X-ray Photoelectron Spectroscopy

Most commonly in XPS instruments the electron energy analyser is positioned so that most of the detected electrons have originated from the sample surface with a trajectory in line with the surface normal. This is called normal emission, and the electrons are said to have a take-off angle of 90° to the surface.

Under these conditions XPS is the most bulk sensitive it can be for a given X-ray energy, with an information depth generally less than about 10nm. It also tends to give the maximum signal, and the geometry is generally well-understood.

Surface sensitivity can be increased

Surface sensitivity can be increased in XPS by varying the angle at which electrons are detected from the surface. This technique is known as Angle-Resolved XPS (ARXPS) and usually requires that samples be tilted to a range of angles from 0° to maybe 80°.

Electrons can only travel a certain distance through a material before some sort of interaction occurs, which is governed by the inelastic mean free path (IMFP) of the electron. By increasing the tilt angle, and therefore increasing the emission angle, we are effectively reducing the information depth of the technique. This is because we are forcing the detection of electrons that have had to travel through more of the material laterally, and therefore can only have escaped from a shallower depth.

This is depicted in the above three graph schematic for the very similar technique of Parallel ARXPS (PARXPS) as employed by the Theta Probe XPS instrument at NEXUS. In this instrument there is no need to tilt the sample, as electrons from emission angles of 20-80° are collected simultaneously.

In this schematic the maximum information depth is indicated by the length of the dashed line in the central graph into the sample surface, and is directly determined by the IMFP. As the emission angle is increased, the length of this line remains the same, as it is governed only by the IMFP, however the depth into the sample that is reaches is now reduced.

At 80° the technique is at its most surface sensitive, and at 20° it is at its most bulk sensitive. By comparing spectra collected at a range of angles, we can identify where a certain species is located in terms of depth. This enables us to:

• perform depth profiling of chemical species without destroying the sample
• calculate the thickness of an overlayer such as an oxide

In the spectra provided, we can see how oxide and metal components on a sample change in relative intensity with emission angle. Since the metal peak is strongest at 20° and weakest at 80°, we can say that the metal is in the bulk, and the oxide is at the surface (as expected).

What can it do for me?

It can:

• provide non-destructive depth-profiling of the top 10nm of a surface
• provide thin-film thickness measurements
• clarify contributions in spectra that come from surface versus the bulk

What are the typical applications?

Typical applications include:

• depth-profiling through thin layers and interfaces with a calculated depth scale
• thin oxide or overlayer thickness measurements
• stratification of multiple thin layers on a substrate

Argon Gas Cluster Ion sources are able to analyse several classes of materials previously inaccessible to XPS depth profiling due to induced damages in the surface and change the chemistry of the analyte.

Case study: Material dependence of argon cluster ion sputter yield in polymers: method and measurements of relative sputter yields for 19 polymers

There is a pressing need for reference data to allow sputter depth-profiling of polymers using cluster and polyatomic ion sources for the quantification of depth in XPS and SIMS. The authors have developed a new method of sputter rate measurement based on a combination of contact masking and white-light interferometry.

The results show a much larger range of sputter yield than might previously have been expected. For example, the sputter yield of PMMA being more than ten times that of polyether ether ketone when using argon ion clusters of around 4 eV/atom, with other polymers being widely distributed between these extremes. Without reference data for sputter rate this wide range could lead to major errors in depth estimation in sputter depth-profiling of polymer coatings, biomaterials, nanostructures, polymer electronic and polymer.

XPS imaging is useful in understanding:

• distribution of chemistries across a surface
• finding the limits of contamination
• examining the thickness variation of an ultra-thin coating

There are two approaches for obtaining XPS images: mapping (serial acquisition) or parallel imaging (parallel acquisition).

Mapping

Serial acquisition of images is based on a two-dimensional, rectangular array of small-area XPS analyses. This method enables measurements of the distribution of elements or chemical states. Serial acquisition is generally slower than parallel acquisition but can collect a range of energies at each pixel compared with collecting only a single energy for parallel acquisition. Using this method, the analysis position is fixed, and the specimen surface is moved with respect to this position.

Parallel imaging

Parallel acquisition method simultaneously images the entire field of view without scanning voltages applied to any spectrometer component. This parallel imaging provides that a set of quantitative images can be acquired in only a few minutes. This method of imaging is faster and produces a better image resolution, of <3 μm, than the mapping method. Then subsequent data processing produces the relative atomic concentration image that can be used to define the elemental and chemical composition as a function of position.

XPS analysis of nano-carbons is not limited to C1s spectra. However, C1s spectra can be problematic because it is generally impossible to uniquely peak fit mixed sp2/sp3 spectra. The differentiated form of the C KLL spectrum allows measurement of the D-parameter, which is a relatively simple measure of sp2:sp3 ratio. The D-parameter will give semi-quantification of sp2 and sp3 amounts in your samples.

Case study: A new approach towards chemical state identification of novel carbons in XPS imaging

The clear identification of allotropes and similar chemical states of carbon in XPS imaging can be made difficult because of the subtle differences observed in spectra, particularly when varying from sp2 to sp3 hybridised carbon. By shifting focus from the commonly analysed C1s region in XPS spectra to the often ignored C KVV region, we utilise the so-called D-Parameter to identify different forms of carbon in a surface.

When this methodology is applied to XPS imaging, the result is a powerful and unambiguous tool for the chemical state identification of carbon in XPS images. Further enhancement by multivariate statistics improves XPS spectral and image quality, and we call this technique Multivariate Auger Feature Imaging. Herein, we have applied this technique to clearly identify in XPS imaging a graphite film mounted on carbon tape.

Further information

For further information, access to DOI: 10.1002/sia.5738

Elucidating detailed information from XPS Data using Multivariate Analysis (MVA) such as principal component analysis (PCA) and partial least square regression (PLS). MVA is one of supporting techniques to diagnose for the interpretation of XPS spectra.

Case study: Multivariate analysis studies of the ageing effect for artists' oil paints containing modern organic pigments

This work demonstrates the potential of surface analysis techniques to contribute to a better understanding for art conservators of the degradation of modern paints. The model paints we select are modern synthetic organic pigments in linseed oil. The results, using a combination of X-ray photoelectron spectroscopy data and multivariate analysis, show good agreement with previous studies using different techniques such as pyrolysis gas chromatography mass spectrometry regarding the ageing effect in oil paints. In addition, we demonstrate that two different modern organic pigments produce a different oxidation status of the linseed oil in the paint matrix.

Further information

Further information, view the Multivariate analysis studies of the ageing effect for artist's oil paints containing modern organic pigments article.

The K-Alpha is a fully integrated, small-spot XPS system for rapid analysis of all sample types with cluster depth profiling.

The K-Alpha has the following features:

• micro-focussed monochromated Al Kα source
• variable spot size (30-400 μm in 5μm steps)
• 180° hemispherical analyser
• 128-channel detector
• scanned and snapshot spectroscopy modes
• dual-beam charge neutralisation
• monoatomic Ar ion and Polyatomic Gas-Cluster Ion Beam (GCIB) source
• cluster sizes of 1000 and 2000 atoms
• depth profiling of metals and oxides as well as polymers and multi-layers
• sample holder in protective atmosphere for samples sensitive to air
• three sample platens available for sample mounting:
  • 60×60 mm plain platen
  • tilt platen for ARXPS
  • rotation platen for depth profiling
  • 20 mm maximum sample thickness

Find out more about the K-Alpha (Thermo Scientific) by visiting the Thermo Scientific website.

The Theta Probe is a Parallel ARXPS system for non-destructive depth-profiling of surfaces coupled with Raman Spectrometer.

The Theta Probe has the following features:

• micro-focussed monochromated Al Kα source
• 250 mm Rowland circle
• 180° spherical sector analyser
• 60° Parallel ARXPS (no need to tilt the sample)
• scanned and snapshot spectroscopy modes
• dual-beam charge neutralisation
• monoatomic Ar ion and Polyatomic Gas-Cluster Ion Beam (GCIB) source
• cluster sizes of 1000 and 2000 atoms
• depth profiling of metals and oxides as well as polymers and multi-layers
• maximum analysis area 70×70 mm
• maximum sample thickness 25 mm
• possibility to be linked with Raman spectrometer, for in situ measurments of XPS with Raman
• multiple sample platens available for sample mounting:
  • 70x70 mm plain plate
  • tilt and rotate holde
  • heating and Cooling stages
• UHV preparation chamber
  
  • monoatomic Ar ion gun
  • sample fracture stage
  • high temperature/pressure gas cell
  • gas dosing
  • prep chamber transfer block, allowing direct access to analysis position

Find out more about the Theta Probe (Thermo Scientific) by visiting the Thermo Scientific website.

The AXIS Nova is the highest energy resolution instrument at NEXUS, capable also of XPS imaging, Ultra-violet photoelectron spectroscopy (UPS) and ISS

The Nova has the following features:

• high power monochromated Al Kα source
• 500mm Rowland circle
• 180° hemispherical analyser (165mm mean radius) combined with spherical mirror analyser
• MCP stack and delay-line detector
• scanned and snapshot spectroscopy modes
• 2D imaging mode (scanned and parallel imaging available) with <3μm spatial resolution
• small-area spectroscopy down to 10μm
• coaxial low energy electron source for charge neutralisation
• HeI/II source for Ultra-Violet Photoelectron Spectroscopy (UPS)
• monoatomic Ar ion and polyatomic coronene source for surface cleaning and depth-profiling
• four sample platens available for sample mounting:
  • 110mm diameter plain platen
  • 0 - 85° tilt platen for ARXPS
  • 360° azimuthal platen with compucentric rotation about the vertical axis
  • deep platen for tall samples

Find out more about the Nova ( Kratos Analytical) by visiting the Kratos Analytical website.