Photoelectron Valence-band Spectra Research Articles

Surface and bulk electronic structures of heavily Mg-doped InN epilayer by hard X-ray photoelectron spectroscopy

To evaluate the polarity, energy band diagram, and oxygen (O) distribution of a heavily Mg-doped InN (InN:Mg+) epilayer with a Mg concentration of 1.0 ± 0.5 × 1020 cm−3, the core-level and valence band (VB) photoelectron spectra are investigated by angle-resolved soft and hard X-ray photoelectron spectroscopies. The InN:Mg+ epilayers are grown by radio-frequency plasma-assisted molecular beam epitaxy. In this doping level, the polarity inversion from In-polar to N-polar occurs with the increase in the Mg flow rate under the same growth conditions, and the VB spectrum clearly indicates the direction of polarity of InN:Mg+, which is N-polar. The energy band diagram is considered to exhibit a two-step downward bending structure due to the coexistence of the n+ surface electron accumulation layer and heavily Mg-doped p+ layer formed in the bulk. The O concentration rapidly increases until ∼4 nm with respect to the surface, which is deduced to be one of the reasons of the formation of the anomalous two-step energy band profile.

Electronic structure of octahedrally coordinated Cr in CrxTiX2 (X = Se, Te) and TixCr1−xSe2

Influence of the covalence, octahedral distortion and the translation symmetry of the chromium ions in octahedral coordination on the electronic structure for compounds under investigation was clarified. Cr L2,3 and Ti L2.3 X-ray absorption spectra (XAS) along with atomic multiplet calculations and resonant Ti 2p-3d and Cr 2p-3d photoelectron spectra of valence band (RXPS) in the intercalated titanium dichalcogenides CrxTiX2 (X=Se, Te) and in the substitution compound TixCr1−xSe2 show the charge transfer between chromium and titanium sublattices.

One-step preparation and photocatalytic performance of vanadium doped TiO2 coatings

In this paper, we have investigated one-step preparation of vanadium doped TiO2 coatings formed by plasma electrolytic oxidation (PEO) of titanium in electrolyte containing 10 g/L Na3PO4·12H2O + 0.5 g/L NH4VO3. The morphology, phase structure, and elemental composition of the formed coatings were characterized by atomic force microscopy (AFM), x-ray diffraction (XRD), and x-ray photoelectron spectroscopy (XPS) techniques. Ultraviolet–visible diffuse reflectance spectroscopy (UV–Vis DRS) was employed to evaluate the band gap energy of obtained coatings. Vanadium doped TiO2 coatings are partly crystallized and mainly composed of anatase phase TiO2, with up to about 2 wt% of vanadium present in the surface layer of the oxide. The valence band photoelectron spectra and UV–Vis DRS showed that vanadium doped TiO2 coatings exhibit notable red shift with respect to the pure TiO2 coatings. The photocatalytic activity was evaluated by monitoring the degradation of methyl orange under simulated sunlight conditions. Photocatalytic activity of vanadium doped TiO2 coatings increases with PEO time. Prolonged PEO times result in higher roughness of obtained coatings, thus increasing surface area available for methyl orange degradation. Vanadium doped TiO2 coatings obtained after 180 s of PEO time exhibit the best photocatalytic activity and about 67% of methyl orange is degraded after 12 h of irradiation under simulated sunlight.

First-principles study of electronic structures and stability of body-centered cubic Ti–Mo alloys by special quasirandom structures

The electronic structures and structural properties of body-centered cubic Ti–Mo alloys were studied by first-principles calculations. The special quasirandom structures (SQS) model was adopted to emulate the solid solution state of the alloys. The valence band electronic structures of Ti–Mo and Ti–Mo–Fe alloys were measured by hard x-ray photoelectron spectroscopy.The structural parameters and valence band photoelectron spectra were calculated using first-principles calculations. The results obtained with the SQS models showed better agreement with the experimental results than those obtained using the conventional ordered structure models. This indicates that the SQS model is effective for predicting the various properties of solid solution alloys by means of first-principles calculations.

Electronic Structure and Magnetic Properties of Iron Doped TiO<sub>2</sub> (Rutile): XPS Measurements and CPA Calculations

X-ray photoelectron spectra of TiO2:Fe are measured. Electronic structure and magnetic properties of rutile doped by iron are calculated in frames of the coherent potential approximation. The main experimental spectral features of TiO2:Fe such as heterovalent cation substitution (Fe3+→Ti4+), decreasing of the band gap value and appearance of additional features at the bottom and top of X-ray photoelectron spectra of valence band in comparison with those for undoped TiO2 are described.

Impact of Mg concentration on energy-band-depth profile of Mg-doped InN epilayers analyzed by hard X-ray photoelectron spectroscopy

The electronic structures of Mg-doped InN (Mg-InN) epilayers with the Mg concentration, [Mg], ranging from 1 × 1019 to 5 × 1019 cm−3 were systematically investigated by soft and hard X-ray photoelectron spectroscopies. The angle-resolved results on the core-level and valence band photoelectron spectra as a function of [Mg] revealed that the energy band of Mg-InN showed downward bending due to the n+ surface electron accumulation and p type layers formed in the bulk. With an increase in [Mg], the energy-band changed from monotonic to two-step n+p homojunction structures. The oxygen concentration rapidly increased at the middle-bulk region (∼4.5 to ∼7.5 nm) from the surface, which was one of the reasons of the transformation of two-step energy band.

Resonance photoelectron spectroscopy of TiX2 (X = S, Se, Te) titanium dichalcogenides

The photoelectron valence band spectra of TiS2, TiSe2, and TiTe2 dichalcogenides are investigated in the Ti 2p-3d resonance regime. Resonance bands in the vicinity of the Fermi energy are found for TiS2 and TiTe2. The nature of these bands is analyzed based on model calculations of the density of electronic states in TiS2, TiSe2, and TiTe2 compounds intercalated by titanium atoms. Analysis of experimental data and their comparison with model calculations showed that these bands have different origins. It is found that the resonance enhancement of an additional band observed in TiS2 is explained by self-intercalation by titanium during the synthesis of this compound. The resonance enhancement in TiTe2 is caused by occupation of the 3d band in Ti.

Electronic structure of single-crystal RMn2O5 multiferroics (R = Y, Bi, Eu, Gd) according to X-ray photoelectron spectroscopy data

Single crystals of orthorhombic multiferroics RMn2O5 (R = Y, Bi, Eu, Gd) and of hexagonal YMnO3 are grown from a solution-melt by mass crystallization. Using an ESCALAB 250 microprobe analysis system, the X-ray photoelectron spectra of valence bands of the multiferroics are obtained. The spectra are interpreted by calculations of the electronic structure of compounds that are close in crystalline structure to the multiferroics studied in this work, as well as by the multiplet structure of rare-earth 4f elements.

Growth and electronic structure of nitrogen-doped graphene on Ni(111)

We report on experimental and theoretical investigations of nitrogen-doped graphene. The incorporation of nitrogen was achieved during chemical-vapor deposition on Ni(111) using pyridine as a precursor. The obtained graphene layers were investigated using photoelectron spectroscopy. By studying C $1s$ and N $1s$ core levels, we show that the nitrogen content is influenced by the growth temperature and determine the atomic arrangement of the nitrogen atoms. Valence-band photoelectron spectra show that the incorporation of nitrogen leads to a broadening of the photoemission lines and a shift of the $\ensuremath{\pi}$ band. Density functional calculations for two possible geometric arrangements, the substitution of carbon atoms by nitrogen and vacancies in the graphene sheet with pyridinic nitrogen at the edges, reveal that the two arrangements have opposite effects on the band structure. For the present experimental approach, vacancies with pyridinic nitrogen are dominant. In the latter case the vacancies generated by the nitrogen doping, not the nitrogen itself, have the main effect on the band structure. By intercalating gold between the doped graphene layer and the Ni(111) substrate electronic decoupling is achieved. After intercalation the doping remains.

Photoemission and LEED study of the Sn/Rh(111) surface—early oxidation steps and thermal stability

We have deposited two monolayers of Sn onto Rh(111) single crystal. After the deposition, no ordered structure was revealed by low energy electron diffraction (LEED). We oxidized the obtained system in a low-pressure oxygen atmosphere at 420 K. The oxidized sample was then gradually heated to study the thermal stability of the oxide layer. We characterized the system by synchrotron radiation stimulated photoelectron spectroscopy and LEED. Valence band and core level photoelectron spectra of rhodium, tin and oxygen were used to study the oxidation of the Sn–Rh(111) surface and its behaviour upon annealing. A low stoichiometric oxide of Sn was created on the surface. The oxidation process did not continue towards creation of SnO2 with higher oxygen dose. The annealing at 970 K caused decomposition of the surface oxide of Sn and creation of an ordered (√3 × √3)R30° Sn–Rh(111) surface alloy.

Valence band offset at GaN/β-Si3N4 and β-Si3N4/Si(111) heterojunctions formed by plasma-assisted molecular beam epitaxy

Ultra thin films of pure β-Si3N4 (0001) were grown on Si (111) surface by exposing the surface to radio- frequency nitrogen plasma with a high content of nitrogen atoms. Using β-Si3N4 layer as a buffer layer, GaN epilayers were grown on Si (111) substrate by plasma-assisted molecular beam epitaxy. The valence band offset (VBO) of GaN/β-Si3N4/Si heterojunctions is determined by X-ray photoemission spectroscopy. The VBO at the β-Si3N4 / Si interface was determined by valence-band photoelectron spectra to be 1.84eV. The valence band of GaN is found to be 0.41±0.05eV below that of β-Si3N4 and a type-II heterojunction. The conduction band offset was deduced to be ~2.36eV, and a change of the interface dipole of 1.29eV was observed for GaN/β-Si3N4 interface formation.

Visible-Light-Active Iron Oxide-Modified Anatase Titanium(IV) Dioxide

Fe(acac)3 is chemisorbed on the surfaces of anatase TiO2 via partial ligand exchange between the acetylacetonate and surface Ti−OH groups [Fe(acac)2/TiO2]. The postheating at 773 K in air forms iron oxide species on the TiO2 surface in a highly dispersed state at a molecular level ((FeOx)m/TiO2). As a result of the iron oxide surface modification, the band gap of TiO2 decreases, while the absorption due to the d−d transition clearly observed for the usual impregnation samples is very weak. (FeOx)m/TiO2 gives rise to a noticeable visible light activity concomitantly with a significant increase in UV light activity, whereas Fe(acac)2/TiO2 hardly responds to visible light. Valence-band X-ray photoelectron spectra of (FeOx)m/TiO2 showed that the band gap narrowing results from the rise in the valence band top with surface modification. Also, photoluminescence spectroscopy indicated that the surface iron oxide species rapidly capture the excited electrons in the conduction band of TiO2 to suppress recombinatio...

Study of concentration-dependent cobalt ion doping of TiO2 and TiO2−xNx at the nanoscale

Experiments with a porous sol-gel generated TiO(2) nanocolloid and its corresponding oxynitride TiO(2-x)N(x) are carried out to evaluate those transformations which accompany additional doping with transition metals. In this study, doping with cobalt (Co(ii)) ions is evaluated using a combination of core level and VB-photoelectron and optical spectroscopy, complementing data obtained from Raman spectroscopy. Raman spectroscopy suggests that cobalt doping of porous sol-gel generated anatase TiO(2) and nitridated TiO(2-x)N(x) introduces a spinel-like structure into the TiO(2) and TiO(2-x)N(x) lattices. TEM and XPS data complemented by valence band-photoelectron spectra demonstrate that metallic cobalt clusters are not formed even at high doping levels. As evidenced by Raman spectroscopy, the creation of a spinel-like structure is commensurate with the room temperature conversion of the oxide and its oxynitride from the anatase to the rutile form. The onset of this kinetically driven process correlates with the formation of spinel sites within the TiO(2) and TiO(2-x)N(x) particles. Despite their visible light absorption, the photocatalytic activity of these cobalt seeded systems is diminished relative to the oxynitride TiO(2-x)N(x).

The interface structure and band alignment at alumina/Cu(Al) alloy interfaces—Influence of the crystallinity of alumina films

Both epitaxial and amorphous ultra-thin alumina films were grown on a Cu-9 at.%Al(1 1 1) substrate by selective oxidation of Al in the alloy in ultra high vacuum. The crystallinity of the alumina films was controlled by oxidation temperature. The photoelectron spectra of Al 2p, O 1s and valence band were measured in-situ during oxidation. The influence of the crystallinity on the interface structure between the alumina films and the substrate was discussed by analyzing the Al 2p spectra composed of multiple peaks. The energy difference between the Fermi level of the substrate and the valence band maximum of the alumina films (band offset) was derived from the valence band spectra. The energy band alignment at the interface between each of the two alumina films and the substrate was revealed by combining the binding energy values of the core levels with the band offset values. The influence of the alumina crystallinity on the band alignment was discussed.

Valence band offset and interface stoichiometry at epitaxial Si3N4/Si(111) heterojunctions formed by plasma nitridation

Ultrathin β-Si3N4(0001) epitaxial films formed by N2-plasma nitridation of Si(111) substrates have been studied by photoelectron spectroscopy using synchrotron radiation. The valence band offset at the β-Si3N4/Si interface was determined by valence-band photoelectron spectra to be 1.8 eV. Furthermore, the Si 2p core-level emissions were analyzed for nitride (Si4+) and subnitride (Si3+ and Si+) components to characterize the interface stoichiometry. In contrast to the interfaces formed by ammonia thermal nitridation and N2-plasma nitridation at room temperature, the interface formed by N2-plasma nitridation at high substrate temperature is very close to subnitride free with an abrupt composition transition.

Cluster study of Al–Co–Ni decagonal quasicrystal

Abstract The electronic structure and atomic arrangement of a decagonal Al–Co–Ni quasicrystal have been investigated in terms of a model unit cluster by using a discrete variational X α (DV-Xα) cluster calculation. The partial electronic density of states (DOS) and overlap population analysis for local clusters around the constituent transition-metals (TM´s) show the energy reduction of both the Co and Ni 3d states but the increase in the anti-bonding character for their TM-Al bonds at the second inner ring TM site of the model cluster in comparison to the most inner ring TM site. Comparing the calculated DOS with the observed valence-band photoelectron and TM Lα X-ray emission spectra, we discuss the occupation at the transition metal sites by Co and Ni.

Interface termination and band alignment of epitaxially grown alumina films on Cu–Al alloy

Epitaxial ultrathin alumina films were grown on a Cu−9 at. % Al(111) substrate by selective oxidation of Al in the alloy in ultrahigh vacuum. The photoelectron spectra of Al 2p and valence band were measured in situ during oxidation. By analyzing multiple peaks of Al 2p, the interface atomic structure was discussed. The energy difference between the Fermi level of the substrate and the valence band maximum of alumina (band offset) was obtained. The relation between the interface atomic structure and the band offset was compared with the reported first-principles calculations. A novel method for controlling the band offset was proposed.

Mn0.06Ge0.94diluted magnetic semiconductor epitaxially grown on Ge(001): Influence ofMn5Ge3nanoscopic clusters on the electronic and magnetic properties

The structural, electronic, and magnetic properties of the ${\mathrm{Mn}}_{0.06}{\mathrm{Ge}}_{0.94}$ diluted magnetic semiconductor, grown at $520\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ by molecular-beam epitaxy on $\mathrm{Ge}(001)2\ifmmode\times\else\texttimes\fi{}1$, have been investigated. Diluted and highly ordered alloys, containing ${\mathrm{Mn}}_{5}{\mathrm{Ge}}_{3}$ nanocrystals, were grown. The valence band photoelectron spectrum of ${\mathrm{Mn}}_{0.06}{\mathrm{Ge}}_{0.94}$ shows a feature located at $\ensuremath{-}4.2\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ below the Fermi level, which is the fingerprint of substitutional Mn atoms in the Ge matrix. Magnetization measurements show the presence of a paramagnetic component due to substitutional Mn atoms and of a ferromagneticlike component due to ${\mathrm{Mn}}_{5}{\mathrm{Ge}}_{3}$ nanocrystallites. The Mn ${L}_{2,3}$ x-ray absorption spectrum of this polyphase film shows no marked multiplet structure, but a bandlike character.

Photoelectron Spectroscopy at the Solid–Liquid Interface of Dye–Sensitized Solar Cells: Unique Experiments with the Solid–Liquid Interface Analysis System SoLiAS at BESSY

At the synchrotron BESSY we run the experimental station SoLiAS, dedicated to solid–liquid interface analysis with soft X-ray induced photoelectron spectroscopy (SXPS). SoLiAS allows wet chemically prepared surfaces to be transferred to the ultra high vacuum without contact with ambient air. In addition in situ (co)adsorption of volatile solvent species onto liquid nitrogen cooled samples is possible. SoLiAS proves to be very useful in analyzing the chemical and electronic structure at the solid–liquid interface of dye–sensitized solar cells. The standard dye RuII(2,2?-bipyridil-4,4?-dicarboxylate)2(NCS)2 was adsorbed from ethanol solution under clean N2 atmosphere in an UHV-integrated electrochemical cell (EC). The standard solvent acetonitrile was adsorbed in situ from the gas phase. For comparison also the nonpolar solvent benzene was adsorbed. Ex situ sintered nanocrystalline anatase substrates as well as in situ deposited polycrystalline TiO2 samples were used, which show a similar distribution of two types of occupied surface states. Distinct reversible changes occur in synchrotron-induced photoelectron valence band and core level spectra when the solvent acetonitrile is adsorbed to pristine and dye-covered TiO2 substrates. TiO2 surface states are quenched and the line width of the dye S2p emission decreases strongly. Based on the experimental results the alignment of the photovoltaic relevant electronic states and a model on the dye–solvent interaction can be deduced that points to the promotion of vectorial charge transfer by increased dye orientation due to solvation.

Electronic structure of Pu carbides: Photoelectron spectroscopy

Valence-band photoelectron spectra of Pu 2C 3 and PuC 0.85 reveal the same type of 5f features below the Fermi level as observed for majority of other Pu systems. They appear at the same energies (0–0.1, 0.5, and 0.85 eV) and their intensity is higher for Pu 2C 3 than for PuC 0.85. They can be attributed to the 5f 5 final-state multiplet originating in the 5f 6 component of the ground state. The higher 5f occupancy (5.48) calculated in Pu 2C 3, explains its non-magnetic character.