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

The characterization of the local bonding configuration of carbon in carbon-based materials is of paramount importance since the properties of such materials strongly depend on the distribution of carbon hybridization states, the local ordering, and the degree of hydrogenation. Carbon 1s near edge X-ray absorption fine structure (NEXAFS) spectroscopy is one of the most powerful techniques for gaining insights into the bonding configuration of near-surface carbon atoms. The common methodology for quantitatively evaluating the carbon hybridization state using C 1s NEXAFS measurements, which is based on the analysis of the sample of interest and of a highly ordered pyrolytic graphite (HOPG) reference sample, was reviewed and critically assessed, noting that inconsistencies are found in the literature in applying this method. A theoretical rationale for the specific experimental conditions to be used for the acquisition of HOPG reference spectra is presented together with the potential sources of uncertainty and errors in the correctly computed fraction of sp(2)-bonded carbon. This provides a specific method for analyzing the distribution of carbon hybridization state using NEXAFS spectroscopy. As an illustrative example, a hydrogenated amorphous carbon film was analyzed using this method, and showed good agreement with X-ray photoelectron spectroscopy (which is surface sensitive). Furthermore, the results were consistent with analysis from Raman spectroscopy (which is not surface sensitive), indicating the absence of a structurally different near-surface region in this particular thin film material. The present work can assist surface scientists in the analysis of NEXAFS spectra for the accurate characterization of the structure of carbon-based materials

Tribological Response of Silicon Oxide-Containing Hydrogenated Amorphous Carbon, Probed across Lengthscales

This work examines the structure and properties of silicon-oxide containing hydrogenated amorphous carbon (a-C:H:Si:O) thin films, and how the structure and properties are responsible for the fundamental tribological response of the material. The films are studied through a range of spectroscopic techniques, focused on the surface-sensitive X-ray photoelectron and near-edge X-ray absorption fine structure spectroscopies. The tribological response is studied at several lengthscales: using macroscale ball-on-flat tribometry, at the nanoscale with sharp diamond-like carbon-coated AFM probes, and at the microscale with steel colloids affixed to AFM cantilevers. The spectroscopic study reveals that the films contain a high fraction of SiOx which leads to a structure rich in sp3 carbon-carbon bonding that affords strong protection against oxidative attack at the elevated temperatures in aerobic environments, which is important for demanding applications. At the macroscale, low friction coefficients are achieved upon the formation of an inherently lubricious, soft and polymeric tribofilm whose composition and structure depends heavily on the sliding environment, while the lubriciousness of the resulting tribofilm does not depend on the environment in which it was formed. Nanoscale experiments demonstrate that the shear strength of a sharp, single asperity contact sliding on a-C:H:Si:O is at least an order of magnitude higher than those estimated from macroscale sliding, raising questions about whether the surface passivation theory of DLC lubricity is sufficient to explain macroscale lubricity. Colloidal AFM experiments show, in situ, that low friction is achieved with the growth of the tribofilm via a combination of reduced adhesion and a precipitous drop in the shear strength, which offset a simultaneous increase in the real area of contact. The compilation of results suggests a model of lubrication which relies on both surface passivation of the counterfaces and the soft and viscoelastic properties of the tribofilm, which reduce the effect on friction of nanoasperity pinning

Interaction of C₆₀ with Tungsten: Modulation of Morphology and Electronic Structure on the Molecular Length Scale

The evolution of morphology and electronic structure in sequential depositions of W and C₆₀ on graphite has been studied by scanning tunneling microscopy/spectroscopy. The deposition sequence decisively controls morphology expression. W deposited on a graphite surface forms small clusters whose morphology is consistent with the predictions of a liquid droplet model in the size regime below 5 nm in diameter; these small clusters then agglomerate without sintering. These agglomerates are immobilized by the subsequent C₆₀ deposition. C₆₀ shows very little interaction with the W-cluster agglomerates, and the formation of typical close packed fullerene islands is observed. The inverse deposition sequence, W deposition on the surface of C₆₀ multilayer islands, leads simultaneously to the formation of ultrasmall W clusters (<i>d</i> < 2 nm) due to limited mobility on the highly corrugated surface, and the intercalation of W in the C₆₀ matrix. The signature of intercalation is cessation of molecule rotation, which is recognized by imaging of molecular orbitals. The electronic structure of C₆₀ is not significantly modified by the presence of W agglomerates, clusters, and intercalation of W. However, if W is deposited on a single layer of C₆₀ its impact on the electronic structure is considerable and expressed in a compression of the band gap, which might be attributable to charge screening due to image charges, or the onset of molecule breakdown. The morphology as well as the electronic structure of this layer is highly inhomogeneous and can be described as a composite of W and C₆₀ due to accumulation of W at the graphite substrate–C₆₀ interface

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

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 sp²-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 sp²-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

Thermally Induced Structural Evolution of Silicon- and Oxygen-Containing Hydrogenated Amorphous Carbon: A Combined Spectroscopic and Molecular Dynamics Simulation Investigation

Silicon- and oxygen-containing hydrogenated amorphous carbon (a-C:H:Si:O) coatings are amorphous thin-film materials composed of hydrogenated amorphous carbon (a-C:H), doped with silicon and oxygen. Compared to a-C:H, a-C:H:Si:O exhibits much lower susceptibility to oxidative degradation and higher thermal stability, making a-C:H:Si:O attractive for many applications. However, the physical mechanisms for this improved behavior are not understood. Here, the thermally induced structural evolution of a-C:H:Si:O was investigated in situ by X-ray photoelectron and absorption spectroscopy, as well as molecular dynamics (MD) simulations. The spectroscopy results indicate that upon high vacuum annealing, two thermally activated processes with a Gaussian distribution of activation energies with mean value <i>E</i> and standard deviation σ take place in a-C:H:Si:O: (a) ordering and clustering of sp² carbon (<i>E</i> ± σ = 0.22 ± 0.08 eV) and (b) conversion of sp³- to sp²-bonded carbon (<i>E</i> ± σ = 3.0 ± 1.1 eV). The experimental results are in qualitative agreement with the outcomes of MD simulations performed using a ReaxFF potential. The MD simulations also indicate that the higher thermal stability of a-C:H:Si:O compared to a-C:H (with similar fraction of sp²-bonded carbon and hydrogen content) derives from the significantly lower fraction of strained carbon–carbon sp³ bonds in a-C:H:Si:O compared to a-C:H, which are more likely to break at elevated temperatures