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Theory and simulation in physics for materials applications

The recent experimental and technological advances have allowed for a better description of materials physical properties implying multiple technological applications. However, those advances also open multiple questions that require theoretical interpretations. The goal of the present symposium is to present to European Materials Community the most recent advances in theoretical and simulation methods to characterize materials.


Theory and simulations in Physics represent nowadays an important contribution to the European Materials Research. Indeed, theoretical characterization from either first principles or approaches that are more empirical are likely to provide important information in parallel to the experimental determinations. In that respect, the scope of this symposium aims at exploring the wide range of theoretical methods developed in the recent years. An important part will be devoted to theoretical and numerical developments to overcome nowadays-physical challenges. Those approaches range from atomic level and first principles methods to mesoscopic physics through tight-binding models and molecular dynamics simulations. In addition, we will consider Quantum Monte-Carlo or hybrid QM/MM methods for larger or biological systems. In parallel, machine-learning algorithms for material screening would give a nice opening on future methodological perspectives in Material Science. Regarding the physical properties, the symposium will focus on standard material properties like structure, electronics, optics or magnetism, thermodynamics, but also on more specific aspects like electronic, spin or heat transport, thermoelectricity, renewable energies and energy harvesting and storage, electron-phonon coupling, mass transport, phonon dynamics. Another important part of the symposium should be devoted obviously to applications in Material Science. Hence, another goal of the symposium is to present a general overview of theory and simulation contribution in the field. For example, we will consider applications in nanosciences and nanostructure materials, bulk, surfaces and interfaces, disordered and low-dimensional materials including graphene and bi-dimensional materials, organic molecules on metallic or oxide surfaces, magnetic and spin cross-over molecules, self-assembled molecular networks, and biological molecules. Applications for future electronics also play an important role in the material community. Therefore, the symposium will be opened to nanoelectronics and molecular electronics and spintronics. In summary, this symposium will provide a wide and unique state of the art overview on the theoretical methods used to describe and characterize materials properties. It aims at having equilibrated contributions from important researchers in the community and young researcher to favor discussions and exchange, and draw some perspectives on the next challenges in the field.

Hot topics to be covered by the symposium:

  • methods and developments in first principle and semi-empirical methods
  • graphene and 2D materials
  • molecular electronics and spintronics, magnetism
  • electronic transport and devices simulations, optical properties
  • electron-phonon coupling, thermoelectricity
  • metal/organic interfaces and framework
  • biological molecules and QM/MM simulations
  • thermodynamics
  • renewable energies and storage
  • mass and heat transport
  • surfaces and interfaces
  • disordered and hybrid organic-inorganic materials

Scientific committee:

  • Tim Frolov, Lawrence Livermore National Laboratory, USA
  • Giorgos Evangelakis, University of Ioannina, Greece
  • Mebarek Alouani, IPCMS, Strasbourg University, France
  • Xavier Blase, Neel Institute, Grenoble, France
  • Daniele Passerone, Empa, Switzerland
  • Hélène Zapolsky, Group of Material Physics, University of Rouen Normandy, France
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Low-dimensional, disordered, and hybrid organic and inorganic materials I : Y. Dappe
Authors : M. Hermanowicz, M.W. Radny
Affiliations : Institute of Physics, Poznan University of Technology, Piotrowo 3, 60-965 Poznan, Poland; Institute of Physics, Poznan University of Technology, Piotrowo 3, 60-965 Poznan, Poland, School of Mathematical and Physical Sciences, University of Newcastle, Callaghan NSW 2308, Australia

Resume : MXenes ? two-dimensional transition metal carbides and nitrides, exfoliated from MAX phases and described by the chemical formula of M(n+1)X(n), n=1-3, have been extensively studied since their discovery in 2011 [1]. The interest in those 2D materials stems from their wide spectrum of electronic and spin properties. They can be engineered to be metallic, semiconducting or, in some cases, exhibit topological features upon proper functionalisation. In this work, a review of structural and electronic properties of materials belonging to this class is given with a focus on possible ways of functionalisation. This includes interfacing, adsorption, external fields as well as mechanical deformation. The model MXenes have been investigated computationally by means of density functional theory within generalised gradient approximation and are discussed in the context of recently published reports. [1] M. Khazaei, A. Ranjbar, M. Arai, T. Sasaki, S. Yunoki, J. Mater. Chem. C, 2017, 5, 2488. The work has been performed under the Project HPC-EUROPA3 (INFRAIA-2016-1-730897), with the support of the EC Research Innovation Action under the H2020 Programme; in particular, MH gratefully acknowledges the support of Dr Stephan Mohr and the computer resources and technical support provided by Barcelona Supercomputing Center. The work has been supported by Polish Ministry of Science and Higher Education (Project No. 06/62/DSPB/2183) and by Poznan Supercomputing and Networking Center (PSNC).

Authors : Ioan Andricioaei
Affiliations : Professor of Chemistry and Professor of Physics University of California Irvine Irvine, California 92697, USA

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.

Authors : Zengqiang Zhai, Olivier Lame, Michel Perez, Julien Morthomas, Claudio Fusco
Affiliations : Zengqiang Zhai; Olivier Lame; Michel Perez; Julien Morthomas; Claudio Fusco

Resume : Abstract: Semicrystalline polymers are used in a broad range of applications. It is largely agreed [1,2] that the mechanical properties of semicrystalline polymers are mainly governed by the presence of molecular connections between the crystallites, namely, chain entanglements and tie molecules (TMs). However, due to the experimental limitation, no quantitative data for the TM concentration has been reported [3]. It is known that both crystallization conditions as well as polydispersity strongly influence the concentration of tie molecules, but no direct experimetal observation is available. Molecular Dynamics (MD) simulation is an excellent tool to overcome the difficulties of experimental measurements and can be used to study the non-equilibrium process of crystallization at the nanoscale. A great deal efforts have been made both experimentally and theoretically to explore the crystallization mechanism of polymers, however, very few of which involved polydispersity [4, 5]. As a matter of fact, the occurrence of polydisperse aggregates is much more common than the existence of pure compounds in a broad spectrum of natural products. In this work, we performed massive MD simulation of seven bidisperse polymer systems with various PDI (polydispersity index), which consist of the same total number of beads (i.e. 100,000), using the algorithm of radical-like polymerization (RLP) [6]. We used a coarse-grained polymer model where polymer chains consist of “beads” representing few structural units, around five to ten carbon groups. The model is based on two potentials: Intra-chain interactions of bonded beads are given by a Finite-Extensible Non-linear Elastic (FENE) potential, all other interactions are modelled by a simple Lennard-Jones (LJ) potential. Isothermal crystallization has been employed, and the mechanism of crystallization has been investigated. We developed a tool to access quantitative analysis of molecular topology. It has been found that the PDI influences the nucleation and growth of crystallites, and then would determine the rate of crystallization and crystallinity. Moreover, the molecular topology also exhibits a dependence on the composition of the disperse systems. Figure 1 shows a snap shot of one of the systems, which looks very much like lamellae. References: 1. Humbert, S.; Lame, O.; Séguéla, R.; Vigier, G. Polymer 2011, 52, 4899-4909. 2. Humbert, S.; Lame, O.; Chenal, J. M.; Rochas, C.; Vigier, G. Journal of Polymer Science Part B: Polymer Physics 2010, 48, 1535-1542. 3. Seguela, R. Journal of Polymer Science Part B: Polymer Physics 2005, 43, 1729-1748. 4. Sliozberg, Y. R.; Kröger, M.; Chantawansri, T. L. The Journal of Chemical Physics 2016, 144, 154901. 5. Li, S.; Register, R. A.; Weinhold, J. D.; Landes, B. G. Macromolecules 2012, 45, 5773-5781. 6. Perez, M.; Lame, O.; Leonforte, F.; Barrat, J.-L. The Journal of chemical physics 2008, 128, 234904.

Low-dimensional, disordered, and hybrid organic and inorganic materials II : M. Hermanowicz
Authors : Abdus Samad, Mohammad Noor-A-Alam, Young-Han Shin
Affiliations : Department of Physics, University of Ulsan

Resume : Since the discovery of graphene, two-dimensional layered materials (e.g. graphene, h-BN, silicene, etc.) have attracted a great deal of attention. However, because of their inversion symmetry (in graphene and silicene) or three-fold rotation symmetry (in h-BN monolayer), those monolayers are not intrinsically polar. Recently we found that an out-of-plane dipole moment as well as piezoelectricity could be induced in those honeycomb lattices merely by chemical functionalization of their surfaces with H and F to break the flatness as well as inversion symmetry. However, the polarization switching in those structures might be practically impossible without breaking any bonds. In order to induce and reverse polarization in two dimensions, we study the substituion with Mn atoms in graphene and the buckled structure of honeycomb binary monolayers such as MoC, WC, WS, and WSe by using density functional theory calculations. The stability of these monolayers is confirmed from formation energies and stable lattice vibrations. The energy difference between the flat and the buckeld MoC is 0.23 eV and the energy barrier from the buckled MoC to the flat MoC is 0.55 eV. Since these energies in two dimensions are much subject to strain, we expect to control the energy barriers by choosing proper bottom substrates. These monolayers also show high in-plane elastic stiffness (C11 for WS is 154.4 N/m) and high spontaneous polarization (e22 for MoC and WC are three times larger than that of monolayer MoS2).

Authors : S. Osella, B. Trzaskowski
Affiliations : Chemical and Biological Systems Simulation Lab, Centre of New Technologies, University of Warsaw, Banacha 2C, 02-097 Warsaw, Poland

Resume : In the last five years, a lot of research effort has been devoted to the creation of hybrid materials which change the electronic properties of one constituent by changing the optoelectronic properties of the other one. The most appealing approach consists on the interaction between organic materials or metals with biological system such as proteins or DNA. Although experimental efforts have already resulted in the formation of a number of stable hybrid bio-organic materials, the main bottleneck of this research field is the formation of the interface between the biological part and the organic/metal one. In particular, the efficiency of the final devices is very low due to problems with the interfacing of such different materials, charge recombination at the interface and the high possibility of losing the function of the biological component which leads to inactivation of the device. Here, we present a multiscale computational design which allow the study of complex interfaces for stable and highly efficient hybrid materials for biomimetic application. In particular, we focus on the use of graphene as organic material/metal and light harvesting protein complexes (Photosystem I) as biological counterpart, linked together via a self-assembly monolayer (SAM) and a biological linker (cytochrome C) to allow flexibility of the whole system, in order to create novel biomimetic materials for solar-to-fuel, bio-transistors or bioorganic electronic applications.

Authors : David Mora-Fonz, Alexander L. Shluger
Affiliations : University College London, University College London

Resume : Amorphous zinc oxide (a-ZnO) is a very important transparent conducting oxide. One of the possible applications of a-ZnO is as a semiconductor layer in thin-film transistors, which are key components in panel displays. Moreover, a-ZnO is the base of very important amorphous electronic materials, such as ZnSnO, InGaZnO, InZnO. We report a detailed theoretical study of the a-ZnO. We provide information on the atomic and electronic structure of a-ZnO spanned across several amorphous structures. More than 500 a-ZnO structures were created using highly accurate interatomic potentials (IP) and a molecular dynamic melt and quench technique. We analyse fourteen cooling rates (0.75-800 K/ps) and eight cell sizes (containing 96-768000 atoms). 90 amorphous structures in total, consisting of 96 and 324 atoms, were subsequently fully re-optimised using a hybrid DFT approach and tested for electron and hole trapping. Our IP amorphous structures show an excellent agreement with hybrid DFT calculations. We tested our hybrid DFT functional by calculating the localised states of the well-known ZnO:LiZn defect, its absorption spectrum and its thermodynamic transition level (Li-O)-/0. Experiments and theoretical studies have shown that ZnO does not have affinity for electron or hole trapping. In agreement with our inverse participation ratio analysis, hybrid DFT calculations for ZnO show that charge trapping is not stable. In the amorphous configuration, however, we show that hole trapping processes take place due to under-coordinated oxygen ions. Trapping energies range from 0.57 to 1.30 eV, with an average of 0.87 eV. We also show that our qualitative results do not change using two different amount (25% and 37.5%) of HF exchange. Electron trapping remains unstable in a-ZnO.

Authors : Puja Adhikari, Mo Xiong, Neng Li, Xiujian Zhao, Paul Rulis, Wai-Yim Ching
Affiliations : Mo Xiong, Neng Li, Xiujian Zhao State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, Wuhan, 430070, P.R. China. Puja Adhikari, Paul Rulis, Wai-Yim Ching Department of Physics and Astronomy, University of Missouri-Kansas City, Kansas City, MO 64110, USA

Resume : Zeolitic imidazolate frameworks (ZIFs) are a rapidly emerging class of versatile porous material with many potential applications. Here, we report the construction of an amorphous ZIF (a-ZIF) model from a near-perfect continuous random network model of a-SiO2. The radial distribution function is in good agreement with measurements for amorphous atZIF-4 but with notable fine differences. The electronic structure and properties of the a-ZIF model are critically compared with those of three crystalline ZIF phases, ZIF-4, ZIF-zni, and ZIF-8, using density functional theory methods. We confirm the retention of the metal tetrahedral bonding coordination in a-ZIF and the nearly identical short-range ordering found in crystalline ZIFs. The considerable Zn−N bond strength plays a key role in retaining the tetrahedrally bonded network structure. The calculated optical properties of a-ZIF show a complex absorption spectrum with an ultralow refractive index n of 1.327 and a plasmon frequency of 15.810 eV.

Mass and heat transport I : V. Fiorentini
Authors : J. Betlej, P. Sowa, R. Kozubski, G.E. Murch, I.V. Belova
Affiliations : M. Smoluchowski Institute of Physics, Jagiellonian University, Lojasiewicza 11 30-348, Krakow; Centre for Mass and Thermal Transport in Engineering Materials, School of Engineering, The University of Newcastle, Callaghan, Australia

Resume : Vacancy-mediated self-diffusion of A- and B-elements in ?triple-defect? B2-ordered ASB1-S binaries was simulated by means of a kinetic Monte Carlo (KMC) algorithm involving atomic jumps to nearest-neighbour (nn) and next-nearest-neighbour (nnn) vacancies. The systems were modelled with an Ising Hamiltonian with nn and nnn pair inteactions completed with local-configuration-dependent migration barriers. The simulated self-diffusion ran in systems showing equilibrium configurations and temperature-dependent vacancy concentrations generated by means of Semi Gand Canonical MC (SGCMC) code. The KMC simulations yielded temperature- and composition-dependent A- and B-atom diffusivities, whose relationship inverted at S < 0,5 ? an effect observed experimentally in Ni-Al intermetallics. The atomistic origins of the phenomenon, as well as of other features of the simulated self-diffusion such as temperature and composition dependences of the correlation factors and activation energies were thoroughly analysed in terms of a number of nanoscopic parameters tunable and monitorable exclusively with atomistic simulations. Roles of diverse factors in the generation of the observed features was elucidated by clearly distinguishing between the equilibrium and kinetic ones.

Authors : Nadège Meyer, Jean-François Wax, Hong Xu
Affiliations : Laboratory LCP - A2MC, Jean Barriol Institute FR-CNRS2843 University of Lorraine, 1 Bd Arago 57070 Metz, France

Resume : A series of investigations has been carried out to calculate accurately the shear viscosity of liquids and liquid mixtures, by means of equilibrium molecular dynamics simulations, via the Green-Kubo relation. Systems studied include alkali metals and alloys, as well as model fluids of Lennard-Jones and binary mixtures. These extensive studies lead to new and simple empirical laws which describe the temperature and density dependence of the shear viscosity of these liquids [1-3]. Furthermore, a simple mapping approximation allowed to predict the viscosity of mixtures from that of an effective one-component fluid [3]. This idea is tested on some real alloys [4]. Last, the validity of the Stokes-Einstein relation, relating the diffusion coefficient and the viscosity, has been examined and confirmed over a wide range of thermodynamic region [1-3]. A possible simple extension of this relation, from pure liquids to mixtures, has been proposed and tested [3]. [1] N. Meyer, H. Xu, and J.-F. Wax, Phys. Rev. B 93, 214203 (2016) [2] N. Meyer, H. Xu, and J.-F. Wax, Phys. Rev. B 96, 094201 (2017) [3] N. Meyer, J.-F. Wax and H. Xu, J. Chem. Phys. 2018 (accepted) [4] N. Meyer, H. Xu, and J.-F. Wax, (2018, submitted)

Authors : Elena V. Levchenko (1) and Alexander V. Evteev (1,2)
Affiliations : (1) School of Mathematical and Physical Sciences, The University of Newcastle, Callaghan, NSW 2308, Australia; (2) School of Engineering, The University of Newcastle, Callaghan, NSW 2308, Australia

Resume : In this contribution, we present a theoretical treatment of the diffusion kinetics in a binary melt carried out in the framework of the Mori-Zwanzig formalism of statistical mechanics [1]. Using analysis based on the generalized Langevin equations for the atomic velocities of species and the interdiffusion flux, we establish relationship between the Onsager coefficient for mass transport and two self-diffusion coefficients of species in a binary melt. The derived relationship naturally accounts for manifestation of microscopic (dynamic) cross-correlation effects in the kinetics of collective diffusion. We demonstrate application of our analysis for interpretation of results of molecular dynamic simulation of diffusion properties of Ni-Al and Cu-Ag melts. We consider Ni-Al and Cu-Ag melts as two opposite cases of binary melts with mixing and demixing tendencies, respectively. Acknowledgements: EVL acknowledges the support of the Australian Research Council (Grant ARC DP170101812). References: [1] E.V. Levchenko, A.V. Evteev, Insight into interrelation between single-particle and collective diffusion in binary melts, Physica A: Statistical Mechanics and its Applications 490 (2018) 1446-1453.

Authors : Andreas Kromik, Elena V. Levchenko, Carlo Massobrio, Alexander V. Evteev
Affiliations : School of Engineering, The University of Newcastle, Callaghan, NSW 2308, Australia; School of Mathematical and Physical Sciences, The University of Newcastle, Callaghan, NSW 2308, Australia; Institut de Physique et Chimie des Matériaux de Strasbourg, Université de Strasbourg, CNRS, UMR 7504, F-67034 Strasbourg, France; School of Engineering, The University of Newcastle, Callaghan, NSW 2308, Australia; School of Mathematical and Physical Sciences, The University of Newcastle, Callaghan, NSW 2308, Australia;

Resume : In this work, we advance a concept for understanding of mass transport in binary melts with the aid of molecular-dynamics assisted databases. As a result, we present for the first time a comprehensive, accurate and self-consistent database of diffusion properties of Ni-Zr melts which is generated a semi-empirical many-body interatomic potential. The reliability of the employed model description of Ni-Zr melts is carefully confirmed against fragmentary experimental data. Statistical mechanical insight into interrelation between single-particle and collective diffusion in a binary melt [1] is further elaborated to understand the cross-correlation between the interdiffusion flux and the force caused by the difference in the average random accelerations of atoms of different species in the short time limit t→0. On this basis, it is found that in the composition range 0.25≲c_Ni≲0.5 both single-particle and collective diffusion dynamics slow down homogeneously upon undercooling of Ni-Zr melts. Furthermore, it is inferred that such homogeneous dynamical slowdown leads to the enhanced glass forming ability of Ni-Zr alloys within this composition range. [1] E.V. Levchenko, A.V. Evteev, Insight into interrelation between single-particle and collective diffusion in binary melts, Physica A: Statistical Mechanics and its Applications 490 (2018) 1446-1453.

Mass and heat transport II : E. Levchenko
Authors : M. B. Maccioni, R. Farris, V. Fiorentini
Affiliations : Dipartimento di Fisica, Università di Cagliari

Resume : The lattice thermal conductivity of the candidate thermoelectric material Mg$_3$Sb$_2$ is studied from first principles, with the inclusion of anharmonic, isotope, and boundary scattering processes. We find that the purely anharmonic conductivity is over an order of magnitude larger than observed experimentally at room temperature; the anomalously low observed conductivity is in fact due to microstructure, i.e. to grain boundary scattering of phonons due to polycrystallinity. Anisotropy is also significant due to the layered structure, as is isotopic scattering, due to the unusual isotopic composition of Mg and Sb; both affect the conductivity at the level of 10\% at room temperature.

Authors : I. Deretzis 1) , K. Huet 2), S. F. Lombardo 1), A. La Magna 1)
Affiliations : 1) CNR-IMM, Catania, Italy 2) Screen-Lasse Paris, France

Resume : Ultra localized annealing techniques, using laser sources, are necessary where heating processes need nanoscale resolution for a plethora of applications in the nano-technology field. Anyhow, the application of such techniques is often hindered by the difficulties in the process control and understanding. In particular, the process conditions often fall in a regime where Fourier law is questionable and the heat propagation should be studied and simulated in term of phonon transport. In order to correctly design processes and control experiments accurate modelling is needed, especially when the reference systems are complex 3D systems made by objects with size in the nm range made of different materials. We present a computation tool developed for the simulation of localized heating and eventual melting processes. The code is designed to efficiently simulate 3D structures with TCAD capabilities for the design of the system. Thermal transport in critical regions (intermediate and large Knudsen number) is ruled by advanced models (derived by the Boltzmann transport equation) whilst in regions with small Knudsen number is ruled by the conventional Fourier law. Heating can be also evaluated by means of self consistent solutions of the Maxwell equations where optical constants are functions of the local average phonons' energy (temperature in quasi-equilibrium conditions). A preliminary calibration is provided for given materials/phases. Applications of the simulations to realistic cases will be discussed and compared with experimental data in order to demonstrate the potentiality of the method.

Authors : Tang-Yu Lai, and Te-Hua Fang*
Affiliations : Department of Mechanical Engineering, National Kaohsiung University of Science and Technology, Kaohsiung 80778, Taiwan.

Resume : The mechanical properties and thermal conductivity of quintuple layers of Bi2Te3 nanofilms are studied by using non-equilibrium molecular dynamics simulation. The interface and and defects on the deformation mechanism and thermal conductivity are achieved. The results indicate that the Bi2Te3 nanofilmse has the ideal Young's modulus and thermal conductivity. The size effect on the mechanical and thermal characteristics are observed . As the interface changed, the structural disorder of atomic arrangement affected the mechanical properties; moreover, the phonons encounter lattice disordered atomic region will produce scattering reduce heat conduction. The results of this investigation are helpful for the potential applications of Bi2Te3 films as thermoelectric devices and energy generation .

Poster session 1 : Y. Dappe, E. Levchenko & G. Ori
Authors : Barbara Pieczyrak, Leszek Jurczyszyn, Pavel Sobotik, Ivan Ostadal, Pavel Kocan
Affiliations : (1,2) Instytut Fizyki Doswiadczalnej, Universytet Wroclawski, pl. Maksa Borna 9, 50-204 Wroclaw, Poland; (3-5) Charles University, Faculty of Mathematics and Physics, Department of Surface and Plasma Science, V Holesovickach 2, 180 00 Praha 8, Czech Republic

Resume : We present a study of two types of atomic defects predicted to dope the Rashba-type spinsplit electronic system of the Tl/Si(111)-1x1 surface - Si-induced vacancies and defects associated with the presence of extra Tl atoms. Structural calculations based on density functional theory (DFT) con firm the stability of the model of the defect induced by an extra Si atom and missing of seven Tl atoms, which was proposed in earlier experimental study. We have found that the electronic structure of such system is in agreement with the experimental data reported earlier. The calculated spatial charge distributions indicate enhancement of the charge around an extra Si atom, which correctly restores topographies of the corresponding scanning tunneling microscopy images while distributions of local density of states of this system correspond to the scanning tunneling spectra. DFT structural calculations let us determine an atomic structure of the defect caused by the presence of an extra Tl atom. The calculated spatial charge distributions show a ring-like feature around the extra Tl atom. The obtained results indicate a charge transfer from the central extra Tl atom to its nearest surrounding, which is in the agreement with earlier spin- and angle- resolved photoemission measurements.

J.PS 1.1
Authors : Ali reza Sasani, Jorge Iniguez, Eric Bousquet
Affiliations : University of Liege, University of Luxembourg; Luxembourg Institute of Science and Technology; University of Liege

Resume : The rare-earth orthoferrites (RFeO3, with R = rare-earth) were discovered in the 1940’s and quickly attracted great interests due to their unique magnetic properties. Recently, there has been a renewed interest in these materials following observation of multiferroicity. Due to the presence of two magnetic sub-lattices and their non-trivial interactions, interesting magnetic structures and magnetic phase transitions are observed i.e. spin reorientation, magnetization reversal, weak ferro-magnetism and magnetoelectricity. Studying and understanding the microscopic origin of these different properties is of great importance to design and optimize their useful responses and to get more fundamental understanding of the underneath physics. Here, we present a first- and second-principles study based on density functional theory (DFT) where we map the DFT calculations to a full Heisenberg Hamiltonian including superexchange, Dzyaloshinskii-Moria (DM) and single ion anisotropy interactions. We also perform spin dynamics to take into account the temperature effects and to calculate Neel temperature and understand the magentization reversal. We also scrutinize the DM interactions in the orthorhombic Pnma structure to find the relation between the different structural distortions and the resulting non-collinear magnetism for each of the sub lattices.

J.PS 1.2
Authors : L.L. Patera, F. Bianchini, C. Africh, C. Dri, G. Soldano, M.M. Mariscal, M. Peressi, G. Comelli
Affiliations : Department of Physics, University of Trieste, Italy; IOM-CNR Laboratorio TASC, Italy; SMN - Centre for Mat. Science and Nanotech., Dept. of Chemistry, University of Oslo, Norway; INFIQC, CONICET and Universidad Nacional de Córdoba, Argentina; IOM-CNR DEMOCRITOS National Simulation Center, Trieste, Italy

Resume : Single adatoms are expected to participate in many processes occurring at solid-gas and solid-liquid interfaces, such as the growth of graphene on metal surfaces. We demonstrate, both experimentally and theoretically, the catalytic role played by single metal adatoms during the synthesis at technologically relevant temperatures (? 700 K) of graphene flakes on Ni(111), mostly characterized by an epitaxial top/hollow-fcc registry with the substrate [1]. Single Ni atoms, diffusing on the metal surface, are temporarily trapped at kink sites along the graphene flake edges and facilitate the incorporation of new C atoms in the graphene network, which thus grows by ordered addition of couples of carbon rows parallel to the edge (Fig. 1). Scanning tunneling microscopy (STM) imaging at the millisecond time scale allowed us to identify the edge structure [2] and individual Ni adatoms, directly capturing their catalytic action. Force-field molecular dynamics (MD) and ab-initio density functional theory (DFT) calculations rationalize the experimental observations, giving a complete description of the growth pathways. Our results unveil the mechanism ruling the activity of a single atom catalyst at work [3]. 1. Bianchini F., Patera L.L., Peressi M., Africh C., Comelli G., J. Phys. Chem. Lett. 5, 467 (2014). 2. Patera L.L., Bianchini F., Troiano G., Dri C., Cepek C., Peressi M., Africh C., Comelli G., Nano Lett. 15, 56 (2015). 3. Patera L.L., Bianchini F., Africh C., Dri C., Soldano G., Mariscal M.M., Peressi M., Comelli G., Science 359, 1243 (2018).

J.PS 1.3
Authors : G.A. Nemnes, T.L. Mitran
Affiliations : Horia Hulubei National Institute for Physics and Nuclear Engineering, 077126 Magurele-Ilfov, Romania

Resume : In the past few years machine-learning techniques have been developed to predict the band gaps in solids [1,2], e.g. the electronic properties of graphene nanoflakes of different shapes [3], while they also provide new clues in crystal structure prediction. We investigate here geometry effects in graphene - hexagonal boron nitride nanoflakes using an artificial neural network (ANN) model. These hybrid systems are know to provide a tunable energy gaps, depending on the proportion of the two materials. Density functional theory (DFT) calculations provide the set of training examples, while using the SIESTA code we can solve efficiently relatively large systems, of a few hundred atoms. The ANN is simulated using FANN library, which allows a flexible implementation. We analyze nanoflake systems with different geometries and determine the energy gap distribution. The trained ANNs provide a correlation between the different shapes and the magnitude of the energy gaps, which may further optimize the design of nanostructured graphene based materials for specific electronic properties. References: [1] J. Lee, A. Seko, K. Shitara, K. Nakayama, I. Tanaka, Phys. Rev. B 93, 115104 (2016) [2] G. Pilania, J.E. Gubernatis, T. Lookman, Comput. Mater. Sci. 129, 156 (2017) [3] M. Fernandez, J. I. Abreu, Hongqing Shi, and A. S. Barnard, ACS Comb. Sci. 18, 661 (2016)

J.PS 1.4
Authors : A. Stamateri, S. Logothetidis
Affiliations : Department of Physics, Nanotechnology Lab LTFN, Aristotle University of Thessaloniki, GR-54124 Thessaloniki, Greece

Resume : The interactions between organic semiconductors and metal layers are critical for efficient charge carrier extraction and hence for the device performance of Organic Electronics (OE). In this work, we employ Density Functional Theory (DFT) calculations to probe from first-principles the structural and the electronic properties of prototype OE materials: P3HT and PC60BM in the proximity of the (111) surface of Ag. We apply different exchange and correlation functionals and examine the energetic stability of various conformations for the adsorption of PC60BM on Ag. We then calculate the charge rearrangement at the interface and associate it with the existence of an interfacial dipole formed upon adsorption of the organic molecules. We investigate the electronic properties of the system in an effort to reveal the exact energy level alignment across the interface. For crystalline P3HT, we employ the same type of calculations on the energetically favored adsorbed geometry. We show that by controlling the adsorption details at the metal/organic systems, the electrostatic potential at the interface can be tuned, and hence the performance of an OE device can be improved. Overall, our work reveals for the first-time the atomic-scale details that underlie charge rearrangement at metal/organic interfaces.

J.PS 1.5
Authors : M. Shafiq1,2, Iftikhar Ahmad1,2 and S. Jalali-Asadabadi3
Affiliations : 1. Department of Physics, Abbottabad University of Science and Technology, Havelian, Abbottabad, Pakistan 2. Center for Computational Materials Science, University of Malakand, Pakistan 3. Department of Physics, Faculty of Science, University of Isfahan, Hezar Gerib Avenue, Isfahan 81744, Iran

Resume : First principle studies of the cubic rare-earth intermetallics RIn3 and RSn3 (R= La, Ce, Pr, Nd) have been carried out within the framework of density functional theory using the full potential linearized augmented plane waves plus local orbital method (FP-LAPW+lo). The calculated structural parameters with different exchange correlation functional are found consistent with the experimental results. The effect of Hubbard potential on the density of states is discussed in details. Furthermore, the SOC effect enhances as one goes from La to Nd in a compound, which demonstrates interesting nature of this effect in periodic table. The elastic constants, bulk moduli, shear moduli, Young?s moduli, anisotropy, Kleinman parameters, Poisson?s ratios, sound velocities for shear and longitudinal waves, and Debye temperatures are calculated and discussed, which reveal that these compounds are ductile in nature.

J.PS 1.6
Authors : Kurelchuk U.N., Vasilyev O.S., Borisyuk P.V.
Affiliations : National Research Nuclear University MEPhI (Moscow Engineering Physics Institute) Kashirskoe shosse, Moscow, Russia, 31115409.

Resume : Ab initio study of thermoelectric properties of noble d-metallic nanoclustered model materials is presented. Semiclassical thermoelectric transport coefficients had been obtained from DFT-calculated band structure. Сonsiderable increasing of Seebeck coefficient had been received numerically, in agreement with experimentally observed trends of nanoclusters properties. In order to develop highly efficient thermoelectric materials relationship of structural, electronic and thermoelectric properties of nanoclusters and nanostructures is discussed.

J.PS 1.7
Authors : Tomasz Wo?niak (1), Alexandra Siklitskaya (2), Magdalena Birowska (3), Nevill Gonzalez Szwacki (3)
Affiliations : (1) Department of Theoretical Physics, Faculty of Fundamental Problems of Technology, Wroc?aw University of Science and Technology, ul. wyb. Wyspia?skiego 27, PL-50-370 Wroc?aw, Poland (2) Institute of Physical Chemistry, Polish Academy of Sciences, ul. Kasprzaka 44/52 PL-01-224 Warsaw, Poland (3) Faculty of Physics, University of Warsaw, ul. Pasteura 5, PL-02-093 Warsaw, Poland

Resume : Using first-principles calculations combined with ab initio molecular dynamics, we predict the existence of kinetically stable nitrogen-rich C1-xNx stripes, which are derivatives of graphitic carbon nitride. The concentration of N in this new carbon nitride stripes reaches x = 0.83, the highest reported so far [1]. The formation energies of the 1D structures are comparable in value to that of the experimentally synthesized layered graphitic C3N4. The electronic properties of the stripes range from semiconducting to metallic depending on the nitrogen content. The semiconducting CN3 stripe has a tunable band gap ranging from 0.7 to 1.1 eV for strains of ±5%. The tensile stiffness of the stripes is quite small if compared to that reported for carbyne and for CN3 is 29.08 eV/Å, a value that is, however, comparable to that of the specific stiffness of diamond [2]. The vibrational and optical properties of the stripes will be also presented and compared to those of known carbon nitride compounds. Our work sheds light on how to design new nitrogen-rich C?N nanostructures. The Polish National Science Centre (NCN) supports the work through the grant UMO-2016/23/B/ST3/03575. Numerical calculations were performed at ICM at the University of Warsaw. [1] S. Zhang et al., J. Phys. Chem. C 120, 3993 (2016). [2] M. Liu at al. ACS Nano 7, 11, 10075-10082 (2013).

J.PS 1.8
Authors : Damian Sobieraj, Jan S. Wróbel, Tomasz Rygier, Duc Nguyen-Manh
Affiliations : Warsaw University of Technology, Warsaw, Poland;Warsaw University of Technology, Warsaw, Poland;Warsaw University of Technology, Warsaw, Poland;Culham Centre for Fusion Energy, Abingdon, United Kingdom

Resume : High-entropy alloys (HEAs) are the new class of materials loosely defined as multicomponent solid solutions containing four or more elements in equal or near equal atomic percentage. In comparison with traditional alloys the disordered structures of these alloys possess significantly larger configurational entropy. The high configurational entropy inhibits the formation of brittle intermetallic phases in favour of multicomponent random solid solutions. Purpose of performed study was to determine and understand, by using ab-initio simulation methods, how short-range ordering, configurational entropy and basic alloy properties in W-Ta-Ti-Cr-V system depend on concentrations of specific elements and temperature. Combination of DFT, Cluster Expansion and Monte Carlo methods was used to perform simulations. This study has revealed, that W-Ta-Ti-V alloys form disordered solid solution in lowest temperature among all considered alloys in W-Ta-Ti-Cr-V systems. The chemical short-range ordering in the equimolar Ta-Ti-V-W alloy vanishes at 700 K.

J.PS 1.9
Authors : Casey N. Brock, Alan F. Wright, D. Greg Walker
Affiliations : Vanderbilt University, Interdisciplinary Materials Science and Sandia National Laboratories; Sandia National Laboratories; Vanderbilt University, Mechanical Engineering

Resume : The study of defect annealing processes in gallium nitride (GaN) will benefit from ab-initio molecular dynamics studies (MD) using plane-wave density functional theory (DFT) and projector augmented wave data sets (PAWs). However, for the large system sizes required for accurate defect studies, there is a strong need to reduce computational expense of the electronic structure calculations. We attempt to reduce this expense by generating low-valence PAWs, i.e. removing electrons from the valence compared to standard PAWs. After setting the desired valence-core partition, we design these PAWs using an automated search for optimal parameters, evaluating each data set using standardized PAW quality metrics such as logderivatives and lattice parameter calculations in the bulk solid. The PAWs are then tested in additional structures post-optimization to ensure transferability to other calculations. Finally, we test the PAWs in benchmark MD defect calculations and compare the accuracy and speed of our custom PAWs to established PAWs in the literature. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy?s National Nuclear Security Administration under contract DE-NA0003525.

J.PS 1.10
Authors : Kazbek S. Baktybekov, Aliya A. Baratova
Affiliations : U.M. Sultangazin Research Space Institute, Munaitpassov 3, 010008, Astana, Kazakhstan; L.N. Gumilyov Eurasian National University, Satpaev street, 2, 010008, Astana, Kazakhstan

Resume : The features of such nonlinear processes as the evolution of radiation defects in crystals, the transfer of the energy of electronic excitation under the influence of an external radiation source are studied. These processes lead to synergistic effects, which allowed to consider the evolution of the structural distribution of particles in the system from the standpoint of multifractal formalism. Complex of programs was developed to simulate the above processes by the method of the cellular automaton of the IV class. The computer program, which realizes the method of multifractal analysis, allowed quantitatively to characterize the structural changes observed in the system. With the usage of computer simulation it is established that the concentration of radiation defects in a crystal subjected to the action of ionizing radiation is the controlling parameter of the process of self-organization of the system, as a result of which clusters are formed from the same type of radiation defects. It was first discovered that a multifractal structure can be formed as a result of the destruction of a chaotic random structure. On the example of two different systems - the structure of radiation defects in an ionic crystal and a system of two types of interacting molecules on the SiO2 surface - it is shown that the phenomenon of "burning out" chaos in a physical system occurs. It is shown that the ratio of the concentrations of interacting reagent molecules and surface temperature are the controlling parameters of the kinetic process and have a significant effect on the nature of the photoprocesses flowing on the surface of a solid. The results of the work can be recommended for the development of technological processes for the production of new materials with the necessary functional properties. This can promote the development of effective techniques for studying the structural and elastic properties of various materials, including porous and fibrous structures, surface layers of metals and alloys after laser treatment, plasma or plastic deformation, the structure of amorphous polymers with a minimum number of test specimens. The developed algorithms for computer modeling of nonlinear processes in condensed media can be modified by adding new modules to the program code and used for further research.

J.PS 1.11
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Development of simulation methods I : C. Massobrio
Authors : Fabrizio Cleri
Affiliations : IEMN CNRS, and Department of Physics, University of Lille, 59652 Villeneuve d?Ascq, France

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).

Authors : Przemysław Jóźwik 1*), Lech Nowicki 2), Cyprian Mieszczyński 2), Renata Ratajczak 2), Anna Stonert 2), Andrzej Turos 2,3), Jacek Jagielski 2,3), Katharina Lorenz 1), Eduardo Alves 1)
Affiliations : 1) IPFN, Instituto Superior Técnico, Universidade de Lisboa, Estrada Nacional 10, 2695-066 Bobadela, Portugal 2) National Centre for Nuclear Research, A. Soltana 7, 05-400 Otwock-Świerk, Poland 3) Institute of Electronic Materials Technology, Wólczyńska 133, 01-919 Warsaw, Poland * On leave from Institute of Electronic Materials Technology, Warsaw, Poland

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

Authors : Woongkyu Jee Scott M. Woodley Alexey A. Sokol
Affiliations : University College London (UCL)

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.

Authors : Stephen Rhatigan, Francesc Illas, Michael Nolan
Affiliations : Tyndall National Institute, University College Cork; Departament de Ciència de Materials i Química Física & Institut de Química Teòrica i Computacional (IQTCUB), Universitat de Barcelona; Tyndall National Institute, University College Cork;

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.

Development of simulation methods II : F. Cleri
Authors : Carlo Massobrio (1), Guido Ori (1), Mauro Boero (1), Assil Bouzid (2), Evelyne Martin (3)
Affiliations : (1) Université de Strasbourg, CNRS, Institut de Physique et Chimie des Matériaux de Strasbourg, UMR 7504, F-67034 Strasbourg, France (2)] Chaire de Simulation à l'Echelle Atomique (CSEA), Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland. (3) Univ. Lille, CNRS, Centrale Lille, ISEN, Univ. Valenciennes, UMR 8520 - IEMN, F-59000 Lille, France

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.

Authors : Fabio Ricci(1), Alexandre Martin(1), Andrés Camilo Garcia Castro(1), Jordan Bieder(2), Xu He(1), Eric Bousquet(1), Matthieu J. Verstraete(1), Philippe Ghosez(1)
Affiliations : (1) Theoretical Materials Physics, Q-MAT, CESAM, Université de Liège, Belgium; (2) CEA DAM-DIF, Arpajon, France

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)

Authors : C. González 1, P. de Andrés 2, F. Flores 1
Affiliations : 1 Universidad Autónoma de Madrid, Department of Theoretical Condensed Matter Physics & Condensed Matter Physics Center (IFIMAC), Madrid, Spain 2 Instituto de Ciencia de Materiales de Madrid-CSIC, Madrid, Spain,

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.

Authors : Peng Chen1,2, Mathieu N. Grisolia3, Hong Jian Zhao1, Otto E. González-Vázquez4,5, L. Bellaiche6, Manuel Bibes3, Bang-Gui Liu2, and Jorge Íñiguez1,5
Affiliations : 1 Materials Research and Technology Department, Luxembourg Institute of Science and Technology (LIST), 5 avenue des Hauts-Fourneaux, L-4362 Esch/Alzette, Luxembourg 2 Beijing National Laboratory for Condensed Matter Physics, Institute of Physics Chinese Academy of Science, Beijing 100190, China 3 Unité Mixte de Physique, CNRS, Thales, Université Paris Sud, Université Paris-Saclay, 1 avenue A. Fresnel, 91767, Palaiseau, France 4 Scientific Computing & Software for Experiments Department, Sincrotrone Elettra, 34149 Basovizza, Trieste, Italy 5 Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), Campus UAB, 08193 Bellaterra, Spain 6 Physics Department and Institute for Nanoscience and Engineering, University of Arkansas, Fayetteville, Arkansas 72701, USA

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.

Development of simulation methods III : G. Ori
Authors : Laura E. Ratcliff, Luigi Genovese
Affiliations : Department of Materials, Imperial College London, London SW7 2AZ, United Kingdom; Univ. Grenoble Alpes, CEA, INAC-SP2M, L_Sim, F-38000, Grenoble, France

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.

Authors : Gökhan Polat, Ziya An?l Erdal, Yunus Eren Kalay
Affiliations : Necmettin Erbakan University, Faculty of Engineering and Architecture, Department of Metallurgical and Materials Engineering, Konya, Turkey Middle East Technical University, Faculty of Engineering, Department of Metallurgical and Materials Engineering, Ankara, Turkey

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.

Authors : Daniel Fritsch(1),(2)
Affiliations : (1)Helmholtz-Zentrum Berlin für Materialien und Energie, D-14109 Berlin, Germany (2)Department of Chemistry, University of Bath, Claverton Down, BA2 7AY Bath, UK

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.

Authors : Francisco C. Franco Jr.
Affiliations : Chemistry Department, De La Salle University, 2401 Taft Avenue, 0922 Manila, Philippines

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.

Electronic transport and device simulation, optical properties : A. Bouzid
Authors : Chathurangi Kumarasinghe , Neophytos Neophytou
Affiliations : School of Engineering University of Warwick Coventry CV4 7AL

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.

Authors : Thomas Frederiksen
Affiliations : Donostia International Physics Center (DIPC), Donostia-San Sebastian, Spain & IKERBASQUE, Basque Foundation for Science, Bilbao, Spain

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); [2] Frederiksen, Paulsson, Brandbyge, Jauho, Phys. Rev. B 75, 205413 (2007); [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).

Authors : K. Ghosh, C. Labbee, C. Dufour and J. Cardin
Affiliations : School of Electronics Engineering (SENSE), VIT University Chennai, Vandalur Kelambakkam Road, Chennai – 600127, India. CIMAP, Normandie Univ, ENSICAEN, UNICAEN, CEA, CNRS, 6 Bd Maréchal Juin, 14050 Caen, Cedex 4, France

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.

Poster session 2 : Y. Dappe, E. Levchenko & G. Ori
Authors : Zhang Li,Li Mu,Zhang Yongqiang,Tan Fuli
Affiliations : Institute of Fluid Physics

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.

J.PS 2.1
Authors : Vladimir V Voronkov
Affiliations : SunEdison Semiconductor - Global Wafers, via Nazionale 59, 39012 Merano Italy

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.

J.PS 2.2
Authors : S.I. Sidorenko1, M.A. Vasylyev2, S.М. Voloshko1, V.V. Yanchuk1, O.I. Kruglov1
Affiliations : 1-National Technical University of Ukraine "KPI name Igor Sikorsky; 2-G.V. Kurdyumov Institute for Metal Physics, N.A.S. of Ukraine

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.

J.PS 2.3
Authors : C. Marculescu , A. Avram, V. Tucureanu, A. Matei, B. Tincu, T. Burinaru, M. Avram
Affiliations : National Institute for Research and Development in Microtechnologies - IMT Bucharest, 126A Erou Iancu Nicolae Street, 077190, Bucharest, Romania

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.

J.PS 2.4
Authors : Sayantan Acharya, Ujjwal Kumar Nandi, and Sarika Maitra Bhattacharyya
Affiliations : Polymer Science and Engineering Division, CSIR-National Chemical Laboratory, Pune 411008, India

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.

J.PS 2.5
Authors : Sven Burger, Nando Farchmin, Martin Hammerschmidt, Sebastian Heidenreich, Philipp Hönicke, Philipp-Immanuel Schneider, Victor Soltwisch, Lin Zschiedrich
Affiliations : JCMwave GmbH, Physikalisch-Technische Bundesanstalt

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.

J.PS 2.6
Authors : Kahyun Hur
Affiliations : Center for Computational Science, Korea Institute of Science and Technology

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.

J.PS 2.7
Authors : B. Majkusiak, J. Walczak
Affiliations : Warsaw University of Technology, Institute of Microelectronics and Optoelectronics

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?.

J.PS 2.8
Authors : Mariem Maiza(1,2,3), Alexis Rucci (1,3), Dinh An Nguyen(2), Nathalie Legrand(2), Philippe Desprez(2), Alejandro A. Franco(1,3,4,5)
Affiliations : 1 Laboratoire de Réactivité et Chimie des Solides (LRCS), UMR CNRS 7314, Université de Picardie Jules Verne, HUB de l?Energie, Rue Baudelocque, 80039 Amiens, France 2 Saft Bordeaux, 111 Boulevard Alfred Daney, 33074 Bordeaux Cedex, France 3 Réseau sur le Stockage Electrochimique de l?Energie (RS2E), FR CNRS 3459, HUB de l?Energie, Rue Baudelocque, 80039 Amiens, France 4 ALISTORE-European Research Institute, FR CNRS 3104, HUB de l?Energie, Rue Baudelocque, 80039 Amiens, France 5 Institut Universitaire de France, 103 Boulevard Saint Michel, 75005 Paris, France

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).

J.PS 2.9
Authors : Andrzej Gawlik, Janusz Bogdanowicz, Andreas Schulze, Jan Misiewicz, Wilfried Vandervorst
Affiliations : Imec, Kapeldreef 75, 3001 Leuven, Belgium KU Leuven, Department of Physics and Astronomy, Institute for Nuclear and Radiation Physics ,Celestijnenlaan 200D, 3001 Leuven, Belgium Department of Experimental Physics, Faculty of Fundamental Problems of Technology, Wroc?aw University of Science and Technology, Wybrze?e Wyspia?skiego 27, 50-370 Wroc?aw, Poland; Imec, Kapeldreef 75, 3001 Leuven, Belgium; Imec, Kapeldreef 75, 3001 Leuven, Belgium; Department of Experimental Physics, Faculty of Fundamental Problems of Technology, Wroc?aw University of Science and Technology, Wybrze?e Wyspia?skiego 27, 50-370 Wroc?aw, Poland; Imec, Kapeldreef 75, 3001 Leuven, Belgium KU Leuven, Department of Physics and Astronomy, Institute for Nuclear and Radiation Physics ,Celestijnenlaan 200D, 3001 Leuven, Belgium

Resume : Understanding the interaction of light with semiconducting nanostructures is of increasing importance for nanoelectronic, photonic and photovoltaic applications. However, the physical mechanisms responsible for the size-dependent optical properties of such structures are not fully understood, as typically studied with purely numerical tools. In this work, we develop an analytical model for the size-dependent optical properties, such as reflectance, of periodic arrays of nm-scale semiconducting fins. Using a mode matching technique, we show that size-dependent optical properties of such structures are due to coupling between diffraction modes existing outside the arrays and waveguide modes inside the arrays. The model is validated with reflectance spectra collected on Si fin arrays of varying geometry, as well as finite-element simulations. A good agreement for wavelengths below 400 nm is achieved. On the other hand, a red-shift of the measured spectra w.r.t. the model is observed for wavelengths above 400 nm. This deviation can be attributed to the slanted sidewalls of the fins which do not affect the near-UV region of the spectra due to the high absorption and a decrease of the refractive index of Si in this range. Finally, we show that for sub-40 nm fins two waveguide modes are sufficient to explain the complex spectra. The generated insight is crucial for the design, fabrication and characterization of nanoelectronic, photonic and photovoltaic devices.

J.PS 2.10
Authors : Fiacre Rougieux
Affiliations : School of Photovoltaics and Renewable Energy Engineering, The University of New South Wales, Sydney NSW2052 Australia

Resume : In silicon, Defects such as interstitial chromium, molybdenum, tungsten and iron have a single known level within the bandgap. Modelling the impact of such defects on the minority carrier lifetime can be simply performed using the simplified Shockley-Read Hall formalism. However, most defects are multivalent defects. This means they can take more than two charge states and have more than one energy level within the bandgap. In silicon, multivalent defects include titanium, vanadium, manganese, cobalt, nickel, palladium, platinum, cobalt, gold and zinc. While often modelled with two or more independent levels in the literature, such defects should in principle be modelled with dependent levels according to the formalism derived by Sah and Shockley and others. Deep Level Transient Spectroscopy measurements are most commonly conducted with injection of only one type of carrier. As such capture mechanisms operating though exciton enhanced Auger phenomena cannot take place as excitons are not present in significant concentrations when only one carrier is injected. This means the capture mechanism measured by DLTS may not be representative of the real capture mechanism in a device with both minority and majority carrier injection. In addition, determination of capture cross sections from DLTS emission data using detailed balance can yield capture cross sections with values orders of magnitude higher than the actual capture cross sections. In this paper, we introduce the challenges in current modelling of the temperature dependence of steady-state recombination at multivalent flaws in semiconductors and provide a more nuanced and complete picture of defect recombination at multivalent defects.

J.PS 2.11
Authors : J. Pineda-Delgado, M.V. Martínez-Contreras, A. Rico-Zavala, A.U. Chávez - Ramírez, L.G. Arriaga, M. P. Gurrola
Affiliations : Centro de investigación y desarrollo Tecnológico en Electroquímica

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].

J.PS 2.12
Authors : Kyeongyoon Lee, Jaewoong Song, Hyunmin Bae, Sunwoong Choi
Affiliations : Hannam University

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.

J.PS 2.13
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Surfaces and interfaces I : L. Razinkovas
Authors : Christopher Gaul, Sebastian Hutsch, Martin Schwarze, Karl Sebastian Schellhammer, Fabio Bussolotti, Satoshi Kera, Gianaurelio Cuniberti, Karl Leo, Frank Ortmann
Affiliations : Technische Universität Dresden

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).

Authors : Adam Paul Karcz, Anne Juul Damø, Kim Dam-Johansen, David Chaiko
Affiliations : Technical University of Denmark, Kgs. Lyngby, Denmark; Technical University of Denmark, Kgs. Lyngby, Denmark; Technical University of Denmark, Kgs. Lyngby, Denmark; FLSmidth Minerals, Midvale, UT, U.S.A.

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.

Authors : Marcin Roland Zem?a, J. S. Wróbel, T. Wejrzanowski, D. Nguyen-Manh
Affiliations : Marcin Roland Zem?a;J. S. Wróbel;T. Wejrzanowski Faculty of Materials Science and Engineering, Warsaw University of Technology, Wo?oska 141, 02-507 Warsaw, Poland; D. Nguyen-Manh CCFE, Culham Centre for Fusion Energy, Abingdon, Oxon OX14 3DB, UK

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.

Authors : Hong Woo Lee, Sang Soo Han
Affiliations : Computational Science Research Center, Korea Institute of Science and Technology (KIST)

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)

Surfaces and interfaces II : F. Ortmann
Authors : Lukas Razinkovas (1), Prithvi Reddy (2), Marcus W. Doherty (2), Chris G. Van de Walle (3), and Audrius Alkauskas (1)
Affiliations : (1) Centre for Physical Sciences and Technology (FTMC), Vilnius LT-10257, Lithuania; (2) Laser Physics Centre, Research School of Physics and Engineering, Australian National University, Canberra, Australia; (3) Materials Department, University of California, Santa Barbara, California 93106- 5050, USA

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).

Authors : Julio Gutierrez, Michael Nolan
Affiliations : Tyndall National Institute, University College Cork, Cork, IReland

Resume : Titanium nitride (TiN) is widely used in industry as a protective coating and a conducting layer due to its hardness, resistance to corrosion and conductivity. TiN can spontaneously form a thin oxide layer when exposed to air, which can modify the properties of the coating. With a limited understanding of the TiO2–TiN interfacial system at present, this contribution presents a comprehensive density functional theory (DFT) study of the the structural and electronic properties of oxidized TiN using the TiN(100)-rutile TiO2(110) interface as a model system. The small lattice mismatch gives good stability to the TiO2–TiN interface after depositing the oxide onto TiN through the formation of interfacial Ti–O bonds. Using DFT+U we show the presence of Ti3+ cations in the TiO2 region, which are preferentially located next to the interface region as well as the rotation of the rutile TiO2 octahedra in the interface structure. The DFT+U TiO2 electronic density of states (EDOS) shows localized Ti3+ defect states forming in the midgap between the top edge of the valence and the bottom of the conduction band. We increase the complexity of our models by the introduction of nonstoichiometric compositions. The vacancy formation energies for Ti in TiN (Evac (Ti) ≥ 4.03 eV) are high but for oxygen removal of multiple interfacial oxygen sites is stable as a result of forming multiple interfacial Ti-O and Ti-N bonds. We further show that a structure with exchanged O and N can lie 0.82 eV higher in energy than the perfect system, suggesting the stability of structures with interdiffused O and N anions at ambient conditions. The presence of N in TiO2 introduces N 2p states localized between the top edge of the O 2p valence states and the midgap Ti3+ 3d states, thus reducing the band gap in the TiO2 region for the exchanged O/N interface EDOS. Finally, we investigate the stability of water adsorbed at a range of coverages on different TiN-TiO2 interfaces and predict the most stable state of adsorbed water over a range of conditions. The outcomes of these simulations give us a most comprehensive insight on the atomic level structure and the electronic properties of oxidized TiN surfaces.

Authors : Susanne G.E.T. Escher, Alexey A. Sokol, Scott M. Woodley
Affiliations : University College London, Department of Chemistry, 20 Gordon Street, London WC1H 0AJ, United Kingdom

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.

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Phase formation, Phase stability and Defects : R. Kozubski
Authors : Hannes Jónsson
Affiliations : Science Institute and Faculty of Physical Sciences, University of Iceland

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).

Authors : Mark Fedorov, Jan Wróbel, Duc Nguyen-Manh, Krzysztof Kurzydłowski
Affiliations : Warsaw University of Technology, Warsaw University of Technology, Culham Centre for Fusion Energy, Warsaw University of Technology

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

Authors : T. Prüfer, K.-H. Heinig
Affiliations : Helmholtz-Zentrum Dresden-Rossendorf, 01328 Dresden, Germany

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

Authors : Tomasz Tarkowski, Nevill Gonzalez Szwacki
Affiliations : Institute of Theoretical Physics, Faculty of Physics, University of Warsaw, Pasteura 5, PL-02093 Warszawa, Poland

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.

Ceramics and semiconductors : C. Kumarasinghe
Authors : Assil Bouzid, Alfredo Pasquarello
Affiliations : Chaire de Simulation à l?Échelle Atomique (CSEA), École Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland

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).

Authors : Daniele Stradi, Jess Wellendorff, Petr A Khomyakov, Ulrik G Vej-Hansen, Maeng-Eun Lee, Søren Smidstrup, Kurt Stokbro
Affiliations : Synopsys A/S Fruebjergvej 3, 2100 København (Denmark)

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]; [2]; [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).

Authors : Chao-Ping Hsu
Affiliations : Institute of Chemistry, Academia Sinica, 128 Sec. 2 Academia Road, Taipei 11529, Taiwan.

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.


Symposium organizers
Elena LEVCHENKOUniversity of Newcastle

School of Mathematical and Physical Sciences, Faculty of Science, University Drive, Callaghan NSW 2308 Australia
Guido ORIInstitut de Physique et Chimie des Matériaux de Strasbourg

IPCMS, CNRS - University of Strasbourg, 23 Rue du Loess, F-67034 Strasbourg, France
Yannick J. DAPPEService de Physique de l’Etat Condensé (SPEC – CNRS – CEA Saclay)

Bât. 771 Orme des Merisiers F-91191 Gif-sur-Yvette, France