Chemically specific identification of carbon in XPS imaging using Multivariate Auger Feature Imaging (MAFI)

Until now, a difficult prospect in XPS imaging has been the identification of similar chemical states of carbon. With the advent of novel nano-carbons such as nanotubes and graphene, the ability to easily and unambiguously identify materials of varying sp2/sp3 nature in XPS spectra and images is becoming increasingly important. We present herein methods for the identification of such species in XPS images by shifting focus from the traditionally analysed C1s region to the X-ray induced carbon Auger feature. By extracting the D-Parameter from XPS data, we have generated what we refer to as "D-Parameter Images", that clearly identify regions of different carbon hybridisation in an image of a graphite flake mounted on carbon tape, and areas of reduced graphene oxide (GO) in a laser-scribed GO film. This method is then enhanced by multivariate analysis, a technique we call "Multivariate Auger Feature Imaging", where the distinction between varying sp2 carbon content on a surface is improved

STUDY OF THE SP2-TO-SP3 RATIO OF DUAL-ION-BEAM SPUTTERED HYDROGENATED AMORPHOUS-CARBON FILMS

Hydrogenated amorphous carbon (a-C:H) thin films have been deposited with a dual-ion-beam sputtering system. The first argon ion beam impinges on the pure pyrolitic graphite target and the second (composed of an ion mixing of argon and hydrogen) directly on the growing film. The sp2 percentages of the films have been deduced from X-ray induced Auger electron spectra. Increasing the hydrogen ion beam energy causes the sp2 percentage to increase whereas on the contrary the Knoop hardness decreases. Moreover a rise in a-C:H hydrogen content causes an increase in the number of sp3 sites

The graphite Core-Valence-Valence Auger Spectrum

The graphite core–valence–valence (CVV) Auger spectrum is analysed, within a one particle approximation, by comparison to the X-ray excited valence band (VB) photoemission spectrum, whose structure is well understood in terms of the graphite density of states (DOS). The aim is to identify the electron states originating the components into which the CVV spectrum is resolved by double differentiation. Contributions assigned to self-folds of VB photoemission features and hence due to transitions coupling electrons from the same DOS feature, dominate the spectrum away from the middle region, while folds between different features are important in the middle region. A single particle approximation proves to be adequate to account for the low binding energy, mostly p-like, part of the spectrum. It brakes down however in the high binding energy, mainly s-like, region where spectral distortion is ascribed to a nonnegligible (2 eV) Coulomb repulsion between final state holes of s-symmetr