All (2002-Present)  SS21  WS21  SS22  WS22  SS23  WS23  SS24

 Karl Franzens University Graz

Graz University of Technology 

Still plenty of room at the bottom: From new nano materials for plasmonics to attosecond spectroscopy of nanoparticles
Florian Lackner
Institut für Experimentalphysik; TU Graz
17:15 - 18:15 Tuesday 22 October 2019 TUG P2

Helium nanodroplets have evolved from a tool for molecular spectroscopy [1, 2] into a versatile approach for the synthesis of multi-component core@shell nanoparticles. [3] In our experiments, we focus on the preparation of particles that contain plasmonic materials. An example are Ag@Au core@shell particles, for which the spectral position of the localized surface plasmon resonance (LSPR) can be controlled by the Ag:Au ratio. [4] A particular advantage of the method is the ability to form new and unconventional plasmonic materials such as K@Au core@shell nanoparticles, a system that can potentially outperform the common standard materials in plasmonics. Our current efforts are geared towards the investigation of interactions between dopant molecules and plasmonic nanoparticles. These experiments are ultimately aiming at surface enhanced Raman spectroscopy (SERS) inside helium nanodroplets.
The new NanoESCA laboratory at the Institute of Experimental Physics, equipped with a photoemission electron microscope with energy filter (EF-PEEM), greatly expands our experimental capabilities. Our current efforts in the NanoESCA lab are focused on Ag@ZnO core@shell nanoparticles. The combination of plasmonic nanoparticles with semiconductors is attracting great interest in photocatalysis and photovoltaics due to the possible enhancement of photo-conversion efficiencies by the plasmonic material. We use pulsed laser sources for two-photon photoemission (2PPE) spectroscopy. Employing 400 nm photons resonant to the LSPR of Ag, PEEM images and photoelectron spectra reveal that the Ag cores retain their plasmonic activity. Furthermore, VUV light with 21 eV photon energy provides access to Zn core-level transitions, sensitive to the local valence electronic structure of Zn, which, hence, enables an investigation of the oxidation of the Zn shell.
Besides time-resolved 2PPE spectroscopy in the NanoESCA, we are aiming at employing XUV transient absorption (XTA) spectroscopy for the investigation of photoinduced dynamics in deposited nanoparticles. I will introduce this method on the example of selected molecules. [5] First results on CoO and NiO demonstrate that it is possible to study dynamics in nanoparticles with XTA spectroscopy even if the particle surface coverage is below a monolayer. Thereby, helium droplet synthesis is the ideal sample preparation method as it allows for a very soft deposition on the required ultrathin substrates.
The obtained results are the first step towards attosecond XTA spectroscopy of nanoparticles, which could shine light on plasmon induced charge-carrier dynamics in core@shell systems with unprecedented temporal resolution.

[1] F. Lackner et al., Phys. Rev. Lett., 113 (15) 153001 (2014), [2] F. Lackner and W.E. Ernst, J. Phys. Chem. Lett. 9, 3561-3566 (2018), [3] P. Thaler et al., J. Chem. Phys. 143, 134201 (2015), [4] F. Lackner et al., Eur. Phys. J D, 73, 104 (2019), [5] F. Lackner et al., J. Chem. Phys. 145 (23), 234313 (2016),