Probing the electronic structure of carbon nanotubes and graphene by resonance Raman Scattering
Raman spectroscopy (RS) is a very useful tool to study carbon nanotubes and graphene, since it furnishes information about the atomic structure, presence of disorder, charge transfer and strain. However, important information about electrons can be also obtained in a Resonance Raman spectroscopy (RRS) investigation, where the energy of the laser excitation can be tuned. RRS is specially interesting in the case of carbon nanotubes due to the enhancement of the Raman signal of a particular (n,m) nanotube when the incident or scattered photon is in resonance with its optical transition and the determination of a resonant Raman map provides the structural (n,m) assignment of the nanotubes present in the samples. We will present resonance Raman maps in different types of chirality-enriched nanotube samples and isotope (13C) enriched samples, and we will show that these maps allow us to investigate the resonance behavior of the G and G´ (or 2D) bands for specific (n,m) nanotubes, allowing us to understand the double resonance Raman process in carbon nanotubes. RRS is also a very useful tool to investigate electrons and phonons near the Dirac point of graphene, due to the specific double resonance (DR) Raman process that occur in these materials. By changing the energy of the incident excitation laser, it is possible to probe different points within the interior of the Brillouin zone. We will present experimental results of the dispersion of electrons and phonons in graphene samples and devices, where an applied electric field and changes in the Fermi level can be controlled by an external gate voltage, showing that Raman spectroscopy is useful to quantify the interaction of graphene with its surrounding environment. We will also show that Raman spectroscopy can be useful to characterize Moiré patterns in twisted graphene layers, in samples grown by CVD on copper foils, due to the unklapp double resonance (U-DR) Raman process. Finally, we will present results of the effect of uniaxial strain in the electronic structure of graphene.