X-ray Photoemission Spectroscopy Analysis Research Articles

Voltage induced reversible and irreversible change of magnetic coercivity in Co/ZnO heterostructure

In this study, the application of bias voltage to 4–8 nm Co/275 nm ZnO heterostructures changed the magnetic behavior reversibly or irreversibly, depending on the different voltage-induced mechanisms. The magnetic coercivity (Hc) monotonically decreased 20% when the small voltages of 0–8 V were applied. The Hc reduction was symmetric with the voltage polarity, and the reversibility was demonstrated by cyclically switching the bias voltage between 0 and 7 V. While a large voltage up to 40 V was applied to the Co/ZnO junction, the current heating effect became considerable and the Co-oxide was formed, as confirmed by depth-profiling X-ray photoemission spectroscopy analysis. The presence of Co-oxide in the Co films induced the irreversible reduction of the Kerr signal and Hc at room temperature. The considerable Hc enhancement at 130 K also indicates the exchange bias coupling effect from the antiferromagnetic Co-oxide.

N–H and S–H insertions over Cu(I)-zeolites as heterogeneous catalysts

N–H and S–H insertion reactions of α-diazoesters into amines and thiols were conducted using various Cu(I)-zeolites, such as zeolite Y, Y USY, ZSM-5, and beta. All the Cu(I)-zeolites successfully catalyzed N–H insertion reactions with high product yields (70–82%) in aqueous solution at room temperature. Interestingly, Cu(I)-USY (Si/Al=30) showed better activity for both N–H and S–H insertion reactions than Cu(I)-Y (Si/Al=2.6), even though they have the same structure and the same +1 oxidation state for Cu. X-ray diffraction, transmission electron microscopy, and X-ray photoemission spectroscopy analysis of the fresh and used catalysts revealed that no noticeable change in the zeolite structure, oxidation state of Cu, or sintering of Cu occurred during the reactions. Furthermore, after being recycled four times, the catalysts showed only minor activity decreases, exhibiting conversion rates 70–80% of those of the fresh catalysts, demonstrating their stability under the current reaction conditions. Temperature programmed reduction experiments showed that reduction of Cu+ to Cu° occurred at ca. 300°C over Cu(I)-USY, while it occurred at ca. 800°C over Cu(I)-Y. The significantly higher activity of Cu(I)-USY than Cu(I)-Y may be due to the more electrophilic Cu centers on Cu(I)-USY, which is highly favorable for ylide formation, and therefore facilitates N–H and S–H insertions.

Radiative mechanism and surface modification of four visible deep level defect states in ZnO nanorods.

Visible luminescence from ZnO nanorods (NRs) is attracting large scientific interest for light emission and sensing applications. We study visible luminescent defects in ZnO NRs as a function of post growth thermal treatments, and find four distinct visible deep level defect states (VDLSs): blue (2.52 eV), green (2.23 eV), orange (2.03 eV), and red (1.92 eV). Photoluminescence (PL) studies reveal a distinct modification in the UV (3.25 eV) emission intensity and a shift in the visible spectra after annealing. Annealing at 600 °C in Ar (Ar600) and O2 (O600) causes a blue and red-shift in the visible emission band, respectively. All samples demonstrate orange emission from the core of the NR, with an additional surface related green, blue, and red emission in the As-Prep, Ar600, and O600 samples, respectively. From PL excitation (PLE) measurements we determine the onset energy for population of the various VDLSs, and relate it to the presence of an Urbach tail below the conduction band due to a presence of ionized Zni or Zni complexes. We measured an onset energy of 3.25 eV for the as prepared sample. The onset energy red-shifts in the annealed samples by about 0.05 to 0.1 eV indicating a change in the defect structure, which we relate to the shift in the visible emission. We then used X-ray photoemission spectroscopy (XPS), and elastic recoil detection analysis (ERDA) to understand changes in the surface structure, and H content, respectively. The results of the XPS and ERDA analysis explain how the chemical states are modified due to annealing. We summarize our results by correlating our VDLSs with specific intrinsic defect states to build a model for PL emission in ZnO NRs. These results are important for understanding how to control defect related visible emission for sensing and electroluminescence applications.

Electronic structure and metal-insulator transition in crystalline magnetic phase-change material Ge1−xFexTe

X-ray photoemission spectroscopy (XPS) and carrier transport measurements were used to investigate the electronic structure of crystalline Ge1−xFexTe films across the metal-insulator transition. Both valence-band XPS analysis and transport properties indicate that the Fermi energy is shifted downward with increasing Fe concentration. This shift is ascribed to that the incorporated Fe atoms give rise to shallow states near the Fermi energy. Temperature-dependent resistivity measurements reveal that Ge1−xFexTe films undergo a transition from insulating to metallic state at Fe concentration x above 0.1. This metal-insulator transition is attributed to disorder-induced Anderson localization.

Modification of fluorous substrates with oligo(ethylene glycol) via “click” chemistry for long-term resistance of cell adhesion

In this work perfluorinated substrates fabricated from SiO2 glass slides are modified with oligo(ethylene glycol) (OEG) units for long-term resistance of cell adhesion purposes, based on fluorous interactions and click chemistry. Specifically, fluorous substrates, prepared by treatment of glass slides with 1H, 1H, 2H, 2H-perfluorodecyltrimethoxysilane (FAS17), were coated with ethynyl-OEG-C8F17, followed by covalent attachment of an azido-OEG via copper-catalyzed azide-alkyne cycloaddition (CuAAC) “click” reaction. We demonstrate that the resultant surface avoid fibrinogen adsorption and resisted cell adhesion for over 14days. X-ray photoemission spectroscopy (XPS) analysis and contact angle goniometry measurements confirm the presence of the OEG molecules on the fluorous substrates. Bright field optical images show total absence of 3T3 fibroblast cells on the OEG modified fluorinated substrate for 1 and 5days, and a remarkably decrease of cell adhesion at 14days.

Electrode metal penetration of amorphous indium gallium zinc oxide semiconductor thin film transistors

Penetration effects of various electrode materials, namely Al, Au, and Cu, on the physical and electrical characteristics of amorphous oxide semiconductor thin film transistors (TFTs) were investigated. Amorphous indium gallium zinc oxide (a-IGZO) TFTs were fabricated with conventional staggered bottom gate structures on a p-type Si substrate. X-ray photoemission spectroscopy (XPS) analysis under the electrode deposition area revealed variations in the oxygen bonding states and material compositions of the a-IGZO layer. Field-emission scanning electron microscopy (FE-SEM) with the line scan of energy dispersive spectroscopy (EDS) showed lateral penetration by the electrode metal. To compare the electrical characteristics of the tested TFTs, the initial current–voltage (I–V) transfer characteristics were examined. In addition, the tested TFTs fabricated using various electrode materials were tested under bias stress to verify the correlations between variations in TFT characteristics and both the metal work function and penetration-induced oxygen vacancies in the channel around the contact area.

X-ray Photoemission Spectroscopy Studies of Cesium Antimonide Photocathodes for Photoinjector Applications

Within the CLIC (Compact Linear Collider) project, feasibility studies of a photoinjector option for the drive beam as an alternative to its baseline design using a thermionic electron gun (Geschonke et al. [1]) are on-going. This R&D program covers both the laser and the photocathode side. Cesium antimonide cathodes were produced at CERN by co-deposition onto copper substrates and characterized by photoemission and by XPS (X-ray Photoemission Spectroscopy) analysis. A systematic study on newly produced and used photocathodes was conducted in order to correlate the surface composition to the photoemissive properties.

Molecular-structure variation of biomolecules irradiated with atmospheric-pressure plasma through plasma/liquid interface

Plasma interactions with organic materials through a plasma/liquid interface have been investigated via the degradation of l-alanine in liquid by irradiation with atmospheric-pressure He plasma. The optical emission spectra of atmospheric-pressure He plasma show considerable emissions of He lines and emissions of O, N, H, and OH radicals attributed to the dissociation of air and water. Variation in the molecular structure of l-alanine in aqueous solution irradiated with the plasma has been observed by Fourier transform infrared (FT-IR) spectroscopy and X-ray photoemission spectroscopy (XPS). The results obtained from FT-IR and XPS analyses show that the atmospheric-pressure He plasma exposure affects the structural variation of l-alanine in solution owing to the oxidation by irradiated radicals from plasma through the plasma/liquid interface.

Effects of nitrogen composition on the resistivity of reactively sputtered TaN thin films

Nanocrystalline tantalum nitride (TaN) thin films have been deposited by reactive direct current magnetron sputtering technique on Si/SiO2 (100) substrate with nitrogen flow rate ranging from 0, 3, 5, 7, 9 to 11 standard cubic centimeter per minute (sccm). Structural properties, surface morphology, chemical composition and and resistivity of the TaN films were investigated by X-ray diffraction (XRD), field emission scanning electron microscopy, X-ray photoemission spectroscopy (XPS) and four-point probe measurements, respectively. In the XRD spectra, a classical formation sequence of tantalum nitride phases in the order of Ta-Ta2N-TaN-Ta4N5 and decreasing amount of metallic Ta were observed with increasing nitrogen flow. The electrical resistivity of the TaN film was found to increase with increasing N/Ta ratio as a result of the increased electron scattering from interstitial N atoms. In the XPS analysis, two groups of Ta4f doublets relating to different TaN phases were observed in the core level spectra of TaN films. No strong coupling was observed between the Ta4f doublets and the Ta4p and the N1s groups. The appropriate nitrogen flow was believed to be helpful in the bonding and formation of stoichiometric TaN. Copyright © 2014 John Wiley & Sons, Ltd.

3D scaffolds from vertically aligned carbon nanotubes/poly(methyl methacrylate) composites via atom transfer radical polymerization

Vertically aligned carbon nanotubes (VACNTs) synthesized by Thermal Chemical Vapour Deposition (TCVD) were modified using an Ar:O2 (97:3) plasma to generate oxygen-containing functional groups on the surface for subsequent modification. X-ray photo-emission spectroscopy (XPS) and micro-Raman analyses confirmed the grafting of those functional groups onto the surface of the nanotubes as well as the removal of amorphous carbon produced and deposited on the VACNT forests during the CVD process. The plasma treated VACNT forests were further modified with 2-bromo-2-methylpropionyl bromide, an atom transfer radical polymerization (ATRP) initiator, to grow poly(methyl methacrylate) (PMMA) chains from the forests via ATRP. Scanning transmission electron microscopy (STEM) of the ensuing VACNT/PMMA composites confirmed the coating of the nanotube forests with the PMMA polymer. 3D scaffolds of polymeric composites with honeycomb like structure were then obtained. Compressive tests have shown that the VACNT/PMMA composite has higher compressive strength than the pristine forest.

Thermal Transformation of 4,4″-Diamino-p-terphenyl on a Si(111)-7 × 7 Surface Analyzed by X-ray Photoemission Spectroscopy and Scanning Tunneling Microscopy

Analysis using X-ray photoemission spectroscopy (XPS) and scanning tunneling microscopy (STM) was carried out for 4,4″-diamino-p-terphenyl (DAT) deposited on a Si(111)-7 × 7 surface with a low coverage at room temperature and subsequently heated at temperatures from 300 to 800 °C. The XPS analysis showed that nitrogen atoms in amino groups of DAT partly reacted with the Si at 350 °C and were transformed to silicon nitride at 600 °C, while carbon atoms in phenyl rings of DAT partly reacted with the Si at 600 °C. The aggregations were found after being heated at 800 °C by STM observation and were attributed to silicon carbide and silicon nitride with carbon atoms in the phenyl rings out of DAT by XPS analysis. The XPS and STM combined analysis revealed the critical temperatures of decomposition and transformation of molecules on substrates with their arrangements and chemical states at a molecular level.

Atomic layer deposition of CeO2/HfO2 gate dielectrics on Ge substrate

We systematically investigated atomic layer deposition (ALD) of HfO2, CeO2 and Ce-doped HfO2 thin films on Ge substrates by using tetrakis dimethylamino hafnium (TDMAH) and tris(isopropyl-cyclopentadienyl)cerium [Ce(iPrCp)3] precursors with H2O. The growth characteristics, chemical and electrical properties were comparatively characterized. On the basis of X-ray photoemission spectroscopy analyses, it was confirmed that the ALD CeO2 on Ge can form a stable interfacial layer composed of Ge1+ and Ge3+, leading to improved interfacial properties. In addition, Ce-doped HfO2 films with various Ce compositions (Ce:Hf=1:1, 1:2, 1:4 and 1:8) were prepared by an ALD supercycle process on Ge substrates. Thereby, we demonstrated that overall electrical properties including dielectric constant, interface state density, hysteresis and leakage current density are significantly improved.

Purification of Carbon Nano-Onions Fabricated by Arc Discharge

Soot containing carbon nano-onions was synthesized by traditional electric arc-discharge method with nickel powder as catalyst. After heating at 400°C for 2 h, the arc discharged soot was soaked in hydrochloric acid solution 8 h, and then ultrasonically dispersed into sodium cholate solution. Such obtained dispersion was filtrated with a medium-speed filter paper, and the filtrate was further filtered with a 0.2 μm polytetrafluoroethylene membrane to collect the purified carbon nano-onions. SEM (scanning electron microscope) and TEM (transmission electron microscope) observation showed that carbon nano-onions with hollow or nickel-encapsulated cavities were observed and they had a wide size distribution with an averaged diameter of 24.4 nm. XRD (X-ray diffraction) and Raman scattering investigations evidenced a high degree graphitization of such obtained carbon nano-onions. The XPS (X-ray photoemission spectroscopy) analysis, survey to surface chemistry, demonstrated that oxygen-containing and nitrogen-containing functional groups and the metal catalyst were removed from the surface of carbon nano-onions after purification.

A strategy to overcome the limits of carbon-based materials as lithium-ion battery anodes

The free-standing Si-coated carbon nanofiber (Si/CNF) mat was fabricated for the anode of lithium ion battery through combining electrospun CNF mat with electrodeposited Si layer. Spaghetti or granule-like Si was obtained by varying the deposition conditions. This Si/CNF mat was directly used as an active material and a current collector as well, which involves neither binders nor additional metal substrate. The best performance was achieved in spaghetti-like Si due to its highly porous nature which can accommodate volume expansion and large surface area which benefit the efficient charge transfer both at Si/CNF interface and at the electrode/electrolyte interface. The optimized Si/CNF mat after annealing at 1000°C delivered a capacity of 870mAhg−1 at 1st discharge and 730mAhg−1 at 50th discharge with a capacity retention of 84%, improving the capacity of pure CNF (280mAhg−1 at the 50th discharge) by almost three times. In addition, corrosion of the current collector no longer exists in our approach. Our X-ray photoemission spectroscopy and electrochemical analysis revealed that the formation of Si–C bond through high temperature annealing can enhance the adhesion between silicon and carbon at the interface which benefits the cyclic performance of anode ultimately.

Fermi-Level Depinning of Ge Schottky Junction Using Se Treatment

Depinning of the Fermi-level at metal/Ge Schottky junctions was achieved by Se treatment at room temperature. Due to the strong Fermi-level pinning close to the valence band edge in Ge, Schottky and Ohmic behaviors were observed in conventional Al Schottky contact s to n- and p-type Ge, respectively. However, Al Schottky contacts to n- and p-type Ge with Se treatment became Ohmic and Schottky, respectively. The transmission electron microscopy (TEM) examination combined with X-ray photoemission spectroscopy (XPS) analysis showed that Se treatment at room temperature yielded the formation of a fairly uniform Se-Ge alloy film with partially ionic binding nature caused by the chemical reaction between Se and Ge. This led to the reduction of surface states of Ge surface along with the passivation of dangling bonds by the incorporation of Se atoms, which was responsible for Fermi-level depinning at Al/Ge junctions.

Fermi-Level Depinning of Ge Schottky Junction Using Se Treatment

Depinning of the Fermi-level at metal/Ge Schottky junctions was achieved by Se treatment at room temperature. Due to the strong Fermi-level pinning close to the valence band edge in Ge, Schottky and Ohmic behaviors were observed in conventional Al Schottky contacts to n- and p-type Ge, respectively. However, Al Schottky contacts to n- and p-type Ge with Se treatment became Ohmic and Schottky, respectively. Electron and hole barrier heights calculated using Richardson plots were found to be 0.15 eV for of Al contacts to Se-treated n- type Ge, and 0.33 eV for Al contacts to Se-treated p-type Ge, respectively. The transmission electron microscopy (TEM) examination combined with X-ray photoemission spectroscopy (XPS) analysis showed that Se treatment at room temperature yielded the formation of a fairly uniform Se-Ge alloy film with partially ionic binding nature caused by the chemical reaction between Se and Ge. This led to the reduction of surface states of Ge surface along with the passivation of dangling bonds by the incorporation of Se atoms, which was responsible for Fermi-level depinning at Al/Ge junctions.

Synthesis and characterization of graphene oxide modified AgBr nanocomposites with enhanced photocatalytic activity and stability under visible light

Novel visible-light-driven graphene oxide (GO)/AgBr nanocomposites were synthesized by a facile solution method. The GO/AgBr nanocomposites were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoemission spectroscopy (XPS) analysis, UV–vis diffuse reflectance spectroscopy (DRS), Fourier transform infrared spectra (FTIR), and Raman spectra. The results indicated that AgBr nanoparticles were evenly distributed on the surface of GO and the heterostructures were formed. The photocatalytic activity of the as-prepared nanocomposites was evaluated by using Rhodamine B (RhB), Methylene blue (MB) and Methyl orange (MO) as target organic pollutants. The nanocomposites exhibit excellent photocatalytic activity for all of three dyes. Compared with bare AgBr particles, the GO/AgBr nanocomposites show better photocatalytic properties toward RhB pollutants. Additionally, the radical scavengers experiment indicated that O2− radicals was the main reactive species for the RhB degradation under visible light. The increased photocatalytic activity of the GO/AgBr nanocomposites was attributed to the strong coupling between GO and AgBr, which facilitated interfacial charge transfer and inhibited electron-hole recombination. A photocatalytic mechanism of GO/AgBr nanocomposites was also proposed.

The Shifts of Band Gap and Binding Energies of Titania/Hydroxyapatite Material

The titania/hydroxyapatite (TiO2/HAp) product was prepared by precipitating hydroxyapatite in the presence of TiO(OH)2 gel in the hydrothermal system. The characteristics of the material were determined by using the measurements such as X-ray photoemission spectroscopy (XPS), X-ray diffraction (XRD), diffuse reflectance spectra (DRS), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and energy dispersive X-ray (EDX). The XPS analysis showed that the binding energy values of Ca (2p1/2, 2p3/2), P (2p1/2, 2p3/2), and O 1s levels related to hydroxyapatite phase whereas those of Ti (2p3/2, 2p1/2) levels corresponded with the characterization of titanium (IV) in TiO2. The XRD result revealed that TiO2/HAp sample had hydroxyapatite phase, but anatase or rutile phases were not found out. TEM image of TiO2/HAp product showed that the surface of the plate-shaped HAp particles had a lot of smaller particles which were considered as the compound of Ti. The experimental band gap of TiO2/HAp material calculated by the DRS measurement was 3.6 eV, while that of HAp pure was 5.3 eV and that of TiO2 pure was around 3.2 eV. The shift of the band gap energy of TiO2 in the range of 3.2–3.6 eV may be related to the shifts of Ti signals of XPS spectrum.

Effects of Nitrogen Content on the Phase and Resistivity of TaN Thin Films Deposited by Electron Beam Evaporation

A series of amorphous electron beam evaporated Ta and TaN films with N/Ta ratio from 0 to 1.15 were deposited on Si/SiO2 substrates at 200°C. As N/Ta ratio increases, the TaN films undergo phase changes from pure metallic Ta to a mixture of Ta, Ta2N, and nitrogen-rich TaN films. The electrical resistivity of the Ta and TaN films increased from 242 µΩ-cm to 1126 µΩ-cm with increasing N/Ta ratio. X-ray diffraction patterns revealed the development of different phases of TaN that are in agreement with the TaN phase diagram. The presence of different phases on the film surface was also confirmed by x-ray photo-emission spectroscopy (XPS) analysis. Groups of Ta4f doublet related to different TaN phases were observed in the core-level spectra of TaN films. Field-emission scanning electron microscopy images revealed that surface morphology also varied with the phase change in TaN films. The N/Ta ratios from energy-dispersive x-ray generally agreed with those from the XPS analysis.

Understanding the Redox Reaction Mechanism of Li2CoPO4f Cathode Material for Achieving Two Lithium Intercalation

Lithium ion batteries have attracted considerable attention as important energy storage and conversion systems for applications including electric vehicles (EVs) and hybrid electric vehicles (HEVs), owing to their high energy and power densities, as well as a long cycle life. Recently, lithium transition metal phosphates, such as LiFePO4, LiMnPO4, and Li3V2(PO4)3, have been considered as potential cathode materials for lithium ion batteries. It is well known that the phosphates display much better electrochemical and thermal stability compared to conventional lithium metal oxides [1]. However, recently, fluorophosphates are being developed as advanced cathode materials display higher operating potential, corresponding to the redox reaction of transition metal, when compared to the respective metal oxides and phosphates [2]. In addition, the highly electronegative fluoride ion helps to improve the cycle stability, which fluorophosphates make attractive materials for high energy batteries. Our current work is focused on maximizing the deliverable discharge capacity of Li2CoPO4F cathode material and to reach theoretical capacity by achieving more than one electron intercalation. The redox couple Co2+/3+ and Co3+/4+ was closely followed during galvanostatic charge-discharge test by x-ray photoemission spectroscopy and x-ray diffraction (XRD) analysis. It was found that the incomplete reduction of cobalt ions during discharge initiated the irreversible capacity loss rather than the electrolyte decomposition being solely responsible. A novel approach was carried out to activate the transition metal ions which resulted in a discharge capacity as high as 230 mAh g-1 at a current rate of 20 mA g-1 for Li/Li2CoPO4F cell. A long plateau at 4.8 V has been observed in the charge cycle with an additional voltage plateau at ~5.0 V, which can be attributed to the two lithium intercalation reaction and has been validated using cyclic voltammetry studies. The detrimental cycle stability observed [3] in Li2CoPO4F has been replaced with a stable cycle performance of > 90 % capacity retention as shown in figure 1. A quasi single phase reaction was confirmed using gravimetric intermittent titration technique in addition to the results from ex-situXRD studies. A discussion based on the obtained results will be presented in detail.