Self-organization of Au colloids assisted by copolymer templates
Résumé
Context For the past fifteen years, the investigations of the Localized Surface Plasmon Resonance (LSPR) for plasmonic nanoparticles (NPs) has opened new perspectives for optical nanosensors. Indeed, there is no longer the need of using a prism in order to excite plasmons. The application possibilities are all the more numerous as the fundamental studies go to the direction of single molecule sensing. What is at stake today is the development of such large scale and low cost devices. Scientific aim and methods The scientific aim is here to use Polystyrene-Polymethylmethacrylate (PS-PMMA) copolymers as templates for small Au NPs (6-12nm large) self-organization onto substrates (see Figure 1). We developed two processes: 1) Incorporation of functionalized Au NPs in copolymer solution and organization in PS domains either by thermal annealing or Langmuir-Blodgett method 2) Incorporation of Au salts in PS-PMMA copolymer solution and in-situ synthesis and organization of Au NPs in PS domains. Results These processes are original routes to organize Au NPs onto substrates in order to fabricate optical nanosensors (see Figure 2). Surprisingly, the extinction optical measurements showed an expected main LSPR peak at about 550nm (for 10nm Au NPs) but also a second or even a third peak at longer wavelengths (respectively ~700nm and ~800nm). Discrete Dipole Approximation (DDA) simulations confirmed the experimental results and tend to argue that the 2nd and/or 3rd peak would come from coupling between very close Au NPs. Conclusion This study opens new routes for large-scale fabrication and organization of Au NPs onto substrates. It could pave the way for developing optical nanosensors when the polymer layer is thin enough.