Daniel Knez

Formation of bimetallic clusters in superfluid helium nanodroplets analysed by atomic resolution electron tomography

Structure, shape and composition are the basic parameters responsible for properties of nanoscale materials, distinguishing them from their bulk counterparts. To reveal these in three dimensions at the nanoscale, electron tomography is a powerful tool. Advancing electron tomography to atomic resolution in an aberration-corrected transmission electron microscope remains challenging and has been demonstrated only a few times using strong constraints or extensive filtering. Here we demonstrate atomic resolution electron tomography on silver/gold core/shell nanoclusters grown in superfluid helium nanodroplets. We reveal morphology and composition of a cluster identifying gold- and silver-rich regions in three dimensions and we estimate atomic positions without using any prior information and with minimal filtering. The ability to get full three-dimensional information down to the atomic scale allows understanding the growth and deposition process of the nanoclusters and demonstrates an approach that may be generally applicable to all types of nanoscale materials.

Thermal instabilities and Rayleigh breakup of ultrathin silver nanowires grown in helium nanodroplets.

Ag nanowires with diameters below 6 nm are grown within vortex containing superfluid helium nanodroplets and deposited onto a heatable substrate at cryogenic temperatures. The experimental setup allows an unbiased investigation of the inherent stability of pristine silver nanowires, which is virtually impossible with other methods due to chemical processes or templates involved in standard production routes. We demonstrate by experiment and by adaption of a theoretical model that initially continuous wires disintegrate into chains of spheres. This phenomenon is well described by a Rayleigh-like breakup mechanism when the substrate is heated to room temperature. Our findings clarify the recent discussions on the cause of the observed segmented patterns, where a breakup during deposition [Gomez et al., Phys. Rev. Lett., 2012, 108, 155302] or mechanisms intrinsic to the helium droplet mediated growth process [Spence et al., Phys. Chem. Chem. Phys., 2014, 16, 6903] have been proposed. The experimental setup confirms the validity of previous suggestions derived from bulk superfluid helium experiments [Gordon et al., Phys. Chem. Chem. Phys., 2014, 16, 25229] for the helium droplet system, and further allows a much more accurate determination of the breakup temperature.

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.

Transformation dynamics of Ni clusters into NiO rings under electron beam irradiation

We report the transformation of nickel clusters into NiO rings by an electron beam induced nanoscale Kirkendall effect. High-purity nickel clusters consisting of a few thousand atoms have been used as precursors and were synthesized with the superfluid helium droplet technique. Aberration-corrected, analytical scanning transmission electron microscopy was applied to oxidise and simultaneously analyse the nanostructures. The transient dynamics of the oxidation could be documented by time lapse series using high-angle annular dark-field imaging and electron energy-loss spectroscopy. A two-step Cabrera-Mott oxidation mechanism was identified. It was found that water adsorbed adjacent to the clusters acts as oxygen source for the electron beam induced oxidation. The size-dependent oxidation rate was estimated by quantitative EELS measurements combined with molecular dynamics simulations. Our findings could serve to better control sample changes during examination in an electron microscope, and might provide a methodology to generate other metal oxide nanostructures.

Inclusions in Si whiskers grown by Ni metal induced lateral crystallization

The formation of Nickel-di-silicide inclusions in silicon whiskers grown during low temperature Ni Metal Induced Lateral Crystallization of amorphous Silicon was studied by High Resolution Transmission Electron Microscopy, Scanning Transmission Electron Microscopy, and Electron Energy Loss Spectroscopy. The heat treatment of the samples lasted for 11 + 11 days at 413 °C for the first 11 days and 442 °C for the rest of the time. The size of the inclusions ranges from just a few atoms to 15–20 nm. It was shown that the NiSi2 inclusions have the form of tetrahedrons, which are bound by {111} coherent interfaces with the Si matrix. These inclusions are homogeneously distributed along the whiskers, and the Ni percentage incorporated in these is 0.035 at. %. The tetrahedral inclusions are formed by trapping NiSi2 clusters at the Si/NiSi2 interface during whisker growth.

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.

Analytical Electron Tomography: Methods and Applications

Electron tomography is a powerful technique for 3D characterization at the nanoscale. Recent developments focus on extracting a wide range of information about a sample in 3D [1]. Of special interest is the combination of electron tomography with spectroscopic techniques EFTEM, EELS and EDS to recover the information present in spectroscopic signals in three dimensions. Analytical electron tomography allows mapping of chemical variations and gradients, approaching the goal of full 3D elemental quantification [2]. Additionally, EELS tomography can be used to extract information about materials properties or chemical bonding [3,4]. In this presentation we will discuss the steps necessary to successfully combine spectroscopy and tomography and show respective applications.