Since 2008, epitaxial graphene growth has been developed in terms of homogeneity and scale by using a 1 ATM argon pressure at high temperature (>1650°C). Until now, it still remains challenging to obtain films with different and controlled characteristics such as the thickness or the doping by tuning the growth parameters. Here, we optimized the epitaxial growth of monolayer graphene (1LG) on 4H-SiC (0001) under a low argon pressure of 10 mbar. This intermediate pressure allows growing a continued 1LG in a short process ~1h30.First, we discuss the initial growth stages from buffer layer to 1LG as a function of annealing temperature (same heating rate). The combined Raman spectroscopy and atomic force microscopy analyses show that a buffer layer, fully covering the Si-face of SiC, forms as the first step of growth. Subsequently, 1LG starts to grow at step edges and continue to cover the buffer layer on terraces with a step-flow growth mechanism. Eventually, reproducible syntheses of 1LG films were achieved by this temperature-controlled growth process. Then, we investigate the structural and electronic properties of the 1LG films. The integrated intensity of G-band in Raman spectra normalized with respect to a HOPG reference, AG/AG-HOPG, of each spectrum in Raman map of our continued graphene film is very close to the experimental value reported for a 1LG, demonstrating the good homogeneity. Atomically resolved scanning tunnellingmicroscopy (STM) evidences a (6x6) superstructure, indicative of 1LG covering a reconstructed interface layer[7]. Regarding the transport measurement, quantum Hall plateau values observed in our graphene layers confirmed both continuity and thickness of the1LG film. Moreover, we estimated a hole concentration of p~10^10 - 10^11 /(cm^2) at 1.7 K in helium atmosphere. We ascribe this unusual doping type to an atmosphere contamination effect since we regain the typical n-doped graphene under vacuum by transport measurement and STM analysis.

Since 2008, epitaxial graphene growth has been developed in terms of homogeneity and scale by using a 1 ATM argon pressure at high temperature (>1650°C). Until now, it still remains challenging to obtain films with different and controlled characteristics such as the number of graphene layers or the doping by tuning the growth parameters. Here, we optimized the epitaxial growth of monolayer graphene (1LG) on 4H-SiC (0001) under a low argon pressure of 10 mbar. This intermediate pressure allows growing a continued 1LG in a short process time ~1h30. First, we discuss the initial growth stages from buffer layer (BL) to 1LG as a function of annealing temperature (same heating rate). The combined Raman spectroscopy and atomic force microscopy analyses show that a BL, fully covering the Si-face of SiC, forms as the first step of growth. Subsequently, 1LG starts to grow at step edges and continue to cover the terraces with a step-flow growth mechanism, as demonstrated. Eventually, reproducible syntheses of 1LG films were achieved by this temperature-controlled growth process. Then, we investigate the structural and electronic properties of the 1LG films. The integrated intensity of G-band in Raman spectra normalized with respect to a HOPG reference, AG/AG-HOPG, of each spectrum in Raman map of our continued graphene film is very close to the experimental value reported for a 1LG, demonstrating the good homogeneity. Atomically resolved scanning tunnelling microscopy (STM) evidences a (6x6) superstructure, indicative of 1LG covering a reconstructed interface layer. Regarding the transport measurement, quantum Hall plateau values observed in our graphene layers confirmed both continuity and thickness of the 1LG film8. Moreover, we estimated by transport measurement and STM analysis a hole concentration of pHall~10^10 - 10^11 cm^-2 at 1.7 K in helium atmosphere. We ascribe this unusual doping type to an atmosphere contamination effect since we regain the typical n-doped graphene under vacuum.

The interactions between the layers of double-walled carbon nanotubes induce a measurable shift of the G bands relative to the isolated layers. While experimental data on this shift in freestanding double-walled carbon nanotubes has been reported in the past several years, a comprehensive theoretical description of the observed shift is still lacking. The prediction of this shift is important for supporting the assignment of the measureddouble-walled nanotubes to particular nanotube types. Here, we report a computational study of the G-band shift as a function of the semiconducting inner layer radius and interlayer separation. We find that with increasing interlayer separation, the G band shift decreases, passes through zero and becomes negative, and further increases in absolute value for the wide range of considered inner layer radii. The theoretical predictions are shown to agree with the available experimental data within the experimental uncertainty.

The understanding of interactions between electrons and phonons in atomically thin heterostructures is crucial for the engineering of novel two-dimensional devices. Electron–phonon (el–ph) interactions in layered materials can occur involving electrons in the same layer or in different layers. Here we report on the possibility of distinguishing intralayer and interlayer el–ph interactions in samples of twisted bilayer graphene and of probing the intralayer process in graphene/h-BN by using Raman spectroscopy. In the intralayer process, the el–ph scattering occurs in a single graphene layer and the other layer (graphene or h-BN) imposes a periodic potential that backscatters the excited electron, whereas for the interlayer process the el–ph scattering occurs between states in the Dirac cones of adjacent graphene layers. Our methodology of using Raman spectroscopy to probe different types of el–ph interactions can be extended to study any kind of graphene-based heterostructure.

A After a brief overview of the discovery and the Raman study of new forms of carbons (intercalated graphite, carbon fiber, fullerenes, carbon nanotubes), the invaluable contribution of late Professor M. Dresselhaus is noted and the 10 reviews and 10 contributions collected to present a picture of the present Raman investigations of graphene and related 2D materials (such as black phosphorus, MoS2) are presented. Methods for numbering the graphene layers, the effects of external perturbations (temperature, pressure, doping and magnetic field) on the phonons of graphene, characterization of the chemical and structural properties of graphene at the nanoscale level by tip-enhanced Raman spectroscopy (TERS), surface enhanced Raman spectroscopy (SERS) and hyperspectral imaging, and applications combining graphene and Raman spectroscopy are addressed.

Raman spectroscopy is commonly used to determine the number of layers of few-layer graphene (FLG)samples. In this work, we focus on the criteria based on the G-band integrated intensity and on the laseroptical contrast. Limitations due to stacking order are discussed and lead to the conclusion that it isnecessary to combine Raman and optical contrast to avoid misinterpretation. Both methods enable todistinguish unambiguously between single layer graphene and multilayer graphene. However, neithereach method separately nor the combination of the two enable a determination of the number of layers forall possible stacking orientations. Importantly, since the two methods always significantly disagree whenthey fail, the comparison of the values deduced by each method allows to discriminate if the determinednumber of layers can be specified or not. Other important parameters (substrate, laser wavelength,objective numerical aperture) are discussed to define a reliable method to determine the number ofgraphene layers in FLG and its domain of validity. The proposed method which combines Raman andoptical contrast measurements, carried out with a 532 nm laser and using a 100x objective with anumerical aperture of 0.9, allows the determination of the number of layers for (up to 5) FLG on thefollowing substrates: (i) glass (soda lime glass or similar with refractive index between 1.50 and 1.55) and(ii) oxidized silicon (SiO2 on silicon, with a SiO2 thickness of 90 ± 5 nm). The method is however limited tohigh quality graphene and FLG with small defect density and low residue.