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What is X-ray Photoelectron Spectroscopy (XPS)?
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- Universallab
- @universallab
With the increasing demand for high-performance materials, surface engineering has become more critical. The surface of a material is where it interacts with the external environment and other materials, making it vital to understand the physical and chemical interactions occurring at or near the surface or interface. This understanding is key to solving many issues related to modern materials. The surface affects various aspects of materials, such as corrosion rates, catalytic activity, adhesion, surface wettability, contact potential barriers, and failure mechanisms.
Surface modification can alter or improve the performance and properties of materials, necessitating the use of surface analysis to understand the surface chemistry and to study the effects of surface engineering. From non-stick coatings on cookware to thin-film electronics and bioactive surfaces, XPS has become one of the standard tools for characterizing surface materials.
X-ray Photoelectron Spectroscopy (XPS), also known as Electron Spectroscopy for Chemical Analysis (ESCA), is a technique used to analyze the chemical properties of material surfaces. XPS can measure the elemental composition, empirical formulas, chemical oxidation states, and electronic states of materials. An XPS spectrum is obtained by directing an X-ray beam onto a solid surface and measuring the kinetic energy of electrons emitted from within 1-10 nm of the analyzed surface. By counting the electrons with specific kinetic energy, you can create a photoelectron spectrum. The peaks in the photoelectron spectrum represent electrons emitted at characteristic energies from atoms. The energy and intensity of these peaks are used for qualitative and quantitative analysis of all surface elements, except for hydrogen.
Surface Characterization
The surface layer refers to a thin layer no more than 3 atomic layers thick (approximately 1 nm), and its thickness varies depending on the material. A thin layer no more than 10 nm thick is called an ultra-thin film, while a thin layer no more than 1 µm is considered a thin film. The rest of the solid is referred to as the bulk material. However, these terms aren't strictly defined, and the distinctions among surface layer, ultra-thin film, and thin film can vary with different materials and applications.
A surface represents a discontinuity between one phase and another, so the physical and chemical properties of the surface differ from those of the bulk material. This difference largely affects the outermost atomic layers. Within a material, an atom is surrounded in all directions by atoms of the same material, following a certain pattern. However, since surface atoms are not surrounded in all directions, they are more reactive than bulk atoms, and they may form bonds.
Surface Characteristics
The performance and modification treatment of materials vary with depth or thickness, which is crucial for specific properties and treatments. Surface analysis can help understand the following areas:
- Semiconductors, Microelectronics
- Microcircuits
- Ultra-thin Films
- Circuit Board Soft Soldering
- Cleaning Treatments
- Thin Film Stabilization
- Passivation Layers
- Lubrication
- Chemicals
- Plastics, Coatings
- Catalysts
- Fibers
- Metals, Steel Industry
- Nitriding, Carburizing
- Corrosion
- Welding
- Material Aging
- Grain Boundary Segregation
- Glass
- Coatings
- Engines, Avionics
- Lubrication
- Corrosion
- Oxidation
- Material Aging, Failure
- Synthetic Fibers
- Adhesives
In certain technical fields, surfaces and surface analysis are significant, including:
Photoemission Process
When atoms or molecules absorb X-ray photons, they emit electrons. The kinetic energy (KE) of these electrons depends on the photon energy (hν) and the binding energy (BE), which is the energy required for the electron to leave the surface.
By measuring the kinetic energy of the emitted electrons, it's possible to identify the types of elements, their chemical states, and the binding energy of electrons near the surface of the material. Binding energy is influenced by various factors, including:
The element from which the electrons are emitted. The orbital from which the electrons are emitted. The chemical environment of the emitting atom.
Since the photoelectron emission cross-section is not dependent on the atomic chemical environment, X-ray Photoelectron Spectroscopy (XPS) can be used as a quantitative analysis technique.
Photoelectron Spectroscopy Techniques
To illustrate the XPS spectrum, a full scan over a wide energy range was conducted on a barium oxide sample. The spectrum contains multiple peaks for barium and oxygen, along with a carbon peak from a surface impurity. With modern XPS spectrometers, it takes about 10 seconds to collect the spectrum of barium oxide. Since barium oxide is an insulator, a low-energy electron beam is introduced to control sample charging during analysis. In the spectrum, a noticeable background appears beneath the peaks, caused by inelastic scattering of electrons before they escape the sample. This inelastic scattering reduces the kinetic energy of electrons and diminishes peak intensity. Background removal is necessary before quantitative analysis of the spectrum.
Chemical states can also be determined from XPS spectra. Here are two approaches to processing the C 1s XPS spectrum of polyethylene terephthalate (PET). The samples were treated differently: one was spin-coated, while the other was cut from PET. There are three distinct chemical states of carbon in PET, revealed by three peaks in the XPS spectrum. The different treatment methods lead to variations in polymer conformation, which affects the XPS spectrum, resulting in a slight shift in the (-O-C-) peak.
