Optical and magneto-optical properties of purely two-dimensional electronic systems-graphene

Despite the massive attention that graphene has attracted in recent years, there are still many unanswered questions about its fundamental properties. In this work we present the results of a series of magneto-optical experiments performed on different graphene systems. The micro-Raman scattering spectroscopy was used as our method of choice, due to its non-invasive character, powerful characterization possibilities and high spatial resolution. The high magnetic fields were used to continuously tune the energy of inter-Landau level electronic excitations into a resonance with other excitations existing in the system. The magnetic field evolution of Raman active inter-Landau level excitations, and the details of the magneto-phonon resonance, gave us important information about the details of the electron-phonon interaction in graphene. Three different types of graphene are studied in this work. The first one consists of graphene flakes that can be found on the surface of graphite. It is possibly the least investigated graphene system, yet the one that shows the highest electronic quality. In Chapter 7 we present results of our magneto-Raman scattering experiments on this system. Our method for locating these flakes with the use of the magnetic field and without it is presented. The evolution of electronic excitations in magnetic fields is discussed. The effects of temperature, excitation wavelength and different coupling to the substrate are shown. We demonstrate that at high magnetic fields a fine-structure of the principal interband electronic excitation develops and discuss it in terms of doping and electron-hole asymmetry. A new type of a resonant electron-phonon interaction is observed, which involve an inter-valley carrier scattering and an emission of a K-point phonon. An analogous process for the phonons from the vicinity of the Γ point is observed.The second studied system consists of a graphene flake encapsulated between two layers of atomically flat hexagonal boron nitride (hBN). It is a representative of a novel class of materials, where different 2D crystals, are stacked on top of each other in a predefined order, to modify some properties of its constituents. Depositing graphene on a thin layer of hBN is expected to largely improve its electronic properties, as compared to graphene deposited on Si/SiO2. In Chapter 8 we present results obtained on such system. We show how spatial mapping with Raman scattering technique can be used for characterization and selective visualization of each constituent of the complex, stacked structures. A first, clear observation of a magneto-phonon resonance and L(-1,1) electronic excitation in an intrinsic, exfoliated graphene is shown. The Fermi velocity dependence on the magnetic field is demonstrated. Also, the Fermi velocity and 2D band energy dependence on the substrate is observed and discussed in terms of dielectric screening of the electron-electron interaction.The last studied system is the CVD grown graphene flake with electrical contacts. In Chapter 9 we show the results of an experiment, where strength of the electron-phonon interaction in a gated, CVD grown, graphene was successfully tuned by the applied gate voltage. We compare these results with the theoretical calculations and show that the intra-band electronic excitations play an important role in the renormalization of the phonon energy.