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Optical control of 4f orbital state in rare earth metals
Get PDFA change of orbital state alters the coupling between ions and their surroundings drastically. Orbital excitations are hence key to understand and control interaction of ions. Rare earth elements with strong magneto crystalline anisotropy MCA are important ingredients for magnetic devices. Thus, control of their localized 4f magnetic moments and anisotropy is one major challenge in ultrafast spin physics. With time resolved x ray absorption and resonant inelastic scattering experiments, we show for Tb metal that 4f electronic excitations out of the ground state multiplet occur after optical pumping. These excitations are driven by inelastic 5d 4f electron scattering, altering the 4f orbital state and consequently the MCA with important implications for magnetization dynamics in 4f metals and more general for the excitation of localized electronic states in correlated material
Active site switching on high entropy phosphides as bifunctional oxygen electrocatalysts for rechargeable robust Zn air battery
No full textHigh entropy materials HEMs offer a quasi continuous spectrum of active sites and have generated great expectations in fields such as electrocatalysis and energy storage. Despite their potential, the complex composition and associated surface phenomena of HEMs pose challenges to their rational design and development. In this context, we have synthesized FeCoNiPdWP high entropy phosphide HEP nanoparticles using a low temperature colloidal method, and explored their application as bifunctional electrocatalysts for the oxygen evolution and reduction reactions OER ORR . Our analysis provides a detailed understanding of the individual roles and transformations of each element during OER ORR operation. Notably, the HEPs exhibit an exceptionally low OER overpotential of 227 mV at 10 mA cm amp; 8722;2, attributed to the reconstructed HEP surface into a FeCoNiPdW high entropy oxyhydroxide with high oxidation states of Fe, Co, and Ni serving as the active sites. Additionally, Pd and W play crucial roles in modulating the electronic structure to optimize the adsorption energy of oxygen intermediates. For the ORR, Pd emerges as the most active component. In the reconstructed catalyst, the strong d d orbital coupling of especially Pd, Co, and W fine tunes ORR electron transfer pathways, delivering an ORR half wave potential of 0.81 V with a pure four electron reduction mechanism. The practicality of these HEPs catalysts is showcased through the assembly of aqueous zinc air batteries. These batteries demonstrate a superior specific capacity of 886 mA h gZn amp; 8722;1 and maintain excellent stability over more than 700 hours of continuous operation. Overall, this study not only elucidates the role of each element in HEMs but also establishes a foundational framework for the design and development of next generation bifunctional oxygen catalysts, broadening the potential applications of these complex materials in advanced energy system
The In Stability of Heterostructures During the Oxygen Evolution Reaction
Get PDFThe urgent need for efficient oxygen evolution reaction OER catalysts has led to the development and publication of many heterostructured catalysts. The application of such catalysts with multiple phases tremendously increases the material design dimensions, and numerous interface related effects can tune the OER performance. In this regard, multiple of these heterostructured electrodes show remarkable OER activities. However, it is not clear if these carefully designed interfaces remain under prolonged OER conditions. Herein, a molecular approach is used to synthesize four different nickel iron phosphide heterostructured materials and deposit them on fluorine doped tin oxide and nickel foam electrodes. The OER performance of the eight electrodes and the reconstruction of the four materials is investigated by in situ spectroscopy after one day of operation, enabled by a freeze quench approach. The most active electrode is also applied under industrial OER conditions and for the value added oxidation of alcohols to ketones. Before catalysis, this electrode comprises crystalline 4 nm nickel phosphide particles on an amorphous iron phosphide matrix. However, after 24 h, a homogenous nickel iron oxyhydroxide phase has formed. This work questions to which extent the design of heterostructures is a suitable strategy for non noble metal OER catalysi
BESSY III overview and its bending sources
Get PDFThe BESSY III project evolves from a pre CDR phase into the CDR phase. And for lattice design, it means, that one of the different Higher Order Achromat MBA lattice candidates has to be chosen as the baseline lattice for the iterations with the construction department. Therefore it is essential that the design of the main and most important components, the bending magnets, will be defined as early as possible. At BESSY III, it is requested, that the bends be used as bending sources in different regimes, the soft X ray lt;2 2 12 and hard in kev , tender the x rays gt;10 keV . In this contribution, we will give an overview of the BESSY III project and its bending sources and discuss briefly the baseline lattic
Magnetization patterns in GaAs Fe33Co67 core shell nanorods
Get PDFWe present a study on the static magnetic properties of individual GaAs core shell nanorods. X ray magnetic circular dichroism combined with photoemission electron microscopy and scanning transmission x ray microscopy were used to investigate the magnetic nanostructures. The magnetic layer is purposely designed to establish a magnetic easy axis neither along the nanostructure s long axis nor perpendicular to it to promote a 3D magnetic helical configuration on the tubular surface. In practice, two types of magnetic textures with in plane magnetization were found inside the nanostructures facets magnetic domains with almost longitudinal or almost perpendicular magnetization with respect to the axis of the tube. We observe that a magnetic field applied perpendicular to the long axis of the nanostructure can add an azimuthal component of the magnetization to the previously almost longitudinal magnetizatio
Electronic Structure of Functionalized Semiconductor Surfaces and Interfaces for Photoelectrochemical Water Splitting
No full textPhotoelectrochemical multi junction devices for direct solar energy conversion have been highly improved during the last decade with solar to hydrogen efficiencies reaching almost up to 20 . However, these efficiencies are still below their expected physical limits, which requires a deeper understanding of band energy diagrams along the functional device interfaces in the vicinity of a liquid electrolyte in order to identify potential and charge transfer losses, that will limit the conversion efficiency of the overall device. For this purpose, model surfaces of classical elemental Si and binary InP semiconductors were prepared and characterized by photoemission spectroscopy with respect to their electronic structure and electronic surface state formation. The interaction of these surfaces with water was investigated by modeling the electrochemical interface in ultra high vacuum using a frozen electrolyte approach. Depending on surface termination and surface state concentration, the surfaces showed a shift in Fermi level towards the vacuum level, indicating an electron injection upon water adsorption by the interaction with unsaturated dangling surface bonds. The contact formation of the photoabsorber to the noble metal catalyst results in an electron depletion layer acting as a barrier for the charge transfer and therefore preventing considerable conversion efficiencies. Using TiO amp; 8322; as buffer layer in between the photoabsorber and catalyst seemed to prevent the strong depletion of the photoabsorber. However, this effect strongly depends on the TiO amp; 8322; film properties, which results from the preparation process. This has to be optimized in order to guarantee a loss free charge transfer from the photoabsorber to the catalyst. The deduced energy band diagrams from modeled interface experiments help to understand the electrochemical performance when using the layer arrangement in a device like setup. However, when method related microstructural effects like lateral inhomogenities or mechanical and structural stability come into play, the performance prediction solely derived from the energy band diagrams of model interfaces seem to fail and cannot fully describe the energetic device complexity at operation condition
Structural and functional insights into class IV lanthipeptide biosynthesis
No full textNatural products have served various functions in human civilization for thousands of years. From food stuffs like caffeine to pharmaceuticals such as penicillin or vancomycin, they are constantly growing field that humans find new utility for each century. One such group is the Ribosomally Synthesized and Post translationally Modified Peptides RiPPs , a broad group exhibiting diverse modifications and bioactivities. RiPPs are, as the name implies, genetically encoded peptides that undergo a series of modifications and finally export from the cell as a mature peptide. Typically, each peptide has its own dedicated biosynthetic machinery that selectively introduces modifications. The selectivity between modification enzymes and the peptide substrate is guided by the amino acid composition of a recognition region in the precursor peptide, which has evolved to recognize its cognate enzymes. The largest family of RiPPs, are the lanthipeptides, which are characterized by the installation of thioether macrocycles termed lanthionines. There are five known classes of lanthipeptides, all categorized by their synthetases that install the lanthionines. However, limited structural information is known about the synthetases, and now four classes have their respective synthetases crystallized, and structures elucidated. Understanding the protein structures will be essential for future lanthipeptide engineering and research. Therefore, the effort toward crystallizing and elucidating the structures of new synthetases is essential for advancing the field. Thus, I focused on a class IV lanthipeptide synthetase from a thermophilic actinomycete with a relatively small gene cluster. The investigation into the class IV lanthipeptide curvocidin began by the cultivation of the native host Thermomonospora curvata but yielded miniscule amounts. Therefore, heterologous expression of the corresponding gene cluster was done in Streptomyces coelicolor to produce curvocidin. Structural elucidation efforts by mass spectrometry, revealed a tricyclic globular topology of the produced lanthipeptide, similar to that of cinnamycin or duramycin, lanthipeptides of a different class. So far, no bioactivity was discovered for curvocidin, and its biological role remains enigmatic. In fact, the class IV lanthipeptides still do not have any reported bioactivity, whereas most other classes have been shown to be antimicrobial, antiallodynic, or morphogenic. In addition, utilizing heterologous expression of the synthetase CuvL and solid phase peptide synthesis of the precursor peptide, enabled us to investigate the mechanism of curvocidin biosynthesis. Most surprisingly, the lanthionine formation followed a non linear pattern, starting at the central lanthionine bridge. This was followed by the adjacent bridge, bringing the N and C terminus of the peptide into proximity and finally closing the last thioether at the centre of the peptide. Exhaustive crystallization efforts resulted in a 2.9 crystal structure of the tri domain lanthipeptide synthetase CuvL, revealing a circular topology. The circular topology is achieved by docking a not before described amp; 946; hairpin motif of the N terminal lyase domain into the C terminal cyclase domain. Additionally, the crystal structure displayed the precursor bound to its central kinase domain. Due to the conformational ensembles that the peptide adopted, we were unable to model the side chains of the peptide. Although, through the advances in artificial intelligence, we were able to use in silico modelling of the synthetase peptide complex to show the interactions. The models highlighted an amphipathic amp; 945; helix conformation on the N terminal region of the peptide, suggesting that these interactions are vital to peptide recognition, which was further confirmed by NMR spectroscopy and showing this to be a prevalent structural motif in class III and IV lanthipeptides for recognitio
Kinetics of reversible deformations during the evaporative drying of silica aerogels
No full textSilica aerogels are highly porous, translucent materials with the lowest thermal conductivities among solids. They are produced via sol gel process and subsequent drying to replace the pore liquid by air without significant alterations of the silica skeleton. During ambient pressure drying APD , evaporation generates capillary forces that put the liquid under tension, which is balanced by the compression of the silica skeleton. This results in substantial drying shrinkage that is made partially reversible by surface modification of silica gels. Past a critical shrinkage, the gels re expand through the spring back effect SBE , demonstrating a remarkable elasticity for a silica based material. However, the description of the evaporation and deformation mechanisms remains mostly theoretical and few experiments were performed to investigate the APD process in operando. This thesis aims to provide an empirical description of the evolution of silica gels properties during drying, thermal treatment and mechanical compression. The main motivation is to unveil the underlying mechanisms of the drying shrinkage and spring back by addressing the kinetics of the gels phase composition during APD. Another incentive is to quantify the capability of silica aerogels to recover large deformations related to drying shrinkage and uniaxial compression and evaluate the corresponding changes in the aerogels nanostructure. Silica gels were prepared by a sol gel process from tetraethyl orthosilicate and were casted as 16 mm tall cylinders, followed by modification with trimethylchlorosilane. Specimen were dried at ambient pressure to produce monolithic aerogels. The average phase composition of the gels during APD was successfully computed by developing a novel quantitative imaging workflow based on in operando X ray micro computed tomography CT . The emergence of the SBE was correlated to an equal volume fraction of silica skeleton, hexane and gas in the gels. To this regard, the re expansion was arguably caused by a local relaxation of the drying stress, indicating a depletion of solvent in some pores. Simulations on unmodified gels supported the incidence of condensation reactions during drying. Further analysis of CT data allowed to map the distribution of liquid and vapor in the gels during drying, which notably uncovered evidence of evaporation of the pore liquid by cavitation. This was supported by estimations using classical nucleation theory, and by separate in operando wide angle X ray scattering experiments showing a significant volume of gas in the gels prior to the SBE. The onset of evaporation by meniscus recession was manifested by a drying front travelling across the specimen and was correlated to a heterogeneous SB
Phase diagram of the antiferromagnetic J1 J2 spin 1 pyrochlore Heisenberg model
No full textWe study the phase diagram of the antiferromagnetic amp; 119869;1 amp; 8722; amp; 119869;2 Heisenberg model on the pyrochlore lattice with amp; 119878; 1 spins at zero and finite temperatures. We use a combination of complementary state of the art quantum many body approaches such as density matrix renormalization group DMRG , density matrix purification, and pseudo Majorana functional renormalization group PMFRG . We present an efficient approach to preserve the applicability of the PMFRG for spin 1 systems at finite temperatures despite the inevitable presence of unphysical spin states. The good performance of our methods is first demonstrated for the nearest neighbor pyrochlore Heisenberg model where the finite temperature behavior of the specific heat and uniform susceptibility show excellent agreement within PMFRG and density matrix purification. Including an antiferromagnetic second neighbor coupling we find that the nonmagnetic ground state phase of the nearest neighbor model extents up to amp; 119869;2 amp; 119869;1 amp; 8764;0.02 within DMRG, beyond which magnetic amp; 119948; 0 long range order sets in. Our PMFRG calculations find the phase transition in a similar regime amp; 119869;2 amp; 119869;1 amp; 8764;0.035 amp; 8290; 8 which, together with the DMRG result, provides a strong argument for the existence of a small but finite nonmagnetic ground state phase in the spin 1 pyrochlore Heisenberg model. We also discuss the origin of discrepancies between different versions of the functional renormalization group concerning the location of this phase transitio