Near-Ambient Pressure X-Ray Photoelectron Spectroscopy (NAP-XPS)

The School of Chemistry houses a SPECS NAP-XPS system. This is a cutting-edge technique which allows for XPS characterisation under realistic conditions. The NAP-XPS is one of only 3 in the UK and ~ 20 worldwide.

What is NAP-XPS?

Schematic of NAP-XPS instrument
Above: Schematic showing the principle of operation of NAP-XPS Below: Camera image of a sample in the NAP cell. The apature is 300 microns in diameter.

XPS is a spectroscopic technique which allows for the chemical composition of the surface of a sample to be determined. Soft X-Rays are fired at the sample and photoelectrons are ejected. These photoelectrons carry information about the elements present in the sample and their bonding environment. Critically, only electrons from the very surface of the sample escape and reach the detector - making XPS a surface sensitive technique. The sample and the detector are kept under high vacuum conditions during measurement (otherwise the photoelectrons would be absorbed by air molecules before reaching the detector).

XPS has been employed for many years to study surfaces and is routinely used in fields such as catalysis, corrosion and electrochemistry where the chemical nature of the surface is critical. However, it has a major drawback - it is a post mortem technique. As the sample must be in high vacuum during the measurement, one can only observe the state of the sample before and after a chemical reaction has occurred, it's not possible to look at the surface during a chemical reaction, which is the most interesting bit!

NAP-XPS represents a revolution in the field, allowing for XPS characterisation of a sample in a gaseous environment. This is achieved by containing the sample in a high pressure cell which is only open to the analyser via a small aperture. A series of pumping stages after this aperture quickly reduce the pressure back down to high vacuum and limits the distance the electrons have to travel through a high pressure of gas. By placing the surface of the sample very close to this aperture, the area under analysis can be in a high pressure of gas whilst also allowing a useable fraction of the emitted photoelectrons to escape and reach the detector.

What can it do?

In the NAP cell

  • Analysis of samples in the presence of a gas (or mixture of gases) up to a total pressure of 25 mbar. Currently available gases: CO2, H2O, O2, H2, CO, NH3. Other gases may be possible by request.
  • Heating/cooling of samples from ~ 0ºC to 700ºC during analysis

In UHV

  • Standard UHV sample prep. (sputter/anneal cycles)
  • Cluster-ion sputtering (depth profiling of fragile samples such as polymers)
  • Dedicated chamber for evaporation of organics etc.
  • LEED

What can't it do?

  • Powder samples. The samples are vertically mounted so must be a single lump of material with no flaky bits!
  • Insulating samples. There is no charge compensation in the NAP cell, so insulating samples will not work.
  • Routine XPS. Due to high demand for NAP experiments, routine XPS is not normally done on the NAP instrument. Routine XPS can be done using the XPS facility at the School of Materials.

How to apply

NAP experiments are very long (usually a week or more), so booking of the equipment directly isn't possible. Instead we run the system in a similar fashion to a synchrotron beamline. Users write an application form and this is assessed by a steering committee. This committee then prioritizes all the applications and grants access accordingly. The steering committee meets quarterly.

To apply, please use this form.

I have written some brief guidance notes on how to write an application. You can access them here: NAP XPS guidance notes

For further information about the application process (or anything else to do with NAP-XPS), please contact Dr. Alex Walton.

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