Fachgebiet Materialmodellierung

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Das Fachgebiet beschäftigt sich mit multi-skaliger Modellierung von Materialstruktur und Materialeigenschaften unter Zuhilfenahme und Weiterentwicklung verschiedener computergestützter Berechnungsmethoden.

Forschungsschwerpunkte beinhalten funktionale Nanoteilchen, Ferroelektrika, transparente and organische Halbleiter sowie nanostrukturierte Metalle und Hochdruckphasen.

Im Bereich der Lehre bieten wir Vorlesungen zur Materialwissenschaft an, wobei der Schwerpunkt im Bereich theoretischer Modelle und computergestützter Beschreibung von Materialien liegt.

Unser Fachgebiet bietet darüber hinaus die Gelegenheit zu Bachelor-, Master- und Promotionsarbeiten an.

Aktuelle Informationen

Polarization-switching dynamics in bulk ferroelectrics with isometric and oriented anisometric pores

Authors: R. Khachaturyan, S Zhukov, J Schultheiß, C Galassi, C Reimuth, J Koruza, H von Seggern and Y A Genenko

Highly porous ferroelectric ceramics possess remarkably less polarizability than dense ceramics; instead they display high tunability of various physical properties. Particularly, the shape and orientation of pores as well as the total porosity exhibit a great effect on the polarization-switching dynamics. In the present work, finite-element simulations of the electric-field distributions and related statistical distributions of local switching times are analysed and compared with the switching characteristics of porous lead zirconate titanate ceramics, extracted from the experiment by means of the inhomogeneous field mechanism model of polarization switching. Surprisingly, the simulated statistical field-distributions turn out to be virtually independent of the pore-size distribution; however, they are sensitive to the anisometric shape and orientation of the pores. Additionally, they exhibit notable broadening with increasing porosity; an effect confirmed by experimental observations.


LaSrMnO4: Reversible Electrochemical Intercalation of Fluoride Ions in the Context of Fluoride Ion Batteries

Authors: Mohammad Ali Nowroozi, Kerstin Wissel†, Jochen Rohrer, Anji Reddy Munnangi, and Oliver Clemens

This article reports on the investigation of LaSrMnO4 with K2NiF4 type structure for use as an intercalation based high voltage cathode material with high capacity for fluoride ion batteries (FIBs). Charging was performed against PbF2 based anodes and shows that fluoride intercalation proceeds stepwise to form LaSrMnO4F and LaSrMnO4F2–x. Ex-situ X-ray diffraction experiments were recorded for different cutoff voltages for a deeper understanding of the charging process, highlighting additional potential of the method to be used to adjust fluorine contents more easily than using conventional fluorination methods. A discharging capacity of approximately 20–25 mAh/g was found, which is ∼4–5 times higher compared to what was reported previously on the discharging of BaFeO2.5/BaFeO2.5F0.5, approaching discharge capacities for conversion based fluoride ion batteries. Density functional theory based calculations confirm the observed potential steps of approximately 1 and 2 V for the first (LaSrMnO4 → LaSrMnO4F) and second (LaSrMnO4F → LaSrMnO4F2–x) intercalation steps against Pb-PbF2, respectively. Additionally, a detailed structure analysis was performed for chemically prepared LaSrMnO4F2–x (x ∼ 0.2), showing strong similarity to the product which is obtained after charging the batteries to voltages above 2 V against Pb-PbF2. It was observed that charging and discharging kinetics as well as coulomb efficiencies are limited for the batteries in the current state, which can be partly assigned to overpotentials arising from the use of conversion based anode composites and the stability of the charged sample toward carbon black and the current collectors. Therefore, the structural stability of LaSrMnO4F2 on the deintercalation of fluoride ions was demonstrated by a galvanostatic discharging to −3 V against Pb-PbF2, which can be used to compensate such overpotentials, resulting in almost complete recovery of fluorine free LaSrMnO4 with a discharge capacity of ∼100 mAh/g. This is the first report showing that selective extraction of fluoride ions from an oxyfluoride matrix is possible, and it highlights that compounds with K2NiF4 type structure can be considered as interesting host lattices for the reversible intercalation/deintercalation of fluoride ions within intercalation based FIBs.

DOI: 10.1021/acs.chemmater.6b05075

Influence of phase transitions and defect associates on the oxygen migration in the ion conductor Na1/2Bi1/2TiO3

Authors: Kai-Christian Meyer and Karsten Albe

Doped or non-stoichiometric Na1/2Bi1/2TiO3 (NBT) exhibits an ion conductivity comparable to yttria stabilized zirconia (YSZ) [M. Li et al., Nat. Mater., 2014, 13, 31–35], with a temperature dependent activation energy. To understand the origin of this temperature dependence we calculated oxygen vacancy migration barriers for three different phases of NBT by means of nudged elastic band calculations within a density functional theory (DFT) approach. We find that for structures with rock-salt ordered A-cations (111-order), the room temperature rhombohedral phase, the intermediate orthorhombic phase and the high temperature tetragonal phase show different migration barriers, decreasing from the rhombohedral to the tetragonal phase. The change in migration barriers from the rhombohedral to tetragonal phase is, however, not large enough to explain the experimentally observed difference. At lower temperatures, the association of oxygen vacancies with either Mg dopants or Bi vacancies increases the activation energy for the migration of oxygen vacancies. Thus, a combination of phase dependent migration barriers and defect association can explain the temperature dependent change in activation energy. Further, when a layered A-cation order (001-order) is present, the oxygen vacancies prefer to be located within the Bi-layer and a fast diffusion along the Bi-layer can occur. Large migration barriers are due to electronic defect states of the migrating oxygen ion.

DOI: 10.1039/c6ta10566a

State transition and electrocaloric effect of BaZrxTi1−xO3: Simulation and experiment

Authors: Yang-Bin Ma, Christian Molin, Vladimir V. Shvartsman, Sylvia Gebhardt, Doru C. Lupascu, Karsten Albe, and Bai-Xiang Xu

We present a systematic study on the relation of the electrocaloric effect (ECE) and the relaxor state transition of BaZrxTi1−xO3 (BZT) using a combination of computer simulation and experiment. The results of canonical and microcanonical lattice-based Monte Carlo simulations with a Ginzburg-Landau-type Hamiltonian are compared with measurements of BaZrxTi1−xO3 (x = 0.12 and 0.2) samples. In particular, we study the ECE at various temperatures, domain patterns by piezoresponse force microscopy at room temperature, and the P-E loops at various temperatures. We find three distinct regimes depending on the Zr-concentration. In the compositional range 0≤x≤0.2, ferroelectric domains are visible, but the ECE peak drops considerably with increasing Zr-concentration. In the range 0.3≤x≤0.7, relaxor features become prominent, and the decrease in the ECE with Zr-concentration is moderate. In the range of high concentrations, x≥0.8, the material is almost nonpolar, and there is no ECE peak visible. Our results reveal that BZT with a Zr-concentration around x=0.12∼0.3 exhibits a relatively large ECE in a wide temperature range at rather low temperature.

DOI: 10.1063/1.4973574

Polarization-Mediated Modulation of Electronic and Transport Properties of Hybrid MoS2–BaTiO3–SrRuO3 Tunnel Junctions

Authors: Tao Li, Pankaj Sharma, Alexey Lipatov, Hyungwoo Lee, Jung-Woo Lee, Mikhail Y. Zhuravlev, Tula R. Paudel, Yuri A. Genenko, Chang-Beom Eom, Evgeny Y. Tsymbal, Alexander Sinitskii, and Alexei Gruverman

Hybrid structures composed of ferroelectric thin films and functional two-dimensional (2D) materials may exhibit unique characteristics and reveal new phenomena due to the cross-interface coupling between their intrinsic properties. In this report, we demonstrate a symbiotic interplay between spontaneous polarization of the ultrathin BaTiO3 ferroelectric film and conductivity of the adjacent molybdenum disulfide (MoS2) layer, a 2D narrow-bandgap semiconductor. Polarization-induced modulation of the electronic properties of MoS2 results in a giant tunneling electroresistance effect in the hybrid MoS2–BaTiO3–SrRuO3 ferroelectric tunnel junctions (FTJs) with an OFF-to-ON resistance ratio as high as 104, a 50-fold increase in comparison with the same type of FTJs with metal electrodes. The effect stems from the reversible accumulation-depletion of the majority carriers in the MoS2 electrode in response to ferroelectric switching, which alters the barrier at the MoS2–BaTiO3 interface. Continuous tunability of resistive states realized via stable sequential domain structures in BaTiO3 adds memristive functionality to the hybrid FTJs. The use of narrow band 2D semiconductors in conjunction with ferroelectric films provides a novel pathway for development of the electronic devices with enhanced performance.

DOI: 10.1021/acs.nanolett.6b04247

Flexocoupling impact on the kinetics of polarization reversal

Authors: Ivan S. Vorotiahin, Anna N. Morozovska, Eugene A. Eliseev, and Yuri A. Genenko

The impact of flexoelectric coupling on polarization reversal and space-charge variation in thin films of ferroelectric semiconductors has been studied theoretically. The relaxation-type Landau-Khalatnikov equation together with the Poisson equation and the theory of elasticity equations have been used to calculate in a self-consistent way the spatial-temporal development of ferroelectric polarization, electric potential, space charge, elastic stresses and strains. The analysis of the obtained results reveals a moderate increase in the flexocoupling influence on the polarization, elastic strain, electric potential, and space-charge development with a decrease in the ferroelectric film thickness. In contrast, the dependence of polarization switching time on the applied electric field is remarkably affected by the flexocoupling strength. The polarization reversal process consists typically of two stages; the first stage has no characteristic time, whereas the second one exhibits a switching time strongly dependent on the applied electric field.

DOI: 10.1103/PhysRevB.95.014104

Departments Award for the best Master Thesis

Our colleague, Ms Leonie Koch, completed her master thesis with distinction and received the Departments Award for the best master thesis with the topic „Computer simulations of ordering effects and dislocation structures in high entropy alloys“.

PhD-graduation in the MM Group

We congratulate our dear colleague Mr. Tobias Brink to his doctoral work on the „Heterogeneities in metallic glasses“.

We thank Mr. Tobias Brink not only for this memorable day with a nice after-defense ceremony but also for all the work he has done within the Materials Modelling group.


Structural origins of the boson peak in metals: From high-entropy alloys to metallic glasses

Authors: Tobias Brink, Leonie Koch and Karsten Albe

The boson peak appears in all amorphous solids and is an excess of vibrational states at low frequencies compared to the phonon spectrum of the corresponding crystal. Until recently, the consensus was that it originated from “defects” in the glass. The nature of these defects is still under discussion, but the picture of regions with locally disturbed short-range order and/or decreased elastic constants has gained some traction. Recently, a different theory was proposed: The boson peak was attributed to the first van Hove singularity of crystal lattices which is only smeared out by the disorder. This new viewpoint assumes that the van Hove singularity is simply shifted by the decreased density of the amorphous state and is therefore not a glass-specific anomaly. In order to resolve this issue, we use computer models of a four-component alloy, alternatively with chemical disorder (high-entropy alloy), structural disorder, and reduced density. Comparison to a reference glass of the same composition reveals that the boson peak consists of additional vibrational modes which can be induced solely by structural disorder. While chemical disorder introduces fluctuations of the elastic constants, we find that those do not lead to sufficient local softening to induce these modes. A boson peak due to a reduction of density could be excluded for the present metallic system.


Anisotropy of self-diffusion in forsterite grain boundaries derived from molecular dynamics simulations

Authors: Johannes Wagner, Omar Adjaoud, Katharina Marquardt and Sandro Jahn

Diffusion rates and associated deformation behaviour in olivine have been subjected to many studies, due to the major abundance of this mineral group in the Earth’s upper mantle. However, grain boundary (GB) transport studies yield controversial results. The relation between transport rate, energy, and geometry of individual GBs is the key to understand transport in aggregates with lattice preferred orientation that favours the presence and/or alignment of specific GBs over random ones in an undeformed rock. In this contribution, we perform classical molecular dynamics simulations of a series of symmetric and one asymmetric tilt GBs of Mg2SiO4 forsterite, ranging from 9.58° to 90° in misorientation and varying surface termination. Our emphasis lies on unravelling structural characteristics of high- and low-angle grain boundaries and how the atomic structure influences grain boundary excess volume and self-diffusion processes. To obtain diffusion rates for different GB geometries, we equilibrate the respective systems at ambient pressure and temperatures from 1900 to 2200 K and trace their evolution for run durations of at least 1000 ps. We then calculate the mean square displacement of the different atomic species within the GB interface to estimate self-diffusion coefficients in the individual systems. Grain boundary diffusion coefficients for Mg, Si and O range from 10−18 to 10−21m3/s, falling in line with extrapolations from lower temperature experimental data. Our data indicate that higher GB excess volumes enable faster diffusion within the GB. Finally, we discuss two types of transport mechanisms that may be distinguished in low- and high-angle GBs.


3D Dislocation Structure Evolution in Strontium Titanate: Spherical Indentation Experiments and MD Simulations

Authors: Farhan Javaid, Alexander Stukowski and Karsten Durst

In the present work, the dislocation structure evolution around and underneath the spherical indentations in (001) oriented single crystalline strontium titanate (STO) was revealed by using an etch-pit technique and molecular dynamics (MD) simulations. The 3D defect structure at various length scales and subsurface depths was resolved with the help of a sequential polishing, etching, and imaging technique. This analysis, combined with load–displacement data, shows that the incipient plasticity (manifested as sudden indenter displacement bursts) is strongly influenced by preexisting dislocations. In the early stage of plastic deformation, the dislocation pile-ups are all aligned in 〈100〉 directions, lying on {110}45 planes, inclined at 45° to the (001) surface. At higher mean contact pressure and larger indentation depth, however, dislocation pile-ups along 〈110〉 directions appear, lying on {110}90 planes, perpendicular to the (100) surface. MD simulations confirm the glide plane nature and provide further insights into the dislocation formation mechanisms by tracing the evolution of the complete dislocation line network as function of indentation depth.


Optimized electrocaloric effect by field reversal: Analytical model

Authors: Yang-Bin Ma, Nikola Novak, Karsten Albe and Bai-Xiang Xu

Applying a negative field on a positively poled ferroelectric sample can enhance the electrocaloric cooling and is a promising method to optimize the electrocaloric cycle. Experimental measurements show that the maximal cooling is not obtained, when the electric field is removed, but reversed to a value corresponding to the shoulder of the P-E loop. This phenomenon cannot be explained if a constant total entropy is assumed under adiabatic conditions. Thus, a direct analysis of entropy changes based on work loss is proposed in this work, which takes the entropy contribution of the irreversible process into account. The optimal reversed field determined by this approach agrees with the experimental observations. This study signifies the importance of considering irreversible process in the electrocaloric cycles.


Flexocoupling impact on size effects of piezoresponse and conductance in mixed-type ferroelectric semiconductors under applied pressure

Authors: Anna N. Morozovska, Eugene A. Eliseev, Yuri A. Genenko, Ivan S. Vorotiahin, Maxim V. Silibin, Ye Cao, Yunseok Kim, Maya D. Glinchuk, and Sergei V. Kalinin

We explore the role of flexoelectric effect in functional properties of nanoscale ferroelectric films with mixed electronic-ionic conductivity. Using a coupled Ginzburg-Landau model, we calculate spontaneous polarization, effective piezoresponse, elastic strain and compliance, carrier concentration, and piezoconductance as a function of thickness and applied pressure. In the absence of flexoelectric coupling, the studied physical quantities manifest well-explored size-induced phase transitions, including transition to paraelectric phase below critical thickness. Similarly, in the absence of external pressure flexoelectric coupling affects properties of these films only weakly. However, the combined effect of flexoelectric coupling and external pressure induces polarizations at the film surfaces, which cause the electric built-in field that destroys the thickness-induced phase transition to paraelectric phase and induces the electretlike state with irreversible spontaneous polarization below critical thickness. Interestingly, the built-in field leads to noticeable increase of the average strain and elastic compliance in this thickness range. We further illustrate that the changes of the electron concentration by several orders of magnitude under positive or negative pressures can lead to the occurrence of high- or low-conductivity states, i.e., the nonvolatile piezoresistive switching, in which the swing can be controlled by the film thickness and flexoelectric coupling. The obtained theoretical results can be of fundamental interest for ferroic systems, and can provide a theoretical model for explanation of a set of recent experimental results on resistive switching and transient polar states in these systems.


Enhanced electrocaloric cooling in ferroelectric single crystals by electric field reversal

Authors: Yang-Bin Ma, Nikola Novak, Jurij Koruza, Tongqing Yang, Karsten Albe and Bai-Xiang Xu

TAn improved thermodynamic cycle is validated in ferroelectric single crystals, where the cooling effect of an electrocaloric refrigerant is enhanced by applying a reversed electric field. In contrast to the conventional adiabatic heating or cooling by on-off cycles of the external electric field, applying a reversed field is significantly improving the cooling efficiency, since the variation in configurational entropy is increased. By comparing results from computer simulations using Monte Carlo algorithms and experiments using direct electrocaloric measurements, we show that the electrocaloric cooling efficiency can be enhanced by more than 20% in standard ferroelectrics and also relaxor ferroelectrics, like Pb(Mg1/3/Nb2/3)0.71Ti0.29O3.


* Geänderte Rufnummern im Fachgebiet Materialmodellierung seit 8. Februar 2016 *

Positive and negative electrocaloric effect in BaTiO3 in the presence of defect dipoles

Authors: Yang-Bin Ma, Anna Grünebohm, Kai-Christian Meyer, Karsten Albe and Bai-Xiang Xu

The influence of defect dipoles on the electrocaloric effect (ECE) in acceptor doped BaTiO3 is studied by means of lattice-based Monte-Carlo simulations using a Ginzburg-Landau type effective Hamiltonian. Oxygen vacancy-acceptor associates are described by fixed local dipoles with orientation parallel or antiparallel to the external field. By a combination of canonical and microcanonical simulations the ECE is directly evaluated. Our results reveal that in the case of antiparallel defect dipoles the ECE can be positive or negative depending on the dipole density. Moreover, a transition from a negative to positive ECE can be observed when the external field increases. These transitions are due to the delicate interplay of internal and external fields and are explained by the domain structure evolution and related field-induced entropy changes. The results are in good qualitative agreement to those obtained by molecular dynamics simulations employing an ab initio based effective Hamiltonian. Finally, a modified electrocaloric cycle, which makes use of the negative ECE in the presence of defect dipoles, is proposed to enhance the cooling effect.


First-principles calculations on structure and properties of amorphous Li5P4O8N3 (LiPON)

Authors: Sabrina Sicolo, Karsten Albe

The structural, electronic and ion transport properties of an amorphous member of the LiPON family with non-trivial composition and cross-linking are studied by means of electronic structure calculations within Density Functional Theory. By a combination of an evolutionary algorithm followed by simulated annealing and alternatively by a rapid quenching protocol, structural models of disordered Li5P4O8N3 are generated, which are characterized by a local demixing in Li-rich and Li-poor layers. These structures have a composition close to what is found experimentally in thin films and contain all the expected diversely coordinated atoms, namely triply- and doubly-coordinated nitrogens and bridging and non-bridging oxygens. The issue of ionic conductivity is addressed by calculating defect formation energies and migration barriers of neutral and charged point defects. Li+ interstitials are energetically much preferred over vacancies, both when the lithium reservoir is metallic lithium and LiCoO2. The competitive formation of neutral Li interstitials when LiPON is contacted with metallic Li results in the chemical reduction of LiPON and the disruption of the network, as recently observed in experiments.


Visualization and Analysis Strategies for Atomistic Simulations
(book chapter)

Author: A. Stukowski

An important aspect of many molecular dynamics studies is the meaningful visualization of computed atomic configurations and trajectories, often contributing a lot to the understanding of the investigated phenomena. This chapter introduces visualization programs and analysis tools that have been developed for working with the output of classical molecular dynamics and other atomistic simulation models. Basic analysis techniques relevant for nanomechanics problems are described, which help to reveal structural phases, defects, and local deformations in materials. Furthermore, this chapter gives an overview of the dislocation extraction algorithm, which is a computational method for the automated detection and identification of dislocation lines in atomistic crystal models.


Bestes Poster beim Poster Award auf dem „International Symposium on Application of Ferroelectrics ISAF/ECAPD/PFM-2016“ für Ruben Khachaturyan.

Deutsch-Ukrainische Akademische Gesellschaft verleiht
Olena Lenchuk den 1. Platz beim PhD Thesis Presentation Contest

Magnetic invisibility of the magnetically coated type-II superconductor in partially penetrated state

Authors: J. Peña-Roche, Y. A. Genenko and A. Badía-Majós

The magnetic behavior of a cylindrical paramagnet/superconductor heterostructure has been studied by numerical simulations. Relying on the variational statement of the critical state model, our results show that magnetic invisibility is compatible with the partial penetration regime in the superconductor. This result accomplishes previous analytic studies that proved a possible perfect undetectability for the full penetration of magnetic flux. For a given geometry, invisibility may be realized only at a certain magnitude of the applied field. Such value decreases with increasing permeability of the magnetic sheath and eventually collapses to zero. This establishes a condition for obtaining realizable invisibility that extends previous expectations relying either on the full penetration ansatz or on perfect diamagnetism.


Fatigue effect on polarization switching dynamics in polycrystalline bulk ferroelectrics

Authors: S. Zhukov, J. Glaum, H. Kungl, E. Sapper, R. Dittmer, Y. A. Genenko and H. von Seggern

Statistical distribution of switching times is a key information necessary to describe the dynamic response of a polycrystalline bulk ferroelectric to an applied electric field. The Inhomogeneous Field Mechanism (IFM) model offers a useful tool which allows extraction of this information from polarization switching measurements over a large time window. In this paper, the model was further developed to account for the presence of non-switchable regions in fatigued materials. Application of the IFM-analysis to bipolar electric cycling induced fatigue process of various lead-based and lead-free ferroelectric ceramics reveals different scenarios of property degradation. Insight is gained into different underlying fatigue mechanisms inherent to the investigated systems.


Reconciling Local Structure Disorder and the Relaxor State in (Bi1/2Na1/2)TiO3-BaTiO3

Authors: Pedro B. Groszewicz, Melanie Gröting, Hergen Breitzke, Wook Jo, Karsten Albe, Gerd Buntkowsky & Jürgen Rödel

Lead-based relaxor ferroelectrics are key functional materials indispensable for the production of multilayer ceramic capacitors and piezoelectric transducers. Currently there are strong efforts to develop novel environmentally benign lead-free relaxor materials. The structural origins of the relaxor state and the role of composition modifications in these lead-free materials are still not well understood. In the present contribution, the solid-solution (100-x)(Bi1/2Na1/2)TiO3-xBaTiO3 (BNT-xBT), a prototypic lead-free relaxor is studied by the combination of solid-state nuclear magnetic resonance (NMR) spectroscopy, dielectric measurements and ab-initio density functional theory (DFT). For the first time it is shown that the peculiar composition dependence of the EFG distribution width (ΔQISwidth) correlates strongly to the dispersion in dielectric permittivity, a fingerprint of the relaxor state. Significant disorder is found in the local structure of BNT-xBT, as indicated by the analysis of the electric field gradient (EFG) in 23Na 3QMAS NMR spectra. Aided by DFT calculations, this disorder is attributed to a continuous unimodal distribution of octahedral tilting. These results contrast strongly to the previously proposed coexistence of two octahedral tilt systems in BNT-xBT. Based on these results, we propose that considerable octahedral tilt disorder may be a general feature of these oxides and essential for their relaxor properties.


Thermodynamics and kinetics of defects in Li2S

Thermodynamics and kinetics of defects in Li2S

Authors: Ashkan Moradabadi and Payam Kaghazchi

Li2S is the final product of lithiation of sulfur cathodes in lithium-sulfur (Li-S) batteries. In this work, we study formation and diffusion of defects in Li2S. It is found that for a wide range of voltages (referenced to metal Li) between 0.17 V and 2.01 V, positively charged interstitial Li (Li+) is the most favorable defect type with a fixed formation energy of 1.02 eV. The formation energy of negatively charged Li vacancy (VLi-) is also constant, and it is only 0.13 eV higher than that of Li+. For a narrow range of voltages between 0.00 V and 0.17 V, the formation energy of neutral S vacancy is the lowest and it decreases with decreasing the cell voltage. The energy barrier for Li+ diffusion (0.45 eV), which takes place via an exchange mechanism, is 0.18 eV higher than that for VLi- (0.27 eV), which takes place via a single vacancy hopping. Considering formation energies and diffusion barriers, we find that ionic conductivity in Li2S is due to both Li+ and VLi-, but the latter mechanism being slightly more favorable.


Functional Interfaces for Transparent Organic Electronic Devices: Consistent Description of Charge Injection by Combining In Situ XPS and Current Voltage Measurements with Self-Consistent Modeling

Functional Interfaces for Transparent Organic Electronic Devices: Consistent Description of Charge Injection by Combining In Situ XPS and Current Voltage Measurements with Self-Consistent Modeling

Authors: Jürgen Gassmann, Sergey V. Yampolskii, Yuri A. Genenko, Thilo C.G. Reusch, and Andreas Klein

The interface properties between Sn-doped In2O3 (ITO) and the organic semiconductor α-NPD are studied using in situ X-ray and ultraviolet photoelectron spectroscopy (XPS, UPS) as well as with in situ current–voltage analysis in combination transport simulations using a self-consistent mean field model. In particular, ITO is sputtered onto α-NPD as required for transparent or inverted organic light-emitting diodes. We identify deposition conditions, which leave the organic molecules intact. The barrier heights determined by XPS/UPS for the inverted interfaces between undoped and doped α-NPD and ITO are 1.0 and 1.1 eV, respectively. These are in good agreement with barrier heights extracted from current–voltage simulations if the band width of the highest occupied molecular orbital (HOMO) is taken into account. The HOMO bandwidth determined by UPS is σUPS = 0.22 eV and that derived from simulations is σsim = 0.23 eV.


Interfaces and interphases in nanoglasses: Surface segregation effects and their implications on structural properties

Interfaces and interphases in nanoglasses: Surface segregation effects and their implications on structural properties

Authors: Omar Adjaoud, Karsten Albe

Metallic nanoglasses can be prepared by cold compaction of amorphous nanoparticles initially condensed in inert gas atmosphere. Experimentally, it has been found that a characteristic feature of nanoglasses is the occurrence of atomic density variations within the microstructure, that cannot be explained by interface induced topological variations in a chemically homogeneous material. Here we present molecular dynamics simulations, which reveal that compositional variations between glass-glass interfaces and the volume material can result from surface segregation effects already present in the primary particles. By comparing results for Pd80Si20 and Cu64Zr36 metallic glasses, we show that amorphous nanoparticles install an inhomogenous elemental equilibrium distribution in the gas phase before they undergo the glass transition into the solid state. A detailed analysis of planar interfaces generated by merging chemically equilibrated surfaces shows that glass-glass interfaces can be understood as interphases of different composition and short-range order, where the local topology, free volume and local composition are intimately linked.


Manipulating dislocation nucleation and shear resistance of bimetal interfaces by atomic steps

Manipulating dislocation nucleation and shear resistance of bimetal interfaces by atomic steps

Authors: R.F. Zhanga, I.J. Beyerlein, S.J. Zheng, S.H. Zhang, A. Stukowski, T.C. Germann

By means of atomistic simulations and interface dislocation theory, the mechanism of dislocation nucleation and shear resistance of various stepped fcc/bcc interfaces are comparatively studied using the Kurdjumov-Sachs (KS) Cu/Nb interface as a prototype. It is found that the introduction of atomic steps at the flat Cu{111}/{110}Nb KS interface does not change the most preferred slip systems, but influences the nucleation sites at the interface during tension loading, indicating that the flat and stepped interfaces possesses comparable energetic barriers for dislocation nucleation. During shear loading, the steps may significantly enhance the resistance to interface sliding by propagating partial dislocations that facilitate the emission and growth of parallel twins via cross slip. When the parallel twins are not favored or are hindered, the interface sliding will dominate in a “climbing peak-to-valley” manner. These results provide an effective pathway to solve the trade-off dilemma between dislocation nucleation and interface sliding by appropriately manipulating atomic steps at the flat interface in the design of high-strength metallic materials.


Influence of Crystalline Nanoprecipitates on Shear-Band Propagation in Cu-Zr-Based MetallicGlasses

Influence of Crystalline Nanoprecipitates on Shear-Band Propagation in Cu-Zr-Based Metallic Glasses

Authors: Tobias Brink, Martin Peterlechner, Harald Rösner, Karsten Albe and Gerhard Wilde

The interaction of shear bands with crystalline nanoprecipitates in Cu-Zr-based metallic glasses is investigated by a combination of high-resolution TEM imaging and molecular-dynamics computer simulations. Our results reveal different interaction mechanisms: Shear bands can dissolve precipitates, can wrap around crystalline obstacles, or can be blocked depending on the size and density of the precipitates. If the crystalline phase has a low yield strength, we also observe slip transfer through the precipitate. Based on the computational results and experimental findings, a qualitative mechanism map is proposed that categorizes the various processes as a function of the critical stress for dislocation nucleation, precipitate size, and distance.


Minimum energy path for the nucleation of misfit dislocations in Ge/Si(0 0 1) heteroepitaxy

Minimum energy path for the nucleation of misfit dislocations in Ge/Si(0 0 1) heteroepitaxy

Authors: O. Trushin, E. Maras, A. Stukowski, E. Granato, S. C. Ying, H Jónsson and T. Ala-Nissila

A possible mechanism for the formation of a 90° misfit dislocation at the Ge/Si(001) interface through homogeneous nucleation is identified from atomic scale calculations where a minimum energy path connecting the coherent epitaxial state and a final state with a 90° misfit dislocation is found using the nudged elastic band method. The initial path is generated using a repulsive bias activation procedure in a model system including 75 000 atoms. The energy along the path exhibits two maxima in the energy. The first maximum occurs as a 60° dislocation nucleates. The intermediate minimum corresponds to an extended 60° dislocation. The subsequent energy maximum occurs as a second 60° dislocation nucleates in a complementary, mirror glide plane, simultaneously starting from the surface and from the first 60° dislocation. The activation energy of the nucleation of the second dislocation is 30% lower than that of the first one showing that the formation of the second 60° dislocation is aided by the presence of the first one. The simulations represent a step towards unraveling the formation mechanism of 90° dislocations, an important issue in the design of growth procedures for strain released Ge overlayers on Si(1 0 0) surfaces, and more generally illustrate an approach that can be used to gain insight into the mechanism of complex nucleation paths of extended defects in solids.

DOI: 10.1088/0965-0393/24/3/035007

Interplay of dislocation-based plasticity and phase transformation during Si nanoindentation

Authors: Zhibo Zhang, Alexander Stukowski, Herbert M. Urbassek

Nanoindentation into single-crystalline Si is modeled by molecular dynamics simulation using a modified Tersoff potential. We observe that the high stress produced during indentation leads to three processes occurring consecutively in the substrate: (i) phase transformation of the original cubic diamond (cd) to the bct5 phase; (ii) generation of dislocations; and (iii) amorphization. The bct5 phase develops along {1 1 1} planes of the cd phase; when these meet, the enclosed volume of cd phase transforms to bct5. The particular role played by a stable tetrahedral structure formed by bct5 {1 1 1} planes and {1 1 1} intrinsic stacking faults in the cd structure is highlighted. The phase transformation to bct5 is partially reversed when dislocations nucleate in the cd phase and locally relieve stresses. The generation and reactions of the uncommon dislocations 1/4〈111〉 and 1/3〈112〉 are discussed.

DOI: 10.1016/j.commatsci.2016.03.039

Compositional and electrical properties of line and planar defects in Cu(In,Ga)Se2 thin films for solar cells – a review

Authors: Daniel Abou-Ras, Sebastian S. Schmidt, Norbert Schäfer, Jaison Kavalakkatt, Thorsten Rissom, Thomas Unold, Thomas Kirchartz, Ekin Simsek Sanli, Peter A. van Aken, Quentin M. Ramasse, Hans-Joachim Kleebe, Doron Azulay, Isaac Balberg, Oded Millo, Oana Cojocaru-Mirédin, Daniel Barragan-Yani, Karsten Albe, Jakob Haarstrich and Carsten Ronning

The present review gives an overview of the various reports on properties of line and planar defects in Cu(In,Ga)(S,Se)2 thin films for high-efficiency solar cells. We report results from various analysis techniques applied to characterize these defects at different length scales, which allow for drawing a consistent picture on structural and electronic defect properties. A key finding is atomic reconstruction detected at line and planar defects, which may be one mechanism to reduce excess charge densities and to relax deep-defect states from midgap to shallow energy levels. On the other hand, nonradiative Shockley–Read–Hall recombination is still enhanced with respect to defect-free grain interiors, which is correlated with substantial reduction of luminescence intensities. Comparison of the microscopic electrical properties of planar defects in Cu(In,Ga)(S,Se)2 thin films with two-dimensional device simulations suggest that these defects are one origin of the reduced open-circuit voltage of the photovoltaic devices.

DOI: 10.1002/pssr.201510440

Dislocation evolution and peak spall strengths in single crystal and nanocrystalline Cu

Authors: Karoon Mackenchery, Ramakrishna R. Valisetty, Raju R. Namburu, Alexander Stukowski, Arunachalam M. Rajendran and Avinash M. Dongare

The dynamic evolution and interaction of defects under the conditions of shock loading in single crystal and nanocrystalline Cu are investigated using a series of large-scale molecular dynamics simulations for an impact velocity of 1km/s. Four stages of defect evolution are identified during shock simulations that result in deformation and failure. These stages correspond to: the initial shock compression (I); the propagation of the compression wave (II); the propagation and interaction of the reflected tensile wave (III); and the nucleation, growth, and coalescence of voids (IV). The effect of the microstructure on the evolution of defect densities during these four stages is characterized and quantified for single crystal Cu as well as nanocrystalline Cu with an average grain size of 6nm, 10nm, 13nm, 16nm, 20nm, and 30nm. The evolution of twin densities during the shock propagation is observed to vary with the grain size of the system and affects the spall strength of the metal. The grain sizes of 6nm and 16nm are observed to have peak values for the twin densities and a spall strength that is comparable with the single crystal Cu.

Influence of microstructure on the cutting behaviour of silicon

Authors: Saurav Goel, Andrii Kovalchenko, Alexander Stukowski, Graham Cross

We use molecular dynamics simulation to study the mechanisms of plasticity during cutting of monocrystalline and polycrystalline silicon. Three scenarios are considered: (i) cutting a single crystal silicon workpiece with a single crystal diamond tool, (ii) cutting a polysilicon workpiece with a single crystal diamond tool, and (iii) cutting a single crystal silicon workpiece with a polycrystalline diamond tool. A long-range analytical bond order potential is used in the simulations, providing a more accurate picture of the atomic-scale mechanisms of brittle fracture, ductile plasticity, and structural changes in silicon. The MD simulation results show a unique phenomenon of brittle cracking typically inclined at an angle of 45°–55° to the cut surface, leading to the formation of periodic arrays of nanogrooves in monocrystalline silicon, which is a new insight into previously published results. Furthermore, during cutting, silicon is found to undergo solid-state directional amorphisation without prior Si–I to Si-II (beta tin) transformation, which is in direct contrast to many previously published MD studies on this topic. Our simulations also predict that the propensity for amorphisation is significantly higher in single crystal silicon than in polysilicon, signifying that grain boundaries eases the material removal process.

Effect of texturing on polarization switching dynamics in ferroelectric ceramics

Authors: Sergey Zhukov, Yuri A. Genenko, Jurij Koruza, Jan Schultheiß, Heinz von Seggern, Wataru Sakamoto, Hiroki Ichikawa, Tatsuro Murata, Koichiro Hayashi and Toshinobu Yogo

Highly (100),(001)-oriented (Ba0.85Ca0.15)TiO3 (BCT) lead-free piezoelectric ceramics were fabricated by the reactive templated grain growth method using a mixture of plate-like CaTiO3 and BaTiO3 particles. Piezoelectric properties of the ceramics with a high degree of texture were found to be considerably enhanced compared with the BCT ceramics with a low degree of texture. With increasing the Lotgering factor from 26% up to 94%, the piezoelectric properties develop towards the properties of a single crystal. The dynamics of polarization switching was studied over a broad time domain of 8 orders of magnitude and was found to strongly depend on the degree of orientation of the ceramics. Samples with a high degree of texture exhibited 2–3 orders of magnitude faster polarization switching, as compared with the ones with a low degree of texture. This was rationalized by means of the Inhomogeneous Field Mechanism model as a result of the narrower statistical distribution of the local electric field values in textured media, which promotes a more coherent switching process. The extracted microscopic parameters of switching revealed a decrease of the critical nucleus energy in systems with a high degree of texture providing more favorable switching conditions related to the enhanced ferroelectric properties of the textured material.

Magnetic structures of the low temperature phase of Mn3(VO4)2 – towards understanding magnetic ordering between adjacent Kagomé layers

Authors: Oliver Clemens, Jochen Rohrer, Gwilherm Nénert

In this article we report on a detailed analysis of the magnetic structures of the magnetic phases of the low temperature (lt-) phase of Mn3(VO4)2 (vMn3V2O8) with a Kagomé staircase structure determined by means of powder neutron diffraction. Two magnetic transitions were found at ∼25 K (HT1 phase, Cmc’a’) and ∼17 K (LT1 phase, Pmc’a’), in excellent agreement with previous reports. The LT1 phase is characterized by commensurate magnetic ordering of the magnetic moments on two magnetic sites of the Mn1a/b (2a +2 d) and Mn2 (8i) ions of the nuclear structure (where for the latter site two different overall orientations of magnetic moments within the ab-plane (Mn2a and Mn2b) can be distinguished. This results in mainly antiferromagnetic interactions between edge-sharing Mn-octahedra within the Kagomé planes. The HT1 phase is characterised by strong spin frustration resulting from the loss of ordering of the magnetic moments of Mn2a/b ions along the b-axis. Both magnetic structures are in agreement with the previously reported ferrimagnetic properties of lt-Mn3(VO4)2 and shed light on the magnetic phase diagram of the compound reported previously. The magnetic structures are discussed with respect to superexchange interaction pathways within the Kagomé layers, which appear to be predominantly antiferromagnetic. The magnetic structures of Mn3(VO4)2 are different compared to those reported for Ni3(VO4)2 and Co3(VO4)2 and represent an unique commensurate way out of spin frustration for compounds with strong antiferromagnetic superexchange interactions within the Kagomé layers. Additionally, we derive a superexchange model, which will be called redox-mediated M–M’(d0)–M superexchange and which can help to understand the exclusively ferromagnetic ordering of adjacent Kagomé layers found only for lt-Mn3(VO4)2.!divAbstract

Atomistic modelling of zirconium and silicon segregation at twist and tilt grain boundaries in molybdenum

Authors: Olena Lenchuk, Jochen Rohrer, Karsten Albe

We investigate the influence of Zr and Si segregation on the cohesive strength of grain boundaries (GBs) in molybdenum using density functional theory calculations. A tilt Sigma5(310)[001] and twist Sigma5[001] GB in bicrystal geometry are chosen as structural models. We determine the site preference of Zr and Si for segregation in these GBs and define the segregation energy. We quantify the effect of solutes on the stability of the GBs against brittle fracture by means of the Griffith criterion (work of separation). Additionally, the intrinsic bond strength of the GB containing a solute is quantified by means of the theoretical strength. The results show that Zr and Si tend to segregate at the GBs if the low-energy insertion sites are available. However, the work of separation is decreased by the presence of Zr and Si and even in the presence of oxygen, there is no increase of the Griffith energy. Contributions of strain and chemical energy are analysed in order to explain our findings.

Investigation of partial discharge in piezoelectric ceramics

Investigation of partial discharge in piezoelectric ceramics

Authors: Tian Hanga, Julia Glauma, Yuri A. Genenko, Toan Phung, Mark Hoffman

Electrical partial discharges were studied in different piezoelectric ceramics. Epoxy material with micro sized cavities was also tested and compared to the piezoelectric ceramics. It is found that compared to epoxy, partial discharge (PD) occurs at relatively lower electric fields for piezoelectric ceramics. The PD inception voltage was found to be lower for materials with higher relative permittivity. This indicates that the intensification of the electric field within the defects is the main cause for the differences in inception field observed for epoxy compared to piezoelectric ceramics. Furthermore, phase resolved PD pattern analysis was carried out for all materials at elevated electric fields. A broad distribution of the discharge event was observed for both epoxy and hard PZT samples, whereas for soft PZT discharge occurs concentrated at electric fields slightly above the coercive field. This intensification of PDs close to the coercive field suggests that PDs may be enhanced due to an increase of the internal field and electron emission rate induced by the domain switching process.

Polarization dynamics variation across the temperature- and composition-driven phase transitions in the lead-free

Polarization dynamics variation across the temperature- and composition-driven phase transitions in the lead-free Ba(Zr02Ti0.8)O32x(Ba0.7Ca0.3)TiO3 ferroelectrics

Authors: Sergey Zhukov, Matias Acosta, Yuri A. Genenko and Heinz von Seggern

The method of thermally stimulated depolarization currents (TSDC) and polarization switching experiments over a large field, time, and temperature regime are used to refine the controversial phase diagram of Ba(Zr0.2Ti0.8)O3−x(Ba0.7Ca0.3)TiO3 and comprehend its relation to ferroelectric and piezoelectric properties. TSDC results suggest the existence of three ferroelectric phases for the composition range of 0.30 ≤ x ≤ 0.60, which can be assigned to the rhombohedral (R), presumably orthorhombic (O), and tetragonal (T) symmetries. Spontaneous polarization is maximal all over the entire intermediate phase region, where the activation barrier for polarization switching is small, not just at R-O or O-T boundaries as might be deduced from previous observations.

Stabilization of the y-Sn phase in tin nanoparticles and nanowires

Stabilization of the γ-Sn phase in tin nanoparticles and nanowires

Authors: N. G. Hörmann, A. Gross, J. Rohrer and P. Kaghazchi

Structures of Sn nanoparticles and nanowires are studied using density functional theory in conjunction with thermodynamic considerations. Besides the low-temperature α and room-temperature β phases, the high-temperature γ phase is considered. Results show that at ambient temperatures for sizes smaller than 50 nm, metallic β- and γ-Sn nanoparticles are more stable than semimetallic α-Sn ones because of their lower surface energies. Moreover, very small Sn nanostructures, exemplified by nanowires, are expected to exhibit the γ phase even at 0 K.

Finite-element simulations of hysteretic alternating current losses in a magnetically coated superconducting tubular wire subject to an oscillating transverse magnetic field

Authors: Y. A. Genenko, H. Rauh and S. Kurdi

Numerical simulations of hysteretic ac losses in a tubular superconductor/paramagnet heterostructure subject to an oscillating transverse magnetic field are performed within the quasistatic approach, calling upon the COMSOL finite-element software package and exploiting magnetostaticelectrostatic analogues. It is shown that one-sided magnetic shielding of a thin, type-II superconducting tube by a coaxial paramagnetic support results in a slight increase of hysteretic ac losses as compared to those for a vacuum environment, when the support is placed inside; a spectacular shielding effect with a possible reduction of hysteretic ac losses by orders of magnitude, however, ensues, depending on the magnetic permeability and the amplitude of the applied magnetic field, when the support is placed outside.


On the origin of anisotropic lithiation of silicon

On the origin of anisotropic lithiation of silicon

Authors: Jochen Rohrer, Ashkan Moradabadi, Karsten Albe, Payam Kaghazchi

Silicon has the highest known theoretical capacity (∼4140 mAhg−1) to store lithium. Among different forms of Si, Si nanowires (SiNWs) are the most promising candidates for the next-generation lithium-ion batteries. Lithiation of SiNWs is a complex process, which is not very well understood. Here, we present density functional theory calculations on Li incorporation in SiNWs using surface and interface geometries. Our results show that, initially, Li intercalation proceeds through Si(110) facets, leading to the formation of an amorphous Li2Si shell. For interfaces between the lithiated (amorphous Li2Si) shell and unlithiated (pristine c-Si) core region we find that the Li intercalation barriers are independent of the actual interface orientation, while interface energies show an orientation dependence similar to surface energies. In particular, a-Li2Si/c-Si(111) is most favorable while a-Li2Si/c-Si(110) is least favorable. Since high-energy interfaces typically show a higher mobility than low-energy interfaces, the experimentally observed anisotropic swelling of SiNWs can be understood on the basis of interface energetics.


Lattice-based Monte Carlo simulations of the electrocaloric effect in ferroelectrics and relaxor ferroelectrics

Authors: Yang-Bin Ma, Karsten Albe, and Bai-Xiang Xu

Canonical and microcanonical Monte Carlo simulations are carried out to study the electrocaloric effect (ECE) in ferroelectrics and relaxor ferroelectrics (RFEs) by direct computation of field-induced temperature variations at the ferroelectric-to-paraelectric phase transition and the nonergodic-to-ergodic state transformation. A lattice-based Hamiltonian is introduced, which includes a thermal energy, a Landau-type term, a dipole-dipole interaction energy, a gradient term representing the domain-wall energy, and an electrostatic energy contribution describing the coupling to external and random fields. The model is first parametrized and studied for the case of BaTiO3. Then, the ECE in RFEs is investigated, with particular focus on the influence of random fields and domain-wall energies. If the strength or the density of the random fields increases, the ECE peak shifts to a lower temperature but the temperature variation is reduced. On the contrary, if the domain-wall energy increases, the peak shifts to a higher temperature and the ECE becomes stronger. In RFEs, the ECE is maximum at the freezing temperature where the nonergodic-to-ergodic transition takes place. Our results imply that the presence of random fields reduces the entropy variation in an ECE cycle by pinning local polarization.

Anomalous compliance and early yielding of nanoporous gold

Authors: Bao-Nam Dinh Ngô, Alexander Stukowski, Nadiia Mameka, Jürgen Markmann, Karsten Albe, Jörg Weissmüller

We present a study of the elastic and plastic behavior of nanoporous gold in compression, focusing on molecular dynamics simulation and inspecting experimental data for verification. Both approaches agree on an anomalously high elastic compliance in the early stages of deformation, along with a quasi immediate onset of plastic yielding even at the smallest load. Already before the first loading, the material undergoes spontaneous plastic deformation under the action of the capillary forces, requiring no external load. Plastic deformation under compressive load is accompanied by dislocation storage and dislocation interaction, along with strong strain hardening. Dislocation-starvation scenarios are not supported by our results. The stiffness increases during deformation, but never approaches the prediction by the relevant Gibson–Ashby scaling law. Microstructural disorder affects the plastic deformation behavior and surface excess elasticity might modify elastic response, yet we relate the anomalous compliance and the immediate yield onset to an atomistic origin: the large surface-induced prestress induces elastic shear that brings some regions in the material close to the shear instability of the generalized stacking fault energy curve. These regions are elastically highly compliant and plastically weak.

Publikation Brink, Sopu, Albe

Solid-state amorphization of Cu nanolayers embedded in a Cu64Zr36 glass

Authors: Tobias Brink*, Daniel Şopu†, and Karsten Albe

Solid-state amorphization of crystalline copper nanolayers embedded in a Cu64Zr36 metallic glass is studied by molecular dynamics simulations for different orientations of the crystalline layer. We show that solid-state amorphization is driven by a reduction of interface energy, which compensates the bulk excess energy of the amorphous nanolayer with respect to the crystalline phase up to a critical layer thickness. A simple thermodynamic model is derived, which describes the simulation results in terms of orientation-dependent interface energies. Detailed analysis reveals the structure of the amorphous nanolayer and allows a comparison to a quenched copper melt, providing further insights into the origin of excess and interface energy.