X-ray Photoelectron Spectroscopy
Also known as: XPS, ESCA, electron spectroscopy for chemical analysis
X-ray Photoelectron Spectroscopy (XPS), also known as Electron Spectroscopy for Chemical Analysis (ESCA), is a surface-sensitive analytical technique that measures the kinetic energies of photoelectrons ejected from a material by high-energy X-rays. Developed by Kai Siegbahn in 1967, XPS determines elemental composition, chemical oxidation states, and chemical bonding within ~10 nanometers of a surface. It is indispensable in materials science for surface characterization, corrosion studies, oxide analysis, and interface chemistry.
Key highlights
- Directly measures binding energy, enabling identification of elemental oxidation states and chemical bonding
- Quantitative elemental concentrations from peak areas and sensitivity factors; typically ±10% accuracy
- Surface-sensitive due to ~1-3 nm photoelectron escape depth, ideal for oxide and interface analysis
- Non-destructive for initial survey; ion sputtering enables depth profiling
- Excellent for differentiating similar elements or different oxidation states of same element
Intuition
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How it works
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When to use it
XPS is ideal for determining surface elemental composition, oxidation states, and identifying chemical compounds on surfaces. Apply to oxides, sulfides, nitrides, and organic layers. Depth profiling by ion sputtering resolves compositional gradients. Avoid for bulk analysis (use ICP-MS) or subsurface features requiring atomic resolution (use STM). Most effective for elements with Z > 3 (above helium); hydrogen and helium cannot be reliably detected.
Strengths & limitations
- Directly measures binding energy, enabling identification of elemental oxidation states and chemical bonding
- Quantitative elemental concentrations from peak areas and sensitivity factors; typically ±10% accuracy
- Surface-sensitive due to ~1-3 nm photoelectron escape depth, ideal for oxide and interface analysis
- Non-destructive for initial survey; ion sputtering enables depth profiling
- Excellent for differentiating similar elements or different oxidation states of same element
- Surface-sensitive only; subsurface structures require destructive depth profiling
- Light elements (H, He, Li) cannot be detected; poor sensitivity for low-Z elements
- Charging effects on insulating samples distort binding energies; grounding or flood-gun compensation needed
- Quantitative analysis requires sensitivity factors and standard samples; relative accuracy ~10-15%
- Photoelectron peak widths limit chemical shift resolution; closely-spaced oxidation states may not resolve
Common pitfalls
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Applications
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Frequently asked
What is the difference between XPS and Auger spectroscopy?
XPS measures photoelectrons ejected by X-rays; Auger spectroscopy measures electrons from Auger processes after electron-beam ionization. XPS is more surface-sensitive and gives better elemental resolution; Auger is spatially localized (useful with SEM). Both require ultra-high vacuum.
How do I correct for charging on insulating samples?
Standard approaches: apply a conductive overlay (carbon, gold), use charge-neutralization floods, or calibrate using standard samples. Flood guns emit low-energy electrons to neutralize positive charges from photoelectron loss.
What does a chemical shift mean in XPS?
A chemical shift is a change in binding energy due to the chemical environment of the atom. For example, Fe3+ has higher binding energy than Fe2+ because more positive charge increases electron binding. Shifts of 1-10 eV distinguish oxidation states and bonding environments.
Can XPS be used for depth profiling?
Yes, with ion sputtering: alternately sputter thin layers and measure XPS to build a depth profile. This reveals compositional gradients and interface structure but is destructive and requires careful interpretation for reactive materials.
Sources
- 1.Siegbahn, K., Nordling, C., Fahlman, A., et al. (1967). ESCA: Atomic, Molecular and Solid State Structure Studied by Means of Electron Spectroscopy. Almqvist and Wiksells.
- 2.Briggs, D., & Seah, M. P. (2003). Practical Surface Analysis by Auger and X-ray Photoelectron Spectroscopy (2nd ed.). John Wiley & Sons.
- 3.Moulder, J. F., Stickle, W. F., Sobol, P. E., & Bomben, K. D. (1992). Handbook of X-ray Photoelectron Spectroscopy. Physical Electronics.
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Cite this page
ScholarGate. (2026, June 3). X-ray Photoelectron Spectroscopy. ScholarGate. https://scholargate.app/materials-science/x-ray-photoelectron-spectroscopy