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Item 3D Impedance Modelling of Metal Anodes in Solid-State Batteries − Incompatibility of Pore Formation and Constriction Effect in Physical-Based 1D Circuit Models(2022-09-08) Eckhardt, Janis K.; Fuchs, Till; Burkhardt, Simon; Klar, Peter J.; Janek, Jürgen; Heiliger, ChristianA non-ideal contact at the electrode/solid electrolyte interface of a solid-state battery arising due to pores (voids) or inclusions results in a constriction effect that severely deteriorates the electric transport properties of the battery cell. The lack of understanding of this phenomenon hinders the optimization process of novel components, such as reversible and high-rate metal anodes. Deeper insight into the constriction phenomenon is necessary to correctly monitor interface degradation and to accelerate the successful use of metal anodes in solid-state batteries. Here, we use a 3D electric network model to study the fundamentals of the constriction effect. Our findings suggest that dynamic constriction as a non-local effect cannot be captured by conventional 1D equivalent circuit models and that its electric behavior is not ad hoc predictable. It strongly depends on the interplay of the geometry of the interface causing the constriction and the microscopic transport processes in the adjacent phases. In the presence of constriction, the contribution from the non-ideal (porous) electrode/solid electrolyte interface to the impedance spectrum may exhibit two signals that cannot be explained when the porous interface is described by a physical-based (effective medium theory) 1D equivalent circuit model. In consequence, the widespread assumption of a single interface contribution to the experimental impedance spectrum may be entirely misleading and can cause serious misinterpretation.Item Auger recombination rates in ZnMgO from first principles(2011) Heinemann, Markus; Heiliger, ChristianWe investigate direct electron-electron-hole interband Auger recombination for wurtzite Zn(1-x)Mg(x)O alloys in the range 0 <= x <= 1. Recombination rates are computed by interpolating the band structure and transition matrix elements from ab initio calculations of bulk ZnO, Zn(0.5)Mg(n0.5)O, and MgO primitive cells. We find that interband Auger recombination is most probable for Mg concentrations around 50%, where ZnMgO does not exist in a stable wurtzite phase. Since, for low Mg concentrations, the calculated Auger coefficients are far below 10(-32) cm(6)/s, we do not expect significant nonradiative loss through direct interband recombination in wurtzite ZnMgO.Item The band alignment of Cu2O/ZnO and Cu2O/GaN heterostructures(2012) Kramm, B.; Laufer, A.; Reppin, D.; Kronenberger, A.; Hering, P.; Polity, A.; Meyer, B. K.Using photoelectron spectroscopy, we investigate the band alignments of the Cu2O/ZnO heterointerface and compare the findings with the corresponding values for Cu2O/GaN. While for Cu2O/ZnO, we find a valence band offset (VBO) of 2.17 eV and a conduction band offset (CBO) of 0.97 eV, both values are considerably reduced for Cu2O/GaN where the numbers are 1.47 eV (VBO) and 0.24 eV (CBO), respectively. The large CBO between ZnO and Cu2O will very likely result in low photovoltaic power conversion efficiencies as is the current status of Cu2O/ZnO solar cells.Item Determination of secondary ion mass spectrometry relative sensitivity factors for polar and non-polar ZnO(2011) Laufer, Andreas; Volbers, Niklas; Eisermann, Sebastian; Potzger, Kay; Geburt, Sebastian; Ronning, Carsten; Meyer, Bruno K.Zinc oxide (ZnO) is regarded as a promising material for optoelectronic devices, due to its electronic properties. Solely, the difficulty in obtaining p-type ZnO impedes further progress. In this connection, the identification and quantification of impurities is a major demand. For quantitative information using secondary ion mass spectrometry (SIMS), so-called relative sensitivity factors (RSF) are mandatory. Such conversion factors did not yet exist for ZnO. In this work, we present the determined RSF values for ZnO using primary (ion implanted) as well as secondary (bulk doped) standards. These RSFs have been applied to commercially available ZnO substrates of different surface termination (a-plane, Zn-face, and O-face) to quantify the contained impurities. Although these ZnO substrates originate from the same single-crystal, we observe discrepancies in the impurity concentrations. These results cannot be attributed to surface termination dependent RSF values for ZnO.Item Diamondoid coating enables disruptive approach for chemical and magnetic imaging with 10nm spatial resolution(2012) Ishiwata, Hitoshi; Acremann, Yves; Scholl, Andreas; Rotenberg, Eli; Hellwig, Olav; Dobisz, Elizabeth; Doran, Andrew; Tkachenko, Boryslav A.; Fokin, Andrey A.; Schreiner, Peter R.; Dahl, Jeremy E. P.; Carlson, Robert M. K.; Melosh, Nick; Shen, Zhi-Xun; Ohldag, HendrikDiamondoids are unique molecular nano-materials with diamond structure and fascinating properties such as negative electron affinity and short electron mean free paths. A thin layer of diamondoids deposited on a cathode is able to act as an electron monochromator, reducing the energy spread of photo-emitted electrons from a surface. This property can be applied effectively to improve the spatial resolution in x-ray photoemission electron microscopy (X-PEEM), which is limited by chromatic aberration of the electron optics. In this paper, we present X-PEEM measurements reaching the technological relevant spatial resolution of 10?nm without the need of expensive and complex corrective optics. Our results provide a simple approach to image surface chemical and magnetic information at nanometer scales by employing diamondoids.Item Electron paramagnetic resonance and photo-electron paramagnetic resonance investigation on the recharging of the substitutional nitrogen acceptor in ZnO(2012) Stehr, J. E.; Hofmann, D. M.; Meyer, B. K.We investigated the substitutional nitrogen center in ZnO single crystals by electron paramagnetic resonance (EPR) and photo-EPR spectroscopy. Aside the three principle hyperfine lines due to the interaction of the N0 (2p5) electron spin with the nitrogen nucleus (I=1, natural abundance 99.6%), we identify additional satellite lines which arise from delta mS=±1 and delta mI=±1, ±2 transitions becoming allowed due to quadrupole interaction. The quadrupole coupling constant e2qQ/h is determined to -5.9MHz with an asymmetry parameter of eta=0.05. These values are somewhat different from those obtained for the nitrogen center in ZnO powders, but are closer to the theoretical calculations of Gallino et al. We further carefully investigated the photon induced recharging of the N centers. We determine the energy hu required for the process NO-+hu -> NO0+ecb- to 2.1±0.05eV, the dependence of the EPR signal intensity on the illumination time shows a mono-exponential behavior which gives evidence that a direct ionization process is monitored.Item Electrospun antimony doped tin oxide (ATO) nanofibers as a versatile conducting matrix(2012) Ostermann, Rainer; Zieba, Roman; Rudolph, Melanie; Schlettwein, Derck; Smarsly, Bernd M.Nanoparticles of ATO (antimony doped tin oxide) were used to produce thick conductive, free standing mats of nanofibers via electrospinning. These fibrous mats were incorporated into polymer films to produce a transparent conducting polymer foil. Moreover, the fiber mats can serve as porous electrodes for electrodeposition of Prussian Blue and TiO2 and were tested in dye-sensitized solar cells.Item Exciton confinement in homo- and heteroepitaxial ZnO/Zn(1-x)Mg(x)O quantum wells with x < 0.1(2011) Laumer, Bernhard; Wassner, Thomas A.; Schuster, Fabian; Stutzmann, Martin; Schörmann, Jörg; Rohnke, Marcus; Chernikov, Alexej; Bornwasser, Verena; Koch, Martin; Chatterjee, Sangam; Eickhoff, MartinZnO/Zn(1 - x)Mg(x)O single quantum well (SQW) structures with well widths d(W) between 1.1nm and 10.4 nm were grown by plasma-assisted molecular beam epitaxy both heteroepitaxially on c-plane sapphire and homoepitaxially on (000 (1) over bar)-oriented bulk ZnO. A significantly reduced Mg incorporation in the top barrier related to the generation of stacking faults is observed for heteroepitaxial samples. Exciton localization is observed for both types of samples, while an enhancement of the exciton binding energy compared to bulk ZnO is only found for homoepitaxial SQWs for 2nm <= d(W) <= 4 nm. Consistently, for homoepitaxial samples, the carrier dynamics are mainly governed by radiative recombination and carrier cooling processes at temperatures below 170 K, whereas thermally activated non-radiative recombination dominates in heteroepitaxial samples. The effects of polarization-induced electric fields are concealed for Mg concentrations x < 0.1 due to the reduction of the exciton binding energy, the screening by residual carriers as well as the asymmetric barrier structure in heteroepitaxial wells.Item Influence of interface termination on the magneto-Seebeck effect in MgO based tunnel junctions(2012) Czerner, Michael; Heiliger, ChristianOn an ab initio level, we investigate the recently observed magneto-Seebeck effect in MgO based tunnel junctions. In particular, we considered ordered CoFe alloys as lead material. Next to the MgO barrier, there are different possible terminations of the CoFe alloy due to the assumed ordered alloy. These results show a strong influence of the termination on the temperature dependence of the magneto-Seebeck effect. In addition, we use a simple model to account for randomly ordered alloys. We propose, that by a controlled treatment of the CoFe/MgO interface the magneto-Seebeck effect can be tuned experimentally.Item An insight into voltage-biased superconducting quantum interference devices(2012) Liu, Chao; Zhang, Yi; Mück, Michael; Krause, Hans-Joachim; Braginski, Alex I.; Xie, Xiaoming; Offenhäusser, Andreas; Jiang, MianhengWe experimentally studied two important parameters of helium-cooled superconducting quantum interference devices (SQUIDs) in the voltage bias mode: the dynamic resistance Rd and the flux-to-current transfer coefficient ?i/?phi, with different junction shunt resistors RJ. We investigated a voltage-biased SQUID using the direct readout current-to-voltage converter scheme involving an operational amplifier. At higher RJ, the flux-to-voltage conversion coefficient ?V/?phi becomes sufficiently large to effectively suppress the room-temperature amplifier´s noise without any need for additional feedback circuits. The McCumber parameter limits the rise of ?V/?phi. We discuss the performance of voltage-biased SQUIDs at different effective McCumber parameters.Item Internal quantum efficiency of III-nitride quantum dot superlattices grown by plasma-assisted molecular-beam epitaxy(2011) Gacevic, Z.; Das, A.; Teubert, Jörg; Kotsar, Y.; Kandaswamy, P. K.; Kehagias, T.; Koukoula, T.; Komninou, P.; Monroy, E.We present a study of the optical properties of GaN/AlN and InGaN/GaN quantum dot (QD) superlattices grown via plasma-assisted molecular-beam epitaxy, as compared to their quantum well (QW) counterparts. The three-dimensional/two-dimensional nature of the structures has been verified using atomic force microscopy and transmission electron microscopy. The QD superlattices present higher internal quantum efficiency as compared to the respective QWs as a result of the three-dimensional carrier localization in the islands. In the QW samples, photoluminescence (PL) measurements point out a certain degree of carrier localization due to structural defects or thickness fluctuations, which is more pronounced in InGaN/GaN QWs due to alloy inhomogeneity. In the case of the QD stacks, carrier localization on potential fluctuations with a spatial extension smaller than the QD size is observed only for the InGaN QD-sample with the highest In content (peak emission around 2.76 eV). These results confirm the efficiency of the QD three-dimensional confinement in circumventing the potential fluctuations related to structural defects or alloy inhomogeneity. PL excitation measurements demonstrate efficient carrier transfer from the wetting layer to the QDs in the GaN/AlN system, even for low QD densities (similar to 10(10) cm(-3)). In the case of InGaN/GaN QDs, transport losses in the GaN barriers cannot be discarded, but an upper limit to these losses of 15% is deduced from PL measurements as a function of the excitation wavelength.Item Intra-excitonic relaxation dynamics in ZnO(2011) Chernikov, Alexej; Koch, Martin; Laumer, Bernhard; Wassner, Thomas A.; Eickhoff, Martin; Koch, Stephan W.; Chatterjee, SangamThe temperature and carrier-density dependent excitonic relaxation in bulk ZnO is studied by means of time-resolved photoluminescence. A rate-equation model is used to analyze the population dynamics and the transitions between different exciton states. Intra-excitonic (n=1) to (n=2) relaxation is clearly identified at low excitation densities and lattice temperatures with a characteristic time constant of 6±0.5 ps.Item Ion conducting particle networks in liquids: modeling of network percolation and stability(2011) Jarosik, Anna; Traub, Uwe; Maier, Joachim; Bunde, ArminNetworks of inorganic particles (here SiO(2)) formed within organic liquids play an important role in science. Recently they have been considered as ´soggy sand´ electrolytes for Li-based batteries with a fascinating combination of mechanical and electrical properties. In this communication we model formation and stability of the networks by Cluster-Cluster Aggregation followed by coarsening on a different time scale. The comparison of computer simulations based on our model with experimental results obtained for LiClO(4) containing polyethylene glycol reveals (i) that the percolation threshold for interfacial conductivity is very small, (ii) that the networks once formed coarsen with a time constant that is roughly independent of volume fraction and size-to a denser aggregate which then stays stable under operating condition. (iii) Trapping of the conducting solvent at high packing is also revealed.Item Microstrip direct current superconducting quantum interference device radio frequency amplifier: Noise data(2012) Schmidt, Bernd; Mück, MichaelA series of about twenty superconducting quantum interference devices (SQUIDs) has been operated as microstrip-SQUID amplifiers (MSAs) at frequencies ranging from 100 MHz to 2GHz to study the dependence of their gain and noise temperature on bias current and flux. The measured values were in good agreement with theory. The observed dependence of MSA gain and noise temperature on bias current and flux resembled the static transfer function of the SQUIDs. The gains are relatively insensitive to changes in bias current and bias flux; the noise temperature is strongly dependent on the bias flux.Item Nitrogen and vacancy clusters in ZnO(2013) Tuomisto, Filip; Rauch, Christian; Wagner, Markus R.; Hoffmann, Axel; Eisermann, Sebastian; Meyer, Bruno K.; Kilanski, Lukasz; Tarun, Marianne C.; McCluskey, Matthew D.Understanding the interaction of group V impurities with intrinsic defects in ZnO is important for developing p-type material. We have studied N-doped ZnO thin films and N-doped bulk ZnO crystals, with positron annihilation spectroscopy, in contrast to earlier studies that have concentrated on N-implanted ZnO crystals. We show that the introduction of N impurities into ZnO, irrespective of whether it is done during the growth of thin films or bulk crystals or through implantation and subsequent thermal treatments, leads to the formation of stable vacancy clusters and negative ion-type defects. Interestingly, the stability of these vacancy clusters is found almost exclusively for N introduction, whereas single Zn vacancy defects or easily removable vacancy clusters are more typically found for ZnO doped with other impurities.Item Optical properties of MgZnO alloys: Excitons and exciton-phonon complexes(2011) Neumann, M. D.; Cobet, C.; Esser, N.; Laumer, Bernhard; Wassner, T. A.; Eickhoff, Martin; Feneberg, M.; Goldhahn, R.The characteristics of the excitonic absorption and emission around the fundamental bandgap of wurtzite Mg(x)Zn(1-x)O grown on c-plane sapphire substrates by plasma assisted molecular beam epitaxy with Mg contents between x = 0 and x = 0.23 are studied using spectroscopic ellipsometry and photoluminescence (PL) measurements. The ellipsometric data were analyzed using a multilayer model yielding the dielectric function (DF). The imaginary part of the DF for the alloys exhibits a pronounced feature which is attributed to exciton-phonon coupling (EPC) similar to the previously reported results for ZnO. Thus, in order to determine reliable transition energies, the spectral dependence is analyzed by a model which includes free excitonic lines, the exciton continuum, and the enhanced absorption due to EPC. A line shape analysis of the temperature-dependent PL spectra yielded in particular the emission-related free excitonic transition energies, which are compared to the results from the DF line-shape analysis. The PL linewidth is discussed within the framework of an alloy disorder model.Item Optical properties of wurtzite/zinc-blende heterostructures in GaN nanowires(2011) Jacopin, G.; Rigutti, L.; Largeau, L.; Fortuna, F.; Furtmayr, F.; Julien, F. H.; Eickhoff, Martin; Tchernycheva, M.The optical and structural properties of wurtzite GaN nanowires containing zinc-blende GaN inclusions of different thicknesses are investigated. Micro-photoluminescence spectra of single nanowires exhibit a series of narrow emission peaks with linewidth as low as 0.8 meV in the interval 3.1-3.42 eV. The peak energy blue-shifts with increasing excitation power following a similar to I(1/3) law due to the progressive band filling and to the screening of the internal field. The quantum confinement in these type-II crystal phase heterostructures was simulated in the framework of a one-dimensional effective mass model, accounting for the internal electrical polarization of the wurtzite GaN. The predicted transition energies are in good agreement with the energy statistics realized on more than 30 single nanowire emission spectra.Item Photocathode device using diamondoid and cesium bromide films(2012) Clay, William A.; Maldonado, Juan R.; Pianetta, Piero; Dahl, Jeremy E. P.; Carlson, Robert M. K.; Schreiner, Peter R.; Fokin, Andrey A.; Tkachenko, Boryslav A.; Melosh, Nicholas A.; Shen, Zhi-XunA photocathode structure is presented that shows promise for use in high brightness electron sources. The structure consists of a metal substrate, a monolayer of a diamondoid derivative, and a thin film of cesium bromide. Diamondoid monolayers reduce the energy spread of electron emitters, while cesium bromide increases the yield and stability of cathodes. We demonstrate that the combined structure retains these properties, producing an emitter with lower energy spread than the corresponding cesium bromide emitter (1.06?eV versus 1.45?eV) and higher yield and stability than un-coated diamondoid emitters.Item Photodissociation of protonated leucine-enkephalin in the VUV range of 8-40 eV(2011) Bari, S.; Gonzalez-Magana, O.; Reitsma, G.; Werner, J.; Schippers, Stefan; Hoekstra, R.; Schlatholter, T.Until now, photodissociation studies on free complex protonated peptides were limited to the UV wavelength range accessible by intense lasers. We have studied photodissociation of gas-phase protonated leucine-enkephalin cations for vacuum ultraviolet (VUV) photons energies ranging from 8 to 40 eV. We report time-of-flight mass spectra of the photofragments and various photofragment-yields as a function of photon energy. For sub-ionization energies our results are in line with existing studies on UV photodissociation of leucine-enkephalin. For photon energies exceeding 10 eV we could identify a new dissociation scheme in which photoabsorption leads to a fast loss of the tyrosine side chain. This loss process leads to the formation of a residual peptide that is remarkably cold internally.Item A review of MBE grown 0D, 1D and 2D quantum structures in a nanowire(2013) Mata, Maria de la; Zhou, Xiang; Furtmayr, Florian; Teubert, Jörg; Gradecak, Silvija; Eickhoff, Martin; Fontcuberta i Morral, Anna; Arbiol, JordiWe review different strategies to achieve a three-dimensional energy bandgap modulation in a nanowire (NW) by the introduction of self-assembled 0D, 1D and 2D quantum structures, quantum dots (QDs), quantum wires (QWRs) and quantum wells (QWs). Starting with the well-known axial, radial (coaxial/prismatic) or polytypic quantum wells in GaN/AlN, GaAs/AlAs or wurtzite/zinc-blende systems, respectively, we move to more sophisticated structures by lowering their dimensionality. New recent approaches developed for the self-assembly of GaN quantum wires and InAs or AlGaAs quantum dots on single nanowire templates are reported and discussed. Aberration corrected scanning transmission electron microcopy is presented as a powerful tool to determine the structure and morphology at the atomic scale allowing for the creation of 3D atomic models that can help us to understand the enhanced optical properties of these advanced quantum structures.