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  • 1.
    Löfgren, Robin
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Pawar, Ravinder
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Larsson, Andreas
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Charged dopants in neutral supercells through substitutional donor (acceptor): nitrogen donor charging of the nitrogen-vacancy center in diamond2018Ingår i: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 20, artikel-id 023002Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Charged defects are traditionally computed by adding (subtracting) electrons for negative (positive) impurities. When using periodic boundary conditions this results in artificially charged supercells that also require a compensating background charge of the opposite sign, which makes slab supercells problematic because of an arbitrary dependence on the vacuum thickness. In this work, we test the method of using neutral supercells through the use of a substitutional electron donor (acceptor) to describe charged systems. We use density functional theory (DFT) to compare the effects of charging the well-studied NV-center in diamond by a substitutional donor nitrogen. We investigate the influence of the donor-N on the NV-center properties as a function of the distance between them, and find that they converge toward those obtained when adding an electron. We analyze the spin density and conclude that the donor-N has a zero magnetic moment, and thus, will not be seen in electron spin resonance. We validate our DFT energies through comparison to GW simulations. Charging the NV-center with a substitutional donor-N enables accurate calculations of slabs, without the ambiguity of using charged supercells. Implantation of donor-N atoms opens up the possibility to engineer NV-centers with the desired charge state for future ICT and sensor applications.

  • 2.
    Löfgren, Robin
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Pawar, Ravinder
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Larsson, Andreas
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    The bulk conversion depth of the NV-center in diamond: computing a charged defect in a neutral slab2019Ingår i: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 21, artikel-id 053037Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The negatively charged nitrogen vacancy (NV-) center in diamond has properties that make it a promising candidate for applications such as a qubit in room temperature quantum computing, single-molecule photoluminescence and NMR sensor, and as a single photon source for quantum cryptography. For many of its uses it is desirable to have the NV-center close to the diamond surface. In this work, we use density functional theory simulations to investigate how the distance of the NV- center to a surface, and its orientation, affect its properties, including the zero-phonon-line. We study the three technologically important surfaces terminated with fluorine, oxygen/hydroxyl and nitrogen. Since the NV-center is charged it requires special measures to simulate within a slab-model. We use the recently proposed charging with a substitutional donor in the diamond lattice resulting in a neutral super-cell, which provides very satisfactory results. We have found that the NV-centers properties converge to bulk values already at 5 angstrom depth.

  • 3.
    Nasibulin, Albert G.
    et al.
    Department of Applied Physics and Center for New Materials, Helsinki University of Technology, Espoo.
    Shandakov, Sergey D.
    Department of Applied Physics and Center for New Materials, Helsinki University of Technology, Espoo.
    Nasibulina, Larisa I.
    Cwirzen, Andrzej
    Laboratory of Building Materials Technology, Faculty of Engineering and Architecture, Helsinki University of Technology, Espoo.
    Mudimela, Prasantha R.
    Department of Applied Physics, Aalto University.
    Habermehl-Cwirzen, Karin
    Laboratory of Building Materials Technology, Faculty of Engineering and Architecture, Helsinki University of Technology, Espoo.
    Grishin, Dmitrii A.
    Mendeleev University of Chemical Technology.
    Gavrilov, Yuriy V.
    Mendeleev University of Chemical Technology.
    Malm, J. E M
    Laboratory of Inorganic Chemistry, Department of Chemistry, Helsinki University of Technology, Espoo.
    Tapper, Unto
    VTT Biotechnology.
    Tian, Ying
    Department of Applied Physics and Center for New Materials, Helsinki University of Technology, Espoo.
    Penttala, Vesa
    Laboratory of Building Materials Technology, Faculty of Engineering and Architecture, Helsinki University of Technology, Espoo.
    Karppinen, Maarit J.
    Laboratory of Inorganic Chemistry, Department of Chemistry, Helsinki University of Technology, Espoo.
    Kauppinen, Esko I.
    Department of Applied Physics and Center for New Materials, Helsinki University of Technology, Espoo.
    A novel cement-based hybrid material2009Ingår i: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 11, artikel-id 23013Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Carbon nanotubes (CNTs) and carbon nanofibers (CNFs) are known to possess exceptional tensile strength, elastic modulus and electrical and thermal conductivity. They are promising candidates for the next-generation high-performance structural and multi-functional composite materials. However, one of the largest obstacles to creating strong, electrically or thermally conductive CNT/CNF composites is the difficulty of getting a good dispersion of the carbon nanomaterials in a matrix. Typically, time-consuming steps of purification and fimctionalization of the carbon nanomaterial are required. We propose a new approach to grow CNTs/CNFs directly on the surface of matrix particles. As the matrix we selected cement, the most important construction material. We synthesized in a simple one-step process a novel cement hybrid material (CHM), wherein CNTs and CNFs are attached to the cement particles. The CHM has been proven to increase 2 times the compressive strength and 40 times the electrical conductivity of the hardened paste, i.e. concrete without sand. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.

  • 4.
    Savini, G.
    et al.
    Department of Chemistry, University of Sussex.
    Heggie, M.I.
    Department of Chemistry, University of Sussex.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Briddon, P.R.
    School of Natural Science, University of Newcastle upon Tyne.
    Electrical activity and migration of 90° partial dislocations in SiC2007Ingår i: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 9, s. 1-13Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    SiC p-i-n diodes exhibit an increase in the voltage drop under forward bias which has been linked with the increased mobility of partial dislocations. Through first-principles calculations, we investigated the Si(g) and C(g) core 90° partials in 4H-SiC. We showed that both dislocations can sustain the asymmetric and symmetric reconstructions along the dislocation line. The latter reconstructions are always electrically active with a half-filled metallic band and are always more likely to migrate with substantially lower activation energies. Further we have suggested that under forward bias, the 90° partials are less mobile than the 30° partial dislocations.

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