Showing papers on "Photoemission spectroscopy published in 2018"
TL;DR: It is demonstrated that 3 × 3 charge-density-wave (CDW) order persists despite distinct changes in the low energy electronic structure highlighted by the reduction in the number of bands crossing the Fermi energy and the corresponding modification of FermI surface topology.
Abstract: We present the electronic characterization of single-layer 1H-TaSe2 grown by molecular beam epitaxy using a combined angle-resolved photoemission spectroscopy, scanning tunneling microscopy/spectroscopy, and density functional theory calculations. We demonstrate that 3 × 3 charge-density-wave (CDW) order persists despite distinct changes in the low energy electronic structure highlighted by the reduction in the number of bands crossing the Fermi energy and the corresponding modification of Fermi surface topology. Enhanced spin-orbit coupling and lattice distortion in the single-layer play a crucial role in the formation of CDW order. Our findings provide a deeper understanding of the nature of CDW order in the two-dimensional limit.
166 citations
04 Jan 2018
TL;DR: In this paper, the authors have grown epitaxial 2D stanene on a single crystal template and determined its crystalline structure synergetically by scanning tunneling microscopy, high-resolution synchrotron radiation photoemission spectroscopy, and advanced first principles calculations.
Abstract: Artificial post-graphene elemental 2D materials have received much attention recently. Especially, stanene, the tin analogue of graphene, is expected to be a robust 2D topological insulator, even above room temperature. We have grown epitaxial 2D stanene on a Ag(1 1 1) single crystal template and determined its crystalline structure synergetically by scanning tunneling microscopy, high-resolution synchrotron radiation photoemission spectroscopy, and advanced first principles calculations. From the STM images, we show that stanene forms a nearly planar structure in large domains. A detailed core-level spectroscopy analysis as well as DFT calculations reveal that the stanene sheet lays over an ordered 2D Ag2Sn surface alloy, but not directly on a bulk-terminated Ag(1 1 1) surface. The electronic structure exhibits a characteristic 2D band with parabolic dispersion due to the non-negligible interaction with the underlying surface alloy.
166 citations
TL;DR: A sizable gap of 129 meV is observed in a 1T'-WSe2 single layer grown on bilayer graphene with in-gap edge state near the layer boundary, leading to an insulator–semimetal transition.
Abstract: Two-dimensional (2D) topological insulators (TIs) are promising platforms for low-dissipation spintronic devices based on the quantum-spin-Hall (QSH) effect, but experimental realization of such systems with a large band gap suitable for room-temperature applications has proven difficult. Here, we report the successful growth on bilayer graphene of a quasi-freestanding WSe2 single layer with the 1T' structure that does not exist in the bulk form of WSe2. Using angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy/spectroscopy (STM/STS), we observe a gap of 129 meV in the 1T' layer and an in-gap edge state located near the layer boundary. The system's 2D TI characters are confirmed by first-principles calculations. The observed gap diminishes with doping by Rb adsorption, ultimately leading to an insulator-semimetal transition. The discovery of this large-gap 2D TI with a tunable band gap opens up opportunities for developing advanced nanoscale systems and quantum devices.
128 citations
TL;DR: MoS2 produced by low-temperature CVD was determined to possess a layered structure with good uniformity, stoichiometry, and a controllable number of layers, which has potential for burgeoning flexible and wearable nanotechnology applications.
Abstract: The efficient synthesis of two-dimensional molybdenum disulfide (2D MoS2) at low temperatures is essential for use in flexible devices. In this study, 2D MoS2 was grown directly at a low temperature of 200 °C on both hard (SiO2) and soft substrates (polyimide (PI)) using chemical vapor deposition (CVD) with Mo(CO)6 and H2S. We investigated the effect of the growth temperature and Mo concentration on the layered growth by Raman spectroscopy and microscopy. 2D MoS2 was grown by using low Mo concentration at a low temperature. Through optical microscopy, Raman spectroscopy, X-ray photoemission spectroscopy, photoluminescence, and transmission electron microscopy measurements, MoS2 produced by low-temperature CVD was determined to possess a layered structure with good uniformity, stoichiometry, and a controllable number of layers. Furthermore, we demonstrated the realization of a 2D MoS2-based flexible gas sensor on a PI substrate without any transfer processes, with competitive sensor performance and mechanical durability at room temperature. This fabrication process has potential for burgeoning flexible and wearable nanotechnology applications.
121 citations
TL;DR: It is shown that the surface of high-quality synthesized molybdenum disulfide (MoS2) is a major n-doping source, with a surface electron concentration nearly four orders of magnitude higher than that of MoS2 inner bulk.
Abstract: Because the surface-to-volume ratio of quasi-two-dimensional materials is extremely high, understanding their surface characteristics is crucial for practically controlling their intrinsic properties and fabricating p-type and n-type layered semiconductors. Van der Waals crystals are expected to have an inert surface because of the absence of dangling bonds. However, here we show that the surface of high-quality synthesized molybdenum disulfide (MoS2) is a major n-doping source. The surface electron concentration of MoS2 is nearly four orders of magnitude higher than that of its inner bulk. Substantial thickness-dependent conductivity in MoS2 nanoflakes was observed. The transfer length method suggested the current transport in MoS2 following a two-dimensional behavior rather than the conventional three-dimensional mode. Scanning tunneling microscopy and angle-resolved photoemission spectroscopy measurements confirmed the presence of surface electron accumulation in this layered material. Notably, the in situ-cleaved surface exhibited a nearly intrinsic state without electron accumulation.
113 citations
TL;DR: Conclusively, the segregated Ge atoms with trivalent bonding in honeycomb configuration form a characteristic two-dimensional germanene-like structure on Ag(111) surface as an overlayer.
Abstract: Large-scale two-dimensional sheets of graphene-like germanium, namely, germanene, have been epitaxially prepared on Ag(111) thin films grown on Ge(111), using a segregation method, differing from molecular beam epitaxy used in previous reports. From the scanning tunneling microscopy (STM) images, the surface is completely covered with an atom-thin layer showing a highly ordered long-range superstructure in wide scale. Two types of protrusions, named hexagon and line, form a (7√7 × 7√7)R19.1° supercell with respect to Ag(111), with a very large periodicity of 5.35 nm. Auger electron spectroscopy and high-resolution synchrotron radiation photoemission spectroscopy demonstrate that Ge atoms are segregated on the Ag(111) surface as an overlayer. Low-energy electron diffraction clearly shows incommensurate “(1.35 × 1.35)”R30° spots, corresponding to a lattice constant of 0.39 nm, in perfect accord with close-up STM images, which clearly reveal an internal honeycomb arrangement with corresponding parameter and ...
108 citations
TL;DR: In this article, the authors investigated the factors limiting the conductivity of fluorine-doped tin dioxide (FTO) produced via atmospheric pressure chemical vapor deposition and showed that significant compensation of donors by acceptors is present with a compensation ratio of 0.5, indicating that for every two donors there is approximately one acceptor.
Abstract: The factors limiting the conductivity of fluorine-doped tin dioxide (FTO) produced via atmospheric pressure chemical vapor deposition are investigated. Modeling of the transport properties indicates that the measured Hall effect mobilities are far below the theoretical ionized impurity scattering limit. Significant compensation of donors by acceptors is present with a compensation ratio of 0.5, indicating that for every two donors there is approximately one acceptor. Hybrid density functional theory calculations of defect and impurity formation energies indicate the most probable acceptor-type defects. The fluorine interstitial defect has the lowest formation energy in the degenerate regime of FTO. Fluorine interstitials act as singly charged acceptors at the high Fermi levels corresponding to degenerately n-type films. X-ray photoemission spectroscopy of the fluorine impurities is consistent with the presence of substitutional F O donors and interstitial F i in a roughly 2:1 ratio in agreement with the compensation ratio indicated by the transport modeling. Quantitative analysis through Hall effect, X-ray photoemission spectroscopy, and calibrated secondary ion mass spectrometry further supports the presence of compensating fluorine-related defects.
85 citations
TL;DR: It is suggested that quantum criticality induced by a collapsing pseudogap phase as a plausible explanation for observed enhancement of electronic specific heat cannot be assigned to the van Hove singularity alone.
Abstract: We present a soft x-ray angle-resolved photoemission spectroscopy study of overdoped high-temperature superconductors. In-plane and out-of-plane components of the Fermi surface are mapped by varyin ...
83 citations
TL;DR: The results suggest that the phosphorene on Au(111) could be a promising candidate, not only for fundamental research but also for nanoelectronics and optoelectronic applications.
Abstract: Exploring stable two-dimensional materials with appropriate band gaps and high carrier mobility is highly desirable due to the potential applications in optoelectronic devices. Here, the electronic structures of phosphorene on a Au(111) substrate are investigated by scanning tunneling spectroscopy, angle-resolved photoemission spectroscopy (ARPES), and density functional theory (DFT) calculations. The substrate-induced phosphorene superstructure gives a superlattice potential, leading to a strong band folding effect of the sp band of Au(111) on the band structure. The band gap could be clearly identified in the ARPES results after examining the folded sp band. The value of the energy gap (∼1.1 eV) and the high charge carrier mobility comparable to that of black phosphorus, which is engineered by the tensile strain, are revealed by the combination of ARPES results and DFT calculations. Furthermore, the phosphorene layer on the Au(111) surface displays high surface inertness, leading to the absence of multi...
82 citations
TL;DR: In this article, a micro-focused angle-resolved photoemission spectroscopy (microARPES) and in situ surface doping were used to manipulate the electronic structure of single-layer WS2 on hexagonal boron nitride (WS2/h-BN).
Abstract: In two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs), new electronic phenomena such as tunable bandgaps1–3 and strongly bound excitons and trions emerge from strong many-body effects4–6, beyond the spin and valley degrees of freedom induced by spin–orbit coupling and by lattice symmetry
7
. Combining single-layer TMDs with other 2D materials in van der Waals heterostructures offers an intriguing means of controlling the electronic properties through these many-body effects, by means of engineered interlayer interactions8–10. Here, we use micro-focused angle-resolved photoemission spectroscopy (microARPES) and in situ surface doping to manipulate the electronic structure of single-layer WS2 on hexagonal boron nitride (WS2/h-BN). Upon electron doping, we observe an unexpected giant renormalization of the spin–orbit splitting of the single-layer WS2 valence band, from 430 meV to 660 meV, together with a bandgap reduction of at least 325 meV, attributed to the formation of trionic quasiparticles. These findings suggest that the electronic, spintronic and excitonic properties are widely tunable in 2D TMD/h-BN heterostructures, as these are intimately linked to the quasiparticle dynamics of the materials11–13. A microfocused angle-resolved photoemission spectroscopy study of single layers of WS2 on hexagonal boron nitride reveals that, upon electron doping, trionic interactions cause a giant increase of the spin splitting in the valence band.
78 citations
TL;DR: In this paper, the authors reported the successful growth on bilayer graphene of a quasi-freestanding WSe$_2$ single layer with the 1T' structure that does not exist in the bulk form of WSe $_2$.
Abstract: Two-dimensional (2D) topological insulators (TIs) are promising platforms for low-dissipation spintronic devices based on the quantum spin Hall (QSH) effect, but experimental realization of such systems with a large band gap suitable for room-temperature applications has proven difficult. Here, we report the successful growth on bilayer graphene of a quasi-freestanding WSe$_2$ single layer with the 1T' structure that does not exist in the bulk form of WSe$_2$. Using angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy/spectroscopy (STM/STS), we observed a gap of 129 meV in the 1T' layer and an in-gap edge state located near the layer boundary. The system's 2D TI characters are confirmed by first-principles calculations. The observed gap diminishes with doping by Rb adsorption, ultimately leading to an insulator-semimetal transition. The discovery of this large-gap 2D TI with a tunable band gap opens up opportunities for developing advanced nanoscale systems and quantum devices.
TL;DR: The Co-doped PSC exhibits excellent optoelectronic properties; the improvements by passivation of electronic trap or sub-band-gap states arising due to the oxygen vacancies in pristine TiO2, enabling faster electron transport and collection are explained.
Abstract: We for the first time report the incorporation of cobalt into a mesoporous TiO2 electrode for application in perovskite solar cells (PSCs). The Co-doped PSC exhibits excellent optoelectronic properties; we explain the improvements by passivation of electronic trap or sub-band-gap states arising due to the oxygen vacancies in pristine TiO2, enabling faster electron transport and collection. A simple postannealing treatment is used to prepare the cobalt-doped mesoporous electrode; UV-visible spectroscopy, X-ray photoemission spectroscopy, space charge-limited current, photoluminescence, and electrochemical impedance measurements confirm the incorporation of cobalt, enhanced conductivity, and the passivation effect induced in the TiO2. An optimized doping concentration of 0.3 mol % results in the maximum power conversion efficiency of 18.16%, 21.7% higher than that of a similar cell with an undoped TiO2 electrode. Also, the device shows negligible hysteresis and higher stability, retaining 80.54% of the initial efficiency after 200 h.
TL;DR: In this paper, angle-resolved photo-emission spectroscopy and scanning tunneling microscopy were used to detect the emergence of a (2 x 2) charge density wave order in single-layer TiTe$_2$ with a transition temperature of 92 $\pm$ 3 K.
Abstract: Two-dimensional materials constitute a promising platform for developing nanoscale devices and systems. Their physical properties can be very different from those of the corresponding three-dimensional materials because of extreme quantum confinement and dimensional reduction. Here we report a study of TiTe$_2$ from the single-layer to the bulk limit. Using angle-resolved photoemission spectroscopy and scanning tunneling microscopy and spectroscopy, we observed the emergence of a (2 x 2) charge density wave order in single-layer TiTe$_2$ with a transition temperature of 92 $\pm$ 3 K. Also observed was a pseudogap of about 28 meV at the Fermi level at 4.2 K. Surprisingly, no charge density wave transitions were observed in 2- and multi-layer TiTe$_2$, despite the quasi-two-dimensional nature of the material in the bulk. The unique charge density wave phenomenon in the single layer raises intriguing questions that challenge the prevailing thinking about the mechanisms of charge density wave formation.
TL;DR: A reactive-barrier-based approach is demonstrated to achieve growth of highly homogeneous single-layer MoS2 on sapphire by the use of a nickel oxide foam barrier during chemical vapor deposition, indicating the successful growth of a highly crystalline and well-oriented MoS 2 monolayer.
Abstract: Single-layer molybdenum disulfide (MoS2) has attracted significant attention due to its electronic and physical properties, with much effort invested toward obtaining large-area high-quality monolayer MoS2 films. In this work, we demonstrate a reactive-barrier-based approach to achieve growth of highly homogeneous single-layer MoS2 on sapphire by the use of a nickel oxide foam barrier during chemical vapor deposition. Due to the reactivity of the NiO barrier with MoO3, the concentration of precursors reaching the substrate and thus nucleation density is effectively reduced, allowing grain sizes of up to 170 μm and continuous monolayers on the centimeter length scale being obtained. The quality of the monolayer is further revealed by angle-resolved photoemission spectroscopy measurement by observation of a very well resolved electronic band structure and spin–orbit splitting of the bands at room temperature with only two major domain orientations, indicating the successful growth of a highly crystalline an...
TL;DR: In this article, a Corundum-structured iridium oxide (α-Ir2O3), showing p-type conductivity, is a strong candidate to form high-quality pn heterojunctions with α-Ga 2O3.
Abstract: Corundum-structured iridium oxide (α-Ir2O3), showing p-type conductivity, is a strong candidate to form high-quality pn heterojunctions with α-Ga2O3. We fabricated α-Ir2O3/α-Ga2O3 pn heterojunction diodes and they showed well-defined rectifying current-voltage (I-V) characteristics with the turn-on voltage of about 2.0 V. The band alignment at the α-Ir2O3/α-Ga2O3 interface was investigated by X-ray photoemission spectroscopy, revealing a staggered-gap (type-II) with the valence- and conduction-band offsets of 3.34 eV and 1.04 eV, respectively. The total barrier height for electrons was about 2.4 eV, which reasonably agreed with the turn-on voltage in the I-V characteristics. This means that electrons are mainly attributed to electrical conduction around the turn-on voltage.Corundum-structured iridium oxide (α-Ir2O3), showing p-type conductivity, is a strong candidate to form high-quality pn heterojunctions with α-Ga2O3. We fabricated α-Ir2O3/α-Ga2O3 pn heterojunction diodes and they showed well-defined rectifying current-voltage (I-V) characteristics with the turn-on voltage of about 2.0 V. The band alignment at the α-Ir2O3/α-Ga2O3 interface was investigated by X-ray photoemission spectroscopy, revealing a staggered-gap (type-II) with the valence- and conduction-band offsets of 3.34 eV and 1.04 eV, respectively. The total barrier height for electrons was about 2.4 eV, which reasonably agreed with the turn-on voltage in the I-V characteristics. This means that electrons are mainly attributed to electrical conduction around the turn-on voltage.
Journal Article•
TL;DR: The authors show that the thermodynamic fission–fusion balance of excitons and electron-hole plasma can be efficiently tuned via the dielectric environment as well as charge carrier doping and observed by photoemission spectroscopy.
Abstract: When electron-hole pairs are excited in a semiconductor, it is a priori not clear if they form a plasma of unbound fermionic particles or a gas of composite bosons called excitons. Usually, the exciton phase is associated with low temperatures. In atomically thin transition metal dichalcogenide semiconductors, excitons are particularly important even at room temperature due to strong Coulomb interaction and a large exciton density of states. Using state-of-the-art many-body theory, we show that the thermodynamic fission–fusion balance of excitons and electron-hole plasma can be efficiently tuned via the dielectric environment as well as charge carrier doping. We propose the observation of these effects by studying exciton satellites in photoemission and tunneling spectroscopy, which present direct solid-state counterparts of high-energy collider experiments on the induced fission of composite particles. Owing to their atomically thin nature, 2D transition metal dichalcogenides host room temperature, strongly bound excitons. Here, the authors show that the thermodynamical balance between fission and fusion of excitons can be tuned by the dielectric environment and charge carrier doping and observed by photoemission spectroscopy.
TL;DR: This work finds that Au(111) breaks the sublattice symmetry of blue phosphorus leading to an orbital-dependent band renormalization upon the formation of a (4 × 4) superstructure and identifies its momentum-resolved electronic structure.
Abstract: Most recently, theoretical calculations predicted the stability of a novel two-dimensional phosphorus honeycomb lattice named blue phosphorus. Here, we report on the growth of blue phosphorus on Au(111) and unravel its structural details using diffraction, microscopy and theoretical calculations. Most importantly, by utilizing angle-resolved photoemission spectroscopy we identify its momentum-resolved electronic structure. We find that Au(111) breaks the sublattice symmetry of blue phosphorus leading to an orbital-dependent band renormalization upon the formation of a (4 × 4) superstructure. Notably, the semiconducting two-dimensional phosphorus realizes its valence band maximum at 0.9 eV binding energy, however, shifted in momentum space due to the substrate-induced band renormalization.
TL;DR: The synthesis of mesoporous Pd@Ru core-shell bimetallic nanorods composed of face-centered cubic Pd and hexagonal close-packed Ru are reported, which exhibit superior catalytic performance and stability for hydrogen evolution reactions.
Abstract: The activity and stability of bimetallic nanocatalysts strongly depend on their structures, compositions, and interfaces. Here, we report the synthesis of mesoporous Pd@Ru core-shell bimetallic nanorods composed of face-centered cubic Pd and hexagonal close-packed Ru. The nanorods have two types of cavities with diameters of 3.0 ± 0.9 and 20.3 ± 8.1 nm. The mutual diffusion process between Ru and Pd is characterized by the high-angle annular dark-field scanning transmission electron microscopy, energy-dispersive X-ray spectroscopy mapping, and the synchrotron radiation photoemission spectroscopy measurements. The mesoporous Pd@Ru nanorods exhibit superior catalytic performance and stability for hydrogen evolution reactions (overpotentials of 30 mV at 10 mA·cm-2 in 1.0 M KOH solution and 37 mV at 10 mA·cm-2 in 0.5 M H2SO4 solution).
TL;DR: In this article, the superconducting gap distribution on the three-dimensional Fermi surfaces of FeSe using synchrotron-based angle-resolved photoemission spectroscopy was studied.
Abstract: Detailed knowledge of the gap function in iron-based superconductors is a requisite to identify the mechanism of superconductivity in these materials. Here, the authors present a systematic study of the superconducting gap distribution on the three-dimensional Fermi surfaces of FeSe using synchrotron-based angle-resolved photoemission spectroscopy. The results imply a significant anisotropy of the superconducting gap on all parts of the FeSe Fermi surface not only in-plane, but also as a function of ${k}_{z}$. The observed in-plane anisotropy can be qualitatively understood in terms of orbital-selective Cooper pairing and nematicity-induced mixing of $s$- and $d$-pairing channels.
TL;DR: In this article, a measurement scheme to access the momentum and energy-resolved spectral function in a quantum gas microscope is proposed, and the spectrum of a single hole excitation in one-dimensional $t$-$J$ models is calculated and analyzed.
Abstract: Quantum gas microscopes are a promising tool to study interacting quantum many-body systems and bridge the gap between theoretical models and real materials. One of the most powerful experimental methods in solids is angle-resolved photoemission spectroscopy (ARPES), which measures the single-particle spectral function. The authors propose a measurement scheme to experimentally access the momentum- and energy-resolved spectral function in a quantum gas microscope. As an example for possible applications, the spectrum of a single hole excitation in one-dimensional $t$-$J$ models is calculated and analyzed. A sharp asymmetry in the distribution of spectral weight, reminiscent of the Fermi arcs observed in the pseudogap phase of cuprates, appears in the case of an isotropic Heisenberg spin chain.
TL;DR: In this paper, the effect of Fe-doping on the structural, optical, magnetic and electronic properties of polycrystalline CeO2 (for 5 and 10% doping concentration of Fecation) samples synthesized by low-temperature solid-state reaction method was reported.
Abstract: The present study reports the effect of Fe-doping on the structural, optical, magnetic and electronic properties of polycrystalline CeO2 (for 5 and 10% doping concentration of Fe-cation) samples synthesized by low-temperature solid-state reaction method. Rietveld refinement of the X-ray diffraction patterns establishes fluorite-type face-centred cubic structure of the Fe-doped CeO2 samples and also confirms successful incorporation of Fe ions in the CeO2 lattice. The UV–Vis–NIR absorption spectra displays reduce band gap energy with rising fluency of Fe-ions, which confirm red shifts in the Fe-doped CeO2 samples. The electronic structure of the pure CeO2 and Fe-doped CeO2 polycrystalline samples have been investigated by X-ray photoemission spectroscopy (XPS). The XPS spectra of Ce 3d reveals the reduction of Ce4+ to Ce3+ states Fe-doped CeO2 samples, which are well supported by the Fe 2p and O 1s spectra. Pure polycrystalline CeO2 displays diamagnetic behaviour at room temperature. Interestingly, 5% Fe-doped CeO2 sample displays S-shape hysteresis loop and establishes room temperature ferromagnetism, whereas, 10% Fe-doped CeO2 sample shows weak ferromagnetic behaviour. A decrement is observed in the magnetization on increasing the doping concentration. The possible reason for ferromagnetism in the Fe-doped CeO2 samples may be incorporation of oxygen vacancies, which are further discussed using F-centre exchange mechanism and double exchange interaction. These experimental findings offer potential opportunities for spintronics and optoelectronics applications by integrating them into device structures and evaluating their performance as a function of their material properties.
TL;DR: In this paper, the influence of ultraviolet light illumination on the structural and luminescent properties of perovskite nanocrystals (NCs) with emissions of 495 and 520 nm was studied.
TL;DR: In this paper, the effect of element doping on the selectivity of CO2 photoreduction was investigated, and it was shown that the Ni-doped ZnCo2O4 atomic layers exhibit a 3.5-time higher CO selectivity.
Abstract: Regulating the selectivity of CO2 photoreduction is particularly challenging. Herein, we propose ideal models of atomic layers with/without element doping to investigate the effect of doping engineering to tune the selectivity of CO2 photoreduction. Prototypical ZnCo2O4 atomic layers with/without Ni-doping were first synthesized. Density functional theory calculations reveal that introducing Ni atoms creates several new energy levels and increases the density-of-states at the conduction band minimum. Synchrotron radiation photoemission spectroscopy demonstrates that the band structures are suitable for CO2 photoreduction, while the surface photovoltage spectra demonstrate that Ni doping increases the carrier separation efficiency. In situ diffuse reflectance Fourier transform infrared spectra disclose that the CO2·− radical is the main intermediate, while temperature-programed desorption curves reveal that the ZnCo2O4 atomic layers with/without Ni doping favor the respective CO and CH4 desorption. The Ni-doped ZnCo2O4 atomic layers exhibit a 3.5-time higher CO selectivity than the ZnCo2O4 atomic layers. This work establishes a clear correlation between elemental doping and selectivity regulation for CO2 photoreduction, opening new possibilities for tailoring solar-driven photocatalytic behaviors.
TL;DR: The studies conclusively establish the stability of uranium pentoxide, in contrast to previous investigations of binary oxides claiming that U(V) occurs only as a metastable intermediate state coexisting with U(IV) and U(VI) species.
Abstract: Thin films of the elusive intermediate uranium oxide U2O5 have been prepared by exposing UO3 precursor multilayers to atomic hydrogen. Electron photoemission spectra measured about the uranium 4f core-level doublet contain sharp satellites separated by 7.9(1) eV from the 4f main lines, whilst satellites characteristics of the U(IV) and U(VI) oxidation states, expected respectively at 6.9(1) and 9.7(1) eV from the main 4f lines, are absent. This shows that uranium ions in the films are in a pure pentavalent oxidation state, in contrast to previous investigations of binary oxides claiming that U(V) occurs only as a metastable intermediate state coexisting with U(IV) and U(VI) species. The ratio between the 5f valence band and 4f core-level uranium photoemission intensities decreases by about 50% from UO2 to U2O5, which is consistent with the 5f 2 (UO2) and 5f 1 (U2O5) electronic configurations of the initial state. Our studies conclusively establish the stability of uranium pentoxide.
TL;DR: It is demonstrated that alloying the Pd(110) surface with submonolayer amounts of Au dramatically accelerates the hydrogen absorption, thereby improving the performance of hydrogen-purifying membranes and hydrogen-storage materials, which is a key for utilizing hydrogen as a carbon-free energy carrier.
Abstract: Enhancement of hydrogen (H) absorption kinetics improves the performance of hydrogen-purifying membranes and hydrogen-storage materials, which is necessary for utilizing hydrogen as a carbon-free energy carrier. Pd–Au alloys are known to show higher hydrogen solubility than pure Pd. However, the effect of Au on the hydrogen penetration from the surface into the subsurface region has not been clarified so far. Here, we investigate the hydrogen absorption at Pd–Au surface alloys on Pd(110) by means of thermal desorption spectroscopy (TDS) and hydrogen depth profiling with nuclear reaction analysis (NRA). We demonstrate that alloying the Pd(110) surface with submonolayer amounts of Au dramatically accelerates the hydrogen absorption. The degree of acceleration shows a volcano-shaped form against Au coverage. This kinetic enhancement is explained by a reduced penetration barrier mainly caused by a destabilization of chemisorbed surface hydrogen, which is supported by density-functional-theory (DFT) calculations. The destabilization of chemisorbed surface hydrogen is attributed to the change of the surface electronic states as observed by angle-resolved photoemission spectroscopy (ARPES). If generalized, these discoveries may lead to improving and controlling the hydrogen transport across the surfaces of hydrogen-absorbing materials.
TL;DR: Time-resolved multiphoton photoemission spectroscopy (MPP) investigates the coherent electron transfer from an interface state that forms upon chemisorption of Ag nanoclusters onto graphite to a σ symmetry interlayer band of graphite.
Abstract: Charge transfer in transduction of light to electrical or chemical energy at heterojunctions of metals with semiconductors or semimetals is believed to occur by photogenerated hot electrons in metal undergoing incoherent internal photoemission through the heterojunction interface. Charge transfer, however, can also occur coherently by dipole coupling of electronic bands at the heterojunction interface. Microscopic physical insights into how transfer occurs can be elucidated by following the coherent polarization of the donor and acceptor states on the time scale of electronic dephasing. By time-resolved multiphoton photoemission spectroscopy (MPP), we investigate the coherent electron transfer from an interface state that forms upon chemisorption of Ag nanoclusters onto graphite to a $\ensuremath{\sigma}$ symmetry interlayer band of graphite. Multidimensional MPP spectroscopy reveals a resonant two-photon transition, which dephases within 10 fs completing the coherent transfer.
TL;DR: This work provides strong evidence that the replica bands observed in the single-layer FeSe/SrTiO_{3} system and several other cases are largely due to the energy loss processes of the escaping photoelectron, resulted from the well-known strong coupling of external propagating electrons to Fuchs-Kliewer surface phonons in ionic materials in general.
Abstract: The recent observation of replica bands in single-layer FeSe/SrTiO_{3} by angle-resolved photoemission spectroscopy (ARPES) has triggered intense discussions concerning the potential influence of the FeSe electrons coupling with substrate phonons on the superconducting transition temperature. Here we provide strong evidence that the replica bands observed in the single-layer FeSe/SrTiO_{3} system and several other cases are largely due to the energy loss processes of the escaping photoelectron, resulted from the well-known strong coupling of external propagating electrons to Fuchs-Kliewer surface phonons in ionic materials in general. The photoelectron energy loss in ARPES on single-layer FeSe/SrTiO_{3} is calculated using the demonstrated successful semiclassical dielectric theory in describing low energy electron energy loss spectroscopy of ionic insulators. Our result shows that the observed replica bands are mostly a result of extrinsic photoelectron energy loss and not a result of the electron phonon interaction of the Fe d electrons with the substrate phonons. The strong enhancement of the superconducting transition temperature in these monolayers remains an open question.
TL;DR: In this paper, the authors developed a semi-analytical theory for the space charge effect in cathode-lens instruments (momentum microscopes, photoemission electron microscopes).
Abstract: Photoelectron spectroscopy, especially at pulsed sources, is ultimately limited by the Coulomb interaction in the electron cloud, changing energy and angular distribution of the photoelectrons. A detailed understanding of this phenomenon is crucial for future pump–probe photoemission studies at (x-ray) free electron lasers and high-harmonic photon sources. Measurements have been performed for Ir(111) at hν = 1000 eV with photon flux densities between ~102 and 104 photons per pulse and μm2 (beamline P04/PETRA III, DESY Hamburg), revealing space-charge induced energy shifts of up to 10 eV. In order to correct the essential part of the energy shift and restore the electron distributions close to the Fermi energy, we developed a semi-analytical theory for the space-charge effect in cathode-lens instruments (momentum microscopes, photoemission electron microscopes). The theory predicts a Lorentzian profile of energy isosurfaces and allows us to quantify the charge cloud from measured energy profiles. The correction is essential for the determination of the Fermi surface, as we demonstrate by means of 'k-space movies' for the prototypical high-Z material tungsten. In an energy interval of about 1 eV below the Fermi edge, the bandstructure can be restored up to substantial shifts of ~7 eV. Scattered photoelectrons strongly enhance the inelastic background in the region several eV below E F, proving that the majority of scattering events involves a slow electron. The correction yields a gain of two orders of magnitude in usable intensity compared with the uncorrected case (assuming a tolerable shift of 250 meV). The results are particularly important for future experiments at SASE-type free electron lasers, since the correction also works for strongly fluctuating (but known) pulse intensities.
TL;DR: Using PL, in conjunction with Raman spectroscopy, the type of oxygen vacancy induced in the nanocrystals on Co doping has been confirmed and the position of the defect levels in the forbidden zone has been studied.
Abstract: X-ray photoemission spectroscopy (XPS), X-ray diffraction (XRD) and transmission electron microscopy (TEM) have been used to study the structural and morphological characteristics of cobalt doped tin(IV) oxide (Sn1−xCoxO2; 0 ≤ x ≤ 0.04) nanocrystals synthesized by a chemical co-precipitation technique. Electronic structure analysis using X-ray photoemission spectroscopy (XPS) shows the formation of tin interstitials (Sni) and reduction of oxygen vacancies (VO) in the host lattice on Co doping and that the doped Co exists in mixed valence states of +2 and +3. Using XRD, the preferential position of the Sni and doped Co in the unit cell of the nanocrystals have been estimated. Rietveld refinement of XRD data shows that samples are of single phase and variation of lattice constants follows Vegard's law. XRD and TEM measurements show that the crystallite size of the nanocrystals decrease with increase in Co doping concentration. SAED patterns confirm the monocrystalline nature of the samples. The study of the lattice dynamics using Raman spectroscopy and Fourier transform infrared (FTIR) spectroscopy shows the existence of many disorder activated forbidden optical phonon modes, along with the corresponding classical modes, signifying Co induced local symmetry breaking in the nanocrystals. UV-Vis spectroscopy shows that the optical band gap has red shifted with increase in doping concentration. The study of Urbach energy confirms the increase in disorder in the nanocrystals with Co doping. Local symmetry breaking induced UV emission along with violet, blue and green luminescence has been observed from the PL study. The spectral contribution of UV emission decreases and green luminescence increases with increase in doping. Using PL, in conjunction with Raman spectroscopy, the type of oxygen vacancy induced in the nanocrystals on Co doping has been confirmed and the position of the defect levels in the forbidden zone (w.r.t. the optical band gap) has been studied.
TL;DR: In this article, a hot-injection colloidal process was used to synthesize triclinic rhenium selenide (ReSe2−x) nanosheets with enhanced HER performance via electronic structure modulation from abundant Se vacancies.
Abstract: Recently, rhenium selenide, a transition metal dichalcogenide, has been electrocatalytically synthesized for use in the hydrogen evolution reaction (HER). However, sample fabrication is still far from satisfactory owing to the plain electrochemical performance and it is still a challenge to synthesize ReSe2 with excellent controllable properties to meet the high level demands of substitution for noble metals. Herein, we report an effective route to fabricate triclinic rhenium selenide (ReSe2−x) nanosheets with enhanced HER performance via electronic structure modulation from abundant Se vacancies, tuned via a hot-injection colloidal process. High resolution transmission electron microscopy (HRTEM), Raman, photoluminescence (PL) and electron paramagnetic resonance (EPR) studies in addition to energy dispersive X-ray spectroscopy (EDX) and inductively coupled plasma-atomic emission spectrometry (ICP-AES) investigations demonstrated that the ReSe2−x nanosheets were composed of 1–3 layers with bountiful Se vacancies. High-resolution synchrotron radiation photoemission spectroscopy (HR-SRPES), ultraviolet photoelectron spectroscopy (UPS), electrochemical tests and theoretical calculations indicate that the controllable generation of the ReSe2−x nanosheets with Se vacancies could notably lead to enlarged pore volumes with densely active catalytic sites based on the electronic structure modification of the nanostructured electrode, and could thus be favorable for the improvement of the HER properties. Specifically, in acidic media, a current density of 10 mA cm−2 can be achieved at an overpotential of 102 mV, and 100 mA cm−2 at 249 mV. Furthermore, the high current density of ∼100 mA cm−2 is preserved at a constant overpotential of 250 mV, even after 12 h of electrolysis.