X-Ray Photoelectron Spectroscopy (XPS)
XPS at a Glance
X-ray photoelectron spectroscopy (XPS) is a surface analysis technique that measures the elemental composition and chemical state of a material using the feedback from an induced photoelectron effect. It is sometimes known by the alternative name Electron Spectroscopy for Chemical Analysis (ESCA).
Right: The new Liquid Phase Photoelectron Spectroscopy system at the NMRC
Applications
- Quantitative elemental composition
- Empirical formula derivation
- Depth profiling
- Chemical state identification
- Electronic state binding energies and densities
- Elemental mapping (XPS imaging)
Images Courtesy of Vladimir Korolkov Photography
How does XPS work?
Samples are exposed to incident X-rays. Photons are absorbed by elements in the top few nanometres of the sample surface, leading to ionization and the emission of core (inner shell) electrons. The kinetic energy distribution of the emitted photoelectrons is measured in the spectrometer and the electron binding energies are determined.
For every element, there will be a characteristic binding energy associated with each core atomic orbital and therefore any given element produces a characteristic peak 'fingerprint' in the photoelectron spectrum.
Peak intensities can be related to the concentration of the given element allowing XPS to provide a quantitative analysis of surface composition.
Changes in oxidation state or chemical environment give rise to small variations in binding energy which in turn leads to small changes in peak position. These shifts are known as chemical shifts. The ability to discriminate between different oxidation states and chemical environments is a big advantage of the XPS technique.
XPS is not sensitive to hydrogen or helium, but can detect all other elements.
Our XPS Facilities
Hosted by the Nanoscale and Microscale Research Centre (nmRC)
Kratos AXIS ULTRA DLD
- Monochromated X-ray source Al Kα emission at 1486.6 eV, plus two non-monochromated sources Mg and Al.
- High throughput multi position programmable stage allows multiple samples in one experiment.
- Argon ion etching for depth profiling or cleaning.
- Chamber pressure better than 5 x10-9 Torr.
- The magnetic immersion lens system allows the area of analysis to be defined by apertures.
- Electrostatic/magnetic lens system (hybrid lens) and a hemispherical analyser (CHA) to sort photoelectrons according to kinetic energy.
- Electron detection and counting with a triple channel plate and delay line detector (DLD).
Kratos Liquid Phase Photoelectron Spectroscopy Machine (LiPPS)
- Kratos Axis Ultra DLD machine with novel and unique, bespoke additional functionality for liquids and specialist samples:
- High energy monochromated Ag source
- Argon gas cluster source for high resolution depth profiling of organic materials (polymers and biological samples) to allow 3D chemical characterisation.
- Bespoke electrochemistry stage for ‘in-situ’ work in ionic liquids.
Publications of Interest
- Eder G., Smith E.F., Cebula I., Heckl W.M., Beton P., Lackinger M., (2013). Solution Preparation of Two Dimensional Covalently Linked Networks by Polymerization of 1,3,5-Tri(4-iodophenyl)benzene on Au(111). ACSnano. 7; 4: 3014–3021
- Villar-Garcia I.J., Smith E.F., Taylor A.W., Qui F., Lovelock K.R.J., Jones R.G., Licence P., (2010). Charging of ionic liquid surfaces under X-ray irradiation: the measurement of absolute binding energies by XPS. Phys. Chem. Chem. Phys. 13: 2797–2808
- Quingpu H., Rutten F.J.M., Smith E., Briggs D., Davies M., Buttery L.D.K., Freeman R., Shakesheff K., (2005). Surface characterization of pre-formed alginate fibres incorporated with a protein by a novel entrapment process. Surf. Interface Anal. 2005; 37: 1077–1081