Ab-initio calculations of diffusion barriers of small vacancy clusters in silicon

Mestrado em Ciência e Engenharia de MateriaisEsta tese apresenta os resultados de um programa de investigação sobre a difusão da lacuna, bi-lacuna e tri-lacuna em silício utilizando simulações numéricas pelo método da teoria do funcional da densidade. Este método está implementado na forma de um programa informático referido como AIMPRO (Ab Initio Modelling PROgram). Para o cálculo dos pontos cela dos mecanismos de difusão foi usado o método “Nudged Elastic Band”. As condições fronteira dos problemas foram impostas recorrendo à utilização de agregados esféricos de silício com 275 atomos, cuja superfície foi saturada por ligações Si-H. As lacunas foram então introduzidas no centro destes agregados. Os valores calculados das barreiras de difusão para a lacuna simples e para a bi-lacuna são respectivamente 0.68 e 1.75 eV. Estes valores apresentam um acordo razoável com os obtidos experimentalmente e obtidos em outros cálculos anteriores. A barreira de difusão da tri-lacuna foi, de acordo com a literatura disponível, calculada pela primeira vêz. O mecanismo de difusão mais favorável apresenta uma barreira de 2.2 eV. No seguimento dos resultados para a lacuna e bi-lacuna, pensamos que este resultado sobrestima a barreira em cerca de 0.25 eV, colocando a nossa melhor estimativa em 1.9- 2.0 eV. Varias fontes de erro nos resultados são comentadas, assim como são sugeridas várias formas de as evitar. ABSTRACT: This work presents the results of a computational investigation into the diffusion of the single vacancy (V) and small vacancy clusters, divacancy (V2) and trivacancy (V3), in silicon. The calculations were performed principally using local density functional theory as implemented by the AIMPRO (Ab Initio Modelling PROgram) code. The Nudged Elastic Band Method was used for elucidating diffusion paths and obtaining the energy barriers for diffusion of the defects considered. Based on ab-initio calculations with H-terminated Si clusters with 275 host atoms, diffusion paths for neutral Vn (n = 1 to 3) defects were found. Calculated values of the activation energy for the diffusion of the Si vacancy and divacancy are 0.68 and 1.75 eV, respectively. These values are in a reasonable agreement with those derived from experimental and previous ab-initio modelling studies. The diffusion of trivacancy in Si has been modelled for the first time. The diffusion barrier of V3 along the proposed diffusion path was found to be about 2.2 eV. This result comes overestimated as the experimental data indicates that the values of diffusion barriers for divacancy and trivacancy in Si should be similar. Probable sources of the calculation errors have been considered and possible ways to surmount these difficulties are proposed

Modification of electronic properties of graphene by interaction with substrates and dopants

First-principles calculations have been carried out to investigate structural and electronic properties of graphene on SiC and diamond substrates and for a study of doping of fluorographene with various surface adsorbates. New insight is given into the problem of the decoupling of the graphene layers from SiC substrates after epitaxial growth. Mechanisms of hydrogen penetration between graphene and SiC(0001) surface, and properties of hydrogen and fluorine intercalated structures have been studied. Energy barriers for diffusion of atomic and molecular hydrogen through the interface graphene layer with no defects and graphene layers containing Stone-Wales defect or two- and four-vacancy clusters have been calculated. It is argued that diffusion of hydrogen towards the SiC surface occurs through the hollow defects in the interface graphene layer. It is further shown that hydrogen easily migrates between the graphene layer and the SiC substrate and passivates the surface Si bonds, thus causing the graphene layer decoupling. According to the band structure calculations the graphene layer decoupled from the SiC(0001) surface by hydrogen intercalation is undoped, while that obtained by the fluorine intercalation is p-type doped. Further, structure and the electronic properties of single and double layer graphene on H-, OH-, and F- passivated (111) diamond surface have been studied. It is shown that graphene only weakly interacts with the underlying substrates and the linear dispersion of graphene pi-bands is preserved. For graphene on the hydrogenated diamond surfaces the charge transfer results in n-type doping of graphene layers and the splitting of conduction and valence bands in bilayer graphene. For the F- and OH-terminated surfaces, charge transfer and doping of graphene do not occur. Finally, the possibility of doping fluorographene by surface adsorbates have been investigated. The structure and electronic properties of fluorographene with adsorbed K, Li, Au atoms, and F4-TCNQ molecule are described. It is shown that adsorption of K or Li atoms results in electron doping of fluorographene, while Au atoms and F4-TCNQ introduce deep levels inside the band gap. The calculated value of the fluorographene work function is extremely high, 7.3 eV, suggesting that p-type doping is difficult to achieve.College of Engineering, Mathematics and Physical Sciences of the University of Exeter

Implanting germanium into graphene

Incorporating heteroatoms into the graphene lattice may be used to tailor its electronic, mechanical and chemical properties. Direct substitutions have thus far been limited to incidental Si impurities and P, N and B dopants introduced using low-energy ion implantation. We present here the heaviest impurity to date, namely $^{74}$Ge$^+$ ions implanted into monolayer graphene. Although sample contamination remains an issue, atomic resolution scanning transmission electron microscopy imaging and quantitative image simulations show that Ge can either directly substitute single atoms, bonding to three carbon neighbors in a buckled out-of-plane configuration, or occupy an in-plane position in a divacancy. First principles molecular dynamics provides further atomistic insight into the implantation process, revealing a strong chemical effect that enables implantation below the graphene displacement threshold energy. Our results show that heavy atoms can be implanted into the graphene lattice, pointing a way towards advanced applications such as single-atom catalysis with graphene as the template.Comment: 20 pages, 5 figure

Some features of reproduction and egg development of <i>Stichaeus grigorjewi</i> from Peter the Great Bay

Reproduction of long shanny Stichaeus grigorjewi is observed in natural conditions and its eggs embryogenesis is investigated in details in aquarium. The embryos and larvae of S. grigorjewi differ from other species by strong development of midbrain that assumes good eyesight forming. Duration of egg development and morphology of embryos and larvae coincide with earlier description made in Hokkaido, Japan, except of body ventral row of 10 melanophores described for S. grigorjewi larvae for the first time. The embryogenesis accelerates sharply under heightened water temperature

INNOVATING CARBON MATERIALS OPEN NEW POSSIBILITIES FOR INCREASING PERFORMANCE OF Li-ion BATTERIES

The purpose of research and development, the results of which are presented in this publication is the modification of carbon materials for use in anode and cathode materials Li batteries with non-aqueous electrolytes. Innovative carbon materials will provide increased energy and safety of lithium current sources, while at the same time reducing the cost of lithium current sources.The purpose of research and development, the results of which are presented in this publication is the modification of carbon materials for use in anode and cathode materials Li batteries with non-aqueous electrolytes. Innovative carbon materials will provide increased energy and safety of lithium current sources, while at the same time reducing the cost of lithium current sources

Electron beam controlled covalent attachment of small organic molecules to graphene

Markevich A, Kurasch S, Lehtinen O, et al. Electron beam controlled covalent attachment of small organic molecules to graphene. NANOSCALE. 2016;8(5):2711-2719.The electron beam induced functionalization of graphene through the formation of covalent bonds between free radicals of polyaromatic molecules and C=C bonds of pristine graphene surface has been explored using first principles calculations and high-resolution transmission electron microscopy. We show that the energetically strongest attachment of the radicals occurs along the armchair direction in graphene to carbon atoms residing in different graphene sub-lattices. The radicals tend to assume vertical position on graphene substrate irrespective of direction of the bonding and the initial configuration. The "standing up" molecules, covalently anchored to graphene, exhibit two types of oscillatory motion bending and twisting - caused by the presence of acoustic phonons in graphene and dispersion attraction to the substrate. The theoretically derived mechanisms are confirmed by near atomic resolution imaging of individual perchlorocoronene (C24Cl12) molecules on graphene. Our results facilitate the understanding of controlled functionalization of graphene employing electron irradiation as well as mechanisms of attachment of impurities via the processing of graphene nanoelectronic devices by electron beam lithography

Three-dimensional ab initio description of vibration-assisted electron knock-on displacements in graphene

peer reviewedTransmission electron microscopy characterization may damage materials, but an electron beam can also induce interesting dynamics. Elastic knock-on is the main electron irradiation damage mechanism in metals including graphene, and although atomic vibrations influence its cross section, only the out-of-plane direction has been considered so far. Here, we present a full three-dimensional first-principles theory of knock-on displacements including the effect of temperature on vibrations to describe dynamics into arbitrary directions. We validate the model with previously precisely measured knock-on damage of pristine graphene, where we show that the isotropic out-of-plane approximation correctly describes the cross section. We then apply our methodology to reversible jumps of pyridinic nitrogen atoms, whose probability under irradiation is measured at 55 and 60 keV. Direct displacement requiring a high emission angle and an alternative pathway via intermittent N adatom creation and recombination are computationally explored but are unable to explain the observed rates, implying stronger inelastic effects at the defect than in pristine graphene

К проектированию ветрозащитных устройств факела распыла полевых опрыскивателей

В статье приведены конструкции различных ветрозащитных устройств и зависимости для расчета их технологических и конструктивных параметров. The article presents the designs of various wind protection devices and dependencies for calculating their technological and design parameters