Structure and Design of Gold Nanosponges

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 [1].
Gold nanosponges
 

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 [2].
 

Research challenges
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.
 

Focused-ion-beam tomography

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 [3] 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.
 

Computer modelling

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 [4], 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 [1].