Accounting for Nanometer-Thick Adventitious Carbon
Contamination in X‑ray Absorption Spectra of Carbon-Based Materials
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Abstract
Near-edge
X-ray absorption fine structure (NEXAFS) spectroscopy
is a powerful technique for characterizing the composition and bonding
state of nanoscale materials and the top few nanometers of bulk and
thin film specimens. When coupled with imaging methods like photoemission
electron microscopy, it enables chemical imaging of materials with
nanometer-scale lateral spatial resolution. However, analysis of NEXAFS
spectra is often performed under the assumption of structural and
compositional homogeneity within the nanometer-scale depth probed
by this technique. This assumption can introduce large errors when
analyzing the vast majority of solid surfaces due to the presence
of complex surface and near-surface structures such as oxides and
contamination layers. An analytical methodology is presented for removing
the contribution of these nanoscale overlayers from NEXAFS spectra
of two-layered systems to provide a corrected photoabsorption spectrum
of the substrate. This method relies on the subtraction of the NEXAFS
spectrum of the overlayer adsorbed on a reference surface from the
spectrum of the two-layer system under investigation, where the thickness
of the overlayer is independently determined by X-ray photoelectron
spectroscopy (XPS). This approach is applied to NEXAFS data acquired
for one of the most challenging cases: air-exposed hard carbon-based
materials with adventitious carbon contamination from ambient exposure.
The contribution of the adventitious carbon was removed from the as-acquired
spectra of ultrananocrystalline diamond (UNCD) and hydrogenated amorphous
carbon (a-C:H) to determine the intrinsic photoabsorption NEXAFS spectra
of these materials. The method alters the calculated fraction of sp<sup>2</sup>-hybridized carbon from 5 to 20% and reveals that the adventitious
contamination can be described as a layer containing carbon and oxygen
([O]/[C] = 0.11 ± 0.02) with a thickness of 0.6 ± 0.2 nm
and a fraction of sp<sup>2</sup>-bonded carbon of 0.19 ± 0.03.
This method can be generally applied to the characterization of surfaces
and interfaces in several research fields and technological applications