Core-level Components Research Articles

Growth Dynamics of Ultrathin Films of Benzo[1,2-b:4,5-b’]dithiophene Derivatives on Au(111): A Photoelectron Spectroscopy Investigation

Ultrathin films of a stereoisomeric mixture of benzo[1,2-b:4,5-b']dithiophene derivatives were grown by thermal evaporation in vacuum on Au(111), and they were studied in situ by photoelectron spectroscopy. X-ray photons from a non-monochromatic Mg Kα conventional X-ray source and UV photons from a He I discharge lamp equipped with a linear polarizer were used. He I photoemission results were compared with density functional theory (DFT) calculations: density of states (DOS) and 3D molecular orbital density distribution. Au 4f, C 1s, O 1s, and S 2p core-level components suggest a surface rearrangement as a function of film nominal thickness, with the variation of the molecular orientation, from flat-laying at the initial deposition to tilted toward the surface normal at coverages exceeding 2 nm. Eventually, the DFT results were exploited in assigning of the valence band experimental structures. Moreover, polarization-dependent photoemission confirmed the tilted arrangement of the molecules, starting at 2 nm. A variation of the work function of 1.4 eV with respect to the clean substrate was measured, together with a valence band offset of 1.3 eV between the organic layer and gold.

Spin-Orbit Coupling Effects in Au 4f Core-Level Electronic Structures in Supported Low-Dimensional Gold Nanoparticles.

Despite their many advantages, issues remain unresolved over the variability in catalytic activities in supported gold nanoparticle (AuNP)-based catalysts, which requires precise characterization to unravel the presence of any fine features. Herein, upon analyzing the Au 4f core-level spin-orbit components in many as-synthesized AuNP-based catalysts, we observed that like deviations in the Au 4f7/2 binding energy positions, both the Au 4f7/2-to-Au 4f5/2 peak intensity and linewidth ratios varied largely from the standard statistical bulk reference values. These deviations were observed in all the as-synthesized supported AuNPs irrespective of different synthesis conditions, variations in size, shape or morphology of the gold nanoparticles, and different support materials. On the other hand, the spin-orbit-splitting values remained almost unchanged and did not show any appreciable deviations from the atomic or bulk standard gold values. These deviations could originate due to alterations in the electronic band structures in the supported AuNPs and might be present in other NP-based catalyst systems as well, which could be the subject of future research interest.

Photoemission investigation of oxygen intercalated epitaxial graphene on Ru(0001)

We study the formation of epitaxial graphene on Ru(0001) using fast x-ray photoelectron spectroscopy during the growth process. The assignment of different C 1s and Ru 3d core level components and their evolution during the growth process gives a detailed insight into the graphene formation and the strongly varying graphene-Ru interaction strength within the large moiré unit cell. Subsequent intercalation of oxygen can be achieved at elevated temperature and the core level spectra show a conversion of the strongly corrugated to quasi free-standing graphene, characterised by a single narrow C 1s component. This conversion and the accompanying flattening of the graphene layer is also confirmed by x-ray photoelectron diffraction. The effect of oxygen intercalation on the electronic structure is studied using angle-resolved photoemission of the valence band states. For graphene/Ru(0001), the strong graphene-substrate hybridisation disrupts the π-band dispersion but oxygen intercalation fully restores the π-band with a strong p-doping that shifts the Dirac point 785 meV above the Fermi level. The doping of the system is highly tunable, as the additional exposure to rubidium can convert the carrier filling to n-type with the Dirac point 970 meV below the Fermi level.

Operando hard X-ray photoelectron spectroscopy study of the Pt/Ru/PbZr0.52Ti0.48O3 interface

We have used hard X-ray photoelectron spectroscopy to probe the Pt/Ru/PbZr0.52Ti0.48O3 (PZT) interface in a Pt/Ru/PZT(220 nm)/Pt/TiO2/SiO2/Si stack. A customized sample-holder allows in-situ photoemission analysis while applying bias to the capacitor. Hard X-rays probe the buried interface between the top electrode and the ferroelectric PZT. The use of operando conditions reveals a polarization-dependent electronic response, most probably due to imperfect screening of the depolarizing field. There is evidence for an additional core level component related to the electrode-PZT interface. Zr oxide nanostructures at the surface of the sol-gel layer may form a ferroelectric dead layer at the interface, affecting device performance.

Ionic liquid ultrathin films at the surface of Cu(100) and Au(111).

Monolayer to multilayer ultrathin films of the ionic liquid (IL) 1-methyl-3-octylimidazolium bis(trifluoromethylsulfonyl)amide have been prepared on Au(111) and Cu(100) surfaces using physical vapor deposition. The ion-surface interactions are studied using a combination of scanning tunnel microscopy, as well as ultraviolet and x-ray photoemission spectroscopies. It is found that the IL does not decompose at the surface of the metals, and that the IL interaction with the Cu(100) surface is much stronger than with the Au(111) surface. As a consequence, STM imaging at room temperature results in more stable imaging at the monolayer coverage on Cu(100) than on Au(111), and work function measurements indicate a large interface dipole upon deposition of a monolayer of IL on Cu. Additional IL depositions on the two surfaces result in two distinct behaviors for the IL core levels: a gradual energy shift of the core levels on Au and a set of two well defined monolayer and multilayer core level components found at fixed energies on Cu, due to the formation of a tightly bound monolayer. Finally, it is proposed that the particularly strong cation-Cu interaction leads to stabilization of the anion and prevents its decomposition at the surface of Cu(100).

High resolution core level spectroscopy of hydrogen-terminated (1 0 0) diamond

Synchrotron-based photoelectron spectroscopy experiments are presented that address a long standing inconsistency in the treatment of the C1s core level of hydrogen terminated (1 0 0) diamond. Through a comparison of surface and bulk sensitive measurements we show that there is a surface related core level component to lower binding energy of the bulk diamond component; this component has a chemical shift of eV which has been attributed to carbon atoms which are part of the hydrogen termination. Additionally, our results indicate that the asymmetry of the hydrogen terminated (1 0 0) diamond C1s core level is an intrinsic aspect of the bulk diamond peak which we have attributed to sub-surface carbon layers.

An Effective Approach to Improving Cadmium Telluride (111)A Surface by Molecular-Beam-Epitaxy Growth of Tellurium Monolayer.

The surface cleansing treatment of non-natural cleavage planes of semiconductors is usually performed in vacuum using ion sputtering and subsequent annealing. In this Research Article, we report on the evolution of surface atomic structure caused by different ways of surface treatment as monitored by in situ core-level photoemission measurements of Cd-4d and Te-4d atomic levels and reflection high-energy electron diffraction (RHEED). Sputtering of surface increases the density of the dangling bonds by 50%. This feature and the less than ideal ordering can be detrimental to device applications. An effective approach is employed to improve the quality of this surface. One monolayer (ML) of Te grown by the method of molecular beam epitaxy (MBE) on the target surface with heating at 300 °C effectively improves the surface quality as evidenced by the improved sharpness of RHEED pattern and a reduced diffuse background in the spectra measured by high-resolution ultraviolet photoemission spectroscopy (HRUPS). Calculations have been performed for various atomic geometries by employing first-principles geometry optimization. In conjunction with an analysis of the core level component intensities in terms the layer-attenuation model, we propose a "vacancy site" model of the modified 1 ML-Te/CdTe(111)A (2 × 2) surface.

Core-Level States of Single-Electron Adsorption Complexes Ge on Si (001) (42)

Calculations (DFT, B3LYP, 6-31 G**) chemical shifts core-level components of germanium atoms, which are included in the surface layer of cluster Si96H84•Ge2, modeling complex molecular adsorption on germanium faces reconstructed Si(001)(4´2), showed that the magnitude of the shift depends on the relative position of atoms Ge. When introducing a single atom in a crystal germanium substrate, this shift is positive, and the introduction of two atoms leads to negative chemical shift. Given the interpretation of the results based on the charge distribution in clusters, the so-called electrostatic approximation.

Evaluation of the fluorinated antisticking layer by using photoemission and NEXAFS spectroscopies

The electronic structures of four kinds of fluorinated self-assembled monolayers (F-SAMs) with different chain length, which were used for an antisticking layer, were investigated by the photoemission and the near-edge X-ray absorption fine structure (NEXAFS) spectroscopies. From the photoemission spectra in the wide and in the C 1s core-level regions, chemical compositions and components of the F-SAMs with different chain length were evaluated. By using the curve fitting analysis of the photoemission spectra in C 1s core-level region, it was found that the CF3 site is located at the top of the surface in the C sites of the F-SAM. From the C K-edge NEXAFS spectra of the F-SAMs as a function of the incidence angle of the excitation photon, it was shown that the σ*(C–F) and σ*(C–C) orbitals in the F-SAMs are parallel and perpendicular to the surface, respectively. This indicates that the C–C chain in (CF2) n part of the F-SAMs is perpendicular to the surface. Based on these results, the electronic structures of the F-SAMs are discussed.

Line shape and composition of the In 3d5/2 core-level photoemission for the interface analysis of In-containing III–V semiconductors

The In 3d5/2 photoelectron spectroscopy peak has been widely used to determine the interface structures of In-containing III–V device materials (e.g., oxidation states). However, an unclear parameter affecting the determination of the energy shifts and number of the core-level components, and therefore, the interpreted interface structure and composition, is still the intrinsic In 3d5/2 peak line shape. It is undecided whether the line shape is naturally symmetric or asymmetric for pure In-containing III–V compounds. By using high-resolution photoelectron spectroscopy, we show that the In 3d5/2 asymmetry arising from the emission at high binding-energy tail is not an intrinsic property of InAs, InP, InSb and InGaAs. Furthermore, it is shown that asymmetry of In 3d5/2 peaks of pure III–V's originates from the natural surface reconstructions which cause the coexistence of slightly shifted In 3d5/2 components with the symmetric peak shape and dominant Lorentzian broadening.

Metal-phthalocyanine ordered layers on Au(110): metal-dependent adsorption energy.

Iron-phthalocyanine and cobalt-phthalocyanine chains, assembled along the Au(110)-(1×2) reconstructed channels, present a strong interaction with the Au metallic states, via the central metal ion. X-ray photoemission spectroscopy from the metal-2p core-levels and valence band high-resolution ultraviolet photoelectron spectroscopy bring to light signatures of the interaction of the metal-phthalocyanine single-layer with gold. The charge transfer from Au to the molecule causes the emerging of a metal-2p core level component at lower binding energy with respect to that measured in the molecular thin films, while the core-levels associated to the organic macrocycle (C and N 1s) are less influenced by the adsorption, and the macrocycles stabilize the interaction, inducing a strong interface dipole. Temperature Programmed Desorption experiments and photoemission as a function of temperature allow to estimate the adsorption energy for the thin-films, mainly due to the molecule-molecule van der Waals interaction, while the FePc and CoPc single-layers remain adsorbed on the Au surface up to at least 820 K.

Evaluation of Fluorinated Self-assembled Monolayer by Using Photoelectron and Near Edge X-ray Absorption Fine Structure Spectroscopies

The electronic structure of fluorinated self-assembled monolayers (F-SAMs) with different chain length was investigated by the using photoelectron and near edge X-ray absorption fine structure (NEXAFS) spectroscopies. From the measurements of the photoelectron spectra in the wide region and in the C 1s core-level region, chemical compositions and components of the F-SAMs with different chain length were clarified. In the C K-edge NEXAFS spectra of the F-SAMs, the several spectral features were observed and the intensity of the features at 292 and 299 eV decreased with increasing the incidence angle of the excitation photon. Based on the results of the photoelectron and NEXAFS spectra measurements, the electronic structure of four kinds of F-SAMs with different chain length is discussed.

Charge transfer in Sr Zintl template on Si(001)

The formation of the half monolayer (ML) Sr Zintl template layer on Si(001) is investigated in a combined experimental and theoretical work consisting of in situ reflection high energy electron diffraction, in situ x-ray photoelectron spectroscopy (XPS), and density functional theory. Starting with clean 2 × 1 reconstructed Si(001), we demonstrate that Sr deposition leads to a charge transfer from the metal to the Si substrate resulting in the disappearance of the asymmetry of Si dimers—an essential structural change that enables direct perovskite epitaxy on Si, and likely, other semiconductors. XPS reveals an unexpected shift to higher binding energy of the Si 2p core-level components, including the bulk. This unusual behavior is attributed to final state effects using first principles calculations. As measured by ultraviolet photoelectron spectroscopy, the deposition of 0.5 ML of Sr lowers the work function of the system by 1.35 eV, and is in good agreement with our theoretical calculations.

Non-local Effects on Oxygen-Induced Surface Core Level Shifts of Re(0001)

The electronic structure of clean and oxygen-covered Re(0001) was investigated by combining high energy resolution core level photoelectron spectroscopy and density functional theory. For the clean surface, we detect two distinct Re 4f7/2 components shifted by 95 meV, which we ascribe to photoemission from the bulk and the top layer of the crystal. Oxygen adsorption at room temperature leads to the appearance of new core level components, which we associate to non-equivalent Re atoms from the topmost layer. In addition to the linear relationship between oxygen-induced core level shifts and number of O–Re bonds, we find that the binding energy shift also depends on the number of oxygen atoms at second and third nearest neighbor positions. This non-local effect is discussed in terms of 5d band center shift and of charge transfer between the metallic substrate and the O atoms.

Synchrotron radiation photoelectron spectroscopy studies of self-organization in As40Se60 nanolayers stored under ambient conditions and after laser irradiation

The influence of air and near bandgap laser irradiation on the surface structure of As40Se60 nanolayers has been investigated by synchrotron radiation photoelectron spectroscopy (SRPES). The As 3d and Se 3d photoemission peaks of the irradiated sample show significant differences in shapes and positions in comparison with those obtained for non-irradiated amorphous films. The experimental data processing and quantification were performed by analyzing As 3d, Se 3d, C 1s and O 1s core-level components obtained by curve fitting. The relative contribution of the As, Se, C and O atoms in different chemical states to the whole As 3d, Se 3d, C 1s and O 1s signals, its structural origins as well as their relation to the As40Se60 surface and subsurface nanolayers structure before and after laser irradiation is analyzed and discussed in detail. An atomic model of induced structural changes on the surface of As40Se60 nanolayers under ambient conditions and after laser irradiation in air is proposed.

Laser induced changes of As50Se50 nanolayers studied by synchrotron radiation photoelectron spectroscopy

The influence of near bandgap laser irradiation on the structure of the As50Se50 thin film has been investigated by synchrotron radiation photoelectron spectroscopy. The As 3d and Se 3d photoemission peaks of the irradiated sample show significant differences in shapes and positions in comparison with those obtained for non-irradiated amorphous film. The experimental data processing and quantifications are performed analyzing As 3d and Se 3d core-level components obtained by curve fitting. The relative contribution of the As and Se atoms in different chemical states to the whole As 3d and Se 3d signal, its structural origins as well as their relation to the As50Se50 nanolayers structure before and after laser irradiation is analyzed and discussed in detail.

Surface composition of clean and oxidized Pd75Ag25(100) from photoelectron spectroscopy and density functional theory calculations

High resolution photoelectron spectroscopy and density functional theory calculations have been used to study the composition of clean and oxidized Pd75Ag25(100). The results for the clean surface confirm earlier reports of surface segregation by Wouda et al. (1998), where the top most layers are rich in Ag. The Pd 3d core level component from the surface region is observed at higher binding energies than the contribution from the bulk which is found to be a signature of Pd embedded in Ag. Low energy electron diffraction and scanning tunneling microscopy measurements reveal that oxidation of the Pd75Ag25(100) surface results in a 5×5R27°-O structure similar to the one reported for Pd(100). The calculations suggest that the stable structure is a PdO(101) monolayer supported on a (100) surface rich in Ag at the interface to the stoichiometric alloy. The calculated core level shifts for the oxidized surface are in good agreement with the experimental observations.

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.

The surface core level shift for lithium at the surface of lithium borate

Abstract The shallow Li 1s core level exhibits a surface-to-bulk core level shift for the stoichiometric Li 2 B 4 O 7 (1 1 0) surface. Angle-resolved photoemission spectroscopy was used to indentify Li 1s bulk and surface core level components at binding energies −56.5±0.4 and −53.7±0.5 eV, respectively. We find photoemission evidence for surface states of Li 2 B 4 O 7 (1 1 0) that exist in the gap of the projected bulk density of states. The existence of surface states is consistent with the large surface-to-bulk core level shift for the Li 1s core.

Surface core level shifts of clean and oxygen covered Ir(111)

We present the results of high resolution core level photoelectron spectroscopy employed to investigate the electronic structure of clean and oxygen covered Ir(111) surface. Ir 4f7/2 core level spectra are shown to be very sensitive to the local atomic environment. For the clean surface we detected two distinct components shifted by 550 meV, originated by surface and bulk atoms. The larger Gaussian width of the bulk component is explained as due to experimentally unresolved subsurface components. In order to determine the relevance of the phonon contribution we examined the thermal behaviour of the core level lineshape using the Hedin–Rosengren theory. From the phonon-induced spectral broadening we found the Debye temperature of bulk and surface atoms to be 298 and 181 K, respectively, which confirms the softening of the vibrational modes at the surface. Oxygen adsorption leads to the appearance of new surface core level components at −200 meV and +230 meV, which are interpreted as due to first-layer Ir atoms differently coordinated with oxygen. The coverage dependence of these components demonstrates that the oxygen saturation corresponds to 0.38 ML, in good agreement with recent density functional theory calculations.