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 AFM 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 BL on terraces with a step-flow growth mechanism, as demonstrated in Fig.1. 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. Regarding transport measurement, quantum Hall plateau values observed in our graphene layers confirmed both continuity and thickness of the 1LG film.

Double-walled carbon nanotubes (DWNTs), consisting of two coaxial and weakly van der Waals coupled single-walled carbon nanotubes (SWNTs), are one of the most ideal and fundamental systems to study the effects of inter-tube coupling on the physical properties in one-dimensional structures.On this poster, we report on the investigations of individual, spatially isolated and suspended double-walled carbon nanotubes (DWNTs) by combination of high-resolution electron microscopy, electron diffraction, Rayleigh spectroscopy and Raman spectroscopy (Figure 1) [1,2]. We first present an overview of the effect of the diameters, inter-tube distance and nature (metallic or semiconducting) on the Raman signature (Radial Breathing Like [1,3,4] and Tangential Modes [4-6]) of DWNTs. We then examine the features of tangential modes and find a clear frequency shift of the inner-layer G modes as a function of the inter-tube distance with respect to the corresponding G modes in equivalent single-walled carbon nanotubes (SWNT).[4-6] These results are understood by considering the effects of the relaxation of the layers and the interaction between the relaxed structures.[7] Finally, we study the experimental excitation dependence of the G modes intensity of the constituent inner and outer SWNTs. In particular, we discuss the effects of the quantum interference between different electronic transitions on the observed behaviors.[8]References[1] D.I. Levshov et al., Carbon, 114, 141 (2017). [2] D.I. Levshov et al., Phys Rev. B 96 (2017), 195410.[3] D.I. Levshov et al., Nano Lett. 11(11) (2011), 4800. [4] H.-N. Tran et al., Adv. Nat. Sci. Nanosci. Nanotechnol. (2017), 8, 15018.[5] D. Levshov et al., J. Phys. Chem. C 119(40) (2015), 23196.[6] D.I. Levshov et al., Phys. Status Solidi B 254(11) (2017), 1700251.[7] V.N. Popov et al., Phys. Rev. B 97 (2018), 165417.[8] H.-N. Tran et al., Phys. Rev. B 95 (2017), 95 (20), 205411

Using an experimental approach that combines high-resolution electron microscopy, electron diffraction and optical spectroscopies, including Raman scattering, Rayleigh spectroscopy and photoluminescence, on individual, spatially isolated and suspended (free-standing) single-walled (SWNTs) and double-walled carbon nanotubes (DWNTs), we have been able to investigate their phonons characteristics and optical properties. In this talk, we present an overview of the most significant results obtained via this approach. Particular attention is paid to the study of DWNTs, a coaxial composite of two SWNTs, which provide a unique model system for studying coupling at the nanoscale. We show how such coupling, originating from the van der Waals interaction between the inner and outer nanotubes, plays a crucial role in determining the characteristics of the collective radial breathing-like modes and G modes in DWNTs (for a review see Ref. 1-2). The effects of quantum interference between different electronic transitions on the experimental behavior of the G-modes intensity are discussed [3]. It must be emphasized that the theoretical predictions of Philippe Lambin and his collaborators have been invaluable in understanding a lot of these experimental data [4].

The goal of this chapter is to review the main information derived from Raman spectroscopy on individual suspended (free-standing) chiralityidentified single-wall carbon nanotubes (SWCNTs) and double-wall carbonnanotubes (DWCNTs) with a special focus on the characteristics of their radial breathing modes and G modes, including their resonance conditions. ForSWCNTs, the different relationships between the radial breathing mode frequency and the inverse of the diameter illustrate the high sensitivity of individual suspended SWCNTs to their environmental conditions. The intrinsicprofiles of the optical longitudinal (LO) and transverse (TO) G modes are unambiguously identified both for metallic and semiconducting chiral and achiral SWCNTs, and the diameter dependence of the LO and TO frequencies are established. In DWCNTs, the intertube coupling, originating from the van der Waals interaction between the inner and outer tubes, plays an important role in determining the features/characteristics of the collectiveradial breathing-like modes and G modes.

Double-walled carbon nanotubes (DWNTs), consisting of two coaxial and weakly van der Waals coupled single-walled carbon nanotubes (SWNTs), are one of the most ideal and fundamental systems to study the effects of inter-tube coupling on the physical properties in one-dimensional structures.On this poster, we report on the investigations of individual, spatially isolated and suspended double-walled carbon nanotubes (DWNTs) by combination of high-resolution electron microscopy, electron diffraction, Rayleigh spectroscopy and Raman spectroscopy (Figure 1) [1,2]. We first present an overview of the effect of the diameters, inter-tube distance and nature (metallic or semiconducting) on the Raman signature (Radial Breathing Like [1,3,4] and Tangential Modes [4-6]) of DWNTs. We then examine the features of tangential modes and find a clear frequency shift of the inner-layer G modes as a function of the inter-tube distance with respect to the corresponding G modes in equivalent single-walled carbon nanotubes (SWNT).[4-6] These results are understood by considering the effects of the relaxation of the layers and the interaction between the relaxed structures.[7] Finally, we study the experimental excitation dependence of the G modes intensity of the constituent inner and outer SWNTs. In particular, we discuss the effects of the quantum interference between different electronic transitions on the observed behaviors.[8]References[1] D.I. Levshov et al., Carbon, 114, 141 (2017). [2] D.I. Levshov et al., Phys Rev. B 96 (2017), 195410.[3] D.I. Levshov et al., Nano Lett. 11(11) (2011), 4800. [4] H.-N. Tran et al., Adv. Nat. Sci. Nanosci. Nanotechnol. (2017), 8, 15018.[5] D. Levshov et al., J. Phys. Chem. C 119(40) (2015), 23196.[6] D.I. Levshov et al., Phys. Status Solidi B 254(11) (2017), 1700251.[7] V.N. Popov et al., Phys. Rev. B 97 (2018), 165417.[8] H.-N. Tran et al., Phys. Rev. B 95 (2017), 95 (20), 205411

We report the direct and unambiguous evidence of the existence of inner semiconducting tube (iSCT) photoluminescence (PL) from measurements performed on four individual free-standing index-identified double-walled carbon nanotubes (DWNTs). Based on thorough Rayleigh scattering, Raman scattering and PL experiments, we are able to demonstrate that the iSCT PL is observed with a quantum yield estimated as a few 10-6 independently of the semiconducting or metallic nature of the outer tube. This result is mainly attributed to ultrafast exciton transfer from inner to outer tube. Furthermore, by carrying out PL excitation experiments on the (14,1)@(15,12) DWNT, we show that the iSCT PL can be detected through the optical excitation of the outer tube (Figure 1) indicating that the exciton transfer can also occur in the opposite way.References[1] D.I. Levshov et al., Phys Rev. B 96 (2017), pp. 195410

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.