Photoemission Line Shape Research Articles

Electronic properties of MnO(110) film grown on Cu (111)

In this study, we investigated MnO thin films grown on Cu(111) using a soft x-ray and UV based spectroscopy, low energy electron diffraction (LEED), scanning tunneling (STM) and scanning transmission electron microscopy (STEM). MnO thin film assumes (110) preferential plane orientation organized in different spatial domains to better adapt to Cu(111) surface and an in-plane/out-of-plane lattice parameter variation up to 11% with respect to bulk counterpart originating a sizeable XLD signal at Mn L2,3 edges. The formation of oxidized Cu region during the MnO thin film growth is not avoidable due to the copper reactivity to oxygen and at the interface the coexistence of MnO islands alternated with Cu2O regions was observed by STM. Mn 2p and Mn 3s core level photoemission line shapes present a slight change in multiplet peak relative intensity with respect bulk MnO counterpart due to the role of Cu substrate in the core level relaxation process. The photon energy dependent photoemission spectra of valence band discriminate the MnO states and indicate a weak influence of the substrate once the film thickness is reduced to 1 nm. These findings open new route in the tuning oxide thin film properties.

Core-level binding energy shifts between interior, terrace and edge atoms in MnO(001) thin films

Understanding the physical origin of core-level photoemission line shapes can offer valuable information about the chemical and physical properties of surfaces. For instance, in a large number of transition metals oxides, changes in the line shape allow the accurate determination of their oxidation states and cation site symmetry. Yet, despite this importance, experimental investigations on core-level shifts have been much neglected in recent years. In order to provide further evidence of the physical relevance, we have, in this contribution, introduced a new aspect of interior-, terrace- and edge- atom core-level binding energy shifts to describe monolayers of MnO(001) films grown on an Au(111) substrate. By this means we were able to distinguish the line shape contributions related to different types of atomic sites. We show that their relative intensities and energy shifts are able to provide information about the relative amount of under coordinated atoms on the surface and, thus give insights into their catalytic properties. Our findings reveal the importance of a detailed surface science characterization to provide the correct interpretation of distinct photoemission line shapes when considering thin film samples as well as nanostructures in general.

Core level photoemission line shape selection: Atomic adsorbates on iron

Robust fitting of core level photoemission spectra is often central to reliable interpretation of X‐ray photoelectron spectroscopy (XPS) data. One key element is employment of the correct line shape function for each spectral component. In this study, we consider this topic, focusing on XPS data from atomic adsorbates, namely, O and S, on Fe(110). The potential of employing density functional theory (DFT) for generating adsorbate projected electronic density of states (PDOS) to support line shape selection is explored. O 1s core level XPS spectra, acquired from various ordered overlayers of chemisorbed O, all display an equivalent asymmetric line shape. Previous work suggests that this asymmetry is a result of finite O PDOS in the vicinity of the Fermi level, allowing O 1s photoexcitation to induce a weighted continuum of final states through electron‐hole pair excitation. This origin is corroborated by O DFT‐PDOS generated for an optimised five‐layer Fe(110)(2 × 2)‐O slab. Adsorbate DFT‐PDOS were also computed for Fe(110) ‐S. As, similar to adsorbed O, there is a significant continuous distribution of states about the Fermi level, it is proposed that the S 2p XPS core levels should also have asymmetric profiles. S 2p XPS data acquired from Fe(110) ‐S, and their subsequent fitting, verify this prediction, suggesting that DFT‐PDOS could aid line shape selection.

Photoelectron Energy Loss in Al(002) Revisited: Retrieval of the Single Plasmon Loss Energy Distribution by a Fourier Transform Method

A Fourier transform (FT) algorithm is proposed to retrieve the energy loss function (ELF) of solid surfaces from experimental X-ray photoelectron spectra. The intensity measured over a broad energy range towards lower kinetic energies results from convolution of four spectral distributions: photoemission line shape, multiple plasmon loss probability, X-ray source line structure and Gaussian broadening of the photoelectron analyzer. The FT of the measured XPS spectrum, including the zero-loss peak and all inelastic scattering mechanisms, being a mathematical function of the respective FT of X-ray source, photoemission line shape, multiple plasmon loss function, and Gaussian broadening of the photoelectron analyzer, the proposed algorithm gives straightforward access to the bulk ELF and effective dielectric function of the solid, assuming identical ELF for intrinsic and extrinsic plasmon excitations. This method is applied to aluminum single crystal Al(002) where the photoemission line shape has been computed accurately beyond the Doniach–Sunjic approximation using the Mahan–Wertheim–Citrin approach which takes into account the density of states near the Fermi level; the only adjustable parameters are the singularity index and the broadening energy Г (inverse hole lifetime). After correction for surface plasmon excitations, the q-averaged bulk loss function, q , of Al(002) differs from the optical value Im[− 1 / e(E, q = 0)] and is well described by the Lindhard–Mermin dispersion relation. A quality criterion of the inversion algorithm is given by the capability of observing weak interband transitions close to the zero-loss peak, namely at 0.65 and 1.65 eV in e(E, q) as found in optical spectra and ab initio calculations of aluminum.

Crystal to Quasicrystal Surface Phase Transition: An Unlocking Mechanism for Templated Growth

In this experimental work we have studied and characterized a surface phase transition on a decagonal Al72Ni11Co17 sample between a crystalline phase, which is induced by sputtering with energetic noble gas ions, and the bulklike decagonal phase. The surface transition to the stable, quasicrystalline phase was followed as a function of temperature by two complementary techniques: high-resolution X-ray photoelectron spectroscopy (HR-XPS) and low-energy electron diffraction (LEED). The observation of shifts in the core–electron binding energies of the constituent atoms and of other photoemission line shape parameters as the surface undergoes the phase transition and the comparison with the structural information obtained from the LEED data enabled us to determine the evolution of the surface from 5 twinned cubic crystalline domains to the decagonal phase. A quasicrystalline chemical environment with some degree of long-range order is observed for Al before the quasicrystalline surface is fully recovered, po...

Electron-phonon correlations on spin texture of gapped helical Dirac fermions

The metallic surface states of a topological insulator support helical Dirac fermions protected by topology with their spin locked perpendicular to their momentum. They can acquire mass through magnetic doping or through hybridization of states on opposite faces of a thin sample. In this case there can be a component of electron spin oriented perpendicular to the surface plane. The electron-phonon interaction renormalizes the dynamics of the charge carriers through their spectral density. It also modifies the gap channel and a second spectral function enters which, not only determines the out of plane spin component, but also comes into in-plane properties. While the out of plane spin component is decreased below the Fermi momentum ($k_F$), the in plane component increases. There are also correlation tails extending well beyond $k_F$. The angular resolved photo-emission line shapes aquire Holstein side bands. The effective gap in the density of states is reduced and the optical conductivity aquires distinct measurable phonon structure even for modest value of the electron-phonon coupling.

General analysis of the angle-resolved photoemission line shape for strongly correlated electron systems

In many cases the standard perturbation approach appears to be too simple to describe precisely the angle-resolved photoemission spectrum of a strongly correlated electron system. In particular, to describe the momentum asymmetry observed in the photoemission spectra of high-${T}_{c}$ cuprates, a phenomenological approach based on an extremely correlated Fermi-liquid model has been recently introduced. Here we analyze the general structure of the Green's function of quasiparticles in strongly correlated electron systems and stress that it is defined not only by the self-energy of Hubbard quasiparticles but also by a strength operator. The latter leads to an additional odd momentum contribution to the spectral function and alone can explain the observed asymmetry. So, the asymmetry of the angle-resolved photoemission spectra can be a measure of the strength of electron correlations in materials.

Normal-state correlated electronic structure of tetragonal FeSe superconductor

Tetragonal FeSe, a prototype iron-chalcogenide superconductor, shows signatures of a strange incoherent normal state. Motivated thereby, we use LDA+DMFT to show how multi-band correlations generate a low-energy pseudogap in the normal state, giving an incoherent metal in good semi-quantitative agreement with observations. Anomalous responses in the normal state, including orbital-dependent effective mass enhancement and photoemission lineshape, are consistently understood.

Structural and electronic characterization of Co nanostructures on Au(332)

Co nanoislands were grown on (332) vicinal surface of Au in UHV using the e-beam evaporation technique. Scanning tunneling microscopy results reveal that Co deposition occurs following an islanding mode for θCo ranging from 0.17 to 0.64 ML. At low coverage nanoislands show a monolayer height, while at higher Co loadings, islands have a maximum bilayer height. XPS measurements rule out the possibility of alloy formation provided that binding energy of Co2p core lines remains unchanged as cobalt loading increases. Also, XPS data reveals that, when subjected to thermal annealing, Co atoms diffuse into Au crystal retaining its chemical nature as before the annealing. Finally, NO adsorption experiments show that Co nanostructures are partially oxidized upon adsorption, as evidenced by changes in core photoemission lineshapes of the Co2p lines. Also, NO adsorption seems to inhibit Co atom diffusion into Au crystal during moderate thermal treatment.

Control of the magnetism of cobalt phthalocyanine by a ferromagnetic substrate

The electronic and magnetic properties of cobalt-phthalocyanine (CoPc) molecules are modified at the interface with a magnetic metallic surface: upon adsorption of CoPc on iron film relevant changes are observed in the C 1$s$ photoemission line shape and on Co ${L}_{2,3}$ edges, and N $K$-edge x-ray absorption spectra. X-ray magnetic circular dichroism shows Co 3$d$ spin-polarized states, i.e., magnetic coupling with the iron substrate at room temperature. The interplay among the electronic and magnetic properties and CoPc adsorption sites is fundamental for tailoring the functionalities of hybrid organic-metallic interfaces.

Anatomy of the self-energy

The general problem of representing the Greens function $G(k,z)$ in terms of self-energy in field theories lacking Wick's theorem is considered. A simple construction shows that a Dyson-like representation with a self-energy $\ensuremath{\Sigma}(k,z)$ is always possible, provided we start with a spectral representation for $G(k,z)$ for finite-sized systems and take the thermodynamic limit. The self-energy itself can then be iteratively expressed in terms of another higher order self-energy, capturing the spirit of Mori's formulation. We further discuss alternative and more general forms of $G(k,z)$ that are possible. In particular, a recent theory, by the author, of extremely correlated Fermi liquids at density $n$, for Gutzwiller projected noncanonical Fermi operators, obtains a new form of the Greens function: $G(k,z)=[(1\ensuremath{-}\frac{n}{2})+\ensuremath{\Psi}(k,z)]/[z\ensuremath{-}{\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{E}}_{k}\ensuremath{-}\ensuremath{\Phi}(k,z)],$ with a pair of self-energies $\ensuremath{\Phi}(z)$ and $\ensuremath{\Psi}(z)$. Its relationship with the Dyson form is explored. A simple version of the two-self-energies model was shown recently to successfully fit several data sets of photoemission line shapes in cuprates. We provide details of the unusual spectral line shapes that arise in this model, with the characteristic skewed shape depending upon a single parameter. The energy distribution curve (EDC) and momentum distribution curve (MDC) line shapes are shown to be skewed in opposite directions, and provide a testable prediction of the theory.

Surface core-level shifts on Ge(111)c(2×8): Experiment and theory

Combining high-resolution photoelectron spectroscopy and density functional theory (DFT) calculations, 3$d$ photoemission line shape and surface core-level shifts have been reinvestigated on the Ge(111)$c$(2 \ifmmode\times\else\texttimes\fi{} 8) surface. It is found that 3$d$ spectra include, in addition to the bulk and three surface-shifted components reported in literature, a component that was not identified in earlier measurements with a lower resolution. The detailed interpretation of these spectra and their line shape is made on the basis of DFT calculations. It is shown that the lowest binding energy component is due to the rest atoms. The higher binding energy emission is caused by the adatoms and the third-layer atoms that are below the adatoms. Finally, the two other surface components originate from the first- and second-layer atoms. The screening effects in the Ge(111)$c$(2 \ifmmode\times\else\texttimes\fi{} 8) are discussed.

Layer-dependent Debye temperature and thermal expansion of Ru(0001) by means of high-energy resolution core-level photoelectron spectroscopy

The layer-dependent Debye temperature of Ru(0001) is determined by means of high-energy resolution core-level photoelectron spectroscopy measurements. The possibility to disentangle three different components in the Ru 3d{sub 5/2} spectrum of Ru(0001), originating from bulk, first-, and second-layer atoms, allowed us to follow the temperature evolution of their photoemission line shapes and binding energies. Temperature effects were detected, namely, a lattice thermal expansion and a layer-dependent phonon broadening, which was interpreted within the framework of the Hedin-Rosengren formalism based on the Debye theory. The resulting Debye temperature of the top-layer atoms is 295{+-}10 K, lower than that of the bulk (T=668{+-}5 K) and second-layer (T=445{+-}10 K) atoms. While these results are in agreement with the expected phonon softening at the surface, we show that a purely harmonic description of the motion of the surface atoms is not valid, since anharmonic effects contribute significantly to the position and line shape of the different core-level components.

LUMO photoemission lineshape in quasi-one-dimensionalC60chains

The low-energy single-particle excitations of highly ordered C60 chains adsorbed on a vicinal copper substrate are investigated by angle-resolved photoemission spectroscopy. The interface state previously identified on C60/Cu(111) shows a one-dimensional dispersion on Cu(553). In contrast, no significant momentum dependence is detected for emission from the lowest unoccupied molecular orbital (LUMO). The LUMO displays similar phonon features as in C60/Cu(111) but it does not peak toward the Fermi level for all considered potassium dopings and its photoemission lineshape is broader than in any other monolayer system investigated so far. This behavior is not easily reconciled with existing theory and indicates that the one-dimensional character of the chains affects the electronic structure of the monolayer in an intricate way.

Spin-charge decoupling and the photoemission line-shape in one-dimensional insulators

The recent advances in angle resolved photoemission techniques allowed the unambiguous experimental confirmation of spin charge decoupling in quasi one dimensional (1D) Mott insulators. This opportunity stimulates a quantitative analysis of the spectral function $A(k,\omega)$ of prototypical one dimensional correlated models. Here we combine Bethe Ansatz results, Lanczos diagonalizations and field theoretical approaches to obtain $A(k,\omega)$ for the 1D Hubbard model as a function of the interaction strength. By introducing a {\it single spinon approximation}, an analytic expression is obtained, which shows the location of the singularities and allows, when supplemented by numerical calculations, to obtain an accurate estimate of the spectral weight distribution in the $(k,\omega)$ plane. Several experimental puzzles on the observed intensities and line-shapes in quasi 1D compounds, like ${\rm SrCuO_2}$, find a natural explanation in this theoretical framework.

XPS, AES, and Electrochemical Study of Iridium Oxide Coating Materials for Cardiovascular Stent Application

The surface composition and morphology of iridium oxide (IrO2) coatings prepared by a multi-step physical vapor deposition process on 316L stainless steel stent substrates has been investigated by XPS, AES, and FESEM. The obtained data show that even though the morphologies of iridium oxide films produced within the examined range of PVD process parameters are almost identical, there is a difference in their surface composition, in particular in the ratios of Ir-O σ and Ir=O π bonds. A correlation between the surface stoichiometries of the IrO2 films and their electrochemical CV and EIS responses has been established. The observed differences in the O 1s photoemission line shape of the investigated iridium oxide films were strikingly reflected in their voltammetric curve features. The implication of these finding on the mechanism of the catalytic decomposition of hydrogen peroxide by IrO2 is discussed.

Theory of photoemission line shape in high-temperature superconductor Bi 2Sr 2CaCu 2O 8+ δ via dipolon mediated electron–electron pairing mechanism

Hitherto unexplained experimentally observed photoemission line shape near (π, 0) in superconductor Bi 2Sr 2CaCu 2O 8+ δ characterized by a peak followed at high energy by a dip and a broad feature has been explained here naturally by dipolon mediated electron–electron pairing mechanism of high- T c superconductors. In the superconducting state the peak is formed by excitation solely of a quasiparticle whereas the broad feature is formed by the excitation of a quasiparticle with concomitant excitation of a dipolon for coupling with the process of creating a photoelectron. In the normal state the photoemission spectra observed in the underdoped, optimally doped and overdoped samples are due to the excitations of O 1,1 and O 1,2 dipolons. The theory predicts the possibility of observing dipolon side bands in photoemission spectra in superconducting state in favorable conditions. We deduce that the dipolon theory provides not only explanation for the observed kink but predicts also the existence of new weak kinks in single-quasiparticle energy dispersion.

Electronic structure at highly ordered organic/metal interfaces: Pentacene on Cu(110)

The electronic structure at highly ordered pentacene monolayer prepared on Cu(110) substrate was studied by angle-resolved ultraviolet photoemission spectroscopy. The valence-level photoemission line shape showed the evidences of (i) formation of the interface states and (ii) two-dimensional energy-band dispersion of the resultant interface states. The lattice constant deduced from the observed energy-band dispersion is consistent with the reported one based on the low-energy electron diffraction experiments. Thus, the observed energy-band dispersion can be ascribed to the in-plane intermolecular energy-band dispersion in the pentacene monolayer on Cu(110). These phenomena may originate from the hybridization between the molecular orbital and the wave function of the substrate surface. Furthermore, work-function change of about $\ensuremath{-}0.9\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ by adsorption of pentacene was observed from the shift of the secondary-electron cutoff. Such a decrease of the work function indicates the formation of a dipole layer at the interface with the molecule positively charged. This direction is opposite to the naive expectation from the electron transfer from the substrate to the molecule, which was suggested from the previous work of core-level photoemission spectroscopy [McDonald et al., Surf. Sci. 600, 1909 (2006)]. This unexpected result may originate from the charge redistribution at the interface due to the induced image charge in the metal and the push back of electrons spilled out from the metal surface by the adsorbed molecules, which may overwhelm the effect of electron transfer.

Electronic structure and anomalous photoemission line-shape of quasi-2D oxide η-Mo 4O 11

The η-Mo 4O 11 compound is a layered two-dimensional (2D) metallic system whose reduced dimensionality originates non-linear properties as charge density wave (CDW) instabilities. We report on synchrotron radiation angle resolved photoemission spectroscopy (ARPES) measurements in order to obtain a detailed picture of the electronic structure of this material. The symmetry of the states near the Fermi level ( E F) has been discussed in relation to the photoemission symmetry selections rules. Our results are in excellent agreement with previous tight-binding calculations and support the hidden nesting concept proposed to explain the CDW instabilities exhibited by this family of compounds. In addition, a very peculiar photoemission line-shape has been found with the presence of localized non-dispersive states. Some possible explanations are discussed.

Angle-resolved photoemission studies of lattice polaron formation in the cuprateCa2CuO2Cl2

To elucidate the nature of the single-particle excitations in the undoped parent cuprates, we have performed a detailed study of Ca{sub 2}CuO{sub 2}Cl{sub 2} using photoemission spectroscopy. The photoemission lineshapes of the lower Hubbard band are found to be well-described by a polaron model. By comparing the lineshape and temperature dependence of the lower Hubbard band with additional O 2p and Ca 3p states, we conclude that the dominant broadening mechanism arises from the interaction between the photohole and the lattice. The strength of this interaction was observed to be strongly anisotropic and may have important implications for the momentum dependence of the first doped hole states.