X-ray Photoelectron Spectroscopy

May 9, 2025

X-Ray Photoelectron Spectroscopy, or XPS, is a technique used to analyze the surface of materials—usually the top 1-10 nanometers. It works by shooting X-rays at a sample, which causes the material to eject electrons (called photoelectrons). By measuring the energy of these electrons, you can figure out what elements are on the surface and learn about their chemical state. XPS has been around since the 1960s and is widely used in chemistry, physics, and materials science to study things like metals, polymers, and even biological samples.

How It Works to Measure the Properties of Materials

Here’s how XPS works in simple terms: You place your sample in a vacuum chamber to avoid interference from air. Then, an X-ray source (usually aluminum or magnesium X-rays) fires at the sample. The X-rays hit the atoms on the surface, and if the energy is high enough, they knock out electrons from the inner shells of those atoms—this is called the photoelectric effect. Each electron has a specific kinetic energy based on the atom it came from and its chemical environment.

A detector measures the kinetic energy of these ejected electrons, and using a formula (binding energy = X-ray energy – kinetic energy – a small correction factor), you calculate the binding energy of the electrons. This binding energy is like a fingerprint—it tells you which element the electron came from (like carbon, oxygen, or iron) and its chemical state (like whether carbon is in a C-C bond or a C=O bond). The whole process is super surface-sensitive because only electrons from the top few nanometers can escape without losing energy.

XPS can measure these properties:

  • Elemental Composition: What elements are on the surface (except hydrogen and helium, which it can’t detect).
  • Chemical State: The bonding environment of elements—like whether iron is metallic or oxidized (Fe²⁺ vs Fe³⁺).
  • Surface Contamination: Any impurities or unwanted layers on the surface.
  • Electronic Structure: Info about the energy levels of electrons, which can reveal how the material conducts or reacts.

It’s often paired with techniques like depth profiling (using argon ions to etch the surface) to see how composition changes deeper into the material.

How Is It Valuable?

XPS is a big deal for a few reasons:

  • It’s great for studying surface chemistry, which is critical in things like coatings, catalysts, or electronics. For example, in 2025, with the push for better batteries, XPS helps researchers analyze the surface of battery electrodes to improve performance.
  • It can spot tiny amounts of elements—down to 0.1% of the surface—which is awesome for finding contamination or defects.
  • It’s used in industries like semiconductors to check the quality of thin films, or in biomedical research to study how proteins stick to implant surfaces.
  • It gives both qualitative (what’s there) and quantitative (how much) data, which makes it super versatile for troubleshooting or innovation.

Basically, XPS helps us understand and improve materials at the atomic level, which is key for advancing tech and solving real-world problems.