Valence Band Studies Research Articles

Hybrid Reduced Graphene Oxide/GaN Nanocolumns on Flexible Niobium Foils for Efficient Photoelectrochemical Water Splitting

We present the photoelectrochemical (PEC) water-splitting properties of pristine and reduced graphene oxide (rGO)-coated GaN nanocolumns (NCs) on flexible niobium (Nb) metal foils. The structural, optical, and electronic structure analyses of rGO-coated GaN-NCs on Nb foils revealed the formation of a rGO/GaN-NCs hybrid structure. Further, the valence-band studies of pure GaN-NCs shows valence-band maxima at ∼3.0 eV below the Fermi level, which decreased to ∼2.8 eV for rGO/GaN-NCs. The PEC measurement performed on pristine GaN-NCs under standard (1 Sun) conditions shows an effective photocatalytic nature with a photocurrent density of ∼60 μA/cm2 at 0.8 V (vs Ag/AgCl) in a 1 M oxalic acid electrolyte, which increases to ∼110 μA/cm2 for rGO/GaN-NCs. The efficient PEC characteristics of rGO/GaN-NCs are attributed to the effective charge separation/transport of photogenerated carriers. Furthermore, transient photocurrent measurement reveals that hybrid and pristine GaN-NCs on flexible metal foil have a fast and stable photoresponse in an aqueous solution. The development of a hybrid nitride nanostructure-based PEC device on flexible metal foils paves the way toward developing scalable PEC devices for hydrogen production applications.

Probing interband and intraband transitions in magneto-optical FeT (T = Cr, Co, Ni) alloys from electronic structure studies

Photoelectron spectroscopy as a function of temperature is used to study interband and intraband transitions in magneto-optical FeT (T = Cr, Co, Ni) alloys. Valence band studies on these alloys indicates a strong d-d hybridization between Fe and T atoms which gives rise to localized valence states and contributes to direct interband transition while the s-d hybridized band contribute to the intraband transition and give rise to the spin polarized conduction electrons. The analysis of core levels shows that the spin orbit coupling is influenced by the near neighbour interaction between Fe and T atoms. The exchange interaction has the temperature dependence which drives the itinerant electron magnetism in FeT alloys. We find that the large density of the itinerant states at the Fermi edge and the large separation between the localized and the itinerant electronic state in FeCr are crucial for the large magneto-optical property reported in this system. In FeCo and FeNi, the overlapped localized and itinerant electronic states make these materials optically anisotropic.

Valence band studies of MoS2 thin films synthesised by electrodeposition method

Thin films of MoS2 were synthesied via electrodeposition. The films were characterized by X-Ray Diffraction, Raman spectroscopy, Field Emission Scanning Electron Microscopy, Optical Spectroscopy and X-ray Photoelectron Spectroscopy. The valence band maximum of the films were calculated from the XPS data. With the XPS and optical spectroscopy data, the band diagrams were calculated. The band alignment with Indium Tin Oxide (ITO) transparent conductive oxide was also deduced from the XPS data. The I-V characteristics of the ITO/MoS2 heterojunction was also studied.

Stress induced modification of electronic band structure and enhanced optical emission in 1-D GaN nanostructures

1-dimensional GaN nanowires (NW) and nanorods (NR) grown using catalyst assisted Chemical Vapour Deposition are investigated for their structural and optical properties, and their electronic band structure. X-ray Diffraction (XRD) studies reveal that biaxial strain increases with aspect ratio, which is corroborated by Raman studies wherein phonon related confinement effects are observed for NR due to the crystal strain. Photoluminescence studies show 3 and 6 times enhancement in the intensity of band edge emission of NW and NR, respectively, signifying the presence of a large number of band tail states. A small red shift in NW (3.43 eV) and large red-shift in NR (3.28eV) is reported, which are attributed to the change in the band gap and band structure of these morphologies. Valence band studies by X-ray Photoelectron Spectroscopy reveal the presence of band tail states for NR and surface bond contraction for NW. The effect of crystal strain on the electronic band structure on optical properties of 1-D GaN nanostructures with different aspect ratios is elucidated.

Investigation of electronic structure of transition metal silicides MnSi1.75 and CoSi for enhanced thermoelectric properties

Transition metal silicides that have low band gap or semimetallic states have gained huge interest due to enhanced thermoelectric efficiency. We present high resolution photoemission studies on two such systems like MnSi1.75 a low band gap semiconductor and CoSi a semimetal to understand the inherent electronic properties. Valence band studies on these systems reveal that there are both localized and delocalized 3d states present where as in the pure transition metals more localized states present near the Fermi level. In both MnSi1.75 and CoSi, the localized 3d states are found to be shifted to higher binding energy due to the strong hybridization with the Si 3s−3p states that gives rise to the hybridization gap in these systems. The delocalized 3d states in both the systems has much lower electronic density near the Fermi level than the pure transition metals that contributes to the conduction. Supporting evidences of delocalized screening in MnSi1.75 and weak correlated satellite in CoSi have been observed in the 2p core-level spectra. We find that the presence of both hybridization gap and the finite carrier concentration at the Fermi level in these transition metal silicides are necessary to have the enhanced thermoelectric properties.

Anomalous magnetic properties of Fe3O4 nanostructures on GaAs substrate probed using X-ray magnetic circular dichroism

Abstract We have studied the magnetic properties and electronic structure of 2 nm Fe3O4 film on GaAs substrates, which evolve in a nano-tip structure and compared it with 150 nm thick Fe3O4 film. Valence band studies reveal a drop in density of states at Fermi level for 2 nm film as compared to thicker film. XMCD studies divulge the unusual high magnetization value of ∼5.1 μB/FU in 2 nm film compared to 3.9 μB/FU observed for bulk Fe3O4 or thicker Fe3O4 films. The excess moment is shown to be due to orbital moment contribution.

Bandgap Engineering and Signature of Ferromagnetism in Ti1−xMnxO2­ Diluted Magnetic Semiconductor Nanoparticles: A Valence Band Study

Diluted magnetic semiconductor Ti1−xMnxO2­ (0.0 ≤ x ≤ 0.06) nanoparticles have been synthesized by sol–gel technique. Phase purity, structural, micro‐structural, and vibrational properties of the samples have been studied by X‐ray diffraction, transmission electron microscopy (TEM), high‐resolution TEM, and Raman spectroscopy. UV–Vis and photoluminescence spectroscopy clearly indicate the tuning of bandgap and appearance of different defect states (oxygen vacancies) with Mn‐doping, respectively. Chemical states and surface stoichiometry of the samples have been probed by X‐ray photoemission spectroscopy (XPS). Shifting of binding energy of Ti2p toward lower value and appearance of Mn2+, Mn3+, and Mn4+confirm Mn doping into TiO2 and also indicate that Mn‐doping reduces the number of oxygen vacancies in the system. Valence band studies have been done by XPS and ultraviolet photoemission spectroscopy (UPS) valence band spectra. Combined result of valence band spectra and optical data reveals shortening of HOMO–LUMO gap with increasing Mn‐concentration. Room temperature ferromagnetism, originating from oxygen vacancies, has been explained on the basis of the bound magnetic polaron (BMP) model. Resistivity measurements have been conducted to examine the semiconducting behavior and to study the electrical conduction mechanism. It is revealed that the thermally activated conduction (Arrhenius) mechanism is valid in the high temperature region whereas Mott's variable‐range hopping (VRH) mechanism is applicable in low temperature region.

A simulation study of valence band offset engineering at the perovskite/Cu2ZnSn(Se1-xSx)4 interface for enhanced performance

Proper valence band offset at the perovskite/hole transport material interface is critical to obtain high performance perovskite solar cells. The Cu2ZnSn(Se1-xSx)4 compound is a potential candidate of hole transport material for perovskite solar cells and exhibits tunable band gap from 0.95 eV for Cu2ZnSnSe4 to 1.5 eV for Cu2ZnSnS4 with different S/(S+Se) ratio, which offers a feasible approach to engineering the valence band offset at the perovskite/Cu2ZnSn(Se1-xSx)4 interface. Here, the valence band offset engineering at the perovskite/Cu2ZnSn(Se1-xSx)4 interface is studied through numerical simulation with SCAPS package. The valence band offset can be tuned from negative value to positive value with different Cu2ZnSn(Se1-xSx)4 composition. With optimized S concentration, a suitable valence band offset (0.27 eV) is obtained, leading to a power conversion efficiency of 20.25%. Further optimization of thickness, defect density, and acceptor density of the Cu2ZnSn(Se1-xSx)4 transport layer is conducted, and power conversion efficiency of 20.77% is obtained. This study here provides a guidance to further optimize the performance of perovskite solar cells with Cu2ZnSn(Se1-xSx)4 hole transport material.

Direct evidence of the existence of Mn3+ ions in MnTiO3

We investigate the room temperature electronic properties of MnTiO3 synthesised by different preparation conditions. For this purpose, we prepared MnTiO3 under two different cooling rates, one is naturally cooled while the other is quenched in liq.nitrogen. The samples were studied using optical absorbance, photoemission spectroscopy and band structure calculations. We observe significant changes in the structural parameters as a result of quenching. Interestingly, in the parent compound, our combined core level, valence band and optical absorbance studies give evidence of the Mn existence in both 2+ and 3+ states. The fraction of Mn3+ ions has been found to increase on quenching MnTiO3 suggests an increase in oxygen non-stoichiometry. The increase in the fraction of the Mn3+ ions has been manifested a) as slight enhancement in the intensity of the optical absorbance in the visible region. There occurs persistent photo-resistance when the incident light is terminated after shining; b) in the behaviour of the features (close to Fermi level) in the valence band spectra. Hence, the combined analysis of the core level, valence band and optical absorbance spectra suggests that the charge carriers are hole like which further leads to the increase in the electrical conductivity of the quenched sample. The present results provide a recipe to tune the optical absorption in the visible range for its applications in optical sensors, solar cell, etc.

XPS valence band study of Na‐silicate glasses: energetics and reactivity

High‐resolution X‐ray photoelectron spectroscopy (XPS) of valence bands of vitreous silica (SiO2(vit)), and Na‐silicate glasses are divisible into 3 sub‐bands. These are the O 2s band, the lower valence band (LVB), and the upper valence band (UVB). Many peaks of the LVB and UVB have line widths (full width at half maximum) of 1.2–1.7 eV, which are commensurate with widths of core level O 1s lines, indicating weak dispersion of these valence band peaks. Two types of oxygen exist in the glasses. There is oxygen bridging 2 Si atoms (bridging oxygen [BO]), and O bonded to Si and Na (nonbridging oxygen [NBO]). The addition of Na to siliceous glasses diminishes electronic density of states of the LVB and enhances density of states of the UVB, a consequence of quenching Si‐BO σ‐bonds of the LVB, creating Si‐NBO σ‐bonds in the UVB, and creating atomic‐like O 2px,y nonbonding orbitals at the top of the UVB or highest occupied molecular orbitals. The process destabilizes the valence bands of the glasses by 2 to 6 eV per Si‐NBO bond formed. All orbitals are destabilized with increased Na2O content. There is effectively complete transfer of Na 3s electrons to NBO but this charge is redistributed among all atoms of the tetrahedron via the 4 Si‐O σ bonds (sp3‐O 2px character). The BE of the Si 2p line decreases more rapidly with Na2O content than any other line monitored, and this preferential accumulation of negative charge on Si causes Si‐O bonds to become weaker through decrease in the force constant of the bonds. The durability of Na‐silicate glasses decreases in response. The destabilization of the highest occupied molecular orbitals makes the Na‐rich glasses more susceptible to attack by reagents such as H+, OH−, and H2O. Copyright © 2017 John Wiley & Sons, Ltd.

Doping induced modifications in the electronic structure and magnetism of ZnO films: Valence band and conduction band studies

The electronic structure of Pulsed Laser Deposited (PLD) ZnO, Zn0.95Fe0.05O (ZFO), Zn0.98Al0.02O (ZAO) and Zn0.93Fe0.05Al0.02O (ZFAO) films were investigated by Photoelectron spectroscopy and X-ray absorption spectroscopy. X-ray diffraction and ϕ-scan measurements show epitaxial c-directional growth of the films. Temperature dependent magnetization and M-H loop measurements show the presence of room temperature magnetic ordering in all the films. Fittings of Fe 2p XPS and Fe L3,2 −edge XAS of ZFO and ZFAO films show the presence of Fe, in both, Fe+2 and Fe+3 states in tetrahedral symmetry. Valence band spectra in resonance mode show resonance photon energy at 56eV showing the presence of Fe2+ state (∼2eV) near the Fermi level. A significant effect of Fe and Al doping on the spectral shape of O K-edge XAS was observed. Results of the Spectroscopic studies reveal that, ferromagnetism in the films is due to the contribution of oxygen deficiency which increases the number of charge carriers that take part in the exchange interaction. Al co-doping with Fe (in ZFAO) results in the enhancement of saturation magnetization by increase in the carrier-mediated ferromagnetic exchange interaction.

Metallic Glass Nano-composite Thin Films for High-performance Functional Applications

Metallic glass nanocomposite thin films were synthesized for an immiscible Ag-Cu alloy system by magnetron sputtering. The structure of the films was unique, consisting of homogeneously dispersed nanocrystallites in an amorphous matrix. The size and volume fraction of the nanocrystallites increased with increasing film thickness resulting in increased elastic modulus and hardness. The high electrical conductivity of the nanocomposite films was examined by a valence-band study, which showed that exchange interaction between Ag and Cu in the nanocomposite structure resulted in enhanced charge carrier concentration. The inverse correlation between electrical conductivity and film thickness was explained by surface and interface scattering of electrons with increasing volume fraction of nanocrystallites. The small temperature dependence of conductivity was attributed to the distorted Fermi surface of the nanocomposite films resulting in a greater contribution from structure scattering, which is temperature-independent.

The SPECIES beamline at the MAX IV Laboratory: afacility for soft X-ray RIXS and APXPS.

SPECIES is an undulator-based soft X-ray beamline that replaced the old I511 beamline at the MAX II storage ring. SPECIES is aimed at high-resolution ambient-pressure X-ray photoelectron spectroscopy (APXPS), near-edge X-ray absorption fine-structure (NEXAFS), X-ray emission spectroscopy (XES) and resonant inelastic X-ray scattering (RIXS) experiments. The beamline has two branches that use a common elliptically polarizing undulator and monochromator. The beam is switched between the two branches by changing the focusing optics after the monochromator. Both branches have separate exit slits, refocusing optics and dedicated permanent endstations. This allows very fast switching between two types of experiments and offers a unique combination of the surface-sensitive XPS and bulk-sensitive RIXS techniques both in UHV and at elevated ambient-pressure conditions on a single beamline. Another unique property of the beamline is that it reaches energies down to approximately 27 eV, which is not obtainable on other current APXPS beamlines. This allows, for instance, valence band studies under ambient-pressure conditions. In this article the main properties and performance of the beamline are presented, together with selected showcase experiments performed on the new setup.

Surface chemical-bonds analysis of silicon particles from diamond-wire cutting of crystalline silicon

The recycling of the Si powder resulting from the kerf loss during silicon ingot cutting into wafers for photovoltaic application shows both significant and achievable economic and environmental benefits. A combined x-ray photoelectron spectroscopy (XPS), attenuated total reflection (ATR)-Fourier transform infrared (FTIR) and micro-Raman spectral analyses were applied to kerf-loss Si powders reclaimed from the diamond wire cutting using different cutting fluids. These spectroscopies performed in suitable configurations for the analysis of particles, yield detailed insights on the surface chemical properties of the powders demonstrating the key role of the cutting fluid nature. A combined XPS core peak, plasmon loss, and valence band study allow assessing a qualitative and quantitative chemical, structural change of the kerf-loss Si powders. The relative contribution of the LO and TO stretching modes to the Si-O-Si absorption band in the ATR-FTIR spectra provide a consistent estimation of the effective oxidation level of the Si powders. The change in the cutting media from deionized water to city water, induces a different silicon oxide layer thickness at the surface of the final kerf-loss Si, depending on the powder reactivity to the media. The surfactant addition induces an enhanced carbon contamination in the form of grafted carbonated species on the surface of the particles. The thickness of the modified surface, depending on the cutting media, was estimated based on a simple model derived from the combined XPS core level and plasmon peak intensities. The effective nature of these carbonated species, sensitive to the water quality, was evidenced based on coupled XPS core peak and valence band study. The present work paves the way to a controlled process to reclaim the kerf-loss Si powder without heavy chemical etching steps.

Photoelectron circular dichroism of isopropanolamine

Spectroscopies based on circular polarized light are sensitive to the electronic and structural properties of chiral molecules. Photoelectron circular dichroism (PECD) is a powerful technique that combines the chiral sensitivity of the circular polarized light and the electronic information obtained by photoelectron spectroscopy. An experimental and theoretical PECD study of the outer valence and C 1s core states of 1-amino-2-propanol in the gas phase is presented. The experimental dichroic dispersions in the photoelectron kinetic energy are compared with theoretical calculations employing a multicentric basis set of B-spline functions and a Kohn-Sham Hamiltonian. In order to understand analogies and differences in the dichroism of structural isomers bearing the same functional groups, a comparison with previous PECD study of valence band of 2-amino-1-propanol is carried out.

Electrical and Optical Properties of CeNi5 Nanoscale Films.

Rare earth compounds are interesting from both a theoretical point of view and for their applications. That is the reason why determining their optical and electrical properties deserves special attention. In this article, we present the conditions we obtained homogenous CeNi5 thin films of nanometer thicknesses. To achieve this goal, our method of choice was laser-induced vaporization, using short and modulated impulses, with electro-optical tuning for the quality factor. The layers that were deposited at a single laser burst had thicknesses between 1.5 and 2.5 nm, depending on the geometry of the experimental setup.Structural and compositional studies of the nanoscale films were made using XRD. The temperature dependence of electrical conductivity was also determined. The following optical properties of the specimens were computed using the Kramers-Krönig framework and discussed: absolute reflection and transmission coefficients for a single wavelength and relative ones for the wide UV-VIS-IR spectra, spectral dependence of the refractive index, and extinction coefficient as real and imaginary parts of the complex refractive index. The valence band studies were made with X-ray photoelectron spectroscopy. All these determinations were well correlated and permitted the evaluation of the energy densities of states in the deeper bands, near the Fermi energy, and at the surface states.

Spin-exchange interaction between transition metals and metalloids in soft-ferromagnetic metallic glasses

High-performance magnetic materials have immense industrial and scientific importance in wide-ranging electronic, electromechanical, and medical device technologies. Metallic glasses with a fully amorphous structure are particularly suited for advanced soft-magnetic applications. However, fundamental scientific understanding is lacking for the spin-exchange interaction between metal and metalloid atoms, which typically constitute a metallic glass. Using an integrated experimental and molecular dynamics approach, we demonstrate the mechanism of electron interaction between transition metals and metalloids. Spin-exchange interactions were investigated for a Fe–Co metallic glass system of composition [(Co1−xFex)0.75B0.2Si0.05]96Cr4. The saturation magnetization increased with higher Fe concentration, but the trend significantly deviated from simple rule of mixtures. Ab initio molecular dynamics simulation was used to identify the ferromagnetic/anti-ferromagnetic interaction between the transition metals and metalloids. The overlapping band-structure and density of states represent ‘Stoner type’ magnetization for the amorphous alloys in contrast to ‘Heisenberg type’ in crystalline iron. The enhancement of magnetization by increasing iron was attributed to the interaction between Fe 3d and B 2p bands, which was further validated by valence-band study.

Core-Shell Vanadium Modified Titania@β-In2S3 Hybrid Nanorod Arrays for Superior Interface Stability and Photochemical Activity.

Core-shell rutile TiO2@β-In2S3 and modified V-TiO2@β-In2S3 were synthesized to develop bilayer systems to uphold charge transport via an effective and stable interface. Morphological studies revealed that β-In2S3 was deposited homogeneously on V-TiO2 as compared to unmodified TiO2 nanorod arrays. X-ray photoelectron spectroscopy (XPS) and electron energy loss spectrometry studies verified the presence of various oxidation states of vanadium in rutile TiO2 and the vanadium surface was utilized for broadening the charge collection centers in host substrate layer and hole quencher window. Subsequently, X-ray diffraction, high-resolution transmission electron microscopy, and Raman spectra confirmed the rutile phases of TiO2 and modified V-TiO2 along with the phases of crystalline β-In2S3. XPS valence band study explored the interaction of valence band quazi Fermi levels of β-In2S3 with the conduction band quazi Fermi levels of modified V-TiO2 for enhanced charge collection at the interface. Photoelectrochemical studies show that the photocurrent density of V-TiO2@β-In2S3 is 1.42 mA/cm(2) (1.5AM illumination). Also, the frequency window for TiO2 was broadened by the vanadium modification in rutile TiO2 nanorod arrays, and the lifetime of the charge carrier and stability of the interface in V-TiO2@β-In2S3 were enhanced compared to the unmodified TiO2@β-In2S3. These findings highlight the significance of modifications in host substrates and interfaces, which have profound implications on interphase stability, photocatalysis and solar-fuel-based devices.

Multiple Phases of Molybdenum Carbide as Electrocatalysts for the Hydrogen Evolution Reaction

AbstractMolybdenum carbide has been proposed as a possible alternative to platinum for catalyzing the hydrogen evolution reaction (HER). Previous studies were limited to only one phase, β‐Mo2C with an Fe2N structure. Here, four phases of Mo‐C were synthesized and investigated for their electrocatalytic activity and stability for HER in acidic solution. All four phases were synthesized from a unique amine–metal oxide composite material including γ‐MoC with a WC type structure which was stabilized for the first time as a phase pure nanomaterial. X‐ray photoelectron spectroscopy (XPS) and valence band studies were also used for the first time on γ‐MoC. γ‐MoC exhibits the second highest HER activity among all four phases of molybdenum carbide, and is exceedingly stable in acidic solution.

Multiple phases of molybdenum carbide as electrocatalysts for the hydrogen evolution reaction.

Molybdenum carbide has been proposed as a possible alternative to platinum for catalyzing the hydrogen evolution reaction (HER). Previous studies were limited to only one phase, β-Mo2C with an Fe2N structure. Here, four phases of Mo-C were synthesized and investigated for their electrocatalytic activity and stability for HER in acidic solution. All four phases were synthesized from a unique amine-metal oxide composite material including γ-MoC with a WC type structure which was stabilized for the first time as a phase pure nanomaterial. X-ray photoelectron spectroscopy (XPS) and valence band studies were also used for the first time on γ-MoC. γ-MoC exhibits the second highest HER activity among all four phases of molybdenum carbide, and is exceedingly stable in acidic solution.