Features
 “Anatomy”Â
of
Raw Chemical State Spectra
Topic #1:Â Why do we collect Chemical State Spectra?
Topic #2:Â Features of a Raw Chemical State Spectrum
Topic #3:Â Two Types of Principle XPS Signals:Â Â Singlet and Doublet
Topic #4:Â Which XPS Signals are used to measure Chemical State Spectra?Â
Topic #5:Â Features of Peak-fitted Chemical State Spectra
Topic #6:Â Useful Reference Tables of BEs and FWHMs
Topic #1 – Why do we collect Chemical State Spectra ?
- A Chemical State Spectrum helps us to decide:
- If an element is present in one or more chemical states / chemical species / oxidation states
- The relative percentage of the different chemical states found by peak-fitting
- If our last experiment produced any changes to the chemical state spectra by comparing before and after spectra
- Survey Spectra are normally run before and after we collect Chemical State Spectra
- If chemical state degradation is suspected, then run a repeating series (10-20 spectra) of that same spectrum over 1 hours time.
- Can Chemical State Spectra of the C (1s) be used to Charge Reference or Charge Correct all High Resolution BEs?
- Yes and No
- The C (1s) BE of the hydrocarbon type of carbon found on all materials has commonly been used to correct for charging that occurs when an insulating sample is exposed to a low voltage beam of electrons from an electron flood gun. This C (1s) BE is defined by the user to occur somewhere between 284.6 eV and 285.2 eV, but there is NO standard or absolute value for the BE of hydrocarbons. In the literature we can find a range of C (1s) BEs. Recently, the more common C (1s) BE is 284.8 eV or 285.0 eV. Again, there is no standard.
- BEs from Conductive or Semi-conductive samples should not and do not need to be charge referenced or charge corrected even though the measured C (1s) BE of the hydrocarbon peak does not appear at 284.8 or 285.0 eV. The surface of various materials that have non-conductive native oxides and various complex semi-conductors produce a Surface Dipole Moment that causes the hydrocarbon C (1s) BE to shift. This is a natural phenomenon that needs to be studied.
- From Chemical State Spectra we can …
- Determine the thickness of one layer if we the peak-fit includes the pure element as one component
- Overlay the raw chemical state spectra and find very subtle changes in chemistry of a before and after pair of samples
- Using advanced software measure thicknesses of several very thin layers
- Potential Degradation of Surface Chemistry during XPS Analysis due to X-rays, Vacuum, or Electron Flood Gun
- If chemical state degradation is suspected, then run a repeating series (10-20 spectra) of that same spectrum over 1 hours time.
- Are High Energy Resolution Spectra the same as Chemical State Spectra? Yes.
- Core Hole Lifetimes are Measured from Peak FWHMs
- Core-hole lifetimes are measured in “eV” units for convenience. These lifetimes are very useful to theoreticians and very advanced measurements.
Topic #2 – Features of a Raw Chemical State Spectrum
Experimental Results from XPS Analyses are plotted on X and Y axes to form a Spectrum
- X-Axis (horizontal) is the Binding Energy “BE” (eV) of Photo-electrons Analyzed
- Y-Axis (vertical) is the total number of Photo-electrons Counted (Cts) at each data point (channel)
Raw Chemical State Spectrum
Raw Chemical State Spectrum – X and Y Values, and Axis Labels
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Typical Experimental Parameters used to Collect a Chemical State Spectrum
Basic Features of a Raw Chemical State Spectrum
Topic #2 – Features of a Raw Chemical State Spectrum
(A very simple example)
One (1) single symmetrical peak from a Principle XPS signal.
Basic Features of Every Chemical State Spectrum |
Simple Chemical State Spectrum – Element “X” | |
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Principle XPS Signal from Oxygen |
NOTEs:
- Element “X” is Oxygen.Â
- Based on well known BE tables, the principle Oxygen signal appears at ~532 eV due to the O (1s) type of photo-electrons. Â
- Because this single peak is symmetrical, this peak probably represents (contains) only one type of Oxygen.Â
- It is possible to add 2 more very small peaks with the same FWHM at the high and low BE ends of this peak, but there are no obvious shoulders that indicate that there might be more than just one single peak. For that reason we would probably use only one “synthetic” peak to fit this raw chemical state spectrum.
- “1s, 2s, 3s, and 4s” type photo-electrons do not have spin-orbit coupling so they can not appear as doublets.
- “s” type photo-electrons are “singlets” due to the absence of spin-orbit coupling.
- A chemical state spectrum is also known as a High (Energy) Resolution Spectrum, or a Narrow Scan.
- This spectrum is from a Principle (Main) Signal that forms a single peak because this type of photo-electron “1s” can NOT have spin-orbit coupling.
Features of a Raw Chemical State Spectra – Continued
Raw Chemical State Spectrum from Element “Y”
(A more complex example)
We see three (3) large peaks and one (1) small peak (total of 4 component-peaks)
Features not seen in above example
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NOTEs: |
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Topic #3:Â Two Types of Principle XPS Signals:
Singlet and Doublet
Raw Chemical State Spectra often have Spin-Orbit Coupling Doublets
These Examples shows Principle XPS Signals that are Split into 2 signals due to Spin-Orbit Coupling
Peak Shapes and Peak Area Ratios of “s”, “p”, “d”, and “f” Type XPS Signals
“s” type signal from electrons in “s” orbital NO spin-orbit splitting coupling is NOT possible a “Singlet” |
“p” type signals from electrons in “p” orbitals Split due to spin-orbit coupling 4:2 peak area ratio a “Doublet” |
“d” type signals from electrons in “d” orbitals Split due to spin-orbit coupling 6:4 peak area ratio a “Doublet” |
“f” type signals from electrons in “f” orbitals Split due to spin-orbit coupling 8:6 peak area ratio a “Doublet” |
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Examples of Raw Chemical State Spectra with Spin-Orbit Coupling Â
Topic #3:
Raw Chemical State Spectrum of pure Indium, “In” metal
that has Spin-Orbit Coupling
Features
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Another simple Chemical State Spectrum but this spectrum has spin-orbit coupled peaks. Most XPS Signals are spin-orbit coupled.
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Topic #3: Raw Chemical State Spectrum of pure Bismuth, “Bi” metal
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Features
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A simple Chemical State Spectrum that involves spin-orbit coupled peaks
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Topic #3:
Raw Chemical State Spectrum of Ionic Chlorine, “KCl”
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Features
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A “simple” Chemical State Spectrum that involves spin-orbit coupled peaks – Cl (2p3/2) and Cl (2p1/2)
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Topic #3:
Raw Chemical State Spectrum of Copper, Cu, metal
that has Spin-Orbit Coupling
Features
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A simple Chemical State Spectrum that has spin-orbit coupled peaks. Most XPS signals are spin-orbit coupled.
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Copper (Cu) Continued |
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Features
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A Very Complex chemical state spectrum from the Cu (2p) Signal of Copper (II) Oxide, CuO
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Topic #4 – Which XPS Signals are used to measure Chemical State Spectra?
and Why?
Essential BASIC Information about Chemical State Spectra
Before looking at any new type of spectral data, scientists need to ask a few basic questions and to visualize various key terms / phrases.
Question:Â Are the following “features” identical ?
Does “Signal” = “Peak” = “Spectrum” ?
Answer:Â No
Science Terminology
- Signal:Â an impulse, or an action that conveys data or information.
- Peak:Â the pointed top (of a mountain)
- Spectrum:Â a collection of colors, as seen in a rainbow, or a range of energies
- Principle: Â the term “principle” has the same meaning as “main”, i.e. Principle Signal = Main Signal
- Chemical State:Â the term “chemical state” has the same meaning as “chemical species”. Go to “Chemical State Definition” for explanation.
XPS Terminology
- Signal: the O (1s) type of photo-electrons is a characteristic type of Principal XPS Signal. The O (1s) signal appears at 532 eV. Each Principle XPS Signal has a single FWHM that can range from 0.3 to 3.0 eV in width. Other examples of Principle XPS Signals are:  C (1s), Si (2p), Fe (2p3), Au (4f7), U (3d5). A single XPS Signal can produce multiple XPS Peaks within a single XPS Spectrum. Those multiple component peaks can be due to the presence of different chemical states / chemical species. This is the major reason why many people use XPS. They use XPS to find different Chemical States on a surface.
- Peak: an XPS Peak is due to just one particular type (or kind) of a chemical material, a chemical species. An XPS Spectrum often has 2 or more XPS peaks. XPS Spectra can have 2 XPS Peaks that are separated by 0.5 eV or > 20 eV.
- Spectrum: a range of binding energies (20-100 eV for chemical state spectra) plotted against the count-rate (or total counts) of photo-electrons emitted is an XPS Spectrum. An XPS Spectrum is a range of BEs within which a single XPS Signal can produce one or more component XPS Peaks that represent one or more different Chemical States / Chemical Species. In special cases, there are overlaps of different XPS Signals which complicates interpretation.
- “s”, “p”, “d”, and “f”: are the different types of electron orbitals that are measured by XPS. The “shell” number of an atom and the “orbital” type of an atom form part of the abbreviation (designation) that is used to describe Principal XPS Signals.
- Shell Numbers: range from 1-6 for peaks observed when using Aluminum X-rays (E=1486.7 eV)
- Spin-Orbit Splitting (Coupling) Terms can be: 1/2, 3/2, 5/2, 7/2. These terms are shown as sub-scripts (1/2, 3/2, 5/2, 7/2) just after the electron orbital type (p3/2 or d5/2 or f5/2)
Examples of XPS Terminology
Signal | Peak | Spectrum | ||
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Topic #4
Which XPS Signals are used to measure Chemical State Spectra? – Continued
Question:Â Why are Principle XPS Signals used to collect Chemical State Spectra?
- The number of XPS Signals produced by an element varies from 2 to 15 when the X-ray source is Al K-alpha which has an Energy = 1486.7 eV. Light elements (low Z) produce 2-5 obvious XPS signals. Heavy elements (Z >55) produce 8-15 XPS signals. Which XPS Signal for an element is the best to use to measure Chemical States that element?
- Principle (Main) XPS Signals have the smallest (narrowest) FWHM (peak-width) so they are the best to use to resolve the presence or absence of different chemical states in a Chemical State Spectrum. All other XPS signals for that element have larger FWHM.
- Principle (Main) XPS Signals are usually the most intense XPS signal(s) for an element. This signal usually has the largest SF (RSF or ASF) value.
- Because the Principle (Main) Signal has the most narrow FWHM, the Principle XPS Signal can provide more accurate and useful chemical state information.
- Because the Principle XPS Signal has the most narrow FWHM, the component-peaks have less peak overlap, and allow us to better resolve the presence or absence of more chemical states.
- Principle (Main) Signals are used to identify the presence or absence of different chemical state spectra because many other scientists over the past 60 years have used the same principle signals to identify and assign chemical states to the characteristic BEs that are resolved from peak-fitting the chemical state spectrum of interest. These Reference BEs are useful guides. The NIST XPS Database has a collection of BEs, no spectra. The PHI handbook and the Crist Handbook are useful reference sources of BEs and actual spectra. The XPS Library has an extensive list of Reference BEs useful for Chemical State Assignments.
- Other examples of Principle XPS Signals are:Â Â C (1s), Si (2p), Fe (2p3), Au (4f7), U (3d5)…..
- A Periodic Table of Principle XPS Signals (shown below) lists the BE of the pure element and the type of Principle XPS Signal that is normally used when we want to measure chemical state spectra. The BE that you see, should be used to help select the range of BEs to be analyzed. The range of BEs is typically 10-15 eV below the BE shown and also 10-15 eV above the BE shown. This makes a spectrum that has a 20-30 eV range of BEs.
- Because nearly all chemical states appear at higher BE than the BE of the pure element, the BE range of the spectrum is adjusted slightly so that the pure element peak BE appears at 2-3 eV to the right at lower BE, That way the raw data appears almost uniformly on the spectrum.
Details about Principle Signals, BEs of Chemicals States, Electronegativity
- A Chemical State Spectrum is also known as a High (Energy) Resolution Spectrum, or a Narrow Scan. A chemical State Spectrum is usually 20-100 eV wide.
- The range of Chemical State BEs produced by a single element, bonded to 1-2 other elements, can range from ~1 eV to 12 eV due to differences in electronegativity of the elements in the group and the type of chemical bonds (single, double, triple),
- As an example:Â Carbon bound to Oxygen, Nitrogen, Sulfur and other elements with different bonds produce chemical state peaks that appear between 282 eV and 294 eV.
- Chemical State Spectra can have multiple XPS Peaks that are due to two different elements bonded together and having electronegativity differences.
- Differences in Electronegativity between two elements have a direct effect on the chemical state BE and the chemical shifts of both elements.
Rules of Thumb – Basic Guidelines to help with peak-fitting and identifying different Chemical States
- Chemical State Shifts usually ~1 eV
- Differences in Chemical States is usually ~ 1 eV
- FWHM for the Principle peak of a pure Elements is usually ~1 eV
- FWHM for the Principle peak of a Chemical Compounds is usually ~1.5- 1.8 eV
Topic #4
Which XPS Signals are used to measure Chemical State Spectra? – Continued
Table of Principal XPS Signals:Â useful to define Binding Energy Ranges
Use the BE of a signal as the center of your spectrum. Make your spectrum 20-30 eV wide. To study the loss and satellite features expand BE to 100 eV.
Keep in mind that most oxidation state BEs are 2-3 higher than the pure element BE.
Topic #5Â –Â Features of Peak-fitted Chemical State Spectra
Features of a Peak-fitted Chemical State SpectrumÂ
Features of a Peak-fit
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Topic #6:Â Useful Reference BE Tables, Chemical Shift Chart
to assist Peak-fitting of Chemical Compounds
Reference Table of BEs and FWHMs to Assist Peak-fitting of Chemical Compounds
Topic #6:Â Useful Reference BE Tables, Chemical Shift Chart
to assist Peak-fitting of Chemical Compounds
Reference Table to Assist Chemical State Assignments of Component Peaks
due to different types of Carbon or Carbon Compounds
Electronegativity Table for Reference Purposes
Topic #6:Â Useful Reference BE Tables, Chemical Shift Chart
to assist Peak-fitting of Chemical Compounds
Summaries of NIST BEs from Chemical Counpounds
used to Assign Chemical States
(assembled by Mark Biesinger)
How do we use Chemical State Spectra?
Common Questions
- What can Chemical State Spectra tell us?
- How can we use Chemical State Spectra?
- What information can we derive from Chemical State Spectra?
- Why do we collect Chemical State Spectra?
- What is the purpose or goal of identifying chemical state spectra?
- Are High Res Spectra the same as Chemical State Spectra?