Carbon 1s X-ray Photoemission Line Shape Analysis of Highly Oriented Pyrolytic Graphite: The Influence of Structural Damage on Peak Asymmetry De-Quan Yang and Edward Sacher* Regroupement Que ´ be ´ cois de Mate ´ riaux de Pointe, De ´ partement de Ge ´ nie Physique, E Ä cole Polytechnique, C.P. 6079, succursale Centre-Ville, Montre ´ al, Que ´ bec H3C 3A7, Canada ReceiVed October 31, 2005. In Final Form: December 13, 2005 C 1s XPS spectra of various highly oriented pyrolytic graphite (HOPG) surfaces, untreated, as well as those treated by keV Ar + beam bombardment and low-energy O 2 ,N 2 , Ar, and H 2 O plasmas, have been systematically studied by comparing two XPS peak-fitting procedures. These procedures treat the spectrum as either (1) the overlap of several symmetric component peaks or (2) a single asymmetric peak. The results indicate that, in the case of HOPG, the asymmetry parameter defining the single peak is directly related to the extent of damage to the alternant hydrocarbon structure of the HOPG surface, as manifested by its correlation with the symmetric peak component due to the damaged HOPG structure. The C 1s XPS line shape in graphites, carbon nanotubes, and vitreous carbons is highly asymmetric. 1-6 We have shown for both HOPG 7 and carbon nanotubes 8 that this asymmetry is due to the presence of definable, quantifiable symmetric component peaks. These peaks are attributed as follows: C1 (284.6 eV, undamaged alternant hydrocarbon structure), C2 (285.6 eV, damaged alternant hydrocarbon structure; the binding energy shift is due to the change in localized electronic states caused by the damage), C3 (286.5 eV, sp 3 free radical defects), C4 (287.8 eV, π* r π shake-up of C2), and C5 (291.4 eV, π* r π shake-up of C1). Some researchers have attributed the C2 component to sp 3 -hybridized carbon, 9 although the evidence 10 after Ar ion bombardment supports our attribution. Detailed support for our attributions may be found in our papers 7,8 as well as in a recent study by others. 10a This deconvolution into component peaks is tedious and time- consuming, especially during surface modification during prolonged treatment. For this and other reasons, many authors treat the C 1s spectrum as a single, asymmetric peak and establish its asymmetry using the Doniach-S ˇ unjic ´ equation, 11 where the photoemission line-shape intensity, I, is Here, Γ is the Gamma function, ǫ is the energy relative to that at the peak height of the unbroadened line, γ is the lifetime width of the hole, and R is the Anderson asymmetry parameter, which is given as where δ i is the Fermi-level phase shift of the lth partial wave. Because the calculation of R from eq 2 is formidable, the practice has been to turn the problem around, and R is evaluated from the asymmetry. Despite the fact that this equation represents the application of the Mahan singularity 12,13 to XPS, which clearly does not hold here, its general form is valid for all asymmetric peak shapes no matter what their origin. Experimentally, R values for graphites have been reported from 0.05 to 0.19; some are found in Table 1. To be able to compare our own data to those of workers who chose to consider the C 1s spectrum to be a single asymmetric peak, we explore here the use of simplified types of Doniach- S ˇ unjic ´ equations to evaluate the R asymmetry parameters of HOPG spectra on which we have already carried out C1-C5 peak separations. We show that because the asymmetry is essentially due to the C2 peak (the other peaks amount to a few percent of the total spectrum) a comparison between the asymmetry indices, obtained from these equations, and the symmetric C2 intensities is meaningful. Advanced Ceramics Inc. type ZYA grade HOPG, 1 × 1 cm 2 , was used for our analyses. XPS was performed in an ESCALab * Corresponding author. E-mail: edward.sacher@polymtl.ca. Tel: (514)- 340-4711, ext. 4858. Fax: (514)340-3218. (1) van Attekum, P. M. Th. M.; Wertheim, G. K. Phys. ReV. Lett. 1979, 43, 1896. (2) Cheung, T. T. P. J. Appl. Phys. 1982, 53, 6857. (3) Cheung, T. T. P. J. Appl. Phys. 1984, 55, 1388. (4) Sette, F.; Wertheim, G. K.; Ma, Y.; Meigs, G.; Modesti, S.; Chen, C. T. Phys. ReV.B 1990, 41, 9766. (5) Smith, R. A.; Armstrong, C. W.; Smith, G. C.; Weightman, P. Phys. ReV. B 2002, 66, 245409. (6) Prince, K. C.; Ulrych, I.; Peloi, M.; Ressel, B.; Cha ´b, V.; Crotti, C.; Comicioli, C. Phys. ReV.B 2000, 62, 6866. (7) Yang, D.-Q.; Sacher, E. Surf. Sci. 2002, 504, 125. (8) Yang, D.-Q.; Rochette, J.-F.; Sacher, E. Langmuir 2005, 21, 8539. (9) (a) Jackson, S. T.; Nuzzo, R. G. Appl. Surf. Sci. 1995, 90, 195. (b) Merel, P.; Tabbal, T.; Chaker, M.; Moisa, M.; Margot, J. Appl. Surf. Sci. 1998, 136, 105. (c) Diaz, J.; Paolicelli, G.; Ferrer, S.; Comin, F. Phys. ReV.B 1998, 54, 8064. (d) Haerle, R.; Riedo, E.; Pasquarello, A.; Baldereschi, A. Phys. ReV.B 2001, 65, 045101. (10) (a) Speranza, G.; Laidani, N. Diamond Relat. Mater. 2004, 13, 451. (b) Takahiro, K.; Terai, A.; Oizumi, S.; Kawatsura, K.; Yamamoto, S.; Naramoto, H. Nucl. Instrum. Methods Phys. Res., Sect. B 2006, 242, 445. (11) Doniach, S.; S ˇ unjic ´, M. J. Phys. C: Solid State Phys. 1970, 3, 285. (12) Mahan, G. D. Phys. ReV. 1967, 163, 612. (13) Nozie `res, P.; de Domenicis, C. T. Phys. ReV. 1969, 178, 1097. I(ǫ) ) Γ(1 -R) (ǫ 2 + γ 2 ) (1 -R)/2 [ cos πR 2 + (1 -R) tan -1 ǫ γ ] (1) Table 1. Reported C 1s r Asymmetry Indices R material reference 0.14 HOPG, ZYB 1 0.065 HOPG 3 0.056 natural graphite 5 0.048 HOPG 13 0.125 HOPG, ZYA 4 0.15 graphite 2 0.19 catalytic carbon, 2 activated carbon, thermal carbon 0.092 HOPG 14 0.19 Ar + -sputtered HOPG 14 R) 2 ∑ l (2l + 1) [ δ l π ] 2 (2) 860 Langmuir 2006, 22, 860-862 10.1021/la052922r CCC: $33.50 © 2006 American Chemical Society Published on Web 12/30/2005