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

Abstract

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.