R.E. Perälä

Yb, Eu, and (Yb + Eu)-stabilized 3 × 1 and 3 × 2 reconstructions on Si(1 1 1)

The atomic arrangement of Si(1 1 1)3 × 1 and 3 × 2 surfaces stabilized by adsorption of divalent rare-earth metals (RE=ytterbium (Yb) and europium (Eu)) in the range of 0.1–0.33 monolayer (ML) has been studied by low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM). The 3 × 2 surface is shown to have the metal coverage of 1/6 ML and possess the atomic arrangement which is independent on the nature of adsorbed RE atoms. Based on the analysis of empty and filled-state STM images, the honeycomb-chain-channel structure is proposed for Yb and Eu-induced 3 × 2 reconstructions, with the adsorption sites of metal atoms being strongly shifted from T4 or H3. A poor ordering of RE atoms in this structure results in 1/2-order LEED streaks in narrow energy range (60–80 eV). The origin of 3 × 1 reconstruction is discussed in terms of present results as well as previously published data.

Structural properties of Bi-stabilized reconstructions of GaInAs(100) surface

Bismuth (Bi) is a surfactant which controls the electronic and structural properties of epitaxially grown GaInAs and GaInP alloys. The authors have studied Bi-terminated surfaces of GaInAs(100) layers, deposited onto InP(100) substrates, by means of scanning tunneling microscopy and spectroscopy and x-ray photoelectron spectroscopy. It appears that Bi stabilizes an interesting (2×1) surface reconstruction on GaInAs(100). Under certain conditions, this reconstruction undergoes a phase transition to (2×4), which has about one-half of the Bi content of the (2×1) surface. Based on the experimental findings, they discuss mechanisms leading to this phase transition.

Formation of an ordered pattern of Bi nanolines on InAs(100) by self-assembly

Self-assembly of uniform patterns of nanolines over large surface areas has been proven to be difficult. The authors report that bismuth (Bi) adsorbate self-assembles into an ordered pattern of Bi nanolines separated by 4.3nm on the Bi-stabilized InAs(100)(2×1). The resulted nanoline surface is studied by scanning tunneling microscopy (STM) and low-energy electron diffraction. The plausible atomic models for the Bi nanolines are proposed on the basis of the STM results. The Bi lines are suggested to consist of two chains of adjacent Bi dimers positioned parallel to the chain and parallel to the Bi dimers of the (2×1) substrate.

Unoccupied electronic states of the FeS2(100) surface studied by inverse photoemission

Inverse photoemission spectra have been measured to study unoccupied electronic states from an FeS2(100) single crystal along the Γ-X symmetry line with normal incidence electrons at energies between 20 and 40 eV. Two main features of the spectra are found, corresponding to direct bulk transitions into unoccupied iron 3d(eg) and sulphur 3p(σ∗) states, respectively, followed by a prominent minimum and a onset of a band with a wide structure due to mainly transitions into Fe 4sp-S 3d bands. These peaks are located at 1.4 and 3.0 eV above the Fermi level for an initial electron energy of 19.8 eV. They disperse according to band-structure calculations. The onset of the Fe 4sp-S 3d bands is about 5.0 eV above the Fermi level. The experimental results obtained here are in a reasonable agreement with band-structure calculations for FeS2, and with previous results of optical, BIS and XAS measurements.

Atomic and electronic structure of the Yb/Ge(111)-(3x2) surface studied by high-resolution photoelectron spectroscopy

Using high-resolution synchrotron-radiation photoelectron spectroscopy and low-energy electron diffraction, we have studied the electronic and structural properties of the Yb-induced Ge(111)-(3x2) reconstruction with a 1/6 monolayer coverage. We found these properties to be similar in many respects to those of the metal-induced Si(111) and Ge(111) reconstructions described previously in terms of the honeycomb chain-channel (HCC) structure. In particular, the Yb/Ge(111)-(3x2) is revealed to have a semiconducting character, the Yb atoms are divalent, and the surface states observed for the Yb/Ge(111) closely resemble those of the Na/Ge(111)-(3x1) in the literature. The Ge 3d core-level analysis, however, shows that the Ge 3d spectra from Yb/Ge(111)-(3x2) drastically differ from corresponding spectra of other Si and Ge HCC reconstructions. An atomic model, based on the general HCC geometry, is proposed for the Yb/Ge(111)-(3x2) surface. In this model, the important structural aspects are a buckling of the Ge = Ge double bond in the top, HCC-reconstructed layer plus a strong rearrangement of the second-layer atoms. (Less)

Segregation, precipitation, and α-α′ Phase separation in Fe-Cr alloys

Segregation, precipitation, and phase separation in Fe-Cr systems is investigated. Monte Carlo simulations using semiempirical interatomic potential, first-principles total energy calculations, and experimental spectroscopy are used. 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 has to be used. Using Exact Muffin-Tin Orbitals method 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 ~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 ~10 at.%. The second atomic layer forms about 0.3 eV barrier for the migration of Cr atoms between bulk and surface atomic layer. To get information on Fe-Cr in larger scales we use semiempirical methods. Using combined Monte Carlo molecular dynamics simulations, based on semiempirical potential, the precipitation of Cr into isolated pockets in bulk Fe-Cr and the upper limit of the solubility of Cr into Fe layers in Fe/Cr layer system is studied. The theoretical predictions are tested using spectroscopic measurements. Hard X-ray photoelectron spectroscopy and Auger electron spectroscopy investigations were carried out to explore Cr segregation and precipitation in Fe/Cr double layer and Fe_0.95Cr_0.05 and Fe_0.85Cr_0.15 alloys. Initial oxidation of Fe-Cr was investigated experimentally at 10^-8 Torr pressure of the spectrometers showing intense Cr_2O_3 signal. Cr segregation and the formation of Cr rich precipitates were traced by analysing the experimental spectral intensities with respect to annealing time, Cr content, and kinetic energy of the exited electron.Iron-chromium alloys, the base components of various stainless steel grades, have numerous technologically and scientifically interesting properties. However, these features are not yet sufficientl ...

Atomistic study of surfaces and interfaces of Fe-Cr and Fe-Cr-Al alloys

Surface and interface properties of Fe-Cr, Fe-Al, and Fe-Cr-Al are studied using Exact Muffin-Tin Orbitals and Monte Carlo methods and with x-ray photoelectron and Auger electron techniques. Surface composition is investigated as a function of oxidation (heating) time. Hard x ray photoelectron spectroscopy (HAXPES) is used to scan non destructively the compositions below the surface. It is found that Cr boosts the Al segregation to the surface.

Segregation, precipitation, and α-α' phase separation in Fe-Cr alloys: a multi-scale modelling approach

Segregation, precipitation, and phase separation in Fe-Cr systems is investigated. Monte Carlo simulations using semiempirical interatomic potential, first-principles total energy calculations, and experimental spectroscopy are used. 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 has to be used. Using Exact Muffin-Tin Orbitals method 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 ~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 ~10 at.%. The second atomic layer forms about 0.3 eV barrier for the migration of Cr atoms between bulk and surface atomic layer. To get information on Fe-Cr in larger scales we use semiempirical methods. Using combined Monte Carlo molecular dynamics simulations, based on semiempirical potential, the precipitation of Cr into isolated pockets in bulk Fe-Cr and the upper limit of the solubility of Cr into Fe layers in Fe/Cr layer system is studied. The theoretical predictions are tested using spectroscopic measurements. Hard X-ray photoelectron spectroscopy and Auger electron spectroscopy investigations were carried out to explore Cr segregation and precipitation in Fe/Cr double layer and Fe_0.95Cr_0.05 and Fe_0.85Cr_0.15 alloys. Initial oxidation of Fe-Cr was investigated experimentally at 10^-8 Torr pressure of the spectrometers showing intense Cr_2O_3 signal. Cr segregation and the formation of Cr rich precipitates were traced by analysing the experimental spectral intensities with respect to annealing time, Cr content, and kinetic energy of the exited electron.Iron-chromium alloys, the base components of various stainless steel grades, have numerous technologically and scientifically interesting properties. However, these features are not yet sufficientl ...

Segregation, precipitation, and alpha-alpha ' phase separation in Fe-Cr alloys

Segregation, precipitation, and phase separation in Fe-Cr systems is investigated. Monte Carlo simulations using semiempirical interatomic potential, first-principles total energy calculations, and experimental spectroscopy are used. 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 has to be used. Using Exact Muffin-Tin Orbitals method 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 ~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 ~10 at.%. The second atomic layer forms about 0.3 eV barrier for the migration of Cr atoms between bulk and surface atomic layer. To get information on Fe-Cr in larger scales we use semiempirical methods. Using combined Monte Carlo molecular dynamics simulations, based on semiempirical potential, the precipitation of Cr into isolated pockets in bulk Fe-Cr and the upper limit of the solubility of Cr into Fe layers in Fe/Cr layer system is studied. The theoretical predictions are tested using spectroscopic measurements. Hard X-ray photoelectron spectroscopy and Auger electron spectroscopy investigations were carried out to explore Cr segregation and precipitation in Fe/Cr double layer and Fe_0.95Cr_0.05 and Fe_0.85Cr_0.15 alloys. Initial oxidation of Fe-Cr was investigated experimentally at 10^-8 Torr pressure of the spectrometers showing intense Cr_2O_3 signal. Cr segregation and the formation of Cr rich precipitates were traced by analysing the experimental spectral intensities with respect to annealing time, Cr content, and kinetic energy of the exited electron.Iron-chromium alloys, the base components of various stainless steel grades, have numerous technologically and scientifically interesting properties. However, these features are not yet sufficientl ...