Four major phases were identified throughout the device cross-sections by EELS bond analyses (Figure (right), E, Figure ). Highly crystalline, graphitized carbon (red, no N) constitutes the transducer layer whereas the sensor layer consists of three N-containing phases, which are a shell phase (yellow) and two less transformed core phases I/II (green/blue) of the same composition but in different bond states . The detailed discussion is given in the supporting information (SI, pp. S8-S11). In conclusion, very high degrees of sp 2 hybridization of both elements in all phases are confirmed with an increase in the early stage of the transformation from core phase II to I (Figure , blue → green). While the spectral data is governed by C=C bonds in the graphitized carbon and predominantly C=N bonds in both core phases with high N content, the dilution of N renders the shell phase a transition state (spectral superposition) between both extremes with both, C=C and C=N bonds present (Figure ). Partial electron transfer from C to N causes the spectral shift by 1.5 eV. The sensor functionality is particularly attributed to those sp 2 -bound N species observed within the shell phase. To distinguish between different pyrrolic/imidazolic/graphitic nitrogen functionalities on the nanometer or even atomic scale is, however, challenging due to the bulkcharacteristics of the spectra with manifold contributions from different atomic configurations.