Researchers at Graz University of Technology (TU Graz) developed a method for assembling plasmonic nanoparticles for specific applications. Their approach was to synthesize so-called core-shell clusters inside helium nanodroplets, with subsequent deposition and controlled oxidation. The researchers sent superfluid helium droplets with an internal temperature of 0.4 K (−273 °C) through a vacuum chamber and selectively introduced individual atoms or molecules into the droplets. The droplets coalesced into a new aggregate that could be deposited on different substrates. The graph illustrates the stepwise synthesis of silver-zinc oxide core-shell clusters. Courtesy of IEP/TU Graz. Using helium-droplet synthesis, the researchers formed core-shell clusters with a 3-nm core of silver and a 1.5-nm-thick shell of zinc oxide. The combination of these materials caused the silver core to absorb light and create a high light-field amplification, called a plasmonic resonance. The electrons in the surrounding zinc oxide moved into an excited state and formed electron-hole pairs — small energy segments that the researchers said could be used for catalysis processes directly on the cluster surface. “The combination of the two material properties increases the efficiency of photocatalysts immensely,” professor Wolfgang Ernst said. “It would be conceivable to use such a material in water splitting for hydrogen production.” The researchers also produced core-shell clusters with a magnetic core of the elements iron, cobalt, or nickel and a shell of gold. The gold also had a plasmonic effect and also protected the magnetic core from unwanted oxidation. The nanoclusters can be influenced and controlled by lasers and by external magnetic fields, the researchers said. The particle size and shape can be controlled from spherical sub-10-nm particles to larger elongated structures. Compared to other synthesis routes, this technique has the advantage of using superfluid helium as a pristine “nanolab” in which the particles are grown without the addition of any solvents or other chemical agents. The team performed temperature-dependent stability measurements and theoretical calculations for both material combinations. Particle morphology and elemental composition were analyzed using scanning transmission electron microscopy (STEM) and energy dispersive x-ray spectroscopy (EDS). The results were further supported by ultraviolet photoelectron spectroscopy (UPS), which indicated a fully oxidized shell layer for the particles studied by STEM. Together with his team, the experimental physicist Wolfgang Ernst of TU Graz succeeded in the targeted synthesis of so-called core-shell clusters using helium-droplet synthesis. Courtesy of Lunghammer/TU Graz. Using two-photon photoelectron (2PPE) spectroscopy, the researchers examined the plasmonic properties of the spherical silver-zinc oxide core-shell particles. They found that when the localized surface plasmon resonance in silver was excited at around 3 eV, plasmonic enhancement led to the liberation of electrons with high kinetic energy. The researchers observed this phenomenon in both silver and silver-zinc oxide particles, showing that even if a silver cluster is covered by the zinc-oxide layer, a plasmonic enhancement can be observed by photoelectron spectroscopy. Nanoparticles and nanoclusters have a catalytically effective surface area that is large compared to their volume. For many applications, this feature could allow material savings while maintaining performance. Because nanomaterials follow the laws of quantum mechanics, nanoparticles of a material can behave completely differently than the same material on a macroscopic scale. Ernst said that he hopes the findings from the experiments will be rapidly transferred into new catalysts “as soon as possible.” The research was published in Nano Research (www.doi.org/10.1007/s12274-020-2961-z).