Accounting for Nanometer-Thick Adventitious Carbon Contamination in X-ray Absorption Spectra of Carbon-Based Materials

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 nanometerscale 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 photo-absorption 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 2 -hybridized carbon from 5 to 20%, and reveals that the adventitious contamination can be

Quantitative Evaluation of the Carbon Hybridization State by Near Edge X-ray Absorption Fine Structure Spectroscopy

“…A detailed description of the experimental procedures for acquiring and processing the NEXAFS data is reported in Ref. [21] and in the Supporting Information.…”

“…Since no theory exists to predict the π*/σ* ratio, a reference material with known sp 2 A graphical representation of the methodology is shown in Figure S.1. Since the C1s→π* transition usually appears in NEXAFS spectra of a-C materials as a distinct spectral feature at around 285.0 eV, its area can be determined by fitting it with a Gaussian synthetic peak 21,47 .…”

“…As for the C1s →σ* absorption feature, its area is usually determined by numerically integrating the spectrum between two limits x 1 and x 2 , which are chosen to represent the σ* contribution to the experimental data. A survey of the published literature revealed that a wide range of integration limit values (x 1 -x 2 ) has been used for computing the area of the C1s →σ* transition: 289-295 eV 67 , 294-301 eV 33,34 , 293-302 eV 30 , 288.6-325 eV 47 , 289-325 eV 46 , and 288.6-320 eV 21 . A quantitative assessment of the influence of the integration limits on the computed fraction of sp 2 -bonded carbon is presented in Section II.…”

“…As a reference, the spectrum of freshly-cleaved HOPG (Figure S.1) sample is commonly employed 21,30,47,52 . One key set of challenges in the use of HOPG as reference material derives from the strong dependence of the π*/σ* ratio on the orientation of the basal planes with respect to the incoming X-ray beam 58 .…”

“…The characterization of the bonding configuration of carbon is usually carried out by XPS through the acquisition and fitting of the C1s spectrum [16][17][18][19][20][26][27][28] . However, the validity of the methodology for the quantitative evaluation of the hybridization state of carbon on the basis of the C1s signal, which relies on fitting it with two distinct features for sp 2 -and sp 3 -bonded carbon 16,26,27 , has been refuted 17,18 , since the binding energy values of the C1s transition for graphite (100% sp 2 -bonded carbon), diamond (100% sp 3 -bonded carbon), and ultrananocrystalline diamond (94±1% sp 3 -bonded carbon 21 ) are not significantly different 17,18 . Instead, insights into the carbon hybridization state in the near-surface region of a-C materials can be gained by XPS through the analysis of the plasmon band near the C1s signal 18,24 , the π-π* shake-up satellites 42,43 , or the X-ray induced C KVV Auger spectrum 17,18,22,23,25,28 .…”