High-resolution X-ray Photoemission Spectroscopy Research Articles

Deep UV transparent conductive Si-doped Ga2O3 thin films grown on Al2O3 substrates

β-Ga2O3 is attracting considerable attention for applications in power electronics and deep ultraviolet (DUV) optoelectronics owing to the ultra-wide bandgap of 4.85 eV and amendable n-type conductivity. In this work, we report the achievement of Si-doped β-Ga2O3 (Si:β-Ga2O3) thin films grown on vicinal α-Al2O3 (0001) substrates with high electrical conductivity and DUV transparency of promising potential as transparent electrodes. The use of Al2O3 substrates with miscut angles promotes step-flow growth mode, leading to substantial improvement of crystalline quality and electrical properties of the Si:β-Ga2O3 films. A high conductivity of 37 S·cm−1 and average DUV transparency of 85% have been achieved for 0.5% Si-doped film grown on a 6° miscut substrate. High-resolution x-ray and ultraviolet photoemission spectroscopy were further used to elucidate the surface electronic properties of the grown Si:β-Ga2O3 films. An upward surface band bending was found at the surface region of Si:β-Ga2O3 films. Interestingly, all the Si:β-Ga2O3 films have a very low work function of approximately 3.3 eV, which makes Si:β-Ga2O3 suitable materials for efficient electron injection. The present Si:β-Ga2O3 films with high conductivity, DUV transparency, and low work function would be useful as the DUV transparent electrode to develop advanced DUV optoelectronic devices.

Biofunctionalization of Porous Titanium Oxide through Amino Acid Coupling for Biomaterial Design.

Porous transition metal oxides are widely studied as biocompatible materials for the development of prosthetic implants. Resurfacing the oxide to improve the antibacterial properties of the material is still an open issue, as infections remain a major cause of implant failure. We investigated the functionalization of porous titanium oxide obtained by anodic oxidation with amino acids (Leucine) as a first step to couple antimicrobial peptides to the oxide surface. We adopted a two-step molecular deposition process as follows: self-assembly of aminophosphonates to titanium oxide followed by covalent coupling of Fmoc-Leucine to aminophosphonates. Molecular deposition was investigated step-by-step by Atomic Force Microscopy (AFM) and X-ray Photoemission Spectroscopy (XPS). Since the inherent high roughness of porous titanium hampers the analysis of molecular orientation on the surface, we resorted to parallel experiments on flat titanium oxide thin films. AFM nanoshaving experiments on aminophosphonates deposited on flat TiO2 indicate the formation of an aminophosphonate monolayer while angle-resolved XPS analysis gives evidence of the formation of an oriented monolayer exposing the amine groups. The availability of the amine groups at the outer interface of the monolayer was confirmed on both flat and porous substrates by the following successful coupling with Fmoc-Leucine, as indicated by high-resolution XPS analysis.

Valence fluctuation in Yb3Si5 probed by synchrotron X-ray photoemission spectroscopy

We report the direct evidence of valence fluctuation in the thermoelectric material Yb3Si5 by using synchrotron X-ray photoemission spectroscopy. The Yb 3d core level photoemission spectrum of a Yb3Si5 single crystal shows clear separation of the Yb2+ peak and Yb3+ multiplet peaks. From the spectral weight of each component, the mean valence of Yb ions is estimated to be +2.67 at 20 K. Furthermore, a Kondo resonance peak just below the Fermi energy is also observed by high-resolution soft X-ray photoemission spectroscopy, suggesting the valence fluctuating state in Yb3Si5.

Preparation, characterization and magnetic properties of Sm0.95Ho0.05FeO3 nanoparticles and their application in the purification of water

Sm0.95Ho0.05FeO3 was successfully prepared in a single phase using the citrate combustion method. The investigated sample was characterized using X-ray diffraction (XRD), a high-resolution transmission electron microscope (HRTEM) and X-ray photoemission spectroscopy (XPS). XRD confirmed that the sample was synthesized in an orthorhombic phase with an average crystallite size of 12 nm. HRTEM indicated that the sample was prepared in the nanoscale with an average particle size of 18 nm. XPS was used to identify the chemical bonds, binding energies and core levels of Sm0.95Ho0.05FeO3. The magnetic properties were studied using a vibrating sample magnetometer and DC magnetic susceptibility using Faraday’s method. The sample has an antiferromagnetic behavior with weak ferromagnetic components. The presence of magnetic Ho3+ ions in the SmFeO3 sample causes the magnetic exchange interaction between the 2p orbital of Fe3+ and the 4d sub-shell of Ho3+ ions. The dependence of pH value on the removal efficiency of Pb2+ from water was studied. The maximum Pb2+ removal efficiency of the Ho-doped SmFeO3 nano perovskite is 26% at pH = 5 and 99% at pH = 8. The Freundlich, Langmuir and Temkin isotherms were studied to understand the adsorption mechanism. The Temkin adsorption isotherm best fitted with the experimental data.

Influence of Co:Fe:Ni ratio on cobalt Pentlandite’s electronic structure and surface speciation

X-ray photoemission spectroscopy (XPS) was used to investigate the electronic structure and oxidation state of transition metals in synthetic cobalt pentlandites (Fe,Co,Ni)9S8 with measured stoichiometries of Fe4.85Ni4.64S8, Co0.13Fe4.68Ni4.71S8, Co2.73Fe3.18Ni3.16S8, Co5.80Fe1.63Ni1.59S8 and Co8.70S8. The addition of cobalt was found to decrease the bond length and hence decrease the unit cell dimensions of the cobalt pentlandite crystal structure by up to 0.18 Å. High resolution XPS S 2p spectra show increases in the binding energies of bulk 5-coordinated (162.1 eV) and surface 3-coordinated (160.9 eV) sulfur (≈0.2 eV) with increasing Co concentration and decreasing bond lengths. As the Co concentration increases, variation in metal site occupation decreases, resulting in smaller S 2p FWHMs due to a dominant single Co-S state rather than mixed Fe-S, Ni-S and Co-S states with similar binding energies. The S 2p high binding energy tail, previously identified as a S 3p- Fe 3d ligand to metal transfer in Fe chalcogenides, shows a marked decrease in intensity as the concentration of Co increases, that is attributed to a decreased probability of ligand-to-metal charge transfer as the eg orbitals are filled. The transition metal XPS 2p spectra were modelled using CTM4XAS to investigate metal site occupation and ligand-to-metal charge transfer. Fe, Co, and Ni were all best simulated using a tetrahedral symmetry and 2+ oxidation state, their 2p3/2 and 2p1/2 peaks occurred at 706.9 and 719.9 eV, 778.2 and 793.1 eV, and 852.8 and 870.0 eV, respectively. A negative charge transfer energy confirms the high binding energy tail results from S3p-Fe3d ligand to metal charge transfer. This increased understanding of the pentlandite electronic structure will provide a basis for the refinement of mineral processing techniques and allow for a reduced environmental impact from limited efficiency.

Crystal Phase, Electronic Structure, and Surface Band Bending of (InxGa1–x)2O3 Alloy Wide-Band-Gap Semiconductors

Ga2O3 is emerging as a promising wide-band-gap semiconductor for high-power electronics and deep ultraviolet optoelectronics. Alloying Ga2O3 with In2O3 leads to the modulation of the band gaps and possible carrier confinement achieved at the heterointerface. In this work, we report a systematic study on the crystallographic phase, electronic structure, and surface band bending of (InxGa1–x)2O3 thin films over the whole composition range of 0 ≤ x ≤ 1 grown on α-Al2O3 (0001) substrates by pulsed laser deposition. It was found that with In content x < 0.2, a monoclinic β-phase of an (InxGa1–x)2O3 film is epitaxially grown on Al2O3 (0001), while a bixbyite phase of the (InxGa1–x)2O3 film grows when x ≥ 0.8. When 0.2 ≤ x < 0.8, a mixed β-phase and bixbyite phase coexist. The evolution of electronic structures of the (InxGa1–x)2O3 films was examined by high-resolution X-ray photoemission spectroscopy (XPS) and optical absorption spectroscopy. For the β-phase films, the optical band gaps decrease from 4.96 eV for Ga2O3 to 4.43 eV for x = 0.4, while for bixbyite (InxGa1–x)2O3, the optical band gaps slightly increase from 3.57 eV for In2O3 to 3.70 eV for x = 0.8. Detailed electronic structure study indicates that the decrease of band gaps of (InxGa1–x)2O3 with an increase of In content mainly results from the upper movement of the valence band edge because the shallow In 4d orbitals introduce a hybridized state with O 2p at the top of the valence band. In addition, the presence of surface electron accumulation (i.e., downward band bending) was identified at the surface region of the (InxGa1–x)2O3 films with x ≥ 0.6, which would provide an opportunity to modulate the surface electronic properties for device applications.

Transparent conductive SnO2 thin films via resonant Ta doping

Transparent conductive oxide (TCO) thin films are highly desired as electrodes for modern flat-panel displays and solar cells. Alternative indium-free TCO materials are highly needed, because of the scarcity and the high price of indium. Based on the mechanism of resonant doping, Ta has been identified as an effective dopant for SnO2 to achieve highly conductive and transparent TCO. In this work, we fabricated a series of Ta-doped SnO2 thin films (Sn1−xTaxO2, x = 0.001, 0.01, 0.02, 0.03) with high conductivity and high optical transparency via a low-cost sol-gel spin coating method. The Sn0.98Ta0.02O2 film achieves the highest electrical conductivity of 855 S cm−1 with a carrier concentration of 2.3 × 1020 cm−3 and high mobility of 23 cm2 V−1 s−1. The films exhibit a very high optical transparency of 89.5% in the visible light region. High-resolution X-ray photoemission spectroscopy and optical spectroscopy were combined to gain insights into the electronic structure of the Sn1−xTaxO2 films. The optical bandgaps of the films are increased from 3.96 eV for the undoped SnO2 to 4.24 eV for the Sn0.98Ta0.02O2 film due to the occupation of the bottom of conduction band by free electrons, i.e., the Burstein-Moss effect. Interestingly, a bandgap shrinkage is also directly observed due to the bandgap renormalization arising from many-body interactions. The double guarantee of transparency and conductivity in Sn1−xTaxO2 films and the low-cost growth method provide a new platform for optoelectronic and solar cell applications.

Electronic structure of a Kondo lattice system CeCuAs2

We study the electronic properties of a Ce-based Kondo material, CeCuAs2 employing high-resolution hard x-ray photoemission spectroscopy. The measurements were done with different photon energies to probe the surface-bulk differences of the electronic structure. Experimental results show significant difference in the hybridization physics for the surface and bulk electronic structures indicating different Ce valency at the surface compared to the bulk. Surface termination appears to play an important role in the correlation physics of this system. In addition, the experimental spectra show loss features due to plasmon excitations. These results bring out complexity of this novel Kondo lattice system that does not show magnetic order down to the lowest temperature studied and have significantly different surface-bulk properties.

Emergence of well-screened states in a superconducting material of the CaFe2As2 family

Coupling among conduction electrons (e.g., Zhang-Rice singlet) are often manifested in the core level spectra of exotic materials such as cuprate superconductors, manganites, etc. These states are believed to play key roles in the ground state properties and appear as low binding energy features. To explore such possibilities in the Fe-based systems, we study the core level spectra of a superconductor $\mathrm{Ca}{\mathrm{Fe}}_{1.9}{\mathrm{Co}}_{0.1}{\mathrm{As}}_{2}$ (CaCo122) in the $\mathrm{Ca}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ (Ca122) family employing high-resolution hard x-ray photoemission spectroscopy. While As core levels show almost no change with doping and cooling, the Ca $2p$ peak of CaCo122 shows reduced surface contribution relative to Ca122 and a gradual shift of the peak position towards lower binding energies with cooling. In addition, we discover the emergence of a feature at the lower binding energy side of the well-screened Fe $2p$ signal in CaCo122. The intensity of this feature grows with cooling and indicates additional channels to screen the core holes. The evolution of this feature in the superconducting composition and its absence in the parent compound suggests relevance of the underlying interactions in the ground state properties of this class of materials. These results reveal another dimension in the studies of Fe-based superconductors and the importance of such states in the unconventional superconductivity in general.

Local excitons in Si/Ge inverted quantum huts (IQHs) embedded Si

We investigate the properties of excitons in the SiGe inverted quantum huts (IQHs) embedded in Si employing high-resolution x-ray photoemission spectroscopy. Ultra-small Si/Ge IQHs (13.3 nm × 6.6 nm) were grown on a Si buffer layer deposited on a Si (001) substrate using molecular beam epitaxy. We study the behavior of the excitons at different depths of the IQH structures by exposing the desired surfaces via controlled sputtering and annealing processes. The Si and Ge core level spectra show interesting properties at different surfaces; additionally, we discover distinct new features at the lower binding energy side of the Ge 3d peak. The emergence of these features is attributed to the final state effects arising from core hole screening by the excitons. The properties of these features in the spectra collected at different locations of the IQHs are found significantly different from each other, indicating the local character of the excitons. These results provide a pathway to study the properties of excitons in such quantum structures. The evidence of the local character of the excitons suggests a type I behavior of the system, which is important for the devices for optoelectronic applications, quantum communications, etc.

Reactivity of C3Hx Adsorbates in Presence of Co-adsorbed CO and Hydrogen: Testing Fischer–Tropsch Chain Growth Mechanisms

The identity of the surface intermediates involved in chain growth during Fischer–Tropsch synthesis remains a topic of ongoing debate. In the present work we use a combination of temperature programmed reaction spectroscopy and high resolution X-ray photoemission spectroscopy to study the reactivity of C3Hx adsorbates on a Co(0001) single crystal surface in order to explore the stabilities of the different C3Hx surface intermediates and to study elementary reaction steps relevant to chain growth and chain termination. Propene (H3C–CH=CH2) and propyl (H3C–CH2–CH2–) adsorbates react below 200 K already, either by desorption of propene or by dehydrogenation to adsorbed propyne (H3C–C≡CH). Co-adsorbed Had and COad do not affect the temperature at which propyl and propene react, but they do suppress the dehydrogenation pathway in favour of propene desorption. Their high reactivity under simulated FTS conditions disqualifies them as feasible intermediates for FTS, which requires long-lived intermediates to match the low monomer formation rate. Propyne, the most stable C3Hx adsorbate in the absence of COad, is hydrogenated to propylidyne (H3C–CH2–C≡) > 230 K when both COad and Had are present. Propylidyne dimerization occurs around 313 K and produces a 3-hexyne (H5C2–C≡C–C2H5) surface intermediate which is hydrogenated to 3-hexene (H5C2–CH=CH–C2H5) above 350 K. These findings are of direct relevance to FTS: they show that the high coverage of COad and Had present during the reaction influence the reactivity of CxHy adsorbates involved in chain growth and ultimately steer product selectivity. The findings provide further experimental support for the previously proposed alkylidyne chain growth mechanism on close-packed cobalt terraces: CO stabilizes CxHy growth intermediates in the alkylidyne form and growth proceeds via coupling of a long chain alkylidyne and methylidyne (CH).

Enhanced ambient stability of exfoliated black phosphorus by passivation with nickel nanoparticles

Since its discovery, the environmental instability of exfoliated black phosphorus (2D bP) has emerged as a challenge that hampers its wide application in chemistry, physics, and materials science. Many studies have been carried out to overcome this drawback. Here we show a relevant enhancement of ambient stability in few-layer bP decorated with nickel nanoparticles as compared to pristine bP. In detail, the behavior of the Ni-functionalized material exposed to ambient conditions in the dark is accurately studied by Transmission Electron Microscopy (TEM), Raman Spectroscopy, and high resolution x-ray Photoemission and Absorption Spectroscopy. These techniques provide a morphological and quantitative insight of the oxidation process taking place at the surface of the bP flakes. In the presence of Ni nanoparticles (NPs), the decay time of 2D bP to phosphorus oxides is more than three time slower compared to pristine bP, demonstrating an improved structural stability within 20 months of observation.

Calcium and phosphorous enrichment of porous niobium and titanium oxides for biomaterial applications

Anodic oxidation of niobium has been investigated in an electrolyte solution containing calcium phosphate and calcium acetate. The oxide layer morphology has been determined by combining 3D Atomic Force Microscopy images and cross-sectional Field Emission SEM measurements. The surface sensitivity of high-resolution X-ray Photoemission Spectroscopy has been exploited to evaluate the chemical composition of the outermost oxide surface, a key feature in view of implant development. Anodic Spark Deposition (ASD) results in the formation of micrometer-thick oxide layers showing a porous morphology suitable for osseointegration purposes. Increasing the maximum potential attained during oxidation leads to an increase of the oxide thickness and pore size accompanied by a noticeable surface enrichment with calcium and phosphorus. Remarkably, a maximal Ca/P ratio of 1.8 was achieved for anodization at 250 V, promising for osseointegration thanks to the similarity with hydroxyapatite stoichiometry. Parallel experiments performed on titanium under the same conditions indicated a maximal Ca/P ratio of 1.3 and pores of smaller dimensions. The comparative analysis suggests that niobium may represent a promising alternative to titanium to be explored for implant development.

Surface-engineered cobalt oxide nanowires as multifunctional electrocatalysts for efficient Zn-Air batteries-driven overall water splitting

Active sites engineering of electrocatalysts is an essential step to improve their intrinsic electrocatalytic capability for practical applications. Here, we demonstrate a facile surface phosphidisation into cobalt oxide (Co3O4) nanowires to boost their active sites, where phosphorus substitution for oxygen atoms greatly increases the number of cobalt (II) cations (Co2+) and oxygen vacancies (VO) active sites. First-principles calculations combined with high-resolution X-ray photoemission spectroscopy, electron spin resonance and X-ray absorption spectroscopy characterization identify that the phosphorus heteroatoms and VO alter the electron density of surface Co atoms in Co3O4 nanowires, resulting in the conversion of cobalt (III) cations (Co3+) to more active Co2+. As a result, the multifunctional catalysts of Co3O4 nanowires exhibit superior catalytic performance in overall water splitting with a low cell voltage of 1.61 V at 10 mA cm−2 and zinc-air batteries with a high open-circuit voltage of 1.41 V and power density of 72.1 mW cm−2. Particularly, two-series-connected Zn-air batteries powerfully drive an electrolyzer system without any additional electrode materials. These experimental and theoretical findings offer a promising approach to constructing active sites in non-precious metal catalysts for efficient and stable electrocatalytic activity.

ZnO:Sb MBE layers with different Sb content-optical, electronic and structural analysis

Understanding how Sb atoms are located in the ZnO crystal lattice and how they affect the layer parameters is essential to obtain p-type conductivity in ZnO:Sb. In this paper, the results of our extensive research (XPS, Raman, X-Ray, SIMS) of Sb doped ZnO films grown by Molecular Beam Epitaxy are presented. It is shown with use of high resolution X-ray Photoemission Spectroscopy (XPS) that three charge states of the Sb impurity: Sb3+, Sb5+, and Sb3− can exist in ZnO:Sb layers. The dominant charge state of Sb ions is 3 + while the contribution of 5 + is very low. The relative fraction of Sb atoms located at the Zn sites is about ∼90%, while ∼10% are incorporated in oxygen sites. It was found that increasing the Sb content in the layers increases the intensity of Raman modes at 511 cm−1, 533 cm−1, and 575 cm−1, which confirms their correlation with Sb doping. Additionally, a blue shift of the E2High Raman mode of ZnO was observed, related to the increase of strain in the oxygen sublattice. The relation between strain and stress for different Sb chemical states (Sb3+, Sb5+, Sb3−) in ZnO is calculated and compared with experimental results.

2-D assembly of supramolecular nanoarchitectures on Mg(0001).

In this work, a Mg(0001) single crystal is used as a novel template to grow 2D supramolecular nano-architectures. By using scanning tunnelling microscopy (STM) and high-resolution X-ray photoemission spectroscopy (HR-XPS), the formation of either a homo-molecular or metal-organic network is reported for carboxylic or amino functionalized molecules, respectively.

Systematic study of electronic properties of Fe-doped TiO2 nanoparticles by X-ray photoemission spectroscopy

The importance of investigating the electronic structure of Fe doped TiO2 nanoparticles lies in understanding their various magnetic and optical applications. In this study Fe doped TiO2 nanoparticles were synthesized by sol–gel method in a wide range of Fe/Ti molar ratios (1, 3, 5, 8, and 10%) and post annealing at 400, 600 and 800 °C in air. The structure and size of nanoparticles were studied by X-ray diffraction and transmission electron microscopy, respectively. Systematic study of the existing states of Fe ions in Fe doped TiO2 and transformation of the existing states as a function of annealing temperature and Fe concentration were carried out utilizing high-resolution X-ray photoemission spectroscopy (XPS). The XPS results showed that Fe was present in all samples while Fe ions were detected in mixed valence (Fe2+ and Fe3+) states. The Fe3+ ions were dominant in the surface region of the nanoparticles. Moreover, the Ti in Fe:TiO2 nanoparticles was assigned to the Ti4+ while a small shift towards lower binding energies was observed upon increasing the annealing temperature and dopant level. This confirms the successful incorporation of Fe into TiO2, and the shifts in binding energies were attributed to the anatase to rutile transformation. The results verify that doping by Fe up to 10% do not exceed the limit of Fe substitutation into TiO2 lattice.

Decarboxylation of Fatty Acids on Anisotropic Au(110) Surfaces

Lowering the oxygen content in biofuels is of vital importance, since high oxygen level content leads to low stability, low heat value, and corruption in engines. Here we report on the thermally activated decarboxylation of fatty acids, raw materials for biofuel production, on an anisotropic Au(110) surface. Due to the one-dimensional (1D) geometrical constraint of the surface reconstruction, linear fatty acid molecules (C30H60O2) are decarboxylated and polymerized at their terminal ends at mild temperatures, resulting in the formation of oxygen-free aliphatic hydrocarbons. Different reaction stages of the decarboxylation were monitored by high-resolution scanning tunneling microscopy and X-ray photoemission spectroscopy. On the basis of density functional theory calculations, a two-step process was proposed for the fatty acid decarboxylation. Our work demonstrates a novel strategy for deoxygenation of fatty acids on a 1D constrained surface as a model catalytic system for producing low oxygen content biofuels.

Thickness-controlled direct growth of nanographene and nanographite film on non-catalytic substrates

Metal-catalyzed chemical vapor deposition (CVD) has been broadly employed for large-scale production of high-quality graphene. However, a following transfer process to targeted substrates is needed, which is incompatible with current silicon technology. We here report a new CVD approach to form nanographene and nanographite films with accurate thickness control directly on non-catalytic substrates such as silicon dioxide and quartz at 800 °C. The growth time is as short as a few seconds. The approach includes using 9-bis(diethylamino)silylanthracene as the carbon source and an atomic layer deposition (ALD) controlling system. The structure of the formed nanographene and nanographite films were characterized using atomic force microscopy, high resolution transmission electron microscopy, Raman scattering, and x-ray photoemission spectroscopy. The nanographite film exhibits a transmittance higher than 80% at 550 nm and a sheet electrical resistance of 2000 ohms per square at room temperature. A negative temperature-dependence of the resistance of the nanographite film is also observed. Moreover, the thickness of the films can be precisely controlled via the deposition cycles using an ALD system, which promotes great application potential for optoelectronic and thermoelectronic-devices.

Magnetite (Fe3O4) nanoparticles: Synthesis, characterization and electrochemical properties

This work reports the synthesis, characterization, and electrochemical properties of magnetite nanoparticles (MNPs). MNPs were prepared by a simple solvothermal method at 200 °C for 8 h using three different concentrations of poly(vinyl)pyrrolidone (PVP). Transmission electron microscopy (TEM) images of the prepared samples reveal agglomerated like-sphere shape with mean particle sizes of ∼ 50–90 nm. The X-ray diffraction (XRD) and selected area electron diffraction (SAED) from TEM confirm the formation of pure phase Fe3O4 nanoparticles. The existence of PVP on the surface of the prepared samples is confirmed by high resolution X-ray photoemission spectroscopy (XPS). Consequently, the electrochemical performances of MNPs prepared using different PVP concentrations were evaluated by using the three-electrode cell system within 6.0M KOH. The results show that the specific capacitances was determined to be 396, 344, and 327 F/g at a scan rate of 2 mV/s and 197, 95, and 75 F/g at a current density of 3.0 A/g for MNPs samples prepared using 0.5 g, 1.0 g, and, 1.5 g of PVP, respectively. MNPs electrode showed the high power density of 777.65 W/kg with 7.36 Wh/kg of energy density addressing in high power density SC region. The cycle stability after 1000 cycles was also tested at 3 A/g and demonstrated the improvement of stability as increasing the amount of PVP. This indicates that the addition of surface element polymer plays a role in the cycle stability of supercapacitor devices.