| 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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| 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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