Xray Photoelectron Spectroscopy

X-ray photoelectron spectroscopy (XPS) is a surface-sensitive, quantitative analytical technique belonging to the broader family of photoemission spectroscopies. It measures the elemental composition, chemical state and electronic structure of the uppermost 5–10 nanometres of a material, corresponding to roughly 50–60 atomic layers. As a method grounded in the photoelectric effect, XPS records the energies and numbers of electrons emitted when a surface is irradiated by X-rays and uses these data to infer chemical and electronic properties. Because chemical state shifts in binding energy reveal the nature of chemical bonding, XPS is widely used across materials science, chemistry, surface engineering, catalysis, biomaterials and cultural heritage science.
XPS is versatile: it can perform surface line profiling, depth profiling when paired with ion-beam etching, and comparative chemical studies of samples in their as-received form or after exposure to heating, reactive gases, ultraviolet radiation, liquids or mechanical cleavage. Routine detection excludes only hydrogen and helium, and parts-per-million sensitivity is achievable under favourable conditions.

Basic Physics

In XPS, a beam of monochromatic or non-monochromatic X-rays impinges on a material, transferring photon energy to core-level electrons. If the photon energy exceeds the electron binding energy, the electron is ejected from the atom. The measured kinetic energy of this emitted electron allows computation of its binding energy through the photoelectric equation:
Ebinding=Ephoton−(Ekinetic+ϕ),E_\text{binding} = E_\text{photon} – (E_\text{kinetic} + \phi),Ebinding​=Ephoton​−(Ekinetic​+ϕ),
where E₍binding₎ is the electron’s binding energy relative to the chemical potential, E₍photon₎ is the known X-ray photon energy (for Al Kα radiation, 1486.7 eV), E₍kinetic₎ is the measured kinetic energy and φ represents a small work-function-like correction that accounts for energy lost as the photoelectron travels from the bulk to the detector. This is essentially a conservation-of-energy relationship. Binding energies derived in this way determine the electronic configuration and oxidation state of the atoms present.
Because most photoelectrons originate from the top few nanometres of the sample, XPS is inherently surface-specific. Although escape depths vary with kinetic energy, a practical sampling depth of around 10 nm is typical.

History

The phenomenon underlying XPS traces to Heinrich Hertz’s 1887 discovery of the photoelectric effect, later explained by Albert Einstein in 1905. The first photoelectron spectra recorded using X-rays appeared in 1907 in experiments undertaken by P. D. Innes, who used an electron analyser and photographic recording to observe broad energy bands.
Subsequent early contributions by Moseley, Rawlinson and Robinson helped clarify these initial results. After the Second World War, Kai Siegbahn and colleagues in Uppsala developed high-resolution electron analysers, enabling the first detailed, chemically informative XPS spectrum in 1954. Siegbahn’s comprehensive studies culminated in the 1967 publication of Electron Spectroscopy for Chemical Analysis (ESCA), a term that long served as a synonym for XPS. With engineering collaboration at Hewlett-Packard, the first commercial monochromatic XPS instrument appeared in 1969. For his pioneering contributions, Siegbahn received the 1981 Nobel Prize in Physics.
Parallel to these developments, ultraviolet photoelectron spectroscopy (UPS) was established by David W. Turner for studying molecular valence electronic structure.

Measurement Principles

A typical XPS spectrum plots the number of detected electrons as a function of binding energy. Each element has a characteristic set of core-level peaks—such as 1s, 2s, 2p, 3s and so on—whose intensities reveal relative quantities of elements within the sampling volume. Developing atomic percentages requires correcting the measured intensities by relative sensitivity factors and then normalising the results. Hydrogen is absent because it cannot be detected by conventional XPS.
XPS not only identifies elements but also determines chemical states. For example, variations in the binding energy of metal–oxygen core levels indicate oxidation states, allowing analysts to determine the proportion of species such as M⁺ or M²⁺ in a metal oxide.

Quantitative Accuracy and Precision

XPS provides excellent relative quantification for homogeneous solids. Absolute quantification requires certified standards and is less commonly attempted. The accuracy of atomic percentages depends on several factors:

• signal-to-noise ratio
• peak intensity and shape
• accuracy of sensitivity factors
• correction for electron transmission functions
• homogeneity of the analysed volume
• electron mean free path and its energy dependence
• extent of surface degradation during measurement

Under optimal conditions, major peaks yield 90–95 per cent accuracy, while weaker peaks (10–20 per cent of the strongest signal) yield 60–80 per cent accuracy. Quantitative precision—the ability to repeat measurements reliably—is essential for meaningful data interpretation.

Detection Limits

Detection sensitivity depends on photoelectron cross section, matrix composition and background noise. Higher atomic number elements generally produce stronger signals. For example, detecting trace gold on silicon is straightforward because the strong Au 4f peak lies on a low background, enabling sensitivity at or below one part per million. By contrast, silicon on gold is more difficult to detect because the Si 2p line lies beneath a large Au background. While limits of 0.01–1 atomic per cent are typical, significantly lower limits can be achieved for favourable systems.

Degradation During Analysis

Sample degradation depends on X-ray wavelength, total X-ray dose, vacuum quality and thermal environment. Metals, alloys, ceramics and most glasses tolerate both monochromatic and non-monochromatic X-rays. However, many polymers, oxygen-rich materials, catalysts and fine organic compounds can degrade.
Non-monochromatic sources generate substantial bremsstrahlung radiation (1–15 keV) and significant heat (often raising sample temperatures by 100–200 °C), which accelerates chemical damage. Monochromatic Al Kα sources greatly reduce these effects and are preferred for sensitive materials.

Experimental Conditions and Techniques

XPS ordinarily requires high vacuum (~10⁻⁶ Pa) or ultra-high vacuum (~10⁻⁷ Pa) to minimise inelastic scattering of photoelectrons. Developments in ambient-pressure XPS have enabled analyses at tens of millibar, supporting the study of catalytic processes and hydrated samples under near-realistic conditions.
For hydrated or biological materials, controlled freezing and sublimation of surface ice layers allow analysis while maintaining structural integrity.
Depth profiling involves alternately sputtering the surface with an ion beam and measuring spectra after each etch step. Line profiling can map chemical variations across a surface, revealing heterogeneity, corrosion patterns or interfacial chemistry.

Applications

XPS finds application in a wide array of fields, including:

• inorganic and organic surface chemistry
• semiconductor and ion-modified materials
• polymer science
• corrosion studies
• catalysis and heterogeneous reaction mechanisms
• coatings, paints, inks and adhesives
• biomaterials, implants, teeth and bone
• cultural heritage analysis of pigments and surface treatments