Peak-Fitting Parameters & BE Calibrations
Used to Peak-fit Chemical Compound Spectra in
The International Spectra-Base of Monochromatic XPS Reference Spectra
Philosophy for Peak-fitting
Our philosophy is to collect spectra under analysis conditions that are practical, readily reproduced, and typically used in laboratories that use monochromatic X-ray sources and work under real world practical analysis conditions. We have assumed that the most XPS laboratories need practical reference spectra and will not spend the time or money to produce and to analyze pure, clean surfaces under ultimate energy resolution conditions. For practical reasons we used the C (1s) spectra from the naturally formed layer of adventitious hydrocarbons because that signal is the “de facto” standard for charge referencing insulating materials.
Peak-Fitting Parameters
Used to Peak-fit Chemical State Spectra in
The International XPS Database of XPS Spectra
After “Data Processing and Charge Shifting” all chemical state spectra, then you must choose the Peak-fitting Parameters for your spectra
The words highlighted using Dark-Red Bold lettering are the methods used on most of the spectra in The International XPS Database of XPS Spectra. The other information is a guide for alternative methods or special methods to help the XPS analyst peak-fit chemical state spectra.
- Start. Preview Spectrum Baseline Region
- Vertically expand all spectra to get a look at the onset upward curving region of the peak at high BE
- This helps us to locate a valid flat baseline region at the low BE end and reveals small peaks at high BE end
- Background Type (BG)
- Linear is preferred (almost flat) for nearly all insulators and a few conductors
- Shirley and Sherwood were developed to be used on conductive materials, and are often used on insulators for convenience, without scientific basis
- Shirley and Sherwood (Proctor) are used when the baseline above the peaks is definitely higher than the lower baseline for conductors
- Shirley was originally designed for use on the Valence Band spectrum of Pure Gold. There is no basis for other uses.
- In various cases neither of these background types will give a best result
- The Tougaard background is not used on any spectra in this database.
- There are other backgrounds that “actively” change the background and the peak-fit to produce a well define result expected by the analyst.
- Background Endpoints and Endpoint Averaging
- Vertically expand the spectrum to look at the baseline
- Vertically expand the spectrum after adding the background to make sure baseline endpoints are in a nearly “flat” or linear region of the baseline
- The low BE endpoint, where the baseline is flat, is usually easy to select visually.
- Use 3-5 data-points (0.2-0.5 eV spread) to make an average baseline endpoint.
- Avoid making any endpoints on the upward edge of any peak
- The high BE endpoint is more difficult to select because of shake-up and satellite peaks. Avoid peaks. Look for a nearly flat region.
- For most correct fit, you should include all shake-up and satellite peaks which can make the peak-fit very challenging to optimize
- Number of Expected Peaks – Always use Minimum (more than 5 for one spin-orbit is probably wrong)
- Use Minimum number of peaks based on obvious shoulders, obvious asymmetry, and peak maxima.
- For C (1s) there are usually 4 peaks
- For O (1s) there are usually 2-3 peaks
- metal oxide peaks range from 529 to 531 eV
- organic oxide peaks range from 531 to 533 eV
- hydroxides and carbonates range from 532 to 533 eV
- adsorbed water in the form of hydroxide groups appear in the 532 to 533 eV range
- For metal peaks, there are often 2 peaks for a “1s” or “2s” peak. One peak dominates, the other is small and is due to degradation or contamination.
- FWHM ranges from 1.2-1.6 eV for most peaks.
- For some conductive compounds the FWHM ranges from 1.0-1.2 eV
- For O (1s) peaks from hydroxides and sulfates the FWHM can range from 1.6 to 1.8 eV
- Full Width at Half of Maximum (Peak-width) – FWHM – based on FWHM tables
- FWHM are expected to be vary similar in any peak-fit for any chemical compound.
- FWHM for individual peaks can be different within a peak-fit by no more than 20% of main peak FWHM
- FWHM for a spectrum that has both a conductor peak and insulator peaks can vary greatly – see next lines of guidance
- FWHM ranges from 1.2-1.6 eV for most peaks.
- Metal FWHM range = 0.4 to 1.0 eV (usually 0.6-0.8 eV)
- Metal Oxide FWHM range = 1.1 to 2.0 eV (usually 1.3-1.6 eV)
- Use same FWHM unless you have reference FWHM
- Gaussian-Lorentzian Peak-shape %s
- Typical G:L for Inorganics (80:20) This is the most common peak-shape
- Typical G:L for Polymers (90:10)
- G:L ratio can be 70:30 for 1s or 2s peaks at high BE >500 eV
- G:L ratio increases to 50:50 at higher BE
- In a few cases G:L was 100:0 for O (1s) or other “s” peaks
- Peak Asymmetry % (Doniach-Sunjic) for conductors and rarely insulators
- Peak asymmetry is seldom used in the XPSDATABASE spectra peak-fits.
- Added smaller peaks to replace asymmetry often gives better result. These peaks are attributed to Core-Valence interactions the same as Doniac-Sunjic.
- Never used on insulators unless using ultra high energy resolution on polymers
- Typically use 15% for conductive materials after initial fit. Often need to constrain FWHM to get best fit.
- Maximum 50% for high BE peaks of conductors
- CasaXPS and other specialty software provide asymmetric Lorentzian (LA) peak-shape that are intended to improve peak-fit on the high BE side of the main peak especially for conductive materials. This is a cosmetic fix. There is no theoretical basis for that shape at this time.
- Peak Area Ratios (Theoretical)
- for “p” orbitals: 2:1
- for “d” orbitals: 6:4
- for “f” orbitals: 8:6
- It is best to use the Scofield cross-sections for high precision peak-fitting of peak area ratios
- None of these peak-fits have used Scofield cross-section area ratios
- Peak Area Constraints for Peak Area Ratios or Empirical Chemistry Ratios
- Need to use whenever there are overlaps, contaminants or other oxidation states to reveal correct ratios and presence or absence of minor components
- BE Constraints (when correct BE is known)
- BEs are constrained to BEs from pure compounds or metals when there are overlaps
- FWHM Constraints (when FWHM control is essential)
- FWHM are constrained to stop peak-fit from increasing beyond 2.0 eV or decreasing to less than 0.4 eV for normal chemical state spectra
- Difference in BE Constraint based on Metal Spin-Orbit splitting (difference is known)
- Example: Si (2p3/2) is 0.60 eV lower than Si (2p1/2)
- Example: Cu (2p3/2) is 20.0 eV lower than Cu (2p1/2)
- Chi-Square that is acceptable
- <4 for low count rate data which is typical for old XPS instruments (SSI S-Probe)
- <15 for very high count rate data which is typical for new XPS instruments
- Decide if Differential Charging Tails are Present or Absent – Low BE side
- Peak-fit tail as a peak, and then delete that peak from peak-fit and label as charging
Peak-Fitting Example-Results
when applying different parameters
Choices for Baseline / Background Type and Endpoint Range
Choose FWHM for First Peak at Lowest BE
Use same FWHM for all peaks for first peak-fit
If one peak is much wider and symmetrical then use a FWHM that is 2X wider just for that peak (use constraint?)
In general, pure metal peaks are 2X more narrow than non-conductive chemical compound peaks (insulators).
Metal FWHM = 0.9 eV, Corresponding Metal Oxide FWHM = 1.7 eV
In general, the largest FWHM for Insulators is 1.8 eV. A few are slightly larger (eg 2.0 eV).
Examples of FWHM For Chemical Compounds and Insulators
Examples of FWHM For Pure Metals and Conductive Materials
Decide Total Number of Peaks in Spectrum – Use Minimum
Three (3) Peak-fitting Examples (A-C)
Difficulty Levels: 1-3
Example A: Level 1 – Peak-fitting of Single Chemical State Spectrum
Example B: Level 2 – Peak-fitting of Complicated Chemical State Spectrum
Example C: Level 3 – Peak-fitting Complicated Spectrum with Constraints