The Real Carbon K-edge

For many years, electron energy-loss (EEL) spectroscopy has been used to analyse the near-edge structure of the carbon ionization K-edge. Amorphous carbon has perhaps one of the most well known carbon ionization edges, with the spectrum featuring a large, broad σ* peak at an energy of ~290–300 eV, and a small, but sharp, π* peak at ~286 eV (Fig. 1a). Recently, the advent of scanning transmission X-ray microscopy (STXM) has given the technique of near-edge X-ray fine structure (NEXAFS) spectroscopy a spatial resolution that can be measured in tens of nanometers. This has created the possibility of a direct comparison of the carbon ionization edge in TEM-EELS with that from NEXAFS for nanoscale amorphous carbon, as shown in Fig. 1. There is a clear discrepancy between the two spectra. Rather than the sharp π* peak at 286 eV of the EEL spectrum, NEXAFS shows a number of smaller peaks from 285–288 eV, indicative of various functional groups. The conclusion can be drawn that NEXAFS gives the “real” spectrum, while the edge structure of the EEL spectrum actually results from radiation damage of the amorphous carbon [1, 2]. Indeed, some results suggest that, to achieve an EEL spectrum from amorphous carbon without radiation damage, the effective spatial resolution of the measurement would be reduced to the level of micrometers [3]. We believe that this renders STXM an inherent advantage over EELS for ionization-edge structure analysis of beam-sensitive carbonaceous materials. STXM’s ability to discriminate functional groups such as aliphatics, quinones, carboxylics, and alcohols is simply unparalleled. Nevertheless, compared to TEM, STXM has many drawbacks, primarily that of poor spatial resolution (see Fig. 2). In our studies of sub-μm carbonaceous aerosol particles, such as soot and amorphous organic carbons, this means missing out on important morphological and structural information. Therefore, we have developed a correlative approach, in which we use STXM to investigate the carbon ionization edge of individual particles, and then analyse the morphology of the exact-same particles with TEM. EDX in the TEM also identifies trace elements, such as Si and S, which provide important clues as to the origin of the aerosol particles. Using this correlative approach, we can identify the type, size, morphology, composition and chemistry of the particles, detailing internal structures and assessing functional groups (Fig. 3). Further use of this methodology can significantly increase our understanding of the origins, nature, and interactions of atmospheric aerosols.