sensing
## 2 Problem Outline
### GFET Characterisation
#### 2.5.1 Structural and Chemical Characterisation  
Raman spectroscopy has emerged as a useful tool for characterising graphene-based surfaces because it offers precise, non-destructive, non-invasive, in situ, and real-time fingerprints based on electron band changes. This approach uses the carbon vibrational modes to gain insights into graphene's structure and electrical properties [127; 128; 129] and investigate the impact of environmental factors, such as temperature, humidity, and gas adsorption on graphene [130]. As shown in Figure 2a, graphene's Raman spectrum is distinguished by three signature peaks identified as D, G, and 2D. The D peak, centred at 1350 cm\({}^{-1}\), correlates with defects and abnormalities in graphene's lattice structure. The G band is optimised around 1580 cm\({}^{-1}\) and is associated with the stretching mode of sp\({}^{2}\)-hybridised carbon atoms and E\({}_{2}\)g phonon mode in the graphene lattice. The 2D peak at 2700 cm\({}^{-1}\) is caused by the second-order scattering of light zone boundary phonons due to a double resonance. The location, intensity, and width of these peaks specify information interpreting the number of layers, defects, and strain in the graphene material [130; 131; 132].  
Fourier transform infrared spectroscopy (FTIR) is used to study the functional groups and chemical bonds [133] and is particularly useful for the analysis of graphene surfaces [134; 135], as it provides critical insights into the nature of the chemical bonding available. The fingerprint region stretches from 500 cm\({}^{-1}\) to 4000 cm\({}^{-1}\), with characteristic features at 1052 cm\({}^{-1}\), 1634 cm\({}^{-1}\), 1728 cm\({}^{-1}\), and 3403 cm\({}^{-1}\) corresponding to C-O-C, C=C, C=O, and -OH functional groups, respectively [136].  
X-ray photoelectron spectroscopy (XPS) is used to quantify the carbon functional groups and evaluate the surface chemistry in graphene materials. This method is promising and precise for determining the chemical functionality of graphene surfaces. XPS identifies and analyses impurities that may adhere to the graphene surface, such as adsorbed gases, residual solvents, and organic residues [137] and the level of carbon hybridisation [138]. These features are crucial for identifying the purity and quality of graphene samples as well as functional groups such as carbonyl, hydroxyl, carboxyl, and epoxy. Understanding the nature and density of these functional groups paves the way for tailoring graphene's chemical reactivity and other features towards biosensing applications. XPS also studies the oxidation level, which provides important information on graphene's stability, electrical structure, and characteristics [137; 139; 140]. Figure 2b shows a typical XPS spectra for rGO [141].  
Figure 2: Graphene characterisation: (**a**) Raman spectroscopy of rGO showing the characteristic D, G, and 2D peaks associated with graphene and its derivatives and (**b**) XPS spectra of rGO with the fitted peak representing different oxygen functional groups that can be used as active sites for surface functionalisation (Reprinted with permission from [141]. 2023, MDPI).