Ultrathin Gold Nanoparticle Superlattices for Nanophotonics

Ultra thin layer of hydrogel beads with gold particles transferred to a glass slide. (Photos: HHU / Christoph Kawan)

In the group of Prof. Dr. med. Matthias Karg at the Institute of Physical Chemistry of Heinrich-Heine-University Düsseldorf (HHU) produces ultrathin, highly ordered layers of hydrogel spheres that enclose gold or silver particles. These structures are of interest for applications in optoelectronics – information and communication technology based on light and nanophotonics. The researchers publish the results of an important step in the direction of so-called “plasmonic nano-lasers” in the latest issue of the journal ACS Applied Materials & Interfaces.

Preparation of the Layer Structure (from the Above): First, a solution of the hydrogel beads with the gold particles contained therein is carefully applied to a water surface. In an instant, this forms an ultrathin, shimmering layer, which can then be lifted off with a glass slide.

The working group “Colloids and Nanooptics” around Prof. Dr. med. Matthias Karg at the Institute of Physical Chemistry has developed a simple yet precise technique for developing highly ordered particle layers. They use tiny, soft and deformable polymer spheres with a hydrogel-like structure. Hydrogels are water-swollen three-dimensional networks. Such structures are known for example from superabsorbents in baby diapers, which are able to bind large amounts of water.

Within these spheres are tiny, only a few nanometers large gold or silver particles, the Kargs team manufactures itself at the HHU from salts of metals in a reduction process. “We can adjust the size of the gold particles very precisely, because the hydrogel shells are permeable to dissolved metal salts, which makes a subsequent overgrowth of the gold nuclei possible.” The structure of these core-shell particles can be compared with a cherry. Here is a hard core surrounded by soft pulp. However, the particles from the laboratory are about one hundred thousand times smaller.

From a dilute solution of these hydrogel beads, the Dusseldorf can then produce thin layers. They put the beads on a water surface, on which a highly ordered and colorful iridescent layer forms by itself. This layer lifts them off the water surface with the help of glass carriers. Through this transfer to the glass shines now the entire glass carriers.

If you look at this layer under the electron microscope, you will see a regular hexagon pattern of small dots. “These are the gold particles in their shells,” explains doctoral student Kirsten Volk, “and we see that these lie in a single, highly ordered layer.” The gold particles provide the color of the layer: they reflect visible light of specific wavelengths interferes and thus gives a different color impression from different angles.

“These thin layers are very exciting for optoelectronics – that is, data transmission and processing using light. With them, it can also be possible to build miniaturized lasers, “says Prof. Karg. Such nano-lasers are only nanometers in size and thus represent a key technology in the field of nanophotonics.

In a study now published in the journal ACS Applied Materials & Interfaces, the Düsseldorf researchers have taken an important hurdle on the way to such nano-lasers. They were able to excite the gold particles to collective vibrations by externally irradiated light. So not every gold particle is excited individually, but all particles vibrate together. This common vibration is the basic requirement for the construction of lasers. The special feature of the published research results is that the particle layers can not only be built very easily and on large surfaces, but are also very thin.   

For optoelectronic applications and nanolaser, the vibrations in the thin layers must be further enhanced. Prof. Karg: “Next we will try to strengthen the suggestion by a targeted doping. In the long run, it could also be possible to realize electrically operable nano-lasers. “