Applying nanoscale metal coatings to thin polymer films is of interest for various promising industrial applications, for example as a means of increasing the efficiency of organic solar cells. The properties of the metal coatings need to be precisely adjusted, depending on the application in question. At DESY, scientists have now studied in real time how the structure of a fine gold coating produced by an industrially important technique known as sputter deposition can be modified and how different rates of sputtering affect the growth of the structures. These findings could simplify and improve the production of tailored metal-polymer interfaces. The researchers around DESY’s Matthias Schwartzkopf and Stephan V. Roth are presenting their findings in the journal ACS Applied Materials & Interfaces.
During sputtering, a solid target is bombarded with ions leading to the release of individual atoms, which can then form an ultra-thin layer of nanoparticles on a second material. “The advantage of this method is that the atoms self-assemble on the surface and therefore adopt the most favourable configuration. In addition, you can adjust the spacing and the size of the nanoparticles more or less at the push of a button,” explains Schwartzkopf. This is why the method has long been used in industrial coating technology.
For their study, the scientist deposited a nanolayer of gold atoms on a polymer substrate made of polystyrene, whereby they varied the rate at which the atoms were deposited while looking for differences in the growth of the layers. “Sputtering gold onto a polymer is rather like a shower of gold atoms raining onto a sheet of plastic. Different structures form on the sheet, depending on whether it ‘rains’ harder or more gently, even when the amount of material deposited remains the same. And depending on what purpose you want to use the material for, you need different structures,” says Schwartzkopf.
Working on the P03 beamline of DESY’s X-ray source PETRA III, they were able to observe the layers in real time as they grew and the structure of the metal films changed. “Particularly at high growth rates, like those used in industrial applications, you need advanced equipment being able to achieve the necessary high temporal resolution in order to observe the processes involved. Using real-time-capable, surface-sensitive X-ray scattering, we were able to observe the way in which the radii of and the spacing between the tiny gold clusters varied,” reports Schwartzkopf.
The scientists discovered two main differences between low and high sputter deposition rates. Firstly, at the start of the deposition process, during so-called nucleation, the density of particles is higher at higher rates, i.e. more clusters of gold atoms are formed, but the individual clusters are smaller. Secondly, at higher deposition rates the so-called percolation threshold is reached sooner, i.e. with thinner layers. This threshold describes the point beyond which all gold clusters can be assumed to be touching each other, meaning there are hardly any gaps left in the layer. This is because at higher deposition rates, branching structures are formed, whereas at lower rates larger blobs are formed which only form interconnected networks at a later stage. This in turn means that metal coatings created at high deposition rates are smoother and more compact.
These observations now allow researchers to deduce what sputter deposition rates are best suited for specific applications. For example, if the material is to be used as a catalyst, its structure ought to consist of many small gold clusters, and this can be achieved using a high sputter rate and small layer thickness. If the material is intended for sensors, on the other hand, a low sputter deposition rate with a comparatively high layer thickness is appropriate, as this results in a structure made up of larger, densely packed clusters.
The researchers now want to study how different properties of the layer change during sputter deposition. For example, they could use spectroscopic methods to examine how the optical properties of the layer differ for different rates. This could provide a better understanding of how the use of sputter deposition can be optimised for industrial applications.