Helium droplet assisted synthesis of plasmonic Ag@ZnO core@shell nanoparticles

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.

Efficient Formation of Size-Selected Clusters upon Pickup of Dopants into Multiply Charged Helium Droplets

“…Some preliminary experiments indicate that this method can also be utilized for the formation of almost mono-dispersed metal nanoparticles consisting of several thousand atoms. Currently we are designing a new experimental setup to deposit such large nanoparticles onto amorphous carbon films and analyze them in a transmission electron microscope, similar to the previous studies in the groups of Ellis [ 24 ] and Ernst [ 13 , 64 , 65 ].…”

“…For small clusters, the addition or removal of a single atom can have a substantial effect on its electronic [ 5 ], optical [ 6 ] and magnetic [ 7 , 8 ] properties as well as on its stability [ 9 ] and chemical reactivity [ 10 , 11 ]. Clusters can be made by either top-down [ 12 ] (breakup of larger particles or bulk) or bottom-up [ 13 ] (condensation of atoms or molecules) approaches. In the case of metal clusters, wet chemical synthesis has been successfully utilized to form specific size-selected clusters but most of the time organic ligands are required to prevent further agglomeration [ 14 ].…”

Metal clusters synthesized in helium droplets: structure and dynamics from experiment and theory

“…The coverage is estimated as 26% based on the ESEM image while the AFM gives the impression of a higher coverage. 80 Fig . 7 Workfunction mapping with the energy resolved photoemission microscope.…”

“…In this case, the helium droplet assisted creation of Ag@Zn particle was the starting point followed by oxidation that was carefully checked by scanning transmission electron microscopy (STEM). 130 Using different droplet beam conditions, smaller spherical core@shell clusters and larger extended structures were deposited on the same substrate exposed to oxygen and imaged as shown in Fig. 19a.…”

“…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. 130 5.3.3 CoO. Cobalt oxides represent another example of a catalytically active material for the oxidation of ammonia to nitric acid.…”