Página 1 dos resultados de 71 itens digitais encontrados em 0.014 segundos

‣ Atomistic modelling of processes involved in poling of PVDF

Ramos, Marta M. D.; Correia, Helena M. G.; Lanceros-Méndez, S.
Fonte: Elsevier Publicador: Elsevier
Tipo: Artigo de Revista Científica
Publicado em /04/2005 Português
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Poling processes play an important role in the design and preparation of many ferroelectric materials for practical uses in the field of sensors and actuators. Particularly, the processing of piezoelectric beta-poly(vinylidene fluoride) (PPVDF) involves mechanical stretching in order to transform the extruded a-phase into the P-phase and poling of this later material in order to optimize the piezoelectric response. This poling process affects the orientation of the dipolar moments of the beta-chains and improves the alpha to beta transformation. Poling processes in general and in PVDF in particular are still quite empirical because a firm understanding of the physical processes involved in poling has not been fully established. In the present work we use a self-consistent quantum molecular dynamics method to study the effect of the electric field on both alpha and beta chains of PVDF. The orientation of the dipolar moments in each chain as a function of an electric field and the accompanying structural modifications due to these reorientations will be the main parameters discussed. The theoretical results will be used to better understand the changes at a molecular level due to the poling process, as observed in FTIR experiments, performed in poled and non-poled beta-PVDF from the same batch.; Fundação para a Ciência e a Tecnologia (FCT) – Programa Operacional “Ciência...

‣ Mesoscopic study of laser absorption by a transparent ceramic

Ribeiro, R. M.; Ramos, Marta M. D.; Stoneham, A. M.
Fonte: Elsevier Publicador: Elsevier
Tipo: Artigo de Revista Científica
Publicado em /02/1998 Português
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The understanding of the processes occurring on the target in pulsed laser deposition (PLD) is crucial for a fast optimisation of the deposition parameters in order to obtain high quality thin films. Phenomenon occurring in the target like the ejection of large particulates that deposit on the substrate or the formation of a rough cone shaped morphology that affect the deposition process cannot be understood in the framework of atomistic simulations, since the processes involve very large volumes. Integration of the heat equations does not seem to be the appropriate approach for the study of PLD, since it ignores the actual ways by which the energy is transferred to the target and transported through it. Mesoscopic modelling provide solutions in an intermediate scale where both results from atomistic studies and methods characteristic of macroscopic modelling are used. We are developing a mesoscopic model for PLD. In this paper, we show the results for the evaporation of a transparent material in which only structural defects can absorb light. The preliminary results show that the generated electric fields play a dramatic role in the process.; Junta Nacional de Investigação, Ciência e Tecnologia (JNICT) - PBIC/C/FIS/2151/95

‣ Atomistic modelling of interfacial bonding at metal-polymer interface

Ramos, Marta M. D.; Almeida, Judite P. P.
Fonte: Elsevier Publicador: Elsevier
Tipo: Artigo de Revista Científica
Publicado em /08/1999 Português
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The chemical bonding at a metal/polymer interface is believed to play an important role in adhesion. The interfacial bonding and consequently adhesion are influenced directly by the way in which the interface is formed. Atomistic modelling of interface formation has the potential to yield significant progress in the field of adhesion between metals and polymers. In order to study the nature of bond formation at metal/polymer interface, the authors have investigated the interaction of aluminium atoms with a trans-polyacetylene fragment. Self-consistent geometry optimization and molecular orbital calculations of adsorbate-substrate system have been carried out within CNDO approximation in a cluster model framework. The results suggest that aluminium dimer (Al-2) bound to a polymer chain is energetically favourable. The adhesion of aluminium dimers to polyacetylene is predicted to be somewhat weaker than that of isolated aluminium atoms to polyacetylene. The compound formation is accompanied by charge transfer between metal and polymer. As a result, charge rearrangement amongst polyacetylene atoms is induced.; Comunidade Europeia (CE). Fundo Europeu de Desenvolvimento Regional (FEDER).; Fundação para a Ciência e a Tecnologia (FCT) - PRAXIS XXI - PRAXIS/2/2.1/FIS/26/94.; Junta Nacional de Investigação...

‣ Modelling the effect of nonplanarity on charge transport along conjugated polymer chains

Correia, Helena M. G.; Ramos, Marta M. D.
Fonte: Elsevier Publicador: Elsevier
Tipo: Artigo de Revista Científica
Publicado em /09/2007 Português
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Conjugated polymers show interesting properties that make them appropriated for nanoelectronics. Several studies of poly(p-phenylene vinylene) (PPV) have suggested that each polymer chain consists of several planar segments, with conjugation length of nanoscale dimension, linked by twists or kinks. A pronounced twist between two planar segments in a PPV chain not only causes loss of main-chain conjugation but it may also alter electron and hole mobility along the chain, which has further implications for the percolation of charge through the polymer film. We used self-consistent quantum molecular dynamics calculations to provide information on the electric field needed to move the injected charges (either electrons or holes) along the planar segments of PPV and to cross the twist between two planar segments perpendicular to each other. Field-dependent charge mobility was also estimated for conjugated segments of various lengths. Our results suggest that electrons can cross the twist between adjacent planar segments for lower applied electric fields than holes if there is no more than one electronic charge (electron or hole) on the PPV chain, otherwise similar fields are needed.; Fundação para a Ciência e a Tecnologia FCT) Programa Operacional “Ciência ...

‣ Modelling the effect of structure and base sequence on DNA molecular electronics

Ramos, Marta M. D.; Correia, Helena M. G.
Fonte: Institute of Physics Publishing (IOP) Publicador: Institute of Physics Publishing (IOP)
Tipo: Artigo de Revista Científica
Publicado em /09/2008 Português
Relevância na Pesquisa
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DNA is a material that has the potential to be used in nanoelectronic devices as an active component. However, the electronic properties of DNA responsible for its conducting behaviour remain controversial. Here we use a self-consistent quantum molecular dynamics method to study the effect of DNA structure and base sequence on the energy involved when electrons are added or removed from isolated molecules and the transfer of the injected charge along de molecular axis when an electric field is applied. Our results have shown that the DNA molecules of poly(C)-poly(G) on B-form and poly(A)-poly(T) on A-form have the highest energy released when one electron is added or removed from them and their Z-form has the lowest energy released. Besides, when an electric field is applied to a charged DNA molecule along its axis there is electron transfer through the molecule, regardless of the number and sign of the injected charge, the molecular structure and the base sequence. Results from these simulations provide useful information that is hard to obtain from the experiments and needs to be considered for a further modelling aiming to improve charge transport efficiency in nanoelectronic devices based on DNA.; Fundação para a Ciência e a Tecnologia (FCT); Programa Operacional “Ciência ...

‣ Quantum and nanoscale modelling of exciton dynamics in polymeric systems

Barbosa, Helder M. C.; Correia, Helena M. G.; Ramos, Marta M. D.
Fonte: American Scientific Publishers Publicador: American Scientific Publishers
Tipo: Artigo de Revista Científica
Publicado em /02/2010 Português
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One of the factors that limit the efficiency of polymer-based optoelectronic devices, such as photovoltaic solar cells and light emitting diodes, is the exciton diffusion within the polymeric network. Due to the amorphous nature the of polymeric materials, the diffusion of excitons is limited by the energetic and spatial disorder in such systems, which is a consequence not only of the chemical structure of the polymer but also from its orphology at nanoscale. To get a deep understanding on how such effects influence exciton dynamics we performed a quantum molecular dynamics simulations to determine the energetic disorder within the polymer system, and Monte Carlo simulations to study exciton diffusion in three-dimensional (3D) polymer networks that present both spatial and energetic disorder at nanometre scale. Our results show clearly that exciton diffusion in poly(p-phenylenevenylene) (PPV) occurs preferentially in the direction parallel to the electrodes surface for a polymer-based optoelectronic devices with the orientation of the conjugated strands similar to those obtained by the spin-coating technique and the decay of such excitons occurs preferentially in longer strands which allow us to get insight on exciton behaviour in polymeric systems that are not possible to be obtained directly from the experiments.; Fundação para a Ciência e a Tecnologia (FCT) – POCTI/CTM/41574/2001...

‣ Theoretical study of the influence of chemical defects on the molecular properties of semiconducting polymers

Ramos, Marta M. D.; Correia, Helena M. G.; Barbosa, Helder M. C.
Fonte: Trans Tech Publications Publicador: Trans Tech Publications
Tipo: Artigo de Revista Científica
Publicado em /01/2010 Português
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Semiconductor polymers are successfully implemented in a broad range of applications such as light emitting diodes, field effect transistors and photovoltaic devices. Most of the achievements reached in the development of these devices were obtained at experimental level, being difficult to identify individually the influence of each factor that limits and controls these devices efficiency. One of the factors that strongly influence the performance of polymer-based devices is the presence of chemical defects in the polymer strands that change their molecular properties. As a result, these polymer strands can work like traps or deep energetic states for charge transport, leading, for instance, to a decrease on charge mobility. At experimental level it is a difficult task to isolate the influence of each type of chemical defects individually on the molecular properties of the polymer strands. It is in this context that theoretical modelling seems to be the most suitable approach to get a deep understanding of the influence of chemical defects on the molecular properties of semiconductor polymers. By performing quantum molecular dynamics calculations we study the influence of chemical defects on the molecular properties of poly(para-phenylenevinylene) (PPV). Our results show clearly a significant difference on the electronic properties of defective polymer strands (e.g. charge carrier localization...

‣ Multi-scale modelling of polymer-based optoelectronic devices

Ramos, Marta M. D.; Correia, Helena M. G.; Barbosa, Helder M. C.
Fonte: SPIE Publicador: SPIE
Tipo: Artigo de Revista Científica
Publicado em /08/2011 Português
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The optimization of polymer-based optoelectronic devices such as light-emitting diodes (LEDs), photodetectors and photovoltaic cells requires the understanding how molecular properties and the spatial arrangement of the conjugated strands affect the electronic processes underlying the functioning of these devices. Since some of the important features are determined largely by the individual molecular strands and other features depend strongly on the nanostructure, a multi-scale modelling of materials and device properties is needed. In this work we discuss the atomistic and nanoscale modelling of charge injection, transport and trapping single-carrier diode based on poly(p-phenylene venylene) (PPV), which also applies to other optoelectronic devices.; This work is part of the research projects POCTI/CTM/41574/2001 and CONC-REEQ/443/EEI/2005, approved by the Portuguese Foundation for Science and Technology (FCT) and support by the European Community Fund FEDER. One of us (H.M.G.C.) is also indebted to FCT for financial support under the post-doctoral grant no. SFRH/BPD/64554/2009.

‣ Multi-scale modelling of polymer-based optoelectronic devices

Ramos, Marta M. D.; Correia, Helena M. G.; Barbosa, Helder M. C.
Fonte: Universidade do Minho Publicador: Universidade do Minho
Tipo: Conferência ou Objeto de Conferência
Publicado em //2011 Português
Relevância na Pesquisa
27.435034%
The optimization of polymer-based optoelectronic devices such as light-emitting diodes (LEDs), photodetectors and photovoltaic cells requires the understanding how molecular properties and the spatial arrangement of the conjugated strands affect the electronic processes underlying the functioning of these devices. Since some of the important features are determined largely by the individual molecular strands and other features depend strongly on the nanostructure, a multi-scale modelling of materials and device properties is needed. In this work we discuss the atomistic and nanoscale modelling of charge injection, transport, trapping and recombination in LEDs.

‣ Computer aided polymer design using multi-scale modelling

Satyanarayana,K. C.; Abildskov,J.; Gani,R.; Tsolou,G.; Mavrantzas,V. G.
Fonte: Brazilian Society of Chemical Engineering Publicador: Brazilian Society of Chemical Engineering
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/09/2010 Português
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The ability to predict the key physical and chemical properties of polymeric materials from their repeat-unit structure and chain-length architecture prior to synthesis is of great value for the design of polymer-based chemical products, with new functionalities and improved performance. Computer aided molecular design (CAMD) methods can expedite the design process by establishing input-output relations between the type and number of functional groups in a polymer repeat unit and the desired macroscopic properties. A multi-scale model-based approach that combines a CAMD technique based on group contribution plus models for predicting polymer repeat unit properties with atomistic simulations for providing first-principles arrangements of the repeat units and for predictions of physical properties of the chosen candidate polymer structures, has been developed and tested for design of polymers with desired properties. A case study is used to highlight the main features of this multi-scale model-based approach for the design of a polymer-based product.

‣ Atomistic modelling of collagen proteins in their fibrillar environment

Streeter, Ian; de Leeuw, Nora H.
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
Publicado em 21/10/2010 Português
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Molecular dynamics simulations can aid studies of the structural and physico-chemical properties of proteins, by predicting their dynamics, energetics, and interactions with their local environment at the atomistic level. We argue that non-standard protocols are needed to realistically model collagen proteins because in their biological state these aggregate to form collagen fibrils, and so they should not be treated as fully solvated molecules. A new modelling approach is presented that can account for the local environment of collagen molecules within a fibril, and which therefore simulates aspects of their behaviour that would not otherwise be distinguished. This modelling approach exploits periodic boundaries to replicate the supermolecular arrangement of collagen proteins within the fibril, in an approach that is more commonly associated with modelling crystalline solids rather than mesoscopic protein aggregates. Initial simulations show agreement with experimental observations and corroborate theories of the fibril’s structure.

‣ A Multiscale Approach to Modelling Drug Metabolism by Membrane-Bound Cytochrome P450 Enzymes

Lonsdale, Richard; Rouse, Sarah L.; Sansom, Mark S. P.; Mulholland, Adrian J.
Fonte: Public Library of Science Publicador: Public Library of Science
Tipo: Artigo de Revista Científica
Publicado em 17/07/2014 Português
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Cytochrome P450 enzymes are found in all life forms. P450s play an important role in drug metabolism, and have potential uses as biocatalysts. Human P450s are membrane-bound proteins. However, the interactions between P450s and their membrane environment are not well-understood. To date, all P450 crystal structures have been obtained from engineered proteins, from which the transmembrane helix was absent. A significant number of computational studies have been performed on P450s, but the majority of these have been performed on the solubilised forms of P450s. Here we present a multiscale approach for modelling P450s, spanning from coarse-grained and atomistic molecular dynamics simulations to reaction modelling using hybrid quantum mechanics/molecular mechanics (QM/MM) methods. To our knowledge, this is the first application of such an integrated multiscale approach to modelling of a membrane-bound enzyme. We have applied this protocol to a key human P450 involved in drug metabolism: CYP3A4. A biologically realistic model of CYP3A4, complete with its transmembrane helix and a membrane, has been constructed and characterised. The dynamics of this complex have been studied, and the oxidation of the anticoagulant R-warfarin has been modelled in the active site. Calculations have also been performed on the soluble form of the enzyme in aqueous solution. Important differences are observed between the membrane and solution systems...

‣ A comparison of finite element and atomistic modelling of fracture

Coffman, Valerie R; Sethna, James P.; Heber, Gerd; Liu, Mu; Ingraffea, Anthony; Bailey, Nicholas P.; Barker, Erin Iesulauro
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 06/03/2008 Português
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Are the cohesive laws of interfaces sufficient for modelling fracture in polycrystals using the cohesive zone model? We examine this question by comparing a fully atomistic simulation of a silicon polycrystal to a finite element simulation with a similar overall geometry. The cohesive laws used in the finite element simulation are measured atomistically. We describe in detail how to convert the output of atomistic grain boundary fracture simulations into the piecewise linear form needed by a cohesive zone model. We discuss the effects of grain boundary microparameters (the choice of section of the interface, the translations of the grains relative to one another, and the cutting plane of each lattice orientation) on the cohesive laws and polycrystal fracture. We find that the atomistic simulations fracture at lower levels of external stress, indicating that the initiation of fracture in the atomistic simulations is likely dominated by irregular atomic structures at external faces, internal edges, corners, and junctions of grains. Thus, cohesive properties of interfaces alone are likely not sufficient for modelling the fracture of polycrystals using continuum methods.; Comment: 18 pages, 11 figures

‣ Approaches to modelling irradiation-induced processes in transmission electron microscopy

Skowron, Stephen T.; Lebedeva, Irina V.; Popov, Andrey M.; Bichoutskaia, Elena
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 21/09/2013 Português
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The recent progress in high-resolution transmission electron microscopy (HRTEM) has given rise to the possibility of in situ observations of nanostructure transformations and chemical reactions induced by electron irradiation. In this article we briefly summarise experimental observations and discuss in detail atomistic modelling of irradiation-induced processes in HRTEM, as well as mechanisms of such processes recognised due to modelling. Accurate molecular dynamics (MD) techniques based on first principles or tight-binding models are employed in the analysis of single irradiation-induced events, and classical MD simulations are combined with a kinetic Monte Carlo algorithm to simulate continuous irradiation of nanomaterials. It has been shown that sulphur-terminated graphene nanoribbons are formed inside carbon nanotubes as a result of an irradiation-selective chemical reaction. The process of fullerene formation in HRTEM during continuous electron irradiation of a small graphene flake has been simulated, and mechanisms driving this transformation analysed.; Comment: 15 pages, 7 figures

‣ Mesoscale modelling of polyelectrolyte electrophoresis

Grass, Kai; Holm, Christian
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 11/02/2009 Português
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The electrophoretic behaviour of flexible polyelectrolyte chains ranging from single monomers up to long fragments of hundred repeat units is studied by a mesoscopic simulation approach. Abstracting from the atomistic details of the polyelectrolyte and the fluid, a coarse-grained molecular dynamics model connected to a mesoscopic fluid described by the Lattice Boltzmann approach is used to investigate free-solution electrophoresis. Our study demonstrates the importance of hydrodynamic interactions for the electrophoretic motion of polyelectrolytes and quantifies the influence of surrounding ions. The length-dependence of the electrophoretic mobility can be understood by evaluating the scaling behavior of the effective charge and the effective friction. The perfect agreement of our results with experimental measurements shows that all chemical details and fluid structure can be safely neglected, and a suitable coarse-grained approach can yield an accurate description of the physics of the problem, provided that electrostatic and hydrodynamic interactions between all entities in the system, i.e., the polyelectrolyte, dissociated counterions, additional salt and the solvent, are properly accounted for. Our model is able to bridge the single molecule regime of a few nm up to macromolecules with contour lengths of more than 100 nm...

‣ A Note on Automatic Kernel Carpentry for Atomistic Support of Continuum Stress

Ulz, Manfred H.; Moran, Sean J.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 09/04/2015 Português
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Research within the field of multiscale modelling seeks, amongst other questions, to reconcile atomistic scale interactions with thermodynamical quantities (such as stress) on the continuum scale. The estimation of stress at a continuum point on the atomistic scale requires a pre-defined kernel function. This kernel function derives the stress at a continuum point by averaging the contribution from atoms within a region surrounding the continuum point. Commonly the kernel weight assignment is isotropic: an identical weight is assigned to atoms at the same spatial distance, which is tantamount to a local constant regression model. In this paper we employ a local linear regression model and leverage the mechanism of automatic kernel carpentry to allow for spatial averaging adaptive to the local distribution of atoms. As a result, different weights may be assigned to atoms at the same spatial distance. This is of interest for determining atomistic stress at stacking faults, interfaces or surfaces. It is shown in this study that for crystalline solids, although the local linear regression model performs elegantly, the additional computational costs are not justified compared to the local constant regression model.

‣ DFT modelling of bulk-modulated carbon nanotube field-effect transistors

Latessa, L.; Pecchia, A.; Di Carlo, A.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 05/12/2005 Português
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We report density-functional theory (DFT), atomistic simulations of the non-equilibrium transport properties of carbon nanotube (CNT) field-effect transistors (FETs). Results have been obtained within a self-consistent approach based on the non-equilibrium Green's functions (NEGF) scheme. Our attention has been focused on a new kind of devices, the so called bulk-modulated CNTFETs. Recent experimental realizations \cite{Chen,Lin_condMat} have shown that such devices can exhibit excellent performances, even better than state-of-the-art Schottky barrier (SB)-modulated transistors. Our calculations have been intended to explore, at an atomistic level, the physical mechanisms governing the transport in these new devices. We emphasize the role that one-dimensional screening has on gate- and drain-induced current modulation mechanisms, pointing out, at the same time, the importance of a correct evaluation of the nanotube quantum capacitance. The operative regimes and the performance limits of the device are analysed, pointing out, at the same time, the role played by the quasi-one-dimensional, short channel effects.; Comment: 12 pages, 14 figures

‣ Multiscale modelling of bionano interface

Lopez, Hender; Brandt, Erik; Mirzoev, Alexander; Zhurkin, Dmitry; Lyubartsev, Alexander; Lobaskin, Vladimir
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 20/11/2015 Português
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In this work we describe a set of Coarse-grained (CG) tools that allow to simulate the uptake of the nanoparticles (NPs) coated with proteins by a lipid bilayer. We describe a CG model to calculate the adsorption energies and the most favorable adsorption orientations of proteins onto a hydrophobic NP. The proposed method is then used to calculate the adsorption energies of two common proteins in human blood onto neutral and negative charged NPs. We also report the effect of the NP radius on the adsorption energies and validate the proposed methodology against full atomistic simulations. We also describe a methodology in which full atomistic simulations of a lipid bilayer and various lipid-cholesterol mixtures are used for the extraction of CG pair potentials. We also compare and validate the predictions of simulations at molecular and CG level. Finally, we present a CG simulation of the interaction a bare NP and of a NP-protein complex with a lipid bilayer.; Comment: To appear in the book "Modelling the Toxicity of Nanoparticles"

‣ Atomistic spin model simulations of magnetic nanomaterials

Evans, Richard F. L.; Fan, Weijia J.; Chureemart, Phanwadee; Ostler, Thomas A.; Ellis, Matthew O. A.; Chantrell, Roy W.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Português
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Atomistic modelling of magnetic materials provides unprecedented detail about the underlying physical processes that govern their macroscopic properties, and allows the simulation of complex effects such as surface anisotropy, ultrafast laser-induced spin dynamics, exchange bias, and mi- crostructural effects. Here we present the key methods used in atomistic spin models which are then applied to a range of magnetic problems. We detail the parallelisation strategies used which enable the routine simulation of extended systems with full atomistic resolution.

‣ Multi-scale modelling of III-nitrides: from dislocations to the electronic structure

Holec, David
Fonte: University of Cambridge; Department of Materials Science and Metallurgy Publicador: University of Cambridge; Department of Materials Science and Metallurgy
Tipo: Thesis; Doctoral
Português
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Gallium nitride and its alloys are direct band gap semiconductors with a wide variety of applications. Of particular importance are light emitting diodes and laser diodes. Due to the lack of suitable lattice-matched substrates, epitaxial layers contain a high density of defects such as dislocations. To reduce their number and to design a device with desired specifications, multilayered systems with varying composition (and thus material properties) are grown. Theoretical modelling is a useful tool for gaining understanding of various phenomena and materials properties. The scope of the present work is wide. It ranges from a continuum theory of dislocations treated within the linear elasticity theory, connects the continuum and atomistic level modelling for the case of the critical thickness of thin epitaxial layers, and covers some issues of simulating the electronic structure of III-nitride alloys by means of the first principle methods. The first part of this work discusses several topics involving dislocation theory. The objectives were: (i) to apply general elasticity approaches known from the literature to the specific case of wurtzite materials, (ii) to extend and summarise theoretical studies of the critical thickness in heteroepitaxy. Subsequently...