Helium droplet assisted synthesis of plasmonic Ag@ZnO core@shell nanoparticles
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stitute of Experimental Physics, Graz University of Technology, A-8010 Graz, Austria Institute of Electron Microscopy and Nanoanalysis & Graz Centre for Electron Microscopy, Graz University of Technology, A-8010 Graz, Austria
© The Author(s) 2020 Received: 21 February 2020 / Revised: 26 June 2020 / Accepted: 27 June 2020
ABSTRACT Plasmonic Ag@ZnO core@shell nanoparticles are formed by synthesis inside helium droplets with subsequent deposition and controlled oxidation. The particle size and shape can be controlled from spherical sub-10 nm particles to larger elongated structures. An advantage of the method is the complete absence of solvents, precursors, and other chemical agents. The obtained particle morphology and elemental composition have been analyzed by scanning transmission electron microscopy (STEM) and energy dispersive X-ray spectroscopy (EDS). The results reveal that the produced particles form a closed and homogeneous ZnO layer around a 2–3 nm Ag core with a uniform thickness of (1.33 ± 0.15) nm and (1.63 ± 0.31) nm for spherical and wire-like particles, respectively. The results are supported by ultraviolet photoelectron spectroscopy (UPS), which indicates a fully oxidized shell layer for the particles studied by STEM. The plasmonic properties of the produced spherical Ag@ZnO core@shell particles are investigated by two-photon photoelectron (2PPE) spectroscopy. Upon excitation of the localized surface plasmon resonance in Ag at around 3 eV, plasmonic enhancement leads to the liberation of electrons with high kinetic energy. This is observed for both Ag and Ag@ZnO particles, showing that even if a Ag cluster is covered by the ZnO layer, a plasmonic enhancement can be observed by photoelectron spectroscopy.
KEYWORDS nanoparticle, helium droplet, plasmonics, photoelectrons
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Introduction
Zinc oxide has become one of the most popular materials for applications in the fields of photocatalysis and optoelectronics due to its intriguing properties. ZnO is a non toxic n-type semiconductor with a wide direct band gap of 3.37 eV, very similar to TiO2 but with a higher absorption efficiency under solar irradiation and a large exciton binding energy of about 60 meV [1]. As a photocatalyst it shows high potential in degradation processes of organic pollutants via the generation of reactive oxygen species and also exhibits antibacterial properties, especially when scaled down to the nano regime [2]. Due to its photoconductivity and photoluminescence characteristics, applications of ZnO in optoelectronics are possible, for example, in ultraviolet (UV) detectors, solar cells, lasers, and various sensing devices [3, 4]. In recent years, miniaturization and engineering of material properties on the nanometer scale have strongly influenced the research on ZnO. Starting with needles and other structures in the micrometer size regime [5], by now the synthesis of ZnO nanoparticles is an important branch of research. An increasing amount of one-, two-, and three-dimensional structures have been synthesized in shapes rang
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