Nano antennae allow to beat Abbe’s diffraction limit: They can focus light to spot sizes far below the wave length of light. The resulting enormous field enhancement and ultra-strong light intensities can be used for basic science, but also allows disruptive applications such as plasmonic water splitting. While advanced and, most importantly, expensive process steps are required to fabricate such artificial nanoantennae, modern materials science allows a cheap, and thus highly attractive alternative, which researchers at TU Ilmenau have just published in Nature Communications Materials .
Porous gold nanoparticles, so-called gold nanosponges, have recently emerged as a new material and possess fascinating optical properties. One of these is the ability to focus light deep below the wavelength, leading to the appearance of characteristic hotspots: Nanosponges are de-facto ensembles of random nanoantennae. Each individual nanosponge has its own internal structure, which is intimately linked to its optical properties and leads to its own set of resonance frequencies and hotspots .
One of the ongoing areas of study is this link between microscopic structure and macroscopic properties, however the required knowledge about the nanosponges’ structure has been notoriously difficult to acquire. In the recently published work, detailed measurements of the chaotic nanometer-sized internal structure of several nanosponges have been carried out and, based on these measurements, a model to automatically produce similar sponge geometries on the computer was created, allowing for much deeper research into the connection between structure and properties.
The project was an interdisciplinary work between material scientists and theoretical physicists of the TU Ilmenau and experimental physicists of the University of Oldenburg as part of a larger cooperation on disordered materials for photonic applications sponsored by the German Research Foundation DFG.
The experimental work was carried out by the group of Peter Schaaf, Dong Wang and Hauke Honig in the clean room facilities of the Centre for Micro- and Nanotechnologies of the TU Ilmenau, Germany.
Focused-ion-beam tomography  was used to cut individual sponges into many thin slices, each only a few nanometers thick. Using electron microscopy, each slice is scanned and the captured images are processed and assembled into a 3D structure. This allowed, for the first time, a detailed look inside a nanosponge. However, the acquisition process must be carried out by trained specialists and is not the simplest of procedures.
The theoretical physics group consisting of Erich Runge, Sebastian Bohm and Malte Grunert simulated the optical properties of the measured nanosponges. This already led to further insight into the nature of the fascinating optical hotspots of real gold nanosponges, as described in the publication.
As each nanosponge is unique, any reliable structure-property theory is of a statistical nature, requiring the recording of dozens of sponges. Previous theoretical work used simple models to represent the nanosponge geometry, but it was not clear how applicable the results were to real nanosponges. The main scientific question of the work was thus the following: Can we develop a more sophisticated geometry creation algorithm tuned around the experimental results? This would allow the generation of a whole library of hundreds of sponges on the computer.
The formation of nanoporous gold is described using the Spinodal demixing in other systems has been successfully modeled using the so-called phase-field method in the past , which was applied to gold nanosponges in this work, leading to excellent results. One of the most critical aspects was the generation of an accurate surface, which is extremely important for the optical properties of the nanosponges. However, eventually, the authors succeeded in finding a reliable and simple, yet powerful geometry creation algorithm. The method is in principle transferable to other nanoporous particles.
Furthermore, the authors identified a set of global morphological properties, such as the number of holes and the fraction of gold in the sponges. When sponges generated using the developed model agree in these properties with those of the experimentally measured sponges, similar optical properties result (of course, considering that each sponge is still somewhat unique).
Finally, through the FIB measurements the authors discovered that the nanosponges are anisotropic, that is, “thicker” at the bottom. The authors implemented this uneven thickness into their geometry creation algorithm in a way that allows for any imaginable structural anisotropy. Effects like this are suspected to play an important role in, e.g., (photo-)catalysis using gold nanosponges, an area of research that is still in its infancy. The authors work is available under open-access at Nature Communications Materials .
 M. Grunert et al., Commun Mater 4, 20 (2023)
 J. Zhong et al., Nano Lett. 18, 4957-4964 (2018)
 K. Hu et al., Philos. Mag. 96, 3322-3335 (2016)
 J. Erlebacher et al., Nature 410, 450-453 (2021)
M. Sc. Malte Grunert
Technische Universität Ilmenau
Department of Mathematics and Natural Sciences
Group of Theoretical Physics