Theory and simulation in physics for materials applications

Resume : The microscopic distribution of mechanical stress provides a unique connection between atomistic simulations and nanoscale mechanics. However, defining the stress field at the atomic scale is far from straightforward, a task overshadowed by several ambiguities. In this talk I will illustrate the standard approach to stress definition, based on the so-called ?virial? formula, and point out the difficulties associated with such a definition. Recent developments based on the covariant definition of the stress field will be illustrated, and the connection with continuum-mechanics definitions of stress and strain will be outlined. Examples of molecular dynamics simulations will concern both condensed-phase systems (grain boundaries, dislocations and microcracks in crystals), and fluid molecular phases (DNA and proteins in solution).

Resume : Analysis of lattice disorder is of high importance in materials science especially for crystals modified by or exposed to charged particles. Interaction with ions causes formation of different defect types. One of the principal methods used for analysis of radiation damage is Rutherford Backscattering Spectrometry in Channeling mode (RBS/C). However, simplified evaluation of RBS/C spectra may mislead or provide incomplete results, particularly for heterostructures or materials containing complex defects. Monte Carlo (MC) simulations have been used for last decades to get information about lattice distortion from RBS/C. One of the most powerful PC software with MC engine is called ‘McChasy’ [1]. The code provides a fitting procedure of RBS/C spectra considering separate depth distributions of simple defects and dislocations as well as substitutions of target atoms. Its recent updates include: - a modern way of obtaining dislocation parameters directly from HRTEM micrographs, - a unique approach of 3D-interaction between ions and target atoms, - computing of thermal vibrations in 3D, - use of a rotation matrix for different orientations of Burger’s vectors, - Xe bubbles in UO2. Application of the McChasy code in the analysis of crystal defects will be described and selected accomplishments presented with special attention paid to ion bombarded ZnO. Possible directions of further development of the code will also be pointed out. [1] A. Turos et al., Nucl. Instr. Meth. B 332 (2014) 50

Resume : The oxides of heavy main-group elements with s2 electron configurations show stereo-chemically active lone pair, which are responsible for the distorted crystal structures and their exciting physical properties of great interest to widely ranging applications. There is however a paucity of physically accurate and computationally affordable models for this class of material. To address this issue, we have developed a novel atomistic model coupled with a one-center Hamiltonian approach to the treatment of ions with active lone pairs. The new model has been implemented and tested in our in-house software, and then applied to study exemplar systems. The common metal ions with such a lone pair are Pb2+ and Sn2+. Unlike simple rocksalt-structured materials (MgO, CdO, etc.), their binary oxides, PbO and SnO, both have non-centrosymmetric structures due to the presence of asymmetrical lone-pair electron densities. Owing to these properties, SnO has been studied as potential thermoelectric and photoelectric materials. Lone-pair electron densities also have an important role in polar perovskites (tetragonal P4mm), PbTiO3 and SnTiO3, which have ferroelectric and piezoelectric properties due to its lone-pair related distortions. Furthermore, the perovskite-structured solar cells are commonly based on lead or tin halide perovskites, both of which show the presence of active lone pairs depending on temperature. Furthermore, there are many other interesting lone pair materials, which have promising chemical and physical properties for applications, which makes the study of lone-pair effects on materials structure and properties crucially important. Most of the introduced materials above were studied using first-principle methods (e.g. DFT), which usually guarantees a sufficient level of accuracy. However powerful and rigorous the method is, because of the computational cost, it has some restrictions on the numbers of atoms can realistically be considered. To deal with the issue, an alternative approach is commonly employed using atomistic level simulations, which are based on semi-empirical interatomic potentials (IP), and can be efficiently applied to simulate large supercells of both perfect bulk and defect containing materials. One of the features of the IP method is the shell model, which imitates polarization of electron densities on ions in a crystal lattice due to its internal electric field. The shell model has shown a lot of success in reproducing crystal structures and their physical properties, the polarizability of the model, however, shows only linear response to the external electric field. This limitation does not usually cause problems when the lattice environment is symmetrical, but when the system becomes strongly asymmetrical, for instance in the presence of lone-pair distortions, it needs some special treatments to reflect the correct physics. Higher-order effects in the lone-pair polarizability are typically dealt with by including fourth-order anharmonic terms, and sometimes introducing unusual interatomic interactions; in cruder models, the shell of lone pair atom might even be neglected. The advantages of our model compared to these previous studies will be shown.

Resume : We have studied (TiO2)35 as a prototype nanoparticle using plane wave density functional theory with on-site Coulomb interactions (DFT U, VASP) and hybrid DFT within a localised basis set periodic approach (FHI-aims). The TiO2 nanoparticle, of size ~2 nm, is constructed from the anatase phase and exhibits a bipyramidal structure with eight (101) facets. We focus on the properties of the nanoparticle which govern the photocatalytic performance, including light absorption, photoexcited charge carrier separation, reducibility and surface reactivity. Our results show that the energy cost to produce a single oxygen vacancy is lower for the nanoparticle relative to the extended anatase (101) surface and indicate that reduction of the nanoparticle will occur at moderate temperatures. The presence of low coordinated O sites in the nanoparticle pushes the valence band edge to higher energy and the emergence of defect states after reduction further narrows the band gap, potentially enhancing the visible light response. Photoexcited electrons and holes localize on low-coordinated Ti and O sites of the nanoparticle and are spatially separated so that recombination is suppressed. The interaction of CO2 is favourable at multiple sites of the stoichiometric and reduced nanoparticle. CO2 adsorbs with a bent geometry and elongated C-O bonds, indicating the potential for activation.

Resume : This talk will feature an overview of what has been achieved in the area of computational material science by the IPCMS atomic-scale simulation team in the framework of first-principles molecular dynamics. This technique has come to a full maturity as a tool providing quantitative information on structural, electronic and dynamical properties of highly diversified classes of materials. Among the available results, we shall exemplify those related to : a) the structure of disordered network-forming materials, with emphasis on binary and ternary chalcogenides, b) the interplay between structural and magnetic properties in hybrid organic-inorganic materials, c) the evolution of nanoparticles adsorbed on surfaces and d) an application to issues of biological interest. Methodological aspects will also be addressed, as the impact of the exchange-correlation fuctionals and the role played in specific cases by the dispersion forces.

Resume : Density Functional Theory calculations, although extremely powerful, are limited to relatively small spatial- and time-scales. The purpose of the MULTIBINIT project is to extend the capabilities of available first-principles codes, to predict properties at the mesoscale and at operating conditions (i.e. at finite temperature and under external mechanical constraints or electric fields) while retaining most of the first-principles predictive power and accuracy. MULTIBINIT is an open-source software, exploiting the so-called “second-principles” approach for lattice dynamics simulations. It is based on atomic potentials fitted on first-principles calculations [1]. In its initial form, it includes harmonic and anharmonic lattice contributions as well as homogeneous strains and their couplings to the lattice. Present developments concern the development of a spin model and its coupling to the lattice. The strength of MULTIBINIT is that it integrates efficient tools for (i) the automatic generation of the model, (ii) the automatic fit of the coefficients from first-principles data, (iii) finite temperature simulations and (iv) the post-process analysis of the results, thanks to the AGATE software. The power of the method will be illustrated on the full-Heusler Fe2VAl system to predict its lattice dynamics and its influence to the thermoelectric power factor. Work supported by the FEDER project LoCoTED. [1] J.C. Wojdel et al. J. Phys.: Cond. Mat., 25, 305401 (2013)

Resume : A new theoretical model for the simulation of the electronic current in a Ballistic Electron Emission Microscope (BEEM) is presented. Based on the Scanning Tunneling Microscope (STM), BEEM measures the electronic cur-rent collected after a metal-semiconductor interface placed far away from the metallic surface. This allows the study of the corre-sponding Schottky barrier (VSB) formed in such interface. The previous proposed models took into consideration only a semi-infinite metallic part joined to the semiconductor by a semiclasical approach, while the revised method, following a layer-by layer procedure within the Keldysh Green function formalism, has beeen generalized to new systems including thin films. In a final step, a semi-infinite semiconductor is connected creating a realistic interface. The three parts of the problem (metal, interface and semiconductor) have been simulated using an accurate ab initio parametrization. As a proof of concept, we have applied our methodogy to analyze two different Au(100)/Ge(100) interfaces. Our theoretical calculations reproduce precise Ultra High Vacuum low-Temperature experimental measurements on such interface without using adjustable parameters. We show that at T = 0 K the theoretical current follows closely the law (V-VSB)^2.1. The inclusion of the temperature effect leads us to identify two slightly different values for the Schottky barrier located at 0.67 and 0.75 eV that we associate to different patches forming the interface.

Resume : We use first-principles methods to investigate the energetics of oxygen-octahedra rotations in ABO3 perovskite oxides. We focus on the short-period, perfectly antiphase or in-phase, tilt patterns that characterize the structure of most compounds and control their physical (e.g., conductive, magnetic) properties. Based on an analytical form of the relevant potential energy surface, we discuss the conditions for the stability of various polymorphs presenting different rotation patterns, and obtain numerical results for a collection of thirty-five representative materials. Our results reveal the mechanisms responsible for the frequent occurrence of a particular structure that combines antiphase and in-phase rotations, i.e., the orthorhombic Pbnm phase displayed by about half of all perovskite oxides, as well as by many nonoxidic perovskites. In essence, the Pbnm phase benefits from the simultaneous occurrence of antiphase and in-phase tilt patterns that compete with each other, but not as strongly as to be mutually exclusive. We also find that secondary antipolar modes, involving the A cations, contribute to weaken the competition between tilts of different types, and thus play a key role in the stabilization of the Pbnm structure. Our results thus confirm and better explain previous observations for particular compounds in the literature. Interestingly, we also find that strain effects, which are known to be a major factor governing phase competition in related (e.g., ferroelectric) perovskite oxides, play no essential role as regards the relative stability of different rotational polymorphs. Further, we discuss why the Pbnm structure stops being the ground state in two opposite limits—namely, for large and small A cations—showing that very different effects become relevant in each case. Our work thus provides a comprehensive discussion and reference data on these all-important and abundant materials, which will be useful to better understand existing compounds as well as to identify new strategies for materials engineering.

Resume : Linear-scaling density functional theory (LS-DFT) methods are becoming increasingly popular due to their ability to overcome the size limitations of standard cubic scaling implementations of DFT, thereby enabling simulations of tens of thousands of atoms. One popular approach to LS-DFT uses a set of localized orbitals, which are optimized to reflect their chemical environment and thus constitute a minimal but highly accurate basis set. This local orbital basis also opens up possibilities beyond reduced computational cost, such as providing a means of analysing and quantifying the length-scale over which the presence of a defect (e.g. a boundary or impurity) in an extended system affects the electronic structure. Using the LS-DFT approach implemented in the BigDFT code, we will show how our method enables one to identify the regions of an extended system which require dedicated optimization of the Kohn-Sham degrees of freedom, and provides the user with a reliable estimation of the errors - if any - induced by the imposed locality. Such a method facilitates an effective reduction of the computational degrees of freedom needed to simulate systems at the nanoscale, while in turn providing a description that can be straightforwardly put in relation with effective or multi-scale models, like tight binding Hamiltonians or QM/MM approaches. We will illustrate our method using defective graphene as an example application.

Resume : Recently, numerous studies have been conducted to model, produce and investigate properties of High Entropy Alloys (HEAs). However, there are limited studies on Low Density High Entropy Alloys (LDHEAs) due to difficulty of production this HEAs with single or dual solid solution phases from low density elements. Therefore, development of LDHEAs is one of the challenging topic in this field. The present study involves design, production and characterization of TiCrMnVAl LDHEA. The design and selection of the proper elements to be used in the alloy are calculated based on thermo-physical calculations, Thermo-Calc software with the HEA database and first-principle calculation with Vienna Ab-initio Simulation Package (VASP) and Molecular Dynamics (MD). VASP and MD simulation of TiCrMnVAl HEA is done by the help of MedeA interface which is thought to form BCC single-phase solid solution as a result of thermo-physical calculation and Thermo-Calc outputs in the limit parameters of HEA forming ability of candidate elements. Then, TiCrMnVAl HEA is produced with copper heart arc-melting set-up. Also, suction casting with 1 and 3 mm cylindrical copper molds is done to investigate the effect of cooling rate on the structure and properties of the TiCrMnVAl HEA. Then, the LDHEA is investigated in details by using electron microscopy (SEM, TEM), X-ray diffraction (XRD) experimentally. The outputs of the modelling and simulations are compared with the experimental results. It is observed that the phases, crystal structure, lattice parameters of TiCrMnVAl HEA is the same with the modelling and simulations. This project is being supported by TUBITAK, under the project number 216M058.

Resume : Density functional theory has proven hugely successful in the calculation of structural properties of condensed matter systems and the electronic properties of simple metals. Band gaps of semiconductors and insulators, however, are often severely underestimated due to limitations in the earliest existing approximate exchange-correlation functionals. Particular improvements are possible by including a fraction of Hartree-Fock exchange, constructing a so-called hybrid functional. The precise proportion of Hartree-Fock exchange is typically treated as an empirical parameter chosen by intuition and experimental calibration. This empiricism can be avoided with a new self-consistent hybrid functional approach for condensed systems, which allows parameter-free hybrid functional investigations. Within this approach the exact amount of Hartree-Fock exchange is identified with the inverse of the dielectric constant, leading to an additional self-consistency cycle in the calculations. Here, we will generally discuss the performance of this new approach in the calculation of structural, electronic, and optical properties of bulk semiconductors. Structural and electronic properties will be compared to theoretical and experimental data, showing considerable improvement with respect to previous approaches. Other particular examples include the applicability of this new approach to defect calculations in ZnO and the amorphous quaternary solid-solution Zn-Sn-Ti oxide.

Resume : Chronic exposure to toxic carbonyl species such as acetaldehyde and formaldehyde are known to cause various health problems including irritation, asthma, cancer, genetic deficiency, among others. Thus, detection of these toxic carbonyl gases is very important and is a subject of interest both experimentally and theoretically. In this study, the interaction of polypyrrole towards toxic carbonyl species: acetaldehyde and formaldehyde, and also to less toxic carbonyl species: acetone and butanone were studied via quantum mechanical calculations. Various oligomers (n = 1,3,5,7, and 9) of pyrrole and pyrrole-gas were investigated using density functional theory (DFT) at the B3LYP-D3(BJ)/6-31G(d) level of theory. The interactions of the carbonyl species with oligopyrrole lead to differences in interaction energies and changes in structural features: H-bond distances, bond angles, and dihedral angles. The changes resulted in variations in the electronic properties of the pyrrole-gas complexes: HOMO/LUMO energies, ionization potentials (IP), electron affinity (EA), and energy gap (EGap). The pyrrole-gas complexes resulted to lower LUMO energies and smaller EGap values compared to pyrrole. Also, it was observed that the smallest carbonyl molecule, formaldehyde, had the lowest LUMO energy and lowest EGap value, while the largest carbonyl molecule, butanone, had the highest LUMO energy and highest EGap value. Furthermore, TDDFT/B3LYP/6-31G(d) calculations showed red-shifted ?max for the pyrrole-gas complexes. The results do not only demonstrate the capability of polypyrrole as toxic carbonyl gas sensor but also its selectivity towards different carbonyl species.

Resume : Half-Heuslers (HHs) have impressive thermoelectric power factors as a result band and orbital degeneracy combined with weak electron-phonon scattering. Investigations into their band structures, reveal that they have multiple bands that can be aligned through different band engineering approaches, giving opportunity to further improve their power factor. In this work we explore the power factor optimization of Co-based p-type HHs TiCoSb, NbCoSn, ZrCoSb and ZrCoBi using ab-initio Density Functional Theory (DFT) calculations and semi-classical Boltzmann transport. For this, we first extract the ab-initio power factor of these materials (using BoltzTrap). We then develop simplified bandstructure models based on the non-parabolic effective mass approximation, considering all relevant local and global band maxima found in ?, L and W points in the Brillouin zone, that match the ab-initio derived power factors reasonably well near the valence band edge. This allows the exploration of the influence of band alignment in a trivial manner. We then calibrate our simple models to experimental data and more involved electron-phonon coupling calculations using Electron Phonon Wannier (EPW) to identify the correct form of the scattering rates, and perform a comprehensive study on the optimal band alignment for maximizing their thermoelectric power factor. Finally, using DFT again we explore alloying and second phasing possibilities (with full-heuslers), that would achieve the identified optimal alignment in the bands of the HHs under consideration.

Resume : The experimental progress to create, characterize, and manipulate a wide range of nanoelectronic systems is complemented by advances in theory and simulation methods to describe their properties at the atomic level. Here I will describe the so-called DFT+NEGF technique, combining nonequilibrium Green's functions (NEGF) to describe open quantum systems and to compute transport properties in combination with density functional theory (DFT) to provide a realistic atomistic and electronic description. More specifically, I will provide an overview of the TranSIESTA [1] and Inelastica [2] DFT+NEGF codes, and describe some of our recent applications of these, e.g., an electron beam splitter realized with crossed graphene nanoribbons [3], inelastic electron tunneling spectroscopy (IETS) experiments of graphene/SiC samples [4], and tip-controlled reversible bond formation/rupture in a single-molecule conductive junction [5]. [1] Brandbyge, Mozos, Ordejón, Taylor, Stokbro, Phys. Rev. B 65, 165401 (2002); Papior, Lorente, Frederiksen, García, Brandbyge, Comp. Phys. Comm. 212, 8-24 (2017); https://launchpad.net/siesta. [2] Frederiksen, Paulsson, Brandbyge, Jauho, Phys. Rev. B 75, 205413 (2007); https://github.com/tfrederiksen/inelastica/. [3] Brandimarte, Engelund, Papior, Garcia-Lekue, Frederiksen, Sánchez-Portal, J. Chem. Phys. 146, 092318 (2017). [4] Minamitani, Arafune, Frederiksen, Suzuki, Shahed, Kobayashi, Endo, Fukidome, Watanabe, Komeda, Phys. Rev. B 96, 155431 (2017). [5] T. Jasper-Tönnies, A. Garcia-Lekue, T. Frederiksen, S. Ulrich, R. Herges, and R. Berndt, Phys. Rev. Lett. 119, 066801 (2017).

Resume : An optimization study on the InAs/GaAs quantum dot (QD) dimension and spacer layer thickness for improving solar cell performance is presented. Kronig Penney model is used to solve the Schrödinger?s equation and compute the bandstructure of the QD layers. For this computation, we have assumed a truncated pyramidal multilayer QD heterostructure exposed to fully concentrated sunlight. Photon flux density for the QD energy bands is simulated taking into account Bose-Einstein mean occupation number of photons for particular radiation energy. This study illustrated a strong dependency of the intermediate energy bands of the QD with change in QD height and spacer thickness, which ultimately can cause a change in the power conversion efficiency. Maximum efficiency can be obtained if the energy of the intermediate band is nearer to the conduction band edge of InAs. This could be obtained with optimized QD height of 2.5 nm and 8 nm GaAs spacer thickness. Further increment in QD height and spacer thickness changes the position of the intermediate band near to the GaAs conduction band edge, which results in decrement in efficiency. Our model in similar agreement with other experimental work can thus serve as an important guideline to the process for fabricating solar cells with higher efficiency.

Resume : Aims: Ni base alloy is of good heat-resisting ability, it is mostly applied in the engine of airplane, the turbojet and the nuclear reactor, so its thermal fatigue damage cannot be avoided. Ni base alloy ceramic composite coating is a mechanic disordered composite; a large number of ceramic particles are distributed in ductile matrix. The experiments show that cracks produce mostly between ceramic particles and Ni base alloy matrix and grow mostly along boundary of phases. Because the thermal expansion coefficients and elastic modulus of Ni base alloy and ceramic particles are different, there will be thermal stresses in both particles and matrix in thermo syphon process. There will be the strange stress and strain field between particles and matrix, so crack easily produces and grows. Methods: Finite element model of Ni-base alloy coating under laser irradiation was constituted based on computational fluid dynamics (CFD) and computational structural dynamics coupling numerical computational methodology. The flow is governed by the 3-D Reynolds averaged Navier-Stokes equations. To split the viscosity flux and the convective flux of the NS equations, the second order central scheme and the ROE scheme were adopted respectively. With the implicit Gauss-Seidel scheme, the code was advanced in time. The turbulence model was used for turbulence simulations. Results: Ni-base alloy coating of aircraft which are irradiated by laser are simulated. The model was proofread with the experiment data in atmosphere condition.

Resume : Hydrogen impurity is a powerful tool to control the material properties of silicon by passivating dopants and other defects. The data on hydrogen diffusion were collected years ago but they are still waiting for a proper analysis, to understand details of this process. A standard model of hydrogen penetration into boron-doped samples from a plasma ambient is in-diffusion of major atomic species H+ limited by their reversible trapping by boron into HB neutral defects. The only material parameter in this problem is a product D+K of the H+ diffusivity D+ and the equilibrium dissociation constant K of the HB defect. However it turns out that the concentration depth profile C(z) and the hole profile p(z) - reported for the same sample annealed at 150oC - cannot be reproduced simultaneously: the C(z) profile corresponds to a much larger D+K in comparison to the p(z) profile. This discrepancy is resolved if there are actually two different kinds of hydrogen ions, H+(1) and H+(2), of different lattice location - one of a larger D+K and the other of a smaller D+K . Accordingly, there are two kinds of passivated boron, HB(1) and HB(2): the second one dominates in the major part of in-diffused region while the first one dominates in the tail part of the profile. A concept of two independent atomic subsystems H(1) and H(2), each involving both positive and neutral charge states, is also useful to account for hydrogen pairing into dimers that becomes essential at a lower doping level.

Resume : For the first time low-energy plasmons reflection spectroscopy was used to determine the coefficient of thermal expansion (CTE) of the polycrystalline FeNi51 alloy surface. The experiment was carried out under ultrahigh vacuum conditions. A detailed analysis of the plasmons energy loss spectra of primary electrons in the 50-600 eV range for surface layers was performed. The method for determining the CTE is based on measuring of the temperature dependence of the energy shifts of the surface and the bulk plasmons in the temperature range from room temperature to 3000C. Taking into account the energy shifts when the alloy sample is heated for the surface and bulk plasmons the following CTE values are obtained: s=7,54х10-5 К-1 і b=4,24х10-5 К-1, respectively.

Resume : Recently, microseparation became an important technology for detecting low levels of harmful bio-agents or for size sorting of different cell types, essential for many applications such as biochemical and environmental assays, micro/nano-manufacturing, and clinical analysis. Microfluidic systems designed for cell separation offer numerous advantages over conventional cell sorting approaches, including reduced sample volume, reduced sample preparation procedures, higher sample throughput, and high spatial resolution. Our interest in this particular study is the development of the Dean’s vortices. Micro and nanoparticles in midelevation migrate transversely outward with the Dean’s vortex, are repelled by the wall lift, and continue to loop back along the top and bottom walls toward the inside wall. The aim of the study is determining the influence of Non-Newtonian fluids on Dean’s vortices formation, considering the above mentioned flow characteristics. In theory, the elastic forces tend to oppose inertial forces; therefore we expect a diminishing strength of the recirculation. The fluids were introduced in curvilinear channel geometry, consisting of a spiral microchannel with one input and three outputs. The microchannels are fabricated in silicon wafers with deep reactive ion etching technology and bonded glass on wafers to seal the microchannels. The Carreau-Yasuda rheological model, that describes the pseudoplastic flow, was implemented in the commercial numerical code ANSYS-FLUENT using a user defined function.

Resume : We present a study of the dynamics of small solute particles in a solvent medium where the solute is much smaller in size, mimicking the diffusion of small particles in crowded environment. The solute exhibits Fickian diffusion arising from non-Gaussian Van Hove correlation function. Our study shows that there are at least two possible origins of this non-Gaussian behaviour: the decoupling of the solute-solvent dynamics and the intermittency in the solute motion, the latter playing a dominant role. In the former scenario when averaged over time long enough to explore different solvent environments, the dynamics recovers the Gaussian nature. In the case of intermittent dynamics the non-Gaussianity remains even after long averaging and the Gaussian behaviour is obtained at a much longer time. Our study further shows that only for an intermediate attractive solute-solvent interaction the dynamics of the solute is intermittent. The intermittency disappears for weaker or stronger attractions.

Resume : The finite-element method (FEM) is the preferred numerical method when electromagnetic fields at high accuracy are to be computed rigorously in nano-optics design. FEM allows for numerical high efficiency (accurate results at low computation times) due to accurate geometry modelling of arbitrary shapes, adaptive meshing strategies and higher-order convergence. Our FEM solver JCMsuite has been used to simulate and design various nanostructures and devices. Among these are microstructured fibers, solar cells, microlithography masks, integrated microlenses, metamaterials, microcavities and plasmonic waveguides. At the Physikalisch-Technische Bundesanstalt (PTB) the solver was tested in various scattering experiments for the characterization of nanostructured surfaces, e.g. EUV/DUV-scatterometry, GISAXS and GIXRF. The flexibility of the solver and the tunability of the numerical precision were found to be advantageous for metrology approaches and outperformed other rigorous simulation methods. We present convergence results of the rigorous Maxwell solver and compare different optimization methods to solve the high-dimensional optimization problems arising in the characterization of nanostructured surfaces.

Resume : Controlling the wave transport properties of materials provides opportunities for improving material properties for a variety of applications including photonics, thermoelectrics, acoustics, and electronics. Here we introduce a rational strategy to control photonic and phononic wave transport utilizing periodic nanostructures derived from bottom-up type block copolymer self-assembly that allows facile and large-scale fabrication of three-dimensionally (3D) structured functional nanomaterials. We utilize finite difference methods to predict photonic and phononic wave transport characteristics in the nanomaterials. Our computational results show that 3D bicontinuous nanostructures provide new opportunities for efficient thermoelectric devices and topological photonics.

Resume : The metal-oxide-semiconductor (MOS) structure with an ultrathin oxide layer (MOS tunnel diode) constitutes an essential part of a broad family of more compound MOS tunnel devices and serves as a basic characterization tool of MOS physical systems. Tunneling though the insulator layer in MOS tunnel devices scaled to the nanometer range for nanoelectronics applications will be additionally affected by the coulomb blockade effect. This paper presents for the first time a model of the MOS tunnel diode including coulomb blockade. The issue of electric potential energy stored in the MOS system, which is essential for the coulomb blockade, is discussed in details with address to literature considerations. Differences between the MOS and MIM (metal-insulator-metal) tunnel diodes are discussed. They results from a potential drop in the semiconductor surface region and additional energy stored in it, and from effects of minority carriers existing in the semiconductor region. The current-voltage and capacitance-voltage characteristics of the MOS tunnel diodes are considered for different parameters of the MOS system and various temperatures. Conclusions are drawn concerning consequences of the coulomb blockade effect on electrical characteristics of the MOS tunnel devices. Acknowledgements: This work has been supported by The National Centre for Research and Development (NCBiR) under grant No. V4-Jap/3/2016 (?NaMSeN?) in the course of ?V4-Japan Advanced Materials Joint Call?.

Resume : Lithium ion batteries (LiBs) constitute a pivotal technology towards the ongoing energetic transition. Deeper understanding of the LiB operation principles can help at preventing failure mechanisms, decreasing their ageing and charging time, by ensuring high safety1. In this work, we revisit, by means of an advanced physical model coupled to experimental characterizations, the lithium transport mechanism inside graphitic electrode materials. Such a mechanism is traditionally modeled under the hypothesis of Fickean diffusion, for which we investigate here its limitations. First, the lithium intercalation/de-intercalation process is examined at a graphite particle level with an in house derived continuum model taking into account the atomistic interactions (Li-C, Li-Li, etc.) within a single plane and between planes. An analytical solution of the associated set of mathematical equations is obtained to quantify parameters crucially affecting the LiB performance, some of them fitted by using in house potentiostatic intermittent titration technique (PITT) experiments. Then, the advanced continuum model is scaled up into a cell level model to investigate the implications of our advanced theory on the response of the battery cells. We present results obtained with both pseudo 2D and a 3D cell2 models, highlighting the role of electrode anisotropy effects on the cell electrochemical performance. Finally, this study highlights the importance of using appropriate physicochemical models to correlate with experiments. References 1. A. A. Franco, RSC Advances, 3, 13027 (2013). 2. A. C. Ngandjong et al., The Journal of Physical Chemistry Letters, 8, 5966?5972 (2017).

Resume : Currently, there is no system that can predict the Proton Exchange Membrane rupture during the process of Electrochemical Hydrogen Compression (EHC), therefore, a fault analysis method has always been used; which also raises the costs, in addition to the existing risks when performing the tests in a real system. In this sense, in the present work it is proposed to use the finite element, thus predicting the membranes mechanical failures before being used in the EHC process. For the mechanical stress analysis in the EHC, we developed a linear elastic-plastic 2D model. The model includes the main components of an electrochemical compressor (membranes and flow plates). The stress and plastic deformation in the membrane were simulated using finite element from the mechanical properties obtained by tensile test in the membranes according to ASTM D882 and taking into account real operating conditions. Stress distributions were obtained using the multiple channel structure design. Experimental measurements of the membrane deformation were obtained using a variation of the differential pressure on the cathode compartment, as well as the chitosan percentage into membrane in order to evaluate the effect of this within the polymer matrix [1], [3].

Resume : The slow crack growth (SCG) behavior of butt-fused polyethylene pipe and polyamide pipe was investigated by dynamic fatigue method. In order to accelerate the fracture time, the experiment was carried out with the dynamic fatigue method and the notch was inserted. However, in the general field, foreign matter may be inserted into the fused portion during the fusing operation of the pipe. To illustrate this, The defect was inserted into the fused portion and the pipe was fused. The tensile test was carried out to confirm that the portion where the defect was inserted was fully welded. It was found that there is no significant difference between the fused part inserted with the defect and the normal fused part. First, an initial V-notch was inserted into the outer diameter of the fused portion of the pipe to confirm the fatigue fracture behavior. Next, the defect was inserted into the fused portion of the pipe, and the fatigue fracture behavior was confirmed. The fracture behaviors of the two types of initial cracks were compared.

Resume : Doping plays a crucial role in semiconductor physics where n-doping is controlled by the ionisation energy of the impurity relative to the conduction band edge. In organic semiconductors, efficient doping may be dominated by various effects, which are presently not well understood. Here, we study n-doping of prototypical C60 and ZnPc with dopants from different classes to understand their efficiency for generating free carriers [1]. We simulate the density of states of the doped systems in the density functional theory framework and calculate the Fermi level position. The results are compared to measurements from direct and inverse photoemission spectroscopy and we find that theoretical and experimental spectra agree very well. From these results, we extract relevant material parameters that influence the doping efficiency. We provide design rules for new efficient host:dopant combinations. We further correlate these results with measured conductivities and thus demonstrate the predictive power of our model in engineering charge transport. [1] C. Gaul et al. Nat. Mater. 17, 439-444 (2018).

Resume : The world’s most abundant copper mineral, chalcopyrite (CuFeS2), is difficult to dissolve during atmospheric leaching using traditional ferric sulfate lixiviants because of its unique physico-chemical properties and the resultant passivating surface products. To overcome this unfortunate circumstance, FLSmidth® has devised a novel approach, which utilizes (1) a surface pretreatment to “activate” the mineral particles and (2) a mechano-chemical Rapid Oxidative Leach (ROL) via a Stirred Media Reactor (SMRt) to mildly attack the particle surfaces. As a result, there is a reduction of surface passivation problems, and this process is able to achieve copper recoveries >97% in under 6 hours. The important contribution of the chemical preconditioning step is of special interest, and it constitutes the incorporation of a few mol% or less of copper (II) ions to dope the mineral and thereby activate chalcopyrite. Because this activation plays a major role in accelerating the leach kinetics, it is critical to understanding the associated phenomena and their contribution in the ROL process is important. To date, the project has employed a number of characterization techniques (XRD, SEM, TEM, and XPS) to study the activation process mechanism, all of which indicate a heavily strained crystal structure due to activation. This modified structure extends beyond the surface layer and into the bulk of the particle. The latest phase of the project involves correlating the experimental observables with density functional theory (DFT) calculations to relate physical changes in the mineral due to addition of copper. Through a better understanding of the mechanisms at play during the activation and subsequent leaching, methods to improve the process will be developed which maximize the efficiency of atmospheric leaching.

Resume : Grain boundaries (GBs) are an immanent components of crystal structure of the structural materials, such as e.g. Fe-Cr steels. Moreover, they have considerable influence on the materials properties, especially on the mechanical one. As a consequence of that, is necessary to investigate effect of radiation-induced defects on the GBs in order to a deeper understanding of the radiation damage. In current study, we investigated characteristics of bcc-Fe and Fe-Cr based tilt GBs interacting with point defects such as vacancy, self-interstitial (SIA, dumbbell-type), and interstitial atoms (He impurities). Several tilt GBs with the rotation axis along [100] and [110] directions were modelled with He impurities, vacancy, and SIA in Fe-Cr. Molecular dynamics (MD) simulations using the interatomic Fe-Cr-He embedded atom model potential were conducted, for twelve GBs, in order to investigate GBs energies, He segregation energies, and the weakening effect of He impurity for several Cr and He concentrations. Furthermore, spin-polarized density functional theory (DFT) calculations focused at two GBs, ?3(111) and ?5(210), allowed to deeper insights into GBs properties. For example, the DFT results show that the presence of He significantly influences the magnetic properties of the system in the relatively distant neighbourhood [1]. The fluctuation of magnetic moments, chemical potentials, formation and migration energies of point defects were studied as a function of distance from GB?s plane. Representative structures of GBs, with Cr content ranging 6-10%, generated using DFT-based Monte Carlo simulations [2] were used to analyse how parameters, such as alloy short-range ordering or local environment, effects on defects properties. [1]. M. R. Zemla, J. S. Wrobel, T. Wejrzanowski, D. Nguyen-Manh, K. J. Kurzydlowski, Nuclear Instruments and Methods in Physics Research Section B Beam Interactions with Materials and Atoms 393:118 (2017). [2]. J. S. Wrobel, D. Nguyen-Manh, M. Y. Lavrentiev, M. Muzyk, S. L. Dudarev, Physical Review B 91 (024108) 2015.

Resume : Black phosphorus (BP) has recently been received significant attention as anode materials for sodium-ion batteries (SIBs) due to the structure similar to graphite with AB stacking bound by van der Waals interactions. And, interlayer distance of BP is greater than that of the graphite (5.4 Å: black phosphorus vs 3.4 Å: graphite), which provides an easier intercalation channel of Na ion than in the graphite. However, BP has a fatal disadvantage of severe volume expansion during charge/discharge process, which is leading to low cycling stability. To overcome the drawback of BP, some researcher recently developed phosphorene/graphene heterostructure (P/G) with remarkable battery performance such as high specific capacity (2,440 mAh g-1) and cyclic life (83% capacity retention after 100 cycles).1 From a theoretical point of view, however, sodiation mechanism in atomic scale has not yet been clearly demonstrated. In this talk, we present an atomistic sodiation mechanism of P/G heterostructure investigated by first-principles calculations, and then compare with the sodiation of BP. Our theoretical study reveals that P/G heterostructure indeed shows a smaller volume expansion (30% lower than in the case of BP) without sacrificing the specific capacity during the sodiation process. In addition, graphene layer in P/G heterostructure provides enhanced electrical conductivity and facile diffusion channel for Na atoms in sodiated NaxP/G structures. Based on theoretical results, we clearly reveal that P/G heterostructures can be attractive anode materials for SIBs with the superior cycling properties and high rate capabilities. 1Jie Sun et al. Nature Nanotechnology 10, 980–985 (2015)

Resume : State-of-the-art theoretical calculations of optical spectra of atoms and small molecules reach accuracies that are often better than experimental resolution. This capability can be used, for instance, to identify rare species in the interstellar medium. The situation with optical spectra of deep defects is very different. On the one hand, these are systems of many electrons and many atoms that are inherently more difficult to treat. On the other hand, there are only a few deep defects with known atomic structure for which accurate spectroscopic data is available. The nitrogen-vacancy (NV) centre in diamond [1] is one such system. In this work we test the accuracy of state-of-the-art first-principles calculations [2] in describing absorption and luminescence lineshapes. We find that in order to obtain fine features in the spectra we have to use defect supercells reaching a few tens of thousands of atoms. This is achieved by using the embedding procedure to construct the dynamical matrix [3]. We also find that coupling to asymmetric e modes (Jahn-Teller effect) affects both the absorption and the luminescence lineshapes. We include this coupling by developing a methodology to treat the vibronic structure with many Jahn-Teller-active modes. Finally, we perform calculations for three different functionals (semi-local functional, hybrid functional with standard parameters, and hybrid functional with parameters set to comply with the Koopmans theorem). Our work thus benchmarks state-of-the-art first-principles calculations of optical spectra of defects and outlines the developments that are still needed. [1] M. W. Doherty et al., Phys. Rep. 528, 1 (2013). [2] C. Freysoldt et al., Rev. Mod. Phys. 86, 253 (2014). [3] A. Alkauskas et al., New J. Phys. 16, 073206 (2014).

Resume : Dynamics and thermodynamics of the interaction of carbon nanotubes with DNA and their effects on DNA properties The interaction of single-walled carbon nanotubes (SWNTs) with DNA is crucial for several nano-materials applications, including SWNT:DNA nanoscale devices or nanosized building blocks for use in nano-switches and nanoscale wiring. In biotechnological applications, modulating the DNA:SWNT interaction is important for SWNT purification, DNA recognition, and ultrafast DNA sequencing, and drug delivery. I will present a study of the the conformational equilibrium and the dynamics between B-to-A forms of double-stranded DNA adsorbed onto single-walled carbon nanotubes (SWNT) using free energy profile calculations based on all-atom molecular dynamics simulations. The potential of mean force of the B-to-A transition of ds-DNA in the presence of an uncharged (10,0) carbon nanotube for two dodecamers with poly-AT or poly-GC sequences is calculated as a function of a root-mean-square-distance metric quantifying the B-to-A transition. The calculations reveal that in the presence of a SWNT DNA favors B-form DNA significantly in both poly-GC and poly-AT sequences. Furthermore, the poly-AT DNA:SWNT complex shows a higher energy penalty for adopting an A-like conformation than poly-GC DNA:SWNT by several kcal/mol. The presence of a SWNT on either poly-AT or poly-GC DNA affects the free energy of the transition such that the B form is favored by as much as 10 kcal/mol. The presence of the marked structural differences between B- and A-DNA have consequence for understanding thermal and conduction properties of composites of nanotubes and DNA used in biomaterials applications.

Resume : One common use of graphene is as a substrate to deposit otherwise not fully stable objects such as nanoclusters. These can then be used in many applications such as catalysis, sensing and data storage. Methods to model the anchoring, structural changes and properties of nanoclusters on graphene as well as their interaction with defects in graphene are then needed. We present a multistep approach to predict and model behaviour of alkali earth oxide nanoclusters on graphene. First, the structures of these clusters were predicted in vacuo using an evolutionary algorithm and data mining coupled with density functional theory and interatomic potentials. We then worked on potentials based on DFT data to model the interaction of graphene with these nanoclusters which will be shown in detail. These include fundamental physical forces which are often omitted in empirical models. With keeping these first steps constant, further investigation can adapt to research questions by e.g. using molecular dynamics or further evolutionary algorithm searches which include the graphene surface, and some results of these further investigations are also presented.

Resume : Theoretical calculations of localized defect states in oxides using a self-interaction corrected density functional are presented. The hole formed near an Al-atom dopant in alpha-quartz has become an important test case for theoretical methodology since generalized gradient approximation (GGA) energy functionals as well as commonly used hybrid functionals, such as PBE0 and HSE06, fail to stabilize the localized hole due to the self-interaction error inherent in Kohn-Sham density functional theory. The present results show that variational, self-consistent calculations using the Perdew-Zunger self-interaction correction involving complex optimal orbitals [1] applied to GGA functional for a 72 atom cell subject to periodic boundary conditions can reproduce well the experimentally deduced lengthening (by 12%) of the Al-O bond to the O-atom where the hole resides as well as the energy of the defect state (calculated to be 1.9 eV above the valence band as compared with measured optical absorption peak at 1.8 eV) and the size of the band gap (9 eV in both experiment and calculations) [2]. Other systems that will be discussed include Li-dopant in MgO where again excellent results, in close agreement with experiments, have been obtained in self-interaction corrected GGA calculations while PBE0 and HSE06 fail to stabilize the localized hole [3]. [1] S. Lehtola, E.O. Jonsson, and H. Jonsson, The effect of complex-valued optimal orbitals on atomization energies with the Perdew?Zunger self-interaction correction to density functional theory, Journal of Chemical Theory and Computation 12, 4296 (2016). [2] H. Gudmundsdottir, E.O. Jonsson and H. Jonsson, Calculations of Al dopant in alpha-quartz using a variational implementation of the Perdew-Zunger self-interaction correction, New Journal of Physics 17, 083006 (2015). [3] E.O. Jonsson et al. (in preparation).

Resume : First wall of fusion reactor is classified as plasma facing material which undergoes the neutron irradiation and needs to withstands all consequent effects including radiation swelling. One of the candidate group of materials for such application are high entropy alloys (HEA), in which the high configurational entropy results in inhibition of the formation of brittle intermetallic phases in favor of multicomponent random solid solutions with very unique properties. Fe-Cr-Mn-Ni HEA sample produced in Oak Ridge National Laboratory has shown improved irradiation swelling resistance compared to conventional single phase Fe-Cr-Ni austenitic alloys [1]. Our research focuses on the phase stability of quaternary magnetic Fe-Cr-Mn-Ni alloy group in whole concentration range with the goal of finding the optimal composition of the alloy as a candidate for the first wall of fusion reactor. Density functional theory simulations have been conducted in order to develop the Cluster Expansion (CE) model for Fe-Cr-Mn-Ni system. Different magnetic configurations have been constructed in order to represent the magnetic alloy behavior. Phase stability at 0K has been analyzed from the points of view of formation and mixing enthlapies. The phase stability at elevated temperatures has been investigated using the Monte Carlo simulations with effective cluster interactions derived from CE method. The short-range order parameters have been investigated as a function of temperature and concentration of constituents. The analysis of order-disorder transition (ODT) temperatures for the whole range of concentrations has enabled the indication of regions with lower ODT temperatures than that observed for the Oak Ridge National Laboratory sample. New stable quaternary ground state has been theoretically predicted. [1] N.A.P. Kiran Kumar et al., Acta Materiala 113 (2016), 230-244

Resume : The single electron transistor (SET) is receiving a lot of research attention as a possible solution for ultra-low powered electronics. So far, manufacturing processes only exists for operation at cryogenic temperatures. For room temperature (RT) operation, it’s necessary to manufacture a tiny (<5nm) Si nano dot with tunnel barriers to electrodes lower then 2nm. The challenge is the synthesis of the needed structures, what can only be realized by a bottom-up process of self-assembled Si dots. Simulation of such fabrication steps needs to take into account effects like diffusion on an atomic level. One very powerful simulation tool for this purpose is the kinetic Monte Carlo method [1], which proved to be predictive, fast and very flexible in the past [2]. For the tasks of our SET manufacturing approach, we replace the atomic Kawasaki exchange by a bond-switching concept, which increases the speed (up to 4 times) without having drawbacks of parallelized versions [3]. Here we will present our approach, performance comparison and apply simulation to phase separation, oxidation etc. The developed methodology can easily be applied to simulate other systems, even in much larger spatial-temporal scale. This work has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 688072. • [1] M. Strobel et al., PRB 64, 245422 • [2] T. Müller et al., APL, 2004 • [3] J. Kelling et al., EPJ ST, 2012

Resume : Crystal structure prediction (CSP) is an field of computational materials science involving artificial intelligence methods to tackle a complex problem of finding the most favorable forms of condensed matter. The evolutionary algorithm (EA) approach combined with density functional theory (DFT) can be successfully used to numerically achieve the goal of finding the "fittest" crystal with given chemical composition. Some implementations of the CSP software using the EA/DFT approach exist [1, 2]. Especially the USPEX code showed to be successful, e.g. in the prediction of the structure of superhard and partially ionic phase of bulk boron [3]. However, the domain of CSP of nanowires (NWs) seems to be still less explored. In this work, we present a currently developed software tool, which initial aim is solving the problem of finding the most stable form of all-boron and boron related NWs via the EA/DFT approach. Therefore we will show: i) how EA techniques can be applied to CSP of NWs by providing examples of recombination/mutation operators and ii) the status of our tool and the road map of development. In the future, we are planning to release our software as an open source package. We gratefully acknowledge support of National Science Centre under grant number UMO-2016/23/B/ST3/03575.

Resume : Modeling electrochemical processes and defects in semiconductor materials through constant Fermi-level ab-initio molecular dynamics Assil Bouzid and Alfredo Pasquarello In the first part of this talk, we focus on modeling electrochemical processes at metal electrode interfaces through the recently developed constant Fermi-level ab-initio molecular dynamics. This technique allows one to simulate systems evolving at constant Fermi energy by controlling the charge transfer between the system and an electron reservoir set at a given potential during the dynamics.[1?3] Based on the constant Fermi-level molecular dynamics and a proper band alignment scheme, we present a new technique to simulate metal/water interfaces under variable bias potential referenced to the standard hydrogen electrode (SHE). Our band alignment scheme yields an accurate SHE level to which the bias potential is referenced. Based on this alignment scheme, we find a potential of zero charge ?_{pzc} ?= 0.22 eV relative to the SHE and a double layer capacitance C_{dl} ? 19 ?Fcm^{?2} in excellent agreement with experimental measurements at the Pt(111) electrode. In addition, we present results related to the Volmer reaction mechanism and the double layer organization at the Pt(111)/water interface as a function of bias potentials ranging from ?0.92 eV to +0.44 eV. [4] In the second part, we show how the computational framework of the constant Fermi-level molecular dynamics can be used as a tool to reveal in a computer-aided way technologically relevant semiconductor defects, which would be difficult to identify through conventional theoretical studies. In this scheme, the Fermi level can be set at any position within the band gap during the defect generation process. Hence, majority defects in varying charge states could be generated depending on the position of the Fermi level in the band gap. This scheme is illustrated in the case of GaAs, and InGaAs/oxide interface [2,3] and relevant defects will be discussed. [1] Assil Bouzid and Alfredo Pasquarello, J. Chem. Theory Comput. 13, 1769 (2017). [2] Assil Bouzid and Alfredo Pasquarello, Phys. Rev. Appl. 8, 014010 (2017). [3] Assil Bouzid and Alfredo Pasquarello, J. Phys.: Condens. Matter 29, 505702 (2017). [4] Assil Bouzid and Alfredo Pasquarello, J. Phys. Chem. Lett. 9, 1880 (2018).

Resume : The aggressive device scaling required to fulfill the requirements of the International Roadmap for Devices and Systems (IRDS) [1] calls for even more advanced simulation tools to describe realistically ultra-scaled device components. Atomistic simulations play a central role in this scenario, as they enable the parameter-free description of new materials and complex physical processes with a level of realism unattainable with conventional technology computer aided design (TCAD) tools. In this talk, the QuantumATK [2,3] software suite will be presented. QuantumATK is an industry-proven platform for atomic-scale modeling of semiconductor materials, nanostructures and nanoelectronic devices. It combines a professional graphical user interface to setup, run and analyze atomistic simulations, with a selection of state-of-the-art atomistic methods, which can be used in a multi-model fashion to achieve complex workflows. In this contribution, several applications of QuantumATK in the context of semiconductor materials and devices will be covered. Initially, the impact of doping on the current-voltage characteristics of truly semi-infinite metal-semiconductor interfaces [4], and of Fermi-level pinning on the transconductance of gated 2D-FET devices made of vdW heterostructures [5], will be discussed. It will then be demonstrated how the QuantumATK multi-model approach allows one to include electron-phonon coupling effects in atomistic large-scale TFET devices [6] to calculate efficiently the electrical properties bulk materials. The use of QuantumATK in the field of spintronics will also be highlighted, in relation to the understanding of the spintronic effects at the interface between topological insulators and ferromagnetic metals [7]. [1] irds.ieee.org; [2] www.synopsys.com/silicon/tcad/quantumatk.html; [3] Stokbro et al., The QuantumATK atomic-scale modelling toolbox, in preparation; [4] Stradi et al., Physical Review B 93, 155302 (2016); [5] Szabo et al., IEEE Electron Device Letters 36, 514 (2015); [6] Gunst et al., Physical Review B 96, 161404 (2017); [7] Marmolejo-Tejada et al., Nano Lett. 17, 5626 (2017).

Resume : In this work, charge mobility was studied with both analytic theory and computer simulation. The mobility in molecular crystals exhibits anisotropy and power-law temperature dependence. We developed an analytic expression to account for the anisotropic mobility observed in molecular crystals. The charge mobility is related to the friction coefficient Drude model, and the latter is expressed in terms of force-force correla- tion function through the fluctuation-dissipation theorem, calculated with a polaron model Hamiltonian. With electronic couplings and reorganization energies derived from first-principle calculation, the temperature dependence of mobility follows a mild power law, μ ∝ T ^−1 (150 − 300K). The anisotropy of mobility was calculated as the ratio of mobility of the fast-axis over slow-axis, μa/μb, was about 30 in a pure point-to-point probe. Meanwhile, we have also observed that systems with large intermolecular electronic coupling (charge transfer integrals) do not necessarily lead to large mobility. We studied several experimentally reported systems and study the effect of delocalized polaron. With Monte Carlo simulation, we showed that large electronic coupling may lead to delocalized polaron, which in terms reduced mobility by its lower energy traps, and lower coupling strengths to the neighbors.