Biomedical subjects

Ferdinand Hofer

Publications and source records attributed to Ferdinand Hofer.

5 recordsLinked to original sources

PubMed2003

Energy-filtering TEM at high magnification: spatial resolution and detection limits.

Energy-filtering TEM (EFTEM) has turned out to be a very efficient and rapid tool for the chemical characterization of a specimen on a nanometer and even subnanometer length scale. Especially, the detection and measurement of very thin layers has become a great application of this technique in many materials science fields, e.g. semiconductors and hard disk technology. There, the reliability of compositional profiles is an important issue. However, the experimentally obtainable spatial resolution strongly influences the appearance of a thin layer in an EFTEM image, when dimensions reach subnanometer levels, which mainly leads to a broadening of the layer in the image. This fact has to be taken into account, when measuring the thickness of such a thin layer. Additionally, the convolution decreases contrast which makes the layer less visible in the image and finally determines the detection limit. In this work we present a systematic study on specifically designed Mn/PdMn multilayer test specimens to explore the practical aspects of spatial resolution and detection limits in EFTEM. Although specific to the ionization edges used, we will present general conclusions about the practical limitations in terms of EFTEM spatial resolution. Additionally, work will be shown about low energy-loss imaging of thin oxide layers, where delocalization is the main factor responsible for broadening.

Journal Article↗

PubMed2003

Elemental occurrence maps: a starting point for quantitative EELS spectrum image processing.

A mechanism for automatic detection, identification and compositional quantification of elements in an EELS spectrum image is described. The method is capable of locating elemental occurrences, discovering signal overlaps, correctly modeling and subtracting the background, or alternatively fitting reference spectra to each image pixel to convert image intensities at any point in a spectrum image to a concentration without almost any operator input, thus paving the way for a completely automated spectrum image analysis. We describe the steps involved in extracting the elemental content in a spectrum image and demonstrate how an image can be derived that clearly reveals the problem zones that prevent accurate results in a subsequent quantification. Such an automatically generated image can then serve as a binary mask, which allows performing selective calculations on certain specimen areas, when applied to the original data set. We demonstrate the feasibility of such an approach by displaying examples computed from ceramic as well as alloy and steel specimens.

Journal Article↗

PubMed2003

Width determination of SiO2-films in Si-based devices using low-loss EFTEM: image contrast as a function of sample thickness.

Energy filtering transmission electron microscopy (EFTEM) has become one of the most efficient tools for specimen characterization at nanometer length scales. EFTEM imaging is most often carried out in the core-loss region but image intensity becomes more and more a limiting factor with decreasing feature size. Alternatively, it is possible to record EFTEM images in the low-loss region, where intensities are essentially higher and where in many cases the images contain material specific contrasts. In this paper we investigate the influence of the important parameters on the material contrast between silicon and silicon dioxide, e.g. specimen thickness, specimen orientation, energy-loss and energy selecting slit width. We show that sample thickness plays an important role and present two methods to calculate material contrast as a function of energy-loss and sample thicknesses. The first method uses spectra taken from both materials at different sample thickness by electron energy-loss spectroscopy, the second calculates contrast directly from a series of energy filtered images. From the results we determine the ideal acquisition parameters for the Si/SiO(2) system and demonstrate imaging at sufficient resolution below 2nm with a test sample of thin SiO(2) layers on Si.

Journal Article↗

PubMed2003

EELS performance measurements on a new high energy resolution imaging filter.

In this paper we characterize the spectroscopy performance of a new high resolution electron energy-loss spectrometer for transmission electron microscopy, recently installed in our lab. With the event of commercially available monochromated transmission electron microscopes, equipped with improved spectrometers, experiments became feasible that allow the study of chemical bonding and electronic structures of atoms and solids at the nanometer level with an energy resolution in the range of 0.2-0.3 eV. This significant enhancement in resolution, however, can only be fully harnessed with an optimized microscope and spectrometer setup and in a suitable local environment. We attempt to quantify some of the limiting factors on our system with emphasis on spectrum mixing, spectrometer aberrations and transmissivity and we sketch proper working conditions in order to achieve the desired performance.

Journal Article↗

PubMed2002

Seasonal- and age-dependent changes of the structure and chemical composition of the spherites in the midgut gland of the harvestmen Gyas annulatus (Opiliones).

Spherites--round laminated membrane bound structures--are located in the cytoplasm of all cell types of the midgut gland in the harvestmen Gyas annulatus. The spherites consist of an organic matrix composed of glycoproteins and proteoglycans whose sugar portion are carboxylated glycosaminoglycans. Different elements are embedded in this matrix, and their presence varies during the life cycle. All spherites in juveniles are composed of alternating concentrically arranged electron dense and electron lucent layers of organic matrix material before overwintering (December). At that time, spherites contain calcium, phosphorus and silicon. Calcium and phosphorus are located in their electron dense layers, and silicon spotwise, mainly peripheral. Material seems to be lost during overwintering of Gyas as electron lucent "empty" layers appear in spherites in March. The "lost" material could be used as energy supply and/or to provide molecules for synthesis processes during non-nourishing overwintering. Spherites do not contain calcium and phosphorus in July and October. These elements seem to have important biological relevance in the formation and hardening of the cuticle during the moultings in spring. In contrast to calcium and phosphorus, silicon is still stored in spherites in July and October but in decreasing concentration, therefore it could be involved in metabolic processes in adult Gyas. In the period from July to the end of their adult life in late autumn, an increasing number of spherites disintegrate and their remnant organic matrix material progressively aggregates with other cellular waste material in one or more huge vacuoles of glandular cells.

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