By computing the double-resonant Raman scattering cross section completely from first principles and including the electron-electron interaction at the GW level, we unravel the dominant contributions for the double-resonant 2D mode in bilayer graphene. We show that, in contrast to previous works, the so-called inner processes are dominant and that the 2D-mode line shape is described by three dominant resonances around the K point. We show that the splitting of the transversal optical (TO) phonon branch in the Γ-K direction, as large as 12  cm(-1) in the GW approximation, is of great importance for a thorough description of the 2D-mode line shape. Finally, we present a method to extract the TO phonon splitting and the splitting of the electronic bands from experimental data.

The practical difficulties to use graphene in microelectronics and optoelectronics is that the available methods to grow graphene are not easily integrated in the mainstream technologies. A growth method that could overcome at least some of these problems is chemical vapour deposition (CVD) of graphene directly on semiconducting (Si or Ge) substrates. Here we report on the comparison of the CVD and molecular beam epitaxy (MBE) growth of graphene on the technologically relevant Ge(001)/Si(001) substrate from ethene (C2H4) precursor and describe the physical properties of the films as well as we discuss the surface reaction and diffusion processes that may be responsible for the observed behavior. Using nano angle resolved photoemission (nanoARPES) complemented by transport studies and Raman spectroscopy as well as density functional theory (DFT) calculations, we report the direct observation of massless Dirac particles in monolayer graphene, providing a comprehensive mapping of their low-hole doped Dirac electron bands. The micrometric graphene flakes are oriented along two predominant directions rotated by 30° with respect to each other. The growth mode is attributed to the mechanism when small graphene “molecules” nucleate on the Ge(001) surface and it is found that hydrogen plays a significant role in this process.

Among the many anticipated applications of graphene, some -such as transistors for Si microelectronics -would greatly benefit from the possibility to deposit graphene directly on a semiconductor grown on a Si wafer. We report that Ge(001) layers on Si(001) wafers can be uniformly covered with graphene at temperatures between 800 • C and the melting temperature of Ge. The graphene is closed, with sheet resistivity strongly decreasing with growth temperature, weakly decreasing with the amount of deposited C, and reaching down to 2 kΩ/2. Activation energy of surface roughness is low (about 0.66 eV) and constant throughout the range of temperatures in which graphene is formed. Density functional theory calculations indicate that the major physical processes affecting the growth are: (1) substitution of Ge in surface dimers by C, (2) interaction between C clusters and Ge monomers, and (3) formation of chemical bonds between graphene edge and Ge(001), and that the processes 1 and 2 are surpassed by CH 2 surface diffusion when the C atoms are delivered from CH 4 . The results of this study indicate that graphene can be produced directly at the active region of the transistor in a process compatible with the Si technology.

The contributions of the two-dimensional phonon dispersion to the double-resonant Raman scattering process in graphene is determined from the line shape of the two-phonon combination mode around 2450 cm(-1). This mode is usually referred to as G* or D + D ''. By combining Raman experiments with excitation energies up to 2.8 eV and a full two-dimensional calculation of the double-resonant Raman process based on fourth-order perturbation, we can describe in detail the composition of this two-phonon mode and explain the asymmetry on the high-frequency side. The asymmetry directly reflects phonon contributions with wave vectors away from the high-symmetry lines in the Brillouin zone. The main peak of this mode originates from the K Gamma high-symmetry line highlighting and supporting two important findings: first, the existence of so-called inner processes and, second, the dominant contribution along the high-symmetry line. DOI: 10.1103/PhysRevB.87.07540

We present and discuss a double-resonant Raman mode in few-layer graphene, which has not been interpreted before and is able to probe the number of graphene layers. This so-called N mode on the low-frequency side of the G mode results from a double-resonant Stokes/anti-Stokes process combining an optical (LO) and an out-of-plane (ZO ) phonon. Simulations of the double-resonant Raman spectra in bilayer graphene show very good agreement with the experiments.Raman spectroscopy belongs to the most widely used methods in graphene research. Raman spectroscopy is used for characterizing graphene regarding defects 1-3 , doping 4,5 , strain 6-9 , crystallographic orientation 10,11 , or interaction with the substrate 12 . In view of fundamental physical properties of graphene, Raman spectroscopy gives information on electron-phonon coupling and scattering rates, optical excitations in graphene, thermal and mechanical properties [13][14][15] . Probably the most popular application of Raman scattering in graphene is the distinction of single-layer graphene from few-layer graphene and graphite via the lineshape of the double-resonant 2D mode 16 . On the other hand, few-layer graphene has recently come into focus, as gated bi-and trilayer graphene offer a tunable band gap 17,18 and bilayer graphene has been demonstrated to give much higher on-off ratios in a field-effect transistor than single-layer graphene 19 . Therefore, it is important to establish a reliable method for the determination of the layer number in few-layer graphene and to identify spectroscopic signatures of the layer-layer interaction. So far, typically the evolution of the 2D-mode lineshape or the absolute Raman intensity of the G mode is used in combination with optical contrast measurements. However, the lineshape of the 2D mode depends strongly on the excitation wavelength 16 , and the G-mode amplitude depends not only on the scattering volume 20,21 , but also on the substrate and optical interference effects 22 . Recently, the rigid-layer shear mode, which is the other Raman-active E 2g phonon mode in graphite, was shown to have a strong frequency dependence on the number of layers in few-layer graphene 23 . The frequency of this mode, however, is below 44 cm −1 . Measurement of this low-frequency mode is therefore difficult and requires non-standard equipment.