TITLE:
Alignment of Vertically Grown Carbon Nanostructures Studied by X-Ray Absorption Spectroscopy
AUTHORS:
Jeannot Mane Mane, François Le Normand, Rolant Eba Medjo, Costel Sorin Cojocaru, Ovidiu Ersen, Antoine Senger, Carine Laffon, Bridinette Thiodjio Sendja, César Mbane Biouele, Germain Hubert Ben-Bolie, Pierre Owono Ateba, Philippe Parent
KEYWORDS:
X-Ray Absorption Spectroscopy, Carbon Nanostructures (CNTs, CNFs, CNWs, CNPs), Plasma- and Hot Filaments-Enhanced Catalytic Chemical Vapor Deposition, Geometrical Model, Angular Dependence
JOURNAL NAME:
Materials Sciences and Applications,
Vol.5 No.13,
November
28,
2014
ABSTRACT: 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.