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) sub strates 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.
Carbon nanotubes (CNTs) have attracted an enormous interest since their first report by Iijima in 1991 [
The different steps of the substrate treatments and CNT growth are recalled in Ref. [
The samples were prepared by deposition of a SiO2 layer (thickness 8 nm) by a Double Electron Cyclotron Resonance (DECR) plasma process on a Si(100) sample (Sb n-doped with
As well, the CNTs growth method using a direct current plasma and hot filaments-enhanced catalytic chemical vapor deposition (DC HF CCVD) process has been fully described elsewhere [
To stop the CNTs growth, the acetylene feedthrough, the polarisation, the filaments and finally the hydrogen feedthrough were subsequently switched off. The references as well as the main characteristics of the sample preparation are displayed in
TEM observations were performed on a TOPCON 002B microscope operating at 200 kV. The samples were scratched with a diamond tip and the material was directly pulled onto an amorphous carbon membrane drilled with holes for direct observations. SEM observations were performed on an XL30S-FEG PHILIPPS working at 3 kV. The nature of the carbon deposit was probed by Raman spectroscopy on a Renishaw apparatus with a He-Ne light source. More structural and spectroscopic data are reported in [
C K-edge measurements were performed at the Laboratoire pour l’Utilisation du Rayonnement Electromagnétique (LURE, ORSAY, France) on the VUV Super-Aco storage ring. They were carried out on the SACEMOR beam line [
Sample | Catalyst | TM Deposition Process S: sputtering; E: evaporation | TM/Si | Pf (W) | Pe (mW) | Pressure (mbars) | T (K) | Nanostructure |
---|---|---|---|---|---|---|---|---|
I Nanot 24 | Co | S | 150 | 10 | 15 | 973 | CNFs with grapheme//substrate | |
II Nanot 29 | Co | S | 150 | 30 | 15 | 973 | CNTs (poorly oriented) | |
III Nanot 30 | Co | E | 0.33 | 150 | 30 | 15 | 973 | CNFs with graphene^substrate |
IV Nanot 31 | Co | E | 0.87 | 150 | 30 | 15 | 973 | CNTs |
V Nanot 36 | Co | E | 100 | 20 | 15 | 973 | CNPs | |
VI Nanot 42 | Co | E | 145 | 20 | 15 | 1083 | CNTs (highly oriented) | |
VII FLN1 | Co | E | 140 | 20 | 15 | 973 | CNTs (medium oriented) | |
VIII FLN2 | Co-Fe | E | 140 | 20 | 15 | 973 | CNTs (highly oriented) | |
IX FLN4 | Co | E | 140 | 20 | 5 | 973 | CNWs |
Samples | Nanostructure | Outer diameter (nm) | Inner diameter (nm) | Length (nm) | Density (mm−2) |
---|---|---|---|---|---|
I | CNFs with grapheme//substrate | 25 | 0 | 110/ | 472 |
II | CNTs (poorly oriented) | ||||
III | CNFs with graphene^substrate | 20 | 0 | 140/ | 494 |
IV | CNTs | 30 | 9 | 375/ | 400 |
V | CNPs | ||||
VI | CNTs (highly oriented) | 25 | 5 | 400/ | 349 |
VII | CNTs (small oriented) | <100 | |||
VIII | CNTs (highly oriented) | 10 | 4 | 187/ | 1000 |
IX | CNWs |
of the line does not exceed 1% of the total signal and can be neglected for materials with a high carbon concentration. The spectra were recorded in the total-electron-yield detection (TEY) and partial electron yield (PEY), the last being expected to be less surface-sensitive. Experiments were carried out in two experimental configurations according to the angle α between the sample and incidence of the light: at normal incidence (α ≈ 0˚ with electric field vector E parallel to the surface,
The spectra were first corrected for the background by substracting it on the preedge low energy side with a linear background contribution. Then the spectra were normalized with regard to the preedge intensity on one side
As the SEM and TEM images clearly illustrate in
catalyst preparation (amount of cartalyst deposited measured by the surface ratio Co/Si, mode of Co deposition, and growth conditions (temperature, plasma power and hot filaments power, pressure)) reported in
These are graphene sheets that merge in the direction normal to the surface (
When the plasma power is high and the catalyst surface concentration is low, then graphene sheets grow in a direction normal to the surface (
They provide hydrogen radicals that are very reactive towards all kinds of amorphous carbon. This is checked in Raman spectra (
To ascertain the reliability of the analysis of CNTs XAS spectra, the XAS spectra of HOPG sample is first recorded. Graphite, with its layered structure and large interlayer separation, is often modelled as a two-dimen- sional solid. In addition, the knowledge of the properties of graphite is a starting point for understanding the structure and properties of many new carbon nanostructures like nanotubes. The two-dimensional nature of graphite results in a strong directionality of the orbitals:
graphite [
In this energy range the contributions of
The small intensity of transition A observed may be explained by incomplete polarisation or by a small sample misalignment. Two parameters
We examine now different carbon nanostructures at GI and NI incidences, respectively.
Feature | GI | NI | Literature | Ref. | Assignation |
---|---|---|---|---|---|
A | 285.5 | 285.4 | 285.5 | 29 | p0 near Q |
A’ | 286.6 | 287.8 | 286.5 | 22, 23, 27, 30, 36, 43 | Free Electron-Like Interlayer States + p*C = C-OH or -(C = C-p*C = O)- |
A” | 288.4 | 288.4 | 288.4 | 22, 23, 27, 36, 43 | p*C=O |
A’” | 288.8 | 288.7 | 289.3 | 22, 23, 27, 28, 32, 43 | s*C-H + -(HO-p*C = O) |
A”” | 290.7 | 290.6 | 290.7 | 27, 28 | s*C-O |
C-H | 291.8 | 291.8 | 291.8 | 23, 24, 30, 43 | Exciton |
B | 292.5 | 292.6 | 292.8 | 29, 30 | s1, s2: G à Q + dipole-allowed |
C + K | 295.5 | 293.8 | 293.8 | 29 | p0 or p1 near G + sC*-O(H) |
D | 297.8 | 297.5 | 297.7 | 29 | s3 - s6: QàP |
E + L | 303.5 | 302.7 | 302.6 | 29 | s7 near Q |
F | 307.5 | 307.9 | 307.2 | 29 | s9 near Q |
G | 308.5 | 311.4 | 311.4 | 29 | s10 near Q |
H | 316.5 | 315.8 | 314.8 | 29 | p4 near Q |
The effect of a thermal treatment can be dramatic on the shape of the carbon K edge absorption spectra. This is illustrated in
After a high vacuum thermal treatment at 500˚C for 3 hours, an absorption spectrum closely resembling to the absorption spectrum of graphite is recorded. Thus two conclusions can be derived from this study: i) XAS absorption spectra are a very sensitive and localized probe of the adsorption on carbon nanostructures and ii) it is required to degas the samples preliminary to a true study of the XAS transitions in carbon nanostructures.
It must be noted that the degassing conditions might not be the same for each carbon nanostructures. Thus it was found (not shown) that the nanostructures that display surface sites not only of the basal plane but also primatic sites, like the graphene arranged in platelet or herringbone in samples I and III, require higher treatment temperatures.
Sample | S// | S^ | H// | H^ | ICNS(GI)/IG(GI) | ICNS(NI)/IG(GI) | Degree of orientation R |
---|---|---|---|---|---|---|---|
HOPG graphite | 0.93 | 0.08 | 3.78 | 0.15 | 1 | 0.04 | −2 |
I | 1.02 | 0.92 | 1.8 | 1.55 | 0.48 | 0.41 | −0,23 |
II | 0.96 | 0.86 | 2.7 | 2.2 | 0.53 | 0.71 | −0.23 |
III | 0.88 | 0.97 | 1.4 | 1.5 | 0.37 0.50 | 0.40 0.54 | 0. 21 |
IV | 0.94 | 1.18 | 1.35 | 1.75 | 0.36 | 0.46 | 0.56 |
V | 1.12 | 0.99 | 2.05 | 1.9 | 0.53 | 0.48 | −0,30 |
VI | 0.64 | 0.9 | 1.3 | 1.7 | 0.34 | 0.45 | 0.61 |
VII | 0.78 | 0.73 | 1.95 | 2 | 0.51 | 0.53 | 0.12 |
VIII | 0.66 | 0.98 | 1.5 | 2.15 | 0.40 | 0.57 | 0.75 |
IX | 0.73 | 0.72 | 1.40 | 1.40 | 0.37 | 0.37 | −0.02 |
From shape analysis of the spectrum the same contributions in the three regions described above can be observed. The main difference comes from a general broadening of the contributions, except the molecular states, that smears out the spectra. This is in agreement with previously reported XAS studies [
Taking into consideration a random orientation of the carbon nanostructures, the XAS spectra would not be dependent on the incident light and the intensity of the feature A may be compared with that of HOPG measured at the magic angle 54.7˚ where no polarization dependence of
whereas the intensity at NI
Then we define a factor of alignment
The absorption intensities are rated by reference to the absorption intensity in HOPG graphite
And more generally for a nanostructure
In the limiting case where,
Here the factor 2 takes into consideration that the
Values of R calculated from Equation (4) are reported in
We have performed a quantitative C K-edge XAS study of the orientation of oriented carbon nanostructures (nanotubes, nanofibers, nanoparticles, nanowall with different orientation of the graphene sheets or shells can be prepared). They are grown on plain SiO2 (8 nm thick)/Si(100) substrates by a Plasma and Hot Filaments-en- hanced Catalytic Chemical Vapor Deposition (PE HF CCVD) process. Using the highly oriented pyrolytic graphite (HOPG) as a starting point model for the understanding of the CNTs properties, we have first recorded and analyzed the HOPG XAS spectra. While applying the C K-edge XAS to the CNTs orientation characterization, we find spectral features very similar to those of HOPG, in agreement with the literature. The XAS spectra are highly sensitive to a previous thermal treatment, as an intense adsorption on the outer wall of the nanotubes may strongly affect the absorption transitions. The morphology and the nature of these carbon nanostructures are characterized by SEM, TEM and Raman spectroscopy. 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
More attention must be deserved to the contribution of capped carbon in addition to carbon sidewalls constituting the major part of these carbon nanostructures.
P. Legagneux (Thales R§D, Orsay) is acknowledged for providing the SiO2/Si(100) samples; M. Accosta and G. Schmerber for sample preparation by sputtering. One of the authors is indebted for a fund from the French Ministry of Foreign Affairs and another one from the AUEF.
Full calculation of the angular dependence of the absorption intensity in the case of carbon nanotubes and carbon nanofibers grown on a flat surface.
The absorption transition
where
where
Let us consider the more general case of carbon nanofibers where the graphitic basal planes are oriented with a conical polar angle
Finally the light impinges the surface at an incidence angle a relative to the
whereas the transition to
which in the case of a linearly polarized light with
and the same as A4 for
In the case of HOPG graphite or graphite planes in the nanostructure parallel to the surface