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2014 Fall Meeting



Computer modelling in nanoscience and nanotechnology: an atomic-scale perspective III

Atomic-scale resolution is nowadays attainable in most modern nanoscience experiments. However, the systematic determination of atomic/molecular coordinates remains far from trivial and particularly challenging, especially when the resolution and/or the capabilities of diffraction or microscopy techniques reach their limits. A viable counterpart on the theoretical side, capable of complementing and enriching the experimental findings is therefore highly desirable in many areas, such as condensed matter physics and chemistry, materials science, biological chemistry, up to health sciences and geophysics, where the atomistic structural information can be readily exploited to explore the interplay between structure and electronic, magnetic, optical, and dynamical (transport) properties. Indeed, the predictive power of atomic scale approaches extends now even to biomedical sciences or geophysics, thanks to the possibility to perform increasingly more extensive and accurate calculations. In these fields, structural determination is not only extremely challenging, because of the increased structural complexity and the very long time-scales involved, but it can be considered as the ultimate goal following synthesis and characterization through spectroscopy techniques.

The main scope of our bi-annual meetings in Warsaw is to follow, critically discuss and review recent advances in the area of atomic-scale modeling of complex materials. This has to be intended as a broad research field (going well beyond the traditional domain of application of solid-state physics and chemistry) in which theoretical methodologies and practical applications, based on the calculation of reliable interatomic forces and energy landscapes, do coexist effectively. As a second goal, the Symposium will set the scene for a thorough understanding of the complex multi-scale nature of materials processes. This can be achieved by investigating the links between the detailed theoretical/computational description at the electronic and atomic scales and alternative approaches suitable for intermediate and large scales.

The symposium will be also be open to significant contributions from the field of macro-molecules of organic and biological interest, and to new computational challenges originating from the vast domains of geophysical materials, non conventional materials for energy harvesting, conversion and storage, or bio-mimetic and natural materials (e.g., wood, paper, bone, silk as well as their artificial counterparts).

This symposium represents a unique opportunity to celebrate, at the European scale, the increasing importance and success of atomistic modelling techniques, including first principle methods, as a tool to complement, elucidate, inspire and guide new experiments.

Some hot topics to be included in the symposium will be (but are not limited to):

  1. First-principles molecular dynamics: what role for linear scaling methods? Is the simulation of metallic systems at finite temperatures fully handled and understood? What are the most recent implementation advances? (e.g. real space methods, new dynamical algorithms, QM/MM…)
  2. How the study of complex magnetic properties (super-exchange, strong correlation effects, spintronics) can be coupled to the determination of the structure, without sacrificing accuracy?
  3. Toward simpler total energy recipes: new applications of effective interatomic potentials in the area of nano-structures?
  4. Can we define general strategies for the coarse-graining of microscopic data? How to approach long-time scales, especially in relation to the application of slow deformation and slow heating/cooling rates?
  5. Is it possible to define computational routes complementary to experimental studies, for the design of atomic clusters, and self-assembled molecular superstructures?
  6. How to identify atomistic mechanisms for diffusion and growth of nanostructures, in combination with the synthesis of e.g.nanoelectronics, biomedical, or energy-storage devices?  

Invited speakers:

With the aim of stimulating the participation especially from younger and promising scientist, the about 12 Invited Talks will be selected among the best submitted abstracts.

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Authors : L. Luneville1, G. Demange2, V Pontikis3, D. Simeone 2
Affiliations : 1CEA/DEN/DANS/DM2S/ LRC CARMEN CEN Saclay 91191 Gif sur Yvette, & CNRS/ SPMS UMR8785 LRC CARMEN, Ecole Centrale de paris, F92292, Chatenay Malabry, France 2CEA/DEN/DANS/DMN/ LRC CARMEN CEN Saclay 91191 Gif sur Yvette, & CNRS/ SPMS UMR8785 LRC CARMEN, Ecole Centrale de paris, F92292, Chatenay Malabry, France,. 3CEA/DSM/IRAMIS 911919 Gif sur Yvette, France

Resume : Ion beam mixing offers the unique opportunity for disordering materials at the atomic scale , thus allowing for studies of steady states which trigger theemergence of new phases absent from the esuilibrium phase diagram1. Molecular Dynamics is extensively used forpredicting the appearance of complex and simple defects, in metals and alloys, however the technique is limited to short times (few picoseconds) and cannot describe satisfactorily steady states establishing at diffusional time scales (few micro seconds)2 and mesoscopic space scales. To overcome these limitations, we propose to combine MD simulations with the Phase Field method for describing the microstructure over few tens of nanometers at the diffusion time escale3. This modelling is here used for the prediction of irradiation induced steady states in the AgCu systeme. Analyzing the results thereby obtained, shows that irradiation slows down the coarsening of Ag rich domains stemming from the spinodal decomposition at equilibrium. Additionally, we found that the size distribution of precipitates and their spatial distribution can both be tuned by appropriately choosing the parameters of the incident beam. References: [1] G Baldinozzi, D Simeone, Phys Rev Lett. 90, 216103 (2003) [2] H Emmerich, H Lowen,et al., Advances in Physics, 92, 1-95 (2012) [3] D. Simeone, G. Demange, L. Lunéville, Phys. Rev. E 88 (2013) 3. [4] M. Briki, J. Creuze, F. Berthier, B. Legrand, Solid State Phenomena 174 (2011) 658.

Authors : Elena V. Levchenko, Alexander V. Evteev, Irina V. Belova and Graeme E. Murch
Affiliations : The University of Newcastle, Australia

Resume : In this contribution, we discuss the formalism of thermotransport in a binary system. We introduce the reduced heat of transport parameter associated with the interdiffusion flux, which is invariant to a change of reference frame and appears to be useful for a direct comparison of simulation and experimental data from different sources. In the presence of a temperature gradient, this parameter characterizes part of the interdiffusion flux that is proportional to the temperature gradient, while in an isothermal binary system it represents the reduced heat flow (pure heat conduction) consequent upon unit interdiffusion flux. Then, we employ this formalism to study the heat transport properties for a model of liquid Ni50Al50 alloy by employing equilibrium molecular dynamics simulations in conjunction with the Green-Kubo method. Our results predict that in the liquid Ni50Al50 alloy in the presence of a temperature gradient Ni tends to diffuse from the cold end to the hot end while Al tends to diffuse from the hot end to the cold end.

Poster session : F. Cleri, R. Kozubski, C. Massobrio, C. Molteni
Affiliations : IEMN, University of Lille I, 59652 Villeneuve d'Ascq (France)

Resume : High-density packing in organic crystal is usually associated with an increase of the coordination between molecules, since the early formulations of molecular symmetry arguments by Kitaigorodski in the late 50s. We contend the validity of this concept for two-dimensional molecular networks self-assembled on solid semiconductor surfaces, by using a combination of scanning tunneling microscopy experiments and multi-scale computer simulations. We study the phase transitions between different polymorphs and we demonstrate that, contrary to simple intuition, the structure with the lowest packing density may correspond indeed to the highest coordination. The subtle competition between intermolecular and surface dispersion forces (often ignored in the discussion of molecular symmetry) is at the origin of such a counter-intuitive result. Having the lowest free energy, such low-density structures spread out macroscopically as the most stable polymorphs over a wide range of molecular concentrations.

Authors : Ricardo Paupitz, Chad E. Junkermeier, Adri C. T. van Duin, Paulo S.Branicio
Affiliations : Departamento de F ́ısica, IGCE, Universidade Estadual Paulista, UNESP, 13506-900, Rio Claro, SP, Brazil; Department of Mechanical and Nuclear Engineering, The Pennsylvania State University, University Park, PA 16802, USA; Institute of High Performance Computing, 1 Fusionopolis Way, #16-16 Connexis, Singapore 138632, Singapore

Resume : We propose and investigate a class of macromolecules based on the architecture of the well known fullerenes. In order to construct these molecules, we used as buiding blocks two dimensional structures: porous graphene and biphenylene- carbon. Density functional-based tight binding methods (DFTB) as well as reactive molecular dynamics (REAXFF) methods were applied in this study to obtain the electronic and structural properties of these molecules. Our calculations predict mechanical stability up to temperatures of 2500K. The atomization energies of carbon structures is predicted to be in the range 0.45 eV/atom to 12.11 eV/atom (values relative to the C60 fullerene), while the BN analogues have atomization energies between -0.17 eV/atom and 12.01 eV/atom (compared to the B12N12 fullerene). Due to their high porosity, these structures may be good candidates for gas storage and/or molecular encapsulation.

Authors : Tiago Botari, Pedro Alves da Silva Autreto, Ricardo Paupitz, Douglas S. Galvao
Affiliations : Applied Physics Department, State University of Campinas (UNICAMP),13083-859 Campinas-SP, Brazil; Applied Physics Department, State University of Campinas (UNICAMP),13083-859 Campinas-SP, Brazil; Physics Departament, Univ. Estadual Paulista (UNESP), 13506-900 Rio Claro-SP, Brazil; Applied Physics Department, State University of Campinas (UNICAMP),13083-859 Campinas-SP, Brazil;

Resume : Healing of nanoholes etched in graphene was already demonstrated experimentally. Graphene healing was observed either at high temperature and at room temperature under low energy STEM observation. It is known that an electron beam can act as a local heat source, what could provide a possible source of energy to promote this effect. Nevertheless, details of the healing mechanism are not clarified yet. In order to address this question, we carried out fully atomistic reactive molecular dynamics simulations under conditions that are reasonably similar to the experimentals ones. WIth our setup it was possible to demonstrate that at very high temperatures, nanoholes can be healed, since carbon atoms have enough thermal energy to overcome the potential barrier present at the edge of structural defects. However, at room temperature, thermal energy is not high enough to drive the healing process. If we consider a local heat source in order to simulate the eff ect of a electron beam, perfect healing can be achieved. Depending on the heating rate, two diff erent perfect healing mechanisms were observed. For low rates, absorbed atoms reconstruct the structure at the nanohole, remaining stable, while for high heating rates, intermediary reconstructions are observed before perfect healing is completed. Our results show that only a carbon source and the heating eff ect of an electron beam are two essential ingredients to explain the healing effect.

Authors : Teresa Moskaliovienė, Arvaidas Galdikas, Akvilė Petraitienė
Affiliations : Physics Department, Kaunas University of Technology, Studentu 50, LT-51368 Kaunas, Lithuania.

Resume : The nitrogen transport in austenitic stainless steels (ASS) and CoCr alloys during plasma nitriding at temperatures below nitrides formation was numerically studied on the basis of the proposed nitrogen diffusion model. The model considers penetration of nitrogen atoms to deeper layers of alloys in presence of internal stresses which are created due interstitial nitrogen expands of the crystal lattice of the alloy. Internal stresses, particularly static pressures, act as additional driving forces for nitrogen diffusion. The influence of internal stresses on nitrogen diffusion is analyzed under the framework of theory based on Onsager’s principle of microscopic reversibility involving the process of barodiffusion for solids. The results show that stress gradient is responsible for making the nitrogen diffuse towards the regions of maximum internal stress thereby accelerating the diffusion of nitrogen atoms to deeper layers of alloys, i.e. resulting of plateau formation in nitrogen depth profiles. It was found that nitrogen diffusion coefficient in ASS and CoCr alloys during nitriding varies with nitrogen concentration according to Einstein-Smoluchowski relation D(CN)~1/CN. It was shown that swelling has significant influence on the nitrogen distribution in plasma nitrided ASS and CoCr alloys. The effects of the hydrogen/nitrogen ratios in the nitriding plasma on the nitrogen depth profiles were analyzed by proposed model.

Authors : Andriy Ostapovets
Affiliations : entral European Institute of Technology – Institute of Physics of Materials, Academy of Sciences of the Czech Republic (CEITEC-IPM), Žižkova 22, 61662 Brno, Czech Republic

Resume : Twinning is important deformation mode in materials with hexagonal close packed structure. Recent experiments have revealed new features of twinning in such materials. Faceting of twin boundaries was observed in magnesium and cobalt; coarse-grained magnesium alloys often contain non-lamellar, irregular shaped twins; twinning without invariant twin planes was also observed in magnesium nano-pillars. Present work is devoted to atomistic simulations of twin growth and migration. Mechanisms that lead to the formation of facets in twin boundaries are studied in present work as well as migration mechanisms of the twin boundaries.

Authors : P.-A. Francioso, E. Lampin, P . L. Palla, E. Lampin and F. Cleri
Affiliations : IEMN UMR CNRS 8520 and University of Lille - CS 60069 - 59625 Villeneuve d'Ascq Cedex- France

Resume : Although crystalline silicon (cSi) and amorphous silica (aSiO2) are the key materials that have driven the development of microelectronics over decades, the details of the interface structure between this cristal and its oxide remain poorly known from the experimental point of view. Atomistic simulations can provide models of the interface, the problem being to characterize these structures in order to correlate some of its features to known characteristics of the interface, such as the dangling bond density. In the present work, we present an alternative characterization by means of an energy profiling across the interface. The procedure allows to identify the evolution of the local environment of silicon atoms from cSi to aSiO2. Moreover, an interface thickness of less than 10 Angstroms can be extracted from the energy profile, in agreement with published measurements by core-level microscopy.

Authors : S.I. Sidorenko1, S.M. Voloshko1, A.M. Gusak2 , D.V. Tyshchenko1
Affiliations : 1National Technical University of Ukraine "Kyiv Polytechnic Institute"; 2Cherkasy National University named after Bogdan Khmelnitsky

Resume : Often reaction diffusion (layers intermediate phase between reacting components growth) occurs at temperatures below 0.7 Tmelt and diffusion along grain boundaries makes significant contribution. Therefore, to describe phases growth kinetics is important for prediction of evolution time of grain structure of intermediate phases. Results of calculations of grains growth in “open system” of “model antiphase domains” are presented. Although real grains theoretically may merge into one (merging), but in practice this rarely happens. It was necessary to propose an algorithm initializing the system, which would avoid boundary with zero energy. As a result, it may be found a pattern of influence of lateral growth antiphase domains on flow rate. Computer simulation of antiphase domains Cu3Au type ordered phase growth competition was carried out by Monte-Carlo algorithm RTA (Residence Times Algorithm). Wondered mesh nodes FCC-lattice each of which can be an atom of copper or gold. The model was adopted by exchange mechanism of diffusion. At the inception of domains average size decreases. Perhaps this is due to thickness of artificially defined cross-domain walls are not equilibrium in initial time extended, reducing the net amount of ordered domains. After initial relaxation average size grow begins (approximately – linearly). For the “open system” antiphase domains (i. e. thin film A3B1 between substance A and substance A1B1) timing atomic jump showes average lateral growth of almost an order of magnitude faster than a "closed system". The model proposed leads to effect of Flux Driven Grain Growth (FDGG).

Authors : Tatsuki Oda1,2, Daiki Yoshikawa2, Masao Obata2, Shinya Haraguchi2, Yusaku Taguchi2, Masahito Tsujikawa2
Affiliations : 1 Institute of Science and Engineering, Kanazawa University, Kanazawa 920-1192, Japan 2 Graduate School of Natural Science and Technology, Kanazawa University, Kanazawa 920-1192, Japan

Resume : The spintronics has grown up intensively to realistic applications in the technology of magnetic random access memory (MRAM) development. Such development has been remarkable in memory density, reading-writing speed, and non-volatile property in cooperation with the technologies of spin-injection and physics of spin transfer torque. The basic physics about magnetism has been developing in the response to electric field (EF) [1,2]. This has emerged as the connection with low power consumption device and small energy scale of magneto-electric effects. The direction to which we make efforts has typically been of interface of metallic magnet and insulator. Toward developing an application, sensitivity or large response to EF may be required. I will discuss a design of interface magnetic anisotropy and its EF effect on the basis of the theoretical works [3,4] with the total energy and electronic structure calculation. [1] D. Chiba et al., Nature. 455, 515 (2008). [2] Weisheit et al., Science 315, 349 (2007). [3] S. Haraguchi et al., J. Phys. D: Appl. Phys., 44, 064005 (2011). [4] M. Tsujikawa et al., J. Appl. Phys., 111, 083910 (2012).

Authors : Fedorov M.M, Pashkevych M.O, Sidorenko S.I., Zamulko S.O.
Affiliations : Fedorov M.M, Pashkevych M.O, Sidorenko S.I., Zamulko S.O.

Resume : Results of last years in theoretical study of vacancy formation energy (VFE) in a variety of materials show the temperature dependence: VFE grows with the growth of the temperature. Previous experimental studies doesn't show such dependence. In order to align future theoretical results with experimental data we propose new model of determining the VFE from first principles considering the temperature effect. We calculate the VFE for Au, Pt and W. Unadjusted VFE is calculated via the Density Functional Theory with the Projector Augmented Wave Method and Generalized Gradient Approximation. Applied exchange-correlation potentials are obtained from Local Density Approximation. Cutoff energy is restricted by 300eV. Structure geometry optimization is conducted by the relaxation of the ions positions with the fixed supercell volume. Brillouin zone integration is performed using 8x8x8 k-points grid, obtained via the Monkhorst-Pack scheme. Optimal supercell consists of 64 atoms in case of Au and Pt, and of 54 atoms in case of W. Adjusting of VFE is made by adding the free energy of phonons (FEP) and the free energy of the thermally excited electrons (FEE) to the unadjusted VFE. FEP is obtained from the phonon frequencies using the small displacements method. FEE is calculated from the density of states on the Fermi level. Obtained results of adjusted VFE show the good convergence with the previous experimental studies VFE, aligning the future theoretical data with experimental result.

Authors : Przemysław Niedzielski, Ewa Raj, Zbigniew Lisik
Affiliations : Lodz University of Technology, Department of Semiconductor and Optoelectronic Devices, Wolczanska 211/215, 90-924 Lodz, POLAND

Resume : Metal-Organic Chemical Vapour Deposition (MOCVD) is one of the epitaxial growth technologies for manufacturing electronic devices. Although, the technique is used frequently, due to high temperatures and huge complexity of the process, a better understanding of the phenomena emerging in the reactor is needed. One of the most important problems during the growth process is to maintain appropriate temperatures within the MOCVD chamber and in the close proximity to the sample surface. Unfortunately, the temperature control is difficult because of rigorous conditions in the reactor chamber and the limited access to perform measurements. Hence, the numerical investigations of the reactor can be used. They can show an influence of various factors such as: carrier gas parameters, flow rates, pressures on temperature field within the reactor chamber. The study presents numerical analysis of thermal-kinetic phenomena in Close Coupled Showerhead reactor with the aid of reduced 3D model proposed by the authors. The model allows reducing computational efforts while keeping the results accuracy. The simulations cover investigations of temperature field within the chamber and processing samples, while changing such parameters as carrier gases, their flow rate, showerhead cooling and susceptor heating. Moreover, the influence of contact thermal resistance between the susceptor and the sample substrate is discussed in the paper. It can be of crucial importance for the growth conditions.

Authors : Federico Comitani, Vittorio Limongelli, Carla Molteni
Affiliations : Kings' College London, Physics Department (UK); University of Naples Federico II, Department of Pharmacy (Italy); King's College London, Physics Department (UK)

Resume : Pentameric ligand-gated ion channels (pLGICs) are important proteins that mediate fast synaptic transmission. Their malfunction leads to serious neuronal disorders; in invertebrates they are involved in insecticide resistance. They are efficient nano-machines, composed of five subunits arranged about an ion permeable pore embedded in the cell membrane, with an extracellular, a transmembrane and an intracellular domain. The channel opens in response to the binding of a ligand (e.g. GABA) to the extracellular domain: ions can the flow across the membrane. The complexity of pLGICs and the limited structural information prevent a detailed understanding of how they function. To elucidate at the atomistic level the ligand-binding activation mechanism, we have investigated the RDL (Resistance to Dieldrin) receptor, an insect GABA-gated pLGICs for which experimental data are available. We have build homology models of its extracellular domain and used docking and molecular dynamics techniques to study the behaviour of GABA in the binding site, including the interaction networks it formed, the conformers it visited and the possible role of water molecules. Moreover, we have used “funnel metadynamics”, a novel methodology for rare events which uses a funnel shaped confining potential to limit the exploration of unbound states in the solvent, to simulate binding and unbinding events and accurately evaluate the binding free energy, in native and mutated systems.

Authors : Niccolo' Corsini, Nicholas D.M. Hine, Peter D. Haynes and Carla Molteni
Affiliations : Imperial College London, Department of Materials and Department of Physics (UK); University of Cambridge, Cavendish Laboratory (UK); King's College London, Physics Department

Resume : Semiconductor nanomaterials display a number of peculiar and tunable properties that distinguish them from their bulk counterparts. Of particular interest is their response to applied pressure, as they transform from one crystalline or amorphous structure to another. Accurate simulations are important for understanding finite size effects in the atomistic mechanisms of phase transformations (difficult to observe clearly in macroscopic experiments), for the opportunity to uncover novel metastable phases stabilized in finite systems, and for potentially innovative applications of nanomaterials. First-principles methods are essential to accurately describe the bond breaking/making in phase transformations and the realistic description of surfaces (often covered by complex surfactants). However the computational cost limits both the length- and time-scales attainable. We have combined an O(N) density functional theory code for large systems and an electronic-enthalpy method to apply pressure to finite systems so to model with quantum mechanical precision processes induced by pressure in nanomaterials, with a focus on Si, Ge, CdS and CdSe nanocrystals.

Authors : Adam J. Zakrzewski
Affiliations : Department of Mathematics and Natural Sciences College of Science, Cardinal Stefan Wyszynski University, ul. Dewajtis 5, 01-815 Warsaw, Poland Institute of Physics, Polish Academy of Sciences, Al. Lotnikow 32/46, 02-668 Warsaw, Poland

Resume : Recent advances in crystal growth methods have made possible the fabrication of various nanostructures with practically arbitrary shapes. Among many others, particular role is played by quantum wells. Quantum wells are basic units of many semiconductor devices including cascade lasers, quantum well photodetectors or electro-absorption modulators. Resonant phenomena in such heterostructures induced by applied electric field are critical for understanding their principles of operation and possible optimization of their design and performance. Resonant states in such heterostructures are usually modelled by the time-independent effective mass Schrödinger equation solved in the complex plane. In this way energy of a given state and its lifetime can be obtained. This work reports on arbitrary precise calculations of such states in quantum wells characterized by arbitrary potential profiles and, moreover, by position-dependent effective masses. This work was partly supported by the European Union within European Regional Development Fund, through Innovative Economy grant POIG.01.01.02-00-008/08

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Advances in computational methods I : mm
Authors : Jorge Íñiguez, Jacek C. Wojdel, Carlos Escorihuela-Sayalero
Affiliations : Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), Campus UAB, 08193 Bellaterra, Spain

Resume : I will describe our development of first-principles (FP) model potentials (MPs) for large-scale lattice-dynamical simulations. Our MPs are applicable to cases in which the atoms in the material preserve their basic connectivity or bonding topology. This allows us to identify a suitable reference configuration and parametrize the energy as a Taylor series for all the atomic distortions with respect to the reference state. Such a textbook approach has many advantages: the MPs are trivial to formulate for any material and physically transparent, the involved approximations are clear, and the accuracy can be improved in a systematic manner. Further, because the energy depends linearly on the MP parameters, we can adopt efficient strategies for their fitting, obtaining a very fast and robust scheme. Also, at variance with the usual perturbative approaches, we express the energy in terms of products of displacement differences, so that translational invariance is guaranteed by construction and the parameter fitting gets further simplified. I will illustrate our scheme with especially challenging cases, namely, ferroic ABO3 perovskite oxides (including nanostructures) undergoing transitions driven by soft phonon modes, which today attract great interest for novel energy-harvesting applications, among others. I will show how our ability to run large-scale simulations with FP accuracy has allowed us to reveal surprising effects that had remained hidden to previous approaches.

Authors : Makoto Nakamura 1, Masao Obata 1, Tetsuya Morishita 2, Tatsuki Oda 1,3
Affiliations : 1 Graduate School of Natural Science and Technology, Kanazawa University, Kanazawa 920-1192, Japan, 2 Nanosystem Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba 305-8568, Japan, 3 Institute of Science and Engineering, Kanazawa University, Kanazawa 920-1192, Japan

Resume : In the computer modelling for either nano- or bio-science, free-energy has become a significant realistically-reachable quantity for describing thermodynamic properties. It is very important to develop a method that enables us to construct free-energy profile without empirical force-fields. Meanwhile, some shortcomings of thermodynamics integration method, which is currently a standard method for constructing free-energy profiles, have been pointed out; poor statistical sampling resulting from a breakdown in the ergodicity, numerical integration as a post-processing, and inconvenient analysis for the multi-dimensional collective variables (reaction coordinates). We have developed a mean-force dynamics approach to construct free-energy profile without empirical potential parameters by combining the logarithmic mean-force dynamics (LogMFD) [1] and first-principles molecular dynamics, calling this new method first-principles LogMFD (FP-LogMFD) [2]. This allows us to sample higher energy states efficiently, to evaluate the free energy at the local point of reaction coordinates without any post-processing, and furthermore to do with keeping a level of accuracy in the first-principles approach. [1] T. Morishita et al., Phys. Rev. E 85, 066702 (2012); J. Comp. Chem. 34, 1375 (2013). [2] M. Nakamura M. Obata, T. Morishita, and T. Oda, J. Chem. Phys. 140, 184110 (2014).

Authors : Y. Gohda(1,2), Z. Torbatian(2), T. Ozaki(3), and S. Tsuneyuki(2,3)
Affiliations : (1) Dept. Mater. Sci. Eng., Tokyo Tech, Yokohama 226-8502, Japan; (2) Dept. Phys., Univ. Tokyo, Tokyo 113-0033, Japan; (3) ISSP, Univ. Tokyo, Kashiwa 277-8581, Japan

Resume : Electrons in rare-earth magnets can be classified as two types: itinerant electrons and localized ones. Fe 3d as well as Nd 5d electrons are examples of the former, while Nd 4f electrons are the latter. In particular, itinerant magnetic states dominate interactions among magnetic sites directly. Thus, itinerant d states are expected to be sensitive to lattice strain, although the current status of the understanding on this issue is being far from complete, in particular for the magnetic anisotropy. In this work, we have investigated strain effects on magnetic properties on the basis of density functional theory for Y2Fe14B, where Y is a prototypical f0 rare earth element [1]. First-principles calculations are performed using the OpenMX code. We changed the lattice constants from the equilibrium values in various manner. We found that the uniform compression enhances the perpendicular magnetic anisotropy. To clarify the origin of this enhancement, we developed a new method to decompose the magnetic-anisotropy energy into contribution from each atomic site as well as from couplings among specific atomic orbitals. This method employs second-order perturbation theory and an on-site approximation for the spin-orbit coupling with the form of l dot s neglecting any off-site interactions. As a result, we clarified that the Fe j2 site plays a significant role in enhancing the magnetic anisotropy. [1] Z. Torbatian, T. Ozaki, S. Tsuneyuki, and Y. Gohda, Appl. Phys. Lett., in press.

Authors : N. Gonzalez Szwacki, J. A. Majewski
Affiliations : Faculty of Physics, University of Warsaw, ul. Hoża 69, 00-681 Warszawa, Poland

Resume : Graphene provided the excitement and impact to explore isolated one-atom thick layers that go beyond graphene. The boron-carbon-nitrogen 2D layers are now widely explored since exhibit a variety of structural and electronic properties that depend on the composition. In this work, we develop strategies for predicting the most stable 2D BxCyNz layers. The idea is to find 2D materials that preserve the honeycomb-like structure and can complement graphene in properties. The modeling of these structures is done using the cluster-expansion method combined with extensive first-principles calculations. Doping of graphene with carbon and boron atoms was studied before, using both first-principles and cluster-expansion methods. The purpose of those studies was mainly to investigate the induced by doping band-gap opening in graphene. For instance, for the Cx 1Nx system two stable ordered semiconducting structures, C12N and C3N (x = 0.08 and 0.25, respectively), have been predicted through the cluster-expansion technique (C12N has a direct band gap of ~1 eV). In this work, we explore systematically the stability and properties of 2D BxCyNz layers with a full spectrum of compositions. For the mentioned above Cx-1Nx system, we predict, for instance, that the most stable layer is obtained for x = 0.66. Interestingly enough, our calculations reveal that the 2D layer splits into 1D stripes. The nanoribbons are not connected with each other (except for weak interactions) and have a band gap of 2.9 eV. Furthermore, the building block of each nanoribbon is a C2N4 molecule with a 1.1 eV/molecule energy gain upon formation of the stripe.

Authors : Zhenwei Li; James Kermode; Alessandro De Vita
Affiliations : King's College London

Resume : Many technologically important, chemically complex phenomena are currently beyond the reach of standard or O(N) first principles molecular dynamics (MD) techniques because the necessary model system sizes are too large, and/or the required simulation times are too long. In most situations, using classical MD is not a viable alternative, as suitably general and accurate “reactive” force fields are not available, nor are fitting databases a priori guaranteed to contain the information necessary to describe all the chemical processes encountered along the dynamics. In this talk I will argue that this situation forces the use of MD techniques capable of incorporating accurate QM information generated at run time during the simulations [1, 2]. It also effectively creates a novel market for dynamical databases coupled with specially-tuned “Machine Learning” force fields which minimise the computational workload by allowing QM subroutine calls only when “chemically novel” configurations are encountered along the system’s trajectory. I will present one such “Learn On the Fly” scheme, effectively unifying First-Principles Molecular Dynamics and Machine Learning into a single, information efficient simulation scheme capable of learning/predicting atomic forces through Bayesian inference [3]. References [1] J.R.Kermode, L.Ben-Bashat, F.Atrash, J.J.Cilliers, D.Sherman and A.De Vita, Nat. Commun. 4, 2441 (2013). [2] A. Gleizer, G. Peralta, J. R. Kermode, A. De Vita and D. Sherman, Phys. Rev. Lett., 112, 115501 (2014). [3] Z. Li, J. R. Kermode and A. De Vita, submitted.

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1,2-dimensional materials and surfaces : mm
Authors : Ioannis Dereztis 1, Giuseppe G.N. Angilella 2, Antonino La Magna 1
Affiliations : 1 Consiglio Nazionale della Ricerche, Istituto per Microelettronica e i Microsistemi Z.I VIII Strada 5 I 95121, Catania, Italy; 2 Department of Physics, University of Catania, Via Santa Sofia 54, 95100, Catania, Italy

Resume : Here we discuss how to extend the domain of application of the Kinetic Lattice Monte Carlo to the large scale (micron for the space and minute for the time) atomistic simulation of system's kinetics characterized by an inner structural transition from one to another crystal structure. The feasibility of this extension is demonstrated for the interesting process of the epitaxital graphene synthesis on silicon carbide (SiC) substrate by selective evaporation at high T of Si atoms. The main steps governing this synthesis process are the following: 1) defective sites are the preferential regions of the sublimation phenomenon, 2) C atoms migrate after the breaking of the Si-C tetrahedral network of bonds and reconfigure in the graphitic lattice after a densification, 3) the densification is mediated by not-ordered weakly bounded configurations, 4) the process can continue leading to the formation of new grephene layers. The proposed method exploits a scaffold composed by inter-penetrating three dimensional (3D) lattices for the characterization of the system's micro-state. Simulated kinetics proceeds by intra-lattice and inter-lattice activated transition events involving single atoms. In the particular case of study, we demonstrate that quantitative predictions of the process evolution as a function of the initial state and the process's parameters can be obtained and readily compared with experimental analyses of processed samples.

Authors : Guilherme Fabris, Ricardo Paupitz
Affiliations : Physics Department, IGCE - Sao Paulo State Unversity Rio Claro - SP - Brazil

Resume : Nanomaterials attracted much attention from materials researchers in the last few decades due to their unique properties. Graphene is a good example of nanomaterial which is considered one of the most promising for developing nanoelectronics in the near future. There is also interest in an inorganic counterpart of graphene, the hexagonal Boron Nitride (h-BN) which is composed of alternate boron and nitrogen atoms in a hexagonal array sheet. In this context, there is a special interest in a hybrid material, which is composed by h-BN lattice with random substitution of some B-N atoms by carbon atoms, the so called h-BNC. Experimentally it is known that large sheets of h-BNC constituted by h-BN and C, distributed randomly, are promising candidates in the electronics area, due to its chemical stability, electronic and mechanical properties. In the present work, we performed a series of simulations for h-BNC structures using molecular dynamics (MD) techniques with reactive force fields (ReaxFF) and also using the Tight Binding Density Functional Theory (DFTB) in order to estimate several mechanical properties for these systems at various C concentrations. Young modulus and poisson coefficient are shown to vary with C substitution percentage from 0% up to 100%. Differences between ReaxFF and DFTB results are discussed.

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Interfaces and heterostructures I : mm
Authors : Claudio Melis, Luciano Colombo
Affiliations : Dipartimento di Fisica, Università di Cagliari

Resume : Poly(dimethylsiloxane) (PDMS) is the most popular elastomeric material since it couples biocompatibility with excellent elastic properties. For this reason considerable efforts are currently being concentrated on the fabrication of stretchable metallic circuits and microelectrodes integrated on PDMS. Recently it has been demonstrated that neutral metallic nanoparticles produced in the gas phase and aerodynamically accelerated in a supersonic expansion can be implanted in PDMS substrate to form a conductive nanocomposite (nc) with novel functional properties[C. Ghisleri et al., J. Phys. D: Appl. Phys 47,015301 (2014)]. While the effectiveness of this novel technique has been already demonstrated few information are available on the ncs elastic properties as a function of the implanted clusters concentration. To this aim we perform dynamic mechanical simulations to estimate the ncs Young modulus for nc samples having different cluster concentrations. The results show that the nanocomposite Young modulus is basically unaffected (with respect to pristine PDMS) up to a cluster concentration of 25%; while above 25 % we observe a Young modulus exponential increase. These results agree well with nanomechanical data based on atomic force microscopy and demonstrate that SCBI can be used to produce metal/polymer nanocomposite materials with reproducible mechanical properties.

Authors : E. Lampin, P . L. Palla, P. A. Francioso and F. Cleri
Affiliations : IEMN UMR CNRS 8520 and University of Lille - CS 60069 - 59625 Villeneuve d'Ascq Cedex- France

Resume : Molecular dynamics (MD) is a statistical mechanics computational approach that provides the opportunity to access basics phenomena involved in the heat transfer at the nanoscale. MD is generally used either to extract bulk conductivities from the heat current fluctuations during a NVE simulation (Green-Kubo or EMD approach), or bulk conductivities and interface resistances from the temperature profile once the stationary regime between a hot and a cold reservoir is reached (direct method or NEMD). We have developed an alternative framework of MD simulations for the study of thermal conductivities and interface resistances. The method, called approach-to-equilibrium MD (AEMD), relies on the Fourier-based exploitation of the transient regime to equilibrium, i.e. flat temperature profile. It is computationally faster than the two other approaches and can be applied to a range of systems. We will present in particular application to bulk Si, Ge and a-quartz and discuss the length dependence originating from micrometric phonon mean free paths in Si. We will also propose different solutions to exploit the temperature transient in the case of interfaces and nanoconstrictions depending on the weight of the interface resistance compared to the conductivity of the materials at both side. Finally we will open perspectives of the approach.

Authors : Ming Hu
Affiliations : 1Institute of Mineral Engineering, Division of Materials Science and Engineering, Faculty of Georesources and Materials Engineering, RWTH Aachen University, 52064 Aachen, Germany 2Aachen Institute for Advanced Study in Computational Engineering Science (AICES), RWTH Aachen University, 52062 Aachen, Germany

Resume : Recently there has been enormous progress in the production of atomic layers that are strictly two-dimensional (2-D) and can be viewed as individual planes of atomic-scale thickness pulled out of bulk materials. Silicon is a leading material in semiconductor industries because of its wide usage in transistors, solar cells, and other electronic devices. Silicene, the silicon equivalent of graphene, has recently attracted significant attention. In this presentation, first I will describe our recent molecular dynamics simulation on thermal transport of atomically thin silicene with improved Stillinger-Weber potential. We found that the silicene has intrinsically extremely low thermal conductivity (~ 10 W/mK at 300 K), which is about 15 times smaller than bulk Si and two orders of magnitude less than that of its carbon counterpart graphene. By quantifying the relative contribution from different phonon polarizations, we show that the phonon transport in silicene is dominated by the longitudinal modes, not the out-of-plane flexural modes as opposed to graphene. Second, I will demonstrate the effect of size (length) and surface functionalization on the thermal transport of silicene. We discovered that, contrary to its counterpart of graphene and despite the similarity of their honeycomb lattice structure, silicene exhibits an anomalous thermal behavior upon surface functionalization: the thermal conductivity of silicene significantly increases with applied hydrogen percentage. Third, by conducting a series of non-equilibrium molecular dynamics simulations, we show that substrates have great effect on the thermal conductivity of silicene. More importantly, substrates can tune the thermal conductivity of silicene in a broad range, which is very important for the thermal management of electronic devices involving silicene. Our findings provide a guide of how to modulate the thermal transport properties of two-dimensional Si with nanoengineering and may be of use in tuning their electronic and optical properties for electronic, thermoelectric, photovoltaic, and opto-electronic applications.

Authors : Alexander V. Evteev, Leila Momenzadeh, Elena V. Levchenko, Irina V. Belova and Graeme E. Murch
Affiliations : The University of Newcastle, Australia

Resume : There are two main approaches currently used for analysing phonon thermal transport in crystals. The first approach employs the Green-Kubo formalism which basically requires the use of molecular dynamics simulation for an equilibrium calculation of the heat current autocorrelation function (HCACF). The second approach is based on employing the Boltzmann transport equation for the phonon distribution function. The first approach is microscopic as it deals with microscopic definition of the heat current. It needs only an appropriate interatomic potential as input and does not require any assumptions about the nature of thermal transport in the model system. Meanwhile, the second approach is used for phenomenological interpretation of experimental and simulation data on phonon thermal transport. In this contribution, we discuss Green-Kubo calculations of the phonon thermal conductivity of crystals with a monatomic unit cell, which revealed: (i) two-stage decay in the HCACF; and (ii) deviation from the inverse temperature dependence of the phonon thermal conductivity at high temperatures towards a more rapid decrease following an exponent close to -1.4. Then we use a general expression for the phonon thermal conductivity of an isotropic solid which follows from the Boltzmann transport equation in order to facilitate analytical treatment of the results of the Green-Kubo calculations.

Interfaces and heterostructures II : mm
Authors : Y. J. Dappe, C. Gonz?lez, J. C. Cuevas
Affiliations : Service de Physique de l'Etat Condens? (CNRS URA2464), IRAMIS, CEA Saclay, 91191 Gif-Sur-Yvette, France; Departamento de F?sica, Universidad de Oviedo, 33006 Oviedo, Spain; Departamento de F?sica Te?rica de la Materia Condensada and Condensed Matter Physics Center (IFIMAC), Universidad Aut?noma de Madrid, 28049 Madrid, Spain

Resume : We present here an exhaustive ab initio study of the use of carbon-based tips as electrodes in single molecule junctions. Motivated by recent experiments, we show that carbon tips can be combined with other carbon nanostructures, such as graphene, to form all-carbon molecular junctions with mol?cules like benzene or C60. Our results show that the use of carbon tips can lead to relatively conductive molecular junctions. However, contrary to junctions formed with standard metals, the conductance traces recorded during the formation of the all-carbon single-molecule junctions do not exhibit clear conductance plateaus, which can be attributed to the inability of the hydrogenated carbon tips to form chemical bonds with the organic molecules. Additionally, we explore here the use of carbon tips for scanning tunneling microscopy and show that they are well suited for obtaining sample images with atomic resolution.

Authors : Jonathan Amodeo* (1), Inas Issa (1,2), Lucile Joly-Pottuz (1), Julien Rethore (2), Karine Masenelli-Varlot (1), Jerome Chevalier (1), Julien Morthomas (1) and Michel Perez (1)
Affiliations : (1) MATEIS, UMR CNRS 5510, INSA-Lyon, F-69621 Villeurbanne, France (2) LAMCOS, UMR CNRS 5259 INSA-Lyon, F-69621 Villeurbanne, France

Resume : The way ceramic nanoparticles behave has recently gained more interest, especially in the field of modern surgery where metallic alloys used for implants and prosthesis are progressively replaced by bio-compatible ceramics to increase their lifetime and reduce ion release responsible for inflammatory reactions. As bio-compatible ceramics are originally made of powders, one may wonder whether size effects could play a role during compaction and sintering and thus, influence the post-processed material mechanical properties. Based on this framework, this study investigates the mechanical behaviour of isolated ceramic nanoparticles using statics and molecular dynamics. Here we model magnesium oxyde MgO as it is a model ceramic, widely studied in terms of elastic properties, slip systems, dislocation characterization and flow behaviour under compression at the macroscopic scale. The ionic system is described using a Buckingham potential and a pairwise Coulomb interaction between point charges. Numerical nano-mechanical tests show that MgO nanoparticles deform up to large strain in comparison to bulk material, without any sign of damage. Deformation proceeds by ½<110>{110} dislocation nucleation and multiplication. Results are interpreted within the framework of small-scale plasticity. Finally, these findings enable the interpretation of recent in situ TEM compression tests performed on MgO <100>-oriented nanocubes at scale about 100 nm.


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Symposium organizers
Carla MolteniKing's College London

Physics Department Strand, London WC2R 2LS UK

+44 20 7848 2170
+44 20 7848 2420
Carlo MASSOBRIOInstitut de Physique et de Chimie des Matériaux, Strasbourg

23, rue du Loess, BP43 F-67034 Strasbourg Cedex 2 France

Fabrizio CLERIInstitut d'Electronique, Microelectronique et Nanotechnologie

Université de Lille I, 59652 Villeneuve d'Ascq, France
Rafal KozubskiInstitute of Physics Jagellonian University

Reymonta 4 30-059 Krakow Poland

+48 12 663 57 16
+48 12 633 70 86