Synchrotron Radiation X-ray Photoelectron Spectroscopy Research Articles

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.

Characterization of nitrided SiC(1 [formula omitted] 00) MOS structures by means of electrical measurements and X-ray photoelectron spectroscopy

Interface nitridation in nitric oxide is a standard technique for improving the performance of silicon carbide (SiC) metal-oxide-semiconductor (MOS) devices. In this study, we focused on m-face SiC MOS structures and investigated the impact of nitridation by means of electrical measurements and X-ray photoelectron spectroscopy (XPS). Gate leakage characteristics were measured in a wide temperature range of about −150 – 100 °C, which clarified that nitridation leads to an unwanted increase in the Fowler-Nordheim leakage current. Scanning XPS measurements revealed that the amount of nitrogen incorporated at the m-face MOS interface could reach about 2.3 times higher than the Si-face MOS interface. The incorporation of nitrogen likely reduces the conduction band offset at the SiO2/SiC interface, thereby increasing the gate leakage. This conclusion was further corroborated in an analysis of band alignment based on synchrotron radiation XPS measurements. Therefore, while nitrided m-face SiC MOS devices exhibit superior on-state performances, they have a limitation in terms of reliability.

Imaging the space-resolved chemical heterogeneity of degraded graphite anode through scanning transmission X-ray microscope

Deeply understanding the chemical heterogeneity of graphite anode is important because it can provide insights into the underlying mechanisms of electrode degradation and failure in battery systems. Herein, the chemical heterogeneity of graphite anode is imaged by X-ray fluorescence mode scanning transmission X-ray microscope (XRF-STXM) to space-resolve the chemical distribution and structure at C, O, F and Na sites. The synchrotron radiation X-ray photoelectron spectroscopy analysis suggests that the solid-electrolyte-interphase (SEI) layer on graphite anodes contains a complex mixture of both organic (-COOR, -C-OR, etc.) and inorganic species (CO32−, LiF, LixPOyFz, etc.), which can vary with depth. Furthermore, XRF-STXM imaging combined with X-ray absorption spectra is used to map out the heterogeneity in the chemical spatial distribution of SEI composition and its location dependence, as well as its strong correlation with the distribution of conductive additives/binders and binders decomposition. The distribution of SEI exhibits a distinctive feature, with the CO32− and LiF species being significantly thicker at edge plane than at basal plane and the distribution being highly non-uniform. The insights gained from the visualization of the spatial chemical distribution will deepen our understanding of SEI, which is crucial for delineating effective strategies for the development of Li-ion batteries.

Formation of high-quality SiO2/GaN interfaces with suppressed Ga-oxide interlayer via sputter deposition of SiO2

While the formation of a GaO x interlayer is key to achieving SiO2/GaN interfaces with low defect density, positive fixed charge is rather easily generated through the reduction of GaO x layer if the annealing conditions are not properly designed. In this study, we minimized the unstable GaO x layer by sputter SiO2 deposition. Negligible GaO x growth was confirmed by synchrotron radiation X-ray photoelectron spectroscopy, even when post-deposition oxygen annealing up to 600 °C was performed. A MOS device with negligible capacitance–voltage hysteresis, stable flat-band voltage, and low leakage current was demonstrated by performing oxygen and forming gas annealing at temperatures of 600 °C and 400 °C, respectively.

Work function lowering of LaB6 by monolayer hexagonal boron nitride coating for improved photo- and thermionic-cathodes

We report a lowering of work function for lanthanum hexaboride (LaB6) by monolayer hexagonal boron nitride (hBN) coating. Photoemission electron microcopy (PEEM) and thermionic emission electron microscopy (TEEM) both revealed that the hBN coated region of a LaB6 (100) single crystal has a lower work function compared to the bare (i.e., non-coated) and graphene coated regions. A broad and uniform brighter image of the hBN coated region in PEEM was quantitatively supported by a 0.4 eV decrease in the work function in photoelectron spectra compared to the bare region. TEEM results were consistent in that the hBN coated region exhibited thermionic emission at 905 °C, whereas the bare and graphene coated regions did not. A larger decrease in the work function for hBN coated LaB6 (100) compared to graphene coated LaB6 (100) was qualitatively supported by our density functional theory calculations. Adding an oxide layer in the calculations improved consistency between the calculation and experimental results. We followed up our calculations with synchrotron-radiation x-ray photoelectron spectroscopy and confirmed the presence of an oxide layer on our LaB6.

Chemical modification of the GaP(0 0 1) surface electric field with sulfide solutions

The surface electric field in GaP(001) passivated with aqueous and alcoholic sulfide solutions is studied by reflectance anisotropy spectroscopy using data on the surface chemical and electronic structure obtained by synchrotron-radiation X-ray photoelectron spectroscopy. Treatment of n-GaP(001) surface with a sulfide solution decreases the surface electric field depending on sulfide solution composition. In particular, passivation with the solution of ammonium sulfide in 2-propyl alcohol turns the total surface electric field almost to zero, while passivation with the aqueous ammonium sulfide solution reduces the surface electric field in a lesser extent. On the other hand, sulfide treatment of p-GaP(001) surface enhances the surface electric field dramatically and this enhancement is nearly independent of the sulfide solution composition. Such changes in the surface electric field can be related to the modification of the dipole associated with chemical bonds in the passivating layer.

In situ observation of the on-surface thermal dehydrogenation of n-octane on Pt(111).

The catalytic dehydrogenation of alkanes constitutes a key step for the industrial conversion of these inert sp3-bonded carbon chains into other valuable unsaturated chemicals. To this end, platinum-based materials are among the most widely used catalysts. In this work, we characterize the thermal dehydrogenation of n-octane (n-C8H18) on Pt(111) under ultra-high vacuum using synchrotron-radiation X-ray photoelectron spectroscopy, temperature-programmed desorption and scanning tunneling microscopy, combined with ab initio calculations. At low activation temperatures, two different dehydrogenation stages are observed. At 330 K, n-C8H18 effectively undergoes a 100% regioselective single C-H bond cleavage at one methyl end. At 600 K, the chemisorbed molecules undergo a double dehydrogenation, yielding double bonds in their carbon skeletons. Diffusion of the dehydrogenated species leads to the formation of carbon molecular clusters, which represents the first step towards poisoning of the catalyst. Our results reveal the chemical mechanisms behind the first stages of alkane dehydrogenation on a platinum model surface at the atomic scale, paving the way for designing more efficient dehydrogenation catalysts.

Electrical properties and energy band alignment of SiO2/GaN metal-oxide-semiconductor structures fabricated on N-polar GaN(0001¯) substrates

The interface properties and energy band alignment of SiO2/GaN metal-oxide-semiconductor (MOS) structures fabricated on N-polar GaN(0001¯) substrates were investigated by electrical measurements and synchrotron-radiation x-ray photoelectron spectroscopy. They were then compared with those of SiO2/GaN MOS structures on Ga-polar GaN(0001). Although the SiO2/GaN(0001¯) structure was found to be more thermally unstable than that on the GaN(0001) substrate, excellent electrical properties were obtained for the SiO2/GaN(0001¯) structure by optimizing conditions for post-deposition annealing. However, the conduction band offset for SiO2/GaN(0001¯) was smaller than that for SiO2/GaN(0001), leading to increased gate leakage current. Therefore, caution is needed when using N-polar GaN(0001¯) substrates for MOS device fabrication.

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.

Impact of nitridation on the reliability of 4H-SiC(112̄0) MOS devices

The leakage current and flat-band voltage (V FB) instability of NO-nitrided SiC(110) (a-face) MOS devices were systematically investigated. Although NO nitridation is effective in improving the interface properties, we found that it reduces the onset field of Fowler–Nordheim current by about 1 MV cm−1, leading to pronounced leakage current. Synchrotron radiation X-ray photoelectron spectroscopy revealed that the nitridation reduces the conduction band offset at the SiO2/SiC interface, corroborating the above finding. Furthermore, systematical positive and negative bias stress tests clearly indicated the enhancement of V FB instability of nitrided a-face MOS devices against electron and hole injection.

Oxidation Mechanisms of Hafnium Overlayers Deposited on an Si(111) Substrate.

The oxidation mechanism of hafnium overlayers on an Si(111) substrate [Hf-Si(111), including the outermost metallic Hf overlayers and interfacial Hf silicides (HfSi and HfSi4)] was investigated via high-resolution synchrotron radiation X-ray photoelectron spectroscopy (SR-XPS) of Hf 4f5/2,7/2, Si 2p1/2,3/2, and O 1s core levels. The atomic-scale interaction of O2 molecules with Hf-Si(111) is discussed by comparing the results obtained following thermal O2 exposures [translational energy (Et) ≈ 0.03 eV] with those obtained following supersonic O2 molecular beam (SOMB) irradiation (Et ≈ 2.2 eV). Metallic Hf and interfacial HfSi were immediately oxidized to HfO2 and Hf (sub)silicates (Hf-O-Si configurations) via trapping-mediated dissociative adsorption. Upon excessive SOMB irradiation, the other interfacial HfSi4 was oxidized via direct dissociation. When oxidation proceeded at the Si(111) substrate via excess SOMB irradiation, volatile Si atoms were emitted from the interfacial SiO2/Si-strained layers. When the volatile Si atoms were trapped in the overlayers, the HfO2 overlayers were converted into completely oxidized Hf silicate layers. However, when the volatile Si atoms passed through the HfO2 overlayers, they reacted with the impinging O2, and the outermost SiO2 deposition layers were formed on HfO2 (or Hf silicate) layers.

Trimethylamine Probes Isolated Silicon Dangling Bonds and Surface Hydroxyls of (H,OH)-Si(001)

To better understand why amines catalyze the reactivity of SiOH with silanes, we examined the adsorption of trimethylamine under a low pressure (10–9–10–8 mbar) and a low temperature (105–160 K) on water-terminated (H,OH)-Si(001), which is both a model surface for adsorption studies and a promising starting substrate for atomic layer deposition. Trimethylamine bonding configurations were determined by combining real-time synchrotron radiation X-ray photoelectron spectroscopy (XPS) and high-resolution electron energy loss spectroscopy (HREELS) with density functional theory (DFT) calculations of core-level ionization energies and vibrational spectra. Both spectroscopies showed that the majority of species are trimethylamine molecules making acceptor H bonds with surface hydroxyls. Moreover, HREELS indicated that the hydrogen-bonding modes (single and double hydrogen acceptor bonds) depend on temperature and/or coverage, which may in turn affect the weakening of the O–H bond, and hence the catalytic effects of trimethylamine. XPS also clearly detected a minority species, trimethylamine, datively bonded to the isolated silicon dangling bonds (a few 1/100th of a monolayer). This species is prone to breaking, and a detailed analysis of the reaction products was made. The reactivity of the electrically active isolated silicon dangling bonds with the amine may impact the Fermi-level position in the gap.

Surface composition of size-selected sea salt particles under the influence of organic acids studied in situ using synchrotron radiation X-ray photoelectron spectroscopy

Synchrotron X-ray photoelectron spectroscopic surface characterisation of size-resolved sea salt aerosol particles revealed Mg enrichment in the particle surface layer which was either enhanced or decreased depending on the organic compound added.

Use of XANES and XPS to investigate the effects of ethanol contamination on anti-wear ZDDP tribofilms

This work uses synchrotron radiation XANES and XPS measurements to investigate the structure of ZDDP tribofilms formed from lubricants contaminated with anhydrous and hydrated ethanol (5%wt). The tribofilms were formed at 40 °C from PAO8 lubricant containing only ZDDP additive onto carbon steel specimens using sliding tests, cylinder-on-flat configuration (line contact) under boundary lubrication. Ethanol increased friction as well as surface damage, detected by an increase in relative porous area. Angle-resolved measurements (15°, 45°, 90°) helped detecting differences along the thickness of the tribofilm. The amount of long-chain phosphates apparently reduced with ethanol, leading to reduced wear performance. The sulphides in the tribofilms were richer in Fe in the presence of ethanol, also indicative of more severe wear.

Silver Nanoparticles Functionalized by Fluorescein Isothiocyanate or Rhodamine B Isothiocyanate: Fluorescent and Plasmonic Materials

This paper presents the synthesis of silver nanoparticles (AgNPs) functionalized with fluorescent molecules, in particular with xanthene-based dyes, i.e., fluorescein isothiocyanate (FITC, λmax = 485 nm) and rhodamine B isothiocyanate (RITC, λmax = 555 nm). An in-depth characterization of the particle–dye systems, i.e., AgNPs–RITC and AgNPs–FITC, is presented to evaluate their chemical structure and optical properties due to the interaction between their plasmonic and absorption properties. UV–Vis spectroscopy and the dynamic light scattering (DLS) measurements confirmed the nanosize of the AgNPs–RITC and AgNPs–FITC. Synchrotron radiation X-ray photoelectron spectroscopy (SR-XPS) was used to study the chemical surface functionalization by structural characterization, confirming/examining the isothiocyanate–metal interaction. For AgNPs–RITC, in which the plasmonic and fluorescence peak are not superimposed, the transient dynamics of the dye fluorescence were also studied. Transient absorption measurements showed that by exciting the AgNPs–RITC sample at a wavelength corresponding to the AgNP plasmon resonance, it was possible to preferentially excite the RITC dye molecules attached to the surface of the NPs with respect to the free dye molecules in the solution. These results demonstrate how, by combining plasmonics and fluorescence, these AgNPs can be used as promising systems in biosensing and imaging applications.

Direct-ARPES and STM Investigation of FeSe Thin Film Growth by Nd:YAG Laser

Research on ultrathin quantum materials requires full control of the growth and surface quality of the specimens in order to perform experiments on their atomic structure and electron states leading to ultimate analysis of their intrinsic properties. We report results on epitaxial FeSe thin films grown by pulsed laser deposition (PLD) on CaF2 (001) substrates as obtained by exploiting the advantages of an all-in-situ ultra-high vacuum (UHV) laboratory allowing for direct high-resolution surface analysis by scanning tunnelling microscopy (STM), synchrotron radiation X-ray photoelectron spectroscopy (XPS) and angle-resolved photoemission spectroscopy (ARPES) on fresh surfaces. FeSe PLD growth protocols were fine-tuned by optimizing target-to-substrate distance d and ablation frequency, atomically flat terraces with unit-cell step heights are obtained, overcoming the spiral morphology often observed by others. In-situ ARPES with linearly polarized horizontal and vertical radiation shows hole-like and electron-like pockets at the Γ and M points of the Fermi surface, consistent with previous observations on cleaved single crystal surfaces. The control achieved in growing quantum materials with volatile elements such as Se by in-situ PLD makes it possible to address the fine analysis of the surfaces by in-situ ARPES and XPS. The study opens wide avenues for the PLD based heterostructures as work-bench for the understanding of proximity-driven effects and for the development of prospective devices based on combinations of quantum materials.

Chemical in‐depth analysis of (Ca/Sr)F2 core–shell like nanoparticles by X‐ray photoelectron spectroscopy with tunable excitation energy

The fluorolytic sol–gel synthesis is applied with the intention to obtain two different types of core–shell nanoparticles, namely, SrF2–CaF2 and CaF2–SrF2. In two separate fluorination steps for core and shell formation, the corresponding metal lactates are reacted with anhydrous HF in ethylene glycol. Scanning transmission electron microscopy (STEM) and dynamic light scattering (DLS) confirm the formation of particles with mean dimensions between 6.4 and 11.5 nm. The overall chemical composition of the particles during the different reaction steps is monitored by quantitative Al Kα excitation X‐ray photoelectron spectroscopy (XPS). Here, the formation of stoichiometric metal fluorides (MF2) is confirmed, both for the core and the final core–shell particles. Furthermore, an in‐depth analysis by synchrotron radiation XPS (SR‐XPS) with tunable excitation energy is performed to confirm the core–shell character of the nanoparticles. Additionally, Ca2p/Sr3d XPS intensity ratio in‐depth profiles are simulated using the software Simulation of Electron Spectra for Surface Analysis (SESSA). In principle, core–shell like particle morphologies are formed but without a sharp interface between calcium and strontium containing phases. Surprisingly, the in‐depth chemical distribution of the two types of nanoparticles is equal within the error of the experiment. Both comprise a SrF2‐rich core domain and CaF2‐rich shell domain with an intermixing zone between them. Consequently, the internal morphology of the final nanoparticles seems to be independent from the synthesis chronology.

High quantum efficiency GaAs photocathodes activated with Cs, O2, and Te

GaAs photocathodes are the primary choice for generating spin-polarized electron beam with high brightness, high polarization, and fast polarization reversal. However, it suffers from short lifetime due to the highly reactive nature of the emission surface, resulting in substantial operational difficulties. Activating GaAs with a more robust material, such as Cs2Te, shows comparable polarization to that of Cs–O activation and increases the lifetime due to the robustness of the Cs2Te layer. However, previously reported photocathodes based on Cs–Te activation on GaAs suffer from 10× lower quantum efficiency (QE) compared to that activated with conventional Cs–O activation. Herein, we report activation recipes for GaAs photocathodes using Cs, O2, and Te. For Cs–Te activation, the QE was 6.6% at 532 nm. For Cs–O–Te activation, the QE was 8.8% at 532 nm and 4.5% at 780 nm. The negative electron affinity of the activated GaAs was directly measured and confirmed by low energy electron microscopy. We also report the activation layer chemical states and stoichiometry using in situ micro-spot synchrotron radiation x-ray photoelectron spectroscopy.

Impact of alkaline and acid treatment on the surface chemistry of a hot-dip galvanized Zn–Al–Mg coating

The surface chemistry of an exemplary hot-dip galvanized Zn–Al–Mg coating comprising 1.6 wt.% aluminum and 1.1 wt.% magnesium was characterized by means of secondary and transmission electron microscopy as well as synchrotron radiation X-ray photoelectron spectroscopy. Lamellae of the coating prepared via focused ion beam technique were examined through energy dispersive X-ray analysis. Further the effect of alkaline and acid treatment respectively was investigated by employing time of flight secondary ion mass spectroscopy, energy dispersive X-ray analysis and glow discharge optical emission spectroscopy. Throughout this work it is shown that both alkaline (NaOH) and acid (H2SO4) solutions alter the chemical composition when applied to the coating surface. With the native Zn–Al–Mg system yielding a complex microstructure covered by a magnesium and aluminum rich oxide film, highly alkaline surroundings (pH > 13) caused a decrease of aluminum at the uppermost surface. Every deployed acid solution (pH < 2.4) lead to a preferred pickling of the magnesium compounds including the oxide layer plus the intermetallic MgZn2 phases of the coating. Exposure to both highly alkaline and acid environments corresponded to a higher proportion of zinc at the coating surface measured by all applied surface analytical tools indicating that the modification of the chemical composition took place on both nanoscopic and microscopic scale.

Formation of BN-covered silicene on ZrB2/Si(111) by adsorption of NO and thermal processes.

We have investigated the adsorption and thermal reaction processes of NO with silicene spontaneously formed on the ZrB2/Si(111) substrate using synchrotron radiation x-ray photoelectron spectroscopy (XPS) and density-functional theory calculations. NO is dissociatively adsorbed on the silicene surface at 300 K. An atomic nitrogen is bonded to three Si atoms most probably by a substitutional adsorption with a Si atom of silicene (N≡Si3). An atomic oxygen is inserted between two Si atoms of the silicene (Si-O-Si). With increasing NO exposure, the two-dimensional honeycomb silicene structure gets destroyed, judging from the decay of typical Si 2p spectra for silicene. After a large amount of NO exposure, the oxidation state of Si becomes Si4+ predominantly, and the intensity of the XPS peaks of the ZrB2 substrate decreases, indicating that complicated silicon oxinitride species have developed three-dimensionally. By heating above 900 K, the oxide species start to desorb from the surface, but nitrogen-bonded species still exist. After flashing at 1053 K, no oxygen species is observed on the surface; SiN species are temporally formed as a metastable species and BN species also start to develop. In addition, the silicene structure is restored on the ZrB2/Si(111) substrate. After prolonged heating at 1053 K, most of nitrogen atoms are bonded to B atoms to form a BN layer at the topmost surface. Thus, BN-covered silicene is formed on the ZrB2/Si(111) substrate by the adsorption of NO at 300 K and prolonged heating at 1053 K.