Helmholtz-Zentrum Berlin für Materialien und Energie

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    Surface dynamics of graphite probed by soft x ray linear dichroism in Auger emission yield

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    Time resolved near edge x ray absorption with Auger electron detection is employed as a surface sensitive probe to study the dynamics of surface atomic motion in graphite upon laser excitation. To this end, ultrafast changes in photoelectron spectra with a focus on carbon KVV Auger intensities are used as probe in a pump probe scheme with pulsed femtosecond near infrared laser excitation of a highly oriented pyrolytic graphite HOPG sample. The dynamics of the HOPG surface excitation at the K edge appears as transient change in Auger yield. Since the directional bond character of carbon is reflected in linear dichroism of x ray absorption, the transient angular variation of the 1s amp; 8594; amp; 960; transitions can be used to uncover the laser induced dynamics of the atomic surface structure which shows a huge anisotropy upon usage of s or p polarized x ray probe pulses, respectivel

    Tracking the phase transformation and microstructural evolution of Sn anode using operando synchrotron X ray energy dispersive diffraction and X ray tomography

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    The intermediate Li Sn alloy phases during de lithiation are identified, and their dynamic phase transformation is unraveled which is further correlated with the volume variation of the Sn at particle and electrode level. Moreover, we find that the Sn particle expansion shrinkage induced particle displacement is anisotropic the displacement perpendicular to the electrode surface z axis is more pronounced compared to the directions x and y axis along the electrode surface. This anisotropic particle displacement leads to an anisotropic volume variation at the electrode level and eventually generates a net electrode expansion towards the separator after cycling, which could be one of the root causes of mechanical detachment and delamination of electrodes during long term operation. The unraveled chemical evolution of Li Sn and deep insights into the microstructural evolution of Sn anode provided here could guide future design and engineering of Sn and other alloy anodes for high energy density Li and Na ion batterie

    Integration of rough RTP absorbers into CIGS perovskite monolithic tandems by NiOx Cu SAM Hole transporting Bi layers

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    We investigate the performance of monolithic copper indium gallium selenide CIGS perovskite tandem solar cells with two different CIGS bottom device absorbers Cu In,Ga Se2 or Cu In,Ga S,Se 2 and with three different hole transporting layers HTLs NiOx SAM, NiOx Cu SAM and SAM alone. NiOx Cu is 2 wt copper doped nickel oxide and SAM is the MeO 2PACz [2 3,6 dimethoxy 9H carbazol 9 yl ethyl]phosphonic acid self assembled monolayer. The CIGSe is fabricated by physical vapor deposition PVD , has a Eg amp; 8764; 1.06eV, and a amp; 963;RMS,PVD amp; 8764; 65 nm, while the CIGSSe is fabricated by rapid thermal processing RTP , has a Eg amp; 8764; 1.01eV, and a amp; 963;RMS,RTP amp; 8764; 120 nm. While the current certified, 24.2 efficient, world record monolithic CIGSe perovskite tandem solar cell has previously been achieved with SAM as a stand alone HTL, this work investigates whether SAM can yield similarly high efficiencies also on industrially compatible, very rough RTP CIGSSe absorbers. We find that the devices with SAM as stand alone HTL suffer from severe FF and Voc losses and that NiOx Cu is needed to act as a shunt quenching layer below that SAM, ensuring conformal coverage of the rough bottom sub cell surface. Within this work the highest achieved in house measured PCEs for the RTP and PVD CIGS based tandems are 21.6 and 23.2 respectively, on a cell area of 1.08 cm2, both of which are obtained with NiOx Cu SAM as an HT

    Solid state NMR study on the structure and dynamics of graphite electrodes in sodium ion batteries with solvent co intercalation

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    The possibility to co intercalate sodium ions together with various glymes in graphite enables its use as a negative electrode material in sodium ion batteries SIBs . However, the storage mechanism and local interactions appearing during this reaction still needs further clarification. 1 H, 13 C and 23 Na ex situ solid state NMR ss NMR experiments are performed to obtain insights into the storage mechanism depending on the state of charge SOC and the electrolyte solvent used. Distinct differences could be seen depending on the SOC, indicating a possible change of the solvation shell, differences in the mobility as well as a phase transition at the plateau. Furthermore, exchange experiments reveal information on the sodium ion transport process in the graphitic lattice. The inferior cycling performance of triglyme 3G compared to diglyme 2G and pentaglyme 5G is also reflected in the ss NMR spectra, showing a reduced mobility and stronger interactions between sodium ions, 3G and graphite already at room temperature R

    Crystal structures of free and ligand bound forms of the TetR AcrR like regulator SCO3201 from Streptomyces coelicolor suggest a novel allosteric mechanism

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    TetR AcrR like transcription regulators enable bacteria to sense a wide variety of chemical compounds and to dynamically adapt the expression levels of specific genes in response to changing growth conditions. Here, we describe the structural characterisation of SCO3201, an atypical TetR AcrR family member from Streptomyces coelicolor that strongly represses antibiotic production and morphological development under conditions of overexpression. We present crystal structures of SCO3201 in its ligand free state as well as in complex with an unknown inducer, potentially a polyamine. In the ligand free state, the DNA binding domains of the SCO3201 dimer are held together in an unusually compact conformation and, as a result, the regulator cannot span the distance between the two half sites of its operator. Interaction with the ligand coincides with a major structural rearrangement and partial conversion of the so called hinge helix amp; 945;4 to a 310 conformation, markedly increasing the distance between the DNA binding domains. In sharp contrast to what was observed for other TetR AcrR like regulators, the increased interdomain distance might facilitate rather than abrogate interaction of the dimer with the operator. Such a reverse induction mechanism could expand the regulatory repertoire of the TetR AcrR family and may explain the dramatic impact of SCO3201 overexpression on the ability of S. amp; 8201;coelicolor to generate antibiotics and sporulat

    Atomistic Simulations of Defects Production under Ion Irradiation in Epitaxial Graphene on SiC

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    Using first principles and analytical potential atomistic simulations, production of defects in epitaxial graphene EG on SiC upon ion irradiation for ion types and energies accessible in helium ion microscope is studied. Graphene SiC systems consisting of the buffer zero graphene layer and SiC substrate, as well as one monolayer and two bilayer additional graphene layers, are focused on. The probabilities for single, double, and more complex vacancies to appear upon impacts of energetic ions in each graphene layer as functions of He and Ne ion energies are calculated and the data are compared with those obtained for free standing graphene. The results indicate that the role of the substrate is minimal for He ion irradiation with energies above 5 amp; 8201;keV, which can be associated with a low sputtering yield from this system upon ion irradiation, as compared with the common Si SiO2 substrate. In contrast, SiC substrate has a significant effect on defect production upon Ne ion irradiation. The results can serve as a guide to the experiments on ion irradiation of EG to choose the optimum ion beam parameters for defect mediated engineering of such systems, for example, for creating nucleation centers to grow other 2D materials, such as h BN, on top of the irradiated E

    Magnetic coupling of guest metallocene molecules with SURMOF 2 host matrix

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    Metal organic frameworks MOFs are crystalline and porous, molecular solids consisting of metal nodes and organic ligands. An interesting example is the MOF 2 system, where Cu2 ions form antiferromagnetically coupled dimers to yield so called paddlewheels. In the case of surface anchored MOF 2 SURMOF 2 systems the Cu2 ions are connected via carboxylate and OH groups in a zipperlike fashion. This unusual coupling of the spin 1 2 ions within the resulting one dimensional chains stabilizes a low temperature ferromagnetic FM phase. In this study, the magnetic properties of two SURMOF 2 systems Cu bdc and Cu bpdc with bdc 1, 4 benzendicarboxylic acid, bpdc 4,4 biphenyldicarboxylic acid were investigated using x ray magnetic circular dichroism both in the absorption and in the scattering geometry. The presence of the FM phase in these SURMOF 2 systems is confirmed. Taking advantage of the element sensitivity of this technique. it was established that the magnetic signal originates from Cu2 ions. After loading of SURMOF 2 with metallocene molecules, no remarkable changes of the magnetic properties of the host matrix were observed. However, the magnetic behavior of the guest molecules, as it turned out, is rather different. In the case of nickelocene loading, a polarization effect was found, resulting in ferromagnetic ordering of the guest molecules. However, in the case of manganocene derivatives, the polarization effect is not observed and these molecules remained in their paramagnetic state. The details of these effects are discusse

    Laser Induced Creation of Antiferromagnetic 180 Degree Domains in NiO Pt Bilayers

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    The antiferromagnetic order in heterostructures of NiO Pt thin films can be modified by optical pulses. After the irradiation with laser light, the optically induced creation of antiferromagnetic domains can be observed by imaging the created domain structure utilizing the X ray magnetic linear dichroism effect. The effect of different laser polarizations on the domain formation can be studied and used to identify a polarization independent creation of 180 domain walls and domains with 180 different N el vector orientation. By varying the irradiation parameters, the switching mechanism can be determined to be thermally induced. This study demonstrates experimentally the possibility to optically create antiferromagnetic domains, an important step towards future functionalization of all optical switching mechanisms in antiferromagnet

    Adsorption of Na Monolayer on Graphene Covered Pt 111 Substrate

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    Modification of graphene electronic properties via contact with atoms of different kind allows for designing a number of functional post silicon electronic devices. Specifically, 2D metallic layer formation over graphene is a promising approach to improving the electronic properties of graphene based systems. In this work we analyse the electronic and spin structure of graphene synthesized on Pt 111 after sodium monolayer adsorption by means of angle resolved photoemission spectroscopy and ab initio calculations. Here, we show that sodium layer formation leads to a shift of the graphene amp; 960; states towards higher binding energies, but the most intriguing property of the studied system is the appearance of a partially spin polarized Kanji symbol like feature resembling the graphene Dirac cone in the electronic structure of adsorbed sodium. Our findings reveal that this structure is caused by a strong interaction between Na orbitals and Pt 5d spin polarized states, where the graphene monolayer between them serves as a mediator of such interactio

    Using internal strain and mass to modulate Dy amp; 8943;Dy coupling and relaxation of magnetization in heterobimetallic metallofullerenes DyM2N C80 and Dy2MN C80 M Sc, Y, La, Lu

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    Endohedral clusters inside metallofullerenes experience considerable inner strain when the size of the hosting cage is comparably small. This strain can be tuned in mixed metal metallofullerenes by combining metals of different sizes. Here we demonstrate that the internal strain and mass can be used as variables to control Dy amp; 8943;Dy coupling and relaxation of magnetization in Dy metallofullerenes. Mixed metal nitride clusterfullerenes DyxY3 amp; 8722;xN Ih C80 x 0 3 and Dy2LaN Ih C80 combining Dy with diamagnetic rare earth elements, Y and La, were synthesized and characterized by single crystal X ray diffraction, SQUID magnetometry, ab initio calculations, and spectroscopic techniques. DyxY3 amp; 8722;xN clusters showed a planar structure, but the slightly larger size of Dy3 in comparison with that of Y3 resulted in increased elongation of the nitrogen thermal ellipsoid, showing enhancement of the out of plane vibrational amplitude. When Dy was combined with larger La, the Dy2LaN cluster appeared strongly pyramidal with the distance between two nitrogen sites of 1.15 1 , whereas DyLa2N C80 could not be obtained in a separable yield. Magnetic studies revealed that the relaxation of magnetization and blocking temperature of magnetization in the DyM2N C80 series M Sc, Y, Lu correlated with the mass of M, with DySc2N C80 showing the fastest and DyLu2N C80 the slowest relaxation. Ab initio calculations predicted very similar g tensors for Dy3 ground state pseudospin in all studied DyM2N C80 molecules, suggesting that the variation in relaxation is caused by different vibrational spectra of these compounds. In the Dy2MN C80 series M Sc, Y, La, Lu , the magnetic and hysteretic behavior was found to correlate with Dy amp; 8943;Dy coupling, which in turn appears to depend on the size of M3 . Across the Dy2MN C80 series, the energy difference between ferromagnetic and antiferromagnetic states changes from 5.6 cm amp; 8722;1 in Dy2ScN C80 to 3.0 cm amp; 8722;1 in Dy2LuN C80, 1.0 cm amp; 8722;1 in Dy2YN C80, and amp; 8722;0.8 cm amp; 8722;1 in Dy2LaN C80. The coupling of Dy ions suppresses the zero field quantum tunnelling of magnetization but opens new relaxation channels, making the relaxation rate dependent on the coupling strengths. DyY2N C80 and Dy2YN C80 were found to be non luminescent, while the luminescence reported for DyY2N C80 was caused by traces of Y3N C80 and Y2ScN C8


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