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Description
Metal nanoparticles on polymer thin films promise various potential applications as functional coatings in solar cells, biosensors, as reflective or antireflective coatings, and in heterogeneous catalysis. Fabricating such nanocluster films with tailored morphology, their characterization and manipulation at the nanoscale are the essential prerequisites and tools for implementing devices in all of these fields. Here, sputter deposition plays a very important role in industrial and nanostructural processing. In order to tune the size-dependent optoelectronic properties, it is mandatory to monitor how the growth kinetic affects the metal film morphology and how it correlates to the optical properties during sputter deposition.
To obtain full control over the nanostructural evolution at the metal-polymer interface and its impact on optoelectronic properties, we employed a combination of in situ time-resolved surface-sensitive X-ray scattering (GISAXS) with in situ UV/Vis Specular Reflectance Spectroscopy (SRS) during sputter deposition of gold (Au) on thin polystyrene films (PS) [1]. We monitored the evolution of the metallic layer morphology according to changes in the key scattering features by geometrical modeling [2] and correlate the nanostructural development to optical properties. The morphological characterization is complemented by X-ray reflectivity and electron microscopy. This enables us to identify the different growth regimes including their specific thresholds and permits better understanding of the growth kinetics of gold clusters and their self-organization into complex nanostructures on polymer thin films.
During sputter deposition, a change in optical reflectivity of the pristine grey-blue PS film occurred ranging from dark blue color due to the presence of isolated Au nanoclusters at the interface to bright red color from larger Au aggregates [3]. Furthermore, a surface diffusion coefficient for Au on PS at room temperature according to the interrupted coalescence model was calculated based on a real-time experiment.
Furthermore, the role of sputter deposition rate in tailoring metal layer morphologies on polymer thin films has been investigated [4]. The deposition rate affects primarily the nucleation process and the adsorption-mediated growth, whereas rather small effects on diffusion-mediated growth processes are observed. Only at higher rates, initial particle densities are higher due to an increasing influence of random nucleation and an earlier onset of thin film percolation occurs. Thus, our findings are of great interest for applications in organic photovoltaics and organic electronics, which benefit from tailored metal-polymer interfaces.
Figure 1: Sputter deposition of Au on PS. Upper row: Selected 2D GISAXS patterns with increasing effective Au film thicknesses δAu. The critical angles of PS (blue) and Au (orange) are indicated by arrows. Middle row: model-based simulation of the GISAXS pattern, based on the object shape sketched in the upper right corners. Lower row: Sketch of the cluster growth morphology with ongoing sputter deposition in the four stages of growth. See [3].
References
[1] M. Schwartzkopf and S.V. Roth, Nanomaterials 6, 239 (2016).
[2] M. Schwartzkopf, A. Buffet , V. Körstgens, K. Schlage, J. Perlich, A. Rothkirch, G. Benecke, E. Metwalli , B. Heidmann, M. Rawolle, G. Herzog, P. Müller-Buschbaum, R. Röhlsberger, R. Gehrke, N. Stribeck and S.V. Roth, Nanoscale, 5 5053-5062 (2013).
[3] M. Schwartzkopf, G. Santoro, C.J. Brett, A. Rothkirch, O. Polonskyi, A. Hinz, E. Metwalli, Y. Yao, T. Strunskus, F. Faupel, P. Müller-Buschbaum and S.V. Roth, ACS Appl. Mater. Interfaces 7, 13547–13556 (2015).
[4] M. Schwartzkopf, A. Hinz, O. Polonskyi, T. Strunskus, F.C. Löhrer, V. Körstgens, P. Müller-Buschbaum, F. Faupel and S.V. Roth, ACS Appl. Mater. Interfaces 9, 5629–5637 (2017).
