Martin Schnedlitz

Synthesis of nanoparticles in helium droplets—A characterization comparing mass-spectra and electron microscopy data

Micrometer sized helium droplets provide an extraordinary environment for the growth of nanoparticles. The method promises great potential for the preparation of core-shell particles as well as one-dimensional nanostructures, which agglomerate along quantum vortices, without involving solvents, ligands, or additives. Using a new apparatus, which enables us to record mass spectra of heavy dopant clusters (>10(4) amu) and to produce samples for transmission electron microscopy simultaneously, we synthesize bare and bimetallic nanoparticles consisting of various materials (Au, Ni, Cr, and Ag). We present a systematical study of the growth process of clusters and nanoparticles inside the helium droplets, which can be described with a simple theoretical model.

Modelling electron beam induced dynamics in metallic nanoclusters

We present a computational scheme to simulate beam induced dynamics of atoms in surface dominated, metallic systems. Our approach is based on molecular dynamics and Monte Carlo techniques. The model is tested with clusters comprised of either Ni, Ag or Au. We vary their sizes and apply different electron energies and cluster temperatures to elucidate fundamental relations between these experimental parameters and beam induced displacement probabilities. Furthermore, we demonstrate the capability of our code to simulate beam driven dynamics by using Ag and Au clusters as demonstration systems. Simulations of beam induced displacement and sputtering effects are compared with experimental results obtained via scanning transmission electron microscopy. The clusters in question are synthesised with exceptional purity inside inert superfluid He droplets and deposited on amorphous carbon supports. The presented results may help to understand electron beam driven processes in metallic systems.

Spectroscopy of gold atoms and gold oligomers in helium nanodroplets

The 6p 2P1/2 ← 6s 2S1/2 and 6p 2P3/2 ← 6s 2S1/2 transitions (D lines) of gold atoms embedded in superfluid helium nanodroplets have been investigated using resonant two-photon ionization spectroscopy. Both transitions are strongly blue-shifted and broadened due to the repulsive interaction between the Au valence electron and the surrounding helium. The in-droplet D lines are superimposed by the spectral signature of Au atoms relaxed into the metastable 2D states. These features are narrower than the in-droplet D lines and exhibit sharp rising edges that coincide with bare atom transitions. It is concluded that they originate from metastable 2D state AuHen exciplexes that have been ejected from the helium droplets during a relaxation process. Interestingly, the mechanism that leads to the formation of these complexes is suppressed for very large helium droplets consisting of about 2 × 106 He atoms, corresponding to a droplet diameter on the order of 50 nm. The assignment of the observed spectral features is supported by ab initio calculations employing a multiconfigurational self-consistent field method and a multi-reference configuration interaction calculation. For large helium droplets doped with Au oligomers, excitation spectra for mass channels corresponding to Aun with n = 2, 3, 4, 5, 7, and 9 are presented. The mass spectrum reveals even-odd oscillations in the number of Au atoms that constitute the oligomer, which is characteristic for coinage metal clusters. Resonances are observed close by the in-droplet D1 and D2 transitions, and the corresponding peak forms are very similar for different oligomer sizes.