Seminar zum systematischen Erfinden

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Seminar zum systematischen Erfinden     

Bandwidth Control and Symmetry Breaking in a Mott-Hubbard Correlated Metal

In Mott materials strong electron correlation yields a spectrum of complex electronic structures. Recent synthesis advancements open realistic opportunities for harnessing Mott physics to design transformative devices. However, a major bottleneck in realizing such devices remains the lack of control over the electron correlation strength. This stems from the complexity of the electronic structure, which often veils the basic mechanisms underlying the correlation strength. This study presents control of the correlation strength by tuning the degree of orbital overlap using picometer-scale lattice engineering. This study illustrates how bandwidth control and concurrent symmetry breaking can govern the electronic structure of a correlated SrVO₃ model system. This study shows how tensile and compressive biaxial strain oppositely affect the SrVO₃ in-plane and out-of-plane orbital occupancy, resulting in the partial alleviation of the orbital degeneracy. The spectral weight redistribution under strain is derived and explained, which illustrates how high tensile strain drives the system toward a Mott insulating state. Implementation of such concepts can push correlated electron phenomena closer toward new solid-state devices and circuits. These findings therefore pave the way for understanding and controlling electron correlation in a broad range of functional materials, driving this powerful resource for novel electronics closer toward practical realization.     

Modification of physicochemical properties of actin filaments suppresses cell fragmentation in the execution phase of staurosporine-induced apoptotic processes

Effects of jasplakinolide (JSP), a stabilizer of F-actin, and latrunculin A (LTA), a destabilizer of F-actin, on a series of events occurring in the execution phase of staurosporine (STS)-induced apoptotic processes were studied using human osteosarcoma 143B cells. Time-dependent apparent increases of the population of cells with collapsed membrane potential of mitochondria (Delta Psi(m)) caused by STS treatment were not due to actual decreases in the Delta Psi(m) per cell, but due to the fragmentation of cells resulting in decreases in the number of active mitochondria per cell. Decreases in the Delta Psi(m) in fragmented cells occurred late in the execution phase. Both JSP and LAT failed to prevent STS-induced release of cytochrome c from mitochondria followed by the activation of caspases 3 and 9, the cleavage of poly (ADP-ribose) polymerase (PARP) and apoptotic nuclear fragmentation. However, both drugs prevented STS-induced apoptotic cell fragmentation and decreases in the Delta Psi(m). These results indicate that physicochemical states of actin filaments play a certain role in the execution phase of STS-induced apoptotic processes.     

A two‐phase combined QoS‐based handoff scheme in a wireless ATM network

We describe the QoS‐based rerouting algorithm that is designed to implement a two‐phase inter‐switch handoff scheme for wireless ATM networks. We propose to use path extension for each inter‐switch handoff, and invoke path optimization when the handoff path exceeds the delay constraint or maximum path extension hops constraint. We study three types of path optimization schemes: combined QoS‐based, delay‐based and hop based path rerouting schemes. Copyright © 2001 John Wiley & Sons, Ltd.     

Development of Synthetic Platelet‐Activating Hydrogel Matrices to Induce Local Hemostasis

Several hemostatic strategies rely on the use of blood components such as fibrinogen and thrombin, which suffer from high cost and short shelf‐life. Here, a cost‐effective synthetic biomaterial is developed for rapid local hemostasis. Instead of using thrombin, thrombin‐receptor‐agonist‐peptide‐6 (TRAP6) is covalently engineered in polyvinyl alcohol (PVA) hydrogels. Soluble PVA‐TRAP6 is first prepared by covalent attachment of cysteine‐containing TRAP6 onto the backbone of PVA‐norbornenes (PVA‐NB) through photoconjugation. Cytotoxicity studies using C2C12 myoblasts indicate that PVA‐NB and PVA‐TRAP6 are nontoxic. Thromboelastography reveals that hemostatic activity of TRAP6 is retained in conjugated form, which is comparable to free TRAP6 solutions with equal concentrations. A 0.1% PVA‐TRAP6 solution can shorten the clotting time (CT) to ca. 45% of the physiological CT. High platelet‐activating efficiency is further confirmed by platelet aggregation assay and flow cytometry (FACS). For potential clinical applications, TRAP6‐presenting hydrogel particulates (PVA‐TRAP6‐P) are developed for local platelet activation and hemostasis. PVA‐TRAP6‐P is prepared by biofunctionalization of photopolymerized PVA‐NB hydrogel particulates (PVA‐NB‐P) with TRAP6. It is demonstrated that PVA‐TRAP6‐P can effectively shorten the CT to ca. 50%. FACS shows that PVA‐TRAP6‐P can activate platelets to a comparable extent as soluble TRAP6 control. Altogether, PVA‐TRAP6‐P represents a promising class of biomaterials for safe hemostasis and wound healing.     

Functionalized Asymmetric Linear Acenes for High‐Performance Organic Semiconductors

A series of compounds from the tetraceno[2,3‐b]thiophene and the anthra[2,3‐b]thiophene family of semiconducting molecules has been made. Specifically, synthetic routes to functionalize the parent molecules with bromo and then hexyl groups are shown. The bromo‐ and hexyl‐functionalized tetraceno[2,3‐b]thiophene and anthra[2,3‐b]thiophene were characterized in the top‐contact thin‐film transistor (TFT) geometry. They give high mobilities, ranging from 0.12 cm² V^?1 s^?1 for α‐n‐hexylanthra[2,3‐ b]thiophene to as high as 0.85 cm² V^?1 s^?1 for α‐bromotetraceno[2,3‐b]thiophene. Notably, grain size increases, going from the shorter anthra[2,3‐b]thiophene core to the longer tetraceno[2,3‐b]thiophene core, with a corresponding increase in mobility. The transition from undesirable 3D to desirable 2D thin‐film growth is explained by the increase in length of the molecule, in this case by one benzene ring, which results in an increase in intralayer interactions relative to interlayer interactions.     

Ferroelectric Materials: Probing Local and Global Ferroelectric Phase Stability and Polarization Switching in Ordered Macroporous PZT (Adv. Funct. Mater. 5/2011)

We describe the characterization, ferroelectric phase stability and polarization switching in strain‐free assemblies of PbZr_0.3Ti_0.7O₃ (PZT) nanostructures. The 3‐dimensionally ordered macroporous structures present uniquely large areas and volumes of PZT where the microstructure is spatially modulated and the composition is homogeneous. Variable temperature powder X‐ray diffraction (XRD) studies show that the global structure is crystalline and tetragonal at room temperature and undergoes a reversible tetragonal to cubic phase transition on heating/cooling. The measured phase‐transition temperature is 50–60 °C lower than bulk PZT of the same composition. The local ferroelectric properties were assessed using piezoresponse force spectroscopy that reveal an enhanced piezoresponse from the nanostructured films and demonstrate that the switching polarization can be spatially mapped across these structures. An enhanced piezoresponse is observed in the nanostructured films which we attribute to the formation of strain free films, thus for the first time we are able to assess the effects of crystallite‐size independently of internal stress. Corresponding polarization distributions have been calculated for the bulk and nanostructured materials using a direct variational method and Landau‐Ginzburg‐Devonshire (LGD) theory. By correlating local and global characterization techniques we have for the first time unambiguously demonstrated the formation of tetragonal and ferroelectric PZT in large volume nanostructured architectures. With the wide range of materials available that can be formed into such controlled architectures we conclude that this study opens a pathway for the effective studies of nanoscale ferroelectrics in uniquely large volumes.     

Benefits of the use of impairment constraint routing in optical networks

In transparent optical networks, the optical signal accumulates the effects of all physical impairments present along the path it traverses. The conventional selection of signal paths based on e.g. shortest path routing without considering the signal quality and its association with the physical impairments does not always provide the optimum solution in terms of network performance such as blocking and resource utilization. This paper proposes an impairment constraint based routing algorithm to achieve an optimal combination of physical and networking performance taking into account all physical linear impairments including noise, chromatic and polarization mode dispersion, crosstalk and filter concatenation effects in an integrated approach. The performance of a typical metropolitan area network is examined and the improvement achieved when using the proposed approach compared to the conventional shortest path routing is demonstrated.     

Suppressed Lattice Disorder for Large Emission Enhancement and Structural Robustness in Hybrid Lead Iodide Perovskite Discovered by High-Pressure Isotope Effect

The soft nature of organic–inorganic halide perovskites renders their lattice particularly tunable to external stimuli such as pressure, undoubtedly offering an effective way to modify their structure for extraordinary optoelectronic properties. Here, using the methylammonium lead iodide as a representative exploratory platform, it is observed that the pressure-driven lattice disorder can be significantly suppressed via hydrogen isotope effect, which is crucial for better optical and mechanical properties previously unattainable. By a comprehensive in situ neutron/synchrotron-based analysis and optical characterizations, a remarkable photoluminescence (PL) enhancement by threefold is convinced in deuterated CD₃ND₃PbI₃, which also shows much greater structural robustness with retainable PL after high peak-pressure compression–decompression cycle. With the first-principles calculations, an atomic level understanding of the strong correlation among the organic sublattice and lead iodide octahedral framework and structural photonics is proposed, where the less dynamic CD₃ND₃⁺ cations are vital to maintain the long-range crystalline order through steric and Coulombic interactions. These results also show that CD₃ND₃PbI₃-based solar cell has comparable photovoltaic performance as CH₃NH₃PbI₃-based device but exhibits considerably slower degradation behavior, thus representing a paradigm by suggesting isotope-functionalized perovskite materials for better materials-by-design and more stable photovoltaic application.     

Cross-layer network formation for energy-efficient IEEE 802.15.4/ZigBee Wireless Sensor Networks

In IEEE 802.15.4/ZigBee Wireless Sensor Networks (WSNs) a specific node (called the PAN coordinator or sink) controls the whole network. When the network operates in a multi-hop fashion, the position of the PAN coordinator has a significant impact on the performance: it strongly affects network energy consumption for both topology formation and data routing. The development of efficient self-managing, self-configuring and self-regulating protocols for the election of the node that coordinates and manages the IEEE 802.15.4/ZigBee WSN is still an open research issue. In this paper we present a cross-layer approach to address the problem of PAN coordinator election on topologies formed in accordance with the IEEE 802.15.4. Our solution combines the network formation procedure defined at the MAC layer by the IEEE 802.15.4 standard with a topology reconfiguration algorithm operating at the network layer. We propose a standard-compliant procedure (named PAN coordinator ELection – PANEL) to self-configure a IEEE 802.15.4/ZigBee WSN by electing, in a distributed way, a suitable PAN coordinator. A protocol implementing this solution in IEEE 802.15.4 is also provided. Performance results show that our cross-layer approach minimizes the average number of hops between the nodes of the network and the PAN coordinator allowing to reduce the data transfer delay and determining significant energy savings compared with the performance of the IEEE 802.15.4 standard.