Plasmonic Nanoparticles in Helium Nanodroplets

The field of plasmonics is emerging as one of the key optical technologies of the 21st century; it holds promise for high-speed, all optical computing and plasmonic nanoparticles are the basis for many intriguing applications ranging from chemical sensing to cancer treatment and diagnostics. The high level of spatial and temporal control over light and photochemistry offered by plasmonics motivates the development of new materials, to be used, for example, for photocatalysis or in solar cells, and thus may have transformative impact on our way to a clean and sustainable society. At the dawn of the age of plasmonics, scientists are seeking for novel materials, designed and tailored for new emerging applications. In the course of this project we will employ a new approach for the synthesis of plasmonic materials by producing plasmonic nanoparticles inside helium nanodroplets. A beam of liquid helium droplets with sizes up to only one micrometer provides an inert, cold and solvent free synthesis environment. As atoms or molecules can be added to the helium droplets sequentially, it is possible to assemble tailored nanoparticles by adding materials with different desired properties step by step to the droplet. In particular, we will explore material combinations for the passivation of nanoparticles which are highly reactive but with very exciting properties for plasmonics. Further experiments will be dedicated to the fabrication of active plasmonic core-shell particles with temperature dependent optical properties. These nanoparticles can then be deposited on any surface that is placed into the helium droplet beam. Furthermore, at the Institute of Experimental Physics of TU Graz we will fabricate core-shell structures in which molecules are encapsulated in between layers of plasmonic materials, known as nanomatryoshkas. These materials are anticipated to enable Raman spectroscopy inside helium nanodroplets by exploiting the ability of plasmonic nanoparticles to concentrate light into very tiny regions around the Raman molecules. From these experiments, we will learn about the properties of new materials, in particular, in a size regime where quantum effects are dominant, and about the interaction between molecules and plasmonic nanoparticles. Understanding the physics of these nanoscale objects is crucial for the design of large scale plasmonic devices and technologies. With the helium droplet synthesis approach, we think that a new, versatile method for the field of plasmonics will be established - a method capable of producing nanoparticles and nanostructures from an unprecedented variety of plasmonic materials with tailored properties.

The project was aiming for the development and characterization of novel plasmonic nanoparticles using helium droplets as synthesis environment. The approach has been used to fabricate nanoparticles and hybrid nanostructures that consist of metals, metal oxides, a variety of organic molecules and combinations thereof. New experimental strategies have been developed, and new fields of applications for helium droplet-based nanoparticle synthesis have been explored. Experiments on Ag@Au core@shell structures revealed that the helium droplet approach is well suited for the fabrication of plasmonic nanoparticles with tunable optical properties, which can be controlled by adjusting the Ag:Au ratio. These particles have been used to fabricate substrates for surface enhanced Raman spectroscopy (SERS). Using energy-sensitive electron microscopy approaches, the plasmonic properties of deposited particles have been characterized and plasmon modes of individual particles have been mapped. The employed ultrathin sub-10 nm substrates sparked new ideas geared towards interdisciplinary and collaborative experiments that combine helium droplet synthesis with time-resolved XUV spectroscopy. An interesting material that has been formed and investigated are Ag@ZnO nanoparticles, which merge a plasmonic metal core with a semiconductor shell. These hybrid structures bear great potential for the improvement of light harvesting materials. A particularly important result is the fabrication of "nanomatryushkas", core@shell@shell nanoparticles, which has been demonstrated by combining gold core particles with layers of solid hexane and dye molecules. These results show that triple-layer particles can be synthesized in helium droplets, which allows for an investigation of plasmon enhancement and quenching effects that occur in these materials. Another key result is presented by the study of potassium clusters in helium droplets, which makes use of a supercontinuum laser-based spectroscopy approach that has been implemented. The results provide detailed insight into the evolution of electronic transitions with potassium cluster size, from discrete bands in small molecules to plasmon modes in nanoparticles. In the course of the project, new classes of dopant materials have been explored, and it has been shown that hybrid nanostructures comprising up to three material layers can be formed with helium droplets. Furthermore, new analysis approaches for the characterization of the fabricated nanostructures have been developed and employed. The results open many new avenues for future research in the field of helium droplet science and beyond.