Valence Band Electronic Structure Research Articles

H2-D2 Exchange Activity and Electronic Structure of Ag x Pd1-x Alloy Catalysts Spanning Composition Space.

Many computational studies of catalytic surface reaction kinetics have demonstrated the existence of linear scaling relationships between physical descriptors of catalysts and reaction barriers on their surfaces. In this work, the relationship between catalyst activity, electronic structure, and alloy composition was investigated experimentally using a Ag x Pd1-x Composition Spread Alloy Film (CSAF) and a multichannel reactor array that allows measurement of steady-state reaction kinetics at 100 alloy compositions simultaneously. Steady-state H2-D2 exchange kinetics were measured at atmospheric pressure on Ag x Pd1-x catalysts over a temperature range of 333-593 K and a range of inlet H2 and D2 partial pressures. X-ray photoelectron spectroscopy (XPS) was used to characterize the CSAF by determining the local surface compositions and the valence band electronic structure at each composition. The valence band photoemission spectra showed that the average energy of the valence band, ε̅v, shifts linearly with composition from -6.2 eV for pure Ag to -3.4 eV for pure Pd. At all reaction conditions, the H2-D2 exchange activity was found to be highest on pure Pd and gradually decreased as the alloy was diluted with Ag until no activity was observed for compositions with x Pd < 0.58. Measured H2-D2 exchange rates across the CSAF were fit using the Dual Subsurface Hydrogen (2H') mechanism to extract estimates for the activation energy barriers to dissociative adsorption, ΔE ads ‡, associative desorption, ΔE des ‡, and the surface-to-subsurface diffusion energy, ΔE ss, as a function of alloy composition, x Pd. The 2H' mechanism predicts ΔE ads ‡ = 0-10 kJ/mol, ΔE des ‡ = 30-65 kJ/mol, and ΔE ss = 20-30 kJ/mol for all alloy compositions with x Pd ≥ 0.64, including for the pure Pd catalyst (i.e., x Pd = 1). For these Pd-rich catalysts, ΔE des ‡ and ΔE ss appeared to increase by ∼5 kJ/mol with decreasing x Pd. However, due to the coupling of kinetic parameters in the 2H' mechanism, we are unable to exclude the possibility that the kinetic parameters predicted when x Pd ≥ 0.64 are identical to those predicted for pure Pd. This suggests that H2-D2 exchange occurs only on bulk-like Pd domains, presumably due to the strong interactions between H2 and Pd. In this case, the decrease in catalytic activity with decreasing x Pd can be explained by a reduction in the availability of surface Pd at high Ag compositions.

Velocity Map Imaging Photoelectron Spectroscopy of Silver Iodide Aerosol Particles.

The valence band electronic structure of isolated silver iodide nanoparticles (AgI NP) was investigated by vacuum-ultraviolet aerosol photoelectron spectroscopy using the velocity map imaging technique (VUV VMI-PES). The VUV VMI-PES results were obtained for polydisperse aerosol produced by aggregation of hydrocolloid of silver iodide particles 8-15 nm in size. The ionization energy of the AgI particles was found to be 6.0±0.1 eV with respect to the vacuum level. The DFT calculations showed that the main contribution to the density of AgI electronic states in the valence region originates from I 5p orbitals. The dependence of the asymmetry parameter on the electron energy showed that the value of the characteristic energy loss of excited photoelectrons was 2.7 eV, which coincided with the band gap of the nanomaterial.

Comparison of core and valence band electronic structures of bulk uracil and 5-halouracils.

Core (C, N, and O 1s regions) and valence band electronic structures of bulk uracil and 5-fluoro-, -chloro-, and -iodouracils were investigated using X-ray photoemission spectroscopy and comprehensively compared with those of 5-bromouracil measured under the same experimental conditions before. The halogenation of uracil shifted the core peaks of the 5-position carbons toward the higher binding energy side and reduced the ionization potentials depending on the type of halogen. Theoretical calculations supported these results. The alterations of electronic properties induced by the halogenation would result in the characteristic properties of 5-halouracils.

Thermoelectric and Magnetic Properties and Electronic Structure of Solid Solutions CuCr1-xLaxS2

The oxidation states of atoms in CuCr1-xLaxS2 (x = 0–0.03) solid solutions were determined using the analysis of Cu2p, Cr2p, S2p, and La3d core level binding energy. The cationic substitution did not significantly affect the charge distribution on matrix elements (Cu, Cr, and S). The oxidation states of the atoms were identified as S2− for sulfur, Cu+ for copper, and Cr3+ for chromium. The cationic substitution in CuCr1-xLaxS2 was found to occur via the isovalent principle. The cationic substitution of CuCrS2 matrix with lanthanum ions led to the enhancement of the Seebeck coefficient comparing CuCr1-xLaxS2 to the initial matrix. The observed enhancement was attributed to the reconstruction of the valence band electronic structure after the cationic substitution. The maximum Seebeck coefficient value of 412 μV/K was measured for CuCr0.985La0.015S2 at 420 K. An increase in the lanthanum concentration to x = 0.03 caused a suppression of the Seebeck coefficient. The synthetic route was found to significantly affect both the magnetic properties and charge carrier concentration. The magnetic properties of CuCr1-xLaxS2 synthesized using metal sulfide reagents cannot be interpreted using the simple isovalent Cr3+ to La3+ cationic substitution model. The defectiveness of the samples and the formation of the impurity CuLaS2 phase could be additional factors that affect the magnetic properties of CuCr1-xLaxS2.

A comprehensive investigation on valence-band electronic structure and linear and nonlinear optical properties of a laser-driven GaAsSb-based Rosen–Morse quantum well

A comprehensive investigation on valence-band electronic structure and linear and nonlinear optical properties of a laser-driven GaAsSb-based Rosen–Morse quantum well

Evidence of electron correlation and weak bulk plasmon in SrMoO3

We investigate the electronic structure of highly conducting perovskite SrMoO3 using valence band photoemission spectroscopy and electronic structure calculations. Large intensity corresponding to coherent feature close to Fermi level is captured by density functional theory (DFT) calculation. An additional satellite at ∼3 eV binding energy remains absent in DFT, hybrid functional (DFT-hybrid) and dynamical mean field theory (DFT + DMFT) calculations. Mo 4d spectra obtained with different surface sensitive photoemission spectroscopy suggest different surface and bulk electronic structures. DFT + DMFT spectral function is in excellent agreement with the coherent feature in the bulk Mo 4d spectra, revealing moderate electron correlation strength. A large plasmon satellite and signature of strong electron correlation are observed in the surface spectra, while the bulk spectra exhibits a weak plasmon satellite.

Magnetic Field Assisted Spark Discharge-Generated Gold Nanostructures: XPS Study of Nitrogen Gas Fate and Chemical Composition of Gold Thin Films

The sparking discharge process utilises high voltage to melt and evaporate tips of electrodes to create particles that can be deposited on substrate. In our research, we examine the influence of a magnetic field and nitrogen flow on gold thin-film formation onto quartz substrate. A positive effect of nitrogen flow and a 0.3 T external magnetic field was observed, in enhancement of surface plasmon band in UV visible and dispersal of nanoparticles without agglomeration. We also detected and described nitrification occurrences of gold measured by XPS at 407 eV and nitridification of quartz substrate on which gold particles are collected. These nitrogen-based chemical reactions occurred during sparking of gold wire inside of ambient air and in the magnetic field, as well during pure nitrogen flow. We measured the valence band electronic structure of gold nanoparticles deposited onto quartz substrate and found that gold thin film prepared in the magnetic field under nitrogen flow has the lowest value of 1.5 eV. Preparation of gold thin films in the magnetic field under nitrogen flow offers a highly dispersed and convenient method for productions of thin films.

Anisotropic magnetization and electronic structure of the first-order ferrimagnet ErCo2 studied by polarization dependent hard X-ray photoemission spectroscopy

The first-order ferrimagnet ErCo2 attracts interest not only because of metamagnetism and magnetocaloric effect just above TC ≈ 32–34 K but also because it is closely related with the itinerant metamagnetism of YCo2 and LuCo2. We study the electronic structure of single crystals with hard X-ray photoemission spectroscopy (HAXPES). Magnetization measurements reconfirm the first-order magnetic transition, metamagnetism, and strong magnetic anisotropy. Calculated ErCo2 band structures of the ferrimagnetic and paramagnetic phases are presented in detail. In the ferrimagnetic state, the density of states just below EF is smaller than in the paramagnetic phase. Valence band spectra in the paramagnetic state show strong polarization dependence. Furthermore, the change across the first-order ferrimagnetic transition in the valence band electronic structures is observed. These experimental data are well described by the band structure calculation incorporated with the polarization dependent cross-sections of orbitals. We further discuss possible effects of electron correlation and spin fluctuation.

Effect of slight cobalt incorporation on the chemical, structural, morphological, optoelectronic, and photocatalytic properties of ZnO thin film

In this study, the effect of low-content cobalt doping on the chemical, structural, morphological, optoelectronic, and photocatalytic properties of ZnO was systematically investigated via XRD, XPS, AES, UPS, PL, UV-vis, and AFM techniques. In this regard, pure and Co (1 at%, 3 at%)-doped ZnO (Co-ZnO#1, Co-ZnO#3) thin films were synthesized using the ultrasonic spray pyrolysis technique. Similarly, the samples were used to coat the inner walls of transparent tubes for MB photocatalytic degradation. AFM-images reveal a high tendency for crystalline grains of both CoZnO surfaces to aggregate to form large ones, increasing the mean roughness. XRD, XPS, and AES, as sensitive and complementary techniques, substantiate the successful Co incorporation as a substitute for Zn, especially with low content (1 at%). UV-vis, PL, and UPS were adopted to study the influence of Co substituting Zn on the optoelectronic properties of ZnO, including the valence band electronic structure, bandgap, mid-bandgap states, excitonic emission, work function, Fermi level, and ionization potential. Besides, PL spectra of both Co-ZnO films reveal the participation of band-to-band transition in UV emissions and a useful intense red-emission at 685 nm, due to the Co incorporation. However, bandgap narrowing, Fermi level lowering, a redshift with considerable fluctuations in excitonic emission, and a perfect quenching of visible emission (400–640 nm) were observed only with Co(1 at%)-doping. These factors combined with the increased grain size, the decreased microstrain, and the low ionization potential (6.80 eV) are suggested to be responsible for improving the photocatalytic efficiency in Co-ZnO#1.

Valence-Band Electronic Structure of ZnO and ZnO:N: Experimental and Theoretical Evidence of Defect Complexes

Undoped and N-doped $\mathrm{Zn}\mathrm{O}$ films are grown by atomic layer deposition under O-rich conditions. Scanning photoelectron microscopy studies carried out on the films cross sections with a state-of-the-art resolution of 130 nm, allowed the study of the electronic structure of individual crystallites. In has been found that crystallites can be divided into two types, that differ in the electronic structure of the valence band. This finding, together with the cathodoluminescence images showing clustering the acceptor- and donor-related emission, unveiled that acceptors and donors are grouped in separate regions of the $\mathrm{Zn}\mathrm{O}$ and $\mathrm{Zn}\mathrm{O}$:$\mathrm{N}$ films. Density-functional-theory (DFT) calculations reveal that the complexes involving zinc vacancy, hydrogen, and nitrogen (in the case of $\mathrm{Zn}\mathrm{O}$:$\mathrm{N}$) modify the density of states in the valence-band region, so the experimentally observed differences in photoelectron spectra between crystallites evidence the grouping of acceptor complexes in some crystallites. DFT calculations also suggest that shallow acceptor states might be of $n\ensuremath{\cdot}{V}_{\mathrm{Zn}}$ origin. Nitrogen willingly joins such complexes and facilitates their formation. The separation of donors and acceptors and its tendency to group along the growth columns is of great relevance for future applications, as it indicates that eventual electronic devices should rather be constructed in a vertical architecture.

Electronic structure of rare-earth semiconducting ErN thin films determined with synchrotron radiation photoemission spectroscopy and first-principles analysis

Erbium nitride (ErN) is an emerging rocksalt rare-earth semiconducting pnictide and has attracted significant interest in recent years for its potential applications in thermoelectric energy conversion, spintronic devices, and for the Gifford-McMahon cryocoolers. Due to the Er intra-$4f$ electronic transition, Er-doped III-nitride semiconductors such as GaN, InGaN, etc. exhibit strong emission in the retina-safe and fiber optical communication wavelength window of 1.54 \ensuremath{\mu}m that is researched extensively for developing solid-state lasers, amplifiers, and light-emitting devices. However, due to ErN's propensity for oxidation in ambient, high-quality ErN thin film growth has been challenging and an in-depth understanding of its electronic structure remains unanswered. In this work, the valence band electronic structure of ErN thin films is measured with normal as well as with resonant synchrotron-radiation photoemission spectroscopy. Photoemission measurements show a valence band maximum and Fermi energy difference of \ensuremath{\sim}2.3 eV in ErN. First-principles density functional theory (DFT) calculations are performed not only to explain the valence band electronic structure but also to determine transport properties such as effective mass and deformation potentials. Strong localized Er-$4f$ states are observed \ensuremath{\sim}6--8 eV below the valence band maxima and the valence band edge is found to exhibit N-$2p$ character. Resonant photoemission data corroborates the DFT calculations. To accurately capture the electronic structure in modeling, beyond generalized gradient approximation (GGA) methods such as (a) Heyd-Scuseria-Ernzerhof hybrid functional and (b) GGA+U Hubbard correction schemes are utilized. Determination of the electronic structure of ErN marks significant progress in developing ErN-based electronic, optoelectronic, and thermoelectric devices.

Phenomenological analysis of the low temperature

The low-temperature phase diagram of a high-temperature cuprate superconductor consists of the superconducting, pseudogap and normal phases is analyzed in yttrium barium copper oxide systems by a phenomenological scheme using the shallow valence band electronic structure. The superconducting and pseudogap phases, which involve condensates, are assumed to be equilibrium systems of the condensate and normal subphases. The condition for phase transitions is given by equality of the total electronic energy gains. Using the mean strength of the pairing energy reproduces the experimental data well and the strength of the pairing energy decreases with increasing charge carrier concentration from the under-doped regime to the over-doped regime. We propose that this result can be explained by the screening effect.

Designing stable 2D materials solely from VIA elements

While the stability of tellurene is generally unsatisfactory among 2D materials, we show that elements from group-VIA may constitute stable 2D materials through the design principle of ionic bonding. In particular, a 2D wide-bandgap semiconductor, TeO3 monolayer with the CrI3-type crystal structure, has been predicted. High kinetic, thermodynamic, and mechanical stabilities have been confirmed by means of first-principles calculations. Remarkably, due to the van Hove singularities in the valence band electronic structures, TeO3 monolayer possesses tunable magnetism and half-metallicity with hole doping or elemental substitution. The corresponding Curie temperatures is above the room temperature according to the results of mean field approximation calculations. Furthermore, the carrier mobilities, exfoliation features, origin of high stability, and possible applications are discussed in detail. The TeO3 monolayer can serve as promising candidates for future nanoelectronics and spintronics applications. SeO3 and SO3 in the same 2D structure are also investigated.

Acetone sensing mechanism of Ar/O2 plasma modified indium oxide electrospun fibers: A combined DFT and experimental study

Surface adsorbed oxygen plays a crucial role in the redox reactions of semiconductor gas sensors. A novel, radio frequency plasma modification method, using mixed Ar/O2, was applied to surface treatment of In2O3 electrospun nanofibers to form an oxide-rich structure. Acetone adsorption behavior on the In2O3 (110) surface was investigated using density functional theory (DFT). With the injection of high-energy Ar/O2 plasma, an unoccupied electronic state was formed in the valence band electronic structure of In2O3. The adsorption energy of acetone on In2O3 after modification was 0.136 eV higher than before plasma modification. The adsorption ability of acetone was greatly improved after modification of Ar/O2 plasma. In2O3 sensors were fabricated before and after plasma modification, and the acetone sensing performances were measured. Gas sensing measurement showed that the Ar/O2 mixed plasma modification caused a unique improvement in the acetone sensing performance of the In2O3 sensors.

Direct observation of spin-resolved valence band electronic states from a buried magnetic layer with hard X-ray photoemission

ABSTRACT We report spin-resolved hard X-ray photoelectron spectroscopy (spin-HAXPES) for a buried Fe thin film in the valence band region. For the spin-HAXPES experiments, we developed an ultracompact built-in Mott-type spin-filter in a sample carrier, which enabled us to use the merit of two-dimensional (2D) multi-channel detector in a recent photoelectron analyser without modifying an apparatus for HAXPES. The effective Sherman function and the single-channel figure of merit (FOM) of the spin-filter were assessed to be −0.07 and 2.0 × 10−4, respectively. By utilizing the 2D detector of the photoelectron analyser, the effective FOM increased by a factor of ~4 × 104 compared to the case when only 1 channel of the 2D detector was used. We have applied spin-HAXPES to MgO(2 nm)/Fe(50 nm)/MgO(001) structures. The spin-HAXPES experiments revealed the majority and minority spin electronic states and the spin polarisation of the buried Fe thin film. Due to the large photoionization cross-section of the 4s orbital of Fe in HAXPES, the spin-resolved spectra mainly reflected the Fe 3d and 4s states. The observed spin-HAXPES and spin polarisation spectral shapes agreed well with the calculated spin-resolved cross-section weighted densities of states and spin polarisation spectra. In contrast, a small discrepancy in the energy scale was recognised due to the electron correlation effects. These results suggest that the electron correlation effects are important in the electronic structure of bulk Fe, and spin-HAXPES is useful for detecting genuine spin-resolved valence band electronic structures of buried magnetic materials.

Perpendicular magnetic anisotropy at the Fe/Au(111) interface studied by Mössbauer, x-ray absorption, and photoemission spectroscopies

The origin of the interfacial perpendicular magnetic anisotropy (PMA) induced in the ultrathin Fe layer on the Au(111) surface was examined using synchrotron-radiation-based M\"{o}ssbauer spectroscopy (MS), X-ray magnetic circular dichroism (XMCD), and angle-resolved photoemission spectroscopy (ARPES). To probe the detailed interfacial electronic structure of orbital hybridization between the Fe 3$d$ and Au 6$p$ bands, we detected the interfacial proximity effect, which modulates the valence-band electronic structure of Fe, resulting in PMA. MS and XMCD measurements were used to detect the interfacial magnetic structure and anisotropy in orbital magnetic moments, respectively. $In$-$situ$ ARPES also confirms the initial growth of Fe on large spin-orbit coupled surface Shockley states under Au(111) modulated electronic states in the vicinity of the Fermi level. This suggests that PMA in the Fe/Au(111) interface originates from the cooperation effects among the spin, orbital magnetic moments in Fe, and large spin-orbit coupling in Au. These findings pave the way to develop interfacial PMA using $p$-$d$ hybridization with a large spin-orbit interaction.

Effect of Y concentration and film thickness on microstructure and electrical properties of HfO2 based thin films

In this work, we introduced a simple solution processing method to prepare yttrium (Y) doped hafnium oxide (HfO2) based dielectric films. The films had high densities, low surface roughness, maximum permittivity of about 32, leakage current < 1.0 × 10−7 A/cm2 at 2 MV/cm, and breakdown field >5.0 MV/cm. In addition to dielectric performance, we investigated the influence of YO1.5 fraction on the electronic structure between Y doped HfO2 thin films and silicon (Si) substrates. The valence band electronic structure, energy gap and conduction band structure changed linearly with YO1.5 fraction. Given this cost-effective deposition technique and excellent dielectric performance, solution-processed Y doped HfO2 based thin films have the potential for insulator applications.

The Diagram of p–n Junction Formed on the n-GaAs Surface by 1.5 keV Ar+ Ion Beam

The core-level and valence band electronic structure of the n-GaAs (100) has been studied by synchrotron-based high-resolution photoelectron spectroscopy after irradiation by an Ar+ ion beam with energy Ei = 1500 eV and fluence Q = 1 × 1015 ions/cm2. Conversion of the conductivity type of the surface layer and formation of a p–n structure have been observed. The p-surface layer thickness (d ~ 5.0 nm) and band structure were experimentally determined from the Ga3d photoelectron spectrum by separation and analysis of the low intense n-type bulk contribution from deeper layers. A band diagram of the p–n junction formed on the n-GaAs surface by Ar+ ion bombardment was reconstructed. The p–n junction proved to be unexpectedly narrow compared to the extended tail of the implanted ion depth distribution.

Aperiodically ordered nano-graphene on the quasicrystalline substrate

Designing exotic structures in low dimensions is key in today’s quest to tailor novel quantum states in materials with unique symmetries. Particularly intriguing materials in this regard are low dimensional aperiodic structures with non-conventional symmetries that are otherwise forbidden in translation symmetric crystals. In our work, we focus on the link between the structural and electronic properties of aperiodically ordered aromatic molecules on a quasicrystalline surface, which has largely been neglected so far. As an exemplary case, we investigate the self-assembly and the interfacial electronic properties of the nano-graphene-like molecule coronene on the bulk truncated icosahedral (i) Al–Pd–Mn quasicrystalline surface using multiple surface sensitive techniques. We find an aperiodically ordered coronene monolayer (ML) film on the i-Al–Pd–Mn surface that is characterized by the same local motifs of the P1 Penrose tiling model as the bare i-Al–Pd–Mn surface. The electronic valence band structure of the coronene/i-Al–Pd–Mn system is characterized by the pseudogap of thebare i-Al–Pd–Mn, which persists the adsorption of coronene confirming the quasiperiodic nature of the interface. In addition, we find a newly formed interface state of partial molecular character that suggests an at least partial chemical interaction between the molecule and the quasicrystalline surface. We propose that this partial chemical molecule–surface interaction is responsible for imprinting the quasicrystalline order of the surface onto the molecular film.

Vertical bonding distances and interfacial band structure of PTCDA on a Sn-Ag surface alloy

Molecular materials enable a vast variety of functionalities for novel electronic and spintronic devices. The unique possibility to alter or substitute organic molecules or metallic substrates offers the opportunity to modify and optimize interfacial properties for almost any desired field of application. For this reason, we extend the successful approach to control molecular interfaces by surface alloying. We present a comprehensive characterization of the structural and electronic properties of the interface formed between the prototypical molecule PTCDA and a Sn-Ag surface alloy grown on an Ag(111) single crystal surface. We monitor the changes of adsorption height of the surface alloy atoms and electronic valence band structure upon adsorption of one layer of PTCDA using the normal incidence x-ray standing wave technique in combination with momentum-resolved photoelectron spectroscopy. We find that the vertical buckling and the surface band structure of the SnAg$_2$ surface alloy is not altered by the adsorption of one layer of PTCDA, in contrast to our recent study of PTCDA on a PbAg$_2$ surface alloy [Phys. Rev. Lett. 117, 096805 (2016)] . In addition, the vertical adsorption geometry of PTCDA and the interfacial energy level alignment indicate the absence of any chemical interaction between the molecule and the surface alloy. We attribute the different interactions at these PTCDA/surface alloy interfaces to the presence or absence of local $\sigma$-bonds between the PTCDA oxygen atoms and the surface atoms. Combining our findings with results from literature, we are able to propose an empiric rule for engineering the surface band structure of alloys by adsorption of organic molecules.