XPS

X-RAY PHOTOELECTRON SPECTROSCOPY (XPS / ESCA) SERVICES

X-ray photoelectron spectroscopy (also called “XPS, “ESCA,” or “Electron Spectroscopy for Chemical Analysis”) is a surface analytical technique used to probe the chemical nature of the outermost 8-10nm of a solid surface.

XPS, as its name implies, is a type of electron spectroscopy that utilizes the photoelectric effect. The kinetic energies of photoelectrons generated by irradiating the sample with soft X-rays are measured. Due to electron scattering, photoelectrons that originated deeper than about ~10 nm are not detected.

Photoelectron kinetic energy is primarily determined by the element and electron orbital in which the electron originally existed and its associated binding energy. It is, therefore, possible to identify and quantify the elements present in the outermost few nanometers of a material by recording a spectrum of counts (or, more commonly, counts per second) vs. energy. Binding energy is generally used for the plots. In many cases, the binding energy of the electrons is subtly but measurably different in one bonding environment/oxidation state than another; this direct measurement of bonding environment, together with its inherently quantitative nature, has led to the widespread application of XPS.

APPLICATIONS AREAS FOR XPS:

X-ray photoelectron spectroscopy (XPS) is highly surface selective and can accommodate any solid sample which is stable in ultra-high vacuum conditions: metals, ceramics, oxides, cured/dried surface coatings, glasses, pharmaceuticals, powders, and many more.

Electron Spectroscopy for Chemical Analysis can be performed to probe numerous chemical properties (including oxidation/nitriding/carburizing behavior, reactivity, catalysis, and layering) of microcircuits, nanoparticles, nanowires, batteries, soldered contacts, cleaning products, motors, corroded materials, adhesives, and fibrous composites. It can also be used in failure analysis in identifying the source of an experimental error (i.e. contamination, destructive sample preparation, or volatilization or reaction at the surface).

MEASUREMENTS COMMON WITH XPS:

  • Determination of which elements are present in a surface (except H and He)
  • Identification of the chemical/bonding states of all elements present
  • Quantification of the elements and chemical states present
  • Angle-resolved measurements to probe compositional variation in outermost 10 nm
  • Sputter depth profiling to probe compositional variation in the outermost 2 um
  • Thin-film characterization: thickness, uniformity, compositional heterogeneity

USES & LIMITATIONS—ADDITIONAL CONSIDERATIONS:

  • What it is great for:
    • Quick & easy sample preparation
    • Rapid data acquisition
    • Accurate quantitative characterization of the near-surface region (30 to 100 Å)
    • Provides elemental composition in parts per thousand and chemical state
    • Often used as a fingerprint especially with non-conducting surfaces
  • Limitations:
    • Lateral resolution is generally poor relative to other techniques (limited to about 10 microns)
    • Sputter depth profiling damages the sample surface and can lead to artifacts (reduction of oxides, preferential sputtering, roughening, mixing, etc.) that must be considered
    • Bonding information cannot be determined in all cases

Example Outputs

Sample survey scan using a monochromatized X-ray source.

High-resolution C1s spectrum showing carbon-oxygen and fluorocarbon species. Each of these may be quantified using straightforward peak fitting procedures.

XPS depth profile obtained using a 3kV Ar+ sputter source

Signal mainly comes from the top few nanometers of the surface

XPS surface nanometers
XPS

INSTRUMENTS WE USE FOR XPS

ThermoScientific ESCALAB 250 X-ray Photoelectron Spectrometer

Outfitted with both monochromatized Al Kα and a non-monochromatized Mg/Zr dual-anode X-ray sources, a UV source for Ultraviolet Photoelectron Spectroscopy (UPS) as well as a digitally controlled ion source for depth profiling and sample cleaning. The system utilizes a double-focusing 180° spherical sector analyzer with magnetic and multi-element electrostatic input lenses focusing emitted electrons to a high-resolution 2D imaging detector or multi-channel spectroscopic detector. Sample stage has 5-axis control, and the sample chamber is equipped with a flood-gun electron source for charge compensation of insulating materials.