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  • 1.
    Almyras, Georgios
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Nanoscale engineering. Linköping University, Faculty of Science & Engineering.
    Sangiovanni, Davide Giuseppe
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. Ruhr Univ Bochum, Germany.
    Sarakinos, Kostas
    Linköping University, Department of Physics, Chemistry and Biology, Nanoscale engineering. Linköping University, Faculty of Science & Engineering.
    Semi-Empirical Force-Field Model For The Ti1-XAlXN (0 ≤ x ≤ 1) System2019In: Materials, ISSN 1996-1944, E-ISSN 1996-1944, Vol. 12, no 2, article id 215Article in journal (Refereed)
    Abstract [en]

    We present a modified embedded atom method (MEAM) semi-empirical force-field model for the Ti1-xAlxN (0 x 1) alloy system. The MEAM parameters, determined via an adaptive simulated-annealing (ASA) minimization scheme, optimize the models predictions with respect to 0 K equilibrium volumes, elastic constants, cohesive energies, enthalpies of mixing, and point-defect formation energies, for a set of approximate to 40 elemental, binary, and ternary Ti-Al-N structures and configurations. Subsequently, the reliability of the model is thoroughly verified against known finite-temperature thermodynamic and kinetic properties of key binary Ti-N and Al-N phases, as well as properties of Ti1-xAlxN (0 amp;lt; x amp;lt; 1) alloys. The successful outcome of the validation underscores the transferability of our model, opening the way for large-scale molecular dynamics simulations of, e.g., phase evolution, interfacial processes, and mechanical response in Ti-Al-N-based alloys, superlattices, and nanostructures.

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  • 2. Chen, Zhuo
    et al.
    Huang, Yong
    Koutná, Nikola
    Gao, Zecui
    Sangiovanni, Davide G.
    Fellner, Simon
    Haberfehlner, Georg
    Jin, Shengli
    Mayrhofer, Paul H.
    Kothleitner, Gerald
    Zhang, Zaoli
    Large mechanical properties enhancement in ceramics through vacancy-mediated unit cell disturbance2023In: Nature Communications, E-ISSN 2041-1723, Vol. 14, no 1Article in journal (Refereed)
    Abstract [en]

    Tailoring vacancies is a feasible way to improve the mechanical properties of ceramics. However, high concentrations of vacancies usually compromise the strength (or hardness). We show that a high elasticity and flexural strength could be achieved simultaneously using a nitride superlattice architecture with disordered anion vacancies up to 50%. Enhanced mechanical properties primarily result from a distinctive deformation mechanism in superlattice ceramics, i.e., unit-cell disturbances. Such a disturbance substantially relieves local high-stress concentration, thus enhancing deformability. No dislocation activity involved also rationalizes its high strength. The work renders a unique understanding of the deformation and strengthening/toughening mechanism in nitride ceramics.

  • 3.
    Dippo, Olivia F.
    et al.
    Univ Calif San Diego, CA 92093 USA.
    Sangiovanni, Davide
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Wenger, Emma
    Univ Calif San Diego, CA 92093 USA.
    Vecchio, Kenneth S.
    Univ Calif San Diego, CA 92093 USA; Univ Calif San Diego, CA 92093 USA.
    Color and pseudogap tunability in multicomponent carbonitrides2022In: Materials & design, ISSN 0264-1275, E-ISSN 1873-4197, Vol. 217, article id 110600Article in journal (Refereed)
    Abstract [en]

    The design and tailoring of material color for both aesthetic and functionality is an ongoing topic of materials science and engineering research. In this work, a method is developed to tune and predict color and pseudogap energy of any compositional variation of B1-rocksalt structured Group 4 and 5 transition metal carbonitride. Optical properties of bulk multicomponent transition metal carbonitrides were characterized using reflectivity spectra. Optical pseudogap energies were extrapolated using the Tauc method, and color appearance was quantified in the Commission Internationale de lEclairage (CIE) Lightness*Chroma*hue (L*C*h) color space. Variations of color parameters chroma and hue were analyzed in terms of pseudogap energies and electronic band structures. Compositional variations were utilized to predictably tune aspects of the electronic structure, including the specificity of electronic transitions and the energy at which they occur, to tailor the materials color appearance and facilitate the formation of new carbonitride colors.

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  • 4.
    Edström, Daniel
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Sangiovanni, Davide G.
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
    Hultman, Lars
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Chirita, Valeriu
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Effects of atomic ordering on the elastic properties of TiN- and VN-based ternary alloys2014In: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 571, no Part 1, p. 145-153Article in journal (Refereed)
    Abstract [en]

    Improved toughness is one of the central goals in the development of wear-resistant coatings. Previous studies of toughness in transition metal nitride alloys have addressed the effects of chemical composition in these compounds. Herein, we use density functional theory to study the effects of various metal sublattice configurations, ranging from fully ordered to fully disordered, on the mechanical properties of VM2N and TiM2N (M2 = W, Mo) ternary alloys. Results show that all alloys display high incompressibility, indicating strong M-N bonds. Disordered atomic arrangements yield lower values of bulk moduli and C11 elastic constants, as well as higher values of C44 elastic constants, compared to ordered structures. We attribute the low C44 values of ordered structures to the formation of fully-bonding states perpendicular to the applied stress. We find that the ductility of these compounds is primarily an effect of the increased valence electron concentration induced upon alloying.

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  • 5.
    Edström, Daniel
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering. Orebro Univ, Sweden.
    Sangiovanni, Davide Giuseppe
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. Ruhr Univ Bochum, Germany.
    Landälv, Ludvig
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering. Sandvik Coromant AB, Sweden.
    Eklund, Per
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Greene, Joseph E
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering. Univ Illinois, IL 61801 USA; Univ Illinois, IL 61801 USA; Natl Taiwan Univ Sci and Technol, Taiwan.
    Petrov, Ivan
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering. Univ Illinois, IL 61801 USA; Univ Illinois, IL 61801 USA.
    Hultman, Lars
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Chirita, Valeriu
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Mechanical properties of VMoNO as a function of oxygen concentration: Toward development of hard and tough refractory oxynitrides2019In: Journal of Vacuum Science & Technology. A. Vacuum, Surfaces, and Films, ISSN 0734-2101, E-ISSN 1520-8559, Vol. 37, no 6, article id 061508Article in journal (Refereed)
    Abstract [en]

    Improved toughness is a central goal in the development of wear-resistant refractory ceramic coatings. Extensive theoretical and experimental research has revealed that NaCl-structure VMoN alloys exhibit surprisingly high ductility combined with high hardness and toughness. However, during operation, protective coatings inevitably oxidize, a problem that may compromise material properties and performance. Here, the authors explore the role of oxidation in altering VMoN properties. Density functional theory and theoretical intrinsic hardness models are used to investigate the mechanical behavior of cubic V0.5Mo0.5N1-xOx solid solutions as a function of the oxygen concentration x. Elastic constant and intrinsic hardness calculations show that oxidation does not degrade the mechanical properties of V0.5Mo0.5N. Electronic structure analyses indicate that the presence of oxygen reduces the covalent bond character, which slightly lowers the alloy strength and intrinsic hardness. Nevertheless, the character of metallic d-d states, which are crucial for allowing plastic deformation and enhancing toughness, remains unaffected. Overall, the authors results suggest that VMoNO oxynitrides, with oxygen concentrations as high as 50%, possess high intrinsic hardness, while still being ductile. Published by the AVS.

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  • 6.
    Edström, Daniel
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Sangiovanni, Davide
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Hultman, Lars
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Chirita, Valeriu
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Petrov, Ivan
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology. Department of Materials Science and the Fredrick Seitz Materials Research Laboratory, University of Illinois, Urbana, USA.
    Greene, Joseph
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology. Department of Materials Science and the Fredrick Seitz Materials Research Laboratory, University of Illinois, Urbana, USA.
    Ti and N adatom descent pathways to the terrace from atop two-dimensional TiN/TiN(001) islands2014In: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 558, p. 37-46Article in journal (Refereed)
    Abstract [en]

    We use classical molecular dynamics and the modified embedded atom method to determine residence times and descent pathways of Ti and N adatoms on square, single-atom-high, TiN islands on TiN(001). Simulations are carried out at 1000 K, which is within the optimal range for TiN(001) epitaxial growth. Results show that the frequency of descent events, and overall adatom residence times, depend strongly on both the TiN(001) diffusion barrier for each species as well as the adatom island-edge location immediately prior to descent. Ti adatoms, with a low diffusion barrier, rapidly move toward the island periphery, via funneling, where they diffuse along upper island edges. The primary descent mechanism for Ti adatoms is via push-out/exchange with Ti island-edge atoms, a process in which the adatom replaces an island edge atom by moving down while pushing the edge atom out onto the terrace to occupy an epitaxial position along the island edge. Double push-out events are also observed for Ti adatoms descending at N corner positions. N adatoms, with a considerably higher diffusion barrier on TiN(001), require much longer times to reach island edges and, consequently, have significantly longer residence times. N adatoms are found to descend onto the terrace by direct hopping over island edges and corner atoms, as well as by concerted push-out/exchange with N atoms adjacent to Ti corners. For both adspecies, we also observe several complex adatom/island interactions, before and after descent onto the terrace, including two instances of Ti islandatom ascent onto the island surface.

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    Ti and N adatom descent pathways to the terrace from atop two-dimensional TiN/TiN(001) islands
  • 7.
    Edström, Daniel
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Sangiovanni, Davide
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Hultman, Lars
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Petrov, Ivan
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology. Department of Materials Science and the Fredrick Seitz Materials Research Laboratory, University of Illinois, Urbana, USA.
    Greene, Joseph
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology. Department of Materials Science and the Fredrick Seitz Materials Research Laboratory, University of Illinois, Urbana, USA.
    Chirita, Valeriu
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    The dynamics of TiNx (x = 1 – 3) admolecule interlayer and intralayer transport on TiN/TiN(001) islands2015In: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 589, p. 133-144Article in journal (Refereed)
    Abstract [en]

    It has been shown both experimentally and by density functional theory calculations that the primary diffusing species during the epitaxial growth of TiN/TiN(001) are Ti and N adatoms together with TiNx complexes (x = 1, 2, 3), in which the dominant N-containing admolecule species depends upon the incident N/Ti flux ratio. Here, we employ classical molecular dynamics (CMD) simulations to probe the dynamics of TiNx (x = 1–3) admolecules on 8 × 8 atom square, single-atom-high TiN islands on TiN(001), as well as pathways for descent over island edges. The simulations are carried out at 1000 K, a reasonable epitaxial growth temperature. We find that despite their lower mobility on infinite TiN(001) terraces, both TiN and TiN2 admolecules funnel toward descending steps and are incorporated into island edges more rapidly than Ti adatoms. On islands, TiN diffuses primarily via concerted translations, but rotation is the preferred diffusion mechanism on infinite terraces. TiN2 migration is initiated primarily by rotation about one of the N admolecule atoms anchored at an epitaxial site. TiN admolecules descend from islands by direct hopping over edges and by edge exchange reactions, while TiN2 trimers descend exclusively by hopping. In contrast, TiN3 admolecules are essentially stationary and serve as initiators for local island growth. Ti adatoms are the fastest diffusing species on infinite TiN(001) terraces, but on small TiN/TiN(001) islands, TiN dimers provide more efficient mass transport. The overall results reveal the effect of the N/Ti precursor flux ratio on TiN(001) surface morphological evolution and growth modes.

  • 8.
    Edström, Daniel
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Sangiovanni, Davide
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. Ruhr University of Bochum, Germany.
    Hultman, Lars
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Petrov, Ivan
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering. University of Illinois, IL 61801 USA.
    Greene, Joseph E
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering. University of Illinois, IL 61801 USA.
    Chirita, Valeriu
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Effects of incident N atom kinetic energy on TiN/TiN(001) film growth dynamics: A molecular dynamics investigation2017In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 121, no 2, article id 025302Article in journal (Refereed)
    Abstract [en]

    Large-scale classical molecular dynamics simulations of epitaxial TiN/TiN(001) thin film growth at 1200 K, a temperature within the optimal range for epitaxial TiN growth, with an incident N-to-Ti flux ratio of four, are carried out using incident N energies E-N = 2 and 10 eV and incident Ti energy E-Ti = 2 eV. To further highlight the effect of E-N, we grow a bilayer film with E-N = 2 eV initially and then switch to E-N = 10 eV. As-deposited layers are analyzed as a function of composition, island-size distribution, island-edge orientation, and vacancy formation. Results show that growth with E-N = 2 eV results in films that are globally overstoichiometric with islands bounded by N-terminated polar 110 edges, whereas films grown with E-N = 10 eV are flatter and closer to stoichiometric. However, E-N = 10 eV layers exhibit local N deficiency leading to the formation of isolated 111-oriented islands. Films grown by changing the incident energy from 2 to 10 eV during growth are more compact than those grown entirely with E-N = 2 eV and exhibit greatly reduced concentrations of upper-layer adatoms, admolecules, and small clusters. Islands with 110 edges formed during growth with E-N = 2 eV transform to islands with 100 edges as E-N is switched to 10 eV. Published by AIP Publishing.

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  • 9.
    Edström, Daniel
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Sangiovanni, Davide
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. Ruhr Univ Bochum, Germany.
    Hultman, Lars
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Petrov, Ivan
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering. Univ Illinois, IL 61801 USA.
    Greene, Joseph E
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering. Univ Illinois, IL 61801 USA.
    Chirita, Valeriu
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Elastic properties and plastic deformation of TiC- and VC-based alloys2018In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 144, p. 376-385Article in journal (Refereed)
    Abstract [en]

    Transition-metal (TM) carbides are an important class of hard, protective coating materials; however, their brittleness often limits potential applications. We use density functional theory to investigate the possibility of improving ductility by forming pseudobinary cubic (MMC)-M-1-C-2 alloys, for which M-1 = Ti or V and M-2 = W or Mo. The alloying elements are chosen based on previous results showing improved ductility of the corresponding pseudobinary nitride alloys with respect to their parent compounds. While commonly-used empirical criteria do not indicate enhanced ductility in the carbide alloys, calculated stress/strain curves along known slip systems, supported by electronic structure analyses, indicate ductile behavior for VMoC. As VMoC layers are sheared along the 1 (1) over bar0 direction on {111} planes, the stress initially increases linearly up to a yield point where the accumulated stress is partially dissipated. With further increase in strain, the stress increases again until fracture occurs. A similar mechanical behavior is observed for the corresponding TM nitride VMoN, known to be a ductile ceramic material [1]. Thus, our results show that VMoC is a TM carbide alloy which may be both hard and ductile, i.e. tough. (C) 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

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  • 10.
    Edström, Daniel
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Sangiovanni, Davide
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Hultman, Lars
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Petrov, Ivan
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering. University of Illinois, IL 61801 USA; University of Illinois, IL 61801 USA.
    Greene, Joseph E
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering. University of Illinois, USA.
    Chirita, Valeriu
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Large-scale molecular dynamics simulations of TiN/TiN(001) epitaxial film growth2016In: Journal of Vacuum Science & Technology. A. Vacuum, Surfaces, and Films, ISSN 0734-2101, E-ISSN 1520-8559, Vol. 34, no 4, p. 041509-1-041509-9Article in journal (Refereed)
    Abstract [en]

    Large-scale classical molecular dynamics simulations of epitaxial TiN/TiN(001) thin film growth at 1200K are carried out using incident flux ratios N/Ti -1, 2, and 4. The films are analyzed as a function of composition, island size distribution, island edge orientation, and vacancy formation. Results show that N/Ti-1 films are globally understoichiometric with dispersed Ti-rich surface regions which serve as traps to nucleate 111-oriented islands, leading to local epitaxial breakdown. Films grown with N/Ti=2 are approximately stoichiometric and the growth mode is closer to layer-by-layer, while N/Ti-4 films are stoichiometric with N-rich surfaces. As N/Ti is increased from 1 to 4, island edges are increasingly polar, i. e., 110-oriented, and N-terminated to accommodate the excess N flux, some of which is lost by reflection of incident N atoms. N vacancies are produced in the surface layer during film deposition with N/Ti-1 due to the formation and subsequent desorption of N-2 molecules composed of a N adatom and a N surface atom, as well as itinerant Ti adatoms pulling up N surface atoms. The N vacancy concentration is significantly reduced as N/Ti is increased to 2; with N/Ti-4, Ti vacancies dominate. Overall, our results show that an insufficient N/Ti ratio leads to surface roughening via nucleation of small dispersed 111 islands, whereas high N/Ti ratios result in surface roughening due to more rapid upper-layer nucleation and mound formation. The growth mode of N/Ti-2 films, which have smoother surfaces, is closer to layer-by-layer. (C) 2016 American Vacuum Society.

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  • 11.
    Edström, Daniel
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering. Orebro Univ, Sweden.
    Sangiovanni, Davide
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. Ruhr Univ Bochum, Germany.
    Hultman, Lars
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Petrov, Ivan
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering. Univ Illinois, IL 61801 USA.
    Greene, Joseph E
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering. Univ Illinois, IL 61801 USA; Natl Taiwan Univ Sci and Technol, Taiwan.
    Chirita, Valeriu
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    TiN film growth on misoriented TiN grains with simultaneous low-energy bombardment: Restructuring leading to epitaxy2019In: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 688, article id 137380Article in journal (Refereed)
    Abstract [en]

    We perform large-scale molecular dynamics simulations of TiN deposition at 1200 K on TiN substrates consisting of under-stoichiometric (N/Ti = 0.86) misoriented grains. The energy of incoming Ti atoms is 2 eV and that of incoming N atoms is 10 eV. The simulations show that misoriented grains are reoriented during the early stages of growth, after which the film grows 001 epitaxially and is nearly stoichiometric. The grain reorientation coincides with an increase in film N/Ti ratio. As the grains reorient, additional nitrogen can no longer be accommodated, and the film composition becomes stoichiometric as the overlayer grows epitaxially.

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  • 12.
    Ekström, Erik
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Hurand, Simon
    Univ Poitiers, France.
    Le Febvrier, Arnaud
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Elsukova, Anna
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Persson, Per O A
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Paul, Biplab
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Eriksson, Fredrik
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Sharma, Geetu
    Rensselaer Polytech Inst, NY 12180 USA.
    Voznyy, Oleksandr
    Univ Toronto Scarborough, Canada.
    Sangiovanni, Davide
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Ramanath, Ganpati
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering. Rensselaer Polytech Inst, NY 12180 USA.
    Eklund, Per
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Microstructure control and property switching in stress-free van der Waals epitaxial VO2 films on mica2023In: Materials & design, ISSN 0264-1275, E-ISSN 1873-4197, Vol. 229, article id 111864Article in journal (Refereed)
    Abstract [en]

    Realizing stress-free inorganic epitaxial films on weakly bonding substrates is of importance for applications that require film transfer onto surfaces that do not seed epitaxy. Film-substrate bonding is usually weakened by harnessing natural van der Waals layers (e.g., graphene) on substrate surfaces, but this is difficult to achieve in non-layered materials. Here, we demonstrate van der Waals epitaxy of stress-free films of a non-layered material VO2 on mica. The films exhibit out-of-plane 010 texture with three inplane orientations inherited from the crystallographic domains of the substrate. The lattice parameters are invariant with film thickness, indicating weak film-substrate bonding and complete interfacial stress relaxation. The out-of-plane domain size scales monotonically with film thickness, but the in-plane domain size exhibits a minimum, indicating that the nucleation of large in-plane domains supports subsequent island growth. Complementary ab initio investigations suggest that VO2 nucleation and van der Waals epitaxy involves subtle polarization effects around, and the active participation of, surface potassium atoms on the mica surface. The VO2 films show a narrow domain-size-sensitive electrical-conductiv ity-temperature hysteresis. These results offer promise for tuning the properties of stress-free van der Waals epitaxial films of non-layered materials such as VO2 through microstructure control (C) 2023 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

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  • 13.
    Ferrari, Alberto
    et al.
    Ruhr Univ Bochum, Germany.
    Sangiovanni, Davide
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. Ruhr Univ Bochum, Germany.
    Rogal, Jutta
    Ruhr Univ Bochum, Germany.
    Drautz, Ralf
    Ruhr Univ Bochum, Germany.
    First-principles characterization of reversible martensitic transformations2019In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 99, no 9, article id 094107Article in journal (Refereed)
    Abstract [en]

    Reversible martensitic transformations (MTs) are the origin of many fascinating phenomena, including the famous shape memory effect. In this work, we present a fully ab initio procedure to characterize MTs in alloys and to assess their reversibility. Specifically, we employ ab initio molecular dynamics data to parametrize a Landau expansion for the free energy of the MT. This analytical expansion makes it possible to determine the stability of the high- and low-temperature phases, to obtain the Ehrenfest order of the MT, and to quantify its free energy barrier and latent heat. We apply our model to the high-temperature shape memory alloy Ti-Ta, for which we observe remarkably small values for the metastability region (the interval of temperatures in which the high-and low-temperature phases are metastable) and for the barrier: these small values are necessary conditions for the reversibility of MTs and distinguish shape memory alloys from other materials.

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  • 14.
    Fiantok, Tomas
    et al.
    Comenius Univ, Slovakia; Comenius Univ, Slovakia.
    Koutna, Nikola
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. TU Wien, Austria.
    Sangiovanni, Davide
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Mikula, Marian
    Comenius Univ, Slovakia; SAS, Slovakia.
    Ceramic transition metal diboride superlattices with improved ductility and fracture toughness screened by ab initio calculations2023In: Scientific Reports, E-ISSN 2045-2322, Vol. 13, no 1, article id 12835Article in journal (Refereed)
    Abstract [en]

    Inherent brittleness, which easily leads to crack formation and propagation during use, is a serious problem for protective ceramic thin-film applications. Superlattice architectures, with alternating nm-thick layers of typically softer/stiffer materials, have been proven powerful method to improve the mechanical performance of, e.g., cubic transition metal nitride ceramics. Using high-throughput first-principles calculations, we propose that superlattice structures hold promise also for enhancing mechanical properties and fracture resistance of transition metal diborides with two competing hexagonal phases, a and ?. We study 264 possible combinations of a/a, a/? or co/co MB2 (where M = Al or group 3-6 transition metal) diboride superlattices. Based on energetic stability considerations, together with restrictions for lattice and shear modulus mismatch (?a < 4%, ?G > 40 GPa), we select 33 superlattice systems for further investigations. The identified systems are analysed in terms of mechanical stability and elastic constants, C-ij, where the latter provide indication of in-plane vs. out of-plane strength ( C-11, C-33 ) and ductility ( C-13 - C-44, C-12 - C-66 ). The superlattice ability to resist brittle cleavage along interfaces is estimated by Griffiths formula for fracture toughness. The a/a-type TiB2 /MB2 (M = Mo, W), HfB2/WB2, VB2/MB2 (M = Cr, Mo), NbB2/MB2 (M = Mo, W), and a/?-type AlB2/MB2 (M = Nb, Ta, Mo, W), are suggested as the most promising candidates providing atomic-scale basis for enhanced toughness and resistance to crack growth.

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  • 15.
    Gambino, Davide
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Sangiovanni, Davide
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. Ruhr University of Bochum, Germany.
    Alling, Björn
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. Max Planck Institute Eisenforsch GmbH, Germany.
    Abrikosov, Igor
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. National University of Science and Technology MISIS, Russia.
    Nonequilibrium ab initio molecular dynamics determination of Ti monovacancy migration rates in B1 TiN2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 96, no 10, article id 104306Article in journal (Refereed)
    Abstract [en]

    We use the color diffusion (CD) algorithm in nonequilibrium (accelerated) ab initio molecular dynamics simulations to determine Ti monovacancy jump frequencies in NaCl-structure titanium nitride (TiN), at temperatures ranging from 2200 to 3000 K. Our results showthat theCDmethod extended beyond the linear-fitting rate-versus-force regime [Sangiovanni et al., Phys. Rev. B 93, 094305 (2016)] can efficiently determine metal vacancy migration rates in TiN, despite the low mobilities of lattice defects in this type of ceramic compound. We propose a computational method based on gamma-distribution statistics, which provides unambiguous definition of nonequilibrium and equilibrium (extrapolated) vacancy jump rates with corresponding statistical uncertainties. The acceleration-factor achieved in our implementation of nonequilibrium molecular dynamics increases dramatically for decreasing temperatures from 500 for T close to the melting point T-m, up to 33 000 for T approximate to 0.7 T-m

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  • 16.
    Gervilla, Víctor
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Nanoscale engineering. Linköping University, Faculty of Science & Engineering.
    Zarshenas, Mohammad
    Linköping University, Department of Physics, Chemistry and Biology, Nanoscale engineering. Linköping University, Faculty of Science & Engineering.
    Sangiovanni, Davide Giuseppe
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Sarakinos, Kostas
    Linköping University, Department of Physics, Chemistry and Biology, Nanoscale engineering. Linköping University, Faculty of Science & Engineering.
    Anomalous versus Normal Room-Temperature Diffusion of Metal Adatoms on Graphene2020In: Journal of Physical Chemistry Letters, ISSN 1948-7185, E-ISSN 1948-7185, The Journal of Physical Chemistry Letters, Vol. 11, no 21, p. 8930-8936Article in journal (Refereed)
    Abstract [en]

    Fabrication of high-performance heterostructure devices requires fundamental understanding of the diffusion dynamics of metal species on 2D materials. Here, we investigate the room-temperature diffusion of Ag, Au, Cu, Pd, Pt, and Ru adatoms on graphene using ab initio and classical molecular dynamics simulations. We find that Ag, Au, Cu, and Pd follow Lévy walks, in which adatoms move continuously within ∼1–4 nm2 domains during ∼0.04 ns timeframes, and they occasionally perform ∼2–4 nm flights across multiple surface adsorption sites. This anomalous diffusion pattern is associated with a flat (<50 meV) potential energy landscape (PEL), which renders surface vibrations important for adatom migration. The latter is not the case for Pt and Ru, which encounter a significantly rougher PEL (>100 meV) and, hence, migrate via conventional random walks. Thus, adatom anomalous diffusion is a potentially important aspect for modeling growth of metal films and nanostructures on 2D materials.

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  • 17.
    Jamnig, Andreas
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Nanoscale engineering. Linköping University, Faculty of Science & Engineering. Univ Poitiers, France.
    Sangiovanni, Davide Giuseppe
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. Ruhr Univ Bochum, Germany.
    Abadias, G.
    Univ Poitiers, France.
    Sarakinos, Kostas
    Linköping University, Department of Physics, Chemistry and Biology, Nanoscale engineering. Linköping University, Faculty of Science & Engineering.
    Atomic-scale diffusion rates during growth of thin metal films on weakly-interacting substrates2019In: Scientific Reports, E-ISSN 2045-2322, Vol. 9, article id 6640Article in journal (Refereed)
    Abstract [en]

    We use a combined experimental and theoretical approach to study the rates of surface diffusion processes that govern early stages of thin Ag and Cu film morphological evolution on weakly-interacting amorphous carbon substrates. Films are deposited by magnetron sputtering, at temperatures T-S between 298 and 413 K, and vapor arrival rates F in the range 0.08 to 5.38 monolayers/s. By employing in situ and real-time sheet-resistance and wafer-curvature measurements, we determine the nominal film thickness Theta at percolation (Theta(perc)) and continuous film formation (Theta(cont)) transition. Subsequently, we use the scaling behavior of Theta(perc) and Theta(cont) as a function of F and T-s, to estimate, experimentally, the temperature-dependent diffusivity on the substrate surface, from which we calculate Ag and Cu surface migration energy barriers E-D(exp) and attempt frequencies nu(exp)(0). By critically comparing E-D(exp) and nu(exp)(0) with literature data, as well as with results from our ab initio molecular dynamics simulations for single Ag and Cu adatom diffusion on graphite surfaces, we suggest that: (i) E-D(exp) and nu(exp)(0) correspond to diffusion of multiatomic clusters, rather than to diffusion of monomers; and (ii) the mean size of mobile clusters during Ag growth is larger compared to that of Cu. The overall results of this work pave the way for studying growth dynamics in a wide range of technologically-relevant weakly-interacting film/substrate systems-including metals on 2D materials and oxides-which are building blocks in next-generation nanoelectronic, optoelectronic, and catalytic devices.

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  • 18.
    Kakanakova-Gueorguieva, Anelia
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Gueorguiev, Gueorgui Kostov
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Sangiovanni, Davide
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Suwannaharn, Nattamon
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Chulalongkorn Univ, Thailand.
    Ivanov, Ivan Gueorguiev
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Cora, Ildiko
    Ctr Energy Res, Hungary.
    Pecz, Bela
    Ctr Energy Res, Hungary.
    Nicotra, Giuseppe
    Ist Microelettron & Microsistemi, Italy.
    Giannazzo, Filippo
    Ist Microelettron & Microsistemi, Italy.
    Nanoscale phenomena ruling deposition and intercalation of AlN at the graphene/SiC interface2020In: Nanoscale, ISSN 2040-3364, E-ISSN 2040-3372, Vol. 12, no 37, p. 19470-19476Article in journal (Refereed)
    Abstract [en]

    The possibility for kinetic stabilization of prospective 2D AlN was explored by rationalizing metal organic chemical vapor deposition (MOCVD) processes of AlN on epitaxial graphene. From the wide range of temperatures which can be covered in the same MOCVD reactor, the deposition was performed at the selected temperatures of 700, 900, and 1240 degrees C. The characterization of the structures by atomic force microscopy, electron microscopy and Raman spectroscopy revealed a broad range of surface nucleation and intercalation phenomena. These phenomena included the abundant formation of nucleation sites on graphene, the fragmentation of the graphene layers which accelerated with the deposition temperature, the delivery of excess precursor-derived carbon adatoms to the surface, as well as intercalation of sub-layers of aluminum atoms at the graphene/SiC interface. The conceptual understanding of these nanoscale phenomena was supported by our previous comprehensiveab initiomolecular dynamics (AIMD) simulations of the surface reaction of trimethylaluminum, (CH3)(3)Al, precursor with graphene. A case of applying trimethylindium, (CH3)(3)In, precursor to epitaxial graphene was considered in a comparative way.

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  • 19.
    Kakanakova-Gueorguieva, Anelia
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Ivanov, Ivan Gueorguiev
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Suwannaharn, Nattamon
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Chulalongkorn Univ, Thailand.
    Chih-Wei, Chih-Wei
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Cora, Ildiko
    Inst Tech Phys & Mat Sci, Hungary.
    Pecz, Bela
    Inst Tech Phys & Mat Sci, Hungary.
    Giannazzo, Filippo
    CNR, Italy.
    Sangiovanni, Davide
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Gueorguiev, Gueorgui Kostov
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    MOCVD of AlN on epitaxial graphene at extreme temperatures2021In: CrystEngComm, ISSN 1466-8033, E-ISSN 1466-8033, Vol. 23, no 2, p. 385-390Article in journal (Refereed)
    Abstract [en]

    The initial stages of metal organic chemical vapor deposition (MOCVD) of AlN on epitaxial graphene at temperatures in excess of 1200 degrees C have been rationalized. The use of epitaxial graphene, in conjunction with high deposition temperatures, can deliver on the realization of nanometer thin AlN whose material quality is characterized by the appearance of luminescent centers with narrow spectral emission at room temperature. It has been elaborated, based on our previous comprehensive ab initio molecular dynamics simulations, that the impact of graphene on AlN growth consists in the way it promotes dissociation of the trimethylaluminum, (CH3)(3)Al, precursor with subsequent formation of Al adatoms during the initial stages of the deposition process. The high deposition temperatures ensure adequate surface diffusion of the Al adatoms which is an essential factor in material quality enhancement. The role of graphene in intervening with the dissociation of another precursor, trimethylgallium, (CH3)(3)Ga, has accordingly been speculated by presenting a case of propagation of ultrathin GaN of semiconductor quality. A lower deposition temperature of 1100 degrees C in this case has better preserved the structural integrity of epitaxial graphene. Breakage and decomposition of the graphene layers has been deduced in the case of AlN deposition at temperatures in excess of 1200 degrees C.

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  • 20.
    Kindlund, H.
    et al.
    Univ Calif Los Angeles, CA 90095 USA.
    Sangiovanni, Davide
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. Ruhr Univ Bochum, Germany.
    Petrov, Ivan
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering. Univ Illinois, IL 61801 USA.
    Greene, Joseph E
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering. Univ Illinois, IL 61801 USA; Natl Taiwan Univ Sci and Technol, Taiwan.
    Hultman, Lars
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    A review of the intrinsic ductility and toughness of hard transition-metal nitride alloy thin films2019In: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 688, article id 137479Article, review/survey (Refereed)
    Abstract [en]

    Over the past decades, enormous effort has been dedicated to enhancing the hardness of refractory ceramic materials. Typically, however, an increase in hardness is accompanied by an increase in brittleness, which can result in intergranular decohesion when materials are exposed to high stresses. In order to avoid brittle failure, in addition to providing high strength, films should also be ductile, i.e., tough. However, fundamental progress in obtaining hard-yet-ductile ceramics has been slow since most toughening approaches are based on empirical trial-and-error methods focusing on increasing the strength and ductility extrinsically, with a limited focus on understanding thin-film toughness as an inherent physical property of the material. Thus, electronic structure investigations focusing on the origins of ductility vs. brittleness are essential in understanding the physics behind obtaining both high strength and high plastic strain in ceramics films. Here, we review recent progress in experimental validation of density functional theory predictions on toughness enhancement in hard ceramic films, by increasing the valence electron concentration, using examples from the V1-xWxN and V1-xMoxN alloy systems.

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  • 21.
    Kindlund, Hanna
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Sangiovanni, Davide
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Jensen, Jens
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Chirita, Valeriu
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Petrov, Ivan
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology. Department of Materials Science and the Fredrick Seitz Materials Research Laboratory, University of Illinois, USA.
    Greene, Joseph
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology. Department of Materials Science and the Fredrick Seitz Materials Research Laboratory, University of Illinois, USA.
    Hultman, Lars
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Effect of WN content on toughness enhancement in V1–xWxN/MgO(001) thin films2014In: Journal of Vacuum Science & Technology. A. Vacuum, Surfaces, and Films, ISSN 0734-2101, E-ISSN 1520-8559, Vol. 32, no 3, p. 030603-Article in journal (Refereed)
    Abstract [en]

    The authors report the growth and mechanical properties of epitaxial B1 NaCl-structure V1-xWxN/MgO(001) thin films with 0 ≤ x ≤ 0.60. The Gibbs free energy of mixing, calculated using density functional theory (DFT), reveals that cubic V1-xWxN solid solutions with 0 ≤ x ≤ 0.7 are stable against spinodal decomposition and separation into the equilibrium cubic-VN and hexagonal-WN binary phases. The authors show experimentally that alloying VN with WN leads to a monotonic increase in relaxed lattice parameters, enhanced nanoindentation hardnesses, and reduced elastic moduli. Calculated V1-xWxN lattice parameters and elastic moduli  (obtained from calculated C11, C12, and C44 elastic constants) are in good agreement with experimental results. The observed increase in alloy hardness, with a corresponding decrease in the elastic modulus at higher x values, combined with DFT-calculated decreases in shear to bulk moduli ratios, and increased Cauchy pressures (C12–C44) with increasing x reveal a trend toward increased toughness.

  • 22.
    Kindlund, Hanna
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Sangiovanni, Davide
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Lu, Jun
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Jensen, Jens
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Chirita, Valeriu
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Birch, Jens
    Department of Materials Science, Fredrick Seitz Materials Research Laboratory, University of of Illinois, Urbana, USA.
    Petrov, Ivan
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology. Department of Materials Science and the Fredrick Seitz Materials Research Laboratory, University of Illinois, Urbana, USA.
    Greene, Joseph
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology. Department of Materials Science and the Fredrick Seitz Materials Research Laboratory, University of Illinois, Urbana, USA.
    Hultman, Lars
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Vacancy-induced toughening in hard single-crystal V0.5Mo0.5Nx/MgO(001) thin films2014In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 77, p. 394-400Article in journal (Refereed)
    Abstract [en]

    Using a combination of experiments and density functional theory (DFT), we demonstrate the first example of vacancy-induced  toughening, in this case for epitaxial pseudobinary NaCl-structure substoichiometric V0.5Mo0.5Nx alloys, with N concentrations 0.55 ≤ x ≤ 1.03, grown by reactive magnetron sputter deposition. The nanoindentation hardness H(x) increases with increasing vacancy concentration from 17 GPa with x = 1.03 to 26 GPa with x = 0.55, while the elastic modulus E(x) remains essentially constant at 370 GPa. Scanning electron micrographs of indented regions show ductile plastic flow giving rise to material pile-up, rather than cracks as commonly observed for hard, but brittle, transition-metal nitrides. The increase in alloy hardness with an elastic  modulus which remains constant with decreasing x, combined with the observed material pile-up around nanoindents, DFT-calculated decrease in shear to bulk moduli ratios, and increased Cauchy pressures (C12-C44), reveals a trend toward vacancy-induced toughening. Moreover, DFT crystal orbital overlap population analyses are consistent with the above results.

  • 23.
    Kindlund, Hanna
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Sangiovanni, Davide
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Martínez-de-Olcoz, L.
    Grupo de Capas Finas e Ingeniería de Superficies, Facultad de Física, Dep. Física Aplicada y Óptica, Universidad de Barcelona, Barcelona, Spain.
    Lu, Jun
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Jensen, Jens
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Birch, Jens
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Petrov, Ivan
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology. Department of Materials Science and the Fredrick Seitz Materials Research Laboratory, University of Illinois, Urbana, USA.
    Greene, Joseph
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology. Department of Materials Science and the Fredrick Seitz Materials Research Laboratory, University of Illinois, Urbana, USA.
    Chirita, Valeriu
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Hultman, Lars
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Toughness Enhancement in Hard Ceramic Thin Films by Alloy Design2013In: APL Materials, E-ISSN 2166-532X, Vol. 1, no 4, p. 042104-Article in journal (Refereed)
    Abstract [en]

    Hardness is an essential property for a wide range of applications. However, hardness alone, typically accompanied by brittleness, is not sufficient to prevent failure in ceramic films exposed to high stresses. Using VN as a model system, we demonstrate with experiment and density functional theory (DFT) that refractory VMoN alloys exhibit not only enhanced hardness, but dramatically increased ductility. V0.5Mo0.5N hardness is 25% higher than that of VN. In addition, while nanoindented VN, as well as TiN reference samples, suffer from severe cracking typical of brittle ceramics, V0.5Mo0.5N films do not crack. Instead, they exhibit material pile-up around nanoindents, characteristic of plastic flow in ductile materials. Moreover, the wear resistance of V0.5Mo0.5N is considerably higher than that of VN. DFT results show that tuning the occupancy of d-t2g metallic bonding states in VMoN facilitates dislocation glide, and hence enhances toughness, via the formation of stronger metal/metal bonds along the slip direction and weaker metal/N bonds across the slip plane.

  • 24.
    Koutna, Nikola
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering. TU Wien, Austria.
    Löfler, Lukas
    Dept Mat Sci, Austria; Rhein Westfal TH Aachen, Germany.
    Holec, David
    Dept Mat Sci, Austria.
    Chen, Zhuo
    Erich Schmid Inst Mat Sci, Austria.
    Zhang, Zaoli
    Erich Schmid Inst Mat Sci, Austria.
    Hultman, Lars
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Mayrhofer, Paul H.
    TU Wien, Austria.
    Sangiovanni, Davide
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Atomistic mechanisms underlying plasticity and crack growth in ceramics: a case study of AlN/TiN superlattices2022In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 229, article id 117809Article in journal (Refereed)
    Abstract [en]

    Interfaces between components of a material govern its mechanical strength and fracture resistance. While a great number of interfaces is present in nanolayered materials, such as superlattices, their fundamental role during mechanical loading lacks understanding. Here we combine ab initio and classical molecular dynamics simulations, nanoindentation, and transmission electron microscopy to reveal atomistic mechanisms underlying plasticity and crack growth in B1 AlN(001)/TiN(001) superlattices under loading. The system is a model for modern refractory ceramics used as protective coatings. The simulations demonstrate an anisotropic response to uniaxial tensile deformation in principal crystallographic directions due to different strain-activated plastic deformation mechanisms. Superlattices strained orthogonal to (001) interfaces show modest plasticity and cleave parallel to AlN/TiN layers. Contrarily, B1-to-B3 or B1-to-B4(B-k) phase transformations in AlN facilitate a remarkable toughness enhancement upon in plane [110] and [100] tensile elongation, respectively. We verify the predictions experimentally and conclude that strain-induced crack growth-via loss of interface coherency, dislocation-pinning at interfaces, or layer interpenetration followed by formation of slip bands-can be hindered by controlling the thicknesses of the superlattice nanolayered components.

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  • 25.
    Levämäki, Henrik
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Bock, Florian
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Sangiovanni, Davide
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Johnson, Lars J. S.
    Sandvik Coromant, Sweden.
    Tasnadi, Ferenc
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Armiento, Rickard
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Abrikosov, Igor
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    HADB: A materials-property database for hard-coating alloys2023In: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 766, article id 139627Article in journal (Refereed)
    Abstract [en]

    Data-driven approaches are becoming increasingly valuable for modern science, and they are making their way into industrial research and development (R&D). Supervised machine learning of statistical models can utilize databases of materials parameters to speed up the exploration of candidate materials for experimental synthesis and characterization. In this paper we introduce the HADB database, which contains properties of industrially relevant chemically disordered hard-coating alloys, focusing on their thermodynamic, elastic and mechanical properties. We present the technical implementations of the database infrastructure including support for browse, query, retrieval, and API access through the OPTIMADE API to make this data findable, accessible, interoperable, and reusable (FAIR). Finally, we demonstrate the usefulness of the database by training a graph -based machine learning (ML) model to predict elastic properties of hard-coating alloys. The ML model is shown to predict bulk and shear moduli for out out-of-sample alloys with less than 6 GPa mean average error.

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  • 26.
    Levämäki, Henrik
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Tasnadi, Ferenc
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Sangiovanni, Davide
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Johnson, L. J. S.
    Sandvik Coromant, Sweden.
    Armiento, Rickard
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Abrikosov, Igor
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. Natl Univ Sci & Technol MISIS, Russia.
    Predicting elastic properties of hard-coating alloys using ab-initio and machine learning methods2022In: npj Computational Materials, E-ISSN 2057-3960, Vol. 8, no 1, article id 17Article in journal (Refereed)
    Abstract [en]

    Accelerated design of hard-coating materials requires state-of-the-art computational tools, which include data-driven techniques, building databases, and training machine learning models. We develop a heavily automated high-throughput workflow to build a database of industrially relevant hard-coating materials, such as binary and ternary nitrides. We use the high-throughput toolkit to automate the density functional theory calculation workflow. We present results, including elastic constants that are a key parameter determining mechanical properties of hard-coatings, for X1-xYxN ternary nitrides, where X,Y ∈ {Al, Ti, Zr, Hf} and fraction . We also explore ways for machine learning to support and complement the designed databases. We find that the crystal graph convolutional neural network trained on ordered lattices has sufficient accuracy for the disordered nitrides, suggesting that existing databases provide important data for predicting mechanical properties of qualitatively different types of materials, in our case disordered hard-coating alloys.

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  • 27.
    Mei, A. B.
    et al.
    University of Illinois, IL 61801 USA; University of Illinois, IL 61801 USA.
    Hellman, Olle
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology. CALTECH, CA 91125 USA.
    Wireklint, N.
    Chalmers, Sweden.
    Schlepuetz, C. M.
    Argonne National Lab, IL 60439 USA.
    Sangiovanni, Davide
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
    Alling, Björn
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Rockett, A.
    University of Illinois, IL 61801 USA; University of Illinois, IL 61801 USA.
    Hultman, Lars
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Petrov, Ivan
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology. University of Illinois, IL 61801 USA; University of Illinois, IL 61801 USA.
    Greene, Joseph E
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology. University of Illinois, IL 61801 USA; University of Illinois, IL 61801 USA.
    Dynamic and structural stability of cubic vanadium nitride2015In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 91, no 5, p. 054101-Article in journal (Refereed)
    Abstract [en]

    Structural phase transitions in epitaxial stoichiometric VN/MgO(011) thin films are investigated using temperature-dependent synchrotron x-ray diffraction (XRD), selected-area electron diffraction (SAED), resistivity measurements, high-resolution cross-sectional transmission electron microscopy, and ab initio molecular dynamics (AIMD). At room temperature, VN has the B1 NaCl structure. However, below T-c = 250 K, XRD and SAED results reveal forbidden (00l) reflections of mixed parity associated with a noncentrosymmetric tetragonal structure. The intensities of the forbidden reflections increase with decreasing temperature following the scaling behavior I proportional to (T-c - T)(1/2). Resistivity measurements between 300 and 4 K consist of two linear regimes resulting from different electron/phonon coupling strengths in the cubic and tetragonal-VN phases. The VN transport Eliashberg spectral function alpha F-2(tr)(h omega), the product of the phonon density of states F(h omega) and the transport electron/phonon coupling strength alpha(2)(tr)(h omega), is determined and used in combination with AIMD renormalized phonon dispersion relations to show that anharmonic vibrations stabilize the NaCl structure at T greater than T-c. Free-energy contributions due to vibrational entropy, often neglected in theoretical modeling, are essential for understanding the room-temperature stability of NaCl-structure VN, and of strongly anharmonic systems in general.

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  • 28.
    Mei, A. B.
    et al.
    Cornell Univ, NY 14853 USA.
    Kindlund, H.
    Univ Calif Los Angeles, CA 90095 USA.
    Broitman, Esteban
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering. SKF, Netherlands.
    Hultman, Lars
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Petrov, Ivan
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering. Univ Illinois, IL 61801 USA.
    Greene, Joseph E
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering. Univ Illinois, IL 61801 USA; Natl Taiwan Univ Sci and Technol, Taiwan.
    Sangiovanni, Davide
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. Ruhr Univ Bochum, Germany.
    Adaptive hard and tough mechanical response in single-crystal B1 VNx ceramics via control of anion vacancies2020In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 192, p. 78-88Article in journal (Refereed)
    Abstract [en]

    High hardness and toughness are generally considered mutually exclusive properties for single-crystal ceramics. Combining experiments and ab initio molecular dynamics (AIMD) atomistic simulations at room temperature, we demonstrate that both the hardness and toughness of single-crystal NaCl-structure VNx/MgO(001) thin films are simultaneously enhanced through the incorporation of anion vacancies. Nanoindentation results show that VN0.8, here considered as representative understoichiometric VNx system, is approximate to 20% harder, as well as more resistant to fracture than stoichiometric VN samples. AIMD modeling of VN and VN0.8 supercells subjected to [001] and [110] elongation reveal that the tensile strengths of the two materials are similar. Nevertheless, while the stoichiometric VN phase cleaves in a brittle manner at tensile yield points, the understoichiometric compound activates transformation-toughening mechanisms that dissipate accumulated stresses. AIMD simulations also show that VN0.8 exhibits an initially greater resistance to both {110} &lt; 1 (1) over bar0 &gt; and {111} &lt; 1 (1) over bar0 &gt; shear deformation than VN. However, for progressively increasing shear strains, the VN0.8 mechanical behavior gradually evolves from harder to more ductile than VN. The transition is mediated by anion vacancies, which facilitate {110} &lt; 1 (1) over bar0 &gt; and {111} &lt; 1 (1) over bar0 &gt; lattice slip by reducing activation shear stresses by as much as 35%. Electronic-structure analyses show that the two-regime hard/tough mechanical response of VN0.8 primarily stems from its intrinsic ability to transfer d electrons between 2nd-neighbor and 4th-neighbor (i.e., across vacancy sites) V-V metallic states. Our work offers a route for electronic-structure design of hard materials in which a plastic mechanical response is triggered with loading. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd.

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  • 29.
    Mei, A. B.
    et al.
    University of Illinois, IL 61801 USA.
    Tuteja, M.
    University of Illinois, IL 61801 USA; University of Illinois, IL 61801 USA.
    Sangiovanni, Davide
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Haasch, R. T.
    University of Illinois, IL 61801 USA; University of Illinois, IL 61801 USA.
    Rockett, A.
    University of Illinois, IL 61801 USA.
    Hultman, Lars
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Petrov, Ivan
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering. University of Illinois, IL 61801 USA.
    Greene, Joseph E
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering. University of Illinois, IL 61801 USA.
    Growth, nanostructure, and optical properties of epitaxial VNx/MgO(001) (0.80 <= x <= 1.00) layers deposited by reactive magnetron sputtering2016In: Journal of Materials Chemistry C, ISSN 2050-7526, E-ISSN 2050-7534, Vol. 4, no 34, p. 7924-7938Article in journal (Refereed)
    Abstract [en]

    VNx/MgO(001) films, similar to 300 nm thick, with x ranging from 1.00 (stoichiometric) to 0.80 are grown by magnetically-unbalanced reactive magnetron sputter deposition in mixed N-2/Ar atmospheres. The combination of lattice-resolution cross-sectional electron microscopy with X-ray diffraction omega = 2 theta, phi-scans, pole figures, and high resolution reciprocal space maps show that VNx layers are epitaxial single crystals which grow cube-on-cube with respect to their substrates: (001)VNx vertical bar vertical bar (001)(MgO) and [100]VNx vertical bar vertical bar [100](MgO). VNx (001) relaxed lattice parameters a(0)(x) decrease linearly from 0.4134 (x = 1.00) to 0.4098 nm (x = 0.80), in agreement with density functional theory (DFT) calculations. Near-stoichiometric VNx layers (0.95 less than or similar to x less than or similar to 1.0) are fully relaxed during growth, while films with lower x values are partially strained as a result of increased anion vacancies impeding dislocation glide. VNx complex dielectric functions epsilon((h) over bar omega) are determined between 0.7 and 4.5 eV using variable-angle spectroscopic ellipsometry and valence states are probed via ultraviolet photoelectron spectroscopy (UPS) in concert with DFT calculations. VN(001) UPS spectra exhibit a feature at binding energies ranging from the Fermi level to 3 eV, together with two peaks deeper in the valence band. These results are consistent with electronic densities of states computed by scaling Kohn-Sham electronic eigenvalues to account for many-body interactions. Imaginary VN(001) dielectric functions epsilon((h) over bar omega) determined by ellipsometry also agree with theoretical values obtained within the random-phase approximation using scaled eigenvalues. Analyses of optical matrix element calculations reveal that VNx dielectric responses are controlled by the phase space for interband transitions; band-structure analyses indicate that epsilon(2)(amp;lt;(hover baramp;gt;omega) spectral features in the infrared-visible range arise primarily from the combination of intraband and d-d transitions, while features at higher energies result primarily from p-d interband transitions. The combined nanostructural and spectroscopic analyses establish that, surprisingly, N vacancies are essentially non-nteracting in high-quality epitaxial VNx containing vacancy concentrations up to similar to 10(22) cm(-3) (x = 0.80).

  • 30.
    Mei, Antonio B.
    et al.
    Cornell Univ, NY 14853 USA; Univ Illinois, IL 61801 USA; Univ Illinois, IL 61801 USA.
    Miao, Ludi
    Cornell Univ, NY 14853 USA.
    Wahila, Matthew J.
    Binghamton Univ, NY 13902 USA.
    Khalsa, Guru
    Cornell Univ, NY 14853 USA.
    Wang, Zhe
    Cornell Univ, NY 14853 USA.
    Barone, Matthew
    Cornell Univ, NY 14853 USA.
    Schreiber, Nathaniel J.
    Cornell Univ, NY 14853 USA.
    Noskin, Lindsey E.
    Cornell Univ, NY 14853 USA.
    Paik, Hanjong
    Cornell Univ, NY 14853 USA.
    Tiwald, Thomas E.
    JA Woollam Co, NE 68508 USA.
    Zheng, Qiye
    Univ Illinois, IL 61801 USA; Lawrence Berkeley Natl Lab, CA 94720 USA; Univ Calif Berkeley, CA 94720 USA.
    Haasch, Richard T.
    Univ Illinois, IL 61801 USA.
    Sangiovanni, Davide
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. Ruhr Univ Bochum, Germany.
    Piper, Louis F. J.
    Binghamton Univ, NY 13902 USA.
    Schlom, Darrell G.
    Cornell Univ, NY 14853 USA; Kavli Inst Cornell Nanoscale Sci, NY 14853 USA.
    Adsorption-controlled growth and properties of epitaxial SnO films2019In: Physical Review Materials, E-ISSN 2475-9953, Vol. 3, no 10, article id 105202Article in journal (Refereed)
    Abstract [en]

    When it comes to providing the unusual combination of optical transparency, p-type conductivity, and relatively high mobility, Sn2+-based oxides are promising candidates. Epitaxial films of the simplest Sn2+ oxide, SnO, are grown in an adsorption-controlled regime at 380 degrees C on Al2O3 substrates by molecular-beam epitaxy, where the excess volatile SnOx desorbs from the film surface. A commensurately strained monolayer and an accompanying van der Waals gap is observed near the substrate interface, promoting layers with high structural perfection notwithstanding a large epitaxial lattice mismatch (-12%). The unintentionally doped films exhibit p-type conductivity with carrier concentration 2.5 x 10(16) cm(-3) and mobility 2.4 cm(2) V(-1)s(-1) at room temperature. Additional physical properties are measured and linked to the Sn2+ valence state and corresponding lone-pair charge-density distribution.

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  • 31.
    Mikula, M.
    et al.
    Comenius University, Slovakia; Institute Mat and Machine Mech SAS, Slovakia.
    Sangiovanni, Davide
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. Ruhr University of Bochum, Germany.
    Plasienka, D.
    Comenius University, Slovakia.
    Roch, T.
    Comenius University, Slovakia.
    Caplovicova, M.
    Slovak University of Technology Bratislava, Slovakia.
    Truchly, M.
    Comenius University, Slovakia.
    Satrapinskyy, L.
    Comenius University, Slovakia.
    Bystricky, R.
    Slovak Academic Science, Slovakia.
    Tonhauzerova, D.
    Comenius University, Slovakia.
    Vlckova, D.
    Comenius University, Slovakia.
    Kus, P.
    Comenius University, Slovakia.
    Thermally induced age hardening in tough Ta-Al-N coatings via spinodal decomposition2017In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 121, no 15, article id 155304Article in journal (Refereed)
    Abstract [en]

    We combine experiments and ab initio density functional theory calculations to investigate the evolution in structural and mechanical properties of TaAlN coatings as a function of the annealing temperature T. Formation of coherent cubic TaN- and AlN-rich nanometer-size domains, occurring during the initial stage of thermally induced phase separation within cubic NaCl-type (B1) TaAlN solid solutions, yields a monotonic increase in hardness from 29 GPa (as deposited coatings) up to a maximum of 35 GPa (+17%) reached after annealing at 1000 degrees C. Further thermal treatment at T amp;gt; 1000 degrees C leads to the transformation of metastable cubic domains into stable hexagonal TaNx and wurtzite AlN phases, thus resulting in hardness reductions. A comparison of our results with those reported in the literature reveals that TaAlN coatings are at least as hard while considerably less stiff (lower elastic moduli) than TiAlN coatings, thus indicating a substantial increase in toughness achieved upon replacing Ti with Ta in the host lattice. Present findings suggest that cubic TaAlN solid solutions are promising candidates for applications in protective coatings possessing both high-temperature hardness and toughness. Published by AIP Publishing.

  • 32.
    Mikula, Marian
    et al.
    Comenius University, Slovakia; Institute Mat and Machine Mech, Slovakia.
    Plasienka, Dusan
    Comenius University, Slovakia.
    Sangiovanni, Davide
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. Ruhr University of Bochum, Germany.
    Sahul, Martin
    Fac Mat Science and Technology STU, Slovakia.
    Roch, Tomas
    Comenius University, Slovakia.
    Truchly, Martin
    Comenius University, Slovakia.
    Gregor, Maros
    Comenius University, Slovakia.
    Caplovic, Lubomir
    Fac Mat Science and Technology STU, Slovakia.
    Plecenik, Andrej
    Comenius University, Slovakia.
    Kus, Peter
    Comenius University, Slovakia.
    Toughness enhancement in highly NbN-alloyed Ti-Al-N hard coatings2016In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 121, p. 59-67Article in journal (Refereed)
    Abstract [en]

    Obtaining high hardness combined with enhanced toughness represents one of the current challenges in material design of hard ceramic protective coatings. In this work, we combine experimental and ab initio density functional theory (DFT) analysis of the mechanical properties of Ti-Al-Nb-N coatings to validate the results of previous theoretical investigations predicting enhanced toughness in TiAIN-based systems highly alloyed (amp;gt;25 at. %) with nitrides of pentavalent VB group elements Nb, Ta, and V. As-deposited Ti1-x-yAlxNbyN coatings (y = 0 divided by 0.61) exhibit single phase cubic sodium chloride (B1) structure identified as TiAl(Nb)N solid solutions. The highest hardness,similar to 32.5 +/- 2 GPa, and the highest Youngs modulus, similar to 442 GPa, are obtained in Nb-free Ti0.46Al0.54N exhibiting pronounced 111 growth-orientation. Additions of Nb in the coatings promote texture evolution toward 200. Nanoindentation measurements demonstrate that alloying TiAlN with NbN yields significantly decreased elastic stiffness, from 442 to similar to 358 divided by 389 GPa, while the hardness remains approximately constant (between 28 +/- 2 and 31 +/- 3 GPa) for all Nb contents. DFT calculations and electronic structure analyses reveal that alloying dramatically reduces shear resistances due to enhanced d-d second-neighbor metallic bonding while retaining strong metal-N bonds which change from being primarily ionic (TiAlN) to more covalent (TiAlNbN) in nature. Overall, Nb substitutions are found to improve ductility of TiAlN-based alloys at the cost of slight losses in hardness, equating to enhanced toughness. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

  • 33.
    Mikula, Marian
    et al.
    Comenius University, Slovakia; Slovak Academic Science, Slovakia.
    Truchly, Martin
    Comenius University, Slovakia.
    Sangiovanni, Davide
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. Ruhr University of Bochum, Germany.
    Plasienka, Dusan
    Comenius University, Slovakia.
    Roch, Tomas
    Comenius University, Slovakia.
    Gregor, Maros
    Comenius University, Slovakia.
    Durina, Pavol
    Comenius University, Slovakia.
    Janik, Marian
    Comenius University, Slovakia.
    Kus, Peter
    Comenius University, Slovakia.
    Experimental and computational studies on toughness enhancement in Ti-Al-Ta-N quaternaries2017In: Journal of Vacuum Science & Technology. A. Vacuum, Surfaces, and Films, ISSN 0734-2101, E-ISSN 1520-8559, Vol. 35, no 6, article id 060602Article in journal (Refereed)
    Abstract [en]

    Design of hard ceramic material coatings with enhanced toughness, which prevents crack formation/propagation leading to brittle failure during application, is a primary industrial requirement. In this work, experimental methods supported by ab initio density functional theory (DFT) calculations and electronic structure analyses are used to investigate the mechanical behavior of magnetron sputtered Ti-Al-Ta-N hard coatings. The as-deposited Ti1-x-yAlxTayN (y = 0-0.60) films exhibit a single phase cubic sodium chloride (B1) structure identified as TiAl(Ta)N solid solutions. While the hardness H of Ti0.46Al0.54N (32.5 +/- 2 GPa) is not significantly affected by alloying with TaN (H of the quaternary nitrides varies between 26 +/- 2 and 35 +/- 4 GPa), the elastic stiffness monotonically decreases from 442 to 354 GPa with increasing Ta contents, which indicates improved toughness in TiAlTaN. Consistent with the experimental findings, the DFT results show that Ta substitutions in TiAlN reduce the shear resistance due to the enhanced occupation of metal-metal bonding states while preserving strong metal-N bonds. The metal-N bonding character, however, is progressively modified from prevalently ionic (TiAlN) toward more covalent (TiAlTaN). (C) 2017 American Vacuum Society.

  • 34.
    Mikula, Marian
    et al.
    Comenius Univ, Slovakia; SAS, Slovakia.
    Uzon, Stela
    Comenius Univ, Slovakia.
    Hudec, Tomas
    Univ Southampton, England.
    Grancic, Branislav
    Comenius Univ, Slovakia.
    Truchly, Martin
    Comenius Univ, Slovakia.
    Roch, Tomas
    Comenius Univ, Slovakia.
    Svec, Peter Jr.
    Slovak Acad Sci, Slovakia.
    Satrapinskyy, Leonid
    Comenius Univ, Slovakia.
    Caplovicova, Maria
    Slovak Univ Technol Bratislava, Slovakia.
    Greczynski, Grzegorz
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Petrov, Ivan
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering. Univ Illinois, IL 61801 USA; Natl Taiwan Univ Sci & Technol, Taiwan.
    Odén, Magnus
    Linköping University, Department of Physics, Chemistry and Biology, Nanostructured Materials. Linköping University, Faculty of Science & Engineering.
    Kus, Peter
    Comenius Univ, Slovakia.
    Sangiovanni, Davide
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Thermally induced structural evolution and age-hardening of polycrystalline V1-xMoxN (x approximate to 0.4) thin films2021In: Surface & Coatings Technology, ISSN 0257-8972, E-ISSN 1879-3347, Vol. 405, article id 126723Article in journal (Refereed)
    Abstract [en]

    Rocksalt-structure (B1) (V,Mo)N alloys are inherently hard and tough ceramics. However, the mechanical properties and thermal stability of (V,Mo)N solid solutions at temperatures greater than or similar to 700 degrees C of relevance for practical applications have not been previously investigated. In this work, we synthesize single-phase B1 polycrystalline V0.57Mo0.43N0.95 coatings to investigate the effects induced by temperature on the nanostructural evolution and hardness (H) of the material. Nanoindentation measurements show that the as-deposited film (H = 23 +/- 3 GPa) becomes approximate to 30% harder (up to 31 +/- 2 GPa) upon annealing at 730 C. Experimental characterization and analyses, based on dispersive X-ray spectroscopy, X-ray diffraction (XRD), and transmission electron microscopy (TEM), reveal that the age-hardening effect originates from decomposition of the solid solution into coherent strained cubic VN-rich/MoN-rich domains. The experimental results are complemented by the composition/temperature (V,Mo)N phase diagram - constructed upon ab initio molecular dynamics free-energies - which indicates that the separation observed in the solid solutions is of spinodal nature. Films annealed at temperatures exceeding 850 degrees C undergo structural coarsening, with formation of hexagonal MoxNy and cubic VN phases, which cause a decrease in hardness to approximate to 22 GPa. Our present findings indicate that (V,Mo)N coatings may offer outstanding mechanical performances during operation at elevated temperatures.

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  • 35.
    Mosyagin, Igor
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. NUST MISIS, Russia.
    Gambino, Davide
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Sangiovanni, Davide
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. Ruhr Univ Bochum, Germany.
    Abrikosov, Igor
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. NUST MISIS, Russia.
    Caffrey, Nuala M.
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. Trinity Coll Dublin, Ireland.
    Effect of dispersion corrections on ab initio predictions of graphite and diamond properties under pressure2018In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 98, no 17, article id 174103Article in journal (Refereed)
    Abstract [en]

    There are several approaches to the description of van der Waals (vdW) forces within density functional theory. While they are generally found to improve the structural and energetic properties of those materials dominated by weak dispersion forces, it is not known how they behave when the material is subject to an external pressure. This could be an issue when considering the pressure-induced structural phase transitions, which are currently attracting great attention following the discovery of an ultrahard phase formed by the compression of graphite at room temperature. In order to model this transition, the functional must be capable of simultaneously describing both strong covalent bonds and weak dispersion interactions as an isotropic pressure is applied. Here, we report on the ability of several dispersion-correction functionals to describe the energetic, structural, and elastic properties of graphite and diamond, when subjected to an isotropic pressure. Almost all of the tested vdW corrections provide an improved description of both graphite and diamond compared to the local density approximation. The relative error does not change significantly as pressure is applied, and in some cases even decreases. We therefore conclude that the use of dispersion-corrected exchange-correlation functionals, which have been neglected to date, will improve the accuracy and reliability of theoretical investigations into the pressure-induced phase transition of graphite.

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  • 36.
    Pshyk, Oleksandr V.
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Li, Xiao
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Petrov, Ivan
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering. Materials Research Laboratory, University of Illinois, Urbana, IL, United States.
    Sangiovanni, Davide Giuseppe
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Palisaitis, Justinas
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Hultman, Lars
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Greczynski, Grzegorz
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Discovery of Guinier-Preston zone hardening in refractory nitride ceramics2023In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 255, article id 119105Article in journal (Refereed)
    Abstract [en]

    Traditional age hardening mechanisms in refractory ceramics consist of precipitation of fine particles. These processes are vital for widespread wear-resistant coating applications. Here, we report novel Guinier-Preston zone hardening, previously only known to operate in soft light-metal alloys, taking place in refractory ceramics like multicomponent nitrides. The added superhardening, discovered in thin films of Ti-Al-W-N upon high temperature annealing, comes from the formation of atomic-plane-thick W disks populating {111} planes of the cubic matrix, as observed by atomically resolved high resolution scanning transmission electron microscopy and corroborated by ab initio calculations and molecular dynamics simulations. Guinier-Preston zone hardening concurrent with spinodal decomposition is projected to exist in a range of other ceramic solid solutions and thus provides a new approach for the development of advanced materials with outstanding mechanical properties and higher operational temperature range for the future demanding applications.

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  • 37.
    Ramanath, Ganpati
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering. Rensselaer Polytech Inst, NY 12180 USA.
    Rowe, Collin
    Rensselaer Polytech Inst, NY 12180 USA.
    Sharma, Geetu
    Rensselaer Polytech Inst, NY 12180 USA.
    Venkataramani, Venkat
    Rensselaer Polytech Inst, NY 12180 USA.
    Alauzun, Johan G.
    Univ Montpellier, France.
    Sundararaman, Ravishankar
    Rensselaer Polytech Inst, NY 12180 USA.
    Keblinski, Pawel
    Rensselaer Polytech Inst, NY 12180 USA.
    Sangiovanni, Davide
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Eklund, Per
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Pedersen, Henrik
    Linköping University, Department of Physics, Chemistry and Biology, Chemistry. Linköping University, Faculty of Science & Engineering.
    Engineering inorganic interfaces using molecular nanolayers2023In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 122, no 26, article id 260502Article in journal (Refereed)
    Abstract [en]

    Advances in interface science over the last 20 years have demonstrated the use of molecular nanolayers (MNLs) at inorganic interfaces to access emergent phenomena and enhance a variety of interfacial properties. Here, we capture important aspects of how a MNL can induce multifold enhancements and tune multiple interfacial properties, including chemical stability, fracture energy, thermal and electrical transport, and electronic structure. Key challenges that need to be addressed for the maturation of this emerging field are described and discussed. MNL-induced interfacial engineering has opened up attractive opportunities for designing organic-inorganic hybrid nanomaterials with high interface fractions, where properties are determined predominantly by MNL-induced interfacial effects for applications.

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  • 38.
    Reeswinkel, Thomas
    et al.
    Rhein Westfal TH Aachen.
    Sangiovanni, Davide
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Chirita, Valeriu
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Hultman, Lars
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Schneider, Jochen M.
    Rhein Westfal TH Aachen.
    Structure and mechanical properties of TiAlN-WNI(x) thin films2011In: Surface & Coatings Technology, ISSN 0257-8972, E-ISSN 1879-3347, Vol. 205, no 20, p. 4821-4827Article in journal (Refereed)
    Abstract [en]

    A combinatorial method was employed to grow TiAlN-WNx films by DC sputtering as well as by High Power Pulsed Magnetron Sputtering (HPPMS) where the W concentration was varied between 10-52 at.% and 7-54 at.%, respectively. Experiments were paired with ab initio calculations to investigate the correlation between composition, structure, and mechanical properties. During all depositions the time averaged power was kept constant. As the W concentration was increased, the lattice parameter of cubic TiAlN-WNx films first increased and then decreased for W concentrations above approximate to 29 at.% (DCMS) and approximate to 27 at.% (HPPMS) as the N concentration decreased. Calculations helped to attribute the increase to the substitution of Ti and Al by W and the decrease to the presence of N vacancies. Youngs modulus and hardness were around 385-400 GPa and 29-31 GPa for DCMS and 430-480 GPa and 34-38 GPa for HPPMS, respectively, showing no significant trend as the W concentration was increased, whereas calculations showed a continuous decrease in Youngs modulus from 440 to 325 GPa as the W concentration was increased from 0 to 37.5 at.%. The presence of N vacancies was shown to increase the calculated Youngs modulus. Hence, the relatively constant values measured may be understood based on N vacancy formation as the W concentration was increased. HPPMS-deposited films exceed DCMS films in Youngs modulus and hardness, which may be a consequence of the larger degree of ionization in the HPPMS plasma. It is reasonable to assume that especially the ionized film forming species may contribute towards film densification and N vacancy formation.

  • 39.
    Reeswinkel, Thomas
    et al.
    RWTH Aachen University.
    Sangiovanni, Davide Giuseppe
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Chirita, Valeriu
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Hultman, Lars
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Schneider, Jochen
    RWTH Aachen University.
    Structure and mechanical properties of TiAlN-WNx thin films2010In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550Article in journal (Refereed)
  • 40.
    Salamania, Janella
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Nanostructured Materials. Linköping University, Faculty of Science & Engineering.
    Calamba Kwick, Katherine
    Sandvik Coromant AB, Stockholm, Sweden.
    Sangiovanni, Davide Giuseppe
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Tasnadi, Ferenc
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Abrikosov, Igor A.
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Rogström, Lina
    Linköping University, Department of Physics, Chemistry and Biology, Nanostructured Materials. Linköping University, Faculty of Science & Engineering.
    Johnson, Lars
    Sandvik Coromant AB, Stockholm, Sweden.
    Odén, Magnus
    Linköping University, Department of Physics, Chemistry and Biology, Nanostructured Materials. Linköping University, Faculty of Science & Engineering.
    High-resolution STEM investigation of the role of dislocations during decomposition of Ti1-xAlxNy2023In: Scripta Materialia, ISSN 1359-6462, E-ISSN 1872-8456, Vol. 229, article id 115366Article in journal (Refereed)
    Abstract [en]

    The defect structures forming during high-temperature decomposition of Ti1-xAlxNy films were investigated through high-resolution scanning transmission electron microscopy. After annealing to 950 °C, misfit edge dislocations a/6〈112〉{111} partial dislocations permeate the interface between TiN-rich and AlN-rich domains to accommodate lattice misfits during spinodal decomposition. The stacking fault energy associated with the partial dislocations decreases with increasing Al content, which facilitates the coherent cubic to wurtzite structure transition of AlN-rich domains. The wurtzite AlN-rich structure is recovered when every third cubic {111} plane is shifted by along the [211] direction. After annealing to 1100 °C, a temperature where coarsening dominates the microstructure evolution, we observe intersections of stacking faults, which form sessile locks at the interface of the TiN- and AlN-rich domains. These observed defect structures facilitate the formation of semicoherent interfaces and contribute to hardening in Ti1-xAlxNy.

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  • 41.
    Salamania, Janella
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Nanostructured Materials. Linköping University, Faculty of Science & Engineering.
    Sangiovanni, Davide Giuseppe
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Kraych, A.
    The Interdisciplinary Centre for Advanced Materials Simulation (ICAMS), Ruhr-Universität Bochum, Bochum, Germany.
    Calamba Kwick, K.M.
    Sandvik Coromant AB, Stockholm, Sweden.
    Schramm, I.C.
    Sandvik Coromant AB, Stockholm, Sweden.
    Johnson, L.J.S.
    Sandvik Coromant AB, Stockholm, Sweden.
    Boyd, Robert
    Linköping University, Department of Physics, Chemistry and Biology, Plasma and Coating Physics. Linköping University, Faculty of Science & Engineering.
    Bakhit, Babak
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Hsu, Tun-Wei
    Linköping University, Department of Physics, Chemistry and Biology, Nanostructured Materials. Linköping University, Faculty of Science & Engineering.
    Mrovec, M.
    The Interdisciplinary Centre for Advanced Materials Simulation (ICAMS), Ruhr-Universität Bochum, Bochum, Germany.
    Rogström, Lina
    Linköping University, Department of Physics, Chemistry and Biology, Nanostructured Materials. Linköping University, Faculty of Science & Engineering.
    Tasnadi, Ferenc
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Abrikosov, Igor A.
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Odén, Magnus
    Linköping University, Department of Physics, Chemistry and Biology, Nanostructured Materials. Linköping University, Faculty of Science & Engineering.
    Elucidating dislocation core structures in titanium nitride through high-resolution imaging and atomistic simulations2022In: Materials & design, ISSN 0264-1275, E-ISSN 1873-4197, Vol. 224, article id 111327Article in journal (Refereed)
    Abstract [en]

    Although titanium nitride (TiN) is among the most extensively studied and thoroughly characterizedthin-film ceramic materials, detailed knowledge of relevant dislocation core structures is lacking. Byhigh-resolution scanning transmission electron microscopy (STEM) of epitaxial single crystal (001)-oriented TiN films, we identify different dislocation types and their core structures. These include, besidesthe expected primary a/2{110}h110i dislocation, Shockley partial dislocations a/6{111}h112i and sessileLomer edge dislocations a/2{100}h011i. Density-functional theory and classical interatomic potentialsimulations complement STEM observations by recovering the atomic structure of the different disloca-tion types, estimating Peierls stresses, and providing insights on the chemical bonding nature at the core.The generated models of the dislocation cores suggest locally enhanced metal–metal bonding, weakenedTi-N bonds, and N vacancy-pinning that effectively reduces the mobilities of {110}h110i and {111}h112idislocations. Our findings underscore that the presence of different dislocation types and their effects onchemical bonding should be considered in the design and interpretations of nanoscale and macroscopicproperties of TiN.

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  • 42.
    Sangiovanni, Davide
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. Ruhr Univ Bochum, Germany.
    Inherent toughness and fracture mechanisms of refractory transition-metal nitrides via density-functional molecular dynamics2018In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 151, p. 11-20Article in journal (Refereed)
    Abstract [en]

    Hard refractory transition-metal nitrides possess unique combinations of outstanding mechanical and physical properties, but are typically brittle. Recent experimental results demonstrated that single-crystal NaCI-structure (B1) V0.5Mo0.5N pseudobinary solid solutions are both hard (similar to 20 GPa) and ductile; that is, they exhibit toughness, which is unusual for ceramics. However, key atomic-scale mechanisms underlying this inherent toughness are unknown. Here, I carry out density-functional ab initio molecular dynamics (AIMD) simulations at room temperature to identify atomistic processes and associated changes in the electronic structure which control strength, plasticity, and fracture in V0.5Mo0.5N, as well as reference B1 TiN, subject to amp;lt;001amp;gt; and amp;lt;110amp;gt; tensile deformation. AIMD simulations reveal that V0.5Mo0.5N is considerably tougher than TiN owing to its ability to (i) isotropically redistribute mechanical stresses within the elastic regime, (ii) dissipate the accumulated strain energy by activating local structural transformations beyond the yield point. In direct contrast, TiN breaks in brittle manner when applied stresses reach its tensile strength. Charge transfer maps show that the adaptive mechanical response of V0.5Mo0.5N originates from highly populated d-d metallic-states, which allow for counterbalancing the destabilization induced via tensile deformation by enabling formation of new chemical bonds. The high ionic character and electron-localization in TiN precludes the possibility of modifying bonding geometries to accommodate the accumulated stresses, thus suddenly causing materials fracture for relatively low strain values. 

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  • 43.
    Sangiovanni, Davide
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. Ruhr Univ Bochum, Germany.
    Mass transport properties of quasiharmonic vs. anharmonic transition-metal nitrides2019In: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 688, article id 137297Article in journal (Refereed)
    Abstract [en]

    I present a development of the color-diffusion algorithm, used in non-equilibrium (accelerated) ab initio molecular dynamics simulations of point-defect migration in crystals [Sangiovanni et al., Phys. Rev. B 93, 094305 (2016)], to determine the temperature dependence of anion vacancy jump frequencies in rocksalt-structure (B1) TiN and VN characterized by quasiharmonic (TiN) vs. strongly anharmonic (VN) lattice dynamics. Over a temperature range [ 0.6"Tm amp;lt; T amp;lt; 0.9 T-m] relatively close to the materials melting points Tm, the simulations reveal that anion vacancy migration in TiN and VN exhibits an Arrhenius-like behavior, described by activation energies EJN = 4.2 0.3 eV and EZN = 3.1 0.3 eV, and attempt frequencies vTN = 8.1015 0.7 s-1 and vvN = 2.1017 c).8s-1. A comparison of activation energies E extracted by Arrhenius linear regression at elevated temperatures with ab initio E,,ca values calculated at 0 Kelvin reveals that, while the nitrogen migration energy amp;ills varies modestly with temperature {AEPN = [E-a(T-m)- Ea(0 K)1/Ea(0 K) 0.1}, the changes in EavN vs. T are considerable (AEavN 1). The temperature-induced variations in vacancy migration energies and diffusivities are discussed in relation to the TiN and VN vibrational properties determined via ab initio molecular dynamics at different temperatures. It is argued that static 0-K calculations, which account for thermal expansion effects within the framework of quasiharmonic transition-state theory, accurately reproduce the finite-temperature mass transport properties of TiN. Conversely, the use of molecular dynamics simulations, which explicit treat lattice vibrations at any temperature of interest, is necessary to achieve reliable atomic diffusivities in B1 VN, a crystal phase dynamically stabilized by anharmonic vibrations [Mei et al., Phys. Rev. B 91, 054101 (2015)].

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  • 44.
    Sangiovanni, Davide
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
    Alling, Björn
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Steneteg, Peter
    Linköping University, Department of Science and Technology, Media and Information Technology. Linköping University, The Institute of Technology.
    Hultman, Lars
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Abrikosov, Igor
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology. National University of Science and Technology, Russia; Tomsk State University, Russia.
    Nitrogen vacancy, self-interstitial diffusion, and Frenkel-pair formation/dissociation in B1 TiN studied by ab initio and classical molecular dynamics with optimized potentials2015In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 91, no 5, p. 054301-Article in journal (Refereed)
    Abstract [en]

    We use ab initio and classical molecular dynamics (AIMD and CMD) based on the modified embedded-atom method (MEAM) potential to simulate diffusion of N vacancy and N self-interstitial point defects in B1 TiN. TiN MEAM parameters are optimized to obtain CMD nitrogen point-defect jump rates in agreement with AIMD predictions, as well as an excellent description of TiNx (similar to 0.7 less than x less than= 1) elastic, thermal, and structural properties. We determine N dilute-point-defect diffusion pathways, activation energies, attempt frequencies, and diffusion coefficients as a function of temperature. In addition, the MD simulations presented in this paper reveal an unanticipated atomistic process, which controls the spontaneous formation of N self-interstitial/N vacancy (N-I/N-V) pairs (Frenkel pairs), in defect-free TiN. This entails that the N lattice atom leaves its bulk position and bonds to a neighboring N lattice atom. In most cases, Frenkel-pair N-I and N-V recombine within a fraction of ns; similar to 50% of these processes result in the exchange of two nitrogen lattice atoms (N-N-Exc). Occasionally, however, Frenkel-pair N-interstitial atoms permanently escape from the anion vacancy site, thus producing unpaired N-I and N-V point defects.

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  • 45.
    Sangiovanni, Davide
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Chirita, Valeriu
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Hultman, Lars
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Toughness enhancement in TiAlN-based quarternary alloys2012In: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 520, no 11, p. 4080-4088Article in journal (Refereed)
    Abstract [en]

    Improved toughness in hard and superhard thin films is a primary requirement for present day ceramic hard coatings, known to be prone to brittle failure during in-use conditions. We use density functional theory calculations to investigate a number of (TiAl)(1-x)MxN thin films in the B1 structure, with 0.06 andlt;= x andlt;= 0.75 obtained by alloying TiAlN with M = V, Nb, Ta, Mo and W. Results show significant ductility enhancements, hence increased toughness, in these compounds. Importantly, these thin films are also predicted to be superhard, with similar or increased hardness values, compared to Ti0.5Al0.5 N. For (TiAl)(1-x)WxN the results are experimentally confirmed. The ductility increase originates in the enhanced occupancy of d-t(2g) metallic states, induced by the valence electrons of substitutional elements (V, Nb, Ta, Mo, W). This effect is more pronounced with increasing valence electron concentration, and, upon shearing, leads to the formation of a layered electronic structure in the compound material, consisting of alternating layers of high and low charge density in the metallic sublattice, which in turn, allows a selective response to normal and shear stresses.

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  • 46.
    Sangiovanni, Davide
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Edström, Daniel
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Hultman, Lars
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Chirita, Valeriu
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Petrov, Ivan
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Greene, Joseph E
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Dynamics of Ti, N, and TiNx (x=1-3) admolecule transport on TiN(001) surfaces2012In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 86, no 15, p. 155443-Article in journal (Refereed)
    Abstract [en]

    We use classical molecular dynamics and the modified embedded atom method formalism to investigate the dynamics of atomic-scale transport on a low-index model compound surface, TiN(001). Our simulations, totaling 0.25 mu s for each case study, follow the pathways and migration kinetics of Ti and N adatoms, as well as TiNx complexes with x = 1-3, which are known to contribute to the growth of TiN thin films by reactive deposition from Ti, N-2, and N precursors. The simulations are carried out at 1000 K, within the optimal range for TiN(001) epitaxial growth. We find Ti adatoms to be the highest-mobility species on TiN(001), with the primary migration path involving jumps of one nearest-neighbor distance d(NN) between adjacent fourfold hollow sites along in-plane andlt; 100 andgt; channels. Long jumps, 2d(NN), are also observed, but at much lower frequency. N adatoms, which exhibit significantly lower migration rates than Ti, diffuse along in-plane andlt; 110 andgt; directions and, when they intersect other N atoms, associatively form N-2 molecules, which desorb at kinetic rates. As expected, TiN and TiN3 complexes migrate at even lower rates with complex diffusion pathways involving rotations, translations, and rototranslations. TiN2 trimers, however, are shown to have surprisingly high diffusion rates, above that of N adatoms and almost half that of Ti adatoms. TiN3 motion is dominated by in-place rotation with negligible diffusion.

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  • 47.
    Sangiovanni, Davide
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
    Edström, Daniel
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Hultman, Lars
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Petrov, Ivan
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology. University of Illinois, Urbana, USA.
    Greene, Joseph E
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology. University of Illinois, Urbana, USA.
    Chirita, Valeriu
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Ab-initio and classical molecular dynamics simulations of N2 desorption from TiN(001) surfaces2014In: Surface Science, ISSN 0039-6028, E-ISSN 1879-2758, Vol. 624, p. 25-31Article in journal (Refereed)
    Abstract [en]

    Ab initio molecular dynamics simulations based on density functional theory show that N adatoms are chemisorbed in threefold sites close to a N surface atom and between the two diagonally opposed neighboring Ti surface atoms on TiN(001). The most probable N adatom reaction pathway, even in the presence of nearby N adatoms, is for the N adatom and N surface atom pair to first undergo several exchange reactions and then desorb as a N2 molecule, resulting in a surface anion vacancy, with an activation barrier Edes of 1.37 eV and an attempt frequency Ades = 3.4 × 1013 s− 1. Edes is essentially equal to the N adatom surface diffusion barrier, Es = 1.39 eV, while As is only three to four times larger than Ades, indicating that isolated N adatoms migrate for only short distances prior to N2 desorption. The probability of N2 desorption via recombination of N adatoms on TiN(001) is much lower due to repulsive adatom/adatom interactions at separations less than ~ 3 Å which rapidly increase to ~ 2 eV at a separation of 1.5 Å. We obtain good qualitative and quantitative agreement with the above results using the modified embedded atom method potential to perform classical molecular dynamics simulations.

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    Ab-initio and classical molecular dynamics simulations of N2 desorption from TiN(001) surfaces
  • 48.
    Sangiovanni, Davide
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
    Edström, Daniel
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Hultman, Lars
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Petrov, Ivan
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology. University of Illinois, Urbana, USA.
    Greene, Joseph E
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology. University of Illinois, Urbana, USA.
    Chirita, Valeriu
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Ti adatom diffusion on TiN(001): Ab initio and classical molecular dynamics simulations2014In: Surface Science, ISSN 0039-6028, E-ISSN 1879-2758, Vol. 627, p. 34-41Article in journal (Refereed)
    Abstract [en]

    Ab initio and classical molecular dynamics (AIMD and CMD) simulations reveal that Ti adatoms on TiN(001) surfaces migrate between neighboring fourfold hollow sites primarily along in-plane less than100greater than channels. less than100greater than and less than110greater than single jumps, as well as less than100greater than double jump rates, obtained directly from MD runs as a function of temperature, are used to determine diffusion activation energies Ea, and attempt frequencies A, for the three preferred Ti adatom migration pathways on TiN(001). From transition rates Aexp[-Ea / (k(B)T)], we determine adatom surface distribution probabilities as a function of time, which are used to calculate adatom diffusion coefficients D(T). AIMD and CMD predictions are consistent and complementary. Using CMD, we investigate the effect on the adatom jump rate of varying the phonon wavelength degrees of freedom by progressively increasing the supercell size. We find that long-wavelength phonons significantly contribute to increasing adatom mobilities at temperatures less than= 600 K, but not at higher temperatures. Finally, by directly tracking the Ti adatom mean-square displacement during CMD runs, we find that Ti adatom jumps are highly correlated on TiN(001), an effect that yields lower D-s values (D-s(corr)) than those estimated from uncorrelated transition probabilities. The temperature-dependent diffusion coefficient is D-s(corr) (T) = (4.5 x 10(-4) Cm-2 s(-1)) exp[-0.55 eV / (k(B)T)].

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    Ti adatom diffusion on TiN(001): Ab initio and classical molecular dynamics simulations
  • 49.
    Sangiovanni, Davide
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Faccio, Ricardo
    Univ Republica, Uruguay.
    Gueorguiev, Gueorgui Kostov
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Kakanakova-Gueorguieva, Anelia
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Correction: Discovering atomistic pathways for supply of metal atoms from methyl-based precursors to graphene surface (vol 25, pg 829, 2023)2023In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 25, no 7, p. 5887-5887Article in journal (Other academic)
    Abstract [en]

    Correction for Discovering atomistic pathways for supply of metal atoms from methyl-based precursors to graphene surface by Davide G. Sangiovanni et al., Phys. Chem. Chem. Phys., 2023, 25, 829-837, https://doi.org/10.1039/D2CP04091C.

  • 50.
    Sangiovanni, Davide
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Faccio, Ricardo
    Univ Republica, Uruguay; Univ Republica, Uruguay.
    Gueorguiev, Gueorgui Kostov
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Kakanakova-Gueorguieva, Anelia
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Discovering atomistic pathways for supply of metal atoms from methyl-based precursors to graphene surface2022In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 25, no 1, p. 829-837Article in journal (Refereed)
    Abstract [en]

    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.

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