Georg Haberfehlner

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.

Correlated 3D Nanoscale Mapping and Simulation of Coupled Plasmonic Nanoparticles.

Electron tomography in combination with electron energy-loss spectroscopy (EELS) experiments and simulations was used to unravel the interplay between structure and plasmonic properties of a silver nanocuboid dimer. The precise 3D geometry of the particles fabricated by means of electron beam lithography was reconstructed through electron tomography, and the full three-dimensional information was used as an input for simulations of energy-loss spectra and plasmon resonance maps. Excellent agreement between experiment and theory was found throughout, bringing the comparison between EELS imaging and simulations to a quantitative and correlative level. In addition, interface mode patterns, normally masked by the projection nature of a transmission microscopy investigation, could be unambiguously identified through tomographic reconstruction. This work overcomes the need for geometrical assumptions or symmetry restrictions of the sample in simulations and paves the way for detailed investigations of realistic and complex plasmonic nanostructures.

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.

Synthesis and morphology of iron-iron oxide core-shell nanoparticles produced by high pressure gas condensation

Core-shell structured Fe nanoparticles (NPs) produced by high pressure magnetron sputtering gas condensation were studied using transmission electron microscopy (TEM) techniques, electron diffraction, electron energy-loss spectroscopy (EELS), tomographic reconstruction, and Wulff shape construction analysis. The core-shell structure, which is composed of an Fe core surrounded by a maghemite (γ-Fe2O3) and/or magnetite (Fe3O4) shell, was confirmed by fast Fourier transform (FFT) analysis combined with EELS. It was found that the particle size and shape strongly depend on the gas environment. Moreover, extensive analysis showed that NPs with a size between 10-20 nm possess a truncated cubic morphology, which is confined by the 6 {100} planes that are truncated by the 12 {110} planes at different degrees. For NPs larger than 20 nm, the rhombic dodecahedron defined by the 12 {110} planes is the predominant crystal shape, while truncated rhombic dodecahedrons, as well as non-truncated and truncated cubic NPs, were also observed. The NPs without truncation showed a characteristic inward relaxation indicating that besides thermodynamics kinetics also plays a crucial role during particle growth.

Quantitative EDXS: Influence of geometry on a four detector system.

The influence of the geometry on quantitative energy dispersive X-ray spectrometry (EDXS) analysis is determined for a ChemiSTEM system (Super-X) in combination with a low-background double-tilt specimen holder. For the first time a combination of experimental measurements with simulations is used to determine the positions of the individual detectors of a Super-X system. These positions allow us to calculate the detectors solid angles and estimate the amount of detector shadowing and its influence on quantitative EDXS analysis, including absorption correction using the ζ-factor method. Both shadowing by the brass portions and the beryllium specimen carrier of the holder severely affect the quantification of low to medium atomic number elements. A multi-detector system is discussed in terms of practical consequences of the described effects, and a quantitative evaluation of a Fayalit sample is demonstrated. Corrections and suggestions for minimizing systematic errors are discussed to improve quantitative methods for a multi-detector system.

3D spatial resolution improvement by dual-axis electron tomography: Application to tri-gate transistors

The performance of semiconductor devices can be linked to geometry and variations of the structure. For transistors in particular, the geometry of the gate stack is essential. In this work we investigate the gate stack of a tri-gate transistor using dual-axis electron tomography. This allows the reconstruction of all surfaces of the gate of the transistor with high resolution and measurement of the local thickness of the gate oxide. While previously, dual-axis electron tomography was employed for reducing missing wedge artifacts, our work demonstrates the potential of dual-axis tomography for improving the resolution of a tomographic reconstruction, even for structures not affected by missing wedge artifacts. By simulations and experiments we show the value of dual-axis tomography for characterization of nanoscale devices as an approach that requires no prior information and that can be easily extended even to more than two tilt axes.

3D Imaging of Gap Plasmons in Vertically Coupled Nanoparticles by EELS Tomography

Plasmonic gap modes provide the ultimate confinement of optical fields. Demanding high spatial resolution, the direct imaging of these modes was only recently achieved by electron energy loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM). However, conventional 2D STEM-EELS is only sensitive to components of the photonic local density of states (LDOS) parallel to the electron trajectory. It is thus insensitive to specific gap modes, a restriction that was lifted with the introduction of tomographic 3D EELS imaging. Here, we show that by 3D EELS tomography the gap mode LDOS of a vertically stacked nanotriangle dimer can be fully imaged. Besides probing the complete mode spectrum, we demonstrate that the tomographic approach allows disentangling the signal contributions from the two nanotriangles that superimpose in a single measurement with a fixed electron trajectory. Generally, vertically coupled nanoparticles enable the tailoring of 3D plasmonic fields, and their full characterization will thus aid the development of complex nanophotonic devices.

In depth nano spectroscopic analysis on homogeneously switching double barrier memristive devices

Memristors based on a double barrier design have been analyzed by various nanospectroscopic methods to unveil details about their microstructure and conduction mechanism. The device consists of an AlOx tunnel barrier and a NbOy/Au Schottky barrier sandwiched between the Nb bottom electrode and the Au top electrode. As it was anticipated that the local chemical composition of the tunnel barrier, i.e., oxidation state of the metals as well as concentration and distribution of oxygen ions, has a major influence on electronic conduction, these factors were carefully analyzed. A combined approach was chosen in order to reliably investigate electronic states of Nb and O by electron energy-loss spectroscopy as well as map elements whose transition edges exhibit a different energy range by energy-dispersive X-ray spectroscopy like Au and Al. The results conclusively demonstrate significant oxidation of the bottom electrode as well as a small oxygen vacancy concentration in the Al oxide tunnel barrier. Possible sc...

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.