Browsing by Subject "TRANSITION-METALS"

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  • Polvi, J.; Heinola, K.; Nordlund, K. (2016)
    N-2 gas is routinely used as a seeding species in fusion plasma to control the amount of power emitted from the plasma by radiation to the tungsten walls of an ITER-like divertor. Nitrogen atoms interact with the plasma-facing materials beryllium and tungsten, and form chemical bonds with the wall surfaces, as well as with plasma hydrogen isotopes, thus raising a special interest in W-N and N-H interactions in the fusion community. In this work we describe the development of an analytical interatomic potential for W-N interactions and benchmark the potential against DFT calculation results for N defects in tungsten.
  • Väyrynen, Katja; Hatanpää, Timo; Mattinen, Miika; Heikkilä, Mikko; Mizohata, Kenichiro; Meinander, Kristoffer; Räisänen, Jyrki; Ritala, Mikko; Leskelä, Markku (2018)
    In this paper, we introduce a new Co precursor for the atomic layer deposition (ALD) of Co metal and other Co containing materials. CoCl2(TMEDA) (TMEDA = N,N,N ',N '-tetramethylethylenediamine) is a diamine adduct of cobalt(II) chloride that is inexpensive and easy to synthesize, making it an industrially viable precursor. Furthermore, CoCl2(TMEDA) shows good volatility at reasonably low temperatures and is thermally stable up to a temperature of, similar to 300 degrees C. We also present a full ALD study for the deposition of CoO thin films using CoCl2(TMEDA) and water as precursors. The process was investigated within a temperature range of 225-300 degrees C. Saturation of the film growth with respect to both precursor pulse lengths was verified. According to X-ray diffraction, the films were a mixture of hexagonal and cubic CoO. No reflections corresponding to Co3O4 were detected. The hexagonal phase is characteristic to nanomaterials only and is not seen in bulk CoO. The crystal structure of the films could be tuned by temperature, water pulse lengths, and type of substrate material. The films deposited at 275 degrees C exhibited 1:1 Co:O stoichiometry and very high purity. The CoO films could be reduced to Co metal at an exceptionally low temperature of 250 degrees C in 10% forming gas. Continuity of the reduced Co films was improved when the CoO film was deposited on TiN instead of native oxide terminated Si. The Co content of a 50 nm reduced metal film was as high as 95 at. %, with negligible amounts of oxygen and hydrogen.
  • Lehtola, Susi (2020)
    recently developed finite-element approach for fully numerical atomic structure calculations [S. Lehtola, Int. J. Quantum Chem. 119, e25945 (2019)] is extended to the description of atoms with spherically symmetric densities via fractionally occupied orbitals. Specialized versions of Hartree-Fock as well as local density and generalized gradient approximation density functionals are developed, allowing extremely rapid calculations at the basis-set limit on the ground and low-lying excited states, even for heavy atoms. The implementation of range separation based on the Yukawa or complementary error function (erfc) kernels is also described, allowing complete basis-set benchmarks of modern range-separated hybrid functionals with either integer or fractional occupation numbers. Finally, the computation of atomic effective potentials at the local density or generalized gradient approximation levels for the superposition of atomic potentials (SAP) approach [S. Lehtola, J. Chem. Theory Comput. 15, 1593 (2019)] that has been shown to be a simple and efficient way to initialize electronic structure calculations is described. The present numerical approach is shown to afford beyond micro-Hartree accuracy with a small number of numerical basis functions, and to reproduce the literature results for the ground states of atoms and their cations for 1
  • Boom, Devin H. A.; Ehlers, Andreas W.; Nieger, Martin; Devillard, Marc; Bouhadir, Ghenwa; Bourissou, Didier; Slootweg, J. Chris (2018)
    In this work, we explored the coordination properties of the geminal phosphinoborane tBu(2)PCH(2)BPh(2) (2) toward different gold(I) precursors. The reaction of 2 with an equimolar amount of the sulfur-based complex (Me2S) AuCl resulted in displacement of the SMe2 ligand and formation of linear phosphine gold(I) chloride 3. Using an excess of ligand 2, bisligated complex 4 was formed and showed dynamic behavior at room temperature. Changing the gold(I) metal precursor to the phosphorus-based complex, (Ph3P) AuCl impacted the coordination behavior of ligand 2. Namely, the reaction of ligand 2 with (Ph3P) AuCl led to the heterolytic cleavage of the gold-chloride bond, which is favored over PPh3 ligand displacement. To the best of our knowledge, 2 is the first example of a P/Bambiphilic ligand capable of cleaving the gold-chloride bond. The coordination chemistry of 2 was further analyzed by density functional theory calculations.
  • Heinola, K.; Djurabekova, F.; Ahlgren, T. (2018)
    Properties of small vacancy clusters in tungsten were studied with first-principles calculations. The binding and formation energies of the vacancy clusters increase with the cluster size. Dynamic characteristics of a di-vacancy were specified between room temperature and 700 K with lattice kinetic Monte Carlo calculations, which were parametrised with the present first-principles results for the dissociation barriers. An Arrhenius fit for the di-vacancy diffusion yielded D = 0.04 exp(-1.65 eV kT(-1)) cm(2) s(-1), and for the mean lifetime, tau = 0.093 exp(1.7 eV) kT(-1) ps. The di-vacancy system was found to be stable up to 500 K, due to the high energy needed for its dissociation. Having a carbon impurity was found to increase the tungsten di-vacancy binding energy.