High-resolution Photoemission Spectroscopy Research Articles

Magnetic and structural properties of Mn-doped Bi2Se3 topological insulators

A thorough investigation is presented of the magnetic and structural properties of Mn-doped Bi2Se3 topological insulators grown by molecular beam epitaxy on top of insulating BaF2 (111) substrates. The magnetic properties have been studied in the temperature range from 2K to 300K in magnetic fields up to 7 T. The systems were further characterized by means of high-resolution X-ray diffraction, electron-microprobe analysis, and X-ray photoemission spectroscopy. Samples with the atomic concentration of Mn up to about 0.06 exhibit an almost perfect crystalline structure while, for higher Mn concentrations, diffuse scattering from defects is observed. Photoemission results suggest a localized non-metallic Mn 3d5 ground state which is weakly or intermediately coupled to the Bi2Se3 environment. The exchange interaction between the Mn moments leads to a ferromagnetic phase at low temperatures with a roughly linear relation between the Curie temperature and the atomic concentration of Mn.

Fast One-Pot Synthesis of MoS2/Crumpled Graphene p-n Nanonjunctions for Enhanced Photoelectrochemical Hydrogen Production.

Aerosol processing enables the preparation of hierarchical graphene nanocomposites with special crumpled morphology in high yield and in a short time. Using modular insertion of suitable precursors in the starting solution, it is possible to synthesize different types of graphene-based materials ranging from heteroatom-doped graphene nanoballs to hierarchical nanohybrids made up by nitrogen-doped crumpled graphene nanosacks that wrap finely dispersed MoS2 nanoparticles. These materials are carefully investigated by microscopic (SEM, standard and HR TEM), diffraction (grazing incidence X-ray diffraction (GIXRD)) and spectroscopic (high resolution photoemission, Raman and UV-visible spectroscopy) techniques, evidencing that nitrogen dopants provide anchoring sites for MoS2 nanoparticles, whereas crumpling of graphene sheets drastically limits aggregation. The activity of these materials is tested toward the photoelectrochemical production of hydrogen, obtaining that N-doped graphene/MoS2 nanohybrids are seven times more efficient with respect to single MoS2 because of the formation of local p-n MoS2/N-doped graphene nanojunctions, which allow an efficient charge carrier separation.

Comparative study of photocatalytic activities of hydrothermally grown ZnO nanorod on Si(001) wafer and FTO glass substrates.

ZnO nanorods have been grown on Si(001) wafer and fluorine-doped tin oxide (FTO) glass substrates for 1 and 4 h with the hydrothermal methods. The morphologies and photocatalytic activities of the ZnO nanorods were found to depend on the substrates. We investigated their properties by using spectroscopic analysis and demonstrated that the shape of nanorod and the ratios of external defects can be controlled by varying the substrates. Our experiments revealed that the nanorods grown on Si(001) have a single-crystalline wurtzite structure with (002) facets and that the number of surface oxygen defects increases with their length as the growth time increases. The nanorods grown on Si(001) have different facets, in particular wider (002) facets, and a higher ratio of the oxygen defect than the nanorods on FTO glass substrate. Moreover, the photocatalytic activities with respect to 2-aminothiophenol (2-ATP) of these nanorods were investigated with high-resolution photoemission spectroscopy (HRPES). We demonstrated that their photocatalytic activity is influenced by the ratios of surface oxygen defects, which varies with the substrate surface.

The Electronic and Adsorption Structures of 2‐Mercaptoethanol and Thioglycolic Acid on the Ge(100) Surface

The adsorption selectivity of 2‐mercaptoethanol (2‐ME) and thioglycolic acid (TGA) was studied on the Ge(100)‐2×1 surface using high‐resolution photoemission spectroscopy (HRPES) and the corresponding density functional theory (DFT) calculations. HRPES analysis results indicated that an OH dissociated SH dissociation bonded structure at low coverage and an O‐dative bonded structure at high coverage are the stable adsorption structures of 2‐ME on the Ge(100) surface. In addition, a SH dissociated OB ‐dative bonded structure at low coverage and a S‐dative bonded structure at high coverage are reasonable adsorption structures of TGA on the Ge(100) surface. Through work function measurements, we also confirmed that the forms of the adsorption structures of 2‐ME and TGA can be determined by n‐type (dissociation‐bonded structure) and p‐type (dative‐bonded structure) properties. Moreover, the adsorption structures of 2‐ME and TGA on the Ge(100)‐2×1 surface were determined through DFT calculations, and the respective stable adsorption structures, transition‐state energy, and reaction pathways were obtained.

Camelback-shaped band reconciles heavy-electron behavior with weak electronic Coulomb correlations in superconductingTlNi2Se2

Using high-resolution photoemission spectroscopy and first-principles calculations, we characterize superconducting TlNi$_2$Se$_2$ as a material with weak electronic Coulomb correlations leading to a bandwidth renormalization of 1.4. We identify a camelback-shaped band, whose energetic position strongly depends on the selenium height. While this feature is universal in transition metal pnictides, in TlNi$_2$Se$_2$ it lies in the immediate vicinity of the Fermi level, giving rise to a pronounced van Hove singularity. The resulting heavy band mass resolves the apparent puzzle of a large normal-state specific heat coefficient (Phys. Rev. Lett. 112, 207001) in this weakly correlated compound.

Bifunctional effects of the ordered Si atoms intercalated between quasi-free-standing epitaxial graphene and SiC(0001): graphene doping and substrate band bending

Bifunctional effects of the Si atoms intercalated between the n-type 6H–SiC(0001) substrate and the zero layer have been disclosed by scanning tunneling microscopy, low-energy electron diffraction, high-resolution synchrotron photoemission spectroscopy and angle-resolved photoemission spectroscopy. As a result of Si intercalation, an ordered Si interfacial layer composed of a Si adlayer and Si adatoms with dangling bonds has been formed under quasi-free-standing epitaxial graphene (EG). It turns out that the SiC(0001) band bending is determined by the Fermi level located close to the lowest states of the upper Hubbard band. The Hubbard bands originate from strong correlation effects of the electrons in the dangling bonds of the Si adatoms ordered on the Si adlayer. The doping level of the decoupled graphene is determined by the amount of charge transferred from the Si adatoms ordered on the Si adlayer to the quasi-free-standing EG.

Photocatalytic oxidation activities of TiO2 nanorod arrays: A surface spectroscopic analysis

The correlation between the defect structures of hydrothermally grown TiO2 nanorods (TNR) and their photocatalytic activity has been investigated by using various surface spectroscopic analysis techniques including scanning transmission X-ray microscopy (STXM) and high-resolution photoemission spectroscopy (HRPES). Defect states related to Ti3+ and oxygen vacancies in TNR were observed when the growth time was over 90min (i.e., those over 500nm in height), and these TNR samples showed photocatalytic oxidation activity with respect to the molecules 2-mercaptoethanol (2-ME) or 2-aminothiophenol (2-ATP), also confirmed by HRPES. The presence of defect structures on the TNR influences on the electronic states of titanium and oxygen, and thus can improve the catalytic oxidation reactions.

(Invited) Band Gap Tunable Monolayer WSe2(1-x)S2x Synthesis, Characterization and Device Applications

“Band gap engineering” in two-dimensional (2D) materials plays an important role in tailoring their physical and chemical properties. The tuning of the band gap is typically achieved by controlling the composition of the semiconductor alloys.1-4 Here, we report the synthesis and electrical properties of monolayer WSe2(1-x)S2x. The photoluminescence (PL) spectroscopy results confirm the tunable band gap in WSe2(1-x)S2x, which is modulated by varying the S content. Atomic-scale analysis was performed and the chemical composition was characterized using high-resolution scanning transmission electron microscopy and X-ray photoemission spectroscopy. The mobility and carrier concentrations were measured in as-grown monolayer-based transistor devices. The emergency of these 2D materials provides grand possibilities for future semiconductor device applications.

Anomalies of a topologically ordered surface.

Bulk insulators with strong spin orbit coupling exhibit metallic surface states possessing topological order protected by the time reversal symmetry. However, experiments show vulnerability of topological states to aging and impurities. Different studies show contrasting behavior of the Dirac states along with plethora of anomalies, which has become an outstanding problem in material science. Here, we probe the electronic structure of Bi2Se3 employing high resolution photoemission spectroscopy and discover the dependence of the behavior of Dirac particles on surface terminations. The Dirac cone apex appears at different binding energies and exhibits contrasting shift on Bi and Se terminated surfaces with complex time dependence emerging from subtle adsorbed oxygen-surface atom interactions. These results uncover the surface states behavior of real systems and the dichotomy of topological and normal surface states important for device fabrication as well as realization of novel physics such as Majorana Fermions, magnetic monopole, etc.

Electronic structure of Fe-based superconductors

Fe-based superconductors have drawn much attention during the last decade due to the presence of superconductivity in materials containing the magnetic element, Fe, and the coexistence of superconductivity and magnetism. Extensive study of the electronic structure of these systems suggested the dominant role of d states in their electronic properties, which is significantly different from the cuprate superconductors. In this article, some of our studies of the electronic structure of these fascinating systems employing high-resolution photoemission spectroscopy is reviewed. The combined effect of electron correlation and covalency reveals an interesting scenario in their electronic structure. The contribution of ligand p states at the Fermi level is found to be much more significant than indicated in earlier studies. Temperature evolution of the energy bands reveals the signature of transition akin to Lifshitz transition in these systems.

Temperature effect on the electronic structure of Nb:SrTiO3 (100) surface**Project supported by the Funds from the Chinese Academy of Sciences (Grant No. 1G2009312311750101) and the National Natural Science Foundation of China (Grant No. 11375228).

The effect of temperature on the electronic structure of Nb-doped SrTiO3 (100) surface is investigated by high-resolution synchrotron radiation photoemission spectroscopy. According to the x-ray photoemission spectroscopy (XPS) results, at an annealing temperature of less than 700 °C, the adsorbed carbon and hydroxyl on the STO surface could be removed, to expose the fresh intrinsic surface with a constant ratio of Ti/O. It is obvious that the STO would be doped by Ca+ impurities of bulks and O vacancies in the surface after annealing at 920 °C for one hour.

Structural, optical and vibrational properties of self-assembled Pbn+1(Ti1−xFex)nO3n+1−δ Ruddlesden-Popper superstructures

Bulk crystals and thin films of PbTi1−xFexO3−δ (PTFO) are multiferroic, exhibiting ferroelectricity and ferromagnetism at room temperature. Here we report that the Ruddlesden-Popper phase Pbn+1(Ti1−xFex)nO3n+1−δ forms spontaneously during pulsed laser deposition of PTFO on LaAlO3 substrates. High-resolution transmission electron microscopy, x-ray diffraction and x-ray photoemission spectroscopy were utilised to perform a structural and compositional analysis, demonstrating that and . The complex dielectric function of the films was determined from far-infrared to ultraviolet energies using a combination of terahertz time-domain spectroscopy, Fourier transform spectroscopy, and spectroscopic ellipsometry. The simultaneous Raman and infrared activity of phonon modes and the observation of second harmonic generation establishes a non-centrosymmetric point group for Pbn+1(Ti0.5Fe0.5)nO3n+1−δ, a prerequisite for (but not proof of) ferroelectricity. No evidence of macroscopic ferromagnetism was found in SQUID magnetometry. The ultrafast optical response exhibited coherent magnon oscillations compatible with local magnetic order, and additionally was used to study photocarrier cooling on picosecond timescales. An optical gap smaller than that of BiFeO3 and long photocarrier lifetimes may make this system interesting as a ferroelectric photovoltaic.

The electronic properties of mixed valence hydrated europium chloride thin film.

We investigate the electronic properties of a model mixed-valence hydrated chloride europium salt by means of high resolution photoemission spectroscopy (HRPES) and resonant photoemission spectroscopy (RESPES) at the Eu 3d → 4f and 4d → 4f transitions. From the HRPES spectra, we have determined that the two europium oxidation states are homogeneously distributed in the bulk and that the hydrated salt film is exempt from surface mixed valence transition. From the RESPES spectra, the well separated resonant contributions characteristic of divalent and trivalent europium species (4f(6) and 4f(7) final states, respectively) are accurately extracted and quantitatively determined from the resonant features measured at the two edges. The partial absorption yield spectra, obtained by integrating the photoemission intensity in the valence-band region, can be well reproduced by atomic multiplet calculation at the M(4,5) (3d-4f) absorption edge and by an asymmetric Fano-like shape profile at the N(4,5) (4d-4f) absorption edge. The ratio of Eu(2+) and Eu(3+) species measured at the two absorption edges matches with the composition of the mixed valence europium salt as determined chemically. We have demonstrated that the observed spectroscopic features of the mixed valence salt are attributed to the mixed-valence ground state rather than surface valence transition. HRPES and RESPES spectra provide reference spectra for the study of europium salts and their derivatives.

Investigation of the interaction between a novel unnatural chiral ligand and reactant on palladium for asymmetric hydrogenation.

We report about the mechanistic studies of the reaction between a newly synthesized (S)-2-((R)-3H-dinaphtho[2,1-c:1',2'-e]azepin-4(5H)-yl)-2-phenylethanol based on the binaphthyl skeleton and (E)-2-methyl-5-phenylpent-2-enoic acid for the asymmetric hydrogenation of α,β-unsaturated acids with heterogeneous palladium catalysts. The specific interactions between the chiral ligand and reactant were investigated in solution with palladium nanoparticles, as well as under ultrahigh vacuum (UHV) conditions on the palladium metal surface in the absence of hydrogen. The reactions were explored using nuclear magnetic resonance (NMR) spectroscopy, scanning tunneling microscopy (STM), and high-resolution photoemission spectroscopy (HRPES) combined with density functional theory (DFT) calculations. A NMR study identified the interaction between both molecules with palladium nanoparticles in solution. In addition, STM and HRPES studies revealed the spatial distribution and configuration of both compounds on the palladium metal surface under UHV conditions. The theoretical results support the experimental results with respect to the interaction energy value. It was found that the reaction between the ligand and reactant occurs with hydrogen bonding on palladium surface, simultaneously. The present study provides mechanistic details of the asymmetric hydrogenation reaction, which bears a correlation between the ligand, reactant, and catalyst during the reaction.

Photo-oxidation activities on Pd-doped TiO2 nanoparticles: critical PdO formation effect

The catalytic activities of three distinct Pd-doped TiO2 (PdTiO2) nanoparticle samples, post-annealed (at 700, 800, and 900°C) after fabrication on silicon substrates, were analyzed with respect to the photo-oxidation of aniline and 2-thiophenecarboxaldehyde. Through a combination of Raman spectroscopy and high-resolution photoemission spectroscopy (HRPES), we demonstrated that the photocatalytic oxidation occurred only when the PdTiO2 nanoparticles were annealed at temperatures above 800°C, that is, when PdO and Ti3+ had partially formed on the PdTiO2 nanoparticles, which indicates that the presence of PdO and Ti3+ is critical for the photocatalytic oxidation. Additionally, we determined the decrease in the band gaps to visible region with PdTiO2 nanoparticles annealed above 800°C from their valence-band spectra, which resulted in photocatalytic activities even under visible light, λ=540nm.

Electronic Structures of CeM2Al10 (M = Fe, Ru, and Os) Studied by Soft X-ray Resonant and High-Resolution Photoemission Spectroscopies

We have performed a photoemission spectroscopy (PES) study of CeM2Al10 (M = Fe, Ru, and Os) to directly observe the electronic structure involved in the unusual magnetic ordering. Soft X-ray resonant (SXR) PES provides spectroscopic evidence of the hybridization between conduction and Ce 4f electrons (c-f hybridization) and the order of the hybridization strength (Ru < Os < Fe). High-resolution (HR) PES of CeRu2Al10 and CeOs2Al10, as compared with that of CeFe2Al10, identifies two structures that can be ascribed to structures induced by the c-f hybridization and the antiferromagnetic ordering, respectively. Although the c-f hybridization-induced structure is a depletion of the spectral intensity (pseudogap) around the Fermi level (EF) with an energy scale of 20-30 meV, the structure related to the antiferromagnetic ordering is observed as a shoulder at approximately 10-11 meV within the pseudogap. The energies of the shoulder structures of CeRu2Al10 and CeOs2Al10 are approximately half of the optical gap (20 meV), indicating that EF is located at the midpoint of the gap.

Estimate of the Coulomb correlation energy in CeAg2Ge2 from inverse photoemission and high resolution photoemission spectroscopy

The occupied and the unoccupied electronic structure of CeAg2Ge2 single crystal has been studied using high resolution photoemission and inverse photoemission spectroscopy, respectively. High resolution photoemission reveals the clear signature of Ce 4f states in the occupied electronic structure which was not observed clearly in our earlier studies. The Coulomb correlation energy in this system has been determined experimentally from the position of the 4f states above and below the Fermi level. Theoretically, the correlation energy has been determined by using the first principles density functional calculations within the generalized gradient approximations taking into account the strong intra-atomic (on-site) interaction Hubbard Ueff term. The calculated valence band shows minor changes in the spectral shape with increasing Ueff due to the fact that the density of Ce 4f state is narrow in the occupied part and is hybridized with the Ce 5d, Ag 4d and Ge 4p states. On the other hand, substantial changes are observed in the spectral shape of the calculated conduction band with increasing Ueff since the density of Ce 4f state is very large in the unoccupied part, compared to other states. The estimated value of correlation energy for CeAg2Ge2 from the experiment and the theory is ≈ 4.2 eV. The resonant photoemission data are analyzed in the framework of the single-impurity Anderson model which further confirms the presence of the Coulomb correlation energy and small hybridization in this system.

Electrochemical, HR-XPS and SERS study of the self-assembly of biphenyl 4,4′-dithiol on Au(111) from solution phase

We report a comparative study of 4,4′-biphenyldithiol adlayers grown on Au(111) substrates in solution phase by different methods. Layers prepared by immersion in solutions of ethanol and n-hexane, with and without the use of a disulfide reducing agent (tris-carboxyethyl phosphine, TCEP), were characterized by electroreductive desorption, impedance spectroscopy, redox activity, high-resolution photoemission spectroscopy, and surface-enhanced Raman spectroscopy. It is shown that the simple immersion in an ethanolic solution leads to the formation of, at least, a bilayer with a high number of SS bonds. The use of n-hexane as solvent produces a drastic reduction of these undesired bonds, and a similar result is obtained if the substrates prepared in the ethanolic solution are then washed with TCEP. The best results were obtained when the reducing agent was added into the ethanol solution, in which case all the characterizations were coincident in the formation of a single layer of standing-up molecules free of SS bonds. The charge of the desorption peak and the relative intensity of the S2p and Au4f photoemission peaks both indicate a surface coverage θ≈0.2. In the case of multilayers formed by immersion in pure ethanolic solutions, the Raman experiments indicate that intralayer SS bonds are not formed, and hence that all the SS bonds are of the interlayer type.

Photoelectrochemical Processes at n-GaAs(100)/Aqueous HCl Electrolyte Interface: A Synchrotron Photoemission Spectroscopy Study of Emersed Electrodes

High-resolution synchrotron photoemission spectroscopy has been applied to detail the electrochemical and photoelectrochemical corrosion reactions at the liquid junction n-GaAs(100)/1 M aqueous HCl solution. Under anodic polarization of 1.8 eV, the main process initiated by the presence of holes in the Ga–As bonding states of the valence band is the formation of soluble gallium chloride complexes and insoluble elemental arsenic on the surface. In addition, arsenic hydroxide forms, which reacts further to soluble HAsO2. In toto, the As/Ga atomic ratio increases, which is accompanied by an increase of the work function. The anodic decomposition reaction is enhanced by illumination as more holes reach the n-semiconductor/electrolyte junction. Under cathodic polarization of 1.5 eV, only minor changes are observed in Ga and As core-level spectra, giving no indication of corrosion, but specific adsorption of hydrated HCl molecules and/or Cl– ions considerably modifies valence band spectra.

Electronic and structural properties of graphene-based metal-semiconducting heterostructures engineered by silicon intercalation

The initial decoupling of the (6√3×6√3)R30° buffer layer also called zero layer graphene (ZLG) on 6H-SiC(0001) by Si intercalation has been investigated by means of high resolution photoemission spectroscopy (HRPES) and microscopy imaging techniques. A combination of complementary techniques has shown that the annealing above 700°C of amorphous Si deposited on ZLG leads to the diffusion of the silicon over the surface. Two competing processes are then observed. Part of the silicon contributes to a progressive decoupling of the ZLG from the substrate (partial decoupling) while the rest agglomerates at the surface to form oriented silicon clusters. After sequences of Si deposition, followed by annealing at 750°C, complete decoupling is observed into quasi-free standing monolayer (ML) graphene. Investigation of the evolution of the C1s and Si2p core levels during the intermediate states shows that the appearance of the graphene contribution coincides with the creation of an extra SiC bulk component, indicating their electronic decoupling. At partial decoupling of the ZLG, we have the coexistence of structurally linked metal-semiconducting materials presenting mutual electronic interactions and composed of nanometric metal-semiconducting heterojunctions.