Femtosecond x-ray diffraction reveals a liquid–liquid phase transition in phase-change materials

6 pags., 5 figs.In phase-change memory devices, a material is cycled between glassy and crystalline states. The highly temperature-dependent kinetics of its crystallization process enables application in memory technology, but the transition has not been resolved on an atomic scale. Using femtosecond x-ray diffraction and ab initio computer simulations, we determined the time-dependent pair-correlation function of phase-change materials throughout the melt-quenching and crystallization process. We found a liquid–liquid phase transition in the phase-change materials AgInSbTe and GeSb at 660 and 610 kelvin, respectively. The transition is predominantly caused by the onset of Peierls distortions, the amplitude of which correlates with an increase of the apparent activation energy of diffusivity. This reveals a relationship between atomic structure and kinetics, enabling a systematic optimization of the memory-switching kinetics.F.Q., A.K., M.N., and K.S.T. gratefully acknowledge financial support from the German Research Council through the Collaborative Research Center SFB 1242 project 278162697 (“Non-Equilibrium Dynamics of Condensed Matter in the Time Domain”), project C01 (“Structural Dynamics in Impulsively Excited Nanostructures”), and individual grant So408/9-1, as well as the European Union (7th Framework Programme, grant no. 280555 GO FAST). M.J.S., R.M., and M.W. acknowledge financial support from the German Research Council through the Collaborative Research Center SFB 917 (“Nanoswitches”) and individual grant Ma-5339/2-1. M.J.S., I.R., and R.M. also acknowledge the computational resources granted by JARA-HPC from RWTH Aachen University under project nos. JARA0150 and JARA0183. M.T., A.M.L., and D.A.R. were supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, through the Division of Materials Sciences and Engineering under contract no. DE-AC02-76SF00515. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under contract DE-AC52-07NA27344. J.L. acknowledges support from the Swedish Research Council. J.S. acknowledges financial support from the Spanish Ministry of Science, Innovation and Universities through research grant UDiSON (TEC2017-82464-R). P.Z. gratefully acknowledges funding by the Humboldt Foundatio

Crystallization of Ge2Sb2Te5 thin films by nano- and femtosecond single laser pulse irradiation

The amorphous to crystalline phase transformation of Ge2Sb2Te5 (GST) films by UV nanosecond (ns) and femtosecond (fs) single laser pulse irradiation at the same wavelength is compared. Detailed structural information about the phase transformation is collected by x-ray diffraction and high resolution transmission electron microscopy (TEM). The threshold fluences to induce crystallization are determined for both pulse lengths. A large difference between ns and fs pulse irradiation was found regarding the grain size distribution and morphology of the crystallized films. For fs single pulse irradiated GST thin films, columnar grains with a diameter of 20 to 60 nm were obtained as evidenced by cross-sectional TEM analysis. The local atomic arrangement was investigated by highresolution Cs-corrected scanning TEM. Neither tetrahedral nor off-octahedral positions of Ge-atoms could be observed in the largely defect-free grains. A high optical reflectivity contrast (~25%) between amorphous and completely crystallized GST films was achieved by fs laser irradiation induced at fluences between 13 and 16 mJ/cm2 and by ns laser irradiation induced at fluences between 67 and 130 mJ/cm2. Finally, the fluence dependent increase of the reflectivity is discussed in terms of each photon involved into the crystallization process for ns and fs pulses, respectively

Ab initio molecular dynamics and materials design for embedded phase-change memory

The Ge2Sb2Te5 alloy has served as the core material in phase-change memories with high switching speed and persistent storage capability at room temperature. However widely used, this composition is not suitable for embedded memories—for example, for automotive applications, which require very high working temperatures above 300 °C. Ge–Sb–Te alloys with higher Ge content, most prominently Ge2Sb1Te2 (‘212’), have been studied as suitable alternatives, but their atomic structures and structure–property relationships have remained widely unexplored. Here, we report comprehensive first-principles simulations that give insight into those emerging materials, located on the compositional tie-line between Ge2Sb1Te2 and elemental Ge, allowing for a direct comparison with the established Ge2Sb2Te5 material. Electronic-structure computations and smooth overlap of atomic positions (SOAP) similarity analyses explain the role of excess Ge content in the amorphous phases. Together with energetic analyses, a compositional threshold is identified for the viability of a homogeneous amorphous phase (‘zero bit’), which is required for memory applications. Based on the acquired knowledge at the atomic scale, we provide a materials design strategy for high-performance embedded phase-change memories with balanced speed and stability, as well as potentially good cycling capability

Phase Transformations and Switching of Chalcogenide Phase-change Material Films Prepared by Pulsed Laser Deposition

The thesis deals with the preparation, characterization and, in particular, with the switching properties of phase-change material (PCM) thin films. The films were deposited using the Pulsed Laser Deposition (PLD) technique. Phase transformations in these films were triggered by means of thermal annealing, laser pulses, and electrical pulses. The five major physical aspects structure transformation, crystallization kinetics, topography, optical properties, and electrical properties have been investigated using XRD, TEM, SEM, AFM, DSC, UV-Vis spectroscopy, a custom-made nanosecond UV laser pump-probe system, in situ resistance measurements, and conductive-AFM. The systematic investigation of the ex situ thermally induced crystallization process of pure stoichiometric GeTe films and O-incorporating GeTe films provides detailed information on structure transformation, topography, crystallization kinetics, optical reflectivity and electrical resistivity. The results reveal a significant improvement of the thermal stability in PCM application for data storage. With the aim of reducing the switching energy consumption and to enhance the optical reflectivity contrast by improving the quality of the produced films, the growth of the GeTe films with simultaneous in situ thermal treatment was investigated with respect to optimizing the film growth conditions, e.g. growth temperature, substrate type. For the investigation of the fast phase transformation process, GeTe films were irradiated by ns UV laser pulses, tailoring various parameters such as pulse number, laser fluence, pulse repetition rate, and film thickness. Additionally, the investigation focused on the comparison of crystallization of GST thin films induced by either nano- or femtosecond single laser pulse irradiation, used to attain a high data transfer rate and to improve the understanding of the mechanisms of fast phase transformation. Non-volatile optical multilevel switching in GeTe phase-change films was identified to be feasible and accurately controllable at a timescale of nanoseconds, which is promising for high speed and high storage density of optical memory devices. Moreover, correlating the dynamics of the optical switching process and the structural information demonstrated not only exactly how fast phase change processes take place, but also, importantly, allowed the determination of the rapid kinetics of phase transformation on the microscopic scale. In the next step, a new general concept for the combination of PCRAM and ReRAM was developed. Bipolar electrical switching of PCM memory cells at the nanoscale can be achieved and improvements of the performance in terms of RESET/SET operation voltage, On/Off resistance ratio and cycling endurance are demonstrated. The original underlying mechanism was verified by the Poole-Frenkel conduction model. The polarity-dependent resistance switching processes can be visualized simultaneously by topography and current images. The local microstructure on the nanoscale of such memory cells and the corresponding local chemical composition were correlated. The gained results contribute to meeting the key challenges of the current understanding and of the development of PCMs for data storage applications, covering thin film preparation, thermal stability, signal-to-noise ratio, switching energy, data transfer rate, storage density, and scalability

Computational Perspective on Intricacies of Interactions, Enzyme Dynamics and Solvent Effects in the Catalytic Action of Cyclophilin A

Cyclophilin A (CypA) is the well-studied member of a group of ubiquitous and evolutionarily conserved families of enzymes called peptidyl–prolyl isomerases (PPIases). These enzymes catalyze the cis-trans isomerization of peptidyl-prolyl bond in many proteins. The distinctive functional path triggered by each isomeric state of peptidyl-prolyl bond renders PPIase-catalyzed isomerization a molecular switching mechanism to be used on physiological demand. PPIase activity has been implicated in protein folding, signal transduction, and ion channel gating as well as pathological condition such as cancer, Alzheimer’s, and microbial infections. The more than five order of magnitude speed-up in the rate of peptidyl–prolyl cis–trans isomerization by CypA has been the target of intense research. Normal and accelerated molecular dynamic simulations were carried out to understand the catalytic mechanism of CypA in atomistic details. The results reaffirm transition state stabilization as the main factor in the astonishing enhancement in isomerization rate by enzyme. The ensuing intramolecular polarization, as a result of the loss of pseudo double bond character of the peptide bond at the transition state, was shown to contribute only about −1.0 kcal/mol to stabilizing the transition state. This relatively small contribution demonstrates that routinely used fixed charge classical force fields can reasonably describe these types of biological systems. The computational studies also revealed that the undemanding exchange of the free substrate between β- and α-helical regions is lost in the active site of the enzyme, where it is mainly in the β-region. The resultant relative change in conformational entropy favorably contributes to the free energy of stabilizing the transition state by CypA. The isomerization kinetics is strongly coupled to the enzyme motions while the chemical step and enzyme–substrate dynamics are in turn buckled to solvent fluctuations. The chemical step in the active site of the enzyme is therefore not separated from the fluctuations in the solvent. Of special interest is the nature of catalysis in a more realistic crowded environment, for example, the cell. Enzyme motions in such complicated medium are subjected to different viscosities and hydrodynamic properties, which could have implications for allosteric regulation and function

Ultrafast transmission electron microscopy of a structural phase transition

Große Hoffnungen für zukünftige Anwendungen im Gebiet der Energieumwandlung werden auf Materialien mit abstimmbaren Eigenschaften und Anregungen gesetzt. Die Funktionalität miniaturisierter Systeme ergibt sich jedoch nicht nur aus den Eigenschaften der einzelnen Materialien, sondern auch aus deren Zusammenspiel und nanoskaliger Strukturierung. Während eine Reihe etablierter experimenteller Techniken in der Lage ist, elektronische Anregungen auf Femtosekunden-Zeit- und Nanometer-Längenskalen zu verfolgen, wurde bisher über keine zeitaufgelöste Nano-Abbildung eines strukturellen Ordnungsparameters berichtet. Die vorliegende kumulative Dissertation behandelt die Entwicklung zeitaufgelöster Dunkelfeld-Bildgebung am Ultraschnellen Transmissions-Elektronenmikroskop (UTEM) in Göttingen. Dieser Ansatz kombiniert Femtosekunden-Zeitauflösung und eine räumliche Auflösung von 5 nm mit einer Empfindlichkeit für die strukturelle Komponente eines Ladungsdichtewellen-Phasenübergangs im 1T-Polytyp des Materials Tantaldisulfid. Ultrakurze Laserpulse induzieren lokal den Phasenübergang, während die raumzeitliche Relaxationsdynamik des strukturellen Ordnungsparameters mit ultrakurzen Elektronenpulsen verfolgt wird. Die Empfindlichkeit für den Ordnungsparameter wird mithilfe einer komplexen Dunkelfeld-Apertur erreicht. In einer ersten Veröffentlichung wird die Technik zur Präparation der dünnen Schichten aus Tantaldisulfid vorgestellt. Die durch Ultramikrotomie gewonnenen Proben sind ideal für Elektronen- und Röntgenexperimente in einer Transmissionsgeometrie, wie die exemplarische Untersuchung von mit Mangan und Eisen interkaliertem Tantaldisulfid zeigt. Statische optische Mikroskopie, Elektronenbeugung und Messungen des zirkularen magnetischen Röntgendichroismus dienen dazu, diese ferromagnetischen Dünnschichten zu charakterisieren und zu bestätigen, dass ihre Eigenschaften denen der ursprünglichen Kristalle entsprechen. Ein zweiter Artikel beschreibt die Umsetzung der zeitaufgelösten Nano-Abbildung. Ein zentraler Aspekt des Experiments ist die Herstellung einer Probe, die das optische Anregungsprofil räumlich strukturiert und gleichzeitig eine stroboskopische Untersuchung des Phasenübergangs in Tantaldisulfid bei Wiederholraten von hunderten Kilohertz ermöglicht. Basierend auf Parametern, die in einem stationären Heizexperiment gewonnen wurden, kann das Verhalten von nanoskaligen Ladungsdichtewellen-Domänen in der freistehenden Dünnschicht in zeitabhängigen Ginzburg-Landau-Simulationen reproduziert werden. Abschließend werden Perspektiven für zukünftige Experimente auf Basis des vorgestellten Ansatzes diskutiert. Ultraschnelle Dunkelfeld-Bildgebung ermöglicht eine Empfindlichkeit auch für weitere strukturelle Freiheitsgrade in komplexen Materialien und wird so zu einem besseren Verständnis aktiv kontrollierter Prozesse auf dem Gebiet der Energieumwandlung beitragen.High hopes are placed on materials with tunable properties and excitations for future applications in energy conversion devices. Functionality of devices, however, not only arises from the properties of individual materials but also from their interplay and nanoscale structuring. While a number of established experimental techniques are capable of tracking electronic excitations on femtosecond time and nanometer length scales, no time-resolved nanoimaging of a structural order parameter had previously been reported. Addressing this challenge, the present cumulative thesis reports on the development and application of a time-resolved dark-field electron microscopy scheme implemented at the Göttingen Ultrafast Transmission Electron Microscope (UTEM). This nanoimaging approach combines femtosecond temporal and 5 nm spatial resolution with sensitivity to the structural component of a charge-density wave phase transition in 1T-polytype tantalum disulfide. Ultrashort laser pulses locally induce the phase transition, while the subsequent spatiotemporal relaxation dynamics of the structural order parameter is tracked using ultrashort electron pulses. Order parameter sensitivity is obtained by means of a dark-field aperture array, tailored to filter the periodicities of the charge-density wave in the diffraction plane of the microscope. In the first publication contributing to this thesis, the preparation technique for the thin films of tantalum disulfide is introduced. Specimens obtained by ultramicrotomy are ideal for electron and x-ray experiments in a transmission geometry, as exemplified by the investigation of manganese- and iron-intercalated tantalum disulfide. Static optical microscopy, electron diffraction and x-ray magnetic circular dichroism measurements serve to characterize these ferromagnetic thin films and to verify that the properties reflect those of the bulk crystals. The second article describes the implementation of the ultrafast nanoimaging approach. A central aspect of the experiment is the design of a specimen that spatially structures the optical excitation pattern and allows for stroboscopic probing of the phase transition in tantalum disulfide at hundreds of kilohertz repetition rates. Based on parameters extracted from a steady-state heating experiment, the optically induced evolution of nanoscale charge-density wave domains in the free-standing thin film is reproduced in time-dependent Ginzburg-Landau simulations. Finally, perspectives for future nanoimaging experiments are discussed. Allowing for sensitivity to further structural degrees of freedom in complex materials, ultrafast dark-field imaging will contribute to a better understanding of actively controlled processes in energy conversion devices.2021-08-1

Diffusion in modified solid-state ionic conductors for energy applications: structure and dynamics

The poor ionic conductivity of candidate solid-state electrolytes in comparison to their liquid counterparts is a critical challenge in the implementation of all solid-state batteries. A strategy that has been widely used to address this issue is the modification of solid-state ionic conductors via chemical doping. However, the complex resultant structures and the presence of secondary phases have meant that understanding how the improved ionic conductivity is achieved remains a challenge. The work in this thesis presents a strategy to characterise the structure and develop an atomic level picture of the ionic diffusion processes in modified solid-state ionic conductors using a combination of powder diffraction and quasi-elastic neutron scattering (QENS). In doing so, this work aims to demonstrate a way to better understand the interplay between the structure of materials and ionic transport and thus facilitate the rational design and optimisation of solid-state ionic conductors for energy applications. The first part of this thesis focuses on NASICON (Na3Zr2Si2PO12), which is among the best performing known solid-state sodium ion conductors. Using neutron powder diffraction (NPD), we present a targeted study characterising the sodium sites to elucidate how fast ionic conduction is achieved in this material. A comparative NPD and preliminary QENS study on Mg-doped NASICON is also presented with the aim of identifying the primary Mg2+ doping site and how this affects sodium diffusion. The second part of this work presents a detailed characterisation of the structure and dynamics of γ-Na3PO4 which is present as a secondary phase in Mg-doped NASICON. As a rotor phase material, γ-Na3PO4 exhibits rotational diffusion of the phosphate units which influences sodium long-range diffusion. Hence, in the development of a model to describe sodium long-range diffusion, we present a strategy which may be extended to characterise ionic diffusion in similar systems where it is necessary to separate contributions from coupled diffusion processes

Structural and Optical Characterization of Solution Processed Lead Iodide Ruddlesden-Popper Perovskite Thin Films

Highly efficient LEDs and photovoltaic cells based on spin coated films of layered Ruddlesden-Popper hybrid perovskites (RPP) have been recently reported. The electronic structure and phase composition of these films remains an open question, with diverse explanations offered accounting for the excellent device performance. Here we report x-ray and optical characterization of hot cast RPP thin films, emphasizing the distribution of structural and electronic properties through the film depth. Our results indicate an at least 70% phase pure n=3 film results from casting a stoichiometric solution of precursors, with minor contributions from n=2 and n=4 phases. We observe a strong correspondence between the predicted single-crystal RPP reciprocal lattice and measured RPP film wide angle scattering pattern, indicating a highly ordered [101] oriented film. This correspondence is broken at the air-film interface where new scattering peaks indicate the existence of a long wavelength structural distortion localized near the films surface. Using transient absorption spectroscopy, we show that the previously detected luminescent mid-gap states are localized on the films surface. Investigating films of varying thickness, we determine the photo-excited carrier dynamics are dominated by diffusion to this interface state, and extract an excitonic diffusivity of 0.18cm2s-1. We suggest that the observed surface distortion is responsible for the creation of luminescent mid-gap states

Synthesis and photophysical properties of neutral and cationic octahedral tungsten iodide clusters

In the course of this thesis, the first crystal structures of cationic and neutral tungsten iodide clusters were solved and published. In the first step the compound [(W6I8)I3(CH3CN)3]I7·I2 featuring the heteroleptic cluster cation [(W6I8)I3(CH3CN)3]+ was synthesized as bulk material. Follow-up experiments starting from [(W6I8)I3(CH3CN)3]I7·I2 yielded the two species [(W6I8)I(CH3CN)5](I3)2(BF4) and [(W6I8)(CH3CN)6](I3)(BF4)3·(H2O). The structures of these two compounds feature the cluster cations [(W6I8)I(CH3CN)5]3+ and [(W6I8)(CH3CN)6]4+. An-ions in the three mentioned compounds are either I3- or I7-, which cause a dark crystal or pow-der color and no obvious photoluminescence. Further conducted reactions gave [W6I8(CH3CN)6](BF4)4·(CH3CN)2 as a yellow powder or plated-shaped crystals. The crystalline compound shows the typical octahedral metal halide cluster photoluminescence with characteristic broad excitation and emission bands. However, in contrast to species bearing electron-withdrawing ligands, emission lifetimes are short, and no quenching in the presence of molecular oxygen is observed. The calculated Electron Local-ization Function (ELF) revealed significantly higher d-electron density above the tungsten atoms and reduced ionicity of the W–N bond for [W6I8(CH3CN)6](BF4)4·(CH3CN)2 compared to (TBA)2[W6I8(CO2C3F7)6], which is a strong quencher. This leads to the conclusion that the ionicity of the W–Ligand bond influences the energetic splitting of the emitting triplet sublevels and therefore also emission lifetimes. Besides the mentioned cationic clusters, the neutral tungsten iodide cluster [W6I12(NCC6H5)2] was also synthesized and characterized for its properties. Due to two benzonitrile and four iodide as apical ligands, the compound is a heteroleptic cluster species. It shows a good hy-drolysis stability and remarkable temperature stability up to 400 °C. Photoluminescence in the solid state is only weakly pronounced with low emission intensity, short lifetime and negligi-ble quantum yield. However, the good hydrolysis and temperature stability as well as the opti-cal band gap of 2.17 eV make the compound a perfect candidate for application in photoca-talysis. This was confirmed in an experiment observing the photocatalytic decomposition of Rhodamine B (Rh B) in aqueous solution. Additional experiments yielded the compounds [W6I8(CH3CN)6](ClO4)4·(CH3CN)2 and [W6I8(DMSO)6](I3)4 featuring cationic clusters in their structures. Further reactions with dif-ferent solvents suggest a high potential to produce many other cationic and neutral tungsten iodide clusters. This would allow for new insights into the photophysical properties of this type of cluster as well as the possibility to discover other highly stable compounds. In addition, with the compounds (nBu4N)2[M6I8(NCO)6] (M = Mo, W), new metal iodide clus-ters are reported. In their structure, NCO- anions are connected to the [M6I8] core as apical ligands via the nitrogen atoms. In solid state, they show the typical cluster photoluminescence with weakly pronounced oxygen quenching. For comparison of photoluminescence properties with the analogue iodides, the compounds (PPN)2[W6Cl14]·(CH2Cl2)2 and (PPh4)2[W6Cl14]·(C3H6O)2 were synthesized. In solid state, the corresponding octahedral tungsten iodide clusters show high quantum yields. In contrast to this, photoluminescence of the synthesized tungsten chloride species is only weakly pro-nounced with short lifetimes and unquantifiable quantum yields. Another contribution of this thesis concerns the synthesis of compounds combining tungsten clusters with metal ions onto which an energy transfer is possible. Reactions with RE-chlorides (RE = Gd, Tb, Eu) produced GdW6Cl15, TbW6Cl15 and EuW6Cl14 as products. Magnetic measurements revealed paramagnetism in correspondence to the magnetic moment of the RE-ion. Photoluminescence spectra recorded of crystalline samples show no emission of the RE-ions. Cooperation with Dr. T. Maulbetsch lead to the synthesis of the compound [Fe(CTP)]2[W6I14]. Photoluminescence experiments at room temperature showed no emission. Experiments con-ducted between 3 K and 100 K in order to clarify if an energy transfer occurs yielded the same results. Further experiments towards the synthesis of supramolecular [W6I14]-metal complex com-pounds lead to crystals of [Fe(CH3CN)6][W6I14]. They show no sign of luminescence but re-veal potential for the synthesis of other compounds based on a metal complex and the [W6I14]2- anion