Author(s)

Jeannot Mane Mane^1,2, François Le Normand¹, Rolant Eba Medjo^3,4, Costel Sorin Cojocaru¹, Ovidiu Ersen¹, Antoine Senger¹, Carine Laffon⁵, Bridinette Thiodjio Sendja^2,3, César Mbane Biouele³, Germain Hubert Ben-Bolie³, Pierre Owono Ateba³, Philippe Parent⁵

¹IPCMS, UMR 7504 CNRS, Strasbourg, France.
²Ecole Nationale Supérieure Polytechnique (National Advanced School of Engineering), Department of Mathematics and Physical Sciences, The University of Yaoundé I, Yaounde, Cameroon.
³Physics Department, Faculty of Science, University of Yaoundé I, Yaounde, Cameroon.
⁴Physics Department, Faculty of Science, University of Douala, Douala, Cameroon.
⁵LURE, UMR CNRS, Centre Universitaire Paris Sud, Orsay, France.

X-Ray Absorption Spectroscopy (XAS) on the carbon K edge of carbon nanostructures (nanotubes, nanofibers, nanowalls) is reported here. They are grown on plain SiO2 (8 nm thick)/Si(100) substrates by a Plasma and Hot Filaments-enhanced Catalytic Chemical Vapor Deposition (PE HF CCVD) process. The morphology and the nature of these carbon nanostructures are characterized by SEM, TEM and Raman spectroscopy. According to conditions of catalyst preparation and DC HF CCVD process, carbon nanotubes (CNTs), carbon nanofibers (CNFs), carbon nanowalls (CNWs), carbon nanoparticles (CNPs) with different orientation of the graphene plans or shells can be prepared. From the angular dependence of the incident light and geometrical morphology of the nanostructures, wide variations of the C K-edge intensity of the transitions to the empty π^* and σ^* states occur. A full lineshape analysis of the XAS spectra has been carried out using a home-made software, allowing estimating the relative proportion of π^* and σ^* transitions. A geometrical model of the angular dependence with the incidence angle of the light and the morphology of the carbon nanostructures is derived. With normalization to the HOPG (Highly Oriented Pyrolytic Graphite graphite) reference case, a degree of alignment can be extracted which is representative of the localized orientation of the graphitic carbon π bonds, accounting not only for the overall orientation, but also for local defects like impurities incorporation, structural defects ... This degree of alignment shows good agreement with SEM observations. Thus CNTs films display degrees of alignment around 50%, depending on the occurrence of defects in the course of the growth, whereas no special alignment can be detected with CNFs and CNPs, and a weak one (about 20%) is detected on CNWs.

X-Ray Absorption Spectroscopy, Carbon Nanostructures (CNTs, CNFs, CNWs, CNPs), Plasma- and Hot Filaments-Enhanced Catalytic Chemical Vapor Deposition, Geometrical Model, Angular Dependence

Mane, J. , Le Normand, F. , Medjo, R. , Cojocaru, C. , Ersen, O. , Senger, A. , Laffon, C. , Sendja, B. , Biouele, C. , Ben-Bolie, G. , Ateba, P. and Parent, P. (2014) Alignment of Vertically Grown Carbon Nanostructures Studied by X-Ray Absorption Spectroscopy. Materials Sciences and Applications, 5, 966-983. doi: 10.4236/msa.2014.513098.