Abstract
In Raman spectroscopy of graphite and graphene, the D band at ∼ 1355 cm−1 is used as the indication of the dirtiness of a sample. However, our analysis suggests that the physics behind the D band is closely related to a very clear idea for describing a molecule, namely bonding and antibonding orbitals in graphene. In this paper, we review our recent work on the mechanism for activating the D band at a graphene edge.
Highlights
Bonding and antibonding orbitals are basic ideas for describing molecules
We show that the bonding and antibonding orbitals in graphene are key factors in the activation mechanism of the D band observed at a graphene edge
In this paper we show that the observed properties of the D band are naturally explained in terms of simple ideas based on molecular orbitals and momentum conservation
Summary
Bonding and antibonding orbitals are basic ideas for describing molecules. Bonding orbitals contribute to the formation of a molecule, whereas antibonding orbitals weaken the bonding and destabilize a molecule. We show that the bonding and antibonding orbitals in graphene are key factors in the activation mechanism of the D band observed at a graphene edge. A model of the D band must at least explain the observed properties: the D band intensity increases only at an armchair edge and is dependent on the laser light polarization. A photo-excited electron passes through two resonance states, which enhances the Raman intensity of a phonon with nonzero wave vector q= 0 This model is not concerned with the details of electron-phonon and electron-light matrix elements, and it does not provide clear explanations of the properties of the D band. This correlation is both an important factor in terms of understanding the D band and an essential feature of graphene. We give some notes on resonant condition in Appendix A
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