A thin layer of carbonaceous material is usually found on the surface of most air exposed samples, this layer is generally known as adventitious carbon. Even small exposures to atmosphere can produce these films. Adventitious carbon is generally comprised of a variety of (relatively short chain [1]) hydrocarbons species with small amounts of both singly and doubly bound oxygen functionality. The source of this carbon has been debated over the years. It does not appear to be graphitic in nature and in most modern high vacuum systems vacuum oils are not readily present (as they have been in the past) [1,2,3,4]. There may be some evidence that CO or CO2 species may play a role in the gradual appearance of carbon on pristine surfaces within the vacuum of the XPS chamber [3].
It’s presence on insulating surfaces provides for a convenient charge reference by setting the main line of the C 1s spectrum to 284.8 eV (although values ranging from 285.0 eV to 284.5 eV have been used in some cases, remember to check for this value when looking for binding energy references in the literature). The error in this value (284.8 eV) is, for most systems, on the order of +/-0.2 eV to 0.3 eV. An in-depth look at the effectiveness of using AdC for charge correction purposes, including standardized fitting procedures, is presented in [5].
Work by Grey et al. [6] has explored the nature of adventitious carbon by XPS and time-of-flight secondary ion mass spectrometry (ToF-SIMS). XPS D-parameter and ToF-SIMS analyses confirms that AdC is not graphitic in nature. An average C 1s spectrum for AdC (Figure 1, Table 1) was derived and shows that, on average, ~ 25 % of the carbon species in AdC is directly associated with oxygen functionality. Similarly, ToF-SIMS analyses show that AdC is comprised of mainly short chain hydrocarbons with some oxygen functionality.
An advanced method for curve-fitting of the C 1s envelope for AdC (Table 2) was developed that included the effects of beta carbons (in this context, the alpha carbon is the carbon directly attached to the oxygen, and the beta carbon is attached to the alpha carbon) and were informed by the configurations of possible volatile organic compounds (VOC) that are the source of most AdC [6]. Using this method in combination with the dataset from [5], the average C–C/C–H AdC aliphatic peak position was shown to be 284.81 eV (+/- 0.25 eV) via verification with a secondary internal reference.
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| Figure 1. Average of 80 adventitious carbon C 1s XPS spectra. |
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| Table 1. Average adventitious carbon C 1s fitting parameters from an average of 80 AdC spectra. |
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| Table 2. Curve-fitting parameters for AdC C 1s including shifted beta peaks (*) (peaks E, F and G). Areas for peaks A, B, C, and D should be left unconstrained. # If peak-shape for peak D is well-defined the FWHM constraint can be removed. |
References:
[1] T.L. Barr, S. Seal,
J. Vac. Sci. Technol. A 13(3) (1995) 1239.
