Universität Stuttgart
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Item Open Access Spectroscopic investigations of the magnetic anisotropy of lanthanide- and cobalt-based molecular nanomagnets(2016) Rechkemmer, Yvonne; Slageren, Joris van (Prof. Dr.)Single-molecule magnets are metal complexes exhibiting an energy barrier for spin reversal, leading to magnetic bistability and slow relaxation of the magnetization. Their potential for practical applications such as high-density magnetic data storage was recognized early on and with the goal of achieving high energy barriers, different kinds of single-molecule magnets have been synthesized. The quadratic dependence of the barrier height on the spin motivated chemists to synthesize metal complexes with very high total spins; however, with limited success. It was shown that high spins come along with low anisotropies and increased interest thus focused on the synthesis and investigation of (mononuclear) complexes of highly anisotropic metal centers, e.g. lanthanide or cobalt complexes. Although rather high energy barriers can be achieved in such systems, practical application remains problematic and has not been realized yet. Reasons are for example the lack of rational design criteria and the complex interplay of different magnetic relaxation pathways. The aim of this work was therefore the comprehensive magnetic and spectroscopic investigation of selected molecular lanthanide and cobalt compounds in order to obtain a deeper insight into the correlation of molecular and electronic structures as well as the corresponding magnetic properties. The applied spectroscopic methods included electron paramagnetic resonance spectroscopy, far-infrared spectroscopy and optical methods. Special emphasis was placed on magnetic circular dichroism (MCD) spectroscopy, which served as a main tool for electronic structure determination. However, since the MCD-spectrometer was not part of the available experimental equipment at the University of Stuttgart, its design, setup and characterization were the first part of this work. In the further course of this work MCD-spectroscopy was employed for the electronic structure determination of selected lanthanide and cobalt compounds. The studied lanthanide compounds were literature-known molecular tetra-carbonates of erbium (1-Er) and dysprosium (1-Dy). Detailed magnetometric studies showed that both 1-Er and 1-Dy are field-induced single-molecule magnets; however, 1-Er and 1-Dy show significant differences in their magnetic relaxation behavior. The magnetic studies were complemented by detailed spectroscopic investigations.The combination of far-infrared-, luminescence- and MCD-spectroscopy allowed for the experimental determination of 48 energy levels for 1-Er and 55 levels for 1-Dy, which built the foundation for the subsequent crystal field analysis and electronic structure determination. In addition, the results of EPR-spectroscopic studies were used for fine-tuning and verifying the respectively determined crystal field parameters. Calculating the magnetic dipole strengths for transitions between the relevant states led to a quantitative understanding of the magnetic relaxation pathways. Besides the investigation of lanthanide compounds, this thesis deals with two classes of cobalt complexes. The first class comprises mononuclear complexes in which one Co(II) ion is ligated by the nitrogen donors of two doubly deprotonated 1,2-bis(methanesulfonamido)-benzene-ligands. Rather acute N-Co-N bite angles indicate strong deviations from ideal tetrahedral symmetry. The static magnetic properties hint at very high energy barriers for spin reversal and with the help of far-infrared spectroscopy, largely negative axial zero-field splitting parameters were determined. The corresponding energy barriers belong to the highest ever reported for 3d-transition metal complexes and investigating the dynamic magnetic properties confirmed single-molecule magnet behavior. The unique magnetic properties were fully explained by analyzing spectroscopic results. The MCD-spectra showed intense signals that were assigned to spin-allowed d-d-transitions. Subsequent crystal field analysis revealed that the strong axial crystal field generated by the ligands leads to a large splitting of the electronic terms and thus in turn to a relatively small energy gap between the electronic ground state and the first excited state. The resulting increase in second-order spin-orbit coupling explains the high energy barriers observed in the studied complexes. The second class of cobalt compounds studied in this work included dimers of distorted octahedrally coordinated Co(II) ions bridged by symmetrical or asymmetrical quinone based bridging ligands. The main focus of investigation lay on the impact of the bridging ligand on the magnetic coupling between the cobalt centers. Thus, the magnetic properties of the complexes were studied with the help of static susceptibility and magnetization measurements and analyzed by means of different models. Depending on the bridging ligand, different signs for the exchange coupling constants were found. The varying signs can be explained by different relative contributions of possible exchange paths, influenced by the different substituents at the bridging ligands or slight geometry differences. The observations indicate that electron withdrawing substituents favor ferromagnetic couplings, which are preferred in the context of molecular magnetism. All in all, it can be concluded that this work provides a contribution to the deeper understanding of the features relevant for single-molecule magnets. The electronic structure determination for selected lanthanide and cobalt complexes applying advanced magnetometric and spectroscopic techniques not only led to an understanding of the static and dynamic magnetic properties but also allowed for the development of design criteria and new approaches for improved single-molecule magnets in the future.Item Open Access Ion beam lithographic and multilayer fresnel zone plates for soft and hard X-rays: nanofabrication and characterization(2015) Keskinbora, Kahraman; Schütz, Gisela (Prof. Dr.)X-ray microscopy has become an important analytical characterization method for a plethora of applications in materials science, physics, chemistry and biology, thanks to the emergence of modern synchrotron radiation facilities. These facilities enable high brilliance, energy tunable, variable polarization X-rays which gives access to mass density, elemental, chemical, electronic and magnetic properties of materials. In the soft X-ray energies nearly all elements can be probed by spectromicroscopic methods. Another important property of synchrotron radiation is the time structure in the ns to ps range, which can be utilized for sophisticated time resolution studies. These opportunities can be combined with high spatial resolution which is determined by the focusing method and the optic. Focusing of X-rays has historically been a difficult task due to strong absorption and weak phase shift of X-rays within matter. The required phase shift of X-rays, which depends on the real part of the complex refractive index, differs from 1 (the vacuum refractive index) only on the order of 10^-2 to 10^-6 and conventional lenses do not work. One very successful X-ray optic is the Fresnel Zone Plate (FZP), a diffractive optic that act as a lens under certain conditions and can focus X-rays to nanometer sized spots. The resolution of the FZP depends on the width of the outermost zone and is highly correlated with the smallest feature that can be fabricated. Conventionally, the e-beam lithography (EBL) is used for production FZPs which could resolve up to 10 nm structures with serious limitations. One difficulty of EBL is its ever increasing complexity for many-step fabrication of smaller features or intricate geometries. Therefore, EBL is mostly constrained to planar, binary geometries with moderate efficiencies strongly decreasing with energy and not effective for hard X-rays. Special 3D geometries in the form of kinoform lenses can theoretically have 100 % focusing efficiencies. Attempts to approximate these geometries via EBL increased the number of process steps even further. The smallest FZP feature size even for low aspect ratios achievable via EBL is fundamentally limited due to the proximity effect which is the interaction and spread of electrons within the resist material. We addressed these issues by focusing our research on alternative FZP fabrication techniques as high-speed ion beam lithography (IBL), and gray scale ion lithography to realize efficient kinoforms. Another approach towards full-material multilayer FZPs with infinite aspect ratio was based on atomic layer deposition (ALD) with subsequent ion beam slicing. Each of these three methods targets specific challenges faced by the e-beam lithography based FZP fabrication techniques. All the fabricated FZPs were tested for their resolution and efficiency performances at a state of the art scanning transmission X-ray microscope at BESSY for soft X-rays and/or at optical test stations at ESRF and PETRA III for hard X-rays. Using IBL the rapid preparation of a 110 nm thick Au FZP with 50 µm diameter and 50 nm ∆r in less than 13 minutes is demonstrated. Employed for X-ray microscopy, the FZP clearly resolved 28.5 nm features with a cut-off of 24.3 nm at ~1120 eV. Additional process improvements were made towards smaller zones with higher zone quality. They allowed the preparation of a FZP with 30 nm outermost half-period remarkably, in about 8 min. This FZP was shown to clearly resolve 21 nm features on a multilayer test object with large room for improvement. This high through-put FZP production route is of special interest not only concerning the low cost and easy availability. A large array of these optical components is attractive, for experiments such as one-shot ultra-high brilliance FEL investigations due to the radiation damage or for instance for coded-aperture arrays for high-angle resolving X-ray astronomy. Towards fabrication of kinoforms for high efficiency X-ray focusing, we have performed various materials optimization studies in order to achieve a high surface quality optic. After various trials the materials were finally optimized and the fabricated lenses achieved more than 14 % absolute diffraction efficiency that is almost 90 % compared to the theoretical prediction. This confirms how closely we were able to replicate the ideal three dimensional surface relief structure for the first time. It was possible to carry out imaging with these lenses with half-pitch resolutions down to 60 nm. The kinoform lenses were tested at the soft X-ray range where a significant absorption is present in materials. These results also potentially pave the way for very high efficiency hard X-ray focusing which can in principle be utilized in laboratory based X-ray sources, X-ray astronomy and the new rising field of X-ray ptychography. To fabricate high resolution ML-FZPs, Al2O3/Ta2O5multilayers, deposited on a smooth glass optical fiber via atomic layer deposition using non-dedicated instruments were carefully cut-out, sliced and polished to a high quality surface finish using focused ion beams. Following the transfer of the slice to a TEM grid as holder the slices were polished to a high surface finish quality, also via a focused ion beam. Fabricated ML-FZPs were synchrotron tested using an in-house constructed 2-axis tilt stage specially designed for aligning ML-FZP with respect to the X-ray optical axis. The results showed that it was possible to resolve 21 nm features in direct imaging at 1200 eV and sub-30 nm focusing at 8 keV. This is the highest demonstrated resolving power for a multilayer type FZP, to date to the best of our knowledge. Results exhibit the potential for high-resolution hard X-ray focusing where this type of optics are especially efficient. For ultra-high resolution hard and soft X-ray imaging, with potentially achievable ∆r of a few nm is well below what can be achieved through any lithography method available today.Item Open Access Structural and electronic properties of nickelate heterostructures(2016) Wrobel, Friederike; Keimer, Bernhard (Prof. Dr.)The fabrication of thin films and multilayers has led to the discovery of novel functional properties which are widely used in electronic devices nowadays. The limit of such a material design is atomic layer-by-layer deposition which was made possible through shuttered molecular beam epitaxy (MBE) growth. In the course of this thesis project a newly developed oxide MBE system was used to grow two different types of nickelate heterostructures, namely superlattices (SLs) consisting of metallic and paramagnetic LaNiO3 sandwiched between a large band-gap insulator and a combination of lanthanum nickelate and cuprate layers into a single hybrid structure. The former type was intensively studied in the last years and a transition to a weakly insulating, antiferromagnetically ordered state was observed in samples where the LaNiO3 thickness had been reduced to only two unit cells. So far little was known about the influence of the growth method on the defects in the samples and consequently on their physical properties. The use of oxide MBE enabled us to improve the overall sample quality of nickelate SLs and to design a novel material. We first optimized the growth of LaNiO3 and thoroughly analyzed the heterostructures by synchrotron-based x-ray diffraction, transmission electron microscopy, and temperature-dependent electrical resistivity. Furthermore we conducted in-depth studies, including x-ray absorption and magneto-transport measurements. The knowledge gained thereby was used grow new, layered nickelate-cuprate hybrid structures with novel electronic and magnetic properties.Item Open Access In situ characterization of phase evolution in LiFePO4(2015) Ohmer, Nils; Maier, Joachim (Prof. Dr.)Among the candidates for electrodes in future Li-based batteries, LiFePO4 (LFP) is one of the most important and most frequently studied materials, undergoing a phase transformation upon delithiation to FePO4 (FP). In spite of the great scientific and practical interest in this material, there is still an extensive debate on the mechanism of this phase transformation and the underlying factors of influence. Within the framework of this thesis, first studies are carried out ex situ on multi-particle, full electrode LFP materials, being electrochemically cycled and analyzed at various states of charge by a combination of highly spatially resolved methods (high-resolution transmission electron microscopy and electron energy loss spectroscopy (HRTEM, EELS)) and integral measurement techniques (analyzing the X-ray diffraction and X-ray absorption near edge structure (XRD, XANES)). This combination of characterization techniques allows one to distinguish between the cycling behaviour of differently sized crystallites within the same electrode. It is found that for electrodes with hydrothermally grown LFP as active material, a particle size dependent cycling behaviour exists, with nanosized particles apparently not participating in the charging process at all. A turbostratic stacking of layers in these nanosized particles is found and identified to be responsible for sluggish lithium insertion and extraction. These higher dimensional defects prevent the small particles from participating in the charging process, most likely by disturbing the lithium diffusion along the 1-dimensional channels, as well as impair the transport along the other directions in the LFP host structure and thus blocking the lithium transport, resulting in a comparibly lower practical capacity during electrochemical cycling. To study the lithium exchange mechanism upon charging a LFP thin film cathode, an all-solid-state thin film battery cell with a lateral design concept is developed and realized by pulsed laser deposition (PLD) and thermal evaporation techniques. Using PLD and shadow masks LFP cathode, Li2O-V2O5-SiO2 (LVSO) electrolyte and LiAl anode thin films are deposited sequentially in a way that the Li transport pathway in the resulting battery is along the X-ray transparent commercial Si3N4 membrane substrate. This enables the usability of synchrotron-based energy resolved scanning transmission X-ray microscopy (STXM) with its high chemical and spatial resolution to perform in situ absorption measurements at the Fe L3 edge. Upon delithiation, a shift in the main absorption feature from 708 to 710 eV is used to fingerprint the change in the local state of charge, identifying areas containing Fe2+ (lithiated) and Fe3+ (delithiated), respectively. The initial lithiation process of a LFP thin film cathode material has been followed by in situ STXM, with a lateral resolution of 30 nm, during electrochemical charging of the thin film battery. The observed initial lithiation process does not follow the classical particle by particle mechanism, typical for multi-particle LFP cathodes, but instead a rather simultaneous, although inhomogeneous, lithiation is observed. The reason for this change in mechanism, compared to multi-particle powder electrodes, is found in mechanical interactions within the thin film upon lithiation, i.e. in the corresponding volume expansion and formation of high energy surfaces, changing the shape of the single-particle chemical potential to a monotone form upon lithiation. This has far-reaching consequences: not only the many-particle mechanism is changed to a concurrent lithiation, but also the single-particle mechanism is changed from a two-phase to a single-phase mechanism upon lithiation. Furthermore, a vanishing hysteresis loop and the disappearing of the memory effect is predicted. These findings are rather general and applicable to all kind of thin films of phase separating intercalation materials, undergoing a volume change upon lithium exchange. To fill the gap in literature on in situ observations of the (L)FP phase evolution on a single-particle level with appreciable space and time resolution, a micrometer-sized all-solid-state thin film battery is built with a defect-chemically well characterized LFP single crystal as cathode material with dimensions of 16x1x0.2 micrometer. Using STXM, the phase evolution along the fast (010) orientation is followed during in situ electrochemical (de)lithiation on a micro-meter scale with a lateral resolution of 30 nm and with minutes of time resolution. Furthermore, the STXM measurements performed on this sample are one of the few experiments ever taken on LFP materials with a well defined defect chemistry, even though fundamentally necessary for an overall understanding of the materials behaviour. This combination discloses not only the mechanism of LFP transformation on a single-particle level, but also the significance of elastic effects on the (de)lithiation process. Using a defect chemical analysis, the position of phase formation is found to be determined by the defect chemical situation, while the growth pattern of both LFP and FP is found to be dominated by elastic effects.Item Open Access Misfit-layered cobalt oxides for thermoelectric energy conversion(2017) Büttner, Gesine; Weidenkaff, Anke (Prof. Dr.)The conversion of waste heat into electrical current by a thermoelectric converter can significantly contribute to a more sustainable usage of our resources. The p-type misfit-layered [Ca2CoO3-δ][CoO2]1.62 is known for its promising conversion efficiency, which yet needs to be improved significantly for commercial applications. The efficiency of a material increases with the Figure of Merit ZT=σα^2/κ, with Seebeck coefficient α, electrical conductivity σ, and thermal conductivity κ. The aim of this thesis is to provide a better understanding of the electrical and the thermal properties of the complex [Ca2CoO3 δ][CoO2]1.62 and to use this understanding to improve the efficiency of converters. Accordingly, (i) the increase of ZT via cation substitution is shown; (ii) a better understanding of the electrical transport above room temperature is developed; (iii) the effect of stoichiometric defects and secondary phases on the thermoelectric properties is investigated. Finally, (iv) [Ca2CoO3 δ][CoO2]1.62 - CaMn0.97W0.03O3 δ - converters are fabricated and the efficiency is increased by a suitable converter design. More specifically, the unexplored influence of Ru and In substitution on the thermoelectric properties of the polycrystalline [Ca2CoO3 δ][CoO2]1.62 is investigated. While In does not have a positive effect, Ru for Co substitution increases ZT up to 20 %. This increase stems from a strong reduction of the thermal conductivity - which is probably induced by resonance scattering - while the decrease of the power factor α^2 σ is minor. The electrical transport mechanism of pure and Ru-substituted [Ca2CoO3 δ][CoO2]1.62 between room temperature and 800 K so far lacks a coherent theoretical model. Surprisingly, the framework of Anderson localization, which was developed to describe conduction in an impurity band of semiconductors, can be applied to the oxide. The Anderson model assumes that transport happens via charge-carrier hopping in a random Coulomb potential. For [Ca2CoO3 δ][CoO2]1.62, charges are considered to hop between Co sites in the CoO2 layer, while the random potential originates from interactions with the mismatched Ca2CoO3 δ layer. The presence of the ionized Ru atoms further alters the Coulomb potential, which increases the activation energy of the transport behavior. This understanding might contribute to the development of better theoretical models for the prediction of the thermoelectric properties of substituted [Ca2CoO3 δ][CoO2]1.62 compounds. A further improvement of the materials efficiency can be achieved by systematic introduction of stoichiometric defects and impurity phases. Here, the unexplored influence of the Co/Ca ratio on the thermoelectric properties of [Ca2 wCoO3 δ][CoO2]1.62, and the effect of Co3O4 impurity phase are investigated. It is shown that an increasing Co/Ca ratio in the [Ca2 wCoO3 δ][CoO2]1.62 phase leads to a larger figure of merit ZT induced by a strong resistivity drop. The decrease of resistivity stems from additional p-type charge carriers created by the formation of Ca vacancies. The Co3O4 impurity phase increases the thermal conductivity of the composite samples and leads to a reduction of ZT when the volume fraction of the Co3O4 phase is increased from 1% to 3%. Hence, the best figure of merit is expected close to the upper phase boundary of the [Ca2 wCoO3 δ][CoO2]1.62 phase. Not only the figures of merit of the materials, but also the design of a thermoelectric converter determines the device efficiency. In a converter, a p-type and a suitable n-type thermoelectric material are connected electrically in series and thermally in parallel. Here, [Ca2 wCoO3 δ][CoO2]1.62 is combined with the n-type CaMn0.97W0.03O3-δ and the device efficiency is improved by a variation of the ratio A_p/A_n of the cross section areas of the legs. The good agreement between the experimental values and the predictions of the compatibility model show the high quality of the fabricated devices and the value of the model for the optimization of the converter design. The adjustment of A_p/A_n improves the power output and the efficiency of the converters, where the best volume and area power densities exceed published high temperature values. The achieved efficiency of 1.08 % at a temperature of 1085 K at the hot side is close to the theoretical expected efficiency and can be further improved via ZT.Item Open Access Electronic transport properties of DNA sensing nanopores : insight from quantum mechanical simulations(2017) Sivaraman, Ganesh; Fyta, Maria (Jun.-Prof. Dr.)The translocation of DNA through nanopores is an intensively studied field as it can lead to a new perspective in DNA sequencing. During this process the DNA is electrophoretically driven through a nanoscale hole in a membrane, and use different sensing schemes to read out the sequence. Within the scope of nanopore sequencing two important sensing schemes relevant to this thesis are: 1.) Tunneling sequencers based on solid state nanopores embedded with gold electrodes 2.) 2D materials beyond graphene For scheme 1, an obvious improvement is to coat the gold electrode with molecules that have high conductance and can form instantaneous hydrogen bond bridges with the translocating polynucleotide thereby improving the transverse current signal. The molecule that we propose is the so called diamondoid which are diamond caged molecules with hydrogen termination. Before applying such a molecule to a nanopore electrode set up, one would like to understand their interaction with DNA and its nucleobases. For this purpose, hydrogen bonded complexes formed between nitrogen doped derivatives of smallest diamondoids (i.e. adamantane derivatives) and nucleobases were investigated using dispersion corrected density functional theory (DFT). Mutated and methylated nucleobases are also taken into consideration in these investigations. DFT calculations revealed that hydrogen bonds are of moderate strength. In addition, starting from the DFT predicted hydrogen bonding configuration for each complex, rotations, and translations along a reference axis was performed to capture variations in the interaction energies along the donor-acceptor groups of the hydrogen bonds. The electronic density of states analysis for the hydrogen bonded complexes revealed distinguishable signatures for each nucleobase, thereby showing the suitability for application in electrodes functionalised with such probe molecules. In the next step, an adamantane derivative is placed on one of the electrode and nucleotides are introduced in such a way that nucleobases form hydrogen bonds with the of the nitrogen group of the adamantane derivatives. Electronic transport calculations were performed for gold electrodes functionalised with 3 different adamantane derivatives. Four pristine nucleotides, one mutated, and one methylated nucleotides were considered. Analysis of the transmission spectra reveal that each of the nucleotides has a unique resonance peak far below the Fermi level. We have also proposed a gating voltage window to sample the resonance peaks of the nucleotide so that they can be distinguished from each other. An alternative to tunneling sequencers would be to use nanopores built in to ultra thin metallic nanoribbons such as graphene. The sequence can be read out from the in-plane current modulation resulting from the local field effect of the translocating nucleotides in the vicinity of the metallic pore edges. But the hydrophobicity of graphene makes it a difficult candidate in aqueous environment. Hence in scheme 2, the aim is to model an ultra thin material that can rectify the hydrophobicity of graphene and can be a very good candidate for current modulation sequencing. Ultra thin MoS2 (2H) monolayer exist as direct band gap semiconductor. Nanopores based on 2H phases have been reported in the literature and are not hydrophobic. By means of chemical exfoliation of the 2H phase, a meta stable 1T phase of MoS2 has also been synthesized by various experimental groups. The 1T phase of MoS2 is metallic. The aim of this thesis is to model a nano-biosensor template based on a hybrid MoS2 monolayer made up of a metallic (1T) phase sandwiched between semiconducting (2H) phase. The sensor that we propose, should have only metallic nanopore edges. As a first step, we have modeled the semiconductor-metal interface, and compared them with experiments. Then an investigation to understand the influence of the increase of the metallic unit on the electronic properties is performed. Since, point defects are highly relevant to electrochemical pore growth, a point sulfur defect analysis is provided to ascertain the weakest point in the sheet. Finally to understand the effect of the interface electronic transport calculations are performed. The transmission spectra reveals a clear asymmetry in the current flow across the interface by means of gating. In the end, the relevance of such a hybrid MoS2 material for nanopore sequencing is discussed.