Core-level Photoelectron Spectroscopy Research Articles

Out-of-equilibrium charge redistribution in a copper-oxide based superconductor by time-resolved X-ray photoelectron spectroscopy

Charge-transfer excitations are of paramount importance for understanding the electronic structure of copper-oxide based high-temperature superconductors. In this study, we investigate the response of a Bi2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_2$$\\end{document}Sr2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_2$$\\end{document}CaCu2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_2$$\\end{document}O8+δ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_{\ extrm{8}+ \\delta }$$\\end{document} crystal to the charge redistribution induced by an infrared ultrashort pulse. Element-selective time-resolved core-level photoelectron spectroscopy with a high energy resolution allows disentangling the dynamics of oxygen ions with different coordination and bonds thanks to their different chemical shifts. Our experiment shows that the O 1s component arising from the Cu–O planes is significantly perturbed by the infrared light pulse. Conversely, the apical oxygen, also coordinated with Sr ions in the Sr-O planes, remains unaffected. This result highlights the peculiar behavior of the electronic structure of the Cu–O planes. It also unlocks the way to study the out-of-equilibrium electronic structure of copper-oxide-based high-temperature superconductors by identifying the O 1s core-level emission originating from the oxygen ions in the Cu–O planes. This ability could be critical to gain information about the strongly-correlated electron ultrafast dynamical mechanisms in the Cu–O plane in the normal and superconducting phases.

Growth optimization, optical, and dielectric properties of heteroepitaxially grown ultrawide-bandgap ZnGa2O4 (111) thin film

Ultrawide bandgap ZnGa2O4 (ZGO) thin films were grown on sapphire (0001) substrates at various growth temperatures with a perspective to investigate the electrical and optical characteristics required for high-power electronic applications. Due to the variation in the vapor pressure of Zn and Ga, severe loss of Zn was observed during pulsed laser deposition, which was solved by using a zinc-rich Zn0.98Ga0.02O target. A pure phase single-crystalline ZGO thin film was obtained at a deposition temperature of 750 °C and an oxygen pressure of 1 × 10−2 Torr. The out-of-plane epitaxial relationship between the sapphire and ZGO thin film was obtained from φ-scan. The x-ray rocking curve of the ZGO thin film grown at 750 °C exhibits a full width at half maximum of ∼0.098°, which indicates a good crystalline phase and quality of the thin film. Core-level x-ray photoelectron spectroscopy of ZGO grown at 750 °C indicated that Zn and Ga were in the 2+ and 3+ oxidation states, respectively, and the atomic ratio of Zn/Ga was estimated to be ∼0.48 from the fitted values of Zn-2p3/2 and Ga-2p3/2. The high-resolution transmission electron microscopy images revealed a sharp interface with the thickness of the ZGO film of ∼265 nm, and the signature of minor secondary phases was observed. The bandgap of the ZGO film at different growth temperatures was calculated from the ultraviolet-diffuse reflectance spectroscopy spectra, and its value was obtained to be ∼5.08 eV for the 750 °C grown sample. The refractive index (n) and the extinction coefficient (k) were determined to be ∼1.94 and 0.023 from the ellipsometric data, respectively, and the real dielectric function (ɛr) was estimated to be ∼6.8 at energy 5 eV. The ultrawide bandgap and dielectric function of ZGO recommend its possible potential applications in deep-ultraviolet optoelectronic devices and high-power electronics.

Surface Structure to Tailor the Electrochemical Behavior of Mixed-Valence Copper Sulfides during Water Electrolysis.

The semiconducting behavior of mixed-valence copper sulfides arises from the pronounced covalency of Cu-S bonds and the exchange coupling between CuI and CuII centers. Although electrocatalytic study with digenite Cu9S5 and covellite CuS has been performed earlier, detailed redox chemistry and its interpretation through lattice structure analysis have never been realized. Herein, nanostructured Cu9S5 and CuS are prepared and used as electrode materials to study their electrochemistry. Powder X-ray diffraction (PXRD) and microscopic studies have found the exposed surface of Cu9S5 to be d(0015) and d(002) for CuS. Tetrahedral (Td) CuII, distorted octahedral (Oh) CuII, and trigonal planar (Tp) CuI sites form the d(0015) surface of Cu9S5, while the (002) surface of CuS consists of only Td CuII. The distribution of CuI and CuII sites in the lattice, predicted by PXRD, can further be validated through core-level Cu 2p X-ray photoelectron spectroscopy (XPS). The difference in the electrochemical response of Cu9S5 and CuS arises predominantly from the different copper sites present in the exposed surfaces and their redox states. In situ Raman spectra recorded during cyclic voltammetric study indicates that Cu9S5 is more electrochemically labile compared to CuS and transforms rapidly to CuO/Cu2O. Contact-angle and BET analyses imply that a high-surface-energy and macroporous Cu9S5 surface favors the electrolyte diffusion, which leads to a pronounced redox response. Post-chronoamperometric (CA) characterizations identify the potential-dependent structural transformation of Cu9S5 and CuS to CuO/Cu2O/Cu(OH)2 electroactive species. The performance of the in situ formed copper-oxides towards electrocatalytic water-splitting is superior compared to the pristine copper sulfides. In this study, the redox chemistry of the Cu9S5/CuS has been correlated to the atomic arrangements and coordination geometry of the surface exposed sites. The structure-activity correlation provides in-depth knowledge of how to interpret the electrochemistry of metal sulfides and their in situ potential-driven surface/bulk transformation pathway to evolve the active phase.

Band alignment and interfacial charge transfer in sol-gel derived anatase/rutile heterophase TiO2: explaining the synergistic photocatalytic activity.

In this study, we have synthesized nanostructured titanium dioxide (TiO2) photocatalysts under different configurations, viz., anatase, rutile and anatase/rutile heterophase (Eg = 3.14-2.96 eV) through a sol-gel route. The photocatalytic performance of the heterophase samples was better than that of their phase-pure counterparts. Photocatalytic performance was maximum for sample T12H (50% rutile) with a rate constant, k = (7.38 ± 1.48) × 10-2 min-1. This is ca. 47% greater than that estimated via an extrapolation method using the values for phase pure samples indicating synergistic effects. High-resolution transmission electron microscope (HRTEM) analysis and the phase-dependent broadening of core-level X-ray photoelectron spectroscopy (XPS) analysis indicate the formation of well-defined anatase/rutile interfaces. Interfacial charge transfer is the critical factor for this synergistic effect. To analyze the charge transfer pathways, the possible band alignment scheme was analyzed through the precise determination of band alignments from XPS, employing the Ti 2p3/2 core level as a reference. We found a staggered rutile band alignment with the valence band edge of rutile lying above that of anatase, indicating the transfer of photogenerated free electrons from the conduction band of rutile to that of anatase and hole transfer from the valence band of anatase to rutile. These inferences are validated through Mott-Schottky analysis as well.

Unraveling the atomic and electronic structure of nanocrystals on superconducting Nb(110): Impact of the oxygen monolayer

The niobium surface is almost always covered by a native oxide layer which greatly influences the performance of superconducting devices. Here we investigate the highly stable niobium oxide overlayer of Nb(110), which is characterized by its distinctive nanocrystal structure as observed by scanning tunneling microscopy (STM). Our ab initio density functional theory (DFT) calculations show that a subtle reconstruction in the surface niobium atoms gives rise to rows of fourfold coordinated oxygen separated by regions of threefold coordinated oxygen. The fourfold oxygen rows are determined to be the source of the nanocrystal pattern observed in STM and the two chemical states of oxygen observed in core-level x-ray photoelectron spectroscopy (XPS) are ascribed to the threefold and fourfold oxygens. Furthermore, we find excellent agreement between the DFT calculated electronic structure with scanning tunneling spectroscopy and valence XPS measurements.

Core-level photoelectron spectroscopy study on the buffer-layer formed in approximately atmospheric pressure argon on n-type and semi-insulating SiC(0001)

Using core-level photoelectron spectroscopy, we studied the surface of SiC(0001)-6√3 × 6√3R30° (buffer layer surface) formed on n-type and semi-insulating substrates via the approximately-atmospheric-pressure graphitization method. When the surface was exposed to air after buffer layer formation, hydrocarbons were adsorbed and oxygen atoms were chemically bonded to the carbon atoms comprising the buffer layer. Subsequently, annealing in ultra-high vacuum and an ethylene ambient allowed the removal of these oxygen atoms and hydrocarbons, and a clean buffer layer surface was obtained. Furthermore, using the peak position of Si 2p, band bending of the buffer layer on the n-type and semi-insulating substrates was estimated. A reasonable value of −0.05 eV was obtained for the n-type substrate. However, for the semi-insulating substrate, the Fermi level (Ef) was located at 0.05 eV below the conduction band minimum (CBM) on the surface. This is anomalous if we interpret the change in the CBM position over Ef from the bulk to the surface to be solely induced by band bending. To interpret this situation, we propose a mechanism in which the n-type region appears near the surface of the SiC bulk under the buffer layer.

Surface valence transition in SmS by alkali metal adsorption

The electronic structure changes of SmS surfaces under potassium (K) doping are elucidated using synchrotron-based core-level photoelectron spectroscopy and angle-resolved photoelectron spectroscopy (ARPES). The Sm core-level and ARPES spectra indicate that the Sm mean valence of the surface increased from the nearly divalent to trivalent states, with increasing K deposition. Carrier-induced valence transition (CIVT) from Sm$^{2+}$ to Sm$^{3+}$ exhibits a behavior opposite to that under conventional electron doping. Excess electrons are trapped by isolated excitons, which is inconsistent with the phase transition from the black insulator with Sm$^{2+}$ to the gold metal with Sm$^{3+}$ under pressure. This CIVT helps to clarify the pressure-induced black-to-golden phase transition in this material, which originates from the Mott transition of excitons.

Electronic and thermal properties of Ag-doped single crystal zinc oxide via laser-induced technique

The doping of ZnO has attracted lots of attention because it is an important way to tune the properties of ZnO. Post-doping after growth is one of the efficient strategies. Here, we report a unique approach to successfully dope the single crystalline ZnO with Ag by the laser-induced method, which can effectively further post-treat grown samples. Magnetron sputtering was used to coat the Ag film with a thickness of about 50 nm on the single crystalline ZnO. Neodymium-doped yttrium aluminum garnet (Nd:YAG) laser was chosen to irradiate the Ag-capped ZnO samples, followed by annealing at 700 °C for two hours to form ZnO:Ag. The three-dimensional (3D) information of the elemental distribution of Ag in ZnO was obtained through time-of-flight secondary ion mass spectrometry (TOF-SIMS). TOF-SIMS and core-level x-ray photoelectron spectroscopy (XPS) demonstrated that the Ag impurities could be effectively doped into single crystalline ZnO samples as deep as several hundred nanometers. Obvious broadening of core level XPS profiles of Ag from the surface to depths of hundred nms was observed, indicating the variance of chemical state changes in laser-induced Ag-doped ZnO. Interesting features of electronic mixing states were detected in the valence band XPS of ZnO:Ag, suggesting the strong coupling or interaction of Ag and ZnO in the sample rather than their simple mixture. The Ag-doped ZnO also showed a narrower bandgap and a decrease in thermal diffusion coefficient compared to the pure ZnO, which would be beneficial to thermoelectric performance.

Self-organized formation of unidirectional and quasi-one-dimensional metallic Tb silicide nanowires on Si(110)

Terbium induced nanostructures on Si(110) and their growth are thoroughly characterized by low energy electron diffraction, scanning tunneling microscopy and spectroscopy, core-level and valence band photoelectron spectroscopy, and angle-resolved photoelectron spectroscopy. For low Tb coverage, a wetting layer forms with its surface fraction continuously decreasing with increasing Tb coverage in favor of the formation of unidirectional Tb silicide nanowires. These nanowires show high aspect ratios for high annealing temperatures or on substrates already containing Tb in the bulk. Both wetting layer and nanowires are stable for temperatures up to 750°C. In contrast to the nanowires, the wetting layer is characterized by a band gap. Thus, the metallic nanowires, which show a quasi-one-dimensional electronic band structure, are embedded in a semiconducting surrounding of wetting layer and substrate, insulating the nanowires from each other.

Valence State Tuning of Gold Nanoparticles in the Dewetting Process: An X-ray Photoelectron Spectroscopy Study.

Gold nanoparticles (AuNPs) are commonly synthesized using the citrate reduction method, reducing Au3+ into Au1+ ions and facilitating the disproportionation of aurous species to Au atoms (Au0). This method results on citrate-capped AuNPs with valence single states Au0. Here, we report a methodology that allows obtaining AuNPs by the dewetting process with three different valence states (Au3+, Au1+, and Au0), which can be fine-tuned with ion bombardment. The chemical surface changes and binding state of the NPs were investigated using core-level X-ray photoelectron spectroscopy (XPS). This is achieved by recording high-resolution Au 4f XPS spectra as a function of ion dose exposure. The results obtained show a time-dependent tuning effect on the Au valence states using low-energy 200 V acceleration voltage Ar+ ion bombardment, and the valence state conversion kinetics involves the reduction from Au3+ and Au1+ to Au0. Proper control of the reduction in the valence states is critical in surface engineering for controlling catalytic reactions.

Properties and device performance of BN thin films grown on GaN by pulsed laser deposition

Wide and ultrawide-bandgap semiconductors lie at the heart of next-generation high-power, high-frequency electronics. Here, we report the growth of ultrawide-bandgap boron nitride (BN) thin films on wide-bandgap gallium nitride (GaN) by pulsed laser deposition. Comprehensive spectroscopic (core level and valence band x-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy, and Raman) and microscopic (atomic force microscopy and scanning transmission electron microscopy) characterizations confirm the growth of BN thin films on GaN. Optically, we observed that the BN/GaN heterostructure is second-harmonic generation active. Moreover, we fabricated the BN/GaN heterostructure-based Schottky diode that demonstrates rectifying characteristics, lower turn-on voltage, and an improved breakdown capability (∼234 V) as compared to GaN (∼168 V), owing to the higher breakdown electrical field of BN. Our approach is an early step toward bridging the gap between wide and ultrawide-bandgap materials for potential optoelectronics as well as next-generation high-power electronics.

Incorporation Site and Valence State of Sn Atoms in Sn-Substituted La(O,F)BiS2 Superconductor

BiS2-based superconductors have attracted considerable attention owing to the possible occurrence of exotic superconductivity. Recently, the superconducting critical temperature (Tc) of La(O,F)BiS2 has been reported to be enhanced with Pb or Sn doping. In this study, we have performed photoelectron holography (PEH) and core-level photoelectron spectroscopy (PES) to clarify the substituted site and valence state of Sn atoms in Sn-substituted La(O,F)BiS2. The PEH study revealed that the Sn atoms are incorporated into the Bi sites. A core-level PES study suggested that the valence state of the substituted Sn atoms is close to that of Sn2+, indicating that the Sn dopants provide hole carriers that compensate the electron carriers introduced by the F doping. These results are fundamental information for understanding the impact of Sn doping on this compound.

Quantification of complex protective surface oxide layer formed during plasma jet exposure of multicomponent ultra-high temperature carbides

Multi-component ultra-high temperature ceramics (MC-UHTCs) has garnered attention due to their improved thermo-mechanical and oxidation properties. MC-UHTC was progressively formed by adding a UHTC component at each step, producing binary, ternary and quaternary UHTC systems, processed by spark plasma sintering (SPS).We investigate the role of multi-elements and thermochemical stability of the surface oxide layer formed during plasma arc jet exposure (T > 2500 °C) of MC-UHTCs using core-level X-ray photoelectron spectroscopy (XPS) correlated with the structural investigation. The results indicate that adding a UHTC component improves oxidation resistance due to the assemblage of refractory phases forming complex mixed oxides via preferential oxidation. XPS spectra of Ta4f, Nb3d, Ti1p, and Hf4f showed that the nature of the chemical bonds switched from metal carbides to metal oxides (and metal oxy-carbides) in the binary system with retained metal carbides in ternary and quaternary UHTCs. The absence of a metal carbide peak in the high-resolution C1s scan of the binary system also supports this finding. The principal complex mixed oxide phases emanated in the exiguous oxidation of MC-UHTCs are quantified using Rietveld analysis. Overall, this study suggests that MC-UHTC is a promising material for possible application in thermal protection systems (TPS).

Recoil lineshapes in hard X-ray photoelectron spectra of large molecules - free and anchored-on-surface 10-aminodecane-1-thiol.

Core-level photoelectron spectroscopy of molecules presents unique opportunities but also challenges in the Hard X-ray Spectroscopy (HAXPES) realm. Here we focus on the manifestation of the photoelectron recoil effects in core-level photoemission spectra, using the independent normal-mode oscillators approach that allows to model and investigate the resulting recoil lineshapes for molecules of large sizes with only a slight computational effort. We model the recoil lineshape for N 1s and C 1s photoemission using the 10-aminodecane-1-thiol molecule as an example. It represents also a class of compounds commonly used in creating self-assembled monolayers (SAMs) on surfaces. Attachment of the -SH head group to the surface is modelled here in a simplified way by anchoring the sulfur atom of a single molecule. The effects of the orientation of photoemission in the molecular frame on the recoil lineshape of such anchored molecules are illustrated and discussed as a possible geometry probe. Time-evolution of the recoil excitations from the initial emission site across the entire molecule is also visualized.

Evaluation of valence band offset and its non-commutativity at all oxide α-Cr2O3/β-Ga2O3 heterojunction from photoelectron spectroscopy

We have investigated the non-commutativity of the band offset in RF magnetron sputter deposited all oxide epitaxial α-Cr2O3/β-Ga2O3 heterojunction (HJ). The core-level x-ray photoelectron spectroscopy technique has been employed to probe the electronic structure of the interface formed between α-Cr2O3 and β-Ga2O3. Valence and conduction band offsets of 2.6 ± 0.2 and 0.9 ± 0.2 eV, respectively, for α-Cr2O3/β-Ga2O3 HJ have been determined from Kraut's method. These values are different from those reported for β-Ga2O3/α-Cr2O3 HJ, thus indicating that the α-Cr2O3/β-Ga2O3 HJ does not follow the band commutativity with respect to the growth sequence of the constituting layers forming the HJ. Furthermore, the band alignment at α-Cr2O3/β-Ga2O3 HJ is still type-II like β-Ga2O3/α-Cr2O3 HJ but with lower band offset values. Therefore, this HJ would also be able to confine the electrons and holes in β-Ga2O3 and α-Cr2O3 layers, respectively, with lower turn on voltage.

Sulfur dioxide interaction with thin iron oxide films on low-index surfaces of iron

The adsorption of sulfur dioxide (SO2) at room temperature on iron oxide surfaces has been studied using core-level photoelectron spectroscopy. A variety of iron oxides, from adsorbate structure to thin film, with different stoichiometries and terminations were grown on common low-index single crystal iron surfaces to model a range of structures in the initial stages of atmospheric oxidation. This permits well-controlled comparisons of differences and similarities in SO2 interaction.Both non-dissociative and dissociative adsorption of SO2 were observed, to different relative and absolute coverages, on all surfaces. The only identified non-dissociated species is SO4. For all surfaces, at least some amounts of atomic sulfur are observed while only for the submonolayer adsorbate structure, also tentative dimerization of sulfur into S2 or formation of S-Osurface is observed. For the two oxides terminated by a complete well-ordered oxygen layer, the absolute sulfur coverage is low. The complete oxygen layer terminated oxide film exhibiting a moiré-pattern had a saturation coverage of one SO4 species per surface super cell, which indicates a tentative preferred adsorption site. The surface showing the highest SO4 formation is the Fe3O4(100) thin film surface with a corrugated row structure. The SO4 formation is suggested between surface oxygen atoms within the same row.

Core and Valence Level Photoelectron Spectroscopy of Nanosolvated KCl.

The solvation of alkali and halide ions in the aqueous environment has been a subject of intense experimental and theoretical research with multidisciplinary interests; yet, a comprehensive molecular-level understanding has still not been obtained. In recent years, electron spectroscopy has been increasingly applied to study the electronic and structural properties of aqueous ions with implications, especially in atmospheric chemistry. In this work, we report core and valence level (Cl 2p, Cl 3p, and K 3p) photoelectron spectra of the common alkali halide, KCl, doped in gas-phase water clusters in the size range of a few hundred water molecules. The results indicate that the electronic structure of these nanosolutions shows a distinct character from that observed at the liquid–vapor interface in liquid microjets and ambient pressure setups. Insights are provided into the unique solvation properties of ions in a nanoaqueous environment, emerging properties of bulk electrolyte solutions with growing cluster size, and sensitivity of the electronic structure to varying solvation configurations.

On the Origin of Reduced Cytotoxicity of Germanium-Doped Diamond-Like Carbon: Role of Top Surface Composition and Bonding.

This work attempts to understand the behaviour of Ge-induced cytotoxicity of germanium-doped hydrogen-free diamond-like carbon (DLC) films recently thoroughly studied and published by Jelinek et al. At a low doping level, the films showed no cytotoxicity, while at a higher doping level, the films were found to exhibit medium to high cytotoxicity. We demonstrate, using surface-sensitive methods—two-angle X-ray-induced core-level photoelectron spectroscopy (ARXPS) and Low Energy Ion Scattering (LEIS) spectroscopy, that at a low doping level, the layers are capped by a carbon film which impedes the contact of Ge species with tissue. For higher Ge content in the DLC films, oxidized Ge species are located at the top surface of the layers, provoking cytotoxicity. The present results indicate no threshold for Ge concentration in cell culture substrate to avoid a severe toxic reaction.

Core level photoelectron spectroscopy of heterogeneous reactions at liquid-vapor interfaces: Current status, challenges, and prospects.

Liquid-vapor interfaces, particularly those between aqueous solutions and air, drive numerous important chemical and physical processes in the atmosphere and in the environment. X-ray photoelectron spectroscopy is an excellent method for the investigation of these interfaces due to its surface sensitivity, elemental and chemical specificity, and the possibility to obtain information on the depth distribution of solute and solvent species in the interfacial region. In this Perspective, we review the progress that was made in this field over the past decades and discuss the challenges that need to be overcome for investigations of heterogeneous reactions at liquid-vapor interfaces under close-to-realistic environmental conditions. We close with an outlook on where some of the most exciting and promising developments might lie in this field.

Core-Level Photoelectron Spectroscopy Study of UTe2

The valence state of UTe$_2$ was studied by core-level photoelectron spectroscopy. The main peak position of the U $4f$ core-level spectrum of UTe$_2$ coincides with that of UB$_2$, which is an itinerant compound with a nearly $5f^3$ configuration. However, the main peak of UTe$_2$ is broader than that of UB$_2$, and satellite structures are observed in the higher binding energy side of the main peak, which are characteristics of mixed-valence uranium compounds. These results suggest that the U 5$f$ state in UTe$_2$ is in a mixed valence state with a dominant contribution from the itinerant $5f^3$ configuration.