Bulk and surface plasmon excitations in amorphous carbon measured by core-level photoelectron spectroscopy

Bulk and surface plasmon excitations in amorphous carbon (a-C) films have been characterized by core-level loss spectroscopy. Atomically smooth a-C surfaces were used in their as-grown state, after UHV annealing and after covalent immobilization of dense molecular monolayers (2-4 × 1014 cm−2), either perfluorinated or labelled with an ester functionality. X-ray photoelectron spectra reveal a sp3-rich hybridization of surface C atoms, with a σ + π plasmon loss distribution centred at 29.5 ± 1 eV, characteristic of a high electron density value. For molecular grafted surfaces, the energy distribution of plasmon losses reveals new contributions in the range 15-25 eV (clearly separated from the energy distribution of the bulk σ + π plasmon loss of a-C) with an increasing loss probability observed at grazing photoemission angles. A simple parameterization method is presented to derive bulk and surface plasmon loss distributions from angular core level loss spectroscopy (XPS) data, without a priori assumptions on the shape of the loss energy distributions.