Chemical bond mapping of carbon by image-spectrum EELS in the second derivative mode

Electron Energy-Loss Near Edge Structure (ELNES) mapping has been performed by using an energy-filtering TEM and the recently developed image-spectrum acquisition technique. The performance of the method in terms of spatial and energy resolution was evaluated by using a Zeiss 902 EFTEM and a specific software for this application. The experimental model was calcite material (CaCO3) deposited on a pure carbon film. This specimen contains two chemical bondings for carbon atoms, namely the CC and the CO bondings. Selected-area EELS spectra of the carbon K-edges showing the characteristic 1sπ∗ transitions could be extracted from 8 nm diameter zones from series of energy-filtered images. The predicted change in the EELS spectrum of sp2 hybridised carbon K-edge due to a chemical shift is clearly visible. The mapping of the 1sπ∗ (CC) molecular transition (peaking at 285 eV) was clearly distinguished from the 1sσ∗ (CO) one at 299 eV. For mapping the 1sπ∗ (CO) at 291 eV as well as the 1ss∗ (CC) molecular transitions, a second derivative method was found to remove the background more correctly with the advantage to reduce the number of images (and therefore the dose). In the state of our knowledge, this represents the first molecular orbital mapping by EFTEM with a 256 × 256 image size where two chemical bondings of an element can be separated.