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«ANNUAL REPORT Riga 2012 Annual Report 2011, Institute of Solid State Physics, University of Latvia. Editor: A.Krumins. Composed matter: A.Muratova. ...»

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12. Ben Gurion University, Beer Sheeva (Prof. A. Aharony, Prof. D. Fuks) Israel

13. Laboratori Nazionali di Frascati (Dr. S. Bellucci, Dr. M. Cestelli-Guidi) Italy

14. Gumilyov National University, Astana (Prof. A. Akilbekov) Kazakhstan

15. FIR Center, University of Fukui (Prof. T. Idehara) Japan

16. Institute of Semiconductor Physics (SPI), Vilnius (Dr. E. Tornau) Lithuania

17. Warsaw University, Dept of Chemistry (Dr A. Huczko) Poland

18. University of Craiova (Dr. D. Constantinescu) Romania

19. St. Petersburg State University (Prof. R.A. Evarestov) Russia

20. Institute of General and Inorganic Chemistry, Russian Academy of Sciences, Moscow (Prof. P.N. Dyachkov)

21. Imperial College London (Prof. M.Finnis) UK

22. University College London (Prof. A.L. Shluger)

23. National University of Lviv (Prof. I. Bolesta and Prof. V. Savchyn) Ukraine

24. Northwestern University, Evanston, Illinois (Prof. M.Olvera de la Cruz) USA 25. University of Maryland, College Park (Dr. G.S. Nusinovich, Dr. M.M. Kukla)

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Fig. 1. Schematic illustration of nanostructured Ni(111) catalyst: (a) side view and (b) top view. Each surface plane is shown with different color to guide eyes. The lower light-blue plane is a mirror plane of symmetrically terminated 5-layer slab.

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The calculated energies of a complete dissociation (Ediss) have been found to be 2.33, 2.17, and 6.40 eV for perfect Ni(111), nanostructured Ni(111), and θ-Al2O3(010) substrates, respectively.

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Fig. 2. Schematic illustration of CH4 dissociation on both Ni(111) and θ-Al2O3(010).

As the result of our simulations, we predict an increase of catalytic activity of nanostructured Ni(111) surface, due to nanofacet formation that potentially can play a role in a predictable growth of CNT (Fig. 3). The key stage of carbon nanotube growth from catalyst is an initial swelling of an island consisting of carbon hexagons and pentagons formed from Cads atoms atop the substrate up to creation of semi-fullerene.

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Fig. 3. Aside (upper) and atop (lower) views of 2D supercells containing CNT of either ac (a) or zz (b) type chirality upon the nanostructured Ni(111) surface.

Our results predict quite effective and reproducible mechanism of growth for carbon nanotubes upon the nickel nanostructured substrate. In absence of catalyst nanoparticles upon the bottom of the nanopores inside alumina membrane the carbon structures could grow from the walls towards the centers of nanopores: either carbon nanoscrolls or rather thick amorphous (soot-like) microtubes. At the bottom level of the multi-scale modeling, ab initio methods can be used for determining the electronic structure of the assumed carbon-metal nanocomposites.

FIRST-PRINCIPLES CALCULATIONS ON SINGLE- AND DOUBLE-WALL

INORGANIC NANOTUBES AND THEIR STRUCTURAL ANALYSIS

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Within the line group irreducible representations developed in collaboration with Prof. R.A.

Evarestov and Dr. A.V. Bandura (St. Petersburg University, Russia) the one-periodic (1D) nanostructures with rotohelical symmetry have been considered for symmetry analysis of single- and double-wall (SW and DW) boron nitride and titania nanotubes (BN and TiO2 NTs) formed by rolling up the stoichiometric two-periodic (2D) slabs of hexagonal structure with the same or opposite orientation of translation and chiral vectors. We have simulated the two sets of commensurate double-wall BN NTs and TiO2 NTs (Fig. 4) with armchair- or zigzagtype chiralities: (n1,n1)@(n2,n2) or (n1,0)@(n2,0), respectively.

a) optimized model of double-wall (6,6)@(12,12) ac-TiO2 NT;

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b) optimized model of double-wall (10,0)@(20,0) zz- TiO2 NT.

Fig. 4. Cross-sections and aside images of hexagonal DW TiO2 NTs corresponding to optimized diameters (i.e., left and right parts of models a) and b), respectively) for armchair and zigzag chiralities. For zz-DW NT (b), there are also shown atoms of the nearest ring behind the cross-section (as considerably more light circles).

Due to a noticeably larger ionic contribution to inter-wall interaction between three-layer OTi-O shells within DW TiO2 NTs their polarization effects are certainly larger than those in double-wall boron nitride nanotubes which results in the higher electron density localization as compared to DW BN NTs. Considerable interaction between the walls in optimal DW NT configurations results in a decrease of band gaps in double-wall nanotubes as compared to those for SW NTs (this decrease is a more pronounced for DW TiO2 NTs).

One-dimensional nanostructures synthesized from complex ternary oxides with a perovskite structure have attracted considerable recent interest due to their unique physical properties and promising novel functionalities as compared to bulk materials. At room temperature SrTiO3 possesses a high symmetry cubic structure and, thus, serves as an excellent model material for a wide class of ABO3 perovskites. Consequently, understanding the behavior of SrTiO3 on the nanoscale is significant for fundamental studies, as well as for shape-controlled synthesis of perovskite nanostructures with predictable properties. Based on ab initio calculations performed in collaboration with Prof. E. Spohr (University of Duisburg-Essen, Germany) and Faculty of Computing (University of Latvia) we predict that the most





energetically stable NTs can be rolled up from (110) nanosheet of rectangular morphology:

Fig. 5. Atomic structure of the most energetically stable SrTiO3 nanotube.

The increase of the Ti–O bond covalency in the outer shell of strontium titanate NT may lead to an enhancement of adsorption properties which means that they can be used in gas-sensing devices. Quantum confinement effects lead to the widening of the NT band gaps, thus, making them attractive for band gap engineering, e.g., in photocatalytic applications.

THEORETICAL SIMULATIONS ON ELECTRIC PROPERTIES FOR JUNCTIONS

OF METALLIC ELECTRODES WITH CARBON NANOTUBES AND GRAPHENE

NANORIBBONS

Yu.N. Shunin, Yu.F. Zhukovskii, S. Bellucci (Laboratori Nazionali di Frascati, Italy) In collaboration with Dr. S. Bellucci (Laboratori Nazionali di Frascati, Italy) within the EC FP7 CATHERINE project, we have developed the model of ‘effective bonds’ in the framework of both cluster approach based on the multiple scattering theory formalism and Landauer theory, which can allow us to predict the resistivity properties for C-Me junctions taking into account chirality effects in the interconnects of single-wall (SW) and multi-wall (MW) CNTs (Fig. 6) as well as single-layer (SL) and poly-layer (PL) GNRs (Fig. 7) with the fitting metals (Me= Ni, Cu, Ag, Pd, Pt, Au) on predefined geometry of carbon nanostructure.

We have also developed the model of inter-shell interaction for the MW CNTs, which allows us to estimate the transparency coefficient as an indicator of possible ‘radial current’ losses.

Fig. 6. Model of CNT - Me interconnect. Fig. 7. GNR (polylayered) - Me interconnect.

Figs. 8 and 9 show the generalized results of simulations on resistance of junctions between various metallic substrates with SW CNT and SL GNR, respectively. It is clear that Ag and Au substrates are more effective electrically while Ni is rather a ‘worse’ substrate for interconnect, although it yields the most effective catalyst for CNT growth.

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Conductance and other current-voltaic parameters depend on the morphology of the nearest shells in MW CNTs and PL GNRs, which results in complications for technological synthesis.

Nevertheless, the corresponding nanodevices possess the stable electric characteristics. We are able now to create a database of combinations for different CNT-Me and GNR-Me junctions taking into account a set of parameters, namely: angle of chirality, CNT diameter, numbers of walls or layers, type of metal substrate (Me), orientation of densely-packed metal substrate, e.g., (100), (111) or (110). Thus, we are able to predict interconnect properties for various configurations of SW and MW CNTs as well as SL and PL GNRs.

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For the first time, we have performed detailed first-principles simulations of perfect and defective uranium mononitride (UN) surfaces and their interaction with oxygen, in collaboration with EC Institute for Transuranium Elements (Karlsruhe, Germany) and Faculty of Computing (University of Latvia). This is relevant for understanding mechanism of UN nuclear fuel exidation in air. Due to a mixed metallic-covalent nature of the chemical bonding in UN, we predicted a high affinity of adsorbed O towards the UN(001) surface.

Indeed, the Ebind values of 6.9-7.6 and 5.0-5.7 eV per O adatom atop the surface U or N atoms, respectively, are accompanied by 0.5-1.2 e charge transfer from the surface towards the O adatom (Fig. 10). The positively charged surface U atom goes outwards, minimizing its distance with the adsorbed O atom while the N atom is strongly displaced from the adsorbed O atom inwards the slab, due to a mutual repulsion between N and O.

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Fig. 10. Schematic top view of O adatoms located atop the surface U atom without (a) and with (b) N vacancy in the proximity of adsorbed O atoms. Numbers in brackets enumerate non-equivalent surface atoms.

Three main migration paths of O upon the UN(001) surface (Fig. 11) are as follows: 1:

between U atom and the nearest N atom, 2: between the two neighboring U atoms, 3: between neighboring N atoms. The most favorable migration trajectory has been optimized to be the line joining the sites atop the nearest surface U atoms and the hollow sites between them (path 2). The corresponding energy barriers found (0.36 eV for the 5-layer slab and 0.26 eV for the 7-layer slab) indicate a high mobility of adsorbed O atoms upon UN. The energy barriers along other two migration trajectories are substantially larger.

Fig. 11. Different oxygen migration paths upon the UN(001) surface (top view).

Both formation energies of uranium and nitrogen vacancies as well as binding energies of oxygen atoms and molecules adsorbed atop the defective UN(001) surface have been estimated too. Presence of the surface nitrogen vacancy closest to the surface U atom (Usurf) results in a low-barrier incorporation of migrating O adatom from position atop Usurf towards this vacancy, which can be considered as a trap. Based on the results of calculations discussed above the following stages of oxygen interaction with the UN surfaces were indentified to explain its easy oxidation: (i) chemisorption of molecular oxygen, (ii) spontaneous breaking of the O2 chemical bond after molecular adsorption, (iii) location of the two newly formed O adatoms atop the adjacent surface U atoms, (iv) high mobility of adsorbed O atoms along the surface, (v) low-barrier incorporation of O into N-vacancies, (vi) stabilization of O atom inside the N-vacancy, (vii) further incorporation of O in pre-existed sub-surface N-vacancies as a result of inter-layer diffusion.

AB INITIO SIMULATIONS OF IMPURITY CLUSTERS IN ODS STEELS

A. Gopejenko, Yu.F. Zhukovskii, Yu. Mastrikov, E.A. Kotomin, P.V. Vladimirov, A. Möslang (Institut für Materialforschung I, Karlsruhe, Germany) V.A. Borodin (Research Center Kurchatov Institute, Moscow, Russia) The understanding of the mechanisms and kinetics of yttria nanoparticle formation in the steel matrix is required of the development of the oxide dispersed strengthened (ODS) steels.

The implementation of the ODS steel for fusion- and advanced fission-reactor blanket structures results in increase of the operation temperature by ~100°C which makes this material very promising these reactors. On the other hand, the mechanical properties and radiation resistance of ODS steels are sensitive to the size and spatial distribution of the oxide precipitates. Therefore, it is necessary to perform a large-scale theoretical modeling of the Y2O3 formation. Large-scale first principles calculations have been performed in collaboration with Dr. A. Möslang and Dr. P.V. Vladimirov (Institut für Materialforschung I, Karlsruhe, Germany) for the γ-Fe lattice containing Y-Y, Y-VFe, VFe-VFe, Y-O (Fig. 12) and O-O pairs as well as different configurations of three-atom clusters Y-O-Y (Fig. 13) and Y-VFe-Y. These calculations are required to accurately estimate the pair- and triple-wise interaction energies necessary for further lattice kinetic Monte Carlo (LKMC) simulations of ODS growth.

Fig. 12. Relaxed 2nd coordination sphere for Fig. 13. Relaxed configuration of 2Y-O configuration of Y-Oint pair. substitute atoms.

The analysis of the pair-wise interactions calculations show that a certain attraction occurs between the Y substitute atom and Fe vacancy, while no bonding occurs between two Y atoms at any distances. The calculations of the interactions between yttrium and oxygen substitute atoms as well as between two oxygen substitute atoms show similar behavior with the highest binding energies at the distance of 1-NN and the decrease of the binding energy with the increase of the inter-defect distance. No significant bonding has been found between the two Fe vacancies located at different distances.

At the same time, we predict location of Fe vacancies in the proximity of impurity atoms.

The calculations on different Y-O-Y cluster configurations clearly show that not only the presence of oxygen atom is required to form certain binding between impurity atoms but also the presence of Fe vacancies favors the growth of the Y2O3 precipitates inside the iron crystalline matrix. This has been proven by the calculations of interactions inside the Y-VFe-Y cluster for which the binding energy has been found to be rather large.

FIRST PRINCIPLES CALCULATIONS OF OXYGEN VACANCY FORMATION



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