Synchrotron X-ray Photoelectron Spectroscopy Research Articles

In-situ Synchrotron x-ray photoelectron spectroscopy study of medium-temperature baking of niobium for SRF application

Abstract In order to determine optimal parameters of vacuum thermal processing of superconducting radiofrequency niobium cavities exhaustive information on the initial chemical state of niobium and its modification upon a vacuum heat treatment is required. In the present work the chemical composition of the niobium surface upon ultra-high vacuum baking at 200–400 °C similar to “medium-temperature baking” and “furnace baking” of cavities is explored in-situ by synchrotron X-ray photoelectron spectroscopy (XPS). Our findings imply that below the critical thickness of the Nb2O5 layer (≈ 1 nm) niobium starts to interact actively with surface impurities, such as carbon and phosphorus. By studying the kinetics of the native oxide reduction, the activation energy and the rate-constant relation have been determined and used for the calculation of the oxygen-concentration depth profiles. It has been established that the controlled diffusion of oxygen is realized at temperatures 200–300 °C, and the native-oxide layer represents an oxygen source, while at 400 °C the pentoxide is completely reduced and the doping level is determined by an ambient oxygen partial pressure. Fluorine (F to Nb atomic ratio is 0.2) after the buffered chemical polishing was found to be incorporated into the surface layer probed by XPS (≈ 4.6 nm), and its concentration increased during the low-temperature baking (F/Nb=0.35 at 230 °C) and depleted at higher temperatures (F/Nb=0.11 at 400 °C). Thus, the influence of fluorine on the performance of mid-T baked, nitrogen-doped and particularly mild-baked (120 °C/48 h) cavities must be considered. The possible role of fluorine in the educed Nb+5 → Nb+4 reaction under the impact of an X-ray beam at room temperature and during the thermal treatment is also discussed. The range of temperature and duration parameters of the thermal treatment at which the niobium surface would not be contaminated with impurities is determined.

Pd nanocubes enclosed by {100} facets for activating electroless Cu deposition on liquid crystal polymer substrates with strong adhesion strength

In this study, we investigate the catalytic performance of palladium (Pd) nanocatalysts with different surface atomic structures for activating electroless copper (Cu) deposition on liquid crystal polymer (LCP) substrates. These nanocatalysts include Pd nanocubes featuring {100} facets of square atomic arrangement as their predominant surface structure and Pd spherical nanoparticles characterized by random atomic arrangements, synthesized using an aqueous method. Our electrochemical measurements reveal that the induction time for the growth of Cu films is significantly shorter when utilizing Pd nanocubes compared to Pd nanospheres. This shorter induction time signifies a quicker initiation of Cu growth, primarily due to the presence of Pd {100} facets. Furthermore, we also develop a surface modification approach involving a crosslinking reaction that utilizes the highly branched polyethylenimine and linear polyvinylpyrrolidone on the Pd nanocubes. This method not only facilitates the adsorption of Pd nanocubes onto the LCP substrates but also achieves a robust peel adhesion strength (630 gf/cm) of the resulting Cu films, exceeding the industrial adhesion goal of 500 gf/cm. Moreover, in-depth synchrotron X-ray photoelectron spectroscopy analysis provides valuable insights into the depth-dependent chemical changes within the Cu films and proves the removal of Cu oxides after the acid treatment.

Lithium-ion battery electrode properties of hydrogen boride.

Recently, hydrogen boride (HB) with a pseudo-two-dimensional sheet structure was successfully synthesized, and it is theoretically predicted to have high potential as a negative electrode material for alkali metal ion batteries, making it a promising new candidate. This study represents the first experimental examination of the negative electrode properties of HB. HB was synthesized via cation exchange from MgB2. The confirmation of HB synthesis was achieved through various spectroscopic experiments, including synchrotron radiation X-ray diffraction and X-ray photoelectron spectroscopy, in addition to direct observation using transmission electron microscopy. The HB electrode was prepared by mixing the HB powder sample with conductive additive carbon black and a polymer binder. A test cell was assembled with the HB electrode as the working electrode, and lithium metal as the counter and reference electrodes, and its battery electrode properties were evaluated. Although reversible charge-discharge curves with good reversibility were observed, the reversible capacity was 100 ± 20 mA h g-1 which is significantly smaller than the theoretical predictions. Nitrogen gas adsorption experiments were performed on the HB powder sample to determine the specific surface area indicating that the HB sheets were stacked together. It is plausible to consider that this stacking structure led to a reduced lithium-ion storage capacity compared to the theoretical predictions.

Surfaces of Atmospheric Droplet Models Probed with Synchrotron XPS on a Liquid Microjet.

ConspectusThe atmosphere is a key part of the earth system comprising myriad chemical species in all basic forms of matter. Ubiquitous nano- and microscopic aerosol particles and cloud droplets suspended in the air play crucial roles in earth's climate and the formation of air pollution. Surfaces are a prominent part of aerosols and droplets, due to the high surface area to bulk volume ratios, but very little is known about their specific properties. Many atmospheric compounds are surface-active, leading to enhanced surface concentrations in aqueous solutions. Their distribution between the surface and bulk may determine heterogeneous chemistry and many other properties of aerosol and cloud droplets, but has not been directly observed.We used X-ray photoelectron spectroscopy (XPS) to obtain direct molecular-level information on the surface composition and structure of aqueous solutions of surface-active organics as model systems for atmospheric aerosol and cloud droplets. XPS is a vacuum-based technique enabled for volatile aqueous organic samples by the application of a high-speed liquid microjet. In combination with brilliant synchrotron X-rays, the chemical specificity of XPS allows distinction between elements in different chemical states and positions within molecular structures. We used core-level C 1s and N 1s signals to identify the alkyl and hydrophilic groups of atmospheric carboxylic acids, alkyl-amines, and their conjugate acids and bases. From this, we infer changes in the orientation of surface-adsorbed species and quantify their relative abundances in the surface. XPS-derived surface enrichments of the organics follow trends expected from their surface activities and we observed a preferential orientation at the surface with the hydrophobic alkyl chains pointing increasingly outward from the solution at higher concentrations. This provides a first direct experimental observation of well-established concepts of surface adsorption and confirms the soundness of the method.We mapped relative abundances of conjugate acid-base pairs in the aqueous solution surfaces from the respective intensities of distinctive XPS signals. For each pair, the protonation equilibrium was significantly shifted toward the neutral form in the surface, compared to the bulk solution, across the full pH range. This represents an apparent shift of the pKa in the surface, which may be toward either higher or lower pH, depending on whether the acid or base form of the pair is the neutral species. The surface shifts are broadly consistent with the relative differences in surface enrichment of the individual acid and base conjugates in binary aqueous solutions, with additional contributions from nonideal interactions in the surface. In aqueous mixtures of surface-active carboxylate anions with ammonium salts at near-neutral pH, we found that the conjugate carboxylic acids were further strongly enhanced. This occurs as the coadsorption of weakly basic carboxylate anions and weakly acidic ammonium cations forms ion-pair surface layers with strongly enhanced local abundances, increasing the probability of net proton transfer according to Le Chatelier's principle. The effect is stronger when the evaporation of ammonia from the surface further contributes to irreversibly perturb the protonation equilibrium, leaving a surplus of carboxylic acid. These surface-specific effects may profoundly influence atmospheric chemistry mediated by aqueous aerosols and cloud droplets but are currently not taken into account in atmospheric models.

Alcohol Vapor Sensor Based on Quasi-2D Nb2O5 Derived from Oxidized Nb2CTz MXenes.

MXenes are two-dimensional (2D) materials with a great potential for sensor applications due to their high aspect ratio and fully functionalized surface that can be tuned for specific gas adsorption. Here, we demonstrate that the Nb2CTz-based sensor exhibits high performance towards alcohol vapors at temperatures up to 300-350 °C, with the best sensitivity towards ethanol. We attribute the observed remarkable chemiresistive effect of this material to the formation of quasi-2D Nb2O5 sheets as the result of the oxidation of Nb-based MXenes. These findings are supported by synchrotron X-ray photoelectron spectroscopy studies together with X-ray diffraction and electron microscopy observations. For analyte selectivity, we employ a multisensor approach where the gas recognition is achieved by linear discriminant analysis of the vector response of the on-chip sensor array. The reported protocol demonstrates that MXene layers are efficient precursors for the derivation of 2D oxide architectures, which are suitable for developing gas sensors and sensor arrays.

High Voltage Ga-Doped P2-Type Na2/3 Ni0.2 Mn0.8 O2 Cathode for Sodium-Ion Batteries.

Ni/Mn-based oxide cathode materials have drawn great attention due to their high discharge voltage and large capacity, but structural instability at high potential causes rapid capacity decay. How to moderate the capacity loss while maintaining the advantages of high discharge voltage remains challenging. Herein, the replacement of Mn ions by Ga ions is proposed in the P2-Na2/3 Ni0.2 Mn0.8 O2 cathode for improving their cycling performances without sacrificing the high discharge voltage. With the introduction of Ga ions, the relative movement between the transition metal ions is restricted and more Na ions are retained in the lattice at high voltage, leading to an enhanced redox activity of Ni ions, validated by ex situ synchrotron X-ray absorption spectrum and X-ray photoelectron spectroscopy. Additionally, the P2-O2 phase transition is replaced by a P2-OP4 phase transition with a smaller volume change, reducing the lattice strain in the c-axis direction, as detected by operando/ex situ X-ray diffraction. Consequently, the Na2/3 Ni0.21 Mn0.74 Ga0.05 O2 electrode exhibits a high discharge voltage close to that of the undoped materials, while increasing voltage retention from 79% to 93% after 50 cycles. This work offers a new avenue for designing high-energy density Ni/Mn-based oxide cathodes for sodium-ion batteries.

Nanoscale iodophoric poly(vinyl amide) coatings for the complexation and release of iodine for antimicrobial surfaces

Wound infection exacerbated by multidrug-resistant (MDR) bacteria has necessitated the need for wound dressing materials with embedded antimicrobial property to prevent the development of chronic wounds. Iodine (I2), typically as a poly(vinyl pyrrolidone)-I2 (PVP–I) complex, is a widely used antiseptic agent in wound care owing to its efficacy, safety, and low probability to cause further resistance. Despite extensive research on PVP–I formulations, there is potentially a large selection of poly(vinyl amide)s that have remained largely unexplored as iodophoric materials, particularly as coatings for wound dressings and medical devices. Therefore, in this study we prepared a series of stable, cross-linked poly(vinyl amide) coatings via plasma polymerization and studied their structure–property relationships for the complexation and release of I2. Plasma polymerized poly(N-vinyl-2-pyrrolidone) (pPVP), poly(N-methyl-N-vinylacetamide) (pPMVA) and poly(N-vinylcaprolactam) (pPVCl) nanoscale coatings were prepared with uniform thicknesses of 330–340 nm and low surface roughness. Complexation of I2 as polyiodides resulted in a significant increase in the pPVCl coating thickness in phosphate buffered saline (PBS), consistent with the Hofmeister effect. Raman spectroscopy confirmed that the I2 was complexed exclusively as the triiodide ion (I3-) in all coatings, which was confirmed by synchrotron X-ray photoelectron spectroscopy (XPS) experiments. Nuclear magnetic resonance (NMR) spectroscopy of poly(vinyl amides) was employed to further prove that the complexing species was hydrogen triiodide (HI3). The loading of I2 species increased from pPVP < pPMVA < pPVCl, with percentage elemental compositions of 1.3, 1.8, and 2.9%, respectively, which correlated with the extent of nitrogen loss during polymerization and oxidation, and was also in agreement with determined negative zeta potential values. However, quantification of the released I2 species over 5 h showed that the greatest release was observed from pPMVA (121.3 µg.cm−2), with pPVP (77.4 µg.cm−2) and pPVCl (74.6 µg.cm−2) releasing a similar amount, highlighting that I2 release from poly(vinyl amide) coatings is a function of the loading and composition of the polymer, not just the I2 complexing amide motif. The I2 complexed pPMVA coated wound dressings were highly effective against Gram-negative Pseudomonas aeruginosa and Gram-positive Staphylococcus aureus, the two major strains associated with wound infection, reducing bacterial viability by up to 4 and 7 (log10 reduction) orders of magnitude. Overall, the poly(vinyl amide) coatings are promising iodophors for the prevention and treatment of wound infections.

Room-Temperature Photoluminescence Mediated by Sulfur Vacancies in 2D Molybdenum Disulfide.

Atomic defects in monolayer transition metal dichalcogenides (TMDs) such as chalcogen vacancies significantly affect their properties. In this work, we provide a reproducible and facile strategy to rationally induce chalcogen vacancies in monolayer MoS2 by annealing at 600 °C in an argon/hydrogen (95%/5%) atmosphere. Synchrotron X-ray photoelectron spectroscopy shows that a Mo 3d5/2 core peak at 230.1 eV emerges in the annealed MoS2 associated with nonstoichiometric MoSx (0 < x < 2), and Raman spectroscopy shows an enhancement of the ∼380 cm-1 peak that is attributed to sulfur vacancies. At sulfur vacancy densities of ∼1.8 × 1014 cm-2, we observe a defect peak at ∼1.72 eV (referred to as LXD) at room temperature in the photoluminescence (PL) spectrum. The LXD peak is attributed to excitons trapped at defect-induced in-gap states and is typically observed only at low temperatures (≤77 K). Time-resolved PL measurements reveal that the lifetime of defect-mediated LXD emission is longer than that of band edge excitons, both at room and low temperatures (∼2.44 ns at 8 K). The LXD peak can be suppressed by annealing the defective MoS2 in sulfur vapor, which indicates that it is possible to passivate the vacancies. Our results provide insights into how excitonic and defect-mediated PL emissions in MoS2 are influenced by sulfur vacancies at room and low temperatures.

Epitaxial growth of gadolinium and samarium thin films and their subsequent facile nitridation at ambient temperatures

Lanthanide nitride species have recently been shown to be easily formed at ambient temperatures and low pressures via the reaction of clean lanthanide surfaces with molecular nitrogen. However, understanding and predicting the nature and efficiency of this process is still in its infancy. In this work we report on the nitridation of the surface of two lanthanide metals, gadolinium (Gd) and samarium (Sm). To that end, epitaxial gadolinium and samarium thin layers are grown on AlN(0001) by molecular beam epitaxy and exposed to molecular nitrogen in high vacuum conditions and ambient temperature. In situ reflection high-energy electron diffraction is used to monitor the growth of Gd in real time, as well as the subsequent exposure to nitrogen, showing a clear transition from a pure Gd surface to a gadolinium nitride (GdN) surface layer. The formation of a nitride layer is further reinforced by magnetic measurements showing clear contributions from the Gd metal and GdN surface layers. Formation of SmN is investigated using synchrotron X-ray photoelectron spectroscopy to probe the surface of Sm pre- and post-nitrogen exposure, showing a change in the surface valence from divalent to trivalent samarium, and confirming the nitridation of the pure Sm surface layer.

Looking Outside the Square: The Growth, Structure, and Resilient Two-Dimensional Surface Electron Gas of Square SnO2 Nanotubes.

Nanotechnology has delivered an amazing range of new materials such as nanowires, tubes, ribbons, belts, cages, flowers, and sheets. However, these are usually circular, cylindrical, or hexagonal in nature, while nanostructures with square geometries are comparatively rare. Here, a highly scalable method is reported for producing vertically aligned Sb-doped SnO2 nanotubes with perfectly-square geometries on Au nanoparticle covered m-plane sapphire using mist chemical vapor deposition. Their inclination can be varied using r- and a-plane sapphire, while unaligned square nanotubes of the same high structural quality can be grown on silicon and quartz. X-ray diffraction measurements and transmission electron microscopy show that they adopt the rutile structure growing in the [001] direction with (110) sidewalls, while synchrotron X-ray photoelectron spectroscopy reveals the presence of an unusually strong and thermally resilient 2D surface electron gas. This is created by donor-like states produced by the hydroxylation of the surface and is sustained at temperatures above 400°C by the formation of in-plane oxygen vacancies. This persistent high surface electron density is expected to prove useful in gas sensing and catalytic applications of these remarkable structures. To illustrate their device potential, square SnO2 nanotube Schottky diodes and field effect transistors with excellent performance characteristics are fabricated.

Synchrotron XPS and Electrochemical Study of Aging Effect on Passive Film of Ni Alloys

To investigate aging effect on the passive film of Ni23Cr15Mo and Ni22Cr9Mo3Nb, synchrotron-based X-ray photoelectron spectroscopy (XPS) was used to analyze the structure and composition of the air-formed passive film on the alloys. The corrosion resistance of the two Ni alloys in 1 M NaCl solution was evaluated with electrochemical cyclic polarization measurement. The synchrotron XPS measurement provided detailed information about chemical states of alloying elements in the passive film, showing that the passive film consists of an inner oxide layer and an outer hydroxide layer. The XPS data allowed precise determination of the chemical composition and the thickness of the outer hydroxide layer, the inner oxide layer, and the underlying subsurface alloy layer. The Cr-oxide in the inner layer grows thicker with aging time, leading to Cr-depletion in the subsurface region. Mo and Nb in the alloy form mixed oxides and hydroxides, and aging in air leads to transformation of the lower valence oxides into higher valence oxides. The freshly formed oxide film exhibits similar barrier properties as the aged oxide film. The stability of the passive film formed on Ni22Cr9Mo3Nb seems to be better than that on Ni23Cr15Mo.

Optimal Composition of Li Argyrodite with Harmonious Conductivity and Chemical/Electrochemical Stability: Fine-Tuned Via Tandem Particle Swarm Optimization.

A tandem (two‐step) particle swarm optimization (PSO) algorithm is implemented in the argyrodite‐based multidimensional composition space for the discovery of an optimal argyrodite composition, i.e., with the highest ionic conductivity (7.78 mS cm−1). To enhance the industrial adaptability, an elaborate pellet preparation procedure is not used. The optimal composition (Li5.5PS4.5Cl0.89Br0.61) is fine‐tuned to enhance its practical viability by incorporating oxygen in a stepwise manner. The final composition (Li5.5PS4.23O0.27Cl0.89Br0.61), which exhibits an ionic conductivity (σ ion) of 6.70 mS cm−1 and an activation barrier of 0.27 eV, is further characterized by analyzing both its moisture and electrochemical stability. Relative to the other compositions, the exposure of Li5.5PS4.23O0.27Cl0.89Br0.61 to a humid atmosphere results in the least amount of H2S released and a negligible change in structure. The improvement in the interfacial stability between the Li(Ni0.9Co0.05Mn0.05)O2 cathode and Li5.5PS4.23O0.27Cl0.89Br0.61 also results in greater specific capacity during fast charge/discharge. The structural and chemical features of Li5.5PS4.5Cl0.89Br0.61 and Li5.5PS4.23O0.27Cl0.89Br0.61 argyrodites are characterized using synchrotron X‐ray diffraction, Raman spectroscopy, and X‐ray photoelectron spectroscopy. This work presents a novel argyrodite composition with favorably balanced properties while providing broad insights into material discovery methodologies with applications for battery development.

Annealing Effect of Absorbing Property for Cs and CsI in Fullerene Investigated by Synchrotron X-ray Photoelectron Spectroscopy

Annealing Effect of Absorbing Property for Cs and CsI in Fullerene Investigated by Synchrotron X-ray Photoelectron Spectroscopy

Nature of arsenic sites in arsenian pyrite- A synchrotron radiation X-ray photoelectron spectroscopy (SRXPS) study

The observed spatial variability of arsenic substituted in pyrite (arsenian pyrite) was considered to study the nature of arsenic environment using synchrotron X-ray photoelectron spectroscopy. As 3d of arsenian pyrite has shown that bulk As, present as As(-I) in arsenian pyrite, may be an As–As dimer rather than as As–S. This argument was made due to shift in the bulk As 3d component to lower binding energy, a feature observed in the As-As dimer of loellingite (FeAs2). The presence of a pronounced tail in the Fe 2p signal of arsenian pyrite, which is absent in loellingite, implies localised arsenic mineralisation in a dominating pyrite host. Possible As cluster formation producing a subtle shift in binding energy to lower values is also put forward. The surface component found at a binding energy of 41.3 eV in the As 3d of arsenian pyrite was ascribed to polymeric sites generated from surface reconstruction initiated upon fracture of As–As or As–S bond. Except for a polymeric sulphur component at a binding energy of 163.2 eV, the S 2p signal of pyrite and arsenian pyrite are equivalent at high excitation energy.

Systematic study of niobium thermal treatments for superconducting radio frequency cavities employing x-ray photoelectron spectroscopy

The structural and chemical composition of the surface layer (100–140 nm) of niobium radiofrequency cavities operating at cryogenic temperature has enormous impact on their superconducting characteristics. During the last years, cavities treated with a new thermal processing recipe, so-called nitrogen infusion, have demonstrated an increased efficiency and high accelerating gradients. The role and importance of nitrogen gas has been a topic of many debates. In the present work we employ variable-energy synchrotron x-ray photoelectron spectroscopy (XPS), to study the niobium surface subjected to the following treatments: vacuum annealing at 800 °C, nitrogen infusion, and vacuum heat treatment as for the infusion process but without nitrogen supply. Careful analysis of XPS energy-distribution curves revealed a slightly increased thickness of the native oxide Nb2O5 for the infused samples (∼3.8 nm) as compared to the annealed one (∼3.5 nm) which indicates insignificant oxygen incorporation into niobium during 120 °C baking and no effect of nitrogen on the formation of oxides or other niobium phases. By conducting an additional in-situ annealing experiment and analyzing the niobium after the failed infusion process, we conclude that the vacuum furnace hygiene particularly during the high-temperature stage is the prerequisite for success of any treatment recipe.

A synchrotron radiation photoelectron spectroscopy study on atomic-scale wet etching of InAs (111)-A and (111)-B in acidic peroxide solutions: surface chemistry versus kinetics

Anisotropy in the wet-chemical surface oxidation of indium arsenide (InAs) (111) in HCl/H2O2 and H2SO4/H2O2 was investigated. The atomic-scale etching kinetics was determined by inductively coupled plasma mass spectrometry and the surface chemistry, and electronic property transformation was probed by post operando synchrotron X-ray photoelectron spectroscopy. The oxidation rate is found to be larger for the (111)-B surface (As-terminated) than for the (111)-A surface (In-terminated). HCl/H2O2 solution has shown lower InAs etch rates than H2SO4/H2O2. The nature of etchant plays a role in the band bending, which is attributed to the formation of different species (sulfates/sulfides, chlorides, and hydroxides) on the surface.

Bidirectional Control of the Band Bending at the (2̅01) and (010) Surfaces of β-Ga2O3 Using Aryldiazonium Ion and Phosphonic Acid Grafting

Beta gallium oxide (β-Ga2O3) is an ultrawide-bandgap semiconductor with one of the highest-known breakdown strengths (∼8 MV/cm) making it a strong candidate for high-efficiency power electronics, deep-ultraviolet photodetectors, and transparent electronic devices. Bare β-Ga2O3 surfaces exhibit a strong upward band bending and electron depletion that is reduced by the formation of a hydroxyl termination on exposure to the atmosphere, although this effect varies with exposure conditions and the processing history of different samples. This work investigates the covalent modification of (2̅01) and (010) β-Ga2O3 surfaces with organic layers, as a mechanism for more effectively controlling their surface band bending. We compare the different electronic effects of grafted layers formed by the electrochemical reduction of 4-nitrobenzenediazonium ions and the spontaneous grafting of octadecylphosphonic acid (ODPA). Atomic force microscopy, synchrotron X-ray photoelectron spectroscopy (XPS), and near-edge X-ray absorption fine structure spectroscopy confirmed the presence of few-nanometer layers of covalently attached nitrophenyl (NP) and ODPA molecules. Valence band XPS showed that NP modification produced upward band bending shifts of +0.51 and +0.53 eV on (2̅01) and (010) β-Ga2O3, respectively, with further increases of +0.35 and +0.20 eV after X-ray-induced reduction of the nitro substituent to amino-like moieties. In contrast, ODPA modification produced downward shifts in surface band bending of −0.36 and −0.07 eV on (2̅01) and (010) β-Ga2O3, respectively. Our study demonstrates that covalently bound NP and ODPA molecules can be used to significantly modify the electronic properties of β-Ga2O3 surfaces, a finding that should prove useful for electronic applications of this material.

Thickness and composition of native oxides and near-surface regions of Ni superalloys

The surface chemistry and thickness of the native oxide, hydroxide, and modified sub-surface layer of three Ni superalloys (alloy 59, 625, and 718) were determined by synchrotron X-ray Photoelectron Spectroscopy (XPS) and X-ray Reflectivity (XRR). Taking advantage of the synchrotron radiation techniques, a procedure for normalizing the photoelectron intensity was employed, which allowed for accurate quantitative analysis revealing a total oxide thickness for all samples of 12–13 Å, a hydroxide layer of 2–3 Å, and a thickness of the sub-surface alloy layer of 20–35 Å. The thickness results were compared to structural atomic models suggesting that the oxide thickness corresponds to four planes of metal cations in the oxide matrix. The XPS data revealed that the native oxides were enriched in Cr3+, Mo(4,5,6)+, and Nb5+, while no Ni oxide was detected. The hydroxide layer mainly contained Ni2+ and Cr3+ hydroxide. The sub-surface layer was enriched in Ni and depleted in Cr, Fe, Mo, and Nb. The obtained oxide composition can be explained using thermodynamics, and it was found that the oxide composition correlates with the enthalpy of oxide formation for the metal elements in the alloys. Finally, the advantages of synchrotron radiation for composition and thickness determination are discussed.

(Invited) Challenges in III-V Surface Passivation and Some Potential Solutions

In many applications, III-V surfaces lie at the buried or embedded interface of an insulator/III-V or metal/III-V junction. The chemical and physical properties of these III-V interfaces are typically different from the uniform III-V bulk crystal structures. Thus, it is difficult to predict the interface behavior (e.g. electronic structure and doping efficiency) on the basis of the established bulk-crystal physics and chemistry. III-V device interfaces often have an amorphous structure and extend over several atomic layers, instead of having a monocrystalline and atomically sharp structure. Reasons for that are discussed in this presentation.Therefore, it is not surprising that the III-V interfaces are a weak part of many practical devices (e.g. laser diode, infrared photodiode, micro-LED, HEMT) causing degradation in the current as well as future III-V applications. III-V interfaces include a high density of point defects, as compared to the uniform bulk crystals areas, which cause the electronic defect levels around the band gap at the interfaces, which further cause the electrical and optical losses. Use of III-V crystals is expected to increase in the areas like LED lighting, infrared optoelectronics, and high-speed transistor circuits, where it is not possible to utilize the Si crystals.Thus it is relevant to investigate and develop the III-V interface properties. The surface or interface passivation means a decrease in the defect-level density or/and in the effects of defect levels. It can be expected that the III-V passivation development requires a multistep procedure combining benefits of the different methods.Some commonly used passivation methods (e.g. wet chemistry, hydrogen forming gas, ALD self cleaning) are summarized in this talk. Our group has studied the issue whether it is useful to include ultrahigh-vacuum (UHV) treatments in a large arsenal of different passivation methods. The traditional surface science has been based on UHV technology. Our group has approached the III-V passivation challenge from a surface-science viewpoint. We have investigated in particular the clean surface properties and the surface oxidation in UHV conditions. Our group has addressed the question whether it is beneficial to pre-oxidize III-V surfaces intentionally because it is very difficult to avoid oxidation of III-V device surfaces. Our results support the previous findings that proper oxidation of III-V interfaces is a potential method to improve the devices performance.We have taken a step toward combining the surface science with the electrical characterization of III-V interfaces to find interconnections between electrical properties and surface-science ones. The high-resolution synchrotron x-ray photoelectron spectroscopy (XPS) has been used. Because XPS is a widely used method, we have addressed specific challenges in XPS analysis and that how the XPS analysis can be developed by including theoretical ab initio calculations. In this talk, it is also attempted to make a comparison to the silicon interface passivation.

Engineering selenium-doped nitrogen-rich carbon nanosheets as anode materials for enhanced Na-Ion storage

Heteroatom-doping is an effective way to regulate the electronic structure of carbon so as to improve their Na-ion storage capability. In all kinds of heteroatoms, selenium (Se)-doping has rarely been investigated even though Se has unique physicochemical properties. Herein, we report a facile fabrication of Se/N-codoped carbon nanosheets (N/Se-CNs) with 10 at% N and 2 at% Se by an oxidation-selenization process. The synchrotron X-ray photoelectron spectroscopy (XPS) measurements indicate that Se doping can significantly change the electronic structures of carbon nanosheets, because Se can act as stronger electron-withdrawing units for Na storage compared with C and N. Therefore, Se can better play the role of improving the conductivity and wettability of the carbon materials. And Se atoms effectively enlarge the interlayer distance of carbon nanosheets to enhance the Na-ion diffusion kinetics and also generate extra active sites for Na-ion storage. Served as anode materials for SIBs, the N/Se-CNs delivers higher specific capacity and better rate capability compared with only N-doped carbon nanosheets. Therefore, Se-doping is an effective way for improvement of Na-storage capability of carbon materials. Moreover, this work should provide a new pathway for design of carbons doped by heteroatoms with large atomic size towards excellent Na-ion storage.