Real-Time Monitoring of Morphology and Optical Properties during Sputter Deposition for Tailoring Metal–Polymer Interfaces

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.

Molecular dynamics simulation of gold cluster growth during sputter deposition

“…Although the resulting values are up to 9 orders of magnitude larger than typical experimental values, the observed nucleation and coalescence of stable clusters is in accordance with the expected physical behavior and the wide-ranging agreement with experimental data for gold on polystyrene from Ref. [20] supports the significance of the results. Nevertheless, it presently remains difficult to perform benchmarks that go beyond comparisons with experimental results.…”

“…This method relies on the analysis of the angledependent intensity structure of scattered X-rays. Most cluster properties were obtained by projecting the scattered structures onto a geometrical model of uniform [20] and MD simulations with selected values for the re-evaporation probability, pre = 1 × 10 −4 , and the fraction of trapped Au particles, γ = 1 × 10 −2 . For deposited thicknesses δ less than about 2 nm, all quantities agree with experimental trends.…”

“…Afterwards, we take the values for D exp and J exp from Ref. [20] to calculate the ratio ξ and finally the deposition rate J. For the mentioned parameters, we obtain the values 1.2 ns for the average time between the deposition of two atoms per nm 2 and 20 ps for the time a diffusing atom to reach a mean squared displacement of 1 nm 2 .…”

“…Typical physical vapor deposition processes of metalpolymer nanocomposites comprise simultaneous deposition of metal and polymer, leading to nanoparticles embedded in a polymer host matrix, or exclusive deposition of metal atoms or clusters onto a prepared polymeric thin film, restricting the nanoparticle formation to the surface and near-surface area of the polymer bulk [15,[17][18][19][20]. Depending on the desired properties of the composite, different deposition methods of both polymer and metal may be advantageous [18,[21][22][23][24].…”

“…[14,20,25]. This experimental method relies on extracting all structural information from the features in the scattering patterns.…”