Christian Liebscher

Interaction of L-Cysteine with ZnO: Structure, Surface Chemistry, and Optical Properties.

Zinc oxide (ZnO) nanoparticles (NPs) were stabilized in water using the amino acid l-cysteine. A transparent dispersion was obtained with an agglomerate size on the level of the primary particles. The dispersion was characterized by dynamic light scattering (DLS), pH dependent zeta potential measurements, scanning transmission electron microscopy (STEM), X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, photoluminescence (PL) spectroscopy, and X-ray absorption fine structure (EXAFS, XANES) spectroscopy. Cysteine acts as a source for sulfur to form a ZnS shell around the ZnO core and as a stabilizer for these core-shell NPs. A large effect on the photoluminescent properties is observed: the intensity of the defect luminescence (DL) emission decreases by more than 2 orders of magnitude, the intensity of the near band edge (NBE) emission increases by 20%, and the NBE wavelength decreases with increasing cysteine concentration corresponding to a blue shift of about 35 nm due to the Burstein-Moss effect.

Electronic structure of metastable bcc Cu–Cr alloy thin films: Comparison of electron energy-loss spectroscopy and first-principles calculations

Metastable Cu-Cr alloy thin films with nominal thickness of 300nm and composition of Cu67Cr33 (at%) are obtained by co-evaporation using molecular beam epitaxy. The microstructure, chemical phase separation and electronic structure are investigated by transmission electron microscopy (TEM). The thin film adopts the body-centered cubic crystal structure and consists of columnar grains with ~50nm diameter. Aberration-corrected scanning TEM in combination with energy dispersive X-ray spectroscopy confirms compositional fluctuations within the grains. Cu- and Cr-rich domains with composition of Cu85Cr15 (at%) and Cu42Cr58 (at%) and domain size of 1-5nm are observed. The alignment of the interface between the Cu- and Cr-rich domains shows a preference for {110}-type habit plane. The electronic structure of the Cu-Cr thin films is investigated by electron energy loss spectroscopy (EELS) and is contrasted to an fcc-Cu reference sample. The experimental EEL spectra are compared to spectra computed by density functional theory. The main differences between bcc-and fcc-Cu are related to differences in van Hove singularities in the electron density of states. In Cu-Cr solid solutions with bcc crystal structure a single peak after the L3-edge, corresponding to a van Hove singularity at the N-point of the first Brillouin zone is observed. Spectra computed for pure bcc-Cu and random Cu-Cr solid solutions with 10at% Cr confirm the experimental observations. The calculated spectrum for a perfect Cu50Cr50 (at%) random structure shows a shift in the van Hove singularity towards higher energy by developing a Cu-Cr d-band that lies between the delocalized d-bands of Cu and Cr.

Correlating Atom Probe Tomography with Atomic-Resolved Scanning Transmission Electron Microscopy: Example of Segregation at Silicon Grain Boundaries

In the course of a thorough investigation of the performance-structure-chemistry interdependency at silicon grain boundaries, we successfully developed a method to systematically correlate aberration-corrected scanning transmission electron microscopy and atom probe tomography. The correlative approach is conducted on individual APT and TEM specimens, with the option to perform both investigations on the same specimen in the future. In the present case of a Σ9 grain boundary, joint mapping of the atomistic details of the grain boundary topology, in conjunction with chemical decoration, enables a deeper understanding of the segregation of impurities observed at such grain boundaries.

In-situ TEM Study of Mechanical Size Effects in TiC Strengthened Steels

It is nowadays well understood that single crystalline metals exhibit a size dependence of the yield stress: with decreasing pillar diameters an increase in the flow stress is measured in micropillar compression tests. This size effect (“smaller is stronger”) is governed by the size of the activated dislocation sources and can be described by a power law equation relating stress and pillar diameter (σ ~ d). For single crystalline metallic pillars containing internal particles three different stress size dependencies are reported in literature: a power law [1, 2], a size-independent (i.e. constant) [3-6], or an intermediate behavior, which can be interpreted as a transition from constant to power law behavior [79]. The differences in size-dependency can be explained by two competing mechanisms, dislocation source activation and dislocation particle interaction [8, 9]. When the stress to activate dislocation sources exceeds the particle strengthening effect, pillars should show the “classical” power law dependence. In the present study we want to elaborate if we can quantitatively deduce the particle strengthening effect of e.g. shearable weak or non-shearable strong particles in metal materials from their transition regimes of size dependency. We have examined this idea in precipitation hardened steel with nanometer-sized titanium carbides (TiC) precipitates.

A Methodology for Investigation of Grain-Boundary Diffusion and Segregation

A grain boundary is the area where two grains of different crystal orientations join together. It is a type of two-dimensional crystal defects where lattice discontinuity and/or chemical inhomogeneity exist. Since the early 1980s, when Watanabe first introduced the idea of ‘grain boundary design’ [1], now often called grain boundary engineering (GBE), there has been an intensive effort to study grain boundaries. The key concept of GBE is by tailoring the structures of grain boundaries to improve the performance of polycrystalline and nanocrystalline materials. Scientists and engineers usually focus on the geometric characters of grain boundaries, which are defined by the orientation relationship between adjacent grains and the spatial orientation of the grain boundary plane. It was found that the properties and behaviors of grain boundaries largely depend on their geometric parameters [2]. For example, special grain boundaries with low-Σ CSL (coincidence-site lattice [3]), i.e. Σ3, Σ5, Σ7, etc. boundaries, have distinct properties such as lower energies, lower tendencies to trap impurities and solutes, lower electrical resistivity, as well as higher resistances to intergranular sliding or degradation as compared to random grain boundaries. Therefore, by tuning the grain boundary character distribution, desired bulk properties can be obtained. In recent years, with the development of advanced analytical techniques such as high-resolution (scanning) transmission electron microscopy (HR(S)TEM) and atom probe tomography (APT), grain boundary chemistry has drawn more and more attentions. Applications of those techniques have shown that grain boundaries can exhibit an array of different chemical states, i.e. clean or segregated with various elements, decorated with complexions or secondary phases, which resulted in significant differences in properties such as mobility, segregation and transport coefficients, mechanical, electric, magnetic, oxidation and corrosion behaviors. Accordingly, using compositional modifications to manipulate the properties of grain boundaries, and ultimately the performance of the material, has become a new concept for material design, termed as grain boundary segregation engineering [4] or complexion engineering [5]. In this context, understanding the formation of different grain boundary chemical states and identifying their compositions is of importance to the material science community.