Ute Salzberger

Linking Microstructure and Nanochemistry in Human Dental Tissues

Mineralized dental tissues and dental pulp were characterized using advanced analytical transmission electron microscopy (TEM) methods. Quantitative X-ray energy dispersive spectroscopy was employed to determine the Ca/P and Mg/P concentration ratios. Significantly lower Ca/P concentration ratios were measured in peritubular dentine compared to intertubular dentine, which is accompanied by higher and variable Mg/P concentration ratios. There is strong evidence that magnesium is partially substituting calcium in the hydroxyapatite structure. Electron energy-loss near-edge structures (ELNES) of C-K and O-K from enamel and dentine are noticeably different. We observe a strong influence of beam damage on mineralized dental tissues and dental pulp, causing changes of the composition and consequently also differences in the ELNES. In this article, the importance of TEM sample preparation and specimen damage through electron irradiation is demonstrated.

Atomic-Scale Quantitative Analysis of Lattice Distortions at Interfaces of Two-Dimensionally Sr-Doped La2CuO4 Superlattices

Using spherical aberration corrected high-resolution and analytical scanning transmission electron microscopy, we have quantitatively studied the lattice distortion and the redistribution of charges in two-dimensionally strontium (Sr)-doped La2CuO4 superlattices, in which single LaO planes are periodically replaced by SrO planes. As shown previously, such structures show Tc up to 35 K as a consequence of local charge accumulation on both sides of the nominal SrO planes position. This is caused by two distinct mechanisms of doping: heterogeneous doping at the downward side of the interface (space–charge effect) and “classical” homogeneous doping at the upward side. The comparative chemical and atomic-structural analyses reveal an interrelation between local CuO6 octahedron distortions, hole spatial distribution, and chemical composition. In particular we observe an anomalous expansion of the apical oxygen–oxygen distance in the heterogeneously doped (space–charge) region, and a substantial shrinkage of the apical oxygen–oxygen distance in the homogeneously doped region. Such findings are interpreted in terms of different Jahn–Teller effects occurring at the two interface sides (downward and upward).

Microstructure, chemistry, and electronic structure of natural hybrid composites in abalone shell

The crystal structure and chemical composition at the inorganic/inorganic and inorganic/organic interfaces in abalone shell (genus Haliotis) were investigated using advanced analytical transmission electron microscopy (TEM) methods. Electron energy-loss near-edge structures (ELNES) of Ca-M2,3, C-K, Ca-L2,3, O-K and low-loss EEL spectra acquired from aragonite and calcite are distinctly different. When comparing biogenic with inorganic material for aragonite, only minor differences in C-K fine structures could be detected. The crystal structure of the mineral bridges was confirmed by ELNES experiments. ELNES and energy-filtered TEM (EFTEM) experiments of regular and self-healed interfaces between nacreous aragonite and prismatic calcite reveal relatively rough transitions. In this work, the importance of TEM specimen preparation and specimen damage on structural features is discussed.

Towards atomically resolved EELS elemental and fine structure mapping via multi-frame and energy-offset correction spectroscopy

Electron energy-loss spectroscopy and energy-dispersive X-ray spectroscopy are two of the most common means for chemical analysis in the scanning transmission electron microscope. The marked progress of the instrumentation hardware has made chemical analysis at atomic resolution readily possible nowadays. However, the acquisition and interpretation of atomically resolved spectra can still be problematic due to image distortions and poor signal-to-noise ratio of the spectra, especially for investigation of energy-loss near-edge fine structures. By combining multi-frame spectrum imaging and automatic energy-offset correction, we developed a spectrum imaging technique implemented into customized DigitalMicrograph scripts for suppressing image distortions and improving the signal-to-noise ratio. With practical examples, i.e. SrTiO3 bulk material and Sr-doped La2CuO4 superlattices, we demonstrate the improvement of elemental mapping and the EELS spectrum quality, which opens up new possibilities for atomically resolved EELS fine structure mapping.

Magnesium-Assisted Continuous Growth of Strongly Iron-Enriched Incisors

Teeth are an excellent example where optimally designed nanoarchitectures with precisely constructed components consist of simple compounds. Typically, these simple constituent phases with insignificant properties show mechanical property amplifications when formed into composite architectures. Material properties of functional composites are generally regulated on the nanoscale, which makes their characterization particularly demanding. Using advanced analytical and imaging transmission electron microscopy techniques, we identified innovative microstructural adjustments combined with astonishing compositional adaptations in incisors of coypu. Unique constituents, recognized as an additional amorphous Fe-rich surface layer followed by a transition zone covering pigmented enamel, provide the required structural stability to withstand repeated mechanical load. The chemically diverse Fe-rich surface layer, including ferrihydrite and iron-calcium phosphates, gives the typical orange-brown coloration to the incisors. Within the spaces between elongated hydroxyapatite crystals in the pigmented enamel, only ferrihydrite was found, implying that enamel pigmentation is a very strictly controlled process. Most significantly, an unprecedentedly high amount of Mg was measured in the amorphous flake-like material within the dentinal tubules of the incisors, suggesting the presence of a (Mg,Ca) phosphate phase. This unusually high influx of Mg into the dentin of incisors, but not molars, suggests a substantial functionality of Mg in the initial formation stages and constant growth of incisors. The present results emphasize the strong mutual correlation among the microstructure, chemical composition, and mechanical properties of mineralized dental tissues.

Correcting the linear and nonlinear distortions for atomically resolved STEM spectrum and diffraction imaging

We report a software tool for post-correcting the linear and nonlinear image distortions of atomically resolved 3D spectrum imaging as well as 4D diffraction imaging. This tool improves the interpretability of distorted scanning transmission electron microscopy spectrum/diffraction imaging data.

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].

STEM SI Warp: a Digital Micrograph script tool for warping the image distortions of atomically resolved spectrum image

Besides conventional imaging techniques, an important capability of modern scanning transmission electron microscopy (STEM) is its integration with microanalysis techniques, such as electron energy loss spectroscopy (EELS) and energy dispersive X-ray spectroscopy (EDXS). In these methods, a focused electron probe raster across the sample and at each probe position imaging signal (s) (ADF or HAADF) and analytical signal(s) can be acquired simultaneously. The marked progress of the instrumentation hardware, i.e., aberration correctors, monochromators, better detectors /cameras, and more stable electron optics, sample holder and instrument environments, has made the chemical analysis at atomic resolution readily possible nowadays. Generally, STEM EELS and EDXS require much higher electron dose and longer dwell time (pixel exposure time) than STEM imaging. As a consequence, the scanning distortion becomes more serious in spectrum imaging. In case of lower-magnification spectrum mapping, these instabilities may be negligible, but at atomic resolution level, instabilities combined with long dwell times may create substantial image distortions, i.e. expansion, compression, and/or shearing of the lattice, that limit the interpretability of the spectrum image.

Oxygen Octahedral Picker: A Digital Micrograph Script Tool for Extracting Quantitative Information From HAADF and ABF Images

The multifaceted magnetic, electrical, and structural functionalities of perovskite oxides are underpinned by the distortions of the crystal lattice [1]. These distortions include the displacement of cations, deformation of oxygen octahedra (BO6, where B is a transition metal atom), and collective tilts of the octahedral network. Controlling and engineering these distortions in the constituent oxides are crucial in designing and fabricating hetero-structures with novel functional properties that are absent in the bulk form. Atomistic understanding of these distortions and elucidation of their influence on the final properties requires imaging and measuring of atomic positions of both cations and oxygen. With the application of spherical aberration (Cs) correctors, sub-Angstrom atomic resolution is nowadays regularly achievable in both TEM and STEM. The recent application of the annular bright-field (ABF) imaging technique in perovskite oxides has become increasingly popular, as it enables simultaneous imaging of heavy and light elements and allows for simultaneous acquisition of other signals [2, 3]. Quantitative analysis of atom-column positions in high-resolution STEM images helps us gain new insight into materials behavior and address material problems. However, there are rare resources about methods for indexing atomic column positions in atomic resolution images, especially for the case of ABF images, as well as for the measurement accuracy.

Surprising high iron enrichment in hard dental tissues of rodents

Biominerals formed by living organisms display highly diverse composition and structure. Many of such biominerals are highly complex composite materials with excellent physical and mechanical properties [1, 2] that cannot be mimicked in the laboratory. These unique masterpiece architectures where organic matrix and crystalline or amorphous minerals are linked together are formed under conditions of moderate temperature, pressure and pH.