Optimization of the carrier concentration in phase-separated half-Heusler compounds

Inspired by the promising thermoelectric properties of phase-separated half-Heusler materials, we investigated the influence of electron doping in the n-type Ti_(0.3−x)Zr_(0.35)Hf_(0.35)NiSn compound. The addition of Nb to this compound led to a significant increase in its electrical conductivity, and shifted the maximum Seebeck coefficient to higher temperatures owing to the suppression of intrinsic carriers. This resulted in an enhancement of both the power factor α^2σ and figure of merit, zT. The applicability of an average effective mass model revealed the optimized electron properties for samples containing Nb. There is evidence in the literature that the average effective mass model is suitable for estimating the optimized carrier concentration of thermoelectric n-type half-Heusler compounds

Nanoplasmonic Sensing for Materials Science

With the rising importance of nanoscience and nanotechnology, there is a need for new sensitive and easy-to-use characterization techniques able to follow processes at the nanoscale. In this thesis different aspects of nanoplasmonic sensing for studying materials science processes at the nanoscale are demonstrated and discussed for the following model systems: oxidation/corrosion of Al and Cu and the solid-liquid phase transition of Sn.Nanoplasmonic sensing relies on the excitation of localized surface plasmons (LSPR) in metal nanoparticles. The resonance details are very sensitive to optical property changes in/on the nanoparticles themselves or in their nano- scale neighborhood, e. g., surface oxidation/corrosion.The corrosion of Al and Cu nanoparticles and thin films was studied using nanoplasmonic sensing in various environments like dry and humid air and liquid water and (for Cu) with and without a corrosion inhibitor. Corrosion kinetics were measured with submonolayer sensitivity – even in the case of very slow corrosion, such as in mildly oxidizing environments and when the metal surface was protected by a corrosion inhibitor.The solid-liquid (melting-freezing) phase transition in Sn nanoparticles was investigated by nanoplasmonic sensing. The undercooling as well as the melting and freezing kinetics were measured and analyzed theoretically.In order to gain broad information about studied systems, it is often desirable to combine several techniques in situ, with the same sample. Nanoplasmonic sensing is very suitable for such combinations. Here, experimental integration was realized of nanoplasmonic sensing with quartz crystal microbal- ance with dissipation monitoring (QCM-D) and with vibrational sum frequency spectroscopy (VSFS).This thesis demonstrates that nanoplasmonic sensing is a highly sensitive, fast, easy-to-use, and versatile technique that can be used to monitor a variety of processes in materials science in situ and in real time

Kinetic Measurements Using Nanoplasmonic Sensing

In this thesis the nanoplasmonic sensing technique was used to study kinetics of (i) the oxidation of Al nanoparticles in air and water and (ii) the solid-liquid phase transition in Sn nanoparticles. The nanoplasmonic sensing technique detects changes of the localized surface plasmon resonance (LSPR) in metal nanoparticles.When light shines on metal nanoparticles, at some wavelength the conduction electrons oscillate in resonance with the light. This resonance is called LSPR. It depends on the electronic structure, size, and shape of the nanoparticle as well as the optical properties of its nanoenvironment.(i) LSPRs in Al nanoparticles were characterized in detail: Extinction, scattering, and absorption efficiencies were determined experimentally for several different nanodisk sizes in the UV-vis-NIR spectral range. The experimental values were shown to be in good agreement with calculations based on the modified long wavelength approximation (MLWA).The oxidation in air of these Al particles was followed for long time periods. The results showed that nanoplasmonic sensing has potential for oxidation studies of metallic nanoparticles. In a more elaborate study, using both the nanoplasmonic sensing technique and quartz crystal microbalance with dissipation monitoring (QCM-D), highly resolved oxidation kinetics were measured for oxidation in water. MLWA model calculations of the LSPR facilitated interpretation of the results. Oxidation of Al nanoparticles in water was found to proceed in several stages, as reported for bulk Al, forming presumably pseudoboehmite (Al2O3\ub7H2O) in the process.(ii) LSPRs in solid and liquid Sn nanoparticles were studied in the temperature range 25-250◦C. Distinct changes of the LSPR features occur when the particles melt and freeze. A large hysteresis between the melting and freezing point is observed. Obviously, nucleation of the solid phase is hindered unless the temperatures is considerably below the melting point. Kinetic measurements of the freezing event at different constant temperatures were performed. Classical nucleation theory with modifications considering the finite nanoparticle dimensions accounts well for the observed behavior