Surface energy and solute strain energy effects in surface segregation

Previous models of surface segregation have generally been based on the assumption that a decrease in surface free energy constitutes the predominant driving force for the phenomenon. In contrast, grain boundary segregation models have been founded on the postulate that the major driving force for that phenomenon is the reduction in lattice strain energy which accompanies the transfer of misfitting solute atoms from the lattice to the boundary. These two concepts have been combined here into a single unified formalism of surface segregation. In addition, the temperature dependence of surface composition of both nickel-rich and gold-rich nickelgold alloys has been measured by Auger electron spectroscopy. Comparisons of the predictions of the combined formalism with the experimental results show excellent agreement between measured and calculated heats of adsorption (segregation). Furthermore, the present formalism provides estimates of the entropies of adsorption which can be used to explain apparent incompatibilities between the behavior of gold-rich and nickel-rich alloys.     

Surface segregation of Al substrate metal on Zr film surface

Segregation of substrate Al on thin Zr film, Zr/Al/Al system was investigated by heating the specimen in a UHV chamber. Dual-cathode magnetron-sputtering source was used for deposition of Zr film as well as thin Al film to avoid aluminum oxide formation at Zr/Al interface. Al segregates on Zr film surface at 730 K. It was found that oxide-free interface between film and substrate is important for segregation in Al system. The diffusion coefficient calculated for surface segregation and inter-metallic compound showed that the grain boundary diffusion and bulk diffusion are very close in Zr/Al/Al system. Hence, it is important to control specimen heating to cause surface segregation by grain boundary diffusion.     

Site competition in surface segregation

Observations have been made, by Auger electron spectroscopy, of site competition between tin, sulphur and calcium co-segregating to the free surface of otherwise pure iron in the temperature range 600–800°C. The correlations between the segregation levels of the species are explained for sub-monolayer and multilayer tin adsorption, in terms of a simple model. In the absence of calcium the reduction in available adsorption sites for tin is a function of both the tin and sulphur coverages. With calcium present the formation of surface precipitates of calcium compounds is observed. Excellent agreement is obtained between the observed interrelated behaviour of the segregants and the predictions of the model, over a wide range of experimental conditions. A similar behaviour is expected, and must be allowed for, in all studies of segregation thermodynamics and kinetics of surfaces and interfaces exhibiting more than two components.     

Surface segregation in alloys: Existence of surface miscibility gaps within regular solution theory

Much attention has been paid to the relationship between the bulk phase diagram and the phenomena of surface segregation. There is no reason to believe, however, that the equilibrium phase boundaries for the surface region are identical to those of the bulk. In this paper, we have derived the surface phase boundaries within the regular solution approximation using pairwise interactions. We find that the concept of phase separation for the surface region is not important if the surface phase is in equilibrium with the bulk and the bulk is in a region of solid solubility. The experimental conditions employed in some studies, however, lead to nonequilibrium states (due to quenching). For these cases, surface phase separation may be expected under typical experimental conditions.     

Surface segregation in HAYNES 230 alloy

The surface segregation in the Ni-based alloy HAYNES 230 was studied by Auger Electron Spectroscopy and X-ray Photoelectron Spectroscopy between 400 and 1100 °C. The qualitative variations of the surface contents of S, P, W, Mo, N, Si, and Mn were determined as a function of annealing temperature and time. It was found that at 925 °C the maximum coverage of sulphur at the alloy surface is in the range 0.06-0.15 monolayers. Chromium evaporation from the HAYNES 230 surface under UHV conditions is clearly evidenced for annealing at 1100 °C.     

Surface segregation and backscattering in doped silicon nanowires

By means of ab initio simulations, we investigate the structural, electronic, and transport properties of boron and phosphorus doped silicon nanowires. We find that impurities always segregate at the surface of unpassivated wires, reducing dramatically the conductance of the surface states. Upon passivation, we show that for wires as large as a few nanometers in diameter, a large proportion of dopants will be trapped and electrically neutralized at surface dangling bond defects, significantly reducing the density of carriers. Important differences between p- and n-type doping are observed. Our results rationalize several experimental observations.     

Kinetics of surface segregation in alloys

The kinetics of surface segregation in ordering alloys are studied with the help of a multilayer model in slabs of different thicknesses. The time evolution of the concentrations of atomic layers perpendicular to the (100) planes of an AB-type BCC ordering alloy (CoFe) are calculated. As a result of the competition between the surface segregation of A atoms and ordering in the bulk, a metastable configuration with two anti-phase boundaries (APB) inside the slab, with odd numbers of layers, was obtained if we started from an ordered initial state with B atoms on the free surfaces (B-termination). The effect of the temperature was also investigated for a slab with 41 layers and from the calculation of the free energies of the metastable and stable states it was shown that the metastable state can exist below T₁ = 0.95T_c, where T_c is the critical temperature of the order-disorder transition in bulk. It was obtained that T₁ decreases with decreasing thickness of the slab, while the difference of free energies of the metastable and stable states slightly increases. The effect of the slab thickness is a typical size effect: the time necessary to reach the steady state decreases with decreasing number of layers.     

Surface gap and surface electronic states in CuGeO3 single crystal

The surface and bulk electronic excitations of CuGeO₃ are investigated by means of electron energy loss and polarized X-ray absorption spectroscopy. CuGeO₃ shows a surface charge transfer gap of about 3.0±0.3 eV. The unoccupied oxygen derived density of states, as probed by X-ray absorption at the O 1s edge, is in good agreement with recent many-body calculations.     

Femtosecond evolution of electronic interactions at the Ni(111) surface

We used time-resolved photoemission spectroscopy to measure the changes in electron self energy for the occupied surface resonance near the surface Brillouin zone center of Ni/W(110) single crystalline films following optical excitations with a fs laser pulse. Binding energy and lifetime display pronounced variations on a 100 fs time scale. We provide evidence that this is directly linked to the laser induced population of electronic levels. PACS 73.20.-r; 73.50.Gr; 78.47.+p; 79.60.Bm     

Surface segregation in decomposing carbide solid solutions

The surface segregation of ZrC carbide from the ZrC_0.82-NbC_0.83 solid solutions has been investigated using the X-ray diffraction, electron microscopy, X-ray microanalysis, and laser mass analysis. A method for including the energy of the interfaces in the free energy of mixing, which ensures the determination of the compositions of the solid solution in which the surface segregation can be observed, has been proposed for the carbide solid solution.     

Waveform interactions and the segregation of concurrent vowels

Two experiments investigated the effects of small values of fundamental frequency difference (delta F0) on the identification of concurrent vowels. As delta F0's get smaller, mechanisms that exploit them must necessarily fail, and the pattern of breakdown may tell which mechanisms are used by the auditory system. Small delta F0's also present a methodological difficulty. If the stimulus is shorter than the beat period, its spectrum depends on which part of the beat pattern is sampled. A different starting phase might produce a different experimental outcome, and the experiment may lack generality. The first experiment explored the effects of delta F0's as small as 0.4%. The smallest delta F0 conditions were synthesized with several starting phases obtained by gating successive segments of the beat pattern. An improvement in identification was demonstrated for delta F0's as small as 0.4% for all segments. Differences between segments (or starting phase) were also observed, but when averaged over vowel pairs they were of small magnitude compared to delta F0 effects. The nature of delta F0-induced waveform interactions and the factors that affect them are discussed in detail in a tutorial section, and the hypothesis that the improvement in identification is the result of such interactions (beat hypothesis) is examined. It is unlikely that this hypothesis can account for the effects observed. The reduced benefit of delta F0 for identification at smaller delta F0's more likely reflects the breakdown of the same F0-guided segregation mechanism that operates at larger delta F0's.     

Atomic structure of PtCu nanoparticles in PtCu/C catalysts from EXAFS spectroscopy data

Deposited electrocatalysts with different distributions of components in PtCu bimetallic nanoparticles involved in their composition were synthesized by simultaneous and sequential reduction of Cu(2+) and Pt(IV) in a carbon suspension. The dependence of the atomic structure of PtCu nanoparticles on the synthesis conditions and the degree of influence of post-treatment was established from analysis of the changes in Fourier transforms of the experimental Pt and Cu EXAFS spectra, as well as the structural parameters obtained by their fitting before and after the treatment of the materials in an acid solution. A technique was proposed for visualizing the atomic structure of synthesized bimetallic nanoparticles. This technique made it possible to determine the character of the distribution of the components over the nanoparticle volume in accordance with the component composition and local atomic structure parameters determined from EXAFS spectroscopy and to obtain the visualization of the distribution of the components in PtCu nanoparticles synthesized by the aforementioned methods.     

Electronic theory of surface segregation in the coherent potential approximation: The surface segregation in CoNi, IrPt and RhPt

The surface segregation in CoNi, IrPt and RhPt is calculated within the coherent potential approximation. It is assumed that the metals form substitutional alloys and only diagonal disorder is taken into account. The condition of the local neutrality resulting in the presence of diagonal potentials in the Hamiltonian is also introduced. The alloys are selected in such a way that: (i) the non-diagonal disorder can be neglected with a relatively good approximation, (ii) the d-band filling is not too large to ensure the procedure of determining diagonal potentials from the local neutrality condition to work properly and (iii) the experimental data are available.     

Antisite-defect-induced surface segregation in ordered NiPt alloy

By means of first principles simulations we demonstrate that tiny deviations from stoichiometry in the bulk composition of the NiPt-L1(0) ordered alloy have a great impact on the atomic configuration of the (111) surface. We predict that at T=600 K the (111) surface of the Ni51Pt49 and Ni50Pt50 alloys corresponds to the (111) truncation of the bulk L1(0) ordered structure. However, the (111) surface of the nickel deficient Ni49Pt51 alloy is strongly enriched by Pt and should exhibit the pattern of the 2x2 structure. Such a drastic change in the segregation behavior is due to the presence of different antisite defects in the Ni- and Pt-rich alloys and is a manifestation of the so-called off-stoichiometric effect.