Substrate supported lattices of metal nanoparticles with controlled inter-particle distances find applications in sensing, optical metamaterials, and plasmonic lasing. These structures are typically fabricated by top-down methods using lithography, for instance, ion-beam, beam or UV photolithography providing nanostructures with dimensions down to a few nanometers. These methods further allow for the fabrication of periodic nanoarrays with controllable inter-particle distances and lattice symmetry. Apart from hexagonal and square lattices, honeycomb structures have gained significant attraction recently.
Researchers lead by Professor Matthias Karg from the Heinrich-Heine-University Düsseldorf in Germany (previous affiliation: University of Bayreuth, Germany), demonstrated the bottom-up preparation of binary plasmonic superstructures via an interface-mediated self-assembly route. The sequential double deposition of individual colloidal monolayers was employed to produce a honeycomb lattice with gold and silver nanoparticles at alternating positions. Their work in now published in the peer-reviewed journal, Colloids and Surfaces A: Physicochemical and Engineering Aspects.
Their experiments are based on noble metal-hydrogel core-shell particles that were prepared in a two-step protocol. In the first step, gold-hydrogel core-shell particles were synthesized by seeded precipitation polymerization of N-isopropylacrylamide in presence of a chemical cross-linker (N,N’-methylenebisacrylamide). In the second step, the gold nanoparticle cores of these core-shell particles were selectively overgrown either by gold or by silver. This growth resulted in [email protected] particles with cores of a total diameter of 75 nm and [email protected] particles with 99 nm total core diameter. Due to the use of the same starting material, the overall hydrodynamic dimensions of the two systems are identical. In order to fabricate highly ordered, non-close packed, plasmonic monolayers from these building blocks, the authors used interface-mediated colloidal self-assembly. When immersed at the air/water interface, both core-shell systems formed homogeneous, macroscopic monolayers spontaneously. Upon transfer of the freely floating monolayers onto solid supports such as microscopy glass slides or TEM grids, substrate supported hexagonally ordered colloidal monolayers were obtained. Investigating the monolayer structure by different microscopy techniques revealed an extraordinary high degree of order with large, single-crystalline domains for both types of core-shell particles. Due to the hydrogel shells – acting as a semitransparent spacer – the metal nanoparticle cores were well-separated with average center-to-center inter-particle distances close to 500 nm.
As the key experiment the authors demonstrated a sequential double deposition of monolayers from the [email protected] and the s[email protected] core-shell particles. This way binary plasmonic honeycomb lattices were created. The extinction of the binary lattice revealed a broad plasmon resonance with the indication of plasmon resonance coupling between the metal cores. Through EDX mapping the authors were able to distinguish between positions of the silver and gold cores. They found that the center-to-center distances in each of the hexagonal sublattices were 470 nm. This was a close match todistances of the individual single monolayers. The center-to-center distance between silver and gold particles in the resulting binary honeycomb lattice was 271 nm.
Tobias Honold1, Kirsten Volk1, Markus Retsch2, and Matthias Karg1,3. Binary plasmonic honeycomb structures: High-resolution EDX mapping and optical properties. Colloids and Surfaces A: Physicochem. Eng. Aspects, volume 510 (2016), pages 198-204.Show Affiliations
- Physical Chemistry − Colloidal Systems, University of Bayreuth, Universitaetsstr. 30, 95440 Bayreuth, Germany.
- Physical Chemistry − Polymer Systems, University of Bayreuth, Universitaetsstr. 30, 95440 Bayreuth, Germany.
- Physical Chemistry I, Heinrich-Heine-University Duesseldorf, Universitaetsstr. 1, 40225 Duesseldorf, Germany.
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