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

The SPECS NAP-XPS system is a technique which allows for XPS characterisation under realistic conditions. XPS is a spectroscopic technique that allows for the chemical composition of the surface analysis of a sample to be determined; this is done through soft X-rays being fired at the sample and photoelectrons ejected.

These ejected photoelectrons carry information about the elements that are present in the sample and their bonding environment. Critically, only electrons from the very surface of the sample escape and reach the detector – thus making XPS a surface-sensitive technique. During the testing process, the sample and the detector are kept under high vacuum conditions during measurement, this is to ensure that the photoelectrons are not absorbed by air molecules before reaching the detector.

XPS has been used for a number of years in order to study surfaces, and it is regularly used in fields such as; catalysis, corrosion and electrochemistry – where the chemical nature of the surface of the sample is critical. However, there is a major drawback – it is used as post mortem technique. As the sample must be kept in high vacuum conditions during the measurement, scientists can only observe the state of the sample before and after a chemical reaction has occurred. It is not possible to look at the surface during a chemical reaction, which is the most interesting and sometimes, most informative part.

NAP-XPS overcomes this limitation by placing the sample inside a special high pressure cell connected to the analyser through several layers of differential pumping. This means that surfaces can be studied in-situ during chemical reactions; for example, following the surface chemistry of a catalyst while it is operating.

NAP-XPS represents a revolution in the field, allowing for the XPS characterisation of a sample in a gaseous environment. This is achieved through the maintenance of 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 so limits the distance that the electrons have to travel through the high pressure of the gas. By placing the surface of the sample close to this aperture, the area under analysis can be in a high pressure of gas, whilst also allowing a usable fraction of the emitted photoelectrons to escape and reach the detector.

XPS is a common analytical tool used for quantitative measurement of the elemental composition as well as specific chemical state information of the surface constituents. It allows us to probe chemical interactions on the atomic level for vapor/solid interfaces. NAP-XPS also allows the investigation of the electronic and structural properties of small organics.

What it can do:

In the NAP cell

  • Analysis of samples in the presence of a gas (or a mixture of gases) up to a total pressure of 25mbar.
  • Heating/cooling of samples from -0ºC to 700ºC during analysis.

In UHV (Ultra High Vacuum)

  • 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 (Low-energy electron diffraction).