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School of Chemistry

Near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS)

The School of Chemistry houses a leading SPECS NAP-XPS system - one of only three in the UK and fewer than 20 worldwide.

The SPECS NAP-XPS system is a technique that allows for XPS characterisation under realistic conditions. XPS is a spectroscopic technique that allows for the chemical composition of the surface of a sample to be determined, with soft X-rays fired at the sample and photoelectrons 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).

Uses

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 part.

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 that 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 while also allowing a usable fraction of the emitted photoelectrons to escape and reach the detector.

What it can do

In the NAP cell

  • Analysis of samples in the presence of a gas (or mixture of gases) up to a total pressure of 25mbar. 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

Limitations

  • 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.

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 prioritises all the applications and grants access accordingly. The steering committee meets quarterly.

Application guidance notes

  • Allocation of time on the NAP-XPS is competitive; we always receive more applications than we have time to allocate, so it's worth investing some time and effort into your proposal.
  • The only thing the panel will judge you on is the application in front of them, so make sure it is a complete summary of what you want to do and that you refer to any previous NAP-XPS experiments in your application.
  • Don't forget to discuss the impact of your work - there has to be some reason why the proposed experiment is important in a broad context.
  • It must be novel - check the literature. If it has been done before, we're not going to do it again.
  • It must exploit the unique capabilities of the instrument. If your experiment is possible using conventional XPS it will have lowest priority.