Correlative Core-Loss and Vibrational EELS for Robust Nanoscale Bonding Quantification in Carbon-Based Materials

Understanding and quantifying bonding configurations in carbon-based materials is essential because hybridization directly governs their mechanical hardness, electrical conductivity, optical response, thermal transport, and chemical stability. The striking property differences between graphite and diamond arise fundamentally from their distinct sp² and sp³ bonding configurations. In technologically relevant carbon systems—including amorphous carbon (a-C), tetrahedral amorphous carbon (ta-C), diamond-like carbon (DLC), and various carbon nanostructures—the relative fraction and spatial distribution of sp² and sp³ bonding critically determine functional performance in applications ranging from protective coatings to electronics and energy storage. Consequently, reliable nanoscale quantification of carbon hybridization has long been a central objective in carbon-based materials characterization. Electron energy-loss spectroscopy (EELS) performed in scanning transmission electron microscopy (STEM) has emerged as a powerful tool for probing bonding configurations in carbon-based materials. In particular, analysis of the carbon K-edge energy-loss near-edge structure (ELNES) provides direct insight into the unoccupied density of states and local electronic structure [1,2]. The π* and σ* features at the C–K edge serve as fingerprints of sp² and sp³ bonding, respectively. It has been demonstrated that the π* peak intensity scales with sp² content and can be quantified through integration methods following appropriate background subtraction and plural-scattering corrections. Over time, these methodologies have been refined to incorporate orientation effects, multiple scattering considerations, and calibration strategies based on graphite and diamond standards [3,4]. Despite these advances, challenges remain to robust and internally consistent hybridization quantification, such as crystallographic anisotropy in graphite. Vibrational EELS has emerged as a complementary technique capable of distinguishing sp² and sp³ stretching modes, based on phononic excitations rather than purely electronic transitions. In this work, we establish a cross-validated experimental framework for determining sp² and sp³ hybridization fractions using both core-loss and vibrational STEM-EELS. Graphite and diamond are employed as well-defined reference standards representing nearly pure sp² and sp³ bonding, respectively. First, core-loss C–K edge spectra are acquired and analyzed to quantify π*/σ* intensity ratios, providing the electronic fingerprints of bonding. These calibrated standards are then applied to determine sp²/sp³ fractions in amorphous carbon and diamond-like carbon. Subsequently, on-axis vibrational EELS measurements are used to resolve phononic contributions associated with various stretching modes. Reference vibrational spectra from graphite and diamond are employed to fit spectra from amorphous carbon, enabling direct comparison between electronic and phononic bonding signatures. By correlating core-loss and vibrational EELS analyses, this study demonstrates a comprehensive approach for robust nanoscale quantification of carbon hybridization. The combined methodology enhances confidence in sp²/sp³ determination while providing deeper insight into the relationship between electronic structure and lattice vibrations in carbon-based materials.