C. T. Koch

Low-loss EFTEM Imaging of Surface Plasmon Resonances in Ag Nanostructures

Understanding how light interacts with matter at the nanometer scale is a fundamental issue in optoelectronics, nanophotonics and nanoplasmonics. The optical properties of metallic nanoparticles are entirely dependent on collective excitations of their valence electrons, known as surface plasmon resonances (SPR), under electromagnetic illumination. Measuring these properties locally at the level of the individual nanoobject in combination with spectral information over the entire visible range constitutes a challenging issue for linking of the global response of the nanoparticles and the underlying structure and morphology. The visualization of localized SPRs on the nanometer scale in combination with spectral information over the entire visible range is of prime importance in the field of biosensors, surface-enhanced Raman spectroscopy, and for the design of metamaterials. But also the explanation of abnormal transmission of light through sub-wavelength holes relies on such information.

ELNES investigations of interfaces in abalone shell

A large number of living organisms produce an infinite variety of materials with excellent (physical, mechanical) properties that are superior when compared to modern, technologically advanced, manmade materials [1]. Their astonishing combination of biominerals (minerals formed inside living organisms) with organic compounds, that are arranged in complex architectures, leads to highly improved materials characteristics compared to their inorganic counterparts [2].

Sub-0.5 eV EFTEM mapping using the Zeiss SESAM

Co-based alloys are used in numerous commercial applications because of their excellent performance under severe environments, which require high-temperature strength, corrosion resistance, wear resistance, etc. In general, these alloys belong to a multi-component system, and the composition and heat and/or mechanical treatments to achieve a desired property are often complex. In this respect, understanding the microstructural changes during the processing is indispensable for the optimal selection of various processing parameters. In particular, the identification of phases in the early stages of phase transformation, which necessitates atomic level observations, is critical to elucidate the origin of the mechanical properties of the alloys.

Study of surface plasmon resonances on assemblies of slits in thin Ag films by low-loss EFTEM imaging

With the ongoing developments in nanotechnology, surface plasmon resonances (SPRs) have started to play a crucial role in many different areas of science. Surface Plasmon resonances are described as collective oscillations in the valence electron density at the surface of a conductor. They have especially received attention in the areas of biosensing in cancer diagnostics [1], near-field Raman spectroscopy [2] and different applications in optoelectronics. In this study, the optical response of a specially perforated thin Ag film is investigated with EFTEM. The experiments were carried out in the 200 kV FEG-TEM Sub-Electron-VoltSub-Angstrom-Microscope (Zeiss SESAM) equipped with an electrostatic monochromator and the in-column MANDOLINE filter [3]. The superior properties of this instrument enable EFTEM imaging in the ultraviolet–near-infrared domain with very high energy resolution and spatial sampling [4]. The Ag specimen was prepared as follows: Using physical vapour deposition, a Ag film with about 100 nm thickness was deposited onto a C film on a standard TEM Cu grid (the dimensions of each mesh is 100 × 100 μm). Focused ion beam (FIB) technique was used to drill different slit structures into the Ag film. The EFTEM series were acquired in the energy loss range from 0.4 eV to 5 eV by using a 0.19 eV energy slit and a step size of 0.2 eV. The EFTEM images were recorded on a 2k × 2k CCD camera with 8 times binning and an acquisition time of 30 sec / image (at each energy loss 3 images were recorded with an exposure time of 10 s and then aligned and averaged). Figure 1(a) shows a zero loss bright-field image of a double-slit structure with dimensions of 200 nm × 1 μm and a separation of 100 nm. A sample image taken from the drift-corrected EFTEM series at an energy loss of 0.6 eV is shown in figure 1(b). The intensity distribution is attributed to a localized plasmon resonance. Such resonances will be discussed and compared with numerical simulations [5].

Surface plasmon resonance effects in a perforated Ag film studied by energy-filtering TEM

The visualization of localized surface plasmon resonances (LSPR) on the nanometer scale in combination with spectral information over the entire visible range is of prime importance in the field of biosensors, surface-enhanced Raman spectroscopy (SERS), aperture-less scanning near-field optical microscopy (SNOM), and for the design of metamaterials. But also the understanding of the abnormal transmission of light through sub- wavelength holes may gain by this technique. With the advent of monochromators and highly dispersive energy filters, energy- filtering TEM has now become available for the study of the optical response of materials. This technique was applied to the detection of band gaps (1) as well as to the study of surface plasmons on metal particles, like Ag nanoprisms (2-4) or Au nanorods (5). Here, the dielectric response of holes in a 100 nm thick Ag film, drilled by using a focused ion beam, is studied by acquiring EFTEM series in the energy range between 0.4 and 4 eV using the Zeiss SESAM microscope (Fig.1). The energy-slit width was 0.2 eV. Apart from multipolar ring-shaped resonances, visible particularly at the isolated holes in the upper row, a number of LSPRs are found which are due to the strong coupling effects between adjacent holes. They sensitively depend on the hole arrangement (6).

Mapping grain boundary potentials in ceramics by nonlinear inline electron holography and impedance spectroscopy

The electrostatic potential arising from charge bound at grain boundary cores in ceramics and the accumulation of space charge in their vicinity is in many cases made responsible for the ion blocking or conducting behavior of electroceramics. While interpretation of impedance spectra of nominally undoped and acceptor-doped SrTiO3 ceramics and bicrystals implies that grain boundaries are positively charged and accompanied by a fairly wide region of negative space charge on both sides, a critical analysis of off-axis and inline electron holography data available in the literature yields very narrow potential profiles of the opposite sign. Some of the advantages of inline holography over off-axis holography are: (a) very simple experimental setup (works in any FEG-TEM, and for small details even with a LaB6 source), (b) possibility to record holograms far away from the specimen edge, (c) large fields of view because there is no need to oversample, and (d) specimen drift may easily be compensated, even during exposure. We will report on the application of a recently developed flux-preserving inline holography reconstruction algorithm [1] which allows the reconstruction of focal series recorded over a large focal range. Fig. 1 shows the (amplified) image distortions of the 15 images within a focal series of a near Σ13 grain boundary in SrTiO3. These image distortions are a side effect of changing the objective lens current by a large amount (the defocus ranged from -16μm to +13μm) and have been determined and corrected for by the reconstruction algorithm. In addition to fitting and correcting relative image distortions the reconstruction algorithm also refines the relative defocus and translation of each image in the course of the reconstruction. The reconstructed phase maps are shown in Figure 2. A double-tilt rotation holder has been used during the experiment. The grain boundary could therefore be aligned with the axis of the specimen holder. Rotating the holder by 18° therefore allowed a tilted projection of the grain boundary projection to be obtained (Fig. 2b). Using the local specimen thickness extracted from Fig. 2b the phase shift (Fig 2a) could be converted into a map of the mean inner potential (Fig. 3). Inline holography results of various bicrystal geometries processed under varying oxygen partial pressures will be presented and will be correlated with grain boundary potential profiles fitted to impedance spectroscopy data as well as analytical TEM data.

Effect of surface orientation on intrinsic island formation on SrTiO3 surfaces

Intrinsic islands were formed on Nb-doped SrTiO3 (100) and (111) surfaces by high-temperature annealing (1200°C) under oxygen atmosphere. The influence of surface orientation on the chemical composition and structure of islands was investigated by using Auger electron spectroscopy, energy-dispersive X-ray spectroscopy as well as electron diffraction in a transmission electron microscope. For a given island size, the same compositional profiles were observed for both surface orientations. While the bigger islands (with a height of several 100 nm) contain almost only Sr as cations, the smaller islands (with a height of few 10 nm) also contain Ti. Identical lattice-plane distances in all islands point to a distinct crystal structure for islands growing on (001) and (111) surfaces.

Low-loss-energy EFTEM imaging of triangular silver nanoparticles

Understanding how light interacts with matter at the nanometer scale is a fundamental issue in optoelectronics and nanophotonics. It is known that the optical properties of nanoparticles are entirely dependent on collective excitations of their valence electrons, known as “surface plasmon resonances” (SPR’s), under electromagnetic illumination. Measuring these properties locally at the level of the individual nano-object constitutes a challenging issue for linking of the global response of the nanoparticles and the underlying structure and morphology.

Band gap mapping using monochromated electrons

The recent development of monochromators for transmission electron microscopes has made valence electron energy-loss spectroscopy (VEELS) a powerful technique to study the semiconductor band structure with high spatial resolution. Albeit difficulties of the band structure measurements were encountered [1], solutions have been demonstrated for several material systems [2]. Taking advantage of the Zeiss SESAM microscope, an energy resolution below 100 meV is achieved routinely [3], which is highly appreciated for band structure measurements using low-loss EELS.

Nonlinear electron inline holography

The phase of an electron wave passing through a specimen can only be measured by interfering it with an external reference wave (off-axis holography) or itself (inline holography). Some of the advantages of inline holography over off-axis holography are: (a) very simple experimental setup (works in any TEM), (b) possibility to record holograms far away from the specimen edge, (c) large fields of view because there is no need to oversample, and (d) specimen drift may easily be compensated, even during exposure. The disadvantage: complicated non-linear equations have to be solved, while off-axis holography is linear. Most focal series reconstruction algorithms available in the literature apply the quasi-coherent approximation to the image formation equation which reduces flux with defocus. This is unphysical and works only for small defoci. We will report on first experimental results obtained with a flux-preserving inline holography reconstruction algorithm [1] which allows the reconstruction of focal series recorded over a large focal range.