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| Figure 1. Graphical summary of the average Zn 2p3/2 binding energies, along with their standard deviations, as reported in the published literature (Table 1). Note that this representation does not convey any information regarding peak widths. Figure adapted from reference [1]. |
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| Table 1. Average Zn 2p3/2 binding energies, Zn L3M4,5M4,5 kinetic energies, and corresponding standard deviations for several zinc species obtained from published literature. The number of references considered is also indicated (#). Table reproduced from [1]. |
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| Figure 2. Zn 2p3/2 core-level spectra for various zinc compounds considered in this study. For reference, vertical lines indicating the approximate binding energy of metallic zinc are overlaid in each panel. [1] |
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| Table 2. Summary of experimentally measured Zn 2p3/2 binding energies, their FWHM, as well as their respective standard deviations. The approximate line shapes according to the LA and DS line shapes in CasaXPS are indicated. Please note that FWHM values reported here were made according to the LA line shape; only subtle variations occur when switching to the DS line shape (typically < 0.1 eV). [1] |
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| Figure 3. Wagner (chemical state) plot of Zn 2p3/2–Zn L3M4,5M4,5 transitions for the zinc compounds examined in this study. Data points from this work are shown as solid symbols, while selected literature values are represented as outlined symbols. Where available, literature averages are shown with corresponding standard deviations. [1] |
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| Table 3. Summary of experimentally measured Zn L3M4,5M4,5 kinetic energies, modified Auger parameter (α’), as well as their respective standard deviations. [1] |
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| Table 4. Zn L3M4,5M4,5 curve fitting parameters. Each component is characterised by the same FWHM value, given in the final column. All components described here use the GL(30) line shape as described in CasaXPS. [1] |
Magnesium 2p binding energy and modified Auger parameter values are shown in Table 1. It is worth noting that accounting for the difference between charge referencing procedures is vital for correct analysis of magnesium compounds (especially for MgO and Mg(OH)2) [1]. For MgO, the peak position for the Mg 2p transition is 49.4 eV when charge referenced to adventitious carbon at 284.8 eV, and 50.8 when referenced to the Mg 2p metal peak at 49.73 eV (or grounded). This has led to a lot of confusion regarding the Mg 2p peak positions to use for analysis. The results from a consistently analyzed dataset are shown in Table 1 together with compiled literature values [1]. Since the chemical sensitivity of the Mg 2p transition is low for magnesium the Auger parameter and anion signals are particularly important to consider for improved speciation.
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| Table 1: Experimental and literature values for the Mg 2p binding energy and the modified Auger parameter for different magnesium compounds. Subscripts and superscripts: “sc” denotes single crystal; “g” denotes grounded and no charge correction was performed; “Mg” denotes floated and the Mg 2p metal peak was used as a charge reference at 49.73 eV; “O 1s” denotes floated and the O 1s lattice peak of MgO or Mg(OH)2 was used as a charge reference at 531.1 eV and 532.7 eV, respectively. Samples with no superscript were floated, and adventitious carbon was used as a charge reference at 284.8 eV. |
The anion fitting parameters—binding energy, peak width, and line shape—obtained from fitting reference samples are reported in Table 2 from a consistently analyzed data set [1]. The O 1s signal can be used to separate MgO and Mg(OH)2 despite their overlapping Mg 2p signals. The O 1s peak position of Mg(OH)2 is +1.6 eV with respect to the main lattice peak of MgO. However, the peak position for Mg(OH)2 can overlap with other environments such as the MgO defective oxide and adventitious carbon, which may need to be accounted for in the analysis.
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| Table 2: Overview of anion fitting parameters, binding energy, peak width, and line shape, from reference data. Subscripts and superscripts: “sc” denotes single crystal; “g” denotes grounded and no charge correction was performed; “Mg” denotes floated and the Mg 2p metal peak was used as a charge reference at 49.73 eV; “ref” denotes reference values when charge referencing to the O 1s lattice peak. Samples with no superscript were floated, and adventitious carbon was used as a charge reference at 284.8 eV. |
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| Figure 1. Survey spectrum of Mg(OH)2 powder where the O 1s ghost peak is clearly visible at 765.7 eV. The figure is reproduced from reference [1]. |
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| XPS detection limits using Al K(alpha) radiation [1]. |
The seminal work of Graham Beamson and Dave Briggs in their "High Resolution XPS of Organic Polymers – The Scienta ESCA300 Database" has been utilized since 1992 as an invaluable resource for the XPS analysis of polymers and organic materials. A summary of carbon 1s binding energies for organic functional groups from this work are presented here. The original work calibrates the binding energy scale to 285.0 eV for aliphatic carbon C 1s. The values presented here are now calibrated to 284.8 eV for aliphatic carbon, in line with recent results [2].
Table 1. Summary of the mean, maximum, and minimum carbon 1s binding energies for different organic functionalities according to the work of Beamson and Briggs [1]. Binding energy calibration presented here have been adjusted to the main aliphatic C 1s peak at 284.8 eV.
The effects of various functional groups on beta carbon binding energies can be significant (Table 2). Note that, in this context, the alpha carbon is the carbon directly attached to the functional group, and the beta carbon is attached to the alpha carbon. These effects have been included in the refinement of the binding energy value for the aliphatic carbon component in adventitious carbon [2].
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| Cs 3d5/2 binding energy values. |
Note: Cs 3d5/2 - 3d3/2 splitting is 13.94 eV
Cs 3s: 1219 eV
Cs 3p3/2: 1002 eV
Cs 3p1/2: 1069 eV
Cs 4s: 234 eV
Cs 4p3/2: 161 eV
Cs 4p1/2: 173 eV
Cs 4d5/2: 77 eV
Cs 4d3/2: 80 eV
Cs 5s: 25 eV
Cs M5N45N45: 931 eV (Al)
Cs M4N45N45: 918 eV (Al)
Analysing indium and indium-based compounds using X-ray Photoelectron Spectroscopy is challenging due to only slight shifts in the binding energies of the commonly used In 3d5/2 core line. A recent paper shares a comprehensive set of reference data for indium and its compounds, covering the In 3d, 3p, and 4d core lines, the In MNN Auger signal, as well as relevant counter ion signals [1]. Valuable tools, such as the modified Auger parameter and chemical state (Wagner) plots, which aid in differentiating indium species are also discussed.
Figure 1 and Table 1 present average literature values for the In 3d5/2 core line, highlighting both the average and standard deviations. These values illustrate the apparent challenge in distinguishing between various indium compounds. Factors like natural line widths, line shapes, and potential errors in charge correction add further complexity to accurate speciation.
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| Figure 1. Average In 3d5/2 literature values for indium compounds. |
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| Table 1. Average In 3d5/2 literature values for indium compounds. |
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| Figure 2 In 3d spectra from [1]. |
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| Table 2. Experimental In 3d5/2 values from [1]. |
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| Figure 3. In MNN spectra for various indium compounds [1]. For reference, vertical lines indicating the kinetic energy (MNN) for metallic indium have been overlaid in each tile. Note that the additional signal present for InPO4 at 414.7 eV was due to Na contamination. |
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| Table 3. In M4N4,5N4,5 and modified Auger parameter values [1]. |
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| Table 4. In MNN Auger peak fitting parameters [1]. |
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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. |
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| Adventitious carbon C 1s curve-fitting parameters [1]. |
Materials of a graphitic nature (e.g., graphite, graphene, carbon nanotubes etc.) will have a C 1s main peak, attributed to C=C, which can be used as a charge reference set to 284.5 eV. An average of values for graphite from 21 references from the NIST database [1] is 284.46 eV with a standard deviation of 0.14 eV. Note that the well characterized value of 284.5 eV for graphitic carbon is also a strong indicator that this value is not appropriate as a value to use for AdC charge referencing. While these types of samples are generally conductive and if they can be mounted in a manor (in electrical contact with the sample stage) to take advantage of this one should do so. However, many of these types of samples come as a small volume of powders or flakes which are very difficult to mount. Usually, we mount these on a double-sided adhesive which works well but electrically isolates the sample. Oxidation of these types of samples (e.g., graphene oxide) or their functionalization (e.g., functionalized CNTs) can result in them behaving less conductively or as a mixed conductive/insulating material. Samples where these materials are mixed with other conducting or insulating compounds can also result in a mixed conductive/insulating sample. For most of these types of samples we now electrically isolate the sample and charge reference to C 1s at 284.5 eV for the graphitic (C=C) peak.[2]
Table 1 from [2] presents general fitting parameters for graphitic, graphene and carbon nanotube type materials. These starting fitting parameters include the main peak asymmetry (defined using an asymmetric Lorentzian (LA) line shape) and π to π* shake-up satellite from a pure graphite standard sample. These fitting parameters are similar to the approach taken by Morgan (Fig. 5, Table 2) [3], Moeini et al. (Table 1) [4], and Gengenbach et al.[5] It is always best to run your own standard (pure graphite, graphene, CNT etc.) to get fitting parameters appropriate for your sample type, instrument and conditions used. Slight differences in the main peak asymmetry and differing shake-up satellite position, shape and intensities are possible for differing classes of graphitic materials. See for example from Morgan[3] where HOPG and nano-onion C 1s spectra show peak-shape differences, likely due to hydrogenation of the sample. However, with this caveat stated, the parameters used based on a graphite standard have worked very well for variety of samples (134) analyzed in the five-year data survey from [2]. Figure 1(A) presents the standard graphite spectrum used to obtain the parameters presented in Table 1. The spectra from Figure 1(B, C and D) show the use of these fitting parameters from Table 1 to effectively model a variety of graphitic component containing materials.
Our recent article [1] in Applied Surface Science Advances highlights methodology developed as a result of years of interactions between many junior and senior X-ray Photoelectron Spectroscopy (XPS) users operating within the CasaXPS spectral processing and interpretation program framework. In particular, discussions arising from a series of workshops have been a significant source for developing the overall XPS data processing concept and are the motivation for creating this work. These workshops organized by the Institut des Matériaux Jean Rouxel (IMN), Nantes gather both experienced and novice users of XPS for a week of discourse in conceptual experiment design and the resulting data processing. However, the framework constructed and utilized within these workshops encouraged the dissemination of knowledge beyond XPS data analysis and emphasized the importance of a multi-disciplinary collaborative approach to surface analysis problem-solving. The material presented here embodies data treatment originating from data made available to the first CNRS Thematic Workshop presented at Roscoff 2013. The methodology described here has evolved over the subsequent workshops in 2016 and 2019 and currently represents the philosophy used in CasaXPS spectral data processing paradigm.
This article also serves as a useful reference descriptor of the CasaXPS software program.
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