X-ray photoelectron spectroscopy and near-edge X-ray-absorption fine structure of C 60 polymer films

Abstract

We report core-level and valence-band X-ray photoelectron spectroscopy (XPS) and carbon K near-edge X-ray-absorption fine structure spectroscopy (NEXAFS) results of plasma-polymerized C 60 . In comparison with evaporated C 60 the C 1s peak is broader and asymmetric for the C 60 polymer and its shake-up satellites diminished. Furthermore, the features of the valence-band as well as the features of the π * antibonding orbitals of the C 60 polymer are broader and reduced in intensity. Changes in the electronic structure are attributed to the polymerization of C 60 , the post-plasma functionalization of the surface by oxygen after exposure to atmosphere, and the occurrence of amorphous carbon. PACS: 73.61.Wp; 68.35.-p; 82.35.+t The fullerene C 60 can be polymerized by different routes including: photo-polymerization [1], hydrostatic pressure [2], charge transfer from alkali-metal dopants to the C 60 molecules [3], plasma [4], and electrochemical process [5].A [2 + 2] cycloaddition via electronically excited triplet states is considered to be the most likely pathway of the polymerization in the photopolymer [1] and subsequent highpressure-high-temperature-(HPHT) induced polymer [6]. The [2 + 2] cycloaddition reaction results in the formation of a cyclobutane ring requiring a sp 2 − sp 3 rehybridization at the 6 − 6 double bonds of C 60 , which form the intermolecular C−C bonds. The formation of the intermolecular bonds leads to a reduced molecular symmetry.In this study, C 60 was polymerized using the plasma deposition technique. The application of radio-frequency (rf) plasma has increased greatly in materials and device processing. However, the physics of the rf plasma and the chemical mechanisms of deposition are not understood in whole, which makes a controlled deposition difficult.Very little is known explicitly about the structure of plasma-polymerized C 60 . In the plasma, various interactions between the vaporized fullerenes and the plasma particles (energetic neutrals, ions, electrons) and photons as well as the already-deposited material occur. The effects of these interactions are dependent on the geometry and the process parameters. Besides the polymerization reaction, undesirable processes such as fullerene fragmentation, substrate heating, re-emission and sputtering of the deposited material, and process gas incorporation have to be considered.Various experimental techniques have been used to characterize the structure of fullerene polymers. In particular, Raman spectroscopy has been widely used [7]. Near-edge X-ray-absorption fine structure spectroscopy (NEXAFS) has proven to be especially useful in examining the electronic structure of carbon materials because it gives a deep insight into the π * and σ * antibonding orbitals and distinguishes between sp 2 and sp 3 bonding with similar sensitivity [8,9].We performed core-level and valence-band X-ray photoelectron spectroscopy (XPS) and C K near-edge X-rayabsorption fine structure spectroscopy to investigate the polymerization and to characterize the chan...

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