Natural and synthetic carbonaceous materials exist in different forms, including diamond, graphite, graphene, nanotubes, fullerenes, diamondoids, amorphous carbon, polymers, and many others. The use of these materials derives from their unique properties (e.g., mechanical, electrical, and optical), which are in part due to the ability of carbon to hybridize in multiple bonding states (i.e., sp³, sp², and sp), and bind to many other elements, such as hydrogen. Owing to the strong dependency of the aforementioned properties on the carbon local bonding configuration and, especially, the carbon hybridization state, the characterization of the structure of carbon-based materials is of paramount importance for establishing processing-structure-properties relationships for these materials. While several techniques have been employed to characterize the amount of sp² and sp³ carbon in these materials, direct comparisons between analytical results are limited. Here, we provide the first, direct comparison between electron energy loss spectra (EELS) and scanning transmission X-ray microscopy/near edge X-ray absorption fine structure (STXM/NEXAFS) spectra acquired on the same thin lamella made of an alloyed version of amorphous carbon, namely silicon- and oxygen-containing hydrogenated amorphous carbon (a-C:H:Si:O). While the fractions of sp² carbon computed from STXM/NEXAFS and EELS spectra are in close agreement, the comparison of NEXAFS spectra acquired in STXM mode with NEXAFS spectra collected in partial electron yield mode on a flat a-C:H:Si:O surface indicated that the destructive preparation of thin lamellae for STXM analyses induces variations in the structure of a-C:H:Si:O, namely the breakage of carbon-silicon and carbon-hydrogen bonds, a change in ordering of sp²-bonded carbon, and an increase in the sp² carbon fraction. These findings can help scientists in the careful interpretation of spectroscopic results obtained from the analysis of samples made of metastable materials after the destructive preparation of specimens for analytical purposes.

Matthew Marcus

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