| 1. |
- Klein, Andreas, et al.
(författare)
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The Fermi energy as common parameter to describe charge compensation mechanisms: A path to Fermi level engineering of oxide electroceramics
- 2023
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Ingår i: Journal of Electroceramics. - 1573-8663 .- 1385-3449. ; 51
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Tidskriftsartikel (refereegranskat)abstract
- Chemical substitution, which can be iso- or heterovalent, is the primary strategy to tailor material properties. There are various ways how a material can react to substitution. Isovalent substitution changes the density of states while heterovalent substitution, i.e. doping, can induce electronic compensation, ionic compensation, valence changes of cations or anions, or result in the segregation or neutralization of the dopant. While all these can, in principle, occur simultaneously, it is often desirable to select a certain mechanism in order to determine material properties. Being able to predict and control the individual compensation mechanism should therefore be a key target of materials science. This contribution outlines the perspective that this could be achieved by taking the Fermi energy as a common descriptor for the different compensation mechanisms. This generalization becomes possible since the formation enthalpies of the defects involved in the various compensation mechanisms do all depend on the Fermi energy. In order to control material properties, it is then necessary to adjust the formation enthalpies and charge transition levels of the involved defects. Understanding how these depend on material composition will open up a new path for the design of materials by Fermi level engineering.
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| 2. |
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| 3. |
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| 4. |
- Canovic, Sead, 1979, et al.
(författare)
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TEM and DFT investigation of CVD TiN/κ–Al2O3 multilayer coatings
- 2007
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Ingår i: Surface and Coatings Technology. - : Elsevier BV. - 0257-8972. ; 202:3, s. 522-531
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Tidskriftsartikel (refereegranskat)abstract
- This paper investigates the interfacial structure in hot-wall CVD TiN/kappa-Al2O3 multilayer coatings using both HREM and DFT modeling. Two multilayers with different thicknesses of the TiN layers (50 and 600 nm) separating the kappa-Al2O3 layers are analyzed. The general microstructure of the two multilayers is relatively similar. The TiN layer in the thicker TiN/kappa-Al2O3 coating is thick enough to be several TiN grains high. This means that epitaxial columns, which are often found in the thinner TiN/kappa-Al2O3 coatings, are not present. However, the orientation relationships at the TiN/kappa-Al2O3 interfaces are the same in both multilayers. The HREM investigations show that kappa-Al2O3 (001) planes can grow directly on flat (111) TiN faces, without any other phases or detectable amounts of impurities, such as sulphur, present. Where the TiN layers are more curved, gamma-Al2O3 can be grown, at least partly stabilized by the cube-on-cube orientation relationship between gamma-Al2O3 and the underlying TiN. The DFT calculations show very similar adsorption strengths for an 0 monolayer positioned on Ti-terminated TiC(111) and TiN(111) surfaces, with preferred adsorption in the fee site. 0 adsorption on N-terminated TiN(111) is much weaker, with preferred adsorption in the top site. Calculated elastic-energy contributions yield a higher stability for kappa-Al2O3 on TiN(111) than on TiC(111) and a higher stability for kappa-Al2O3 than for alpha-Al2O3 on both TiC and TiN. This indicates that the observed higher stability Of kappa-Al2O3 on TiC(111) than on TiN(111) is not due to the lattice mismatch, while the preferred epitaxial growth of kappa-Al2O3 over alpha-Al2O3 can be partly attributed to the mismatch.
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| 5. |
- Rohrer, Jochen, 1978, et al.
(författare)
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Ab initio structure modelling of complex thin-film oxides: thermodynamical stability of TiC/thin-film alumina
- 2010
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Ingår i: Journal of Physics Condensed Matter. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 22:1, s. 015004-
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Tidskriftsartikel (refereegranskat)abstract
- We present a strategy to identify energetically favourable oxide structures in thin-film geometries. Thin-film candidate configurations are constructed from a pool of sublattices of stable and metastable oxide bulk phases. Favourable stoichiometric compositions and atomic geometries are identified by comparing total and Gibbs free energies of the relaxed configurations. This strategy is illustrated for thin-film alumina on TiC, materials which are commonly fabricated by chemical vapour deposition (CVD) and used as wear-resistant multilayer coatings. Based on the standard implementation of ab initio thermodynamics, with an assumption of equilibrium between molecular O2 and the oxide, we predict a stability preference of TiC/alumina configurations that show no binding across the interface. This result is seemingly in conflict with the wear-resistant character of the material and points towards a need for extending standard ab initio thermodynamics to account for relevant growth environments.
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| 6. |
- Rohrer, Jochen, 1978, et al.
(författare)
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Ab initio thermodynamics of deposition growth: Surface terminations of TiC(111) and TiN(111) grown by chemical vapor deposition
- 2010
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Ingår i: Physical Review B - Condensed Matter and Materials Physics. - 2469-9950 .- 2469-9969. ; 82:4
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Tidskriftsartikel (refereegranskat)abstract
- We present a calculational method to predict terminations of growing or as-deposited surfaces as a function of the deposition conditions. Such characterizations are valuable for understanding catalysis and growth phenomena. The method combines ab initio density-functional-theory calculations and experimental thermodynamical data with a rate-equations description of partial pressures in the reaction chamber. The use of rate equations enables a complete description of a complex gas environment in terms of a few, (experimentally accessible) parameters. The predictions are based on comparisons between free energies of reaction associated with the formation of surfaces with different terminations. The method has an intrinsic nonequilibrium character. In the limit of dynamic equilibrium (with equal chemical potential in the surface and the gas phase) we find that the predictions of the method coincide with those of standard ab initio based equilibrium thermodynamics. We illustrate the method for chemical vapor deposition of TiC (111) and TiN (111), and find that the emerging termination can be controlled both by the environment and the growth rate.
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| 7. |
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| 8. |
- Rohrer, Jochen, 1978, et al.
(författare)
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Computational scheme for ab-initio predictions of chemical compositions interfaces realized by deposition growth
- 2010
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- We present a novel computational schemeto predict chemical compositions at interfacesas they emerge in a growth process.The scheme uses the Gibbs free energy of reaction associatedwith the formation of interfaces with a specific compositionas predictor for their prevalence.It explicitly accounts for the growth conditions by rate-equation modeling of the deposition environment.We illustrate the scheme for characterizing the interfacebetween TiC and alumina.
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| 9. |
- Rohrer, Jochen, 1978, et al.
(författare)
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Computational scheme for ab-initio predictions of chemical compositions interfaces realized by deposition growth
- 2011
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Ingår i: Computer Physics Communications. - : Elsevier BV. - 0010-4655. ; 182:9, s. 1814-1818
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Tidskriftsartikel (refereegranskat)abstract
- We present a novel computational scheme to predict chemical compositions at interfaces as they emerge in a growth process. The scheme uses the Gibbs free energy of reaction associated with the formation of interfaces with a specific composition as predictor for their prevalence. It explicitly accounts for the growth conditions by rate-equation modeling of the deposition environment. The Bell-Evans-Polanyi principle motivates our emphasis on an effective nonequilibrium thermodynamic description inspired by chemical reaction theory. We illustrate the scheme by characterizing the interface between TiC and alumina. Equilibrium thermodynamics favors a nonbinding interface, being in conflict with the wear-resistant nature of TiC/alumina multilayer coatings. Our novel scheme predicts that deposition of a strongly adhering interface is favored under realistic conditions.
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| 10. |
- Rohrer, Jochen, 1978
(författare)
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Formation stability and electronic structure of surfaces and interfaces from first principles
- 2010
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis deals with two closely interwoven aspects of first-principle(density functional theory) materials modeling:(1)~prediction of atomic structure & chemical composition,and (2)~prediction of electronic properties.In the first part, we focus on atomic structure (AS) and chemical composition (CC) of surface and interface systems.These systems have a large range of technical applications,building on both mechanical strength and electron behavior.Surface and interface systems are often fabricatedin complex gas-phase deposition environments.Characterizing and predicting AS and CCis an important challenge and understanding of how these result ina growth environment isof particular interest.We formulate a novel nonequilibrium thermodynamic methodto predict AS and CC as a function of the deposition environment.The method combines first-principle calculations withchemical reaction theoryand rate-equation modeling.We implement this method and use it to illustrate its predictive powerfor characterizing AS and CCat industrially relevant interfaces between alumina and titanium carbide,grown by chemical vapor deposition.Our predictions of AS and CC result in adhesion properties that agree withthe wear-resistant nature of TiC/alumina multilayers;equilibrium predictions do not.This result suggests that our method is a useful theoretical toolfor characterizing materials whose AS and CC is determinedby the specific deposition conditions.In the second part, we investigate the relevance of van der Waals (vdW)interactions for electronic properties.We focus on vdW binding in graphene overlayers at silicon carbide surfacesand in multilayers of graphane (a fully hydrogenated derivative of graphene).These materials are promising candidates for future electronicdevices.Performing band-structure calculations and wave-function analysis,we find that vdW binding to a neighboring layer or substratecan significantly alter the electronic behavior and in particular the band structure.Our calculations predict stronglocal band-gap modifications in insulating graphane multilayersdue to vdW interactions.We also documentthat vdW binding effectively amounts to a dopingof graphene overlayers at SiC surfaces.
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| 11. |
- Rohrer, Jochen, 1978, et al.
(författare)
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Relative stability of 6H-SiC(0001) surface terminations and formation of graphene overlayers by Si evaporation
- 2010
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- We present density functional theory (DFT) calculations for 6H-SiC{0001} surfaces with different surface stackings, terminations and reconstructions.We compare the relative stability of different (0001) and different (000-1)surfaces in terms of their surface free energies.Removing surface and subsurface Si atoms, we simulate the formation ofgraphene and graphene-like overlayers by Si evaporation.We find that overlayers with a different nature of bonding arepreferred at the two non-equivalent surface orientations.At (0001), a chemically bonded, highly strained and buckled film is predicted.At (000-1), a van der Waals (vdW) bonded overlayer is preferred.We quantify the vdW binding and show that it can have a doping effect onelectron behavior in the overlayer.
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| 12. |
- Rohrer, Jochen, 1978, et al.
(författare)
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Stacking and band structure of van der Waals bonded graphane multilayers
- 2010
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- We use density functional theoryand the van der Waals density functional (vdW-DF) methodto determine the binding separation in bilayer and bulk graphaneand study the corresponding electronic band structure.The calculated binding separation (distance between center-of-mass planes)and binding energy are 4.5-5.0 Å (4.5-4.8 Å) and 75-102 meV/cell (93-127 meV/cell)in the bilayer (bulk), depending on the choice of vdW-DF version.We obtain the corresponding band diagrams using ordinary GGA calculationsfor the geometries specified by our vdW-DF results.We find significant band-gap modifications by up to -1.2 eV (+4.0 eV)in various regions of the Brillouin zone,produced by the bilayer (bulk) formation.The possibility of such large modifications signalsa potential of vdW-induced band-gap engineering in other materials
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| 13. |
- Rohrer, Jochen, 1978, et al.
(författare)
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Stacking and band structure of van der Waals bonded graphane multilayers
- 2011
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Ingår i: Physical Review B - Condensed Matter and Materials Physics. - 2469-9950 .- 2469-9969. ; 83:16
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Tidskriftsartikel (refereegranskat)abstract
- We use density functional theory and the van der Waals density functional (vdW-DF) method to determine the binding separation in bilayer and bulk graphane and study the changes in electronic band structure that arise with the multilayer formation. The calculated binding separation (distance between center-of-mass planes) and binding energy are 4.5 -5.0 angstrom (4.5 - 4.8 angstrom) and 75 - 102 meV/cell (93 - 127 meV/cell) in the bilayer (bulk), depending on the choice of vdW-DF version. We obtain the corresponding band diagrams using calculations in the ordinary generalized gradient approximation for the geometries specified by our vdW-DF results, so probing the indirect effect of vdW forces on electron behavior. We find significant band-gap modifications by up to -1.2 eV (+ 4.0 eV) in various regions of the Brillouin zone, produced by the bilayer (bulk) formation.
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| 14. |
- Rohrer, Jochen, 1978
(författare)
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Structure of thin-film oxides: an ab initio study of TiC/Alumina
- 2008
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Oxides and oxide films play a major role in present-daytechnologies. Identification and analysis of their atomic andelectronic structure are important to develop new functionalmaterials. At the same time, the ionic character, complexity,and structural flexibility make accurateatomic structure determinationsdifficult, both from the experimental and the theoreticalpoint of view.This licentiate thesis presents an efficientand general method to identify promisingcandidates for oxide thin films and to study theirstructural elements using ab initiodensity functional theory calculations.Thin films generated from building blocksof the complex bulk structure of (meta-)stableoxides form a well-defined network of initial configurations.Strong atomic relaxations are predicted andcharacterized by ab initio calculations.We order the resulting,relaxed thin-film candidatesaccording to variations in ab initiototal energy and ab initio thermodynamic Gibbsfree energy. The relaxed structures for the mostfavorable films provide insight on the atomicconfiguration of the truly stable and metastable films.The method is illustrated and testedfor thin-film alumina (Al2O3)on a titanium carbide substrate (TiC).The TiC/alumina system plays an importantrole for wear-resistant coatings grown bychemical-vapor deposition (CVD).The bulk structures of the stable alpha-and metastable kappa-Al2O3lead to the identification of 38 initial thin-filmconfigurations for a given film thickness n,including three different stoichiometric compositions,Al_{4n-4}O_{6n}, Al_{4n-2}O_{6n}, and Al_{4n}O_{6n}.The results of extensive density functional theorystudies show that the energetically most favorablethin films differ heavily from theirinitial structures and possess up to 60 %tetrahedrally coordinated Al ions.This is considerably more than in the mainbulk-alumina crystal structures. The method iscapable of catching thin film candidates withstructural building blocks that are not explicitlyincluded in the network of initial configurations.The results of this licentiate thesis can haveimplications for the theory description of CVD growth ofwear-resistant TiC/alumina multilayer coatings. We find thatthe thermodynamically favored TiC/Al_{4n-4}O_{6n} systems,stable in the physically relevant range of the oxygen chemicalpotential, separate into a tightly bound O-monolayer onTiC (TiC/O) and a weakly bound Al_{4(n-1)}O_{6(n-1)}overlayer. Strongly binding filmsare predicted to be stabilized onlyat extreme UHV conditions or by Ti defects between the tightlybound O-monolayer and the alumina overlayer. The thesissuggests that the nonequilibrium nature of the CVD growthenvironment plays an important role in securing the necessarystrength of the TiC/alumina binding and the optimal aluminanucleation.
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| 15. |
- Rohrer, Jochen, 1978, et al.
(författare)
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Understanding adhesion at as-deposited interfaces from ab initio thermodynamics of deposition growth: thin-film alumina on titanium carbide
- 2010
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Ingår i: Journal of Physics Condensed Matter. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 22:47, s. 472001-
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Tidskriftsartikel (refereegranskat)abstract
- We investigate the chemical composition and adhesion of chemical vapour deposited thin-film alumina on TiC using and extending a recently proposed nonequilibrium method of ab initio thermodynamics of deposition growth (AIT-DG) (Rohrer and Hyldgaard 2010 Phys. Rev. B 82 045415). A previous study of this system (Rohrer et al 2010 J. Phys.: Condens. Matter 22 015004) found that use of equilibrium thermodynamics leads to predictions of a non-binding TiC/alumina interface, despite its industrial use as a wear-resistant coating. This discrepancy between equilibrium theory and experiment is resolved by the AIT-DG method which predicts interfaces with strong adhesion. The AIT-DG method combines density functional theory calculations, rate-equation modelling of the pressure evolution of the deposition environment and thermochemical data. The AIT-DG method was previously used to predict prevalent terminations of growing or as-deposited surfaces of binary materials. Here we extend the method to predict surface and interface compositions of growing or as-deposited thin films on a substrate and find that inclusion of the nonequilibrium deposition environment has important implications for the nature of buried interfaces.
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