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  • 1.
    Araujo, C. Moysés
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Kapilashrami, Mukes
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Jun, Xu
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Jayakumar, Onattu D.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Nagar, Sandeep
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Wu, Yan
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Århammar, Cecilia
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Johansson, Börje
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Belova, Lyubov
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Ahuja, Rajeev
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Gehring, Gillian A.
    Rao, K. Venkat
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Room temperature ferromagnetism in pristine MgO thin films2010Ingår i: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 96, nr 23Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Robust ferromagnetic ordering at, and well above room temperature is observed in pure transparent MgO thin films (<170 nm thick) deposited by three different techniques. Careful study of the wide scan x-ray photoelectron spectroscopy rule out the possible presence of any magnetic contaminants. In the magnetron sputtered films, we observe magnetic phase transitions as a function of film thickness. The maximum saturation magnetization of 5.7 emu/cm(3) is measured on a 170 nm thick film. The films above 500 nm are found to be diamagnetic. Ab initio calculations suggest that the ferromagnetism is mediated by cation vacancies.

  • 2. Blomqvist, A.
    et al.
    Århammar, Cecilia
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Pedersen, H.
    Silvearv, F.
    Norgren, S.
    Ahuja, Rajeev
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Understanding the catalytic effects of H(2)S on CVD-growth of alpha-alumina: Thermodynamic gas-phase simulations and density functional theory2011Ingår i: Surface & Coatings Technology, ISSN 0257-8972, E-ISSN 1879-3347, Vol. 206, nr 7, s. 1771-1779Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The catalytic effect of H(2)S on the AlCl(3)/H(2)/CO(2)/HCl chemical vapor deposition (CVD) process has been investigated on an atomistic scale. We apply a combined approach with thermodynamic modeling and density functional theory and show that H(2)S acts as mediator for the oxygenation of the AI-surface which will in turn increase the growth rate of Al(2)O(3). Furthermore we suggest surface terminations for the three investigated surfaces. The oxygen surface is found to be hydrogenated, in agreement with a number of previous works. The aluminum surfaces are Cl-terminated in the studied CVD-process. Furthermore, we find that the AlClO molecule is a reactive transition state molecule which interacts strongly with the aluminum and oxygen surfaces.

  • 3. Huang, L. M.
    et al.
    Århammar, Cecilia
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Araujo, José
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Silvearv, F.
    Ahuja, Rajeev
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Tuning magnetic properties of In2O3 by control of intrinsic defects2010Ingår i: Europhysics letters, ISSN 0295-5075, E-ISSN 1286-4854, Vol. 89, nr 4Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The electronic structure and magnetic properties of In2O3 with four kinds of intrinsic point defects (O vacancy, In interstitial, O interstitial, and In vacancy) have been theoretically studied using the density functional theory. The defect energy states of the O vacancy and In interstitial are close to the bottom of conduction band and act as shallow donors, while the defect energy states of the In vacancy and O interstitial are just above the top of the valence band and act as shallow acceptors. Without addition of any magnetic ions, all the hole states are completely spin polarized, while the electron states display no spin polarization. This implies that semiconducting In2O3 can display magnetic ordering, purely due to the intrinsic defects. However, the formation energies for neutral p-type defects are too high to be thermodynamically stable at reasonable temperatures. Nevertheless, it is shown that negative charging can greatly decrease the formation energies of p-type defects, simultaneously removing the local magnetic moments. We conlcude that V-In''' and O-I '' will be the dominant compensating defects as In2O3 is doped with TM ions, such as Sn, Mo, V and Cr. This result is consistent with the general view that the p-type defect is a key feature to mediate ferromagnetic coupling between transition metal ions of dilute concentration in metal oxides.

  • 4. Moysés Araújo, C.
    et al.
    Kapilashrami, Mukes
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Jun, Xu
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Jayakumar, Onattu D.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Nagar, Sandeep
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Wu, Yan
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Århammar, Cecilia
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Johansson, Börje
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Belova, Lyubov
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Ahuja, Rajeev
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Rao, K Venkat.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Unusual ferromagnetism above room temperature in undoped thin films and nanoparticles of MgOManuskript (Övrigt vetenskapligt)
  • 5. Nisar, Jawad
    et al.
    Århammar, Cecilia
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Jämstorp, Erik
    Ahuja, Rajeev
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Optical gap and native point defects in kaolinite studied by the GGA-PBE, HSE functional, and GW approaches2011Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 84, nr 7, s. 075120-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The electronic structure of kaolinite with and without intrinsic defects has been studied by the Perdew-Burke-Ernzerhof (PBE) and Heyd-Scuseria-Ernzerhof (HSE) functionals and by the G0W0 approach. The band gap of defect-free kaolinite was estimated to between 6.2 and 8.2 eV. Analysis of the formation energy of native point defects in kaolinite was carried out under different growth conditions. When the PBE defect formation energy as a function of temperature is considered, the hydroxyl vacancy is compensated by a hydrogen vacancy at a formation energy of 0.45 eV at oxygen-rich and hydrogen-poor conditions. The hydroxyl vacancy acts as a donor whereas the hydrogenvacancy acts as an acceptor, both inducing states in the band gap. The HSE03 hybrid functional increases the defect formation energy and tends to localize and move these states away from the band edges, as compared to the other two methods. Our results imply that intrinsic defects will tune the band gap of kaolinite and influence properties related to its band structure such as the cation retention capability and drug release.

  • 6.
    Århammar, Cecilia
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Atomistic modelling of functional solid oxides for industrial applications: Density Functional Theory, hybrid functional and GW-based studies2011Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
    Abstract [en]

    In this Thesis a set of functional solid oxides for industrial applications have been addressed by first principles and thermodynamical modelling. More specificially, measurable quantities such as Gibbs free energy, geometry and electronic structure have been calculated and compared when possible with experimental data. These are crystalline and amorphous aluminum oxide (Al2O3), Zirconia (ZrO2), magnesium oxide (MgO), indiumoxide (In2O3) and Kaolinite clay (Al2Si2O5(OH)4).

    The reader is provided a computation tool box, which contains a set of methods to calculate properties of oxides that are measurable in an experiment. There are three goals which we would like to reach when trying to calculate experimental quantities. The first is verification. Without verification of the theory we are utilizing, we cannot reach the second goal -prediction. Ultimately, this may be (and to some extent already is) the future of first principles methods, since their basis lies within the fundamental quantum mechanics and since they require no experimental input apart from what is known from the periodic table. Examples of the techniques which may provide verification are X-Ray Diffraction (XRD), X-ray Absorption and Emission Spectroscopy (XAS and XES), Electron Energy Loss Spectroscopy and Photo-Emission Spectroscopy (PES). These techniques involve a number of complex phenomena which puts high demands on the chosen computational method/s. Together, theory and experiment may enhance the understanding of materials properties compared to the standalone methods. This is the final goal which we are trying to reach -understanding. When used correctly, first principles theory may play the role of a highly resolved analysis method, which provides details of structural and electronic properties on an atomiclevel. One example is the use of first principles to resolve spectra of multicomponentsamples. Another is the analysis of low concentrations of defects. Thorough analysis of the nanoscale properties of products might not be possible in industry due to time and cost limitations. This leads to limited control of for example low concentrations of defects, which may still impact the final performance of the product. On example within cutting tool industry is the impact of defect contents on the melting point and stability of protective coatings. Such defects could be hardening elements such as Si, Mn, S, Ca which diffuse from a steel workpiece into the protective coating during high temperature machining. Other problems are the solving of Fe from the workpiece into the coating and reactions between iron oxide, formed as the workpiece surface is oxidized, and the protective coating.

    The second part of the computational toolbox which is provided to the reader is the simulation of solid oxide synthesis. Here, a formation energy formalism, most often applied to materials intended in electronics devices is applied. The simulation of Chemical Vapour Deposition (CVD) and Physical Vapor Deposition (PVD) requires good knowledge of the experimental conditions, which can then be applied in the theoretical simulations. Effects of temperature, chemical and electron potential, modelled concentration and choice of theoretical method on the heat of formation of different solid oxides with and without dopants are addressed in this work. A considerable part of this Thesis is based upon first principles calculations, more specifically, Density Functional Theory (DFT) After Kohn and Pople received the Nobel Prize in chemistry in 1998, the use of DFT for computational modelling has increased strikingly (see Fig. 1). The use of other first principles methods such as hybrid functionals and the GW approach (see abbreviations for short explanations and chapter 4.5 and 5.3.) have also become increasingly popular, due to the improved computational resources. These methods are also employed in this Thesis.

  • 7.
    Århammar, Cecilia
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Study of solid oxide systems from first principles calculations: energetics and magnetic properties of native defects and dopants in ZrO2 and MgO2009Licentiatavhandling, sammanläggning (Övrigt vetenskapligt)
  • 8.
    Århammar, Cecilia
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Araujo, C. Moyses
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Rao, K. Venkat
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Norgren, S.
    Johansson, Börje
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Ahuja, Rajeev
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Energetics and magnetic properties of V-doped MgO bulk and (001) surface: A GGA, GGA+U, and hybrid density functional study2010Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 82, nr 13, s. 134406-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In this work, a first-principles study of the energetic and magnetic properties of V-doped MgO is presented, where both the bulk and (001) surface were investigated. It is found that V assumes a high-spin state with a local moment of about 3 mu(B). In the bulk, the interaction between these local moments is very short ranged and the antiferromagnetic (AFM) ordering is energetically more favorable. The formation of V-V-Mg-V defect clusters is found to weaken the antiferromagnetic coupling in bulk MgO, degenerating the AFM and ferro-magnetic state. However, these clusters are high in energy and will not form at equilibrium conditions. By employing the GGA+U approach, with U=5 eV, the V 3d states on the (001) surface are shifted below the Fermi level, and a reasonable surface geometry was achieved. A calculation with the hybrid HSE03 functional, contradicts the GGA+U results, indicating that the V-MgO surface should be metallic at this concentration. From the energetics it is concluded that, at the modeled concentration, VxOy phases will limit the solubility of V in MgO at equilibrium conditions, which is in agreement with previous experimental findings. In order to achieve higher concentrations of V, an off-equilibrium synthesis method is needed. Finally, we find that the formation energy of V at the surface is considerably higher than in the bulk and V is thus expected to diffuse from the surface into the bulk of MgO.

  • 9.
    Århammar, Cecilia
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Moysés Araújo, C.
    Ahuja, Rajeev
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Energetics of Al doping and intrinsic defects in monoclinic and cubic zirconia: First-principles calculations2009Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 80, nr 11, s. 115208-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    First-principles theory within the supercell approach has been employed to investigate Al doping and intrinsic defects in monoclinic and cubic zirconia. The effect of oxygen chemical potential and Fermi level on the formation energy and on the defect concentration have been taken into account. The formation of oxygen vacancies is found to be energetically more favorable in the cubic than in the monoclinic phase under the same oxygen chemical potential and Fermi energy. In both phases, substitutional Al decays from neutral charge state into the charge state -1, with the transition energy just above to the top of the valence band. Our findings indicate that by confining the Fermi energy to the region between the middle of the band gap and the bottom of the conduction band, high Al solubility could be achieved, although formation of Al is likely followed by the formation of interstitial oxygen. Furthermore, the concentration of Al with charge state -1 along with the equilibrium Fermi energy have been calculated in a self-consistent procedure. Here, the possible compensating defects with the relevant charge states have been considered. The obtained concentrations of Al and oxygen vacancies follow the experimental trend but underestimates experimental data. When the formation of defect clusters, composed by two substitutional Al and one oxygen vacancy, are considered, good quantitative agreement with experimental values of both Al and oxygen vacancy concentration is achieved. The results suggest that defect clusters will be formed as a result of Al doping in cubic phase of ZrO2, whereas the concentration of defect clusters is negligible in the monoclinic phase, both in accordance with experiment.

  • 10.
    Århammar, Cecilia
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Moysés Araújo, C.
    Ahuja, Rajeev
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Structure and electronic properties of Al-doped zirconia polymorphs from first principlesManuskript (Övrigt vetenskapligt)
  • 11.
    Århammar, Cecilia
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Moysés Araújo, C.
    Rao, K Venkat.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Johansson, Börje
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Ahuja, Rajeev
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    First-principles investigation of magnetic properties of V-doped MgO: bulk and (001) surfaceManuskript (Övrigt vetenskapligt)
  • 12.
    Århammar, Cecilia
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Pietzsch, Annette
    Bock, Nicolas
    Holmstroem, Erik
    Araujo, C. Moyses
    Grasjo, Johan
    Zhao, Shuxi
    Green, Sara
    Peery, T.
    Hennies, Franz
    Amerioun, Shahrad
    Foehlisch, Alexander
    Schlappa, Justine
    Schmitt, Thorsten
    Strocov, Vladimir N.
    Niklasson, Gunnar A.
    Wallace, Duane C.
    Rubensson, Jan-Erik
    Johansson, Börje
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Ahuja, Rajeev
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Unveiling the complex electronic structure of amorphous metal oxides2011Ingår i: Proceedings of the National Academy of Sciences of the United States of America, ISSN 0027-8424, E-ISSN 1091-6490, Vol. 108, nr 16, s. 6355-6360Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Amorphous materials represent a large and important emerging area of material's science. Amorphous oxides are key technological oxides in applications such as a gate dielectric in Complementary metal-oxide semiconductor devices and in Silicon-Oxide-Nitride-Oxide-Silicon and TANOS (TaN-Al2O3-Si3N4-SiO2-Silicon) flash memories. These technologies are required for the high packing density of today's integrated circuits. Therefore the investigation of defect states in these structures is crucial. In this work we present X-ray synchrotron measurements, with an energy resolution which is about 5-10 times higher than is attainable with standard spectrometers, of amorphous alumina. We demonstrate that our experimental results are in agreement with calculated spectra of amorphous alumina which we have generated by stochastic quenching. This first principles method, which we have recently developed, is found to be superior to molecular dynamics in simulating the rapid gas to solid transition that takes place as this material is deposited for thin film applications. We detect and analyze in detail states in the band gap that originate from oxygen pairs. Similar states were previously found in amorphous alumina by other spectroscopic methods and were assigned to oxygen vacancies claimed to act mutually as electron and hole traps. The oxygen pairs which we probe in this work act as hole traps only and will influence the information retention in electronic devices. In amorphous silica oxygen pairs have already been found, thus they may be a feature which is characteristic also of other amorphous metal oxides.

  • 13.
    Århammar, Cecilia
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Silvearv, F.
    Norgren, Susanne
    Pedersen, Henrik
    Ahuja, Rajeev
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    A theoretical study of point defects incorporated into CVD-grown α-aluminaManuskript (preprint) (Övrigt vetenskapligt)
    Abstract [en]

    The energetics and electronic structure of a number of defects; C, Cl, H and S in alpha-Al2O3 is investigated. These species are present in the gas phase during the Chemical Vapor Deposition (CVD) and little is known of their solubility in alpha-Al2O3. It is found that not only the hydrogen interstitial, which is already wellknown for its dual action as a donor and acceptor defect will be amphotheric, but also the carbon and sulfur interstitial may gain both negative and positive charge states. However, at the CVD equilibrium conditions, charge compensation between the different defects will most likely not take place. For this a non equilibrium method such as Plasma Enhanced CVD or Physical Vapor Deposition (PVD) is needed.

1 - 13 av 13
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