| 11. |
- Kuronen, A., et al.
(författare)
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Segregation, precipitation, and alpha-alpha ' phase separation in Fe-Cr alloys
- 2015
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 92:21
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Tidskriftsartikel (refereegranskat)abstract
- Iron-chromium alloys, the base components of various stainless steel grades, have numerous technologically and scientifically interesting properties. However, these features are not yet sufficiently understood to allow their full exploitation in technological applications. In this work, we investigate segregation, precipitation, and phase separation in Fe-Cr systems analyzing the physical mechanisms behind the observed phenomena. To get a comprehensive picture of Fe-Cr alloys as a function of composition, temperature, and time the present investigation combines Monte Carlo simulations using semiempirical interatomic potential, first-principles total energy calculations, and experimental spectroscopy. In order to obtain a general picture of the relation of the atomic interactions and properties of Fe-Cr alloys in bulk, surface, and interface regions several complementary methods have to be used. Using the exact muffin-tin orbitals method with the coherent potential approximation (CPA-EMTO) the effective chemical potential as a function of Cr content (0-15 at. % Cr) is calculated for a surface, second atomic layer, and bulk. At similar to 10 at. % Cr in the alloy the reversal of the driving force of a Cr atom to occupy either bulk or surface sites is obtained. The Cr-containing surfaces are expected when the Cr content exceeds similar to 10 at. %. The second atomic layer forms about a 0.3 eV barrier for the migration of Cr atoms between the bulk and surface atomic layer. To get information on Fe-Cr in larger scales we use semiempirical methods. However, for Cr concentration regions less than 10 at. %, the ab initio (CPA-EMTO) result of the important role of the second atomic layer to the surface is not reproducible from the large-scale Monte Carlo molecular dynamics (MCMD) simulation. On the other hand, for the nominal concentration of Cr larger than 10 at. % the MCMD simulations show the precipitation of Cr into isolated pockets in bulk Fe-Cr and the existence of the upper limit of the solubility of Cr into Fe layers in Fe/Cr layer systems. For high Cr concentration alloys the performed spectroscopic measurements support the MCMD simulations. Hard x-ray photoelectron spectroscopy and Auger electron spectroscopy investigations were carried out to explore Cr segregation and precipitation in the Fe/Cr double layer and Fe0.95Cr0.05 and Fe0.85Cr0.15 alloys. Initial oxidation of Fe-Cr was investigated experimentally at 10(-8) Torr pressure of the spectrometers showing intense Cr2O3 signal. Cr segregation and the formation of Cr-rich precipitates were traced by analyzing the experimental atomic concentrations and chemical shifts with respect to annealing time, Cr content, and kinetic energy of the exited electron.
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| 12. |
- Lang, J. J. K., et al.
(författare)
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Ab initio and scanning tunneling microscopy study of an indium-terminated GaAs(100) surface : An indium-induced surface reconstruction change in the c(8x2) structure
- 2010
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 81:24, s. 245305-
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Tidskriftsartikel (refereegranskat)abstract
- Technologically useful indium- (In) terminated c(8 x 2)-reconstructed GaAs(100) substrate surface has been studied by first-principles calculations and scanning tunneling microscopy (STM) measurements. Our total-energy calculations demonstrate the stability of four different so-called zeta a structures with In monomer rows and In coverage between 0.5 and 2 monolayers on the GaAs(100) substrate. Thus, we introduce a surface system, which stabilizes the zeta a reconstruction. Furthermore, an interesting trend is found. Atomic structure of the c(8 x 2) reconstruction depends on the surface-layer cation and substrate volumes, which, in principle, allows to tune the surface structure by cation adsorption. This phenomenon is related to the peculiar c(8 x 2) atomic surface structure, which shows mixed surface layer, including both anions and cations, and uncommon metallic-type cations in the zeta a structure, which do not show covalent bonds. Our results predict a structural transition from the zeta structure to the zeta a structure as the surface cation size is increased at 0 K. The found transition is probably related to the disordered surface structures (consisting of zeta and zeta a building blocks) found experimentally by x-ray diffraction at room temperature. Comparison of the STM images, calculated for various c(8 x 2) models, with the former and present measured STM images of In/GaAs(100) c(8 x 2) supports the presence of stable zeta a reconstructions.
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| 13. |
- Punkkinen, M. P. J., et al.
(författare)
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Calculation of surface core-level shifts within complete screening : Problems with pseudohydrogenated slabs
- 2008
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 77:24
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Tidskriftsartikel (refereegranskat)abstract
- By considering GaAs(110) and Si(100)(2x1) surfaces, it is shown that the use of the pseudohydrogen atoms to saturate the dangling bonds at one side of the slab modeling a free surface can lead to distorted surface core-level shifts within the complete screening picture. The effect is linked to the polarization of the slab and to the change in the bulklike electronic structure close to pseudohydrogenated part of the slab. It is demonstrated that these problems can be avoided if the pseudohydrogenated slab is large enough and the bulk reference layer is properly chosen. One easy way to control these errors is to monitor the planar-averaged potential energy curve of the slab.
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| 14. |
- Punkkinen, Marko P. J., et al.
(författare)
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Core-level shifts of InP(100)(2 x 4) surface : Theory and experiment
- 2009
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Ingår i: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 603:16, s. 2664-2668
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Tidskriftsartikel (refereegranskat)abstract
- Surface core-level shifts (SCLSs) of the (2 x 4)-reconstructed InP(100) surface with the established mixed In-P dimer structure have been investigated by first-principles calculations and photoelectron spectroscopy. Theoretical values were calculated using both the local density approximation (LDA) and the generalized gradient approximation (GGA) for the exchange-correlation energy functional. The obtained theoretical values are quite similar within both approximations. The found differences originate in the tiny structural differences. It is concluded that the expansion or contraction of the crystal lattice has smaller effect on the SCLSs than the geometrical details of the reconstruction, which suggests that the Madelung potential has the dominant effect on the SCLSs. The results support the presence of a P 2p peak at higher binding energy (BE) compared to bulk peak, as proposed with recent measurements [P. Laukkamen, J. Pakarinen, M. Ahola-Tuomi, M. Kuzmin, R. E. Perala, I. J. Vayrynen, A. Tukiainen, V. Rimpilainen, M. Pessa, M. Adell, J. Sadowski, Surf. Sci. 600 (2006) 3022], and reveal several hitherto not reported SCLSs. The calculated SCLSs reproduce the measured spectra within reasonable accuracy. Furthermore, the atomic origins of the InP(100)(2 x 4) SCLSs are solved. In particular, it is shown that the lowest SCLS of P 2p, of the InP(100)(2 x 4) arises from the topmost In-P dimers.
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| 15. |
- Punkkinen, Marko P. J., et al.
(författare)
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Stability and structure of rare-earth metal and Ba-induced reconstructions on a Si(100) surface
- 2009
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 80:23
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Tidskriftsartikel (refereegranskat)abstract
- We have studied, by means of ab initio calculations, the energetics and the atomic and electronic structures of various reconstructions induced by rare-earth metals (RE=Eu, Nd, Sm, and Yb) and Ba on Si(100) in the coverage range up to 0.5 monolayer. It is shown that Si dimer buckling is an important structural element for such systems, leading frequently to oblique surface lattice symmetries. The strong metal atom-silicon binding favors the increased amount of metal atoms per unit surface area, i.e., the (2x3) reconstruction with two metal atoms per unit cell is found to be energetically unstable with respect to the (2x1) reconstruction with three metal atoms per the same surface area [Eu/Si(100) and Yb/Si(100)]. The influence of the atomic size and the valence of the adsorbates is also investigated. In particular, it is found that an increase in atomic size stimulates the metal-metal repulsion, stabilizing the (2x3) configuration [Ba/Si(100)]. In the case of trivalent metals, the stabilization of the (2x3) is mediated by the loss of semiconducting state in the competing phases [Sm/Si(100) and Nd/Si(100)]. Our results demonstrate the importance of many factors, which account for the abundance of RE/Si(100) reconstructions. Finally, prominent atomic models are proposed for (2x3) and (2x6) reconstructions, and the character of the wavy "(1x2)" reconstruction is discussed. The simulated scanning tunneling microscopy images for the proposed (2x6) reconstruction are in a particularly good agreement with the complex experimental images.
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| 16. |
- Punkkinen, M. P. J., et al.
(författare)
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Surface core-level shifts of GaAs(100)(2x4) from first principles
- 2007
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 76:11
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Tidskriftsartikel (refereegranskat)abstract
- First-principles calculations show that measured surface core-level shifts (SCLSs) of the GaAs(100)(2x4) surfaces can be described within the initial state effects. The calculated As 3d and Ga 3d SCLSs for the beta 2 and alpha 2 reconstructions of the GaAs(100)(2x4) surfaces are in reasonable agreement with recent measurements. In particular, the results confirm that both the lower and the higher binding energy SCLSs, relative to the bulk emission in the As 3d photoelectron spectra, are intrinsic properties of the GaAs(100)(2x4) surfaces. The most positive and most negative As shifts are attributed to the third layer As atoms, which differs from the previous intuitive suggestions. In general, calculations show that significant SCLSs arise from deep layers, and that there are more than two SCLSs. Our previously measured As 3d spectra are fitted afresh using the calculated SCLSs. The intensity ratios of the SCLSs, obtained from the fits, show that as the heating temperature of the GaAs(100)(2x4) surface is increased gradually, the area of the alpha 2 reconstruction increases on the surface, but the beta 2 phase remains within the whole temperature range, in agreement with previous experimental findings. Our results show that the combination of the experimental and theoretical results is a prerequisite for the accurate analysis of the SCLSs of the complex reconstructed surfaces.
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