Core Level Binding Energy Shifts Research Articles

Fluorinated Phosphorene: Electrochemical Synthesis, Atomistic Fluorination, and Enhanced Stability.

Phosphorene has attracted great interest due to its unique electronic and optoelectronic properties owing to its tunable direct and moderate band-gap in association with high carrier mobility. However, its intrinsic instability in air seriously hinders its practical applications, and problems of technical complexity and in-process degradation exist in currently proposed stabilization strategies. A facile pathway in obtaining and stabilizing phosphorene through a one-step, ionic liquid-assisted electrochemical exfoliation and synchronous fluorination process is reported in this study. This strategy enables fluorinated phosphorene (FP) to be discovered and large-scale, highly selective few-layer FP (3-6 atomic layers) to be obtained. The synthesized FP is found to exhibit unique morphological and optical characteristics. Possible atomistic fluorination configurations of FP are revealed by core-level binding energy shift calculations in combination with spectroscopic measurements, and the results indicate that electrolyte concentration significantly modulates the fluorination configurations. Furthermore, FP is found to exhibit enhanced air stability thanks to the antioxidation and antihydration effects of the introduced fluorine adatoms, and demonstrate excellent photothermal stability during a week of air exposure. These findings pave the way toward real applications of phosphorene-based nanophotonics.

Effects of rhenium on graphene grown on SiC(0001)

We study the effects of Rhenium (Re) deposited on epitaxial monolayer graphene grown on SiC(0001) and after subsequent annealing at different temperatures, by performing high resolution photoelectron spectroscopy (PES) and angle resolved photoelectron spectroscopy (ARPES). The graphene-Re system is found to be thermally stable. While no intercalation or chemical reaction of the Re is detected after deposition and subsequent annealing up to 1200°C, a gradual decrease in the binding energy of the Re 4f doublet is observed. We propose that a larger mobility of the Re atoms with increasing annealing temperature and hopping of Re atoms between different defective sites on the graphene sample could induce this decrease of Re 4f binding energy. This is corroborated by first principles density functional theory (DFT) calculations of the Re core-level binding energy shift. No change in the doping or splitting of the initial monolayer graphene electronic band structure is observed after Re deposition and annealing up to 1200°C, only a broadening of the bands.

Origin of the core-level binding energy shifts in Au nanoclusters

We investigate the shifts of the core-level binding energies in small gold nanoclusters by using {\it ab initio} density functional theory calculations. The shift of the 4$f$ states is calculated for magic number nanoclusters in a wide range of sizes and morphologies. We find a non-monotonous behavior of the core-level shift in nanoclusters depending on the size. We demonstrate that there are three main contributions to the Au 4$f$ shifts, which depend sensitively on the interatomic distances, coordination and quantum confinement. They are identified and explained by the change of the on-site electrostatic potential.

Core-level binding energy shifts as a tool to study surface processes on LaAlO3/SrTiO3

X-ray photoelectron spectroscopy (XPS) is used to study the adsorbed species on LalAlO3/SrTiO3 surfaces combined with computational modeling. Core level binding energy shifts are calculated from first-principles for various terminations of LaAlO3 (LAO) surfaces with adsorbed H and OH in different concentrations and with or without the presence of oxygen vacancies on the surface. Final state relaxation effects in the presence of the core hole are included. Energies of formation and adsorption for the various species and relevant reaction energies are calculated. These are used to interpret the changes in XPS spectra of an LAO film under different growth and annealing treatments. We find a peak with 2eV binding energy shift toward higher binding energy in the O1s spectrum cannot be associated with OH as is commonly assumed. Instead it is assigned to carbonaceous species by studying the corresponding changes in the C1s spectrum. The OH and H species are found to be too strongly bound to be removed at 500°C.

(Invited) Determining Solid/Liquid Junction Energetics By Operando Ambient-Pressure X-Ray Photoelectron Spectroscopy

The performance of a photoelectrochemical solar cell (PEC) depends strongly on the electrochemical nature of the semiconductor/electrolyte junction. Operando ambient pressure X-ray photoelectron spectroscopy (OAP-XPS) investigation of solid/liquid junctions provides quantitative understanding of the energetic relations in PECs [1-3]. We demonstrate that OAP-XPS can be used to determine these relationships for solid/liquid systems. The OAP-XPS data can be analyzed to determine the relationship between the energy bands in a semiconductor electrode and the redox levels in the solution as well as directly analyze the energetics of the double layer at a metal-aqueous solution interface. From the response of the core level binding energy shifts and the broadening to variations in applied potential, we obtain the energetics of the solid/electrolyte interface. All major conditions for semiconductor-electrolyte contacts including accumulation, depletion, and Fermi-level pinning were observed, and the flat-band potential was determined. The data are compared to theoretical evaluations of the potential in the solid and electrolyte near the interface. 1. H.J. Lewerenz et al., Electrochimica Acta, 211, 711–719 2016 2. M. F. Lichterman et al., Journal of the Electrochemical Society, 163(2), H139–H146, 2016 3. M. F. Lichterman et al., Energy & Environmental Science, 8(8), 2409–2416, 2015

In Situ X-ray Photoelectron Spectroscopy Study of Lithium Interaction with Graphene and Nitrogen-Doped Graphene Films Produced by Chemical Vapor Deposition

It is commonly accepted that the presence of nitrogen atoms in a graphene lattice improves many properties of carbon materials and particularly enhances their electrochemical capacity in Li ion batteries. Here, we present model experiments for revealing the difference in interaction of lithium with N-doped and N-free graphene samples by monitoring the changes in their electronic states after the deposition of Li vapors. Graphene and N-doped graphene films have been grown by chemical vapor deposition on copper substrates using methane and acetonitrile as precursors. The electronic structure of the films transferred onto SiO2/Si substrates was examined by X-ray photoelectron spectroscopy (XPS) before and after deposition of lithium from a Li evaporation source under vacuum conditions. A comparison between two graphene samples using in situ XPS measurements has detected a higher accumulation of lithium on the N-doped graphene, which implies its high prospects in energy storage applications. Analysis of the XPS core-level binding energy shifts showed that charge density donated by lithium is localized near the nitrogen defects, especially around the nitrogen atoms directly substituting for carbon atoms.

Predicting core level binding energies shifts: Suitability of the projector augmented wave approach as implemented in VASP.

Here, we assess the accuracy of various approaches implemented in Vienna ab initio simulation package code to estimate core-level binding energy shifts (ΔBEs) using a projector augmented wave method to treat core electrons. The performance of the Perdew-Burke-Ernzerhof (PBE) and the Tao-Perdew-Staroverov-Scuseria (TPSS) exchange-correlation density functionals is examined on a dataset of 68 molecules containing B→F atoms in diverse chemical environments, accounting for 185 different 1s core level binding energy shifts, for which both experimental gas-phase X-ray photoemission (XPS) data and accurate all electron ΔBEs are available. Four procedures to calculate core-level shifts are investigated. Janak-Slater transition state approach yields mean absolute errors of 0.37 (0.21) eV at PBE (TPSS) level, similar to highly accurate all electron ΔSCF approaches using same functionals, and close to XPS experimental accuracy of 0.1 eV. The study supports the use of these procedures to assign ΔBEs of molecular moieties on material surfaces of interest in surface science, nanotechnology, and heterogeneous catalysis. © 2017 Wiley Periodicals, Inc.

Ab initiodensity functional theory study on the atomic and electronic structure of GaP/Si(001) heterointerfaces

The atomic and electronic band structures of GaP/Si(001) heterointerfaces were investigated by ab initio density functional theory calculations. Relative total energies of abrupt interfaces and mixed interfaces with Si substitutional sites within a few GaP layers were derived. It was found that Si diffusion into GaP layers above the first interface layer is energetically unfavorable. An interface with Si/Ga substitution sites in the first layer above the Si substrate is energetically the most stable one in thermodynamic equilibrium. The electronic band structure of the epitaxial GaP/Si(001) heterostructure terminated by the $(2\ifmmode\times\else\texttimes\fi{}2)$ surface reconstruction consists of surface and interface electronic states in the common band gap of two semiconductors. The dispersion of the states is anisotropic and differs for the abrupt Si-Ga, Si-P, and mixed interfaces. Ga $2p$, P $2p$, and Si $2p$ core-level binding-energy shifts were computed for the abrupt and the lowest-energy heterointerface structures. Negative and positive core-level shifts due to heterovalent bonds at the interface are predicted for the abrupt Si-Ga and Si-P interfaces, respectively. The distinct features in the heterointerface electronic structure and in the core-level shifts open new perspectives in the experimental characterization of buried polar-on-nonpolar semiconductor heterointerfaces.

Bonding and Electron Energy-Level Alignment at Metal/TiO2 Interfaces: A Density Functional Theory Study

Metal/TiO2 interfaces have been extensively studied because of their importance in electronic devices, electrochemical cells, and photocatalysis. In this article, we present our studies on electronic structures for anatase TiO2(001)/fcc-metal(001) (metal = Pt, Pd, or Au) interfaces using first-principles calculations. It is demonstrated that the Schottky barrier height depends on the metal work function and significantly decreases at an interface with strong adhesion between the metal and TiO2. The sizable reduction of the barrier height is a consequence of dipole formation at the interface due to electron transfer from TiO2 to the metal. The formation of dipoles at the Pt/TiO2 interface is supported by our experimental results for a core-level binding-energy shift in Pt clusters loaded on the surface of TiO2. Differences in the bonding and antibonding characters of metal–O bonds for the three metals are discussed based on the projected densities of states given by our density-functional theory calculations.

Interpreting core-level spectra of oxidizing phosphorene: Theory and experiment

We combine ab initio density functional theory calculations with the equivalent cores approximation to determine core-level binding-energy shifts at phosphorus sites caused by oxidation of phosphorene. We find that presence of oxygen increases the core-level binding energies of P atoms and expect binding-energy shifts of up to 6 eV in highly defective geometries. We have identified likely binding geometries of oxygen that help to interpret the observed core-level photoemission spectra in samples at different stages of oxidation and allow us to determine the fractions of specific local geometries.

Synchrotron radiation photoemission study of Pb1−xCdxTe crystal with local structure

The paper presents photoemission study of core level binding energy shifts caused by local crystalline structure collapse in cubic Pb1−xCdxTe crystal. Photoemission spectra of two kinds of semiconductor samples are compared. The first one is ternary crystal of Pb0.94Cd0.06Te with the frozen rock salt structure where the crystalline local structure collapse is expected due to the difference of ion radii of Cd and Pb cations. The second sample was the CdTe(22nm)/PbTe(6nm)/CdTe(4μm)/GaAs(111)B nanostructure grown by molecular beam epitaxy (MBE) method, where crystalline local structure is not expected to be created. The photoemission spectra show that for the crystal with local structure the electron binding energies of cations are higher (e.g. +0.2eV) whereas for anions they are lower (e.g. −0.08eV) than in the multilayer structure. A model is proposed to explain obtained results by the local crystalline structure collapse in Pb0.94Cd0.06Te crystal.

Atomic-Scale Insight into the Metal–Support Interaction: A Case for Ag Nanoparticles on Ordered ZrO2(111) Thin Films

Metal–support interaction plays an important role in heterogeneous catalysis. We studied the Ag–zirconia interaction by investigating the growth, electronic structure, and thermal stability of Ag nanoparticles on thin ZrO2(111) film surfaces by low-energy electron diffraction, synchrotron radiation photoemission spectroscopy, and X-ray photoelectron spectroscopy. The ZrO2(111) films were epitaxially grown on Pt(111). A three-dimensional growth mode with a nearly constant particle density of ∼2.0 × 1012 particles/cm2 was found for Ag growth on ZrO2(111) at 300 K. The binding energy (BE) of Ag 3d5/2 shifts to a higher BE up to 0.5 eV when decreasing the Ag coverages. The Auger parameter analysis shows that the primary contribution to the Ag 3d core level BE shift is the final state effect, indicating a very weak interaction between Ag nanoparticles and ZrO2(111). Thermal stability experiments demonstrate that Ag particles undergo serious sintering before they desorb from the zirconia surface. In addition, l...

Characterization of the Acetonitrile Aqueous Solution/Vapor Interface by Liquid-Jet X-ray Photoelectron Spectroscopy

We report photoelectron spectroscopy measurements from binary acetonitrile–water solutions, for a wide range of acetonitrile mole fractions (xCH3CN = 0.011–0.90) using a liquid microjet. By detecting the nitrogen and carbon 1s photoelectron signal of CH3CN from aqueous surface and bulk solution, we quantify CH3CN’s larger propensity for the solution surface as compared to bulk solution. Quantification of the strong surface adsorption is through determination of the surface mole fraction as a function of bulk solution, xCH3CN, from which we estimate the adsorption free energy using the Langmuir adsorption isotherm model. We also discuss alternative approaches to determine the CH3CN surface concentration, based on analysis of the relative amount of gas- versus liquid-phase CH3CN, obtained from the respective photoelectron signal intensities. Another approach is based on the core-level binding energy shifts between liquid- and gas-phase CH3CN, which is sensitive to the change in solution surface potential an...

Charge spill-out and work function of few-layer graphene on SiC(0 0 0 1)

We report on the charge spill-out and work function of epitaxial few-layer graphene on 6 H-SiC(0 0 0 1). Experiments from high-resolution, energy-filtered x-ray photoelectron emission microscopy (XPEEM) are combined with ab initio density functional theory calculations using a relaxed interface model. The work function values obtained from theory and experiments are in qualitative agreement, reproducing the previously observed trend of increasing work function with each additional graphene plane. Electron transfer at the SiC/graphene interface through a buffer layer (BL) causes an interface dipole moment which is at the origin of the graphene work function modulation. The total charge transfer is independent of the number of graphene layers, and is consistent with the constant binding energy of the SiC component of the C 1s core-level, measured by XPEEM. Charge leakage into a vacuum depends on the number of graphene layers, explaining why the experimental, layer-dependent C 1s graphene core-level binding energy shift does not rigidly follow that of the work function. Thus, a combination of charge transfer at the SiC/graphene interface and charge spill-out into the vacuum resolves the apparent discrepancy between the experimental work function and C 1s binding energy.

Metal nanoparticle size distribution in hybrid organic/inorganic films determined by high resolution X-ray photoelectron spectroscopy

We report on the application of high resolution X-ray photoelectron spectroscopy (HR-XPS) to provide a fast identification of the size distribution of metal nanoparticles (NPs) embedded in a polymer matrix. An accurate spectral analysis was performed on the metal photoelectron core level to determine the specific calibration curve which relates the metal NP core level binding energy shift to its size, which was independently measured by transmission electron microscopy. We have fully characterized the binding energy shifts in the case of silver NPs on a polythiophene based polymer layer. This work shows how this procedure can be applied to characterize multimodal size distributions of metal NPs on a statistical adequate sample area, without having typical experimental limitations of a TEM experiment. Moreover, this technique can give access to chemical analysis and by alternating ion beam sputtering and XPS analysis, the NP diffusion along the growth direction can be revealed.

Integrated X-ray photoelectron spectroscopy and DFT characterization of benzene adsorption on Pt(111), Pt(355) and Pt(322) surfaces

We systematically investigate the adsorption of benzene on Pt(111), Pt(355) and Pt(322) surfaces by high-resolution X-ray photoelectron spectroscopy (XPS) and first-principle calculations based on density functional theory (DFT), including van der Waals corrections. By comparing the adsorption energies at 1/9, 1/16 and 1/25 ML on Pt(111), we find significant lateral interactions exist between the benzene molecules at 1/9 ML. The adsorption behavior on Pt(355) and Pt(322) is very different. While on Pt(355) a step species is clearly identified in the C 1s spectra at low coverages followed by occupation of a terrace species at high coverages, no evidence for a step species is found on Pt(322). These different adsorption sites are confirmed by extensive DFT calculations, where the most favorable adsorption configurations on Pt(355) and Pt(322) are also found to vary: a highly distorted across the step molecule is found on Pt(355) while a less distorted configuration adjacent to the step molecule is deduced for Pt(322). The theoretically proposed C 1s core level binding energy shifts between these most favorable configurations and the terrace species are found to correlate well with experiment: for Pt(355), two adsorbate states are found, separated by ~0.4 eV in XPS and 0.3 eV in the calculations, in contrast to only one state on Pt(322).

Binding Modes of Fluorinated Benzylphosphonic Acids on the Polar ZnO Surface and Impact on Work Function

The interfaces formed between a zinc-terminated polar zinc oxide (0002) surface and a series of chemisorbed fluorinated benzylphosphonic acids have been studied at the density functional theory level. The results indicate that there occur substantial changes in the adsorption energy and surface work function whether the binding mode is bidentate or tridentate. Also, the trends and magnitude of the various factors that determine the total modifications in work function markedly vary between the two binding modes. We have also calculated the oxygen core-level binding energy shifts of the PO3 moiety with respect to the oxygen atoms in bulk ZnO; good agreement is obtained between the calculated values of the core-level binding energy shifts for the tridentate binding mode and available X-ray photoelectron spectroscopy data.

Direct experimental determination of the K2pand Cl2pcore-level binding energy shifts between molecular and solid KCl: Line broadening effects

Using a technique used previously for metals, the K $2p$ and Cl $2p$ photoelectron spectra of KCl have been measured from the molecular vapor and nonconductor solid simultaneously---a direct measurement for a nonconductor. The Cl $2p$ core level in bulk-solid KCl has a 0.79-eV larger binding energy than its molecular KCl counterpart, but the K $2p$ core level in bulk-solid KCl has a 3.75-eV smaller binding energy than in molecular KCl. After correction for the instrumental resolution, the Cl $2p$ experimental linewidths (FWHM) for both molecular (0.40 eV) and solid spectra (0.56 eV) are much smaller than the corresponding K $2p$ molecular and solid linewidths (0.88 and 0.93 eV, respectively). It is concluded that final-state vibrational splitting is the dominant factor in determining both molecular and solid-state linewidths, with phonon broadening being a smaller effect for the solid.

Alloying and surface composition of model Pd–Ag films synthesized by electroless deposition

PdAg model films and real composite membranes were prepared by sequential electroless deposition on top of porous stainless steel supports. Their surface properties were investigated by X-ray photoelectron spectroscopy (XPS), Angle resolved XPS and sputtering depth profile. It was shown that the surface of the alloy was strongly enriched in silver after the annealing treatment up to 500 °C on hydrogen stream. A relationship between the Ag 3d5/2 core-level binding energy shift and the silver surface composition was observed from the XPS data obtained with the model samples. The surface composition of real membranes after both hydrogen permeation and reaction experiments showed silver enrichment, in agreement with the data obtained from the model sample.

Crystallography Aided by Atomic Core‐Level Binding Energies: Proton Transfer versus Hydrogen Bonding in Organic Crystal Structures

Ionic bond or hydrogen bridge? Brønsted proton transfer to nitrogen acceptors in organic crystals causes strong N1s core-level binding energy shifts. A study of 15 organic cocrystal and salt systems shows that standard X-ray photoelectron spectroscopy (XPS) can be used as a complementary method to X-ray crystallography for distinguishing proton transfer from H-bonding in organic condensed matter.