2018 Fall Meeting
Atomic-scale design protocols towards energy, electronic, catalysis and sensing applications
Functional materials are of fundamental importance for the advances of new technologies. The objective of this symposium is to discuss the state of the art of atomic-scale simulative and experimental protocols to design novel functional nanostructured materials.
Nanostructured materials are essential building blocks for the fabrication of new devices for energy harvesting/storage, sensing, catalysis, magnetic and optoelectronic applications. However, due to the increase of technological needs, it is essential to identify new functional materials and improve the properties of the existing ones. Device performance can be enhanced tailoring the properties of materials at the nanoscale. Intensive research activities have been devoted to the synthesis of nanomaterials and to the characterization of their properties during the last years. However, the discovery of novel functional materials – non-linear photonic crystals, multiferroics, layered Van der Waals and perovskite-like heterostructures, nanotubes, functionalized surfaces, thin films, interfaces, etc. – requires that we understand the atomic structural principles governing the response to external stimuli. In this perspective, the symposium will focus on the recent developments of design strategies for smart materials. Particular emphasis will be made on synergistic investigations involving experimental, materials informatics, and computational approaches, which can provide the fundamental understanding of these materials as well as new insights necessary to guide and accelerate the search of materials with targeted functionalities. The symposium will be a unique opportunity to bring together experimental and computational researchers from various communities (physicists, chemists, engineers, computational and materials informatics scientists) who could pave the way to a new generation of functional materials.
Hot topics to be covered by the symposium:
- Design of 2D nanomaterials (nanoparticles, nanotubes, Transition Metal Dichalcogenides)
- Design of 3D nanomaterials (molecular crystals and perovskite-based materials for energy harvesting/storage devices
- Layered heterostructures and interfaces
- Quantitative prediction of structural response to external stimuli
- Catalytic, sensing, electronic, photonic and optoelectronic applications
- Instrumentation and analysis technique development (hardware, software)
Confirmed invited speakers:
- Albert Bruix, TU Munich, Theoretical Chemistry, Germany
- Aleix Comas Vives, ETH Zürich, Switzerland
- Vincenzo Fiorentini, Università di Cagliari, Italy
- Cesare Franchini, University of Vienna, Austria
- Luca Ghiringhelli, Fritz Haber Institute, Germany
- Axel Gross, Institute of Theoretical Chemistry, Ulm University, Germany
- Geoffroy Hautier, Université Catholique de Louvain, Belgium
- Yuqing Huang, Linköping University, Sweden
- Jorge Íñiguez, Luxemburg Institute of Science and Technology, Luxemburg
- Nuria Lopez, Institut Català d'Investigaciò Quìmica, Spain
- Pablo Ordejon, Institut Català de Nanociència i Nanotecnologia, Spain
- Rossitza Pentcheva, Universität Duisburg-Essenadd, Germany
- Jonathan Skelton, University of Bath, United Kingdom
- David C. Smith, University of Southampton, United Kingdom
- Taylor D. Sparks, University of Utah, USA
- Alessandro Stroppa, CNR-SPIN L’Aquila, Italy
- Matthias Vandichel, Chalmers, Sweden
- David Lewis, University of Manchester, United Kingdom
- Steven May, Drexel University, USA
- Michele Saba, Università di Cagliari, Italy
- Spyros Yannopoulos, Institute of Chemical Engineering Sciences, Greece
- Hongbin Zhang, Technische Universität Darmstadt, Germany
Selected papers are published online: https://doi.org/10.1021/acs.inorgchem.9b01785
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Session 1 : -
Authors : Pablo Ordejón
Affiliations : Catalan Institute of Nanoscience and Nanotechnology (ICN2), BIST and CSIC Campus UAB, E08193 Cerdanyola del Valles, Barcelona, Spain
Resume : I will present an overview of the capabilities of the SIESTA code, which allows to perform DFT simulations in complex materials. The favorable scaling of SIESTA permits to tackle very large problems even with modest computational resources. SIESTA is well parallelized, and contains a number of features that make is specially attractive. I will show some illustrations of these features, including a QM/MM implementation, and the possibility of computing non-equilibrium electronic transport properties in nanodevices. Particular applications, such as the study of the feasibility of using nanopores in graphene for the sequencing of DNA chains (by means of the measurement of the resistance of the graphene sheet) will be used as an illustration.
Authors : E. Serpini, P. Nicolini, E. Ukraintsev, A. Rota, B. Rezek[3,4], T. Polcar[2,5], S. Valeri
Affiliations :  Dipartimento di Scienze Fisiche, Informatiche e Matematiche, Università degli Studi di Modena e Reggio Emilia, Via Università 4, 41121 Modena, Italy;  Department of Control Engineering, Czech Technical University, Karlovo nám?stí 13, 12135, Prague 2, Czech Republic;  Institute of Physics, ASCR, Cukrovarnická 10, 162 00 Prague 6, Czech Republic;  Faculty of Electrical Engineering, Czech Technical University, Technická 2, 166 27 Prague 6, Czech Republic;  Engineering Materials, Faculty of Engineering and Environment, University of Southampton, Southampton SO17 1BJ, United Kingdom
Resume : One third of energy produced by industrial countries is lost as friction. High wear caused by friction means ca. 35% of industrial production is used to replace degraded products, whilst causing the breakdown of machinery, resulting in safety risks and environmental pollution. Controlling and reducing friction is a fundamental step in attaining the sustainable development of our society, as detailed in the Brundtland report. In this contribution I will present the results of a study on the nanometric sliding of molybdenum disulfide against itself both from the experimental and from the computational point of view. The differences between ordered material (single crystal) and disordered material (sputtered coating) were investigated. Tribological experiments were performed using Lateral Force Microscopy. Atomic Force Microscopy tips modified by sputter deposition of molybdenum disulfide were used for the first time. This feature opened up the possibility for close comparison with classical molecular dynamics simulations. In both cases, the coefficient of friction for the ordered system in inert conditions was found to be smaller than for disordered system. This result demonstrates the impact of morphology at the nanoscale and highlights the importance of molecular dynamics as a diagnostic and predictive tool in nano-friction. Furthermore, experiments show that the effects of the environment on nanoscale friction are reduced with respect to the macroscale case. These findings can expedite the process of fabricating molybdenum disulfide-based coatings with superior tribological properties, with the ultimate aim of reducing the energy dissipation due to friction.
Authors : Michael-Marcus Schmitt, Yajun Zhang, Alain Mercy He Xu Eric Bousquet and Philippe Ghosez
Affiliations : Theoretical Materials Physics, Q-MAT, CESAM, Université de Liège, Liège, Belgium
Resume : Rare-earth nickelates (RNiO3), rare-earth manganites (RMnO3), and alkaline-earth ferrites (AMnO3), are perovskites sharing the same formal single-electron occupation of the transition metal eg orbitals. At high temperatures, these oxides show a metallic behavior. On cooling, they typically exhibit a metal to insulator transition (MIT) but through different mechanisms. On the one hand, RMnO3 perovskites show an orbital-order type MIT and Jahn-Teller distortion of the oxygen octahedra while ReNiO3 and CaFeO3 exhibit a charge-order type MIT and breathing distortion of the oxygen octahedra. We show that the tendency to either charge- or orbital-order type transition is not an exclusive intrinsic property of each family of compound but has to be thought as tunable. From ab-initio calculations, we first clarify the structurally triggered mechanism at the origin of the charge-order type MIT in ferrites and nickelates. We then demonstrate that an alternative orbital-order MIT can be engineered under tensile epitaxial strain in these compounds. Finally, we propose different possibilities in the eg1 family to create new multiferroic materials, highlighting how fundamental understanding of the interplay between magnetic, electronic properties and structural distortions can lead to new design paths toward exciting materials.
Authors : Y.Q. Huang, Y.X. Song, S.M. Wang, I.A. Buyanova, W.M. Chen
Affiliations : Y.Q. Huang,I.A. Buyanova,W.M. Chen (Department of Physics, Chemistry and Biology, Linköping University) Y.X. Song (Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences) S.M. Wang (Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences && Department of Microtechnology and Nanoscience, Chalmers University of Technology)
Resume : A combined effect of a strong spin-orbit interaction and preserved time-reversal symmetry in a 3D topological insulator (TI) has led to the topological surface state (TSS) with locked spin and momentum. Taking advantage of the so-called circular photon galvanic effect (CPGE), it had been shown by us and other groups that helicity-dependent photocurrent (HPC) can be generated on the surface of a 3D TI [1-3]. In this work, by exploring the CPGE in a macroscale Bi2Te3 thin film epitaxially grown on GaAs substrate, we have discovered a significant HPC contribution from a semiconductor-TI interface. In our hybrid system, the HPC is found to originate from both the in-plane and out-of-plane spin texture of the TSS, which can be separated by varying the incidence angle of the excitation light. Using photocurrent excitation spectroscopy and time-resolved photoluminescence spectroscopy, we provide compelling experimental proofs for a novel HPC component that arises from injection of spin-polarized carriers from GaAs. Such HPC component mainly occurs at the GaAs/Bi2Te3 interface, which has an opposite direction as compared with the original HPC with Bi2Te3 alone. In the normal incidence condition, the interface HPC dominates over the latter and the direction of the total HPC is found to be reversed. Furthermore, we show that in the hybrid system the magnitude and direction of the HPC can be controlled by inducing precession of the injected electron spins around an external magnetic field. These discoveries not only open a new playground for intriguing physics but also promises enriched spin functionalities by integrating TI with conventional semiconductors, which may pave a way to a new generation of hybrid devices for opto-spintronic applications. References:  J. W. McIver, D. Hsieh, H. Steinberg, P. Jarillo-Herrero, and N. Gedik, Nat. Nanotechnol. 7, 96 (2011).  C. Kastl, C. Karnetzky, H. Karl, and A. W. Holleitner, Nat. Commun. 6, 6617 (2015).  Y. Q. Huang, Y. X. Song, S. M. Wang, I. A. Buyanova, and W. M. Chen, Nat. Commun. 8, 15401 (2017).
Session 2 : -
Authors : Matthias Vandichel
Affiliations : Division of chemical physics and competence centre for catalysis
Resume : PtSn alloys are promising for the preferential oxidation of CO in the presence of H2 (PROX), used in hydrogen purification. The rate accelerating effect of PtSn compared to Pt has previously been attributed to ligand effects, thus electronic modification of Pt. Only recently been questioned as PtSn has been observed to segregate during typical conditions for CO oxidation. To elucidate the segregation hypothesis, various systems are herein investigated with ab initio thermodynamics using Density Functional Theory (DFT). In addition, first-principles microkinetic modeling is applied to compare the rate of CO oxidation at an SnO2/Pt interface and a bare Pt surface. The co-catalytic role of an SnO2 rim is clearly manifested in an enhanced activity at low temperatures . Alternative SnO2/Pt3Sn interfaces are investigated by studying their CO oxidation and O2 regeneration  The barrier for CO oxidation via a Mars van Krevelen mechanism is found to be lower on finite SnO2 and (SnO2)3 units as compared to the bulk-like model. However, the regeneration of the finite systems is associated with higher barriers for O2 dissociation which may become the rate limiting step in the low temperature regime where the metal surface can be assumed to be CO covered . The presentation will also address possible deactivation routes of Pt3Sn nanoparticles covered with CO . References: 1. M. Vandichel, A. Moscu, H. Grönbeck (2017) ACS Catalysis 7 (11):7431-7441. doi:10.1021/acscatal.7b02094 2. M. Vandichel, H. Grönbeck, Topics in Catalysis (2018), in press 3. M. Vandichel, H. Grönbeck, Journal of Catalysis, submitted
Authors : Seungchul Kim, Heechae Choi
Affiliations : Korea Institute of Science and Technology; Virtual Lab Inc.
Resume : Tremendous efforts have been continuing to improve the performance of TiO2 photocatalyst. As a typical way, we have studied defects and impurity doping in TiO¬2 in two different approaches for improving its performance. First approach is to lower the band gap for high absorption rate, and second is to synthesize fine size of polymorphic particles for high carrier separation rate. Doping of W and N, and intrinsic defects at highly reducing condition (e.g. black TiO2) were considered in the first approach. We explain irregular behavior of N-doped and W-N co-doped TiO2 in absorption rate and photocatalytic activity, and find intrinsic pair defect that might be, at least in part, responsible for the absorption property and improved photocatalytic activity of black TiO2. In the second approach, we doped W, a bandgap narrower, with Sn. Sn is a promoter of anatase-to-rutile phase transformation (ART). W dopant improves the photo-absorption, but it also makes space charge layer near the surface, where electron-hole separation occurs, thinner. Indeed, rutile-anatase polymorphic TiO2 is hard to be small due to high surface energy of rutile phase. Therefore, it is difficult task to get synergetic effects of bandgap narrower and high carrier separation rate of polymorphic particles. This synergetic effect is achieved by co-doping Sn with W. We synthesized ~ 10 nm sized nanoparticles of anatase-rutile mixed phase, and such samples show higher activity than doped TiO2 and commercially available P25.
Authors : Pussana Hirunsit , Boon Siang Yeo
Affiliations : Pussana Hirunsit, National Nanotechnology Center (NANOTEC), National Science and Technology Development Agency (NSTDA), Thailand Science Park, Pathum Thani, Thailand 12120 ; Boon Siang Yeo, Department of Chemistry, Faculty of Science, National University of Singapore, 3 Science Drive 3, Singapore 117543 Boon Siang Yeo, Solar Energy Research Institute of Singapore, National University of Singapore, 7 Engineering Drive 1, Singapore 117574
Resume : Electrochemical reduction of CO2 is a promising process which can reduce carbon footprint and bring us toward neutral CO2 cycle. CO2 waste can be converted to various valuable hydrocarbon fuels and chemicals. The key advantage of the electrochemical process is that the required energy supply is low and can be discontinuous which could be provided by renewable energy resources such as solar cells. Copper is known to catalyze CO2 electroreduction best compared to other pure metals. Recently, extensive researches have been exploring alternatives for catalysts to increase selectivity and reactivity of CO2 electrochemical reduction. This work applies density functional theory (DFT) calculations to demonstrate the mechanistic workings of sulfur dopant in tuning the CO2 reduction pathway on Cu towards formate production, which could form the basis of new strategies for the design of other highly selective CO2 reduction electrocatalysts by using p-block dopants. The CuSx catalyst has experimentally showed that it can reduce CO2 to formate with a remarkable faradaic efficiency of 75% and current density of -9.0 mA/cm2 at -0.9 V vs. the reversible hydrogen electrode while other hydrocarbon products and CO are significantly suppressed. The *COOH and HCOO* are the key intermediates which determine the selective CO and formate production, respectively. The presence of sulfur on copper surface weakens adsorption of *COOH and HCOO* intermediates to a higher degree when their coverage increases compared to pure Cu(111) surface. This results in the favorability of the potential-limiting step for formate production and the suppression of CO production on CuSx surface. Also, the onset potential of hydrogen evolution reaction (HER) is also more delayed on CuSx catalyst compared to Cu(111) surface, thus the H2 production is less on CuSx surface.
Authors : Tayor D. Sparks, Steven K. Kauwe, Marcus Parry, Jake Graser, Jakoah Brgoch, Aria Tehrani Mansouri, Anton Olinyk
Affiliations : University of Utah; University of Utah; University of Utah; University of Utah; University of Houston; University of Houston; University of Houston
Resume : Over 7 years ago the White House announced the Materials Genome Initiative (MGI) asking computational materials scientists and experimentalists to find ways to “discover, develop, manufacture, and deploy materials twice as fast at a fraction of the cost.” High throughput computation and experiments have made some progress but we are still far from the MGI goal. However, the emerging field of Materials Informatics offers a completely new and under-utilized approach via machine learning and big data approaches to materials problems. In this talk, I’ll describe the promise, challenges, and opportunities that this new approach affords materials scientists. Specifically, I describe some of the new data-driven tools we are developing in our group as well as tools developed in conjunction with Citrine Informatics such as the “Materials Recommendation Engine.” These tools allow us to reduce the risk associated with exploring chemical whitespace for new, interesting materials and enable rapid material discovery. I’ll demonstrate the utility of machine learning with specific examples in thermoelectrics, superhard materials, crystal structure classification, and thermochemical data prediction.
Session 3 : -
Authors : Benjamin Geisler, Rossitza Pentcheva
Affiliations : Department of Physics, University of Duisburg-Essen
Resume : Oxides are a promising materials class for thermoelectric applications due to their chemical and thermal stability and environmental friendliness, but also owing to their complex correlated behavior. Nanostructuring and reduced dimensionality has been proposed as a promising route to further optimization of the thermoelectric response . By combining DFT+U calculations and Boltzmann transport theory using the BoltzTrap code we explore the implications of interface polarity and confinement on the thermoelectric properties of nickelate superlattices. Taking as an example LaNiO3/SrTiO3(001) superlattices, we demonstrate that compatible n- and p-type materials can be realized by selective choice of the layer stacking at the polar interfaces . On the other hand, a strongly enhanced thermoelectric response is obtained in nonpolar LaNiO3/LaAlO3(001) superlattices due to the metal-to-insulator transition occurring as a result of confinement . Last but not least we discuss the effect of localized electrostatic doping in La2CuO4/ LaNiO3(001) superlattices .  L. D. Hicks and M. S. Dresselhaus, Phys. Rev. B 47, 12727 (1993).  B. Geisler, A. Blanca-Romero and R. Pentcheva, Phys. Rev. B 95, 125301 (2017).  B. Geisler and R. Pentcheva, Phys. Rev. Materials 2, 055403 (2018).  F. Wrobel, B. Geisler, Y. Wang, G. Christiani, G. Logvenov, M. Bluschke, E. Schierle, P. A. van Aken, B. Keimer, R. Pentcheva, and E. Benckiser, Phys. Rev. Materials 2, 035001 (2018).
Authors : Umberto Terranova, Claire Mitchell, Meenakshisundaram Sankar, David Morgan, Nora H. de Leeuw
Affiliations : School of Chemistry, Cardiff University
Resume : Pyrrhotites Fe(1-x)S (0< x< 0.125), the most common iron sulfides in nature after pyrite, are minerals with promising catalytic properties. Like pyrite, pyrrhotites are very reactive towards molecular oxygen, which is easily incorporated into their surfaces. In order for the catalytic mechanisms to be understood properly, it is essential to achieve a detailed knowledge of the oxidised substrate available to the reactants. In this work, after using X-ray photoelectron spectroscopy to show that the catalytically relevant surfaces contain oxidic species, we use density functional theory to investigate the early oxidation mechanism of the prismatic surfaces of troilite FeS, i.e. the stoichiometric end-member of the pyrrhotite group. We find that atomic oxygen adsorbs in Fe-O-Fe bridging motifs which are thermodynamically stable under ambient conditions. During the first oxidation steps, the formation of the S-O bond is less favoured than Fe-O, suggesting that the sulfur oxides observed experimentally form only subsequently. Our calculations predict, moreover, the possible substitution of sulfur for oxygen and a clustering growth of the oxidic units. In agreement with experiment, the oxidation of troilite is exothermic, where the equilibrium between adsorption and substitution is influenced by the presence of Fe vacancies.
Authors : Jonathan Michael Skelton
Affiliations : Department of Chemistry, University of Bath, Claverton Down, Bath BA2 7AY, UK
Resume : The design and development of responsive “smart” materials is a key challenge in contemporary materials science, with applications ranging from chemical sensing and environmental monitoring to personal medicine and fashion. Selectively trapping long-lived metastable states provides a new and relatively unexplored, but highly-promising, route to materials with selective responses to a range of environmental stimuli. In this talk, I will discuss three examples. Square-planar Pt complexes based on tridentate “pincer” ligands produce needle-like crystals where the Pt-Pt stacking and colour can be perturbed by diffusion of small molecules through the crystallites, and the solvent selectivity and response controlled via the ligand chemistry. Solid-state linkage isomers show a photoinduced change in ligand binding through a single-crystal-to-single-crystal (SCSC) phase transition. Using time-resolved single-crystal X-ray diffraction and kinetic modelling, I will show how the metastable-state lifetimes can be tuned through orders of magnitude by varying the temperature. I will also present a family of thermochromic organic cocrystals that show a series of temperature-induced SCSC transitions, enabled by molecular disorder introduced with a novel crystal-engineering strategy. Finally, I will also highlight how a combination of spectroscopy, crystallography and modelling can together yield the atomistic understanding needed to exploit metastability as a design strategy in the future.
Poster Session : -
Authors : Ivan Kondov (1), Patrick Faubert (2), Claas Müller (2)
Affiliations : (1) Steinbuch Centre for Computing, Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany; (2) Universtiy of Freiburg, Department of Microsystems Engineering – IMTEK, Laboratory for Process Technology, Georges-Köhler-Allee 103, 79110 Freiburg, Germany
Resume : Oxygen reduction reaction (ORR) is the electrochemical process at the cathode in polymer electrolyte fuel cells and lithium-air batteries. Important for the construction of sustainable fuel cells is to identify candidates for novel ORR catalysts that not only satisfy the requirements for catalytic activity and durability under the cell operation conditions, but are also free of precious metals and rare elements. Recent development of alkaline polymer electrolytes opens up possibilities to use nickel alloys as an alternative cathode catalyst. We study the catalytic activity of nickel-based catalyst materials employing a simple model for the ORR . As activity descriptors we use the effective reversible and critical electrochemical potentials calculated from first principles using density functional theory. To describe the active surface of the catalyst we employ atomistic structure models, such as periodic slabs of different sizes. For high-throughput virtual screening of various surface structures, we have developed a workflow framework which will be presented. We find that catalytic activity of the nickel surface sites can be considerably influenced via doping and decoration of the surface with Cr oxides [1, 2] and oxides of other transition metals, such as Ti, V, Mn, Fe and Co. Based on these results we identify the factors contributing to activity enhancement and suggest specific modifications to increase catalyst performance significantly. The presented framework can be used for efficient screening of candidate structures for precious-metal free ORR catalysts that may be later synthesized and characterized. The produced data could help finding even better candidates using machine learning algorithms.  I. Kondov, P. Faubert and C. Müller, Electrochim. Acta 236, 260 (2017);  P. Faubert, I. Kondov, D. Qazzazie, O. Yurchenko and C. Müller, MRS Commun. 8, 160 (2018).
Authors : Benjamin J. Irving, Tomas Polcar
Affiliations : Czech Technical University in Prague, University of Southampton
Resume : Low-dimensional materials have recently attracted immense interest due to their fascinating physical properties and potential for application in diverse fields such as (opto)electronics, energy harvesting and dry lubrication. Transition metal dichalcogenides (TMDs), of general form MX2 (M = Mo, W; X = S, Se, Te), are posited as being some of the best solid-state lubricants currently available. They exhibit a lamellar structure in which covalently-bonded MX2 layers are held together by weak van der Waals forces, which, together with very low ideal shear strengths (i.e., the maximum load applied parallel to the face of the material that can be resisted prior to the onset of sliding) render them suitable for use in the mitigation of friction. Our extensive density functional calculations highlight the dependence of important nanomechanical properties of TMDs on their chemical composition and bilayer orientation (sliding direction); in particular, our calculations underscore the intrinsic relationship between incommensurate layers and superlubricity. Our latest calculations have focused on TMD-based van der Waals heterostructures (e.g. WS2 sliding on MoS2), with the aim of formalizing the relationship between fundamental quantum chemical parameters of the constituent elements and the nanomechanical properties of the material. Ultimately, we wish to improve the predictive capabilities of in silico methods during the material design process.  B. Irving, P. Nicolini and T. Polcar, Nanoscale, 2017, 9, 5597-5607
Authors : Kuo-Lun Tai, Guan-Min Huang, Wen-Wei Wu
Affiliations : Department of Materials Science and Engineering, National Chiao Tung University, No.1001, University Rd., East Dist., Hsinchu City 300, Taiwan
Resume : Two dimensional materials are now in the research front of material science. Among them, few-layered molybdenum disulfide possesses great potential in semiconductor and energy storage application. It is crucial to realize the morphological stability of the materials under externally applied stimulus. Since even slightly morphological change can influence the physical and chemical properties of materials significantly. Here, a nano-sculpting technique of atomic layers MoS2 via in-situ transmission electron microscopy (TEM) was provided. We placed CVD-grown MoS2 on the TEM chips and the experiment was conducted at high vacuum environment upon heating. During the process, live observation of cutting and shaping of layered MoS2 were captured through fast framing skill. Also, the structural evolution of the atomic layers MoS2 were revealed at atomic scale. Furthermore, we found the unique behavior such as scrolling and wrinkling and the original flat layered structure ended up a unique heterostructure. The results shows that more active site are formed in MoS2 and it is beneficial for functioning as a catalyst. This study not only supplied a distinct method to explore the fundamental mechanisms but also exhibited the potential application of MoS2 in nanoscale.
Authors : Yoon-Su Shim1, Segi Byun2, Jong Min Yuk1, Jung-Joon Yoo3, Chan-Woo Lee4
Affiliations : 1Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea 2Center for Convergence Property Measurements, Korea Research Institute of Standards and Science (KRISS), 267 Gajeong-ro, Yuseong-gu, Daejeon 34113, Republic of Korea 3Separation and Conversion Materials Laboratory, Energy Efficiency and Materials Research Division, Korea Institute of Energy Research (KIER), 152 Gajeong-ro, Yuseong-gu, Daejeon 34129, Republic of Korea 4R&D Platform Center, Korea Institute of Energy Research (KIER), 152 Gajeong-ro, Yuseong-gu, Daejeon 34129, Republic of Korea
Resume : Electrochemical capacitors have drawn valuable attention as a new class of micro-power sources. However, they have limited their applications in micro-energy storage due to their low volumetric energy densities. Here, Birnessite (KMnO2)/reduced graphene oxide (RGO) hybrid device were prepared via a two-step fabrication process involving a simple chemical redox reaction and post-thermal annealing to form a multi-layer structured film. With this versatile method, GO sheets can act as either a growth template for the highly capacitive KMnO2 nanosheets or as a mechanical support for the production of compact hybrid films. Enhanced electrochemical characterization of the KMnO2/RGO hybrid film revealed a remarkable ultrahigh volumetric capacitance (493 F/cm3 at 10 mV/s), ultrahigh energy and power density (13.3 mWh/cm3 at 2.5 A/cm3 and 22.6 W/cm3 at 58 A/cm3, respectively) and a semi-permanent cycle life (97% capacitance retention) in an aqueous electrolyte system. In addition, we investigated atomic-level reaction mechanisms of the KMnO2/RGO capacitor using density-functional theory calculations (DFT). Various materials properties of KMnO2/RGO including stoichiometric deviation, energetics of K formation, electron redistribution, local atomic environments, etc have been explored in atomic level, and the origin of high capacitance of the KMnO2/RGO is elucidated.
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Session 4 : -
Authors : A. Filippetti (1), A. Urru (1,2), F. Ricci (3), M. B. Maccioni (1), J. Iniguez (4), V .Fiorentini (1)
Affiliations : 1. U Cagliari, 2. SISSA, 3. U Louvain-l.-N., 4 LIST
Resume : Layered perovskites AnBnO3n 2 exhibit a natural internal stacking in blocks of n octahedra along (110), and are interesting players in the field of multiferroicity and beyond [1-4]. Our showcase are two n=5 materials which qualify as the first example of ferroelectric metal and, respectively, ferromagnetic multiferroic metal: Bi5Ti5O17, which exhibits coexisting metallicity and spontaneous polarization (P along the stacking direction) as well as a switchable depolarizing field in a finite system ; and Bi5Mn5O17, a ferromagnet with coexisting metallicity and spontaneous polarization (P transverse to the stacking direction) thanks to an unusual sheet-like Fermi surface . In both cases the polar distortion involves Bi-O bonding-driven off-centering. Further, the n=4 material La2Ti2O7 (a band insulator and ferroelectric, due to uncompensated rotations) becomes upon magnetic doping either a ferroelectric weak-ferromagnet (La2Mn2O7) with anomalously large linear magnetoelectric coupling  or a ferroelectric ferromagnet (V-doped La2Ti2O7 [2,3]) with magnetization inversion upon polarization inversion, i.e. quintessential magnetoelectricity. Refs:  A. Filippetti, V. Fiorentini, F. Ricci, P. Delugas, and J. Iniguez: Prediction of a native ferroelectric metal, Nature Comm. 7, 11211 (2016).  M. Scarrozza, M. B. Maccioni, G. M. Lopez, and V. Fiorentini, Topological multiferroics, Phase Trans. 88, 953 (2015)  M. Scarrozza, A. Filippetti, and V. Fiorentini: Ferromagnetism and orbital order in a topological ferroelectric, Phys. Rev. Lett. 109, 217202 (2012); Multiferroicity in vanadium-doped LaTiO: insights from first principles, Eur. Phys. J. 86, 128 (2013)  A. Urru et al., to be published
Authors : T. Prüfer, K.-H. Heinig, W. Möller
Affiliations : Helmholtz-Zentrum Dresden-Rossendorf, 01328 Dresden, Germany
Resume : For future low power-consumption nanoelectronics, a room-temperature single-electron transistor may be configured by placing a small (few nm diam.) Si nanodot in a thin (<10 nm) SiO2 interlayer in Si. This can be achieved by ion-irradiation induced interface mixing, which turns the oxide layer into metastable SiOx, and subsequent high-temperature thermal decomposition which leaves, for a sufficiently small mixed volume, a single Si nanodot in the SiO2 layer. Corresponding ion mixing simulations have been performed using the binary collision approximation (BCA), followed by kinetic Monte-Carlo (KMC) simulations  of the decomposition process, with good qualitative agreement with the structures observed in related experiments. Quantitatively, however, the BCA simulation appears to overestimate the mixing effect. This is attributed to the neglect of the positive entropy of mixing of the Si-SiO2 system, i.e. the immiscibility counteracts the collisional mixing by “up-hill diffusion” . Consequently, intermitting KMC diffusion steps have been introduced into the BCA mixing simulation, resulting in an excellent predictive power for the irradiation step of the production process. This work has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 688072. •  W. Möller et al., NIM B, 322, 23–33 •  M. Strobel et al., PRB 64, 245422 •  B. Liedke et al., NIM B 316 (2013) 56–61
Authors : Francesco Foggetti, Sergey Artyukhin
Affiliations : Italian Institute of Technology, Genova, Italy - University of Genova, Italy; Italian Institute of Technology, Genova
Resume : Ferroelectric domain walls are emerging as robust 2D systems with promising functionality. Recent scanning tunneling and impedance microscopy studies revealed DC and AC conductivity, 2D electron gas and modified chemistry at ferroelectric domain walls. We study phonons localized at ferroelectric domain walls, and scattering of bulk phonons off ferroelectric domain walls using continuum theory and discrete models. We also discuss signatures of these phenomena in scanning impedance microscopy data.
Authors : Liam McDonnell(1), David Smith(1), Chung Che Huang(2), Qingsong Cui(2)
Affiliations : 1. Physics and Astronomy , University of Southampton, Southampton, United Kingdom; 2. Optoelectronics Research Centre, University of Southampton , Southampton , United Kingdom.
Resume : The electronic and optical properties of monolayer Transition Metal Dichalcogenides such as MoS2, and WS2 are dominated by excitonic effects due to large binding energies from 2D confinement, spin splitting of the valence band, and inversion symmetry breaking. Previous Raman spectroscopy studies have investigated excited excitonic states1, symmetry dependent exciton-phonon coupling 2, thermal conductivity 3, and interlayer coupling4. However these studies are often limited by low excitation energy resolution or are only performed at a single temperature. We present a temperature dependent resonance Raman spectroscopy study on CVD grown monolayer WS2 flakes using excitation energies from 1.86eV to 2.16eV in resonance with the A exciton. Resonance Raman profiles were obtained for a range of temperatures from 4K to 295K, and for multiple phonons including A1’, E’, 2ZA, LA, 2LA, and an unassigned peak attributed to two phonon Raman scattering. The high energy resolution of the resonance Raman profiles allowed observation of both excitons and trions at 4K for the first time, with energies 2.04eV and 2.063eV respectively. Modelling of the Raman scattering amplitude demonstrates that exciton-trion scattering is also required to explain the results. The amplitudes of the scattering terms allow the coupling strength of different phonons to excitons, trions, and exciton-trion scattering to be probed. In addition we discuss how two-phonon scattering peaks allow us to use large wavevector phonons to probe the dark exciton density of states. The resonance profiles for two-phonon, large-wavevector processes are shown to be well described by the same initial and final state resonance model used to describe the single phonon processes. This indicates that dark excitons in WS2 probed in these experiments are a continuum of states.
Session 5 : -
Authors : Jorge Íñiguez
Affiliations : Luxembourg Institute of Science and Technology
Resume : I will discuss antiferroelectric (AFE) materials whose anomalous response to applied electric fields makes them attractive, for example, as capacitors for pulse-power applications that require a fast energy release. After summarizing the basic phenomenology and our current understanding of classic compounds like PbZrO3 , I will address the question of how to induce AFE behavior by suitable chemical doping of a ferroelectric (FE) material, showing representative first-principles results for BiFeO3-RFeO3 solid solutions [2,3,4], where R is a lanthanide. I will then focus on the representative case of BiFeO3-NdFeO3 mixtures, to show that it is possible to predict optimum conditions (composition and cationic Bi-Nd order, direction of appied electric field) yielding enhanced functional properties of such AFEs . In particular, I will discuss the optimization of the room-temperature polarization vs. electric field hysteresis loop, which determines the performance in energy applications and can be simulated by using first-principles-based effective models that I will briefly introduce . Time allowing, I will outline current prospects on tuning AFE behaviors, or even inducing novel ones, in nano-structured materials. Work done in collaboration with many colleagues, particularly the group of L. Bellaiche at U. Arkansas (USA). Work at LIST funded by Luxembourg National Research Fund.  First-principles study of the multimode antiferroelectric transition of PbZrO3, Jorge Íñiguez, Massimiliano Stengel, Sergey Prosandeev and L. Bellaiche, Physical Review B 90, 220103(R) (2014).  First-principles investigation of the structural phases and enhanced response properties of the BiFeO3-LaFeO3 multiferroic solid solution, O. E. González-Vázquez, Jacek C. Wojdeł, Oswaldo Diéguez and Jorge Íñiguez, Physical Review B 85, 064119 (2012).  Finite-temperature properties of rare-earth-substituted BiFeO3 multiferroic solid solutions, Bin Xu, Dawei Wang, Jorge Íñiguez and L. Bellaiche Advanced Functional Materials 25, 552 (2015).  Electric phase diagram of bulk BiFeO3, Massimiliano Stengel and Jorge Íñiguez, Physical Review B 92, 235148 (2015).  Designing lead-free antiferroelectrics for energy storage, Bin Xu, Jorge Íñiguez and L. Bellaiche, Nature Communications 8, 15682 (2017).  Novel nanoscale twinned phases in perovskite oxides, S. Prosandeev, Dawei Wang, Wei Ren, Jorge Íñiguez and L. Bellaiche, Advanced Functional Materials 23, 234 (2013).
Authors : Anne A. Y. Guilbert, Jenny Nelson
Affiliations : Department of Physics and Centre for Plastic Electronics, Imperial College London, SW7 2AZ, London, United Kingdom; Department of Physics and Centre for Plastic Electronics, Imperial College London, SW7 2AZ, London, United Kingdom
Resume : Many processes in organic devices, such as charge transport, are critically influenced by the packing of the molecules, and thus ultimately influenced by the crystal structure of the semi-crystalline semiconductors. Semi-crystalline materials are likely to exhibit different polymorphs, depending on the structure of the side chains, the molecular weight and the processing conditions. As a consequence, designing new materials for improved efficiency is a truly multi-parameter problem and often yields disappointing results. Being able to predict the likely crystal polymorph of a given molecule as a function of temperature, or in other words, drawing the phase diagram of the material, is therefore an important challenge. In this work, we choose as a model system 3-hexylthiophene (3HT) oligomers of different length. We represent the molecules by a modified version of the force field by Moreno et al.1 We investigate potential crystal structures by selecting a number of space groups that are consistent with the likely trans-geometry of the 3HT oligomers and then building a crystal. We find the unit cell parameters for each space group (stacking, lamellar stacking distances and the unit cell angles), by scanning the parameter space using molecular mechanics simulations. We rank the minimised crystal structures according to the Helmholtz free energy, rather than potential energy, in order to evaluate the results against thermodynamic data. Finally, we compare our results with a thermodynamic study of 3HT oligomers.2,3 This step allows us to validate our method and to ultimately accept or reject the proposed crystal structures. 1. M. Moreno, M. Casalegno, G. Raos, S. V. Meille, R. J. Po J. Phys. Chem. B, 2010, 114, 1591-1602. 2. F. P. V. Koch, P. Smith, M. Heeney J. Am. Chem. Soc. 2013, 135, 13695-13698 3. F. P. V. Koch, M. Heeney, P. Smith J. Am. Chem. Soc. 2013, 135, 13699-13709
Authors : Nina Tymińska,1 Angel T. Garcia-Esparza,1 Rabih Al Rahal Al Orabi,2 Tangui Le Bahers1
Affiliations : 1 Univ Lyon, ENS de Lyon, CRNS, Université Claude Bernard Lyon 1, Laboratorie de Chimie UMR 5182, F-69342 Lyon, France; 2 Department of Physics and Science of Advanced Materials Program, Central Michigan University, Mt. Pleasant, MI 48859, USA.
Resume : One of the most significant challenges in the development of water splitting device is finding efficient new solar-energy absorber materials. For an efficient conversion of the energy from a source of visible light a given semiconductor must fulfill strict requirements such as: an appropriate band gap, high dielectric constant, good charge carrier mobility and suitable band positions to perform hydrogen and oxygen evolution water splitting half-reactions. Therefore, these properties are of crucial importance in designing new materials for solar energy conversion. Computational chemistry, mainly based on Density Functional Theory (DFT), has proven to be highly reliable to reproduce these properties, generally difficult to obtain experimentally. Consequently, a step forward in designing new materials for the aforementioned application can be done by using DFT to predict in silico properties of never synthetized semiconductors. In this talk, the properties of YTaON2 and YTiO2N, that were never characterized experimentally, will be presented based on DFT calculations. The discussion will include considerations on the thermodynamic stability of the proposed polymorphs constructed by varying anion ordering along with comparisons with the known parent compounds: LaTaON2 and LaTiO2N. DFT results shows that YTaON2 has a direct 2.1 eV band gap, advantageous dielectric, and transport properties as compared to the LaTaON2 analogue, previously proposed as promising visible light absorber semiconductor for solar-fuels.
Authors : Geoffroy Hautier
Affiliations : Université catholique de Louvain
Resume : Essential materials properties can now be assessed through ab initio methods. When coupled with the exponential rise in computational power, this predictive power provides an opportunity for large-scale computational searches for new materials. We can now screen thousands of materials by their computed properties even before the experiments. This computational paradigm allows experimentalists to focus on the most promising candidates, and enable researchers to efficiently and rapidly explores new chemical spaces. In this talk, I will present the challenges as well as opportunities in materials discovery in high-throughput ab initio computing for opto-electronic materials. I will especially focus on results on p-type transparent conducting materials but also on very recent work on electrides. The impact of high-throughput computing is multiplied when the generated data is shared with free and easy access. I will finish my talk by presenting the Materials Project (http://www.materialproject.org), a collaborative project which precisely targets such a data dissemination.
Authors : Anchalee Junkaew,1* Masahiro Ehara,2 Phornphimon Maitarad,3 Kajornsak Faungnawakij,1 Supawadee Namuangruk,1*
Affiliations : 1National Nanotechnology Center (NANOTEC), National Science and Technology Development Agency (NSTDA), 111 Thailand Science Park, Pathum Thani 12120, Thailand 2Institute for Molecular Science, Nishigo-naka 38, Myodai-ji, Okazaki, Aichi 444-8585, Japan 3Research Center of Nano Science and Technology, Shanghai University, Shanghai 200444, P. R. China
Resume : Anatase titanium dioxide (TiO2) has been extensively applied as a support and a catalyst in various applications. Contrast to the stability, the reactive (001) facet becomes more attractive for catalytic reactions than the stable (101) surface. Herein, the catalytic and electronic charge nature of the anatase-TiO2 (001) and (101) surfaces were elucidated by using a plane-wave based density functional theory (DFT). Our investigation indicates that the (001) facet reveals promising catalytic properties for a selective catalytic reduction of NO using NH3 (SCR-NH3 of NO) and a H2S desulfurization at low temperatures. Their rate determining steps require activation energies approximately 13 to 15 kcal/mol. The obtained results agree well with the experimental observation that active sites, i.e. five-fold-coordinate Ti atoms (Ti5c) and two-fold-coordinate O atoms (O2c), play important roles on the NH3-SCR of NO reaction over the (001) surface. The insights into mechanisms and charge properties assure the superior catalytic performance of this (001) surface compared to the (101) surface. According to this finding, the anatase-TiO2 (001) catalyst is not only promising for these pollutant removal, but it is also attractive for using in other catalytic applications.
Authors : Aleix Comas-Vives, Lucas Foppa, Christophe Copéret
Affiliations : Department of Chemistry and Applied Biosciences, ETH Zürich, Vladimir-Prelog-Weg 1-5, CH-8093, Zürich, Switzerland
Resume : The first step of Fischer-Tropsch Synthesis (FTS), which converts syngas (CO/H2) to hydrocarbons within the gas-to-liquid technology, consists in the activation of CO in a CO*/H* adlayer on the surface of metallic catalysts (Ru, Co, Fe). Since FTS activity and selectivity depend on CO cleavage, the elucidation of reaction mechanisms and active sites for this transformation has been a very active field of research, with static DFT approaches playing a central role.(1-3) Two main mechanisms for CO cleavage have been proposed for Ru catalysts based on static Density Functional Theory (DFT) calculations: the direct C–O cleavage (on step-edge sites) and the hydrogen-assisted route (on flat surfaces). For the former CO cleavage on different surface sites present on Ru nanoparticles, we recently found there is a particle size effect that can be rationalized via first principles calculations combined with bonding analysis.(1) Nevertheless, in order to compare the two proposed mechanisms, it is needed to go beyond the static simulation approach by introducing adlayer and coverage effects using mobile and reactive co-adsorbates, under finite temperatures since the diffusion of surface species and the interaction of the reaction intermediates with the co-adsorbates can modulate the reaction path. Here, we evaluate how finite temperature effects influence CO cleavage mechanisms during Ru FTS using ab initio molecular dynamics (AIMD) simulations on Ru flat and stepped model surfaces covered with a CO*/H* adlayer.(4) 1. L. Foppa, C. Copéret, A. Comas-Vives, J. Am. Chem. Soc. 2016, 138, 16655. 2. B. T. Loveless, C. Buda, M. Neurock, E. Iglesia, J. Am. Chem. Soc. 2013, 135, 6107. 3. G. T. K. Gunasooriya, A. P. van Bavel, H. P. C. E. Kuipers, M. Saeys, ACS Catal. 2016, 6, 3660. 4. L. Foppa, M. Iannuzzi, C. Copéret, A. Comas-Vives, 2018, submitted.
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Session 7 : -
Authors : Axel Gross
Affiliations : Institute of Theoretical Chemistry, Ulm University, 89069 Ulm, Germany
Resume : Understanding processes at electrochemical electrode-electrolyte interfaces is crucial in order to improve devices in electrochemical energy storage and conversion, such as batteries or fuel cells. In spite of this relevance, our knowledge about microscopic structures and processes at these interfaces is still rather limited. The theoretical description of these interfaces is hampered by at least two factors. i) In electrochemistry, properties of the electrode-electrolyte interfaces are governed by the electrode potential which adds considerable complexity to the theoretical treatment since charged surfaces have to be considered. ii) The theoretical treatment of processes at solid-liquid interfaces necessitates a proper description of the liquid which in principle requires to perform computationally expensive statistical averages. I will in particular focus on how, despite these obstacles, the electrochemical environment can be appropriately and efficiently taken into account in theoretical first-principles studies. For example, the presence of the electrolyte can be treated in a grand-canonical approach as a reservoir. Furthermore, in order to design interfaces with improved properties, the concept of descriptors can be rather helpful. It will be discussed whether the height of metal self-diffusion barriers can serve as a descriptor for the occurrence of dendrite growth in batteries which is one of the major issues in the safe operation of batteries.
Authors : Marco Catanzaro, Remedios Cortese, Sophie Hermans, Dario Duca
Affiliations : Marco Catanzaro, Dr. Remedios Cortese, Prof. Dario Duca Dipartimento di Fisica e Chimica Università degli Studi di Palermo Viale delle Scienze Ed. 17, I-90128, Palermo, Italy; Sophie Hermans Université catholique de Louvain, Institut de la Matière Condensée et des Nanosciences (IMCN), Place Louis Pasteur 1/3, B-1348 Louvain-la-Neuve, Belgium
Resume : In order to evaluate the potential use of metal-free hexagonal boron nitride (h-BN) nanosheet catalysts, a study was performed, within the frame of density functional theory (DFT), on the formic acid (FA) dehydrogenation to CO2 and H2 that occurs on hydrogen-activated h-BN defects. In the BN material model, the activated site constellations were obtained by removing adjacent B, N couples and partially hydrogenating them. Different mechanistic hypotheses were considered. In particular, the possibility of preferentially dehydrogenating O or C atoms of FA, either by N or B sites of h-BN was evaluated. The calculated energy profiles would suggest that the expected mechanism firstly involves the dehydrogenation of the C atom and then the dehydrogenation of the O one, through a five-terms cyclic transition state. In the latter, the carboxylic oxygen was bound to a B site of h-BN. On the grounds of the theoretical results obtained, it was conceived an experimental study, which included defective h-BN nanosheet catalyst synthesis as well as its morpho-structural characterization and the FA catalytic decomposition analysis. In this case, the conversion of FA to CO2 and H2 clearly increased along with the hydrogenation degree of the boron nitride based catalyst, seeming to experimentally confirm the H2 defect activation, promoting the dehydrogenation processes.
Authors : Janne Kalikka
Affiliations : Tampere University of Technology, Laboratory of Physics, Finland
Resume : Chalcogenide phase-change (PC) materials are widely used in data storage due to their ability to switch reversibly between crystalline (electrically conductive, optically reflective) and amorphous (resistive, non-reflective) phases. The switching is achieved with a short electric or laser pulse, and the current state can be read with a less powerful one. The memory is non-volatile, and rewritable in timescale of nanoseconds. A paper published in 2011  reported the same behaviour in crystalline heterostructure with the switching between two crystalline states that behaved not only similarly but with better operating characteristics than the alloy material. Today's simulation methods allow screening of different materials with good accuracy. However, the number of possible layer sequences in a chalcogenide heterostructure is high enough to make 'brute-force' screening not feasible. A genetic algorithm is known to perform well when searching for a global minimum in a complex landscape with a large number of local minima. The results of genetic algorithm search for the most stable heterostructure with Ge2Sb2Te5 overall chemical composition are discussed . Based on the search, a potential structure for the phase-change material is suggested, with a XRD pattern that agrees with experimental pattern of an epitaxially grown structure.  Robert E. Simpson et al., Nat. Nanotechnol., 6, 501–505 (2011).  J. Kalikka et al., Nanoscale 8, 18212-18220 (2016).
Authors : Luca M. Ghiringhelli, Matthias Scheffler
Affiliations : Fritz Haber Institute of the Max Planck Society, Berlin, Germany
Resume : The number of possible materials is practically infinite, while only few hundred thousands of (inorganic) materials are known to exist and for few of them even basic properties are systematically known. In order to speed up the identification and design of new and novel optimal materials for a desired property or process, strategies for quick and well-guided exploration of the materials space are highly needed. A desirable strategy would be to start from a large body of experimental or theoretical data, and by means of “data-analytics” methods, to identify yet unseen patterns or structures in the data. Specifically I will describe how to spot yet unseen patterns or structures in the data, by identifying the key atomic and collective physical actuators by compressed sensing and machine learning. This enables us to build maps of materials where different regions correspond to materials with different properties. As the connections between actuators and materials properties are intricate, an attempt to describe the relationship in terms of an insightful physical model may be pointless. I will demonstrate our methodology for describing and predicting 2D topological insulators, the metal/insulator classification, catalytic CO2 activation, and more.
Session 8 : -
Authors : Cesare Franchini
Affiliations : University of Vienna
Resume : In this talk, we shall discuss the application of single-particle and quasiparticle approaches for the calculation of the properties of complex materials from a first principles perspective. Beside reviewing the fundamental theories we describe the technical procedures to obtain quantitative predictions without adjustable parameters using self-consistent hybrid functionals (scPBE0), GW, the Bethe-Salpeter equation (BSE), and DFT+U with interaction parametrs U/J derived from the constrained random phase approximation. Specific topics include: band gaps and band structure, convergence protocols in GW (comparison between the conventional approach based on an incremental variation of a specific set of parameters and the basis-set extrapolation scheme), and optical properties using the BSE and model-BSE methods. Most of the examples will focus on (spin-orbit) transition metal oxides, where the entanglement of orbital, spin and lattice interactions gives rise to collective effects which are difficult to model and understand, but are of key importance for practical functionalizations.
Authors : Belviso Florian, Cammarata Antonio
Affiliations : Department of Control Engineering, Faculty of Electrical Engineering, Czech Technical University in Prague, Karlovo Náměstí 13, 121 35 Prague, Czech Republic.
Resume : We investigated the atomic scale tribological properties of transition metal dichalcogenides (TMDs), using ab-initio techniques. Such compounds are formed by triatomic layers with MX2 stoichiometry (M: transition metal cation, X: chalcogen anion) held together by van der Waals forces. We considered 6 prototypical MX2 TMDs (M=Mo, W; X=S, Se, Te) with hexagonal P63/mmc symmetry, focusing on how specific phonon modes contribute to their intrinsic friction. Within the DFT framework, we described the exchange-correlation interaction energy by means of the PBE functional, including long range dispersion interactions in the Grimme formulation (DFT-D3 van der Waals). We identified and disentangled the electro-structural features that determine the intra- and inter-layer motions affecting the intrinsic friction by means of electro-structural descriptors such as orbital polarization, bond covalency and cophonicity.1 We show how the phonon modes affecting the intrinsic friction can be adjusted by means of an external electrostatic field. In this way, the electric field turns out to be a knob to control the intrinsic friction. The presented outcomes are a step forward in the development of layer exfoliation and manipulation methods, which are fundamental for the production of TMD-based optoelectronical devices and nanoelectromechanical systems.  Cammarata, Antonio and Polcar, Tomas (2015) DOI 10.1039/C5RA24837J
Authors : Victor Claerbout
Affiliations : Czech Technical University in Prague, CVUT
Resume : It is estimated that 23% of all energy consumed is lost on energy dissipation due to friction and wear. The control of energy loss under tribological conditions is then mandatory for a sustainable development. To this aim, tribological research has focused on transition metal dichalcogenides (TMD) which are considered to be among the best solid lubricants due to their lamellar structure. Specifically, molybdenum disulfide has revealed its super low friction behavior . However, a full understanding of the mechanism behind this behavior remains lacking. In this contribution we aim to elucidate the phenomena taking place at the nanoscale when two layers of molybdenum disulfide slide one atop of another. In particular, by means of molecular dynamics simulations, we studied the effect of rotational sliding anisotropy  (i.e., the changing frictional behavior upon a non-zero misfit angle between the two layers or varying the sliding direction) on the energy dissipation due to friction. We simulated different sliding conditions (varying e.g. normal load) in order to highlight their effect on the lubricating properties. These results will help on the one hand to identify the fundamental mechanisms that govern friction at an atomistic level in TMD’s, as well as providing guidelines for the design of novel layered materials with improved tribological properties.
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Session 9 : -
Authors : Albert Bruix, Karsten Reuter
Affiliations : Technical University of Munich
Resume : The surfaces of many late transition metals are oxidized under ambient conditions or at increased oxygen pressures, which has strong implications for corrosion and catalysis. Stable O-enriched states resulting from oxidation may consist of the metal surface with high concentration of adsorbed O atoms, the corresponding metal-oxide, or something in between (e.g. surface oxides). The quantitative atomistic modeling of oxidation reactions catalyzed on transition metals often therefore requires accounting for the reactivity on more than one stable phase. In this work, CO oxidation on Pd(100) and the corresponding surface oxide phase PdO(101)/Pd(100) formed on it is addressed by means of Density Functional Theory calculations and kinetic Monte Carlo simulations. We use a novel multi-lattice microkinetic modeling approach to investigate the role of the metal and oxide phases and how their coexistence under different operation conditions affects catalytic performance. This also allows probing the relevance of sites at the interface between the two phases and the effect of phase transitions on the overall reactivity. We furthermore show how results from this two-phase model can be generalized to understand the kinetics of multifunctional catalysts combining a metal, an oxide, and their interface.
Authors : Huseyin Sener Sen and Tomas Polcar
Affiliations : Department of Control Engineering, Czech Technical University in Prague
Resume : A common goal for materials employed in nuclear environments is to exhibit the highest radiation tolerance. The lifetimes of current and even more of new-build and future reactors are largely determined by materials issues such as embrittlement and swelling in fuel cladding and structural components. Radiation tolerance-related requirements are further boosted by the higher performance expected for Generation IV fission reactors and fusion reactors, which will expose materials to much higher numbers of atomic displacements then current and near-feature reactors. In the process of energy production via fission (and, to some extent, by fusion), both fuel components and structural materials are subject to substantial radiation damage, which initially appears in the form of local intrinsic point defects within the material (i.e. vacancies and interstitials). The point defects agglomerate, interact with the underlying microstructure and lead to undesirable effects such as blistering and radiation-induced embrittlement, which render the materials unsuitable for the desired application. Another important factor is represented by helium embrittlement. He originates from the transmutation of reactor elements which can release alpha particles that acquire electrons to become helium atoms. He is insoluble and mobile in most metals and migrates to grain boundaries and interfaces where it forms bubbles leading to embrittlement. Many of the materials being developed for deployment in these environments are based on incremental improvements to existing materials such as Oxide Dispersion Strengthened (ODS) steels. Here, we present nanoscale Zr-Nb metallic multilayer composite as s a model material to understand the role of interfaces, defects and He in radiation environments using density functional theory in an attempt to create radiation damage tolerant, self-healing material.
Authors : Andrea Castelli, Davide Spirito, Davide Altamura, Mirko Prato, Cinzia Giannini, Giulia Biffi, Sergey Artyukhin, Luca Ceseracciu, Roman Krahne, Liberato Manna and Milena Arciniegas
Affiliations : Nanochemistry Department, Optoelectronics, Quantum Materials Theory, and Materials Characterization Facility. Istituto Italiano di Tecnologia, Via Morego 30, 16163 Genova, Italy.
Resume : High compressive strains have been used as a way to tune physical properties of three-dimensional perovskite crystals. Pressure in the GPa range generates structural distorsions in perovskite lattice, affecting the electronic structure and modifying optical and transport properties. Surprisingly, stacks of hyrid organic-inorganic perovskite flakes demonstrate tunability of optical spectra at much lower pressures, in the range of tens of MPa. Pristine flakes are near-white emitting, and in-situ photoluminescence experiments during loading and unloading in a mechanical pressure cell reveal drastic change in their optical emission spectrum, particularly an enhancement of the blue emission. Using first principles simulations, we are able to reproduce the main features of the optical properties and to analyze their variations under compressive strain.
Authors : Feng-Ren Fan , Hua Wu , Dmitrii Nabok , Shunbo Hu , Wei Ren , Claudia Draxl , Alessandro Stroppa 
Affiliations :  Laboratory for Computational Physical Sciences (MOE), State Key Laboratory of Surface Physics, and Department of Physics, Fudan University, Shanghai 200433, China;  Physics Department and IRIS Adlershof, Humboldt-Universität zu Berlin, Zum Groβen Windkanal 6, D-12489 Berlin, Germany;  Department of Physics, and International Center of Quantum and Molecular Structures, Shanghai University, Shanghai 200444, China;  CNR-SPIN, Via Vetoio, L’Aquila 67100, Italy
Resume : Hybrid organic-inorganic compounds attract a lot of interest for their flexible structures and multifunctional properties. For example, they can have coexisting magnetism and ferroelectricity whose possible coupling gives rise to magnetoelectricity. Here using first-principles computations, we show that, in a perovskite metal-organic framework (MOF), the magnetic and electric orders are further coupled to optical excitations, leading to an Electric tuning of the Magneto-Optical Kerr effect (EMOKE). Moreover, the Kerr angle can be switched by reversal of both ferroelectric and magnetic polarization only. The interplay between the Kerr angle and the organic-inorganic components of MOFs offers surprising unprecedented tools for engineering MOKE in complex compounds. Note that this work may be relevant to acentric magnetic systems in general, e.g., multiferroics.
Department of Control Engineering, Karlovo Náměstí, 13, 121 35, Prague 2, Czech Republiccammaant@fel.cvut.cz
Department of Materials Science and Engineering, 2220 Campus Drive, Evanston, Illinois 60208, USAdanilo.firstname.lastname@example.org
Dipartimento di Fisica e Chimica, Viale delle Scienze ed.17, 90128, Palermo, Italyremedios.email@example.com