Near-surface Region Research Articles

Unidimensional Approximation of the Diffuse Electrical Layer in the Inner Volume of Solid Electrolyte Grains in the Absence of Background Ions.

In this paper we continue working on our theory of electrical double layers resulting exclusively from dissociation of a solid electrolyte, which we previously proposed as a medium for catalytic interaction between solid cellulose and solid acid catalysts of hydrolysis. Two theoretical unidimensional models of the inner grain volume are considered: an infinitely long cylindrical pore, and a gel electrolyte near a grain outer surface. Despite the model simplicity, the predictions for the cylindrical pore case are in semi-quantitative agreement with literature data on electroosmotic experiments, adequately explaining high proton selectivity of sulfonic membranes, and decline of such selectivity at high background acid concentration. The gel model predicts less concentrated diffuse layer in comparison to electrolytes with impenetrable skeleton (e. g., sulfonated carbons). This suggests limited suitability of gel electrolytes as catalysts if a substrate cannot diffuse into the gel bulk and the reaction is thereby spatially limited to the near-surface region, for example if a substrate is solid like aforementioned cellulose.

Visualization of microstructural mechanisms in nanocrystalline ferrite during grinding

Microstructural changes in the near-surface regions of a material determine its mechanical properties and consequently also its tribological behavior. This work is a study of the microstructural development of nanocrystalline ferrite subjected to a grinding process using molecular dynamics simulations. We visualize the work piece by producing various types of computational tomographic sections that are colored according to the grain orientation, the local temperature, the stress in grinding direction, as well as the atomic flow velocities. In particular, we introduce “differential EBSD” tomographs to highlight the changes to the microstructure caused by the grinding process, allowing us to detect even subtle differences in lattice orientation and small distances in grain boundary migration. We use our visualization approach to discuss the acting microstructural mechanisms in a load- and time-resolved fashion, spanning a wide range of grinding conditions from mild to severe. In addition to removed matter, we observe lattice rotation originating at the surface and advancing deeper into the work piece with increasing load, grain growth by grain boundary migration, and the transient formation of unstable small new grains.

Exchange Reactions at Mineral Interfaces.

Exchange reactions are a family of chemical reactions that appear when mineral surfaces come into contact with protic solvents. Exchange reactions can also be understood as a unique interaction at mineral interfaces. Particularly significant interactions occurring at mineral surfaces are those with water and CO2. The rather complex process occurring when minerals such as calcium silicate hydrate (C-S-H) phases come into contact with aqueous environments is referred to as a metal-proton exchange reaction (MPER). This process leads to the leaching of calcium ions from the near-surface region, the first step in the corrosion of cement-bound materials. Among the various corrosion reactions of C-S-H phases, the MPER appears to be the most important one. A promising approach to bridging certain problems caused by MPER and carbonation is the passivation of C-S-H surfaces. Today, such passivation is reached, for instance, by the functionalization of C-S-H surfaces with water-repelling organic films. Unfortunately, these organic films are weak against temperature and especially weak against abrasion. Exchange reactions at mineral interfaces allow the preparation of intrinsic, hydrophobic surfaces of C-S-H phases just at room temperature via a metal-metal exchange reaction.

Consequences of Different Mechanical Surface Preparation of Ni-Base Alloys during High Temperature Oxidation.

The influence of surface roughness on its high temperature oxidation for an Ni-base superalloy was studied using laser profilometry, atomic force microscopy, mass change measurements, glow-discharge optical emission spectrometry, scanning electron microscopy, X-ray diffraction, and positron annihilation methods. The isothermal and cyclic air oxidation tests were performed at 1000 °C and showed dependence of oxidation behavior on surface roughness. Smoother surfaces oxidation resulted in the formation of a multilayered oxide scale consisting of NiO, Cr2O3, and internally oxidized Al2O3 while a rougher surface formed protective Al2O3 scale. The factors responsible for different oxidation behaviors were determined as higher concentration of vacancies and increased residual stresses in the near-surface region of studied alloys.

Silver nanoparticles decorated grassy ZnO coating for photocatalytic activity enhancement

The ZnO/Ag composite photocatalysts were prepared via the deposition of Ag on the surface of the grassy ZnO by a simple immersion reduction method. The dosage of Ag in the composites was adjusted by the concentration of Ag ions, immersion time and the content of the reducing agent. The grassy ZnO substrate presented a hexagonal wurtzite structure and a uniform dispersion. The band gap of the ZnO/Ag composites was decreased than that of grassy ZnO due to the surface plasma resonance (SPR) effect caused by Ag modification. The photocurrent response of the composites with proper amount of Ag loading was greatly improved by the SPR effect of Ag and subsequently enhanced light absorption, which resulted in the separation of electron-hole pairs in the near-surface region of the neighboring semiconductor. The ZnO/Ag composites had excellent photocatalytic efficiency in decolorization of RhB under UV irradiation.

Red luminescence in H-doped β−Ga2O3

The effects of hydrogen incorporation into beta-Ga2O3 thin films have been investigated by chemical, electrical and optical characterization techniques. Hydrogen incorporation was achieved by remote plasma doping without any structural alterations of the film; however, X-ray photoemission reveals major changes in the oxygen chemical environment. Depth-resolved cathodoluminescence (CL) reveals that the near-surface region of the H-doped Ga2O3 film exhibits a distinct red luminescence (RL) band at 1.9 eV. The emergence of the H-related RL band is accompanied by an enhancement in the electrical conductivity of the film by an order of magnitude. Temperature-resolved CL points to the formation of abundant H-related donors with a binding energy of 28 +/- 4 meV. The RL emission is attributed to shallow donor-deep acceptor pair recombination, where the acceptor is a VGa-H complex and the shallow donor is interstitial H. The binding energy of the VGa-H complex, based on our experimental considerations, is consistent with the computational results by Varley et al [J. Phys.: Condens. Matter, 23, 334212, 2011].

Filiform corrosion behaviour on machined AA7150 aluminium alloy

Abstract The microstructure and the filiform corrosion behaviour of machined AA7150 aluminium alloy were investigated using scanning and transmission electron microscopies combined with potentiodynamic polarization and filiform corrosion testing, respectively. It is found that the grain refinement, redistribution of alloying elements, and elements segregation at grain boundaries are evident within the near-surface region on the machined AA7150 aluminium alloy. The corrosion susceptibility of machining introduced near-surface deformed layer is significantly improved caused by the modified microstructure associated with severe deformation. Filiform corrosion resistance on the machined surface is obviously decreased, due to the surface roughness associated with machining tracks and the presence of the electrochemically more active near-surface deformed layer introduced by machining.

Direct Observation of Hole Carrier-Density Profiles and Their Light-Induced Manipulation at the Surface ofGe

We demonstrate that, by using low-energy positive muon ($\mu^+$) spin spectroscopy as a local probe technique, the profiles of free charge carriers can be directly determined in the accumulation/depletion surface regions of p- or n-type Ge wafers. The detection of free holes is accomplished by measuring the effect of the interaction of the free carriers with the $\mu^+$ probe spin on the observable muon spin polarization. By tuning the energy of the low-energy $\mu^+$ between 1 keV and 20 keV the near-surface region between 10 nm and 160 nm is probed. We find hole carrier depletion and electron accumulation in all samples with doping concentrations up to the $10^{17}$ cm$^{-3}$ range, which is opposite to the properties of cleaved Ge surfaces. By illumination with light the hole carrier density in the depletion zone can be manipulated in a controlled way. Depending on the used light wavelength $\lambda$ this change can be persistent ($\lambda = 405, 457$ nm) or non-persistent ($\lambda = 635$ nm) at temperatures $< 270$ K. This difference is attributed to the different kinetic energies of the photo-electrons. Photo-electrons generated by red light do not have sufficient energy to overcome a potential barrier at the surface to be trapped in empty surface acceptor states. Compared to standard macroscopic transport measurements our contact-less local probe technique offers the possibility of measuring carrier depth profiles and manipulation directly. Our approach may provide important microscopic information on a nanometer scale in semiconductor device studies.

Nonthermal phase transitions in irradiated oxides

It is predicted theoretically that various oxides (Al2O3, MgO, SiO2 and TiO2) under ultrafast excitation of the electronic system exhibit nonthermal phase transitions. In the bulk, Al2O3 transiently forms a superionic phase via nonthermal phase transition, MgO and SiO2 disorder, TiO2 experiences solid–solid phase transition while thermal effects lead to melting. In the finite-size samples and near-surface regions, MgO undergoes solid–solid phase transition at lower doses than those required for atomic disorder. All studied oxides but TiO2, if allowed to expand, exhibit a lower damage threshold, whereas in TiO2 expansion releases the stress and prevents solid–solid phase transition thereby increasing the damage threshold up to the melting one. The results suggest that a nonthermal phase transition is a general response of oxides to sufficiently high ultrafast electronic excitation. A comparison with nonadiabatic simulations demonstrates that Born–Oppenheimer approximation systematically overestimates damage thresholds, and in some cases misses a phase transition entirely.

Effect of rapid thermal annealing on damage of silicon matrix implanted by low-energy rhenium ions

The structural, electronic, and magnetic properties of low-energy rhenium implanted c-Si are examined for the first time. The damage created by rhenium ions and the following partial reconstruction of the silicon host matrix after rapid thermal annealing (RTA) are investigated as a function of the fluence. Rutherford backscattering spectrometry (RBS) results reveal that the implanted ions are located in the near-surface region with the distribution maximum at about 23 nm below the surface. The analysis of rhenium-depth distribution using the McChasy code shows that the implanted Re-ions are located in the interstitial lattice positions. The RTA leads to a partial recovery of the silicon crystal structure. According to the RBS results, the formed inclusions are not coherent with the silicon host matrix causing an increase of the lattice distortion. Analysis of channeled RBS/c spectra carried out by the McChasy code revealed different levels of bent channels in damaged regions suggesting bimodal distribution of inclusions in the silicon. Studies of high-resolution X-ray photoelectron spectroscopy (XPS) conducted after the RTA showed the shift of Re 4f7/2 binding energy (BE) by +0.68 and + 0.85 eV with respect to metallic rhenium for the samples with lower/higher fluencies, respectively. Complex XPS, density functional theory (DFT) simulations, and transmission electron microscopy (TEM) data analysis allowed us to conclude that the near-surface layer of the sample (∼10 nm) consists of nanoinclusions with cubic and/or hexagonal ReSi. In the middle area of the samples, much larger nanoinclusions (>10/20 nm for higher/lower fluencies, respectively) containing pure metallic rhenium inside are formed. The RTA increases the magnetic moment of the sample with the lower dose nearly 20-fold, whereas in the sample with the higher dose a 3-fold increment is observed only. The magnetic response of the examined systems after the RTA indicates a presence of magnetic interactions between the nanoinclusions resulting in the system exhibiting super-spin glass or super-ferromagnetism.

Tritium removal from the LHD first-wall by the hydrogen plasma discharge

Abstract During the 19th experimental campaign in the Large Helical Device (LHD) in 2017, deuterium (D) plasma experiments were performed for the first time in the LHD. Tritons were generated by the D-D reaction in the core plasma during the plasma discharges. To remove the tritium in plasma facing surfaces for safety in maintenance, hydrogen (H) plasma experiments were performed for one month after the four months of D plasma experiments. In this study, the removal effect of the tritium by H plasma discharges was investigated. Six stainless steel (SUS316L) samples were introduced into a position of the first-wall of the LHD in the last month of the D experiments, which were exposed to the D plasma for one month and then, three samples were continued to exposure to the following H plasma for another one month. The experimental results showed that one-half of the tritium in the surface region of the SUS sample was removed, which was measured by the TIPT (Tritium imaging plate technique). Further, hydrothermal treatments were performed and the results showed that the tritium retention in the bulk of the SUS materials was also reduced. Meanwhile, the carbon content in the near-surface region was also reduced after H plasma experiments, which reduces the possibility of the formation of the C H chemical bond.

Relationship between the hydroxyl termination and band bending at (2¯01)β−Ga2O3 surfaces

Synchrotron x-ray photoelectron spectroscopy was used to explore the relationship between the hydroxyl termination and band bending at the $(\overline{2}01)$ surface of $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{G}{\mathrm{a}}_{2}{\mathrm{O}}_{3}$ bulk single crystals. All as-received $(\overline{2}01)$ surfaces were terminated with OH groups, with H/OH binding to surface ${\mathrm{O}}_{\mathrm{s}}/\mathrm{G}{\mathrm{a}}_{\mathrm{s}}$ atoms. Removal of this native OH termination produced a large upward shift in band bending of up to 1.0 eV, consistent with strong electron depletion and a semiconductor to insulator-like transition in the near-surface region. Simple surface treatments were used to control the size and stability of the band bending of as-received $(\overline{2}01)$ surfaces by modifying the nature of the OH termination. NaOH (${\mathrm{H}}_{2}\mathrm{S}{\mathrm{O}}_{4}$) treatment consistently produced upward (downward) shifts in band bending and a significant increase (decrease) in the thermal stability of the OH termination that was associated with an increase in the relative density of $\mathrm{G}{\mathrm{a}}_{\mathrm{s}}\text{\ensuremath{-}}\mathrm{OH}$ (${\mathrm{O}}_{\mathrm{s}}\text{\ensuremath{-}}\mathrm{H}$) species. Annealing in wet ${\mathrm{O}}_{2}$ (at 600 \ifmmode^\circ\else\textdegree\fi{}C) produced an extremely stable OH termination and the strongest downward shift in band bending. These effects, combined with the relatively slow dissociation of ${\mathrm{H}}_{2}\mathrm{O}$ on bare $(\overline{2}01)$ surfaces, allowed the preparation of surfaces with significant variations in band bending that may prove useful in optimizing the properties of $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{G}{\mathrm{a}}_{2}{\mathrm{O}}_{3}$ metal-semiconductor contacts and heterojunctions. A comparison of two methods used to determine the absolute band bending at semiconductor surfaces confirmed that bare $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{G}{\mathrm{a}}_{2}{\mathrm{O}}_{3}(\overline{2}01)$ surfaces are characterized by strong upward band bending (\ensuremath{\sim}0.5 to 1.0 eV) and an electron depletion layer that can be completely removed by the hydroxylation of the surface.

NaF/RbF-Treated Cu(In,Ga)Se2 Thin-Film Solar Cell Absorbers: Distinct Surface Modifications Caused by Two Different Types of Rubidium Chemistry.

The underlying beneficial mechanism of heavy alkali postdeposition treatment (PDT) of Cu(In,Ga)Se2 thin-film solar cell absorbers that led to new record efficiencies in recent years is studied using photoelectron spectroscopy. Excitation energies between 40.8 eV and 6 keV were used to examine the near-surface region of Cu(In,Ga)Se2 thin-film solar cell absorbers that underwent NaF and combined NaF/RbF PDT. The already Cu-deficient surface region after NaF PDT, which is modeled as a Cu:(In + Ga):Se = 1:5:8 phase, shows further depletion after NaF/RbF PDT and seems to incorporate some Rb. Additionally, we have found strong indications for the NaF/RbF PDT-induced formation of a Rb-In-Se-type compound with a 1:1:2 stoichiometry partially covering the absorber surface. The electronic Cu(In,Ga)Se2 structure is modified due to the RbF treatment, with a pronounced shift in the valence band maximum away from the Fermi level in the immediate vicinity of the surface.

Effects of dopant concentration on microstructure and strain states of in-situ phosphorus-doped epitaxial silicon films during dry oxidation

The adjustability of the incorporation and distribution of P dopants in Si materials has led to their application in various electronic and photovoltaic devices. In particular, P-doped Si films have been investigated for dopant-induced phenomena such as oxidation enhancement and dopant segregation during thermal oxidation. Despite their technical as well as scientific importance of dopant behaviors in P-doped Si films, these were hardly observed due to low P incorporation. In this study, we prepared and oxidized P-doped epitaxial Si (Si:P) films grown on the Si(100) substrate using in-situ P-doped deposition technique to incorporate high P concentration up to 9.6% (4.8 × 1021 cm−3) in the films, exceeding solid solubility limits (~3 × 1020 cm−3). We found that in oxidized samples with P concentration greater than 6.2%, defects were generated in the near-surface region of Si:P layers and segregated P was observed at the defected areas. The chemical bonding states of P at the oxidized Si:P film increased in the P 2p peak intensity at the interface between the oxide and the Si:P layer, indicating P segregation at this interface. In the strain states of Si:P films, the decrease in the out-of-plane lattice parameter was observed, while maintaining the constant in-plane lattice parameter, implying strain release due to decreased P concentration in the Si:P layer after dry oxidation.

Observation of iron diffusion in the near-surface region of magnetite at 470 K

The authors study interdiffusion of Fe-cations between homoepitaxially grown 57Fe3O4 films and (001) oriented Fe3O4 substrates in the temperature range of 470-770 K under high vacuum by neutron reflectivity and time-of-flight secondary ion mass spectrometry. The paper estimates diffusion lengths in the order of 20-50 \AA{} and diffusion constants in the order of 10-20 m2/s.

Unraveling and Mapping the Mechanisms for Near-Surface Microstructure Evolution in CuNi Alloys under Sliding.

The origin of friction and wear in polycrystalline materials is intimately connected with their microstructural response to interfacial stresses. Although many mechanisms that govern microstructure evolution in sliding contacts are generally understood, it is still a challenge to ascertain which mechanisms matter under what conditions, which limits the development of tailor-made microstructures for reducing friction and wear. Here, we shed light on the circumstances that promote plastic deformation and surface damage by studying several face-centered cubic CuNi alloys subjected to sliding with molecular dynamics simulations featuring tens of millions of atoms. By analyzing the depth- and time-dependent evolution of the grain size, twinning, shear, and stresses in the aggregate, we derive a deformation mechanism map for CuNi alloys. We verify the predictions of this map against focused ion beam images of the near-surface regions of CuNi alloys that were experimentally subjected to similar loading conditions. Our results may serve as a tool for finding optimum material compositions within a specified operating range.

Analyzing Near-Surface Regions of Hydrophobic and Long-Term Weathered Natural Stones at Microscopic Scale

The visual appearance of building structures is an important attribute which reflects the character and identity of a region. Due to the influence of weathering, the surfaces of building stones alter, leading to aesthetic changes of the material surface such as discoloration or darkening. In this study, near-surface regions of weathered Baumberger (BST), Schleeriether (SST), and Obernkirchener Sandstones (OKS) have been analyzed at the microscopic scale in order to investigate the intensity and the extent of visual as well as structural changes and how both can be affected due to the presence of surface treatments with hydrophobing agents. It could be detected that aesthetic changes appeared already after 2 years of outdoor exposure, with the slightest variations on BST surfaces, followed by SST and OKS. The use of hydrophobing agents leads to a reduction in surface darkening in the short term. After long-term weathering, no significant changes are visible, as similar values in total color difference (ΔE*) were measured. Biogenic growth and the formation of black weathering crusts are the main reasons for color alterations in the case of the examined stones. The surface damages occur especially on calcareous (BST) followed by clayey (SST) and quartzitic (OKS) stone surfaces.

SrTiO[formula omitted] optical waveguides fabricated by carbon ion irradiation at various fluences

We report the fabrication of planar optical waveguide in SrTiO3 crystals by 6.0 MeV carbon ion irradiation with different fluences. The dark mode spectra were measured by the prism-coupling method at 633 nm and the near-field intensity distributions were measured by the end-face coupling method. The reflectivity calculation method (RCM) was used for reconstructing refractive index profile and the finite-difference beam propagation method (FD-BPM) was used to simulate the near-field intensity distributions. SRIM was used to simulate the DPA caused by carbon ion irradiation; Micro-Raman spectra, X-ray Diffraction (XRD) and Rutherford back scattering/channeling (RBS/Channeling) were used to study the lattice damage induced by the carbon ion irradiation with different fluences in the near-surface region.

Sandwiched structure of hot-deformed Nd-Fe-B permanent magnets processed by Nd-Cu eutectic alloys diffusion

A series of sandwiched structures with different near-surface mass fractions x (x = 3 wt%, 4 wt%, 5 wt%) was employed to develop high-coercivity hot-deformed Nd-Fe-B magnets by the addition of 2 wt% Nd-Cu eutectic alloys via adjusting the middle thickness and near-surface thickness. The designed magnet with a pronounced composite structure shows a 23% increase in coercivity with a 6% loss of remanence by adjusting the sandwiched structure at 4 wt% Nd-Cu eutectic alloys in the near-surface regions. The results indicate that the near-surface Nd-Cu-rich “shell” structure can effectively suppress the magnetization reversal of overall magnets, enhancing the coercivity. With the help of loading stress, Nd-Cu liquid enriched at the near-surface regions of the sample is infiltrated into the Nd-Cu-lean middle region, resulting in a concentration gradient. Microstructure characterizations further demonstrate that the infiltrated Nd–Cu eutectic plays a critical role in inhibiting grain growth and intergranular magnetic interaction. The optimized microstructure features suppress the reversed magnetization process, which makes a positive contribution to coercivity.

Simulating the outer layers of rapidly rotating stars

ABSTRACT This paper presents the results of a set of radiative hydrodynamic simulations of convection in the near-surface regions of a rapidly rotating star. The simulations use microphysics consistent with stellar models, and include the effects of realistic convection and radiative transfer. We find that the overall effect of rotation is to reduce the strength of turbulence. The combination of rotation and radiative cooling creates a zonal velocity profile in which the motion of fluid parcels near the surface is independent of rotation. Their motion is controlled by the strong up and down flows generated by radiative cooling. The fluid parcels in the deeper layers, on the other hand, are controlled by rotation.