In this paper, we discuss the low-frequency range of the Raman spectrum of individual suspended index-identified single-walled (SWCNTs) and double-walled carbon nanotubes (DWCNTs). In SWCNTs, the role of environment on the radial breathing mode (RBM) frequency is discussed. We show that the interaction between the surrounding air and the nanotube does not induce a RBM upshift. In several DWCNTs, we evidence that the low-frequency modes cannot be connected to the RBM of each related layer. We discuss this result in terms of mechanical coupling between the layers which results in collective radial breathing-like modes. The mechanical coupling qualitatively explains the observation of Raman lines of radial breathing-like modes, whenever only one of the layers is in resonance with the incident laser energy.

In this article, we report the synthesis of ultra-long carbon nanotubes (CNTs) by thermal chemical vapour deposition method. Ultra-long, individual and aligned CNTs were directly grown on a flat silicon substrate. The orientation of the nanotubes was found parallel to the gas flow direction. The ultra-long CNTs were grown with different transition metallic salts, such as nickel chloride, iron (III) chloride, cobalt acetate and ruthenium acetate, as the catalysts. The influence of the growth conditions, such as growth temperature, reactive gas flow on the length and alignment of the CNTs was studied in detail. By using different catalysts, ultra-long single-walled carbon nanotubes (SWCNTs) or multi-walled carbon nanotubes (MWCNTs) were successfully grown. These ultra-long CNTs were transferred to other substrates by two methods. (1) The first method is to use polydimethylsiloxane as a stamp. (2) The second method is to use KOH as an etching agent. The diameter and length of the CNTs were characterised by transmission electron microscope, scanning electron microscope, atomic force microscope and Raman spectroscopy. The results indicate that the length of the CNTs can reach up to 4 mm. The diameter of the SWCNTs is in the range of 0.7-2.1 nm and the diameter of the MWCNTs is approximately 150 nm.

The influence of the temperature and precursor pressure on the defect density of singlewalled carbon nanotubes (SWCNTs) grown by catalytic chemical vapor deposition was studied for several catalyst-precursor couples. The SWCNT defect density was assessed by studying the Raman D band. In situ Raman monitoring was used to determine experimental conditions allowing the preparation of samples free of pyrolytic carbon and not altered by air exposure. The most striking feature is that the Arrhenius plots of the IG/ID ratio systematically display a convex shape, i.e. the apparent activation energy decreases with increasing temperature. From HRTEM observations and oxidation experiments, this evolution of the D band features is ascribed to the catalytic growth of long SWCNTs with few defects at high temperature and of short and defective SWCNTs and carbon structures at low temperature. The convex Arrhenius behavior is well accounted by two kinetic models: (i) a model considering a change of intermediate states as a function of the temperature (for instance due to a phase transition of the catalyst particle or a change of intermediate carbon species) and (ii) a model considering a high-temperature process of defect creation (for instance by reaction with reactive gas species).

résumé

Présentation des objectifs et des premier résultats de la thèse

Catalytic chemical vapor deposition (CCVD) is a major method for the synthesis of carbon nanotubes (CNTs) and nanowires. Despite considerable progress in the control of the carbon nanotube length, orientation and structure, the fundamental processes involved in the nanotube nucleation, growth and termination still remain poorly understood. Rationalization is made difficult because CCVD combines the difficulties of different fields (heterogeneous catalysis, crystal growth, thin film deposition). The complexity of the problem especially arises when one considers the different chemical and physical processes involved, operating both on surface and in gas-phase, intricate at different time and spatial scales (molecular, nano, macro). Developing realistic growth models clearly requires the combination of theoretical and simulation works with reliable in situ data. We chose to investigate the growth of SWNT using in situ Raman measurements because SWNT provide an intense, specific and structure-dependent Raman signature. We initiated a systematic study of the growth conditions for different catalyst-precursor couples. From our observations, at least two conditions are required for nucleating SWNT: the catalyst metal particle must be in a reduced state and they must reach a critical carbon concentration. Once reduced, catalyst particles quickly restructure, most noticeably at high temperatures. Being in kinetic competition with SWNT growth, catalyst restructuring reduces the lifetime and the yield of SWNT growth and shifts the SWNT distribution toward larger diameters. In addition to SWNT, we evidenced the by-formation of disordered carbon nanostructures (DCN) by Raman and HRTEM. Their relative proportion is dependent on the kinetic equilibrium between the supply of carbon atoms from the gas precursor and their rearrangement at the surface of the catalyst particle. These processes also influence the kinetics of SWNT growth and its deactivation due to catalyst encapsulation by DCN. By studying the defect-induced D band of each structure (SWNT and DCN) as a function of the growth temperature, activation energies for the healing of defects can be extracted (figure 1). Radial Breathing Modes (RBM) can also be exploited to monitor the distribution of SWNT diameter. In addition to the catalyst preparation, we observed that the growth conditions (temperature and precursor pressure) significantly influence the diameter distribution (figure 2). Most noticeably, the growth of small diameter nanotubes is promoted in a narrow domain of the parameters of growth. In situ Raman experiments were performed to identify the processes influencing the diameter distribution. References [1] M. Picher, E. Anglaret, V. Jourdain, Diamond, 19 (2010), pp. 581-585. [2] M. Picher, R. Arenal, V. Jourdain, E. Anglaret, submitted. [3] M. Picher, E. Anglaret, R. Arenal, V. Jourdain, Nano Letters, 9 (2009), pp. 542–547. [4] M. Picher, E. Anglaret, R. Arenal, V. Jourdain, submitted.