Universität Stuttgart
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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 Strukturelle und spektroskopische Eigenschaften epitaktischer FeMn/Co Exchange-Bias-Systeme(2015) Schmidt, Mathias; Goering, Eberhard (PD Dr.)Das Thema dieser Arbeit bestand in der Präparation und Charakterisierung des Exchange-Bias Systems FeMn/Co. Hierbei wurden mittels Molekularstrahlepitaxie zwei sich durch ihre kristalline Orientierung unterscheidende FeMn/Co-Probensysteme auf (100)-orientiertem Magnesiumoxid hergestellt. Zur Erzeugung einer flachen Schichtstruktur mit einer ausgeprägten kristallinen Ordnung war der Einsatz zweier Pufferschichten (Pt,Cu) notwendig. Bei Substrattemperaturen oberhalb von 900 K eine (100)-orientierte Schicht erschaffen, bei niedrigeren Temperaturen um 670 K entstand eine (111)-Orientierung. Untersuchungen der Kristallstruktur ergaben ein epitaktisches Wachstum des (100)-orientierten Probensystems (HTPt) mit großen Kristallitstrukturen, während für das (111)-orientierte System (NTPt) eine vierfach entartete Untergitterstruktur mit kleineren Kristalliten entstand. Es entstand ein wohldefinierter und reproduzierbarer Herstellungsprozess, bei dem sämtliche Schichtparameter gezielt verändert werden konnten. Anschließend wurden magnetometrische Untersuchungen der Probensysteme durchgeführt. Mittels SQUID-Magnetometrie wurde die Temperaturabhängigkeit dieser beiden Parameter untersucht. Es stellte sich eine stärkere Temperaturabhängigkeit des NTPt-Probensystems heraus, die der kleineren Kristallitgröße und der höheren Aktivierung von Pinnningzentren in den Korngrenzen zugeschrieben werden konnte. Dann wurde die magnetische Anisotropie der Probensysteme untersucht, dies geschah mittels eines MOKE-Systems. Es zeigte sich eine stärkere magnetokristalline Anisotropie des HTPt-Systems verglichen mit dem NTPt-System, die mit der ausgeprägteren kristallinen Ordnung in den magnetischen Schichten korreliert. Für eine ausführlichere Charakterisierung wurde auf die FORC (First Order Reversal Curves)-Methode zurückgegriffen. Dieses Verfahren erbrachte den Nachweis der asymmetrischen Magnetisierungsumkehr nicht nur parallel und antiparallel zur Feldkühlrichtung sondern auch für identische Projektionen auf die Feldkühlrichtung. Dieses Verhalten lässt auf eine nicht parallele Anordnung der leichten Richtungen von Ferromagnet und Antiferromagnet schließen. Neben der Asymmetrie der Magnetisierungsumkehr konnte auch das unterschiedliche Ummagnetisierungsverhalten beider Probensysteme analysiert werden. Die Auftrennung der irreversiblen und reversiblen Magnetisierungsbeiträge mittels FORC erbrachte für das HTPt-System irreversible Anteile über den gesamten Winkelbereich, während für das NTPt-System über nahezu den gesamten Winkelbereich reversible und somit rotationsbasierte Mechanismen identifiziert werden konnten. Zuletzt wurden die Probensysteme Röntgenabsorptionsmessungen unter Ausnutzung des Röntgenzirkulardichroismus (XMCD) unterzogen. Einerseits wurde ein Vergleich beider Probensysteme erstellt, andererseits auch Veränderungen der magnetischen Eigenschaften durch gezielte Manipulationen der antiferromagnetischen Struktur untersucht. Diese bestanden in Veränderungen der Dicke der Cu-Pufferschicht sowie in der Änderungen der Zusammensetzung des Antiferromagneten. Die Ergebnisse der Absorptionsmessungen wurden mit Hilfe der Summenregeln analysiert, um die Beiträge von magnetischem Spin- und Bahnmoment zu separieren und quantitativ zu untersuchen. Sowohl Eisen als auch Mangan zeigen ein XMCD-Differenzsignal von unkompensierten rotierbaren magnetischen Momenten. Die Magnetisierung findet sich nahe der Grenzfläche, in den tiefensensitiveren TFY-Messungen konnten keine unkompensierten Momente nachgewiesen werden. Verglichen mit den für Volumenproben reiner Elemente erhaltenen Ergebnissen wurde eine Zunahme des Bahndrehmomentes an beiden Kanten festgestellt. Die Menge an unkompensierten rotierbaren Spinmomenten nimmt bei Reduktion der magnetokristallinen Anisotropie des Antiferromagneten zu, dies ist besonders an der Eisenkante der Fall, auch wenn der Effekt ebenfalls an der Mangankante feststellbar ist. Durch eine auf der Intensität des Absorptionssignals basierende Abschätzung wurde die effektive Dicke der rotierbaren Schicht ermittelt, sie beträgt je nach Probensystem bis zu drei Monolagen für vorliegenden Exchange-Bias. Zuletzt wurde aus den gemessenen Spektren das sogenannte „Branching Ratio“ ermittelt, das Rückschlüsse auf den Erwartungswert der Spin-Bahn-Kopplung in den untersuchten Probensystemen zulässt. Hierbei ergibt sich für Mangan generell ein höherer Wert der Spin-Bahn-Kopplung verglichen mit den Messungen an der Eisenkante. Die Summe der beobachteten Effekte legt zur Erklärung der nahe der Grenzfläche im Antiferromagneten stattfindenden Abläufe ein Wechselspiel der globalen magnetokristallinen Anisotropie der Probensysteme mit einer lokal erhöhten Anisotropie an Stellen mit gestörter Kristallsymmetrie wie Fehlstellen oder Korngrenzen nahe. Letztere führt zum Ausbildung von gepinnten magnetischen Momenten, die durch den Feldkühlprozess eine unidirektionale Ausrichtung erhalten und den Exchange-Bias verursachen.Item Open Access Phase behavior of colloidal monolayers on one-dimensional periodic and quasiperiodic light fields(2015) Zaidouny, Lamiss; Bechinger, Clemens (Prof. Dr.)We experimentally investigate under real space conditions, crystalline and quasicrystalline colloidal assemblies in the presence of one dimensional light field potentials. By using scanning optical tweezers, such potential landscapes can – in contrast to interference patterns – be controlled in situ without realignment of optical components. In this thesis we have reported on the creation of colloidal crystalline phases in one-dimensional periodic potential landscapes. We use a suspension of silica particles and bromobenzene in order to realize a wide range of line spacings relative to the lattice constant. Such a suspension is characterized by low electrical polarity and therefore the Debye screening length of the system is about 4.6 µm. This large distance as compared to aqueous suspensions, results in the formation of crystals with lattice constants up to 20 µm. In the presence of the laser pattern, a rich variety of crystal-crystal and crystal-disordered structures are observed. The observed phases are stabilized by the competition of optical and electrostatic forces and thus strongly depend on both the periodicity and the laser intensity of the light field. In the second part of the thesis our experiments demonstrate that a smooth transition between one-dimensional crystals to quasicrystals is possible by the formation of a periodic average structure (PAS) on 1D quasiperiodic substrate potential. This intermediate phase is robust with respect to parameter variations. Contrary to atomic quasicrystals, the formation of PAS does not require complex or even anisotropic pair potentials. Obviously, PAS can be generally expected in situations where periodic and quasiperiodic ordering principles compete against each other. PAS of higher-dimensional quasicrystalline structures, can be experimentally achieved e.g. in decagoconal colloidal monolayers formed in the presence of five interfering beams. In contrast to the 1D case, two-dimensional systems have no simple 1-1 mapping between periodic and quasiperiodic structures, hence experimental colloidal studies will provide direct information how crystal to quasicrystal transitions are achieved under such conditions. Finally, it should be mentioned, that in addition to the phases observed here, other structures can form. For example, at higher particle concentrations and smaller Debye screening lengths, phases with interstitial particle strings between the quasiperiodic or periodic lines should be observed. In addition, inverse tweezing with quasiperiodic light potentials is also applicable for such colloidal systems.Item Open Access Non-equilibrium work distribution for interacting colloidal particles under friction(2015) Gomez-Solano, Juan Ruben; July, Christoph; Mehl, Jakob; Bechinger, ClemensWe experimentally investigate the non-equilibrium steady-state distribution of the work done by an external force on a mesoscopic system with many coupled degrees of freedom: a colloidal crystal mechanically driven across a commensurate periodic light field. Since this system mimics the spatiotemporal dynamics of a crystalline surface moving on a corrugated substrate, our results show general properties of the work distribution for atomically flat surfaces undergoing friction. We address the role of several parameters which can influence the shape of the work distribution, e.g. the number of particles used to locally probe the properties of the system and the time interval to measure the work. We find that, when tuning the control parameters to induce particle depinning from the substrate, there is an abrupt change of the shape of the work distribution. While in the completely static and sliding friction regimes the work distribution is Gaussian, non-Gaussian tails show up due to the spatiotemporal heterogeneity of the particle dynamics during the transition between these two regimes.Item Open Access Transient dynamics of a colloidal particle driven through a viscoelastic fluid(2015) Gomez-Solano, Juan Ruben; Bechinger, ClemensWe study the transient motion of a colloidal particle actively dragged by an optical trap through different viscoelastic fluids (wormlike micelles, polymer solutions, and entangled λ-phage DNA). We observe that, after sudden removal of the moving trap, the particle recoils due to the recovery of the deformed fluid microstructure. We find that the transient dynamics of the particle proceeds via a double-exponential relaxation, whose relaxation times remain independent of the initial particle velocity whereas their amplitudes strongly depend on it. While the fastest relaxation mirrors the viscous damping of the particle by the solvent, the slow relaxation results from the recovery of the strained viscoelastic matrix. We show that this transient information, which has no counterpart in Newtonian fluids, can be exploited to investigate linear and nonlinear rheological properties of the embedding fluid, thus providing a novel method to perform transient rheology at the micron-scale.