Cryogenic Break-Junction Characterization of Single Organic Molecules

In this thesis, a cryogenic mechanically controllable break-junction (MCBJ) setup is developed and used for the characterization of single organic molecules at different temperatures. Molecules are the building blocks of life and among others responsible for charge transfer in biological processes,e.g. photosynthesis in plants and metabolism in humans. Since these processes are not yet completely understood, single molecules are highly interesting systems to study. Furthermore, due to their astonishing properties they might be used as electronic components in future technologies. This approach is called “molecular electronics”, existing representatives that use thin or thick layers of organic molecules are for instance light emitting diodes (OLED) or liquid crystal displays (LCD). However, single molecule devices are still not in use, and in order to build such systems the single molecules have to be understood, especially their electronic properties. Since the size of single molecules is typically of the order of nm, their characterization requires an appropriate setup. In this thesis a MCBJ is developed that traps single molecules between two electrodes, and thus allows to characterize its electronic features. The first aim of this thesis is the construction and development of a cryogenic MCBJ setup, consisting mainly of three parts: (i) the sophisticated sample holder, suitable for measurements inside the helium-flow cryostat, including a piezo-positioner to bend the sample, (ii) an automated setup to control the measurement parameters and acquire the data (e.g. temperature, applied voltage, measured current, piezo-position), and (iii) the development of suitable break-junction samples consisting mainly of a lithographically prepared Au bridge (with nm size dimensions). The three parts together allow measurements of molecules with a position control in the pm regime, currents ranging from below pA to mA, and temperatures ranging from 4 K to room temperature. The second aim is to develop and to establish adequate procedures for break-junction measurements, regarding mounting of the sample, deposition of molecules and measurement techniques. For the latter one, conductance–position characteristics (CPCs), current–voltages characteristics(IVC), standard CPC histograms and contour histograms are developed, automated and tested in reference measurements without molecules. The third aim is the measurement of simple “test-bed” molecules. Hexanedithiol and benzenedithiol are chosen as representatives for simple alkanes and conjugated molecules, respectively. CPCs of the rod-like hexanedithiol show a distinct peak in agreement with literature values, while BeDT exhibits a more complex behavior. IVCs are performed on the molecules and molecular levels are obtained. The curves demonstrate the quality of the measurement techniques and the analysis methods. Finally, the fourth aim is the measurement of the temperature dependent electronic properties of the more complex molecules terphenyldithiol (TPT) and porphyrine (TPyP). The latter one plays an important role in biological processes and represents a promising candidate for molecular electronics. We find, that TPT exhibits a distinct and strongly temperature dependent conductance peak, while the level of the molecular orbital is independent of temperature. It is shown that for TPT a transition from direct tunneling to “hopping” mechanism takes place around a temperature of 100K. The TPyP displays unusual CPCs. Only tilted plateaus are observed, that are indicative for clustering of the molecule. Contour histograms demonstrate the presence of these clusters in the break-junction. IVCs of TPyP reveal a temperature dependent electronic or vibrational mode. It shifts towards higher voltages with increasing temperature and is therefore only detectable belowT $\lesssim$ 180K. Above this temperature reordering of the molecules and the molecule-Au bonds leadto large noise in the IVC at large voltages. The MCBJ represents an elegant way to detect interesting electronic properties of single molecules. Here, a major aspect is the great stability of the metal-molecule-metal system that allows extensive characterization even in the case of complex molecules. In this thesis it is shown, that lower temperatures improve the performance of the MCBJ, and, even more crucial, temperature dependent measurements allow a deeper insight in the charge transfer of single molecules

The multiplicity of massive stars in the Orion Nebula cluster as seen with long-baseline interferometry

The characterization of multiple stellar systems is an important ingredient for testing current star formation models. Stars are more often found in multiple systems, the more massive they are. A complete knowledge of the multiplicity of high-mass stars over the full range of orbit separations is thus essential to understand their still debated formation process. Observations of the Orion Nebula Cluster can help to answer the question about the origin and evolution of multiple stars. Earlier studies provide a good knowledge about the multiplicity of the stars at very small (spectroscopic) and large separations (AO, speckle) and thus make the ONC a good target for such a project. We used the NIR interferometric instrument AMBER at VLTI to observe a sample of bright stars in the ONC. We complement our data set by archival NACO observations of \theta 1 Ori A to obtain more information about the orbit of the close visual companion. Our observations resolve the known multiple systems \theta 1 Ori C and \theta 1 Ori A and provide new orbit points, which confirm the predicted orbit and the determined stellar parameters for \theta 1 Ori C. Combining AMBER and NACO data for \theta 1 Ori A we were able to follow the motion of the companion from 2003 to 2011. We furthermore find hints for a companion around \theta 1 Ori D and a previously unknown companion to NU Ori. With a probability of ~90% we can exclude further companions with masses of > 3 Msun around our sample stars for separations between ~2 mas and ~110 mas. We conclude that the companion around \theta 1 Ori A is most likely physically related to the primary star. The newly discovered possible companions further increase the multiplicity in the ONC. For our sample of two O and three B-type stars we find on average 2.5 known companions per primary, which is around five times more than for low-mass stars.Comment: accepted by A&

VLTI status update: a decade of operations and beyond

We present the latest update of the European Southern Observatory's Very Large Telescope interferometer (VLTI). The operations of VLTI have greatly improved in the past years: reduction of the execution time; better offering of telescopes configurations; improvements on AMBER limiting magnitudes; study of polarization effects and control for single mode fibres; fringe tracking real time data, etc. We present some of these improvements and also quantify the operational improvements using a performance metric. We take the opportunity of the first decade of operations to reflect on the VLTI community which is analyzed quantitatively and qualitatively. Finally, we present briefly the preparatory work for the arrival of the second generation instruments GRAVITY and MATISSE.Comment: 10 pages, 7 figures, Proceedings of the SPIE, 9146-1

The Astropy Problem

The Astropy Project (http://astropy.org) is, in its own words, "a community effort to develop a single core package for Astronomy in Python and foster interoperability between Python astronomy packages." For five years this project has been managed, written, and operated as a grassroots, self-organized, almost entirely volunteer effort while the software is used by the majority of the astronomical community. Despite this, the project has always been and remains to this day effectively unfunded. Further, contributors receive little or no formal recognition for creating and supporting what is now critical software. This paper explores the problem in detail, outlines possible solutions to correct this, and presents a few suggestions on how to address the sustainability of general purpose astronomical software

Influence of Hygrothermal Aging on Poisson’s Ratio of Thin Injection-Molded Short Glass Fiber-Reinforced PA6

The hygrothermal aging of short glass fiber-reinforced polyamide 6 materials (PA6 GF) represents a major problem, especially in thin-walled components, such as in the automotive sector. In this study, therefore, the thickness and the glass fiber content of PA6 GF materials were varied and the materials were exposed to hygrothermal aging. The temperature and relative humidity were selected in the range from −40 °C up to 85 °C, and from 10% up to 85% relative humidity (RH). In the dry-as-molded state, the determined Poisson’s ratio of the PA6 GF materials was correlated with the fiber orientation based on computer tomography (MicroCT) data and shows a linear dependence with respect to the fiber orientation along and transverse to the flow direction of the injection molding process. With hygrothermal aging, the value of Poisson’s ratio increases in the flow direction in the same way as it decreases perpendicular to the flow direction due to water absorption

Fracture mechanics and statistical mechanics of reinforced elastomeric blends

Elastomers are found in many applications ranging from technology to daily life applications for example in tires, drive systems, sealings and print rollers. Dynamical operation conditions put extremely high demands on the performance and stability of these materials and their elastic and flow properties can be easily adjusted by simple manipulations on their elastic and viscous properties. However, the required service life suffers often from material damage as a result of wear processes such as abrasion and wear fatigue, mostly caused by crack formation and propagation. This book covers interdisciplinary research between physics, physical chemistry, material sciences and engineering of elastomers within the range from nanometres to millimetres and connects these aspects with the constitutive material properties. The different chapters describe reliable lifetime and durability predictions based on new fracture mechanical testing concepts and advanced material-theoretical methods which are finally implemented in the finite element method for structural simulations. The use of this approach allows a realistic description of complex geometrical and loading conditions which includes the peculiarities of the mechanical behaviour of elastomeric materials in detail. Furthermore, this approach demonstrates how multi-scale research concepts provide an ambitious interdisciplinary challenge at the interface between engineering and natural sciences. This book covers the interests of academic researchers, graduate students and professionals working in polymer science, rubber and tire technology and in materials science at the interface of academic and industrial research

Stress Generated Modifications of Structural and Morphologic Properties of Epitaxial SrTiO 3 Films on Sapphire

Owing to the strong connection between strain and ferroelectricity, large shifts of the Curie temperature and polarization are observed in strained ferroelectric material. In this work, the effects of lattice-mismatch induced stress upon the crystallographic structure, strain, and generation of different types of defects in epitaxial SrTiO3 films on CeO2 buffered sapphire are examined and discussed in context with the resulting impact of strain on the polarization of the ferroelectric layers. Depending on the thickness of the SrTiO3 layer, characteristic changes in their structural perfection and crystallographic orientation with respect to the substrate system are observed. For thin films, misfit dislocations partially compensate the stress in the SrTiO3 layer, whereas cracks develop in thicker SrTiO3 films. The structural modifications and the formation of defects are explained in a model based on lattice misfit induced stress and energy considerations. It is demonstrated that intrinsic mismatch and thermal mismatch must be considered to explain strain dependent effects such as induced ferroelectricity and modifications of the permittivity of these complex heteroepitaxial layer systems

Electronic characterization of polar nanoregions in relaxor-type ferroelectric NaNbO3 films

Strained NaNbO3 films of different thicknesses are epitaxially grown on (110) NdGaO3 substrates. A detailedanalysis of the permittivity of these films demonstrates that strain not only leads to a modification of thepermittivity and the ferroelectric transition temperature, it also results in a pronounced relaxor-type behavior andallows a direct estimation of the size and mobility of the polar nanoregions (PNRs). The compressive strain reducesthe transition temperature to 125 K and enhances the corresponding permittivity up to ε ≈ 1500 for the thinnestfilm. Since the strain relaxes with increasing film thickness, both effects, reduction of phase transition temperatureand enhancement of ε, depend on the thickness of the film. The films show a characteristic frequency and electricfield dependence of ε, which is discussed in terms of the Vogel-Fulcher equation and Rayleigh law, respectively.Using the electric field dependence of the resulting freezing temperature TVF, allows a direct estimation of thevolume of the PNRs at the freezing temperature, i.e. from 70 to 270 nm3. Assuming an idealized spherical shapeof the PNRs, diameters of a few nanometers (5.2–8 nm) are determined that depend on the applied ac electricfield. The irreversible part of the polarization seems to be dominated by the presence and mobility of the PNRs. Itshows a characteristic peak at low temperature around TVF, vanishes at a temperature where the activation energyof the PRNs extrapolates to zero, and shows a frequency dispersion that is characteristic for relaxor-type behavior