Showing papers by "Technical University of Berlin published in 2010"

Lattice-strain control of the activity in dealloyed core–shell fuel cell catalysts

Abstract: Electrocatalysis will play a key role in future energy conversion and storage technologies, such as water electrolysers, fuel cells and metal-air batteries. Molecular interactions between chemical reactants and the catalytic surface control the activity and efficiency, and hence need to be optimized; however, generalized experimental strategies to do so are scarce. Here we show how lattice strain can be used experimentally to tune the catalytic activity of dealloyed bimetallic nanoparticles for the oxygen-reduction reaction, a key barrier to the application of fuel cells and metal-air batteries. We demonstrate the core-shell structure of the catalyst and clarify the mechanistic origin of its activity. The platinum-rich shell exhibits compressive strain, which results in a shift of the electronic band structure of platinum and weakening chemisorption of oxygenated species. We combine synthesis, measurements and an understanding of strain from theory to generate a reactivity-strain relationship that provides guidelines for tuning electrocatalytic activity.

The Mechanism of Water Oxidation: From Electrolysis via Homogeneous to Biological Catalysis

Abstract: Striving for new solar fuels, the water oxidation reaction currently is considered to be a bottleneck, hampering progress in the development of applicable technologies for the conversion of light into storable fuels. This review compares and unifies viewpoints on water oxidation from various fields of catalysis research. The first part deals with the thermodynamic efficiency and mechanisms of electrochemical water splitting by metal oxides on electrode surfaces, explaining the recent concept of the potential-determining step. Subsequently, novel cobalt oxide-based catalysts for heterogeneous (electro)catalysis are discussed. These may share structural and functional properties with surface oxides, multinuclear molecular catalysts and the catalytic manganese–calcium complex of photosynthetic water oxidation. Recent developments in homogeneous water-oxidation catalysis are outlined with a focus on the discovery of mononuclear ruthenium (and non-ruthenium) complexes that efficiently mediate O2 evolution from water. Water oxidation in photosynthesis is the subject of a concise presentation of structure and function of the natural paragon—the manganese–calcium complex in photosystem II—for which ideas concerning redox-potential leveling, proton removal, and OO bond formation mechanisms are discussed. The last part highlights common themes and unifying concepts.

Hydrothermal carbonization of biomass: A summary and discussion of chemical mechanisms for process engineering

Abstract: Hydrothermal carbonization can be defined as combined dehydration and decarboxy lation of a fuel to raise its carbon content with the aim of achieving a higher calorific value. It is realized by applying elevated temperatures (180–220°C) to biomass in a suspension with water under saturated pressure for several hours. With this conversion process, a lignite-like, easy to handle fuel with well-defined properties can be created from biomass residues, even with high moisture content. Thus it may contribute to a wider application of biomass for energetic purposes. Although hydrothermal carbonization has been known for nearly a century, it has received little attention in current biomass conversion research. This review summarizes knowledge about the chemical nature of this process from a process design point of view. Reaction mechanisms of hydrolysis, dehydration, decarboxylation, aromatization, and condensation polymerization are discussed and evaluated to describe important operational parameters qualitatively. The results are used to derive fundamental process design improvements. Copyright © 2010 Society of Chemical Industry and John Wiley & Sons, Ltd

Optimal Control of Partial Differential Equations: Theory, Methods and Applications

Abstract: Optimal control theory is concerned with finding control functions that minimize cost functions for systems described by differential equations. The methods have found widespread applications in aeronautics, mechanical engineering, the life sciences, and many other disciplines. This book focuses on optimal control problems where the state equation is an elliptic or parabolic partial differential equation. Included are topics such as the existence of optimal solutions, necessary optimality conditions and adjoint equations, second-order sufficient conditions, and main principles of selected numerical techniques. It also contains a survey on the Karush-Kuhn-Tucker theory of nonlinear programming in Banach spaces. The exposition begins with control problems with linear equation, quadratic cost function and control constraints. To make the book self-contained, basic facts on weak solutions of elliptic and parabolic equations are introduced. Principles of functional analysis are introduced and explained as they are needed. Many simple examples illustrate the theory and its hidden difficulties. This start to the book makes it fairly self-contained and suitable for advanced undergraduates or beginning graduate students. Advanced control problems for nonlinear partial differential equations are also discussed. As prerequisites, results on boundedness and continuity of solutions to semilinear elliptic and parabolic equations are addressed. These topics are not yet readily available in books on PDEs, making the exposition also interesting for researchers. Alongside the main theme of the analysis of problems of optimal control, Troltzsch also discusses numerical techniques. The exposition is confined to brief introductions into the basic ideas in order to give the reader an impression of how the theory can be realized numerically. After reading this book, the reader will be familiar with the main principles of the numerical analysis of PDE-constrained optimization.

Synthesis of a carbon nitride structure for visible-light catalysis by copolymerization

Abstract: and nonmetallic elements (N, C, B) creates localized/ delocalized states in the band gap and thus extends its optical absorption to the visible region, but doping usually comes with accelerated charge recombination and lower stability of the doped materials. Meanwhile, various other inorganic, non-TiO2-based, visible-light catalysts have been developed (e.g., metal oxides, nitrides, sulfides, phosphides, and their mixed solid solutions), whereby Ga, Ge, In, Ta, Nb, and W are the main metal constituents. However, sustained utilization of solar energy calls for the development of more abundant and stable catalysts working with visible light, and this has remained challenging so far. Recently, a polymeric semiconductor on the basis of a defecteous graphitic carbon nitride (g-C3N4), was introduced as a metal-free photocatalyst which fulfills the basic requirements for a water-splitting catalyst, including being abundant, stable, and responsive to visible light. In the following, we use the notation “g-C3N4” to describe this class of materials rather than the idealized structure. The most active system is in fact presumably an N-bridged “poly(tri-s-triazine)”, already described by Liebig as “melon”. A semiconductor structure with band edges straddling the water redox potential was revealed for melon by DFT calculations, albeit electrochemical analysis is still awaited. g-C3N4 is considered to be the most stable phase of covalent carbon nitride, and facile synthesis of the melon substructure from simple liquid precursors and monomers allows easy engineering of carbon nitride materials to achieve the desired nanostructures via soft-chemical processing routes and methods. For instance, a high surface area (67–400 mg ) can be imparted on g-C3N4 materials by polymerization of cyanamide on a silica template, which results in photocatalytically more active g-C3N4 nanostructures. [8] Metal-doped gC3N4 can also be conveniently obtained by polymerization of dicyandiamine in the presence of metal salts, and thus multifunctionalization of such materials for a variety of applications can be achieved. Most importantly, the electronic and optical properties of carbon nitride, regarded as a polymer semiconductor, are in principle adjustable by organic protocols. Such organic protocols have been widely used to control the performance of traditional p-conjugated polymers, for example, to improve solar-cell efficiencies by constructing copolymerized donor–acceptor structures, or to modify electronic properties by co-blending with p/n-type organic dopants. Our aim was to use such organic modifications to extend the insufficient light absorption of g-C3N4 (a result of its large band gap of 2.7 eV, which corresponds to wavelengths shorter than 460 nm) towards the maximum of the solar spectrum. Here we demonstrate that the optical absorption of carbon nitride semiconductor materials is extendable into the visible region up to about 750 nm by simple copolymerization with organic monomers like barbituric acid (BA). The electronic and photoelectric properties of the modified carbon nitrides were then investigated to elucidate their enhanced activity for hydrogen production from water containing an appropriate sacrificial reagent with visible light. In principle, BA can be directly incorporated into the classical carbon nitride condensation scheme (Scheme 1). New carbon nitride structures were therefore synthesized by dissolving dicyandiamide with different amounts of BA in water, followed by thermally induced copolymerization at 823 K. For simplicity, the resulting samples are denoted CNBx, where x (0.05, 0.1, 0.2, 0.5, 1, 2) refers to the weighedin amount of BA. The structure, texture, and electrochemical properties of these materials were characterized, and their photochemical performance analyzed. Their XRD patterns (Figure S1, Supporting Information) are dominated by the characteristic (002) peak at 27.48 of a graphitic, layered structure with an interlayer distance of d = [*] J. Zhang, X. Chen , Prof. X. Fu, Prof. X. Wang State Key Laboratory Breeding Base of Photocatalysis Fuzhou University, Fuzhou 350002 (China) E-mail: xcwang@fzu.edu.cn

How to Explain Individual Classification Decisions

Abstract: After building a classifier with modern tools of machine learning we typically have a black box at hand that is able to predict well for unseen data. Thus, we get an answer to the question what is the most likely label of a given unseen data point. However, most methods will provide no answer why the model predicted a particular label for a single instance and what features were most influential for that particular instance. The only method that is currently able to provide such explanations are decision trees. This paper proposes a procedure which (based on a set of assumptions) allows to explain the decisions of any classification method.

Combining Brain-Computer Interfaces and Assistive Technologies: State-of-the-Art and Challenges.

Abstract: In recent years, new research has brought the field of electroencephalogram (EEG)-based brain–computer interfacing (BCI) out of its infancy and into a phase of relative maturity through many demonstrated prototypes such as brain-controlled wheelchairs, keyboards, and computer games. With this proof-of-concept phase in the past, the time is now ripe to focus on the development of practical BCI technologies that can be brought out of the lab and into real-world applications. In particular, we focus on the prospect of improving the lives of countless disabled individuals through a combination of BCI technology with existing assistive technologies (AT). In pursuit of more practical BCIs for use outside of the lab, in this paper, we identify four application areas where disabled individuals could greatly benefit from advancements in BCI technology, namely, “Communication and Control”, “Motor Substitution”, “Entertainment”, and “Motor Recovery”. We review the current state of the art and possible future developments, while discussing the main research issues in these four areas. In particular, we expect the most progress in the development of technologies such as hybrid BCI architectures, user–machine adaptation algorithms, the exploitation of users’ mental states for BCI reliability and confidence measures, the incorporation of principles in human–computer interaction (HCI) to improve BCI usability, and the development of novel BCI technology including better EEG devices.

Functional Materials: From Hard to Soft Porous Frameworks

Abstract: This Review aims to give an overview of recent research in the area of porous, organic-inorganic and purely organic, functional materials. Possibilities for introducing organic groups that exhibit chemical and/or physical functions into porous materials will be described, with a focus on the incorporation of such functional groups as a supporting part of the pore walls. The number of organic groups in the network can be increased such that porous, purely organic materials are obtained.

The Hybrid BCI

Abstract: Nowadays, everybody knows what a hybrid car is. A hybrid car normally has two engines to enhance energy efficiency and reduce CO2 output. Similarly, a hybrid brain-computer interface (BCI) is composed of two BCIs, or at least one BCI and another system. A hybrid BCI, like any BCI, must fulfill the following four criteria: (i) the device must rely on signals recorded directly from the brain; (ii) there must be at least one recordable brain signal that the user can intentionally modulate to effect goal-directed behaviour; (iii) real time processing; and (iv) the user must obtain feedback. This paper introduces hybrid BCIs that have already been published or are in development. We also introduce concepts for future work. We describe BCIs that classify two EEG patterns: one is the event-related (de)synchronisation (ERD, ERS) of sensorimotor rhythms, and the other is the steady-state visual evoked potential (SSVEP). Hybrid BCIs can either process their inputs simultaneously, or operate two systems sequentially, where the first system can act as a "brain switch". For example, we describe a hybrid BCI that simultaneously combines ERD and SSVEP BCIs. We also describe a sequential hybrid BCI, in which subjects could use a brain switch to control an SSVEP-based hand orthosis. Subjects who used this hybrid BCI exhibited about half the false positives encountered while using the SSVEP BCI alone. A brain switch can also rely on hemodynamic changes measured through near-infrared spectroscopy (NIRS). Hybrid BCIs can also use one brain signal and a different type of input. This additional input can be an electrophysiological signal such as the heart rate, or a signal from an external device such as an eye tracking system.

Carbon Dioxide and Formic Acid - The couple for an environmental-friendly hydrogen storage?

Abstract: In search for future energy supplies the application of hydrogen as an energy carrier is seen as a prospective issue. However, the implementation of a hydrogen economy is suffering from several unsolved problems. Particularly challenging is the storage of appropriate amounts of hydrogen. In this context the utilization of carbon dioxide–formic acid for hydrogen storing is discussed.

Mechanism of gold nanoparticle formation in the classical citrate synthesis method derived from coupled in situ XANES and SAXS evaluation.

Abstract: Although gold nanoparticles (GNP) are among the most intensely studied nanoscale materials, the actual mechanisms of GNP formation often remain unclear due to limited accessibility to in situ-derived time-resolved information about precursor conversion and particle size distribution Overcoming such limitations, a method is presented that analyzes the formation of nanoparticles via in situ SAXS and XANES using synchrotron radiation The method is applied to study the classical GNP synthesis route via the reduction of tetrachloroauric acid by trisodium citrate at different temperatures and reactant concentrations A mechanism of nanoparticle formation is proposed comprising different steps of particle growth via both coalescence of nuclei and further monomer attachment The coalescence behavior of small nuclei was identified as one essential factor in obtaining a narrow size distribution of formed particles

Temperature Dependence of the Nitrogen-Vacancy Magnetic Resonance in Diamond

Abstract: The temperature dependence of the magnetic-resonance spectra of nitrogen-vacancy (NV-) ensembles in the range of 280-330 K was studied. Four samples prepared under different conditions were analyzed with NV- concentrations ranging from 10 ppb to 15 ppm. For all samples, the axial zero-field splitting (ZFS) parameter D was found to vary significantly with temperature, T, as dD/dT=-74.2(7) kHz/K. The transverse ZFS parameter E was nonzero (between 4 and 11 MHz) in all samples, and exhibited a temperature dependence of dE/(EdT)=-1.4(3)x10{-4} K-1. The results might be accounted for by considering local thermal expansion. The temperature dependence of the ZFS parameters presents a significant challenge for diamond magnetometers and may ultimately limit their bandwidth and sensitivity.

Metal-Free Heterogeneous Catalysis for Sustainable Chemistry

Abstract: The current established catalytic processes used in chemical industries use metals, in many cases precious metals, or metal oxides as catalysts. These are often energy-consuming and not highly selective, wasting resources and producing greenhouse gases. Metal-free heterogeneous catalysis using carbon or carbon nitride is an interesting alternative to some current industrialized chemical processes. Carbon and carbon nitride combine environmental acceptability with inexhaustible resources and allow a favorable management of energy with good thermal conductivity. Owing to lower reaction temperatures and increased selectivity, these catalysts could be candidates for green chemistry with low emission and an efficient use of the chemical feedstock. This Review highlights some recent promising activities and developments in heterogeneous catalysis using only carbon and carbon nitride as catalysts. The state-of-the-art and future challenges of metal-free heterogeneous catalysis are also discussed.

An Android Application Sandbox system for suspicious software detection

Abstract: Smartphones are steadily gaining popularity, creating new application areas as their capabilities increase in terms of computational power, sensors and communication. Emerging new features of mobile devices give opportunity to new threats. Android is one of the newer operating systems targeting smartphones. While being based on a Linux kernel, Android has unique properties and specific limitations due to its mobile nature. This makes it harder to detect and react upon malware attacks if using conventional techniques. In this paper, we propose an Android Application Sandbox (AASandbox) which is able to perform both static and dynamic analysis on Android programs to automatically detect suspicious applications. Static analysis scans the software for malicious patterns without installing it. Dynamic analysis executes the application in a fully isolated environment, i.e. sandbox, which intervenes and logs low-level interactions with the system for further analysis. Both the sandbox and the detection algorithms can be deployed in the cloud, providing a fast and distributed detection of suspicious software in a mobile software store akin to Google's Android Market. Additionally, AASandbox might be used to improve the efficiency of classical anti-virus applications available for the Android operating system.

Network Security: A Decision and Game-Theoretic Approach

Abstract: Covering attack detection, malware response, algorithm and mechanism design, privacy, and risk management, this comprehensive work applies unique quantitative models derived from decision, control, and game theories to understanding diverse network security problems It provides the reader with a system-level theoretical understanding of network security, and is essential reading for researchers interested in a quantitative approach to key incentive and resource allocation issues in the field It also provides practitioners with an analytical foundation that is useful for formalising decision-making processes in network security

Towards a Cure for BCI Illiteracy

Abstract: Brain–Computer Interfaces (BCIs) allow a user to control a computer application by brain activity as acquired, e.g., by EEG. One of the biggest challenges in BCI research is to understand and solve the problem of “BCI Illiteracy”, which is that BCI control does not work for a non-negligible portion of users (estimated 15 to 30%). Here, we investigate the illiteracy problem in BCI systems which are based on the modulation of sensorimotor rhythms. In this paper, a sophisticated adaptation scheme is presented which guides the user from an initial subject-independent classifier that operates on simple features to a subject-optimized state-of-the-art classifier within one session while the user interacts the whole time with the same feedback application. While initial runs use supervised adaptation methods for robust co-adaptive learning of user and machine, final runs use unsupervised adaptation and therefore provide an unbiased measure of BCI performance. Using this approach, which does not involve any offline calibration measurement, good performance was obtained by good BCI participants (also one novice) after 3–6 min of adaptation. More importantly, the use of machine learning techniques allowed users who were unable to achieve successful feedback before to gain significant control over the BCI system. In particular, one participant had no peak of the sensory motor idle rhythm in the beginning of the experiment, but could develop such peak during the course of the session (and use voluntary modulation of its amplitude to control the feedback application).

Rational Extension of the Family of Layered, Covalent, Triazine‐Based Frameworks with Regular Porosity

Abstract: 2010 WILEY-VCH Verlag Gmb Porous solids are an important class of materials in sorption and chromatography applications and more recently they have also become relevant for applications in catalysis (as active catalyst or support) and storage (gas storage in microporous, batteries). Controlled construction of such extended porous frameworks from suitable molecular building blocks paves the way for establishing a connection betweenmolecular and solid properties in a well-defined, pre-ordered chemical environment. Rational synthesis of extended arrays of organicmatter in bulk, in solution, in crystals, and in thin films has always been a paramount goal in chemistry. The classical synthetic tools to obtain long-range regularity are, however, limited to non-covalent interactions, in contrast to the structurally more random character of covalent polymerization reactions. The most challenging hurdle in the synthesis of extended, yet precisely defined 2D and 3D structures based on covalent chemistry is widely believed to be the requirement that the reaction linking individual organic constituents should be reversible, allowing the scaffold to arrange to the thermodynamic, well-ordered product rather than the kinetic, amorphous structure. Reports by Yaghi et al. have shown that two kinds of condensation reactions yielding planar, six-membered boroxine rings (B3O3) and five-membered BO2C2 rings meet this criterion and thus can be utilized as covalent linkers between organic units to generate 2D and 3D covalent organic frameworks (COFs). Successful synthesis of COF materials from light, non-metallic, molecular building blocks is appealing for applications such as gas storage and catalysis – in particular because they hold the promise of being completely inert with respect to water, unlike many (metal organic) frameworks known from literature. Additionally, the large number of inexpensive organic compounds offers modularity, in which desirable features such as surface functionalization and tunable pore sizes can conceptually be easily achieved. Previously, we showed that the polytrimerization reaction of 1,4-dicyanobenzene in zinc chloride is an alternative way towards covalently linked, covalent, triazine-based frameworks (CTF-1). Although other porous, organic networks based on the triazine linker have been synthesized since, CTF-1 remained unique in particular because it not only exhibited permanent microand mesoporosity but also crystallinity. In the following, we shall report on a secondmember in the class of covalent, triazine-based frameworks (termed CTF-2), which was obtained from the ionothermal condensation of 2,6-naphthalenedinitrile (Fig. 1). This find should corroborate our previous assertion that the triazine ring (C3N3) is a fruitful and modular linking-unit in the synthesis of extended and ordered layered frameworks. In planning the synthesis, we drew on our previous experiences with the cyclotrimerization of dinitrile compounds in a zinc chloride salt melt, and we chose 2,6-naphthalenedicarbonitrile, because of its rigidity and its relative chemical stability resulting from its extended aromatic p-system. The later trait was of special significance, since the reaction conditions involve both elevated temperatures ( 400 8C) and the partaking of a catalytically active species, opening up potential pathways to many undesired decomposition and condensation reactions. One such pathway of aromatic nitrile decomposition involves C H bond cleavage, but also the oxidative halogenation of the aromatic unit in the presence of a metal halide. Homolytic cleavage of the carbon nitrile bond at temperatures above 400 8C and the associated depletion of nitrogen content within the structure were observed previously for a set of studied model compounds in this type of reaction. The framework CTF-2 was synthesized heating zinc chloride and 2,6-naphthalenedicarbonitrile in a quartz glass ampoule at 400 8C for 40 h. The set-up in a closed system was chosen since the monomer starts to sublimate at temperatures around 220 8C. The typical yields of this reaction are in the range of 80–85%, suggesting that some of the side reactions outlined above do occur. In the FTIR spectrum we see that the intensity of the carbonitrile band at 2225 cm 1 decreases significantly after 20 h and diminishes further following the course of the reaction, while bands at 1535 and 1315 cm , indicative of aromatic C N stretching and breathing modes in the triazine unit, appear (c.f. Supporting Information). The C solid-state NMR (CP/MAS) experiment performed on CTF-2 further confirmed the presence of sp carbons from the triazine unit ( 169 ppm) and the naphthalene (133, 128 ppm) as well as sp carbons from unreacted carbonitriles (110 ppm), which are most likely situated at the terminal edges of the condensed material (Supporting Information). Elemental analysis revealed a molecular ratio of C36.0H18.8N5.4, which is close to the theoretical composition of a perfectly condensed, infinite framework, namely C36H18N6. The washing procedure outlined in the experimental section left a

Henshin: advanced concepts and tools for in-place EMF model transformations

Abstract: The Eclipse Modeling Framework (EMF) provides modeling and code generation facilities for Java applications based on structured data models. Henshin is a new language and associated tool set for in-place transformations of EMF models. The Henshin transformation language uses pattern-based rules on the lowest level, which can be structured into nested transformation units with well-defined operational semantics. So-called amalgamation units are a special type of transformation units that provide a forall-operator for pattern replacement. For all of these concepts, Henshin offers a visual syntax, sophisticated editing functionalities, execution and analysis tools. The Henshin transformation language has its roots in attributed graph transformations, which offer a formal foundation for validation of EMF model transformations. The transformation concepts are demonstrated using two case studies: EMF model refactoring and meta-model evolution.

Spectral Analysis of Signed Graphs for Clustering, Prediction and Visualization.

Abstract: We study the application of spectral clustering, prediction and visualization methods to graphs with negatively weighted edges. We show that several characteristic matrices of graphs can be extended to graphs with positively and negatively weighted edges, giving signed spectral clustering methods, signed graph kernels and network visualization methods that apply to signed graphs. In particular, we review a signed variant of the graph Laplacian. We derive our results by considering random walks, graph clustering, graph drawing and electrical networks, showing that they all result in the same formalism for handling negatively weighted edges. We illustrate our methods using examples from social networks with negative edges and bipartite rating graphs.

Nucleation and Growth of Gold Nanoparticles Studied via in situ Small Angle X-ray Scattering at Millisecond Time Resolution

Abstract: Gold nanoparticles (AuNP) were prepared by the homogeneous mixing of continuous flows of an aqueous tetrachloroauric acid solution and a sodium borohydride solution applying a microstructured static mixer. The online characterization and screening of this fast process (∼2 s) was enabled by coupling a micromixer operating in continuous-flow mode with a conventional in-house small angle X-ray scattering (SAXS) setup. This online characterization technique enables the time-resolved investigation of the growth process of the nanoparticles from an average radius of ca. 0.8 nm to about 2 nm. To the best of our knowledge, this is the first demonstration of a continuous-flow SAXS setup for time-resolved studies of nanoparticle formation mechanisms that does not require the use of synchrotron facilities. In combination with X-ray absorption near edge structure microscopy, scanning electron microscopy, and UV−vis spectroscopy the obtained data allow the deduction of a two-step mechanism of gold nanoparticle formati...

The Berlin Brain–Computer Interface: Non-Medical Uses of BCI Technology

Abstract: Brain–computer interfacing (BCI) is a steadily growing area of research. While initially BCI research was focused on applications for paralyzed patients, increasingly more alternative applications in healthy human subjects are proposed and investigated. In particular, monitoring of mental states and decoding of covert user states have seen a strong rise of interest. Here, we present some examples of such novel applications which provide evidence for the promising potential of BCI technology for non-medical uses. Furthermore, we discuss distinct methodological improvements required to bring non-medical applications of BCI technology to a diversity of layperson target groups, e.g., ease of use, minimal training, general usability, short control latencies.

Covalent Triazine Framework as Catalytic Support for Liquid Phase Reaction

Abstract: An important goal in the preparation of highly active supported metal particles is the enhancement of the metal support interaction, providing a more stable catalyst, especially for liquid phase reactions as the leaching and reconstruction of the active phase causes deactivation. In this work, a covalent triazine framework (CTF) as support for Pd nanoparticles is compared to activated carbon (AC), the typical support used in liquid phase reactions. The results indicate that the presence of the N-heterocyclic moieties on the surface of the frameworks is beneficial for improving the stability of Pd nanoparticles during the liquid phase glycerol oxidation. Pd/CTF showed better activity and in particular better stability when compared to Pd supported on activated carbon (AC).

Carrier Multiplication in Graphene

Abstract: Graphene as a zero-bandgap semiconductor is an ideal model structure to study the carrier relaxation channels, which are inefficient in conventional semiconductors. In particular, it is of fundamental interest to address the question whether Auger-type processes significantly influence the carrier dynamics in graphene. These scattering channels bridge the valence and conduction band allowing carrier multiplication, a process that generates multiple charge carriers from the absorption of a single photon. This has been suggested in literature for improving the efficiency of solar cells. Here we show, based on microscopic calculations within the density matrix formalism, that Auger processes do play an unusually strong role for the relaxation dynamics of photoexcited charge carriers in graphene. We predict that a considerable carrier multiplication takes place, confirming the potential of graphene as a new material for high-efficiency photodevices.

Multi-UAV Cooperation and Control for Load Transportation and Deployment

Abstract: This paper deals with the cooperation and control of multiple UAVs with sensing and actuation capabilities. An architecture to perform cooperative missions with a multi-UAV platform is presented. The interactions between UAVs are not only information exchanges but also physical couplings required to cooperate in the joint transportation of a single load. Then, the paper also presents the control system for the transportation of a slung load by means of one or several helicopters. Experimental results of the load transportation system with one and three helicopters are shown. On the other hand, the UAVs considered in the platform can also deploy small objects, such as sensor nodes, on different locations if it is required. This feature along with the whole platform architecture are illustrated in the paper with a real multi-UAV mission for the deployment of sensor nodes to repair the connectivity of a wireless sensor network.

Large-format, high-speed, X-ray pnCCDs combined with electron and ion imaging spectrometers in a multipurpose chamber for experiments at 4th generation light sources

Abstract: Fourth generation accelerator-based light sources, such as VUV and X-ray Free Electron Lasers (FEL), deliver ultra-brilliant (∼1012–1013 photons per bunch) coherent radiation in femtosecond (∼10–100 fs) pulses and, thus, require novel focal plane instrumentation in order to fully exploit their unique capabilities. As an additional challenge for detection devices, existing (FLASH, Hamburg) and future FELs (LCLS, Menlo Park; SCSS, Hyogo and the European XFEL, Hamburg) cover a broad range of photon energies from the EUV to the X-ray regime with significantly different bandwidths and pulse structures reaching up to MHz micro-bunch repetition rates. Moreover, hundreds up to trillions of fragment particles, ions, electrons or scattered photons can emerge when a single light flash impinges on matter with intensities up to 1022 W/cm2. In order to meet these challenges, the Max Planck Advanced Study Group (ASG) within the Center for Free Electron Laser Science (CFEL) has designed the CFEL-ASG MultiPurpose (CAMP) chamber. It is equipped with specially developed photon and charged particle detection devices dedicated to cover large solid-angles. A variety of different targets are supported, such as atomic, (aligned) molecular and cluster jets, particle injectors for bio-samples or fixed target arrangements. CAMP houses 4π solid-angle ion and electron momentum imaging spectrometers (“reaction microscope”, REMI, or “velocity map imaging”, VMI) in a unique combination with novel, large-area, broadband (50 eV–25 keV), high-dynamic-range, single-photon-counting and imaging X-ray detectors based on the pnCCDs. This instrumentation allows a new class of coherent diffraction experiments in which both electron and ion emission from the target may be simultaneously monitored. This permits the investigation of dynamic processes in this new regime of ultra-intense, high-energy radiation—matter interaction. After an introduction into the salient features of the CAMP chamber and the properties of the redesigned REMI/VMI spectrometers, the new 1024×1024 pixel format pnCCD imaging detector system will be described in detail. Results of tests of four smaller format (256×512) devices of identical performance, conducted at FLASH and BESSY, will be presented and the concept as well as the anticipated properties of the full, large-scale system will be elucidated. The data obtained at both radiation sources illustrate the unprecedented performance of the X-ray detectors, which have a voxel size of 75×75×450 μm3 and a typical read-out noise of 2.5 electrons (rms) at an operating temperature of −50 °C.