ABSTRACT
Graphene attracts tremendous interest in the scientific community owing to its unique electronic, optical, and mechanical properties. In the previous chapters, the transport properties characterized by electrical measurement and the chemical properties for application in energy storage have been introduced. In this chapter, we introduce the optical studies of graphene using Raman, infrared (IR), and visible spectroscopy. Graphene can present good optical contrast under specially constructed substrates despite the fact that it is only one-atom thick. Raman and visible/IR spectroscopic studies, which are based on using the optical microscope to locate the exact sample position, can be easily carried out in air ambient with high efficiency and high throughput. Raman spectroscopy is one of the
most commonly used methods for probing various properties of graphene, such as thickness and stacking geometry, doping, defects, edge chirality, and strain. These properties will be introduced in Sections 7.2.1−7.2.5. Despite the fact that IR spectroscopy is not used as common as Raman spectroscopy, it is significant for characterizing the chemical species as well as probing the low-energy regime of electronic band structure in graphene. This will be introduced briefly in Section 7.3. 7.2 Raman Spectroscopic Features of GrapheneEven for pristine graphene, the electronic properties show strong dependence on thickness, stacking geometry, and edge chirality. Furthermore, graphene’s electronic properties can be intentionally tuned by introducing defects, doping, and applying strain. Raman spectroscopy provides a fast and easy-to-operate technique that can characterize the above. Normally, Raman spectroscopy analyzes the features of lattice vibrational modes and hence it is very sensitive to the atomic arrangement and phonon structures, and less sensitive to electronic structure. However, for graphene and carbon based materials in general, Raman scattering is very sensitive to the electronic energy levels due to the strong resonance effect. Our discussion below focuses on the electronic properties of graphene instead of the atomic structures. Graphene and its derivative structures normally contain two prominent Raman active peaks, called the G and the 2D bands, located at ~1580 and 2700 cm−1, respectively (red curve in Fig. 7.1). The G band arises from the E2g vibrational symmetry corresponding to the zero-momentum phonon at Γ point (in-plane transverse optical phonon branch) [1]. The 2D band is the second order of D band which originates from the breathing mode of A1g symmetry at around K point. Remember that the first order D band is silent in Raman measurement for the perfect hexagonal sp2-based carbon crystal structure, and it can only be observed in the presence of disorders/defects as shown in the blue curve in Fig. 7.1 (indicated by the green circle). The G, 2D, and D bands have rich physics and can practically guide us to analyze the detailed electronic properties of graphene. In addition, some other weak Raman active bands can also be used to assist in analyzing graphene’s properties. In the
following sections, we will introduce Raman spectroscopic studies of graphene’s thickness, stacking geometry, doping, defects, edge chirality, and strain.
