Energy-loss near-edge structure changes with bond length in carbon systems

We show that when the graphene planes of graphite are uniformly expanded, thereby increasing the C - C bond length to 1.7 A, the sigma(*) edge onset of the energy-loss near-edge structure (ELNES) spectrum shifts to lower energies by almost 5 eV, meanwhile the pi(*) edge shifts by less than 0.2 eV. The shift of the sigma(*) edge demonstrates that for bond lengths which are typical of some carbon systems such as amorphous carbon, it is possible to find sigma(*) features in the ELNES spectra at energies as low as 286-288 eV. Calculations on 64-atom amorphous carbon (a-C) and amorphous carbon nitride model structures characterized by a wide range of bond lengths confirm this. Most of the sp(2)/sp(3) quantification techniques that are available overlook this issue of sigma(*) contamination of the pi(*) region and assume that all features within this energy range are entirely of pi(*) origin. We show that the effect of bond length variation on the pi(*) spectrum of graphite and a-C is minor, thereby supporting the reliability of the former spectrum for sp(2)/sp(3) quantification purposes, as was recently demonstrated [see J. T. Titantah and D. Lamoen, Phys. Rev. B 70, 075115 (2004)].