Daniela Lopes Mafra

We explore a CVD transfer technique that abandons both the intermediate membrane and chemical etching of the metal catalyst. This method is fast, simple and is a necessary route towards roll-to-roll production of large-area CVD graphene sheets at high quality and low cost. Such integration is a step forward to the economical and industrial scale production of graphene and enables technology for flexible electronics and optoelectronics.

The Raman spectra of graphene samples exhibit a band around 2700 cm-1, the so called G' band, that is ascribed to a double resonance Raman process involving electrons and phonons in the vicinity of the Dirac point. A dispersive behavior in the position and shape of this band is observed when we change the laser energy used in the Raman experiment, showing that it can be used to probe experimentally the dispersion of electrons and phonons near the Dirac point of graphene. We will present a resonance Raman investigation of monolayer and bilayer graphene using many different laser lines in the visible and near IR range. By the analysis of the dispersive behavior of the G' band we can obtain information about the electronic structure of monolayer and bilayer graphene, such as the intralayer and interlayer tight-binding parameters. Our results reveals a significant asymmetry between the electronic dispersion in the valence and conduction bands of bilayer graphene. We are also abl...

A mass-related symmetry breaking in isotopically labeled bilayer graphene (2LG) was investigated during in-situ electrochemical charging of AB stacked (AB-2LG) and turbostratic (t-2LG) layers. The overlap of the two approaches, isotopic labeling and electronic doping, is powerful tool and allows to tailor, independently and distinctly, the thermal-related and transport-related phenomena in materials, since one can impose different symmetries for electrons and phonons in these systems. Variations in the system's phonon self-energy renormalizations due to the charge distribution and doping changes could be analyzed separately for each individual layer. Symmetry arguments together with first-order Raman spectra show that the single layer graphene (1LG), which is directly contacted to the electrode, has a higher concentration of charge carriers than the second graphene layer, which is not contacted by the electrode. These different charge distributions are reflected and demonstrated...

The interaction of electrons and phonons is a fundamental issue for understanding the physics of graphene, resulting in the renormalization of phonon energy as a function of Fermi energy, which has been ascribed to the breakdown of the adiabatic approximation. In this work we study the behavior of the optical phonon modes in different bilayer graphene devices by applying bottom

The dispersion of phonons and the electronic structure of graphene systems can be obtained experimentally from the double-resonance (DR) Raman features by varying the excitation laser energy. In a previous resonance Raman investigation of graphene, the electronic structure was analyzed in the framework of the Slonczewski-Weiss-McClure (SWM) model, considering the outer DR process. In this work we analyze the data

The Raman spectroscopy has been widely used to study carbon materials. In this work the dispersion of phonons and the electronic structure of graphene systems can be obtained experimentally from the double-resonance (DR) Raman features by varying the excitation laser energy. The electronic structure was analyzed in the framework of the Slonczewski-Weiss-McClure (SWM) model, considering both the outer and inner

The G' (or 2D) Raman band of AB stacked bilayer graphene comes from a double resonance Raman (DRR) process and is composed of four peaks (P(11), P(12), P(21), and P(22)). In this work, the integrated areas (IA) of these four peaks are analyzed as a function of the laser power for different laser lines. We show that the dependence of the IA of each peak on temperature is different for each distinct laser excitation energy. This special dependence is explained in terms of the electron-phonon coupling and the relaxation of the photon-excited electron. In this DRR process, the electron is scattered by an iTO phonon from a K to an inequivalent K' point of the Brillouin zone. Here, we show that this electron relaxes while in the conduction band before being scattered by an iTO phonon due to the short relaxation time of the excited electron, and the carrier relaxation occurs predominantly by emitting a low-energy acoustic phonon. The different combinations of relaxation processes determine the relative intensities of the four peaks that give rise to the G' band. Some peaks show an increase of their IA at the expense of others, thereby making the IA of the peaks both different from each other and dependent on laser excitation energy and on power level. Also, we report that the IA of the G' mode excited at 532 nm, shows a resonance regime involving ZO' phonons (related to the interlayer breathing mode in bilayer graphene systems) in which a saturation of what we call the P(12) process occurs. This effect gives important information about the electron and phonon dynamics and needs to be taken into account for certain applications of bilayer graphene in the field of nanotechnology.

The Raman spectra of graphene samples exhibit a band around 2700 cm$^{-1}$, the so called G$'$ band, that is ascribed to a double resonance Raman process involving electrons and phonons in the vicinity of the Dirac point. A dispersive behavior in the position and shape of this ...

ABSTRACT Micro-Raman experiments as a function of magnetic field up to 15 T were performed on a set of natural graphene flakes on Si/SiO2 substrates and multilayer epitaxial graphene grown on a carbon face of SiC. Pronounced oscillations of the G-band position and linewidth attributed to crossings of this mode with Landau levels were observed in epitaxial graphene. Calculated phonon energy and broadening oscillations obtained from the phonon's Green function show good agreement with the results obtained for SiC samples, in line with a previous report [1]. For graphene flakes, the field evolution of the G-band is strongly sample-dependent, and may also depend on the position of the focal spot. A splitting of G-band in two peaks was observed in some cases for B>12 T. Our results suggest the large sensitivity of graphene electron-phonon interaction to both magnetic field and local conditions. [1] C. Faugeras et al., Phys. Rev. Lett. 103, 186803 (2009).