Molecular probes challenge prevalent view of zinc oxide surfaces

Zinc oxide can be tailored for many different applications, many of which depend on how other materials interact with the zinc oxide surface. This interaction is governed by the presence of hydroxyl (OH) groups, which form due to interactions with the air or water. However, investigating the nature and structure of these OH groups requires a highly sensitive instrument that can distinguish between the oxygen atoms in the OH groups and those in the zinc oxide itself. Dr Jens Niederhausen of Helmholtz-Zentrum Berlin and his colleagues from the University of Tübingen and Soochow University brought their ZnO samples to Diamond's I09 beamline, which is designed for high-resolution studies of atomic and electronic structures of surfaces and interfaces.

In the X-ray Standing Waves (XSW) technique, a beam of X-rays directed at the sample under investigation reflects back. The incident and reflected waves combine to form an interference pattern. The resulting waveform appears to be standing still, in that the peaks and troughs do not move in space. For these experiments, Dr Niederhausen combined XSW with X-ray Photo Emission Spectroscopy (XPS), which is a surface-sensitive technique that measures the electrons emitted from a surface in the X-ray beam. 

Dr Niederhausen explains:

David Duncan, the senior beamline scientist on I09, carried out a complementary experiment on the same samples, using Photoelectron Diffraction (PhD). Dr Niederhausen continued:

Theoretical models tell us that the OH groups are on top of the ZnO surface. However, the results of these experiments did not agree. The OH groups weren't where they were expected to be.

Simulated adsorption geometries for three of the models considered in this work. The horizontal lines indicate for each case the expected overlap between the X-ray standing wave and the oxygen atoms in the OH groups (orange) as well as the atoms that make up the PTCDI molecules.

There were no theoretical models that explained the experimental results. However, measuring the vertical position of the OH groups above the ZnO surface assumed that the ZnO surface was perfectly flat. Real-world conditions are a little less ideal, so Dr Niederhausen developed a new technique to get more accurate measurements.

He looked for an organic molecule that could form as complete and pure a monolayer on the zinc oxide substrate as possible. The perfect choice turned out to be PTCDI, which is a stable, planar (2D) molecule. He repeated his experiments with the new samples, measuring the vertical distance between the OH groups and the PTCDI monolayer. Again, the results showed that the OH groups were not on the zinc oxide surface.

Were they acting as a spacer between the two different materials? Researchers from Graz University of Technology in Austria carried out state-of-the-art computational physics to model that scenario. Again, the model did not match the experimental results. In contrast, assuming direct contact between PTCDI and the ZnO surface gave a very good match. So we don't know the structure of the OH groups on this ZnO surface, but we can rule out that they sit on top.

Dr Niederhausen is excited by these unexpected results. He says:

To find out more about the I09 beamline, or to discuss potential applications, please contact Principal Beamline Scientist Tien-Lin Lee: tien-lin.lee@diamond.ac.uk.