Abstract
The reproducible low-cost fabrication of functional metal-polymer nanocomposites with tailored optoelectronic properties for advanced applications remains a major challenge in applied nanotechnology. To obtain full control over the nanostructural evolution at the metal-polymer interface and its impact on optoelectronic properties, we employed combined in situ time-resolved microfocus grazing incidence small angle X-ray scattering (μGISAXS) with in situ UV/vis specular reflectance spectroscopy (SRS) during sputter deposition of gold on thin polystyrene films. On the basis of the temporal evolution of the key scattering features in the real-time μGISAXS experiment, we directly observed four different growth regimes: nucleation, isolated island growth, growth of larger aggregates via partial coalescence, and continuous layer growth. Moreover, their individual thresholds were identified with subnanometer resolution and correlated to the changes in optical properties. During sputter deposition, a change in optical reflectivity of the pristine gray-blue PS film was observed ranging from dark blue color due to the presence of isolated nanoclusters at the interface to bright red color from larger Au aggregates. We used simplified geometrical assumptions to model the evolution of average real space parameters (distance, size, density, contact angle) in excellent agreement with the qualitative observation of key scattering features. A decrease of contact angles was observed during the island-to-percolation transition and confirmed by simulations. Furthermore, a surface diffusion coefficient according to the kinetic freezing model and interfacial energy of Au on PS at room temperature were calculated based on a real-time experiment. The morphological characterization is complemented by X-ray reflectivity, optical, and electron microscopy. Our study permits a better understanding of the growth kinetics of gold clusters and their self-organization into complex nanostructures on polymer substrates. It opens up the opportunity to improve nanofabrication and tailoring of metal-polymer nanostructures for optoelectronic applications, organic photovoltaics, and plasmonic-enhanced technologies.
Highlights
The investigated preparation of homogeneous metal−polymer nanocomposites by sputter deposition of Au on PS thin films matches the primary challenge of nanotechnology for reproducible, low-cost fabrication with tailored optical properties over a large area
The observed antireflective behavior with its maximum at 0.95 ± 0.1 nm thickness suggests a promising range for effective resonant cluster layers in organic photovoltaics (OPV) applications to increase their light harvesting capabilities
The adjustable optical thin film properties induced by the cluster layer can be applied to tune the optical response of functional polymer surfaces or anticounterfeiting security features.[5]
Summary
Since the dawn of nanotechnology, one of the primary objectives has been to obtain a fully controlled process to fabricate nanostructures on a larger scale with tailored properties using self-organizing principles.[1,2] Hereby, gold as a noble metal with remarkable relativistic quantum chemistry[3] combined with excellent chemical stability presents various promising advanced applications in form of supported nanoclusters, for example, in heterogeneous catalysis and optoelectronics.[4,5] The final device performance, in particular for optoelectronics, strongly depends on cluster size distribution, shape, surface chemistry, and arrangement.[6,7] In catalysis, the perimeter of a gold cluster creates catalytic-active binding sites at the interface, for example, enabling the oxidative conversion of toxic CO into CO2 even at a temperature of 40 K.8,9 an increase in light harvesting ability has been reported by introducing noble metal cluster layers in photovoltaics.[5,10−12]. During sputter deposition of Co, Metwalli et al showed that a selectively decoration of PS domains occurs on microphase-separated diblock copolymer films.[34] Schlage et al followed the evolution of the magnetic state during Fe sputter deposition onto a highly ordered, nanoporous PS containing diblock copolymer resulting in a magnetic antidot array.[35] In the case of sputter deposition of Au on similar diblock copolymer thin films, a preferential accumulation of Au at PS domains occurs.[22] This selective wetting behavior is primarily attributed to the differences in surface mobility and interaction of the Au adatoms with the PS domains.[39,40] In a recent study, the selectivity of Au on PS domains was exploited to fabricate directional hierarchically nanostructures with optical anisotropy via glancing angle sputter deposition.[41] exploring the metal−polymer interaction during self-organization of Au on PS homopolymer under controllable and reproducible conditions like sputter deposition forms a 2-fold scientific platform On one hand, this model system serves to investigate practical preparation conditions and functional applications of metal−polymer nanocomposites.[13,28,38] On the other hand, it contributes to tackle fundamental scientific questions such as metal subsurface diffusion and its impact on the polymer thin films stability.[40,42,43]. We have followed in situ and in real-time the morphological evolution of the nanostructured Au film and the related optical properties in the UV/vis regime during radio frequency (RF) sputter deposition by combining microfocus GISAXS (μGISAXS) together with UV/vis specular reflectance spectroscopy (SRS) measurements; the latter being a very sensitive method to study changes in the optical properties of thin films.[44,45] The combination of low deposition rates during RF sputter deposition with the high time resolution achieved during continuous metal layer deposition allows for observing early metal growth stages and extracting important morphological parameters as well as the thresholds involved in the nanostructure growth kinetics with subnanometer resolution.[25,26,41,46] All this information is significant for tuning the nanostructures for a large variety of organic optoelectronics such as OPVs and OLEDs, which is in turn beneficial for their low-cost fabrication, device performance, and optimized use of noble metals in general
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