Белорусская энергетика: современное состояние и перспективы

Материалы III Междунар. науч. конф. студентов, аспирантов и молодых ученых, Гомель, 20 мая 2010 г

Electronic and optical properties of core-shell InAlN nanorods: a comparative study via LDA, LDA-1/2, mBJ and G0W0G_0W_0 methods

Currently, self-induced InAlN core-shell nanorods enjoy an advanced stage of accumulation of experimental data from their growth and characterization as well as a comprehensive understanding of their formation mechanism by the ab initio modeling based on Synthetic Growth Concept. However, their electronic and optical properties, on which most of their foreseen applications are expected to depend, have not been investigated comprehensively. $G_0W_0$ is currently regarded as a gold-standard methodology with quasi-particle corrections to calculate electronic properties of materials in general. It is also the starting point for higher-order methods that study excitonic effects, such as those based on the Bethe-Salpeter equation. One major drawback of $G_0W_0$, however, is its computational cost, much higher than density-functional theory (DFT). Therefore, in many applications, it is highly desirable to answer the question of how well approaches based on DFT, such as e. g. LDA, LDA-1/2, and mBJ, can approximately reproduce $G_0W_0$ results with respect to the electronic and optical properties. Thus, the purpose of the present paper is to investigate how the DFT-based methodologies LDA, LDA-1/2, and mBJ can be used as tools to approximate $G_0W_0$ in studies of the electronic and optical properties of scaled down models of core-shell InAlN nanorods. For these systems, we observed that band gaps, density of states, dielectric functions, refractive indexes, absorption and reflectance coefficients are reasonably well described by LDA-1/2 and mBJ when compared to $G_0W_0$, however, at a much more favorable computational cost.Comment: The following article has been submitted to The Journal of Chemical Physics. After it is published, it will be found at https://publishing.aip.org/resources/librarians/products/journals

Structural and electronic properties of organic and organometallic clusters

Tese de doutoramento em Física (Física Teórica) apresentada à Fac. de Ciências e Tecnologia de CoimbraO trabalho de investigação apresentado nesta dissertação visa compreender e analisar as propriedades estruturais e electrónicas de fulerenos, agregados mistos de carbono com átomos de metais de transição e ainda agregados mistos de silício com átomos de metais de transição. Foram utilizados métodos quânticos, predominantemente a Teoria do Funcional da Densidade (DFT) nas suas Aproximações da Densidade Local (LDA) ou do Gradiente Generalizado (GGA). O modelo de Tight-Binding (TB) foi utilizado nos estudos de agregados envolvendo um número de átomos mais elevado. Com efeito, nas últimas duas décadas foi reconhecido que os métodos ab initio e particularmente a DFT proporcionam uma descrição quantitativa de muitas propriedades associadas ao estado fundamental e a estados excitados de diferentes átomos, moléculas, agregados e sólidos, tornando-se desta forma num instrumento que permite prever eficazmente a estabilidade ou não de novos materiais com propriedades pré-definidas. Neste contexto, a maioria do trabalho apresentado nesta tese é exploratório e permitiu tirar conclusões bem definidas para os agregados estudados (e os materiais deles derivados). Basicamente foram estudados três grupos de agregados: fulerenos, metalocarbohedrenos (metcars) e agregados de silício com um átomo de transição

A perspective on thermal stability and mechanical properties of 2D Indium Bismide from ab initio molecular dynamics

Identification and synthesis of 2D topological insulators is particularly elusive. According to previous ab initio predictions 2D InBi (Indium Bismide) is a material exhibiting topological properties which are combined with a band gap suitable for practical applications. We employ ab initio molecular dynamics (AIMD) simulations to assess the thermal stability as well as the mechanical properties such as elastic modulus and stress-strain curves of 2D InBi. The obtained new knowledge adds further characteristics appealing to the feasibility of its synthesis and its potential applications. We find that pristine 2D InBi, H-InBi (hydrogenated 2D InBi) as well as 2D InBi heterostructures with graphene are all stable well above room temperature, being the calculated thermal stability for pristine 2D InBi 850 K and for H-InBi in the range above 500 K. The heterostructures of 2D InBi with graphene exhibit thermal stability exceeding 1000 K. In terms of mechanical properties, pristine 2D InBi exhibits similarities with another 2D material, stanene. The fracture stress for 2D InBi is estimated to be similar to 3.3 GPa (similar to 3.6 GPa for stanene) while elastic modulus of 2D InBi reads similar to 34 GPa (to compare with similar to 23 GPa for stanene). Overall, the thermal stability, elastic, and fracture resistant properties of 2D InBi and its heterostructures with graphene appear as high enough to motivate future attempts directed to its synthesis and characterization.Funding Agencies|Swedish National Infrastructure for Computing (SNIC) at the National Supercomputer Center (NSC) in Linkoping [SNIC 2020/5-146, SNIC 2020/14-17]; Swedish Research Council [2018-05973]; [VR2017-04071]</p

Benzene, coronene, and circumcoronene adsorbed on gold, and a gold cluster adsorbed on graphene: Structural and electronic properties

Density functional theory (DFT) calculations were performed in order to investigate the stability and the electronic structure of graphene-gold interfaces. Two configurations were studied: a gold cluster interacting with graphene and different polycyclic aromatic hydrocarbon (PAH) molecules, namely, C6H6 (benzene), C24H12 (coronene), and C54H18 (circumcoronene) adsorbed on an Au(111) surface. Nonlocal interactions were accounted for by using the semiempirical DFT-D2 method of Grimme. A limited set of calculations were also performed by using the first-principles van der Waals density functional method (vdW-DF). Adsorption distances around 3 angstrom and electronic charge transfer values of about (3-13) x 10(-3)e(-) per carbon atom were predicted for all systems. No major changes resulting from the adsorption of the gold cluster were detected in the graphenes density of states. The DFT-D2 results involving the adsorption of the PAH molecules on gold show an estimated binding energy of 73 meV per carbon atom, as well as an electronic charge loss of 0.10 x 10(-2) e(-), also per carbon atom, for an extended graphene sheet adsorbed on a gold surface. The modeling of the adsorption of C6H6 molecule on a gold surface suggests that the vdW-DF method provides more accurate results for the binding energies of such systems, in comparison to pure DFT calculations, which do not take the nonlocal interactions into account, as well as to simulations employing the DFT-D2 method.Funding Agencies|Swedish Foundation for International Cooperation in Research and Higher Education (STINT)|YR2009-7017|Swedish Research Council (VR)|

Ab initio molecular dynamics of atomic-scale surface reactions: insights into metal organic chemical vapor deposition of AlN on graphene

Metal organic chemical vapor deposition (MOCVD) of group III nitrides on graphene heterostructures offers new opportunities for the development of flexible optoelectronic devices and for the stabilization of conceptually-new two-dimensional materials. However, the MOCVD of group III nitrides is regulated by an intricate interplay of gas-phase and surface reactions that are beyond the resolution of experimental techniques. We use density-functional ab initio molecular dynamics (AIMD) with van der Waals corrections to identify atomistic pathways and associated electronic mechanisms driving precursor/surface reactions during metal organic vapor phase epitaxy at elevated temperatures of aluminum nitride on graphene, considered here as model case study. The results presented provide plausible interpretations of atomistic and electronic processes responsible for delivery of Al, C adatoms, and C-Al, CHx, AlNH2 admolecules on pristine graphene via precursor/surface reactions. In addition, the simulations reveal C adatom permeation across defect-free graphene, as well as exchange of C monomers with graphene carbon atoms, for which we obtain rates of approximate to 0.3 THz at typical experimental temperatures (1500 K), and extract activation energies Eexca = 0.28 +/- 0.13 eV and attempt frequencies A(exc) = 2.1 (x1.7(+/- 1)) THz via Arrhenius linear regression. The results demonstrate that AIMD simulations enable understanding complex precursor/surface reaction mechanisms, and thus propose AIMD to become an indispensable routine prediction-tool toward more effective exploitation of chemical precursors and better control of MOCVD processes during synthesis of functional materials.Funding Agencies|Swedish Research Council (VR) through FLAG-ERA JTC project GRIFONE [VR 2015-06816, VR 2017-04071]; Olle Engkvist Foundation</p

Fullerene-like CSx: A first-principles study of synthetic growth

Fullerene-Like (FL) Sulpho-Carbide (CSx) compounds have been addressed by first principles calculations. Geometry optimization and cohesive energy results are presented for the relative stability of precursor species such as C2S, CS2, and C2S2 in isolated form. The energy cost for structural defects, arising from the substitution of C by S is also reported. Similar to previously synthesized FL-CNx and FL-CPx compounds, the pentagon, the double pentagon defects as well as the Stone-Wales defects are confirmed as energetically feasible in CSx compounds.Original Publication:Cecilia Goyenola, Gueorgui Kostov Gueorguiev, Sven Stafström and Lars Hultman, Fullerene-like CSx: A first-principles study of synthetic growth, 2011, CHEMICAL PHYSICS LETTERS, (506), 1-3, 86-91.http://dx.doi.org/10.1016/j.cplett.2011.02.059Copyright: Elsevier Science B.V., Amsterdam.http://www.elsevier.com

CF(x): A first-principles study of structural patterns arising during synthetic growth

Structural and bonding patterns arising from the incorporation of fluorine atoms in a graphene-like network relevant to the deposition of carbon fluoride (CF(x)) films were addressed by first-principles calculations. We find that large N-member (N = 8-12) rings, defects by sheet branching, and defects associated with bond rotation pertain to CF(x). The cohesive energy gains associated with these patterns are similar to 0.2-0.4 eV/at., which is similar to those for a wide range of defects in other C-based nanostructured solids. Fullerene-like CF(x) is predicted for F concentrations below similar to 10 at.%, while CF(x) compounds with higher F content are predominantly amorphous or polymeric.Funding Agencies|Swedish Governmental Agency for Innovation Systems (VINNOVA)||European Research Council (ERC)||</p

Discovering atomistic pathways for supply of metal atoms from methyl-based precursors to graphene surface

Conceptual 2D group III nitrides and oxides (e.g., 2D InN and 2D InO) in heterostructures with graphene have been realized by metal-organic chemical vapor deposition (MOCVD). MOCVD is expected to bring forth the same impact in the advancement of 2D semiconductor materials as in the fabrication of established semiconductor materials and device heterostructures. MOCVD employs metal-organic precursors such as trimethyl-indium, -gallium, and -aluminum, with (strong) metal-carbon bonds. Mechanisms that regulate MOCVD processes at the atomic scale are largely unknown. Here, we employ density-functional molecular dynamics - accounting for van der Waals interactions - to identify the reaction pathways responsible for dissociation of the trimethylindium (TMIn) precursor in the gas phase as well as on top-layer and zero-layer graphene. The simulations reveal how collisions with hydrogen molecules, intramolecular or surface-mediated proton transfer, and direct TMIn/graphene reactions assist TMIn transformations, which ultimately enables delivery of In monomers or InH and CH3In admolecules, on graphene. This work provides knowledge for understanding the nucleation and intercalation mechanisms at the atomic scale and for carrying out epitaxial growth of 2D materials and graphene heterostructures.Funding Agencies|Swedish Research Council [2017-04071]; Swedish Research Council (VR) [2016-05156]; Competence Center Functional Nanoscale Materials (FunMat-II) (Vinnova) [IB2018-7520]; Swedish Foundation for International Cooperation in Research and Higher Education STINT; [VR-2015-04630]; [VR-2021-04426]</p