Publications in High Impact Journals
We are committed to keeping up to date with materials research. With a continuing tradition of excellence, we are trying to bridge the gap between academic research and practical problem solving as well as industrial needs for 65 years. The investigation of materials and materials related problems is mostly based on light, electron and ion microscopy. Please find here our most recent papers.
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 characterisation will thus aid the development of complex nanophotonic devices.
G. Haberfehlner, F.-P. Schmidt, G. Schaffernak, A. Hörl, A. Trügler, A. Hohenau, F. Hofer, J. R. Krenn, U. Hohenester, G. Kothleitner
Tomographic imaging of the photonic environment of plasmonic nanoparticles
The photonic local density of states (LDOS) governs the enhancement of light-matter interaction at the nanoscale, but despite its importance for nanophotonics and plasmonics experimental LDOS imaging remains extremely challenging. Here we introduce a tomography scheme based on electron microscopy that allows retrieval of the three-dimensional LDOS of plasmonic nanoparticles with nanometer spatial and sub-eV energy resolution. From conventional electron tomography experiments we obtain the three-dimensional morphology of the nanostructure, and use this information to compute an expansion basis for the photonic environment. The expansion coefficients are obtained through solution of an inverse problem using as input electron-energy loss spectroscopy images. We demonstrate the applicability of our scheme for silver nanocuboids and coupled nanodisks, and resolve LDOS enhancements with extreme sub-wavelength dimensions in hot spots located at roughness features or in gaps of coupled nanoparticles.
A. Hörl, G. Haberfehlner, A. Trügler, F.-P. Schmidt, U. Hohenester, G. Kothleitner
Direct-Write 3D Nanoprinting of Plasmonic Structures
While we recently showed a computer aided design for the reliable fabrication of complex 3D nanostructures via Focused Electron Beam Induced Deposition (FEBID) (DOI: 10.1021/acsnano.6b02108) we here demonstrate the applicability of such FEBID materials for plasmonic applications.
Specifically, FEBID´s direct-write capability of 3D nanoprinting on virtually any material and surface morphology was used to create complex 3D nanostructures of compact and pure gold. Theses freestanding gold architectures revealed strong plasmonic activity and pave the way for a new generation of 3D nanoplasmonic architectures that can be printed on-demand.
R. Winkler, F.-P. Schmidt, U. Haselmann, J. D. Fowlkes, B. B. Lewis, G. Kothleitner, P. D. Rack, H. Plank
Direct-Write Fabrication of Cellulose Nano-Structures via Focused Electron Beam Induced Nanosynthesis
In many areas of science and technology, patterned films and surfaces play a key role in engineering and development of advanced materials. Here, we introduce a new generic technique for the fabrication of polysaccharide nano-structures via focused electron beam induced conversion (FEBIC). For the proof of principle, organosoluble trimethylsilyl-cellulose (TMSC) thin films have been deposited by spin coating on SiO2 / Si and exposed to a nano-sized electron beam. It turns out that in the exposed areas an electron induced desilylation reaction takes place converting soluble TMSC to rather insoluble cellulose. After removal of the unexposed TMSC areas, structured cellulose patterns remain on the surface with FWHM line widths down to 70 nm. Systematic FEBIC parameter sweeps reveal a generally electron dose dependent behavior with three working regimes: incomplete conversion, ideal doses and over exposure. Direct (FT-IR) and indirect chemical analyses (enzymatic degradation) confirmed the cellulosic character of ideally converted areas. These investigations are complemented by a theoretical model which suggests a two-step reaction process by means of TMSC → cellulose and cellulose → non-cellulose material conversion in excellent agreement with experimental data. The extracted, individual reaction rates allowed the derivation of design rules for FEBIC parameters towards highest conversion efficiencies and highest lateral resolution.
T. Ganner, J. Sattelkow, B. Rumpf, M. Eibingener, D. Reishofer, R. Winkler, B. Nidetzky, S. Spirk, H. Plank
Edge Mode Coupling within a Plasmonic Nanoparticle
The coupling of plasmonic nanoparticles can strongly modify their optical properties. Here, we show that the coupling of the edges within a single rectangular particle leads to mode splitting and the formation of bonding and antibonding edge modes. We are able to unambiguously designate the modes due to the high spatial resolution of electron microscopy-based electron energy loss spectroscopy and the comparison with numerical simulations. Our results provide simple guidelines for the interpretation and the design of plasmonic mode spectra.
F.-P. Schmidt, H. Ditlbacher, A. Hohenau, U. Hohenester, F. Hofer, J.R. Krenn
Self-organized Sr Leads to Solid State Twinning in Nano-scaled Eutectic Si Phase
In this paper we propose a new mechanism for twin nucleation in the eutectic Al-Si alloy with trace Sr impurities. Observations made by sub-angstrom resolution scanning transmission electron microscopy and X-ray probing proved the presence of <110> Sr columns which are preferentially located at twin boundaries. Density functional theory simulations indicate that Sr atoms bind in the Si lattice only along the <110> direction; the preferential positions are at the first and second nearest neighbors for interstitial and substitutional Sr, respectively. Density functional theory total energy calculations confirm that twin nucleation at Sr columns is energetically favorable. Hence, twins may nucleate in Si precipitates after solidification, which provides a different perspective to the currently accepted mechanism suggesting that twin formation occurs during precipitate growth.
M. Albu, A. Pal, C. Gspan, R.C. Picu, F. Hofer & G. Kothleitner
Simulation-Guided 3D Nanomanufacturing via Focused Electron Beam Induced Deposition
Focused Electron Beam Induced Deposition (FEBID) is one of the very few techniques that enables a direct-write, bottom-up synthesis of free-standing 3D nanostructures on virtually any substrate. In the past, the fabrication of simple architectures has mostly been achieved by time-consuming trial and error approaches. To turn around the situation we introduced in our paper a hybrid Monte Carlo-continuum simulation for the predictable fabrication of complex 3D structures at the micro- and nanoscale. Additionally, a 3D computer-aided design (CAD) program is introduced, which allows initial design prior to any experiment which ultimately turns around the master-slave relation between design and fabrication. By that, FEBID is leveraged into the status of a true 3D Nanoprinter which will generically pave the way for entirely new applications that require 3D architectures.
J. D. Fowlkes, R. Winkler, B.B. Lewis, M.G. Stanford, H. Plank, P.D. Rack
Spatial localization of membrane degradation by in situ wetting and drying of membranes in the scanning electron microscope
A special in-house designed and fabricated in situ wetting stage was used to investigate the impact of chemicals on polymeric membranes. These flat sheet microfiltration membranes are widely used for separation purposes in medical as well as in beverage industries. Because of their broad applicability it is necessary to preserve their performance which is done by the frequent cleaning and disinfection with chemicals. Nevertheless over time, a loss in performance can be observed. The new investigation method allows to obtain spatial resolved results about the membrane degradation caused by different chemicals. In contrast, typically used methods provide only parameters which are mostly integrated over the whole membrane cross-section. In this paper we were able to show that the used in situ wetting investigation enables the detection of membrane degradation and the ability to identify the layer which is most affected by the chemical treatment. Additionally, the results were verified by Fourier transform infrared spectroscopy (FT-IR) and a method based in the absorption of small water droplets. The acquired results help to understand the mechanism of the degradation process in polymeric membranes due to chemicals and provide information for the membrane development.
M. Nachtnebel, H. Fitzek, C. Mayrhofer, B. Chernev, P. Pölt