Surface segregation of InGaAs films by the evolution of reflection high-energy electron diffraction patterns

Surface segregation is studied via the evolution of reflection high-energy electron diffraction （RHEED） patterns under different values of As4 BEP for InGaAs films. When the As4 BEP is set to be zero, the RHEED pattern keeps a 4x3/（nx3） structure with increasing temperature, and surface segregation takes place until 470 ℃ The RHEED pattern develops into a metal-rich （4x2） structure as temperature increases to 495℃. The reason for this is that surface segregation makes the In inside the InGaAs film climb to its surface. With the temperature increasing up to 515℃, the RHEED pattern turns into a GaAs（2x4） structure due to In desorption. While the As4 BEP comes up to a specific value （1.33 x 10-4 Pa-1.33 x 10-3 Pa）, the surface temperature can delay the segregation and desorption. We find that As4 BEP has a big influence on surface desorption, while surface segregation is more strongly dependent on temperature than surface desorption.

Integral PVA-PES Composite Membranes by Surface Segregation Method for Pervaporation Dehydration of Ethanol

A facile surface segregation method was utilized to fabricate poly（vinyl alcohol）-polyethersulfone （PVA-PES） composite membranes. PVA and PES were first dissolved in dimethyl sulfoxide （DMSO）, then casted on a glass plate and immersed in a coagulation bath. During the phase inversion process in coagulation bath, PVA spontaneously segregated to the polymer solution/coagulation bath interface. The enriched PVA on the surface was further crosslinked by glutaraldehyde. Scanning electron microscopy （SEM）, X-ray photoelectron spectroscopy （XPS） and energy dispersive spectrometer （EDS） confirmed the integral and asymmetric membrane structure with a dense PVA-enriched surface and a porous PES-enriched support, as well as the surface enrichment of PVA. The coverage fraction of the membrane surtace by PVA reacned up to 86.8% when me PVA content m me membrane recipe was 16.7% （by mass）. The water contact angle decreased with the increase of PVA content. The effect of coagulation bath type on membrane structure was analyzed. The membrane pervaporation performance was evaluated by varying the PVA content, the annealing temperature, feed concentration and operation temperature. The membrane exhibited a fairly good ethanol dehydration capacity and long-term operational stability.

Effects of rapid thermal annealing on crystallinity and Sn surface segregation of Ge1-xSnx films on Si (100) and Si (111)

Germanium-tin films with rather high Sn content （28.04% and 29.61%） are deposited directly on Si （100） and Si （111） substrates by magnetron sputtering. The mechanism of the effect of rapid thermal annealing on the Sn surface segregation of Ge1-xSnx films is investigated by x-ray photoelectron spectroscopy （XPS） and atomic force microscopy （AFM）. The x-ray diffraction （XRD） is also performed to determine the crystallinities of the Ge1-xSnx films. The experimental results indicate that root mean square （RMS） values of the annealed samples are comparatively small and have no noticeable changes for the as-grown sample when annealing temperature is below 400℃. The diameter of the Sn three-dimensional （3D） island becomes larger than that of an as-grown sample when the annealing temperature is 700℃. In addition, the Sn surface composition decreases when annealing temperature ranges from 400℃ to 700℃. However, Sn bulk compositions in samples A and B are kept almost unchanged when the annealing temperature is below 600℃. The present investigation demonstrates that the crystallinity of Ge1-xSnx/Si （111） has no obvious advantage over that of Ge1-xSnx/Si （100） and the selection of Si （111） substrate is an effective method to improve the surface morphologies of Ge1-xSnx films. We also find that more severe Sn surface segregation occurs in the Ge1-xSnx/Si （111） sample during annealing than in the Ge1-xSnx/Si （100） sample.

Surface segregation of copper in antibacterial ferritic stainless steel

By using auger electron spectroscopy （AES） and diffusion theory to analyze the surface segregation of copper in antibacterial ferritic stainless steel, establishing a diffusion model, and calculating the activation energy of diffusion of the copper in ferrite, the affect of surface segregation on the antibacterial capabilities were researched. The results show that the concentration of the copper surface at 973 K and 1 073 K could be expressed asln X^sCu/X^bCu = k0 √Dt/d（-△Hv^Cu＋△Hf^tron＋ △Hs^Cu）/3RT , with the parameters relating to the concentration of the diffusion layer, the coefficient of diffusion, the length of diffusion, the latent heat of evaporation and the latent heat of fusion. The activation energy of diffusion of copper in ferrite is approximately 221. 688 kJ/mol. The antibacterial property of the steel is improved as the surface segregation of the copper is increased. At 1 073 K for 60 min, the concentration of the surface copper is over three times higher than the basic concentration. The antibacterial property of the stainless steel can reach approximately 99.9%.

SAM AND XPS STUDIES ON SURFACE SEGREGATION OF SUBSTRATE ELEMENTS IN Au-Ni-KOVAR ALLOY SYSTEM

The surface segregation of substrate elements through the Au layer in the Au-Ni-Kovar sys- tem specimens heated at 250—350℃ for 0.5—1.5 h has been studied by SAM and XPS. Visual evidence of the mechanism for Ni and Co surface segregation of the complicated sys- tem has been given,i.e.Ni and Co penetrated the gold layer mainly by grain boundary diffu- sion and then covered the Au layer by surface diffusion.The strdy results of the chemical states of surface segregation elements not only indicate that oxygen adsorption and oxidation reaction are the driving force for the surface segregation of Ni and Co,but also show that the above segregation can result in water absorption on surfaces.

Understanding the surface segregation behavior of bimetallic CoCu toward HMF oxidation reaction

Surface segregation is ubiquitous in multi-component materials and is of great important for catalysis but little is known on the surface structure under graphene encapsulation.Here,we show that the graphene encapsulated CoCu performs well for the electrocatalytic oxidation of 5-hydroxymethylfurfural(HMF)to2,5-furandicarboxylic acid(FDCA)with the onset potential before 1.23 VRHEand a nearly 100%selectivity of FDCA under 1.4 VRHE.From the experimental results,the unprecedented catalytic performance was attributed to local structural distortion and sub-nanometer lattice composition of the CoCu surface.We accurately show the dispersed Cu doped Co_(3)O_(4) nano-islands with a lot of edge sites on the bimetallic Co-Cu surface.While,the gradient components effectively facilitate the establishment of built-in electric field and accelerate the charge transfer.Theoretical and experimental results reveal that the surface Co and neighbouring Cu atoms in sub-nanometer lattice synergistically promote the catalysis of HMF.This work offers new insights into surface segregation in tuning the element spatial distribution for catalysis.

SURFACE SEGREGATION IN Mo-La ALLOY WIRE

The surface segregation in Mo-4wt-% La alloy wire has been studied by using Auger elec- tron spectroscopy.The process of the segregation is that the La atoms diffuse from the grain boundaries to surface.The experimental results were analysed by kinetics of grain boundary diffusion.The diffusion activation energy E_(?)=1.6 eV was obtained.

Calculation of the Surface Segregation of Cu-0.3 at. pct Au Alloy

Monte Carlo simulation technique with the energetics described by the embedded atom method has been used to calculate the surface segregation of Cu-0.3 at. pct Au alloy at T=800 K. It is found that the component Au in the first surface layer (i.e.(100), (110) and (111) faces) is about 50～60 times as large as that in the bulk.

Preparation of pH-responsive membranes with amphiphilic copolymers by surface segregation method

Novel pH-responsive membranes were prepared by blending pH-responsive amphiphilic copolymers with polyethersulfone(PES) via a nonsolvent-induced phase separation(NIPS) technique. The amphiphilic copolymers bearing Pluronic F127 and poly(methacrylic acid)(PMAA) segments, abbreviated as PMAA n–F127–PMAA n,were synthesized by free radical polymerization. The physical and chemical properties of the blend membranes were evaluated by scanning electron microscopy(SEM), Fourier transform infrared(FTIR) spectrum, water contact angle, Zeta potential and X-ray photoelectron spectroscopy(XPS). The enrichment of hydrophilic PMAA segments on the membrane surfaces was attributed to surface segregation during the membrane preparation process. The blend membranes had signi ficant p H-responsive properties due to the conformational changes of surface-segregated PMAA segments under different pH values of feed solutions. Fluxes of the blend membranes were larger at low p H values of feed solutions than that at high pH values. The pH-responsive ability of the membranes was enhanced with the increase of the degree of PMAA near-surface coverage.

Effects of concurrent grain boundary and surface segregation on the final stage of sintering: the case of Lanthanum doped yttriastabilized zirconia

Dopants play a critical role in tailoring the microstructure during sintering of compacts. These dopants may form solid solution within the bulk, and/or segregate to the grain boundaries（GBs） and the solidvapor interfaces（free surfaces）, each causing a distinct energetic scenario governing mass transports during densification and grain growth. In this work, the forces controlling the dopant distribution, in particular the possibility of concurrent segregation at both surfaces and GBs, are discussed based on the respective enthalpy of segregation. An equation is derived based on the minimum Gibbs energy of the system to determine enthalpy of segregation from experimental interface energy data, and the results applied to depict the role of La as a dopant on the interface energetics of yttria stabilized zirconia during its final stage of sintering. It is shown that La substantially decreases both GB and surface energies（differently）as sintering progresses, dynamically affecting its driving forces, and consequent grain growth and densification in this stage.

Surface and transport properties of Cu-Sn-Ti liquid alloys

The lack of experimental data and / or limited experimental information concerning both surface and transport properties of liquid alloys often require the prediction of these quantities. An attempt has been made to link the thermophysical properties of a ternary Cu-Sn-Ti system and its binary Cu-Sn, Cu-Ti and SnoTi subsystems with the bulk through the study of the concentration dependence of various thermodynamic, structural, surface and dynamic properties in the frame of the statistical mechanical theory in conjunction with the quasi-lattice theory （QLT）. This formalism provides valuable qualitative insight into mixing processes that occur in molten alloys.

Research of the behaviour of O chemisorption on the （110） surface of Rhx-Pt1-x alloy

An atomic group model of the disordered binary alloy Rhx-Pt1-x has been constructed to investigate surface segregation. According to the model, we have calculated the electronic structure of the Rhx-Pt1-x alloy surface by using the recursion method when O atoms are adsorbed on the Rhx-Pt1-x （110） surface under the condition of coverage 0.5. The calculation results indicate that the chemical adsorption of O changes greatly the density of states near the Fermi level, and the surface segregation exhibits a reversal behaviour. In addition, when x 〈 0.3, the surface on which O is adsorbed displays the property of Pt; whereas when x 〉 0.3 it displays the property of Rh.

XPS Investigation of Segregation of Sb in SnO_2 Powders

Antimony doped tin-oxide powders were prepared by controlled precipitation. Surface compositions were determined by X-ray photoelectron spectroscopy （XPS）. The results indicate that enrichment of dopant at the surface of nanoparticles depends on total doping concentration and annealing temperature. Doped antimony shows the tendency to diffuse to surface when annealing at high temperatures. But same amount of Sb atoms are retained on the surface for powders annealing at different temperatures. The higher enrichment at higher temperature is caused by the decreasing of surface areas. Variation of the conductivities of SnO2 powders is also discussed.

Surface Layer of Ni_(68)P_(18)Cr_(14) Amorphous Alloy

In Ni_(68)P_(18)Cr_(14) amorphous alloy, the dominant component elements Ni and P were in lower content in the surface layer, while Cr was enriched. There was a P enrichment region just below the surface oxide layer, which was supposed to enhance the surface segregation of Cr. Nickel was less oxidized than Cr.

Directional migration behavior of cerium during sintering process of mischmetal doped cemented carbide

Three observation methods were used to investigate the existing form and the behavior of rare earth during the sintering process of high activity mischmetal （RE, with lanthanum and cerium） doped WC-8%Co-0.048%RE（mass fraction） alloy with low carbon-containing level by scanning electron microscopy （SEM） and energy dispersive X-ray spectroscopy （EDXS）, considering the fact that the addition amount of rare earth in the alloy is very minute. The directional migration process and mechanism of cerium were discussed. First, the sinter skin （surface） is observed. oxide on the sinter skin, and lanthanum in these cerium observed, and lanthanum containing phase/micro-zone in It is shown that there exists a dispersedly distributed cerium containing enrichment positions is very minute. Secondly, the polished section is the alloy is identified. Finally, based on the fact that the fracture of cemented carbide is resulted from the heterogeneous phase or other defects within the microstructure, the fracture surface is observed and cerium containing phase/micro-zone in the fracture source approximately 260 μm from the surface is identified. These combined observations reveal adequately the fact that lanthanum and cerium get separated and cerium predominantly migrates towards the surface during the sintering process.

Enhancing Structural Stability and Pervaporation Performance of Composite Membranes by Coating Gelatin onto Hydrophilically Modified Support Layer

The interfacial compatibility of composite membrane is an important factor to its structural stability, andseparation performance. In this study, poly （ether sulfone） （PES） support layer was first hydrophilically modified with poly（vinyl alcohol） （PVA） via surface segregation during the phase inversion process. Gelatin （GE） was then cast on the PVA-modified PES support layer as the active layer followed by crosslinking to fabricate composite membranes for ethanol dehydration. The enrichment of PVA on the surface of support layer improved interfacial compatibility of the as-prepared GE/PVA-PES composite membrane. The water contact angle measurement and X-ray photoelectron spectroscopy （XPS） data confirmed the surface segregation of PVA with a surface coverage density of -80%. T-peel test showed that the maxima/force to separate the support layer and the active layer was enhanced by 3 times compared with the GE/PES membrane. The effects of PVA content in the support layer, crosslinking of GE active layer and operating parameters on the pervaporative dehydration performance were investigated. The operational stability of the composite membrane was tested by immersing the membrane in ethanol aqueous solution for a period of time. Stable pervaporation performance for dehydration of 90% ethanol solution was obtained for GE/PVA-PES membrane with a separation factor of -60 and a permeation flux of -1910 g.m^-2.h1 without peeling over 28 days immersion.

Melting Behaviour of Core-Shell Structured Ag-Rh Bimetallic Clusters

The melting behaviour of four typical core-shell structured 309-atom Ag-Rh bimetallic clusters, with decahedral and icosahedral geometric configurations, is investigated by using molecular dynamics simulation, based on the Sutton-Chen potential. The initial atomic configurations are obtained from semi-grand canonical ensemble Monte Carlo simulations. It is found that the melting point temperature Tm increases with the mole fraction of Rh in the bimetallic clusters, and Tm of Ag-Rh icosahedral clusters is higher than those of Ag-Rh decahedral clusters with the same Rh mole fraction. It is also found that the Ag atoms lie on the surface of Ag-Rh bimetallic clusters even after melting.

Surface-tuning nanoporous AuCu_(3) engineering syngas proportion by electrochemical conversion of CO_(2)

The direct electrochemical conversion of CO_(2) to syngas with controllable composition remains challenging. In this work, driven by concentration gradient, a simple air-heating aided strategy has been developed to adjust surface composition of the self-supporting nanoporous AuCu_(3) alloy. According to Fick First Law, the interior Cu atoms of the AuCu_(3) alloy with Au-rich surface gradually segregated outwards during heating, realizing Cu-rich surface eventually. Correspondingly, the competing electrocatalytic CO_(2) reduction (ECR) to CO and hydrogen evolution reactions (HER) were tactfully balanced on these alloy surfaces, thus achieving proportion-tunable syngas (CO/H2). Density functional theory (DFT) calculations on the Gibbs free energy change of the COOH* and H* (ΔGCOOH*, ΔGH*) on the alloy surfaces were conducted, which are generally considered as the selectivity descriptors for CO and H2 products, respectively. It shows ΔGCOOH* gradually increases in contrast to the decreased ΔGH* with more Cu on the surface, suggesting H2 is more favored over Cu sites, which is consistent with the declining CO/H2 ratio observed in the experiments. This study reveals that the surface composition controls ECR activity of nanoporous AuCu_(3) alloy, providing an alternative way to the syngas production with desirable proportion.

Gas adsorbate-induced Au atomic segregation and clustering from Cu(Au)

Surface compositional and phase segregation in an alloy can change its functionality, especially for applications where surface structure and chemistry play a vital role.For instance, the surface status of alloy catalysts significantly affects their catalytic performance for both heterogeneous and electrochemical processes. Surface segregation is believed to be driven by the difference in surface energy to reduce the total free energy of the alloy. However, the atomistic processes during the segregation process remain elusive, especially for gas molecule-induced segregation, where both structural and chemical reordering may occur. Herein, we achieved in-situ atomic-scale visualization of the surface segregation behaviors of a solid solution Cu(Au) alloy under the CO gas by an aberration-corrected environmental transmission electron microscope. CO-induced Cu(Au) surface ordering structures largely change the surface chemistry of the alloy. Further gas exposure at elevated temperature could facilitate Au atom diffusion through a specific "atomic channel" structure for dealloying and clustering on the surface. The segregated Au nanoparticles show rich phase and morphological dynamics interacting with the alloy surface, where the gas adsorption also plays an important role. These atomic insights provide direct evidence for the surface segregation and dealloying mechanisms of bimetallic alloys, and highlight the role of gas adsorbate in these surface processes.

Ignition-proof mechanism of magnesium alloy added with rare earth La from first-principle study

The surface segregation of La and its effect on the oxygen adsorption on a Mg （0001） surface for a coverage 0=-0.25 monolayer were performed by using first-principles calculations. The calculated results showed that La atoms preferred occupying surface sites to the bulk sites, which suggested the La surface segregation. When oxygen atoms adsorbed on a pure or La alloyed Mg （0001） surface, certain amount of heat would release, and La alloying made the heat released less, which might increase the ignition point of Mg alloy. Both Mg and La had strong atomic affinity with oxygen, so the oxidation film of Mg-La alloys consisted of MgO, La2O3. The denser La2O3 turned oxide film into free and close structure, and prevented oxygen from passing through the oxidation film. The La-O covalent bonding could explain why La2O3 was compact, and resulted in good ignition-proof of Mg-La alloys.