Revisiting the EELS analyses and its coupling with multi‐wavelength Raman spectroscopy: the case of hydrogenated amorphous carbon thin films
Authors: Lajaunie Luc , Pardanaud Cédric , Martin Céline , Pascal Puech , Hu Cheng , Biggs Mark James , Arenal Raul 
Published: December 2016

Thanks to the long‐term stability of their properties, hydrogenated amorphous carbon (a:C‐H) thin films are very promising materials for numerous applications including coatings for spatial applications.
1
In order to improve their performances, a full understanding of their local chemistry is highly required. Fifteen years ago, according to the seminal work of Ferrari
et al.
,
2
EELS was the most used technique to get such kind of quantitative information on these materials. Nowadays the complexity of the physics phenomena behind EELS is well known
3
and this technique is regarded as time‐consuming and difficult to interpret properly. Other optical techniques such as Raman spectroscopy are now clearly favored by the scientific community. However they still lack of the spatial resolution that EELS in a STEM offers for getting direct chemical information.

a‐C:H thin films, with a thickness around 300 nm, were deposited on a Si wafer and submitted to isothermal annealing at 500°C with different annealing times up to 2500 minutes. The hydrogen content was monitored by multi‐wavelength (MW) Raman using a set of reference materials. To determine the sp
2
fraction (sp
2
%) from core‐loss EELS, the
R
ratio (

R = I
π*(ΔE)
/I(
π*(ΔE)

+
σ*(ΔE)

) was determined first by taking into account the asymmetry of the π* character (
Fig. 1a
).
4,5
This value was then normalized by the maximum
R
value (R
REF
) that could be obtained from a HOPG sample in the same experimental condition using relativistic calculations (
Fig. 1b
).
6
When needed, this method was also slightly modified to take into account the contribution of heterospecies. In addition, the mass density and the oxygen content was derived from low‐loss and core‐loss spectra, respectively.

The EELS C‐K edge spectra (
Fig. 2a)
present all a typical signature of amorphous carbons. However, the intensity of the massif above 292 eV differs from sample to sample and clearly highlights a slight variation of the sp
2
%. The samples annealed 2500 min also presents a supplementary peak (red arrow in
Fig. 2a
), which is related to the oxidation of the thin film. As expected, the sp
2
% increases with the annealing time (
Fig. 2b
). This effect is related to the H desorption of the thin films as monitored by Raman spectroscopy. Two samples do not follow this trend: the as‐deposited sample and the sample annealed 2500 minutes. This latter presents a strong oxidation, leading to a decrease of the sp
2
%. On the other hand, the as‐deposited sample shows variation of the C‐K edge fine structures (
Fig. 3a
) highlighting chemical inhomogneities in the thin film. This sample presents a strong gradient of the sp
2
% induced by the deposition process (
Fig. 3b
) which is cured with the annealing time.

All these results will be detailed together with the influence of the oxidation on the chemical and physical properties. In addition, the coupling of MW Raman, infrared and EELS spectroscopies to extract a wealth of chemical information will be discussed. Our results provide a complete combination of C‐hybridization, spatial elemental analyses and structural defects studies for shedding light on these complex materials.
7,8