New design of polyphenylene dendrimers for full-color light-emitting diodes

Dendritic systems, and in particular polyphenylene dendrimers, have recently attracted considerable attention from the synthetic organic chemistry community, as well as from photophysicists, particularly in view of the search for synthetic model analogies to photoelectric materials to fabricate organic light-emitting diodes (OLEDs), and even more advanced areas of research such as light-harvesting system, energy transfer and non-host device. Geometrically, dendrimers are unique systems that consist of a core, one or more dendrons, and surface groups. The different parts of the macromolecule can be selected to give the desired optoelectronic and processing properties. Compared to small molecular or polymeric light-emitting materials, these dendritic materials can combine the benefits of both previous classes. The high molecular weights of these dendritic macromolecules, as well as the surface groups often attached to the distal ends of the dendrons, can improve the solution processability, and thus can be deposited from solution by simple processes such as spin-coating and ink-jet printing. Moreover, even better than the traditional polymeric light-emitting materials, the well-defined monodisperse distributed dendrimers possess a high purity comparable to that of small molecules, and as such can be fabricated into high performance OLEDs. Most importantly, the emissive chromophores can be located at the core of the dendrimer, within the dendrons, and/or at the surface of the dendrimers because of their unique dendritic architectures. The different parts of the macromolecule can be selected to give the desired optoelectronic and processing properties. Therefore, the main goals of this thesis are the design and synthesis, characterization of novel functional dendrimers, e.g. polytriphenylene dendrimers for blue fluorescent, as well as iridium(III) complex cored polyphenylene dendrimers for green and red phosphorescent light emitting diodes. In additional to the above mentioned advantages of dendrimer based OLEDs, the modular molecular architecture and various functionalized units at different locations in polyphenylene dendrimers open up a tremendous scope for tuning a wide range of properties in addition to color, such as intermolecular interactions, charge mobility, quantum yield, and exciton diffusion. In conclusion, research into dendrimer containing OLEDs combines fundamental aspects of organic semiconductor physics, novel and highly sophisticated organic synthetic chemistry and elaborate device technology.rn

Fingering instability down the outside of a vertical cylinder

We present an experimental and numerical study examining the dynamics of a gravity-driven contact line of a thin viscous film traveling down the outside of a vertical cylinder of radius R. Experiments on cylinders with radii ranging between 0.159 and 3.81 cm show that the contact line is unstable to a fingering pattern for two fluids with differing viscosities, surface tensions, and wetting properties. The dynamics of the contact line is studied and results are compared to previous studies of inclined plane experiments in order to understand the influence substrate curvature plays on the fingering pattern. A lubrication model is derived for the film height in the limit that ε = H/R≪1, where H is the upstream film thickness, and in terms of a Bond number ρgR3/(γH), and the linear stability of the contact line is analyzed using traveling wave solutions. Curvature controls the capillary ridge height of the traveling wave and the range of unstable wavelength when ε = O(10-1), whereas the shape and stability of the contact line converge to the behavior one observes on a vertical plane when ε ≤ O(10-2). The most unstable wave mode, cutoff wave mode for neutral stability, and maximum growth rate scale as 0.45 where = ρgR2/γ ≥ 1.3, and the contact line is unstable to fingering when ≥ 0.56. Using the experimental data to extrapolate outside the range of validity of the thin film model, we estimate the contact line is stable when <0.56. Agreement is excellent between the model and the experimental data for the wave number (i.e., number of fingers) and wavelength of the fingering pattern that forms along the contact line.

Characterization of vascular cavity coatings and microscopic tube-shaped structures from Upper Cretaceous dinosaur bone: evidence of a bacterial origin for dinosaur blood vessels""

Recent claims of blood vessels extracted from dinosaur fossils challenge classical views of soft-tissue preservation. Alternatively, these structures may represent postdepositional,diagenetic biofilms that grew on vascular cavity surfaces within the fossil. Similar red, hollow, tube-shaped structures were recovered from well-preserved and poorly-preserved (abraded, desiccated, exposed) Upper Cretaceous dinosaur fossils in this study. Integration of light microscopy, scanning electron microscopy, and energy dispersive x-ray spectroscopy was used to compare these vessel structures to the fossils from which they are derived. Vessel structures are typically 100-400 μm long, 0.5-1.5 μm thick, 10-40 μm in diameter and take on a wide range of straight, curved, andbranching morphologies. Interior surfaces vary from smooth to globular and typically contain spheres, rods, and fibrous structures (< 2 μm in diameter) incorporated into the surface. Exterior surfaces exhibit 2-μm-tall converging ridges, spaced 1-3 μm apart, that are sub-parallel to the long axis of the vessel structure. Fossil vascular cavities are typically coated with a smooth or grainy orange layer that shows a wide range of textures including smooth, globular, rough, ropy, and combinations thereof. Coatings tend to overlay secondary mineral crystals and framboids, confirming they are not primary structures of the fossil. For some cavity coatings, the surface that had been in contact with the bone exhibits a ridged texture, similar to that of vessel structures, having formed as a mold of the intravascular bone surface. Thus, vessel structures are interpreted as intact cavity coatings isolated after the fossil is demineralized. The presence of framboids and structures consistent in size and shape with bacteria cells, the abundance of iron in cavity coatings, and the growth of biofilms directly from the fossil that resemble respective cavity coatings support the hypothesis that vessel structures result from ironconsuming bacteria that form biofilms on the intravascular bone surfaces of fossil dinosaur bone. This also accounts for microstructures resembling osteocytes as some fossil lacunae are filled with the same iron oxide that comprises vessel structures andcoatings. Results of this study show that systematic, high-resolution SEM analyses of vertebrate fossils can provide improved insight on microtaphonomic processes, including the role of bacteria in diagenesis. These results conflict with earlier claims of dinosaurblood vessels and osteocytes.

Self-adjoint extensions for confined electrons: From a particle in a spherical cavity to the hydrogen atom in a sphere and on a cone

In a recent study of the self-adjoint extensions of the Hamiltonian of a particle confined to a finite region of space, in which we generalized the Heisenberg uncertainty relation to a finite volume, we encountered bound states localized at the wall of the cavity. In this paper, we study this situation in detail both for a free particle and for a hydrogen atom centered in a spherical cavity. For appropriate values of the self-adjoint extension parameter, the bound states localized at the wall resonate with the standard hydrogen bound states. We also examine the accidental symmetry generated by the Runge–Lenz vector, which is explicitly broken in a spherical cavity with general Robin boundary conditions. However, for specific radii of the confining sphere, a remnant of the accidental symmetry persists. The same is true for an electron moving on the surface of a finite circular cone, bound to its tip by a 1/r1/r potential.

Experimental results on gravity driven fully condensing flows in vertical tubes, their agreement with theory, and their differences with shear driven flows' boundary-condition sensitivities

This doctoral thesis presents the experimental results along with a suitable synthesis with computational/theoretical results towards development of a reliable heat transfer correlation for a specific annular condensation flow regime inside a vertical tube. For fully condensing flows of pure vapor (FC-72) inside a vertical cylindrical tube of 6.6 mm diameter and 0.7 m length, the experimental measurements are shown to yield values of average heat transfer co-efficient, and approximate length of full condensation. The experimental conditions cover: mass flux G over a range of 2.9 kg/m2-s ≤ G ≤ 87.7 kg/m2-s, temperature difference ∆T (saturation temperature at the inlet pressure minus the mean condensing surface temperature) of 5 ºC to 45 ºC, and cases for which the length of full condensation xFC is in the range of 0 < xFC < 0.7 m. The range of flow conditions over which there is good agreement (within 15%) with the theory and its modeling assumptions has been identified. Additionally, the ranges of flow conditions for which there are significant discrepancies (between 15 -30% and greater than 30%) with theory have also been identified. The paper also refers to a brief set of key experimental results with regard to sensitivity of the flow to time-varying or quasi-steady (i.e. steady in the mean) impositions of pressure at both the inlet and the outlet. The experimental results support the updated theoretical/computational results that gravity dominated condensing flows do not allow such elliptic impositions.

A Surface Displacement Analysis for Volcan Pacaya from October 2001 through March 2013 by Means of 3-D Modeling of Precise Position GPS Data

Volcán Pacaya is one of three currently active volcanoes in Guatemala. Volcanic activity originates from the local tectonic subduction of the Cocos plate beneath the Caribbean plate along the Pacific Guatemalan coast. Pacaya is characterized by generally strombolian type activity with occasional larger vulcanian type eruptions approximately every ten years. One particularly large eruption occurred on May 27, 2010. Using GPS data collected for approximately 8 years before this eruption and data from an additional three years of collection afterwards, surface movement covering the period of the eruption can be measured and used as a tool to help understand activity at the volcano. Initial positions were obtained from raw data using the Automatic Precise Positioning Service provided by the NASA Jet Propulsion Laboratory. Forward modeling of observed 3-D displacements for three time periods (before, covering and after the May 2010 eruption) revealed that a plausible source for deformation is related to a vertical dike or planar surface trending NNW-SSE through the cone. For three distinct time periods the best fitting models describe deformation of the volcano: 0.45 right lateral movement and 0.55 m tensile opening along the dike mentioned above from October 2001 through January 2009 (pre-eruption); 0.55 m left lateral slip along the dike mentioned above for the period from January 2009 and January 2011 (covering the eruption); -0.025 m dip slip along the dike for the period from January 2011 through March 2013 (post-eruption). In all bestfit models the dike is oriented with a 75° westward dip. These data have respective RMS misfit values of 5.49 cm, 12.38 cm and 6.90 cm for each modeled period. During the time period that includes the eruption the volcano most likely experienced a combination of slip and inflation below the edifice which created a large scar at the surface down the northern flank of the volcano. All models that a dipping dike may be experiencing a combination of inflation and oblique slip below the edifice which augments the possibility of a westward collapse in the future.

Investigating the sensitivity of hurricane intensity and trajectory to sea surface temperatures using the regional model WRF

The influence of sea surface temperature (SST) anomalies on the hurricane characteristics are investigated in a set of sensitivity experiments employing the Weather Research and Forecasting (WRF) model. The idealised experiments are performed for the case of Hurricane Katrina in 2005. The first set of sensitivity experiments with basin-wide changes of the SST magnitude shows that the intensity goes along with changes in the SST, i.e., an increase in SST leads to an intensification of Katrina. Additionally, the trajectory is shifted to the west (east), with increasing (decreasing) SSTs. The main reason is a strengthening of the background flow. The second set of experiments investigates the influence of Loop Current eddies idealised by localised SST anomalies. The intensity of Hurricane Katrina is enhanced with increasing SSTs close to the core of a tropical cyclone. Negative nearby SST anomalies reduce the intensity. The trajectory only changes if positive SST anomalies are located west or north of the hurricane centre. In this case the hurricane is attracted by the SST anomaly which causes an additional moisture source and increased vertical winds.

Surface-patterned micromechanical elements by polymer injection molding with hybrid molds

Hybrid molds enable the fabrication of polymeric parts with features of different length scales by injection molding. The resulting polymer microelements combine optical or biological functionalities with designed mechanical properties. Two applications are chosen for illustration of this concept: As a first example, microelements for optical communication via fiber-to-fiber coupling are manufactured by combining two molds to a small mold insert. Both molds are fabricated using lithography and electroplating. As a second example, microcantilevers (μCs) for chemical sensing are surface patterned using a modular mold composed of a laser-machined cavity defining the geometry of the μCs, and an opposite flat tool side which is covered by a patterned polymer foil. Injection molding results in an array of 35 μm-thick μCs with microscale surface topographies. In both cases, when the mold is assembled and closed, reliefs are transferred onto one surface of the molded element whose outlines are defined by the micromold cavity. The main advantage of these hybrid methods lies in the simple integration of optical surface structures and gratings onto the surface of microcomponents with different sizes and orientations. This allows for independent development of functional properties and combinations thereof.

On vertical electric fields at lunar magnetic anomalies

We study the interaction between a magnetic dipole mimicking the Gerasimovich magnetic anomaly on the lunar surface and the solar wind in a self-consistent 3-D quasi-neutral hybrid simulation where ions are modeled as particles and electrons as a charge-neutralizing fluid. Especially, we consider the origin of the recently observed electric potentials at lunar magnetic anomalies. An antimoonward Hall electric field forms in our simulation resulting in a potential difference of <300V on the lunar surface, in which the value is similar to observations. Since the hybrid model assumes charge neutrality, our results suggest that the electric potentials at lunar magnetic anomalies can be formed by decoupling of ion and electron motion even without charge separation.

Induction of root-resistance by leaf-herbivory follows a vertical gradient

Leaf-herbivory can lead to systemic changes in root metabolism and resistance. As yet, it is unknown if these changes affect the whole root system, or if they are more pronounced in the upper root parts, which are closer to the actual site of attack. As this spatial aspect may be an important determinant of the interactions that can be expected to occur within the rhizosphere, we investigated if leaf-herbivore induced root resistance differs between upper and lower roots of maize. We also tested if the density of leaf-herbivores correlates with intensity of the root response. The systemic increase in resistance was found to be more pronounced in the upper than the lower roots and was independent of leaf herbivore density. The results suggest that there is a vertical gradient in the strength of the root response following leaf-herbivory, and that soil organisms living closer to the surface may be more affected by leaf-attack than the ones living in deeper soil layers.

Numerical study of the laser-tip coupling in surface plasmon assisted stacked-double-gate field emitter arrays

Recently, sub-wavelength-pitch stacked double-gate metal nanotip arrays have been proposed to realize high current, high brightness electron bunches for ultrabright cathodes for x-ray free-electron laser applications. With the proposed device structure, ultrafast field emission of photoexcited electrons is efficiently driven by vertical incident near infrared laser pulses, via near field coupling of the surface plasmon polariton resonance of the gate electrodes with the nanotip apex. In this work, in order to gain insight in the underlying physical processes, the authors report detailed numerical studies of the proposed device. The results indicate the importance of the interaction of the double-layer surface plasmon polariton, the position of the nanotip, as well as the incident angle of the near infrared laser pulses.

Effects of Surface Forcing on Interannual Variability of the Fall Phytoplankton Bloom in the Gulf of Maine Revealed Using a Process-Oriented Model

SeaWiFS (Sea-viewing Wide Field-of-view Sensor) chlorophyll data revealed strong interannual variability in fall phytoplankton dynamics in the Gulf of Maine, with 3 general features in any one year: (1) rapid chlorophyll increases in response to storm events in fall; (2) gradual chlorophyll increases in response to seasonal wind-and cooling-induced mixing that gradually deepens the mixed layer; and (3) the absence of any observable fall bloom. We applied a mixed-layer box model and a 1-dimensional physical-biological numerical model to examine the influence of physical forcing (surface wind, heat flux, and freshening) on the mixed-layer dynamics and its impact on the entrainment of deep-water nutrients and thus on the appearance of fall bloom. The model results suggest that during early fall, the surface mixed-layer depth is controlled by both wind-and cooling-induced mixing. Strong interannual variability in mixed-layer depth has a direct impact on short-and long-term vertical nutrient fluxes and thus the fall bloom. Phytoplankton concentrations over time are sensitive to initial pre-bloom profiles of nutrients. The strength of the initial stratification can affect the modeled phytoplankton concentration, while the timing of intermittent freshening events is related to the significant interannual variability of fall blooms.