Ultraviolet Photoemission Spectroscopy Spectra Research Articles

Interfacial electronic state between hexagonal ZnO and cubic NiO.

The interface of two dissimilar materials gives rise to a myriad of interesting structural, magnetic, and electronic properties that may be utilized to produce novel materials with unique characteristics and functions. In particular, growing a cubic oxide film on top of a hexagonal oxide substrate results in such unique properties due to the conflict of their respective stabilization mechanisms within the interface layer. This study aims to elucidate the electronic properties of the interface between hexagonal ZnO and cubic NiO by analyzing the interface electronic states within epitaxial NiO films grown on ZnO substrates, expressed in the form of ultraviolet photoemission spectroscopy (UPS) for valence band structure and X-ray absorption spectroscopy (XAS) spectra for conduction band structure. This is accomplished through a modeling approach in which the film, substrate, and interface signals are assumed to be related to each other by a set of mathematical equations, and then rearranging and modulating the equations to obtain unique UPS and XAS spectra that depict the interface electronic states.

Surfactant-Encapsulated Polyoxometalate Complex as a Cathode Interlayer for Nonfullerene Polymer Solar Cells

Surfactant-Encapsulated Polyoxometalate Complex as a Cathode Interlayer for Nonfullerene Polymer Solar Cells

Many-body photoemission theory for organic molecular crystals

A many-body photoemission theory based on Keldysh nonequilibrium Green's functions is developed and applied to photoemission from extended states of organic molecules and their crystals, in particular excited by ultraviolet and soft X-ray radiation. In this energy region, electron-phonon interaction and radiation filed screening play some important roles. To incorporate the latter effects, we should use quantum electrodynamics (QED) approach. To discuss the electron-phonon interaction, we apply the Hedin equation including the screened Coulomb interaction mediated by the electron-phonon interaction. This theoretical framework gives us a unified picture of the UPS spectra from organic solids. Furthermore some illustrative numerical calculations are shown to discuss the relative importance of the multiple scatterings, Debye-Waller factors, the peak width of the quasi-particles, and the angular dependence of the phonon satellites. We demonstrate some specific features of ultraviolet photoemission spectroscopy (UPS) spectra from particularly anisotropic organic solids.

Electron microscopy and ultraviolet photoemission spectroscopy studies of native oxides on GaN(0001)

We investigated native oxides on GaN(0001) using high-resolution scanning transmission electron microscopy (STEM) and ultraviolet photoemission spectroscopy (UPS). As a result, it was confirmed that the native oxides on both n-type and p-type GaN(0001) with a thickness of ∼1 nm were not amorphous but crystalline with lattice-matched structures to GaN, as we reported previously [Y. Irokawa et al., Jpn J. Appl. Phys. 56, 128004 (2017)]. Moreover, the UPS spectra of native oxides on n-type GaN(0001), which were similar to the reported clean GaN(0001) spectra, showed the existence of surface states at the valence band maximum (VBM), reflecting the defective quality of the native oxides.

Trigonal (1T) and hexagonal (2H) mixed phases MoS2 thin films

Molybdenum di-sulfide thin films of thickness <1500 Å, were prepared by the dip-coating technique at different baking temperatures within the range 400 °C–450 °C using methanolic solution of ammonium molybdate and ammonium thiocyanate and hence studied their different properties using X-ray diffraction (XRD), UV-vis spectroscopy, Photoluminescence (PL), X-ray photoelectron spectroscopy (XPS), ultraviolet photoemission spectroscopy (UPS) and Current-Voltage (I-V) relationship measurements. XRD result shows that the MoS2 films are mixed metallic octahedral prismatic 1T- and trigonal prismatic bulk crystal 2H-phases. The I-V relationships show higher electrical conductivity for the films prepared at 400 °C and 450 °C than the films prepared at 425 °C. XPS measurement also shows the formation of mixed phases of MoS2 thin films as revealed by XRD. We have estimated the valence band maximum (VBM) and work functions from higher and lower energy range of UPS (He-I) spectra of MoS2 thin films prepared at 425 °C and 450 °C. The VBM are ∼4.4 eV & ∼4.2 eV and their work functions are ∼3.5 eV & ∼3.3 eV respectively that are correlated with conductivity and formation of mixed phases of MoS2 thin films.

Anode engineering of highly efficient polymer solar cells using treated ITO

ITO substrates were treated with organic solvent cleaning(OSC), SC1 treatment[V(NH4OH):V(H2O2): V(H2O)=1:1:5], O2 plasma and UV ozone, respectively. Combined investigations of atom force microscopy(AFM), water contact angle measurements, ultraviolet photoemission spectroscopy(UPS) and X-ray photoemission spectroscopy(XPS) demonstrated that UV ozone treatment could give rise to the smoothest surface, the most hydrophilic property and the highest work function(WF) of ITO due to the removal of hydrophobic C―O impurity from the ITO surface and the enrichments of more oxygen on the ITO surface. When PEDOT:PSS film[(poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate)] was deposited on the ITO substrates treated with UV ozone, it showed a lower root-mean- square roughness in AFM images, a higher transmission in UV-Vis transmission spectra and a higher WF in UPS spectra than the PEDOT:PSS films deposited on the ITO substrates treated by other three methods. As a result, the power conversion efficiency of polymer solar cells(PSCs) based on PTB7:PC71BM as an active layer and ITO treated by UV ozone as an anode can reach 8.48% because of the simultaneously improved short circuit current, open circuit voltage and fill factor compared to the PSCs with ITO treated with other three methods.

Quasiparticle interfacial level alignment of highly hybridized frontier levels: H2O on TiO2(110).

Knowledge of the frontier levels' alignment prior to photoirradiation is necessary to achieve a complete quantitative description of H2O photocatalysis on TiO2(110). Although H2O on rutile TiO2(110) has been thoroughly studied both experimentally and theoretically, a quantitative value for the energy of the highest H2O occupied levels is still lacking. For experiment, this is due to the H2O levels being obscured by hybridization with TiO2(110) levels in the difference spectra obtained via ultraviolet photoemission spectroscopy (UPS). For theory, this is due to inherent difficulties in properly describing many-body effects at the H2O-TiO2(110) interface. Using the projected density of states (DOS) from state-of-the-art quasiparticle (QP) G0W0, we disentangle the adsorbate and surface contributions to the complex UPS spectra of H2O on TiO2(110). We perform this separation as a function of H2O coverage and dissociation on stoichiometric and reduced surfaces. Due to hybridization with the TiO2(110) surface, the H2O 3a1 and 1b1 levels are broadened into several peaks between 5 and 1 eV below the TiO2(110) valence band maximum (VBM). These peaks have both intermolecular and interfacial bonding and antibonding character. We find the highest occupied levels of H2O adsorbed intact and dissociated on stoichiometric TiO2(110) are 1.1 and 0.9 eV below the VBM. We also find a similar energy of 1.1 eV for the highest occupied levels of H2O when adsorbed dissociatively on a bridging O vacancy of the reduced surface. In both cases, these energies are significantly higher (by 0.6 to 2.6 eV) than those estimated from UPS difference spectra, which are inconclusive in this energy region. Finally, we apply self-consistent QPGW (scQPGW1) to obtain the ionization potential of the H2O-TiO2(110) interface.

Valence electronic structure of sublimated Fe4single-molecule magnets: an experimental and theoretical characterization

The valence electronic structures of two single-molecule magnets (SMMs), [Fe4(L)2(dpm)6] and [Fe4(L)2(pta)6], (Hdpm = dipivaloylmethane, Hpta = pivaloyltrifluoroacetone, L3− = Ph–C(CH2O)33−), are investigated by means of ultraviolet photoemission spectroscopy (UPS) and ab initio calculations. The experimental UPS spectra of both compounds are analysed and compared with the total density of states (TDOS) computed with the hybrid functional PBE0. The substitution of half of the methyl groups in [Fe4(L)2(dpm)6] with fluorine atoms in [Fe4(L)2(pta)6] unexpectedly affects the spectrum shape in the Fermi region, thus becoming a useful fingerprint of the two SMMs. Moreover, a computational protocol at DFT + U level of theory is assessed on both compounds, which is in good agreement with the experimental spectroscopic and magnetic data. The basis for the future modelling of the adsorption of Fe4 clusters on surfaces is established.

Optimization of a High Work Function Solution Processed Vanadium Oxide Hole-Extracting Layer for Small Molecule and Polymer Organic Photovoltaic Cells

We report a method of fabricating a high work function, solution processable vanadium oxide (V2Ox(sol)) hole-extracting layer. The atmospheric processing conditions of film preparation have a critical influence on the electronic structure and stoichiometry of the V2Ox(sol), with a direct impact on organic photovoltaic (OPV) cell performance. Combined Kelvin probe (KP) and ultraviolet photoemission spectroscopy (UPS) measurements reveal a high work function, n-type character for the thin films, analogous to previously reported thermally evaporated transition metal oxides. Additional states within the band gap of V2Ox(sol) are observed in the UPS spectra and are demonstrated using X-ray photoelectron spectroscopy (XPS) to be due to the substoichiometric nature of V2Ox(sol). The optimized V2Ox(sol) layer performance is compared directly to bare indium–tin oxide (ITO), poly(ethyleneoxythiophene):poly(styrenesulfonate) (PEDOT:PSS), and thermally evaporated molybdenum oxide (MoOx) interfaces in both small molec...

Ni2MnGa(100) ferromagnetic shape memory alloy: A surface study

Ni2MnGa(100) single crystal studied using low energy electron diffraction (LEED) and ultraviolet photoemission spectroscopy (UPS) exhibits interesting modification of the surface properties that are mainly influenced by surface composition as well as intrinsic effects. In the martensite phase, the LEED spot profiles show presence of an incommensurate modulation for the stoichiometric surface. In contrast, a commensurate modulation is observed for Mn-excess Ni–Mn–Ga surface. A pre-martensite phase is identified at the surface. Both the surface martensitic and pre-martensitic transition temperatures decrease as the Mn content increases. The UPS spectra in the austenite phase exhibit systematic change in shape as a function of surface composition that can be related to changes in the hybridization between Ni and Mn 3d states. The spectra in the martensite phase exhibit interesting modifications near the Fermi level, which has been compared to density of states calculated for a modulated structure by ab-initio density functional theory. Intrinsic surface properties dissimilar from the bulk are enhanced hysteresis width of the martensite transition and increased pre-martensitic transition temperature.

Determination of Electronic Structure of Oxide–Oxide Interfaces by Photoemission Spectroscopy

A method has been developed to use the finite escape depth of the photoelectrons emitted in ultraviolet photoemission spectroscopy (UPS) to determine the electronic density-of-states at the interface between two dissimilar metal oxides. Ultrathin films of one oxide are grown heteroepitaxially, one monolayer at a time, on a single-crystal substrate of the other oxide, and UPS spectra are taken after each complete monolayer. By comparing experimental UPS spectra with calculated spectra based on specific models of the interfacial structure, the interfacial density-of-states can be extracted. The two oxide systems studied here are NiO-Fe(3)O(4) and CoO-Fe(3)O(4). The former system is found to have an atomically abrupt interface, with no significant density of interface states. For CoO, however, an interfacial electronic spectrum, different from that of either the substrate or the overlayer, is found. The spatial extent and possible origin of those interfacial states is discussed.

Valence band structure of the icosahedral Ag-In-Yb quasicrystal

The valence band structure of the icosahedral $(i)$ Ag-In-Yb quasicrystal, which is isostructural to the binary $i\text{-Cd-Yb}$ system, is investigated by ultraviolet photoemission spectroscopy (UPS). Experimental results are compared with electronic-structure calculations of a cubic approximant of the same phase. UPS spectra from the fivefold, threefold, and twofold $i\text{-Ag-In-Yb}$ surfaces reveal that the valence band near to the Fermi level is dominated by $\text{Yb}\text{ }4f$-derived states, in agreement with calculations. The spectra also exhibit peaks which are surface core level shifted, caused by changes in the electronic structure in surface layers. Calculations yield a pseudogap in the density of states due to a hybridization of the $\text{Yb}\text{ }5d$ band with the $\text{Ag}\text{ }5p$ and $\text{In}\text{ }5p$ bands. Both experimental and calculated band features are very similar to those of Cd-Yb. The modification of the band structure after surface treatment by sputtering and by oxidation is also studied. Additionally, the work function of $i\text{-Ag-In-Yb}$ measured from the width of UPS spectrum is found to be almost unaffected by surface orientation, but increases after sputtering or oxidation.

Surface Structural Study on Ionic Liquids Using Metastable Atom Electron Spectroscopy

The surface structures of ionic liquids 1-CnH2n+1-3-methylimidazolium tetrafluoroborate (BF4−), trifluoromethanesulfonate (OTf) and bis(trifluoromethylsulfonyl)amide (TFSA) with n = 4, 8, and 10 were investigated by ultraviolet photoemission spectroscopy (UPS) and metastable atom electron spectroscopy (MAES). The MAES spectra reveal that the anions and cations share the surface of the room-temperature ionic liquids (RTILs) with a shorter alkyl chain at n = 4 although the alkyl chain layer tends to cover the outermost surface in the case of RTILs with longer alkyl chains of n = 8 and 10. Comparison of UPS and MAES spectra demonstrates that the nonpolar groups such as the alkyl chain and the CF3 group point toward the vacuum side, while the polar groups such as the SO3 and SO2 groups face toward the bulk side. A double-layered structure at the surface, which consists of an alkyl chain layer and a polar layer containing anions and imidazolium rings, is proposed from these observations.

A study of the interaction between perylene and the TiO 2(1 1 0)-(1 × 1) surface-based on XPS, UPS and NEXAFS measurements

The interaction between a semi-large aromatic hydrocarbon compound (perylene) and the TiO 2(1 1 0)-(1 × 1) surface under ultra high vacuum conditions has been probed by X-ray photoemission spectroscopy (XPS), ultraviolet photoemission spectroscopy (UPS) and near-edge X-ray absorption fine structure (NEXAFS) methods. UPS measurements of the adsorbate system have been compared with an experimental UPS spectrum of perylene in the gas phase and a calculated spectrum obtained by means of density functional theory (DFT) methods. NEXAFS results of perylene molecules adsorbed on TiO 2(1 1 0)-(1 × 1) were compared with data from an α-phase perylene single crystal. A novel analysis of the valence data has been employed to show that no strong chemical interaction takes place between perylene and the TiO 2(1 1 0)-(1 × 1) surface. Furthermore, angle-dependent NEXAFS measurements and the growth curve results suggest that the perylene molecules are oriented flat down onto the TiO 2 substrate due to weak van der Waals interactions.

Electronic structure ofCa3Co4O9studied by photoemission spectroscopy: Phase separation and charge localization

We report on an electronic structure study of a quasi-two-dimensional Co oxide ${\text{Ca}}_{3}{\text{Co}}_{4}{\text{O}}_{9}$ with ${\text{Ca}}_{2}{\text{CoO}}_{3}$ rocksalt layers and ${\text{CoO}}_{2}$ triangular lattice layers by means of x-ray photoemission spectroscopy (XPS), ultraviolet photoemission spectroscopy (UPS), configuration-interaction calculation on a ${\text{CoO}}_{6}$ cluster model, and unrestricted Hartree-Fock calculation on a multiband $d\text{\ensuremath{-}}p$ model. The $\text{Co}\text{ }2p$ XPS spectrum shows that the Co valence of the rocksalt layer is similar to that of the triangular lattice layer. The cluster-model analysis of the $\text{Co}\text{ }2p$ XPS spectrum indicates that the Co sites of the rocksalt and triangular lattice layers commonly have charge-transfer energy $\ensuremath{\Delta}$ of $\ensuremath{\sim}1.0\text{ }\text{eV}$, $d\text{\ensuremath{-}}d$ Coulomb interaction $U$ of $\ensuremath{\sim}6.5\text{ }\text{eV}$, and transfer integral $(pd\ensuremath{\sigma})$ of $\ensuremath{\sim}\ensuremath{-}2.3\text{ }\text{eV}$. The $\text{Co}\text{ }3d$ ${t}_{2g}$ peak in the valence-band XPS spectrum remains sharp even above the spin-state transition temperature at $\ensuremath{\sim}380\text{ }\text{K}$, indicating that the spin-state transition is different from the low-spin to intermediate-spin or high-spin transitions in perovskite ${\text{LaCoO}}_{3}$. The line shape of the UPS spectrum near the Fermi level can be reproduced by the combination of unrestricted Hartree-Fock results for the charge-ordered insulating (COI) and paramagnetic metallic (PM) states. The analysis shows that the phase separation between the COI and PM states plays important roles in ${\text{Ca}}_{3}{\text{Co}}_{4}{\text{O}}_{9}$.

Photoemission electronic states and magnetic properties of Dy(Co 1− xFe x) 2

The valence band electronic states of Dy(Co 1− x Fe x ) 2 Laves phases with x = 0, 0.05, 0.1, 0.7 and 1.0 were measured by the ultraviolet photoemission spectroscopy (UPS) in room temperature. The Curie temperatures were determined from the electrical resistivity measurements and from the temperature dependence of the hyperfine fields by 57Fe Mössbauer spectroscopy. The UPS spectra showed the very pronounced composition dependent emission at the Fermi energy E F, the broad band within 0 and −4 eV binding energy interval below E F and two strong features at about −6.5 and −9.0 eV. The spectra were compared with the band structure calculations using the full-potential linearized augmented plane waves (FLAPW) method. From the comparison we concluded that the intensities at E F are correlated with the values of the Curie temperatures, the broad band comes entirely from different 3d Co and/or Fe states and the strong two features come entirely from 4f electrons of dysprosium.

Morphology and electronic properties of the pentacene on cobalt interface

In this paper, we report the structural and electronic properties of pentacene thin films grown on a polycrystalline Co film using atomic force microscopy and ultraviolet photoemission spectroscopy (UPS), respectively. Investigation of this type of interface is of importance for the engineering of hybrid organometallic spintronic devices for which the use of spin polarized electrodes is a prerequisite. Uniform single crystalline areas of pentacene as large as several micrometers, with molecules arranging almost perpendicular to the substrate, were obtained. For the electronic properties at this interface, we have found an energy barrier for the hole injection of about 1eV, in spite of the fact that the ionization potential of pentacene reported previously equals the work function of Co. A shift of the vacuum level of the same magnitude has also been observed. A comparison of the UPS spectra of the pentacene films with the gas phase spectrum directly indicates that hybridization effects are present at this interface.

Direct observation of surface potential change due to hydrogen termination of CVD diamond surface by metastable-induced electron spectroscopy

Metastable-induced electron spectroscopy (MIES) together with ultraviolet photoemission spectroscopy (UPS) was applied to the analysis of the surface electronic structure of chemical-vapor-deposited diamond films. The films were grown in a microwave plasma, and their surfaces were terminated by hydrogen. The MIES spectrum measured at an as-deposited surface contains peaks due to adsorbates. When this surface was annealed at 400 °C, those peaks were depressed, and the spectrum showed a similar structure to that of UPS. Once the surface was re-hydrogenated, the MIES spectrum rose up at lower energies than the UPS spectrum did for surfaces annealed at lower temperatures. Finally after annealing at 1000 °C, the cutoff energies of MIES and UPS converged at nearly the same values. The result demonstrates that the MIES detects a surface potential which changes locally at the hydrogen-terminated surfaces.

Doping Effect of Tetrathianaphthacene Molecule in Organic Semiconductors on Their Interfacial Electronic Structures Studied by UV Photoemission Spectroscopy

The n-type doping ability of the electron-donating organic molecule tetrathianaphthacene (TTN) was examined by ultraviolet photoemission spectroscopy (UPS) for two kinds of matrix organic semiconductors, hexadecafluorophthalocyaninatozinc (F16ZnPc) and tris(8-hydroxyquinoline)aluminum (Alq3). The observed dependence of the UPS spectra suggests that TTN acts as a good donor in F16ZnPc, while the effect was not so significant for Alq3. These results and the consideration of the energy parameters of these molecules indicate that a close match between the highest occupied molecular orbital of the dopant and the lowest unoccupied molecular orbital of the matrix is important for efficient n-type doping.

N-Type doping effect on the electronic structure of organic semiconductor: doping of tetrathianaphthacene (TTN) into hexadecafluorophthalocyaninatozinc (F 16ZnPc) by co-evaporation

The effect of n-type doping of hexadecafluorophthalocyaninatozinc (F 16ZnPc) with an electron-donating organic molecule tetrathianaphthacene (TTN) was investigated by ultraviolet photoemission spectroscopy (UPS). We measured the UPS spectra over a wide range of the film thickness for pristine F 16ZnPc, pristine TTN, and TTN-doped F 16ZnPc. From the change of the energy of the vacuum level (VL), we could deduce the energy level alignment at organic/metal interfaces. The results for pristine F 16ZnPc showed (1) abrupt shift of VL at small thickness corresponding to dipole layer formation, and (2) more gradual shift of VL, which might correspond to the band bending to align the Fermi level by residual impurity acting as unintentional dopant. For F 16ZnPc doped with TTN, the region (2) is merged into the region (1), indicating that the thickness of the band-bending region of the doped F 16ZnPc was much reduced from that of pristine F 16ZnPc. This demonstrates that F 16ZnPc was effectively doped with carriers from the TTN molecules. It was also found that the energy of the highest occupied molecular orbital of F 16ZnPc was shifted to higher binding energy by doping TTN molecules. These results suggest that TTN acts as a good donor in F 16ZnPc.