Basic operating principles of the VG

Escalab II X-ray Photoelectron Spectrometer

 

Note: Please do not attempt to use this equipment without adequate training.  There is the potential to cause severe damage to the machine, and yourself, if the correct protocol is not followed.

 

X-ray photoelectron spectroscopy is a UHV surface technique able to identify the elemental composition of the outermost atomic layers of a solid (2-10 nm). The fundamental process is based on the photoelectric effect, the ejection of electrons from the material as a consequence of the adsorption of very short wave length radiation.

 

For XPS, these are ‘soft’ X-rays of 200 – 2000 eV photon energy although the X-rays are usually monochromated (Al X-rays have an energy of 1486.3 eV). This means that the emitted photoelectrons have a particular kinetic energy (Ek) which is measured by a detector. Ek can be related to the binding energy (EB) of the electron in the solid by the equation.

 

EB = hν - Ek - Φ

 

where Φ is the spectrometer work function and hν the energy of the incident radiation.Therefore, the binding energy is a direct measure of the energy required to remove the electron from material and is specific to the orbital of the element from which it came. This is what gives XPS the power to measure not only elemental information but relate peak intensity to an amount of this element within the sample.

 

The exact binding energy can be dependent upon both oxidation states and the local chemical and visible environment. Atoms of higher oxidation state exhibit a higher binding energy due to the extra coulombic interaction between the photo-emitted electron and the ion core. This ability to discriminate between different oxidation states and chemical environments is one of the major strengths of the XPS technique.

 

The analysis chamber for XPS needs to be operated at ultra high vacuum (UHV) conditions to remove as much adsorbed gas from the sample surface as possible prior to analysis and to keep a clear a path for ejected electrons to reach the detector.  The operating pressure in the main chamber on the XPS machine is ~1 × 10-9 mbar.  In order to reach this pressure, the sample must move into the main chamber through various vacuum chambers to ensure the sample is fully evacuated, so as to preserve the vacuum in the main analysis chamber.

The sample is inserted initially into a rotary pumped chamber, and reaches a pressure of ~10-3 mbar before moving to the preparatory chamber.  This chamber is diffusion pumped and will reach a pressure of ~10-8 mbar once samples are fully evacuated.  This can take up to a few hours depending on factors such as number of samples, surface area of samples etc. 

Carousels containing up to 6 samples can be inserted at one time, carousels and samples are moved between chambers using a built in lever.  It is possible to drop samples/carousels if it is done without caution, so never attempt to move samples until you are sufficiently trained to carry it out yourself.  Samples can only be retrieved by bringing the chamber up to atmospheric pressure, and this happens rarely.

To analyse a sample, it’s position to the X-ray gun needs to be optimised with respect to the X,Y and Z directions so as to maximise the number of electrons reaching the detector.  Achieving the maximum number of counts possible will reduce the signal to noise ratio and minimise the error within the data.

On collection of XPS data, it may become apparent that the peaks are shifted from the expected binding energy position. This arises as a consequence of the build-up of a positive charge at the surface of non-conducting specimens - the rate of photo-electron loss is greater than that of their replacement from within the specimen.  It produces a retarding field at the surface that will shift the peaks by lowering the kinetic energy of ejected electrons.  This can be fixed by using a known peak as a reference.  The C1s peak is often used for this, as all samples can be found to contain carbon to some extent.  Setting this peak to 285.0 eV will allow all other peaks to be scaled by the same amount.

 

A survey scan such is carried out to assess the elements present, then more high resolution scans can be carried out on areas of interest.

Assigment of peaks can be tricky and open to interpretation, therefore some knowledge of what you expect to be present in your sample is required to achieve the best fit of data.  It is also necessary to know about the other types of peak that can be present in your spectrum (e.g satellite peaks, Auger peaks, ghost peaks etc).

Please contact the Surface Analysis Facility for any help or if you have any questions regarding XPS.

 

Some good books for reference are:

Practical Surface Analysis -Auger and X-ray Photoelectron Spectroscopy
D. Briggs and M. P. Seah (Editors), Wiley Interscience, 1990 (2nd ed.)

An Introduction to Surface Analysis by XPS and AES

John F. Watts and John Wolstenholme, Wiley Interscience, 2003

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