Graphene offers X-ray photoelectron spectroscopy a window of opportunity

X-ray photoelectron spectroscopy (XPS) is among the most sensitive and informative surface analysis techniques. However, it requires high vacuum equipment for operation because electrons can only penetrate a short distance in dense media such as air. An international team led by CNST researchers working with collaborators from Elettra Sincrotrone Trieste, Italy and the Technical University of Munich, Germany have overcome this limitation by employing the fact that graphene is transparent to electrons (electrons can pass through it largely unimpeded) but is impermeable to gas or liquids. The researchers used graphene covering a small opening to separate a liquid sample cell from the high vacuum conditions of the electron spectrometer. They were able to demonstrate that good quality XPS data can be recorded from liquid using the approach. They evaluated the electron transparency of graphene membranes quantitatively and compared it with theoretical predictions. Additionally, the researchers were also able to spectroscopically measure in situ the chemistry of bubble formation due to radiation-induced splitting of water into oxygen and hydrogen. Because the bubble formation is a frequent unwanted process, the measurement of the onset of bubble formation sets an upper limit for the intensities of the X-rays (or electrons) which can be used in this approach. The researchers’ work potentially fills a much needed gap. Assessing the chemical status of surfaces and interfaces immersed in liquids or atmospheric pressure gas is very much needed for many applications such as biomedical research, electrochemical energy devices, and catalysis. The current state of the art of high pressure XPS is to use sophisticated, expensive and bulky differentially pumped stages in front of the electron energy analyzer. Only a few instruments of this kind are currently available worldwide but there is great need for these experimental capabilities. The researchers’ design is far simpler and has the potential to reduce costs to the level that this type of measurement could be afforded by many more labs. As often happens with new technologies, there remain some challenges and limitations. The graphene adhesion to the surface of the apertures needs to be improved, as do the radiation and electrochemical stability of the atomically thin graphene. The researchers plan to work on these challenges in order to develop better techniques for clean and non-disruptive transfer of graphene windows to the supporting openings.