Core-level Photoelectron Spectroscopy Research Articles

Layer-by-Layer Graphene Growth on β-SiC/Si(001).

The mechanism of few-layer graphene growth on the technologically relevant cubic-SiC/Si(001) substrate is uncovered using high-resolution core-level and angle-resolved photoelectron spectroscopy, low-energy electron microscopy, and microspot low-energy electron diffraction. The thickness of the graphitic overlayer supported on the silicon carbide substrate and related changes in the surface structure are precisely controlled by monitoring the progress of the surface graphitization in situ during high-temperature graphene synthesis, using a combination of microspectroscopic techniques. The experimental data reveal gradual changes in the preferential graphene lattice orientations at the initial stages of the few-layer graphene growth on SiC(001) and can act as reference data for controllable growth of single-, double-, and triple-layer graphene on silicon carbide substrates.

Formation of graphene-capped cobalt silicides

Intercalation synthesis of graphene-capped cobalt silicides has been studied in situ by core-level photoelectron spectroscopy with synchrotron radiation and low-energy electron diffraction. It is found that at 400 °C the intercalation of graphene/Ni(1 1 1) with cobalt takes place in a wide range of coverage up to 17 ML Co. Graphene is strongly coupled with the upper layer of Co atoms and stabilizes the fcc structure of the intercalated Co(1 1 1) film. Subsequent intercalation of graphene/Co/Ni(1 1 1) with silicon results in the formation of an ordered Co3Si surface phase of the (√3 × √3)R30° structure and then of Co2Si silicide and Co-Si solid solution. Our experimental data and DFT calculations have shown that the coupling between the graphene layer and the surface silicide is rather weak and the properties of quasi-freestanding graphene are preserved.

Electronic structure of β-Ta films from X-ray photoelectron spectroscopy and first-principles calculations

The electronic structure and chemical bonding of β-Ta synthesized as a thin 0 0 1-oriented film (space group P4¯21m) is investigated by 4f core level and valence band X-ray photoelectron spectroscopy and compared to α-Ta bulk. For the β-phase, the 4f7/2 peak is located at 21.91 eV and with the 4f5/2 at 23.81 eV which is 0.16 eV higher compared to the corresponding 4f peaks of the α-Ta reference. We suggest that this chemical shift originates from electron screening, higher resistivity or strain in the β-Ta film. Furthermore, the 5d–5s states at the bottom of the valence band are shifted by 0.75 eV towards higher binding energy in β-Ta compared to α-Ta. This is a consequence of the lower number of nearest neighbors with four in β-Ta compared to eight in the α-Ta phase. The difference in the electronic structures, spectral line shapes of the valence band and the energy positions of the Ta 4f, 5p core-levels of β-Ta versus α-Ta are discussed in relation to calculated states of β-Ta and α-Ta. In particular, the lower number of states at the Fermi level of β-Ta (0.557 states/eV/atom) versus α-Ta (1.032 states/eV/atom) that according to Mott’s law should decrease the conductivity in metals and affect the stability by charge redistribution in the valence band. This is experimentally supported from resistivity measurements of the film yielding a value of ∼170 μΩ cm in comparison to α-Ta bulk with a reported value of ∼13.1 μΩ cm.

Valence and core-level photoelectron spectroscopy studies of Fe2P: Effect of P segregation on the surface electronic structure

The effect of P segregation on the electronic structure of Fe2P has been studied by soft X-ray photoelectron spectroscopy (PES). As the Fe2P surface was sputtered by Ar+ ion (2 kV, 10 min), the amount of P atoms in the surface region was reduced. The amount of surface P atoms was further reduced by annealing the sputtered surface at elevated annealing temperatures up to about 400 °C, and it was increased by annealing at temperatures higher than 400 °C. It was deduced from the analyses of the P 2p core-level PES spectra that desorption of P atoms weakly bonded to the surface, which are thought to be formed through the sputtering process, proceeded when the surface was annealed at temperatures lower than 500 °C. On the other hand, it was also deduced from the PES study that the segregation of bulk P atoms toward the surface proceeded when the annealing temperature was higher than 300 °C. These two processes were competing at 300–500 °C, leading to the minimum amount of surface P atoms at approximately 400 °C. The PES spectra were measured after annealing the sputtered surface at various temperatures, and it was found that the stabilization of the 3d states of surface Fe atoms due to the bonding with surrounding P atoms was ineffective irrespective of the amount of P atoms on Fe2P.

Mapping Valence Band and Interface Electronic Structure Changes during the Oxidation of Mo to MoO3 via MoO2 and MoO3 Reduction to MoO2: A NAPPES Study

Tuning the surface energetics, especially work function (ϕ) of the materials, is of a great deal of interest for a wide range of surface- and interface-based devices and applications. How the ϕ of a solid surface changes under the reaction conditions is of paramount interest to the chemists, particularly in the areas of surface-dependent phenomena such as, catalysis and electrochemistry. In the present study, by using the valence band and core-level photoelectron spectroscopy, surface electronic changes from Mo to MoO3 via MoO2 was studied under relevant near-ambient pressure (NAP) and high temperature conditions. A very significant change in ϕ from Mo to MoO3 was observed and it is well corroborated with the changes in gas-phase vibrational features of O2 in both near-ambient pressure ultraviolet photoelectron spectra (NAPUPS) as well in NAP X-ray photoelectron spectroscopy. Reversible changes in the electronic structure is observed when MoO3 was reduced in H2 to MoO2. On the basis of the extent of oxida...

Tuning of magnetic behaviour in nitrogenated graphene oxide functionalized with iron oxide

Two-dimensional graphene oxide (GO) nanomaterials offer interesting physical/chemical properties and have been explored for potential use in electronics, magnetic, catalysis, and energy storage applications, because of its high charge-carrier mobility and high specific surface area. This study investigates the electronic and magnetic properties of nitrogenated graphene oxide (GO-Nx), and nitrogenated graphene oxide functionalized with iron oxide (GO-Nx:Fe). Four different nitrogen (N) precursor viz. ammonium hydroxide (NH4OH), hexamethylenetetramine (C6H12N4), acetonitrile (C2H3N), and carbamide (CH4N2O) are used to synthesis “GO-Nx” using chemical route. As C6H12N4 based-GO-Nx shows higher ferromagnetism, thus it is further functionalized with iron oxide using three different iron (Fe) precursors viz. ferric oxide (Fe2O3), ferroso-ferric oxide (Fe3O4), and iron oxide-hydroxide (FeOOH) to tune this room temperature ferromagnetism (RTFM). The electronic structure of “GO-Nx” and “GO-Nx:Fe” are characterized using C 1s, O 1s, N 1s and Fe 2p core-level X-ray photoelectron spectroscopy (XPS) and their corresponding magnetic behaviours are correlated with the SQUID-measured M-H loops. The magnetic moment changes due to the conversion and formation of different nitrogen bonded carbon along with the different phase of iron oxide. The tuning of magnetization in “GO-Nx” and “GO-Nx:Fe” using different N-precursors and/or Fe-precursors is an efficient route to tailor the electronic/magnetic properties of graphene-oxide for different electronic and magnetic device applications.

Time-resolved electron spectroscopy for chemical analysis of photodissociation: Photoelectron spectra of Fe(CO)5, Fe(CO)4, and Fe(CO)3.

The prototypical photoinduced dissociation of Fe(CO)5 in the gas phase is used to test time-resolved x-ray photoelectron spectroscopy for studying photochemical reactions. Upon one-photon excitation at 266 nm, Fe(CO)5 successively dissociates to Fe(CO)4 and Fe(CO)3 along a pathway where both fragments retain the singlet multiplicity of Fe(CO)5. The x-ray free-electron laser FLASH is used to probe the reaction intermediates Fe(CO)4 and Fe(CO)3 with time-resolved valence and core-level photoelectron spectroscopy, and experimental results are interpreted with ab initio quantum chemical calculations. Changes in the valence photoelectron spectra are shown to reflect changes in the valence-orbital interactions upon Fe-CO dissociation, thereby validating fundamental theoretical concepts in Fe-CO bonding. Chemical shifts of CO 3σ inner-valence and Fe 3p core-level binding energies are shown to correlate with changes in the coordination number of the Fe center. We interpret this with coordination-dependent charge localization and core-hole screening based on calculated changes in electron densities upon core-hole creation in the final ionic states. This extends the established capabilities of steady-state electron spectroscopy for chemical analysis to time-resolved investigations. It could also serve as a benchmark for how charge and spin density changes in molecular dissociation and excited-state dynamics are expressed in valence and core-level photoelectron spectroscopy.

Evolution of optical properties and band structure from amorphous to crystalline Ga2O3 films

The optical properties and band structure evolution from amorphous to crystalline Ga2O3 films was investigated in this work. Amorphous and crystalline Ga2O3 films were obtained by changing the growth substrate temperatures of pulsed laser deposition and the crystallinity increase with the rising of substrate temperature. The bandgap value and ultraviolet emission intensity of the films increase with the rising of crystallinity as observed by means of spectrophotometer and cathodoluminescence spectroscopy. Abrupt bandgap value and CL emission variations were observed when amorphous to crystalline transition took place. X-ray photoelectron spectroscopy core level spectra reveal that more oxygen vacancies and disorders exist in amorphous Ga2O3 film grown at lower substrate temperature. The valence band spectra of hard X-ray photoelectron spectroscopy present the main contribution from Ga 4sp for crystalline film deposited at substrate temperature of 500 oC, while extra subgap states has been observed in amorphous film deposited at 300 oC. The oxygen vacancy and the extra subgap density of states are suggested to be the parts of origin of bandgap and CL spectra variations. The experimental data above yields a realistic picture of optical properties and band structure variation for the amorphous to crystalline transition of Ga2O3 films.

First layer water phases on anatase TiO2(101)

The anatase TiO2(101) surface and its interaction with water is an important topic in oxide surface chemistry. Firstly, it benchmarks the properties of the majority facet of TiO2 nanoparticles and, secondly, there is a controversy as to whether the water molecule adsorbs intact or deprotonates. We have addressed the adsorption of water on anatase TiO2(101) by synchrotron radiation photoelectron spectroscopy. Three two-dimensional water structures are found during growth at different temperatures: at 100 K, a metastable structure forms with no hydrogen bonding between the water molecules. In accord with prior literature, we assign this phase to chains of disordered molecules. Growth 160 K results in a metastable structure with expressed hydrogen bonding between the water molecules. At 190 K, the water molecules become disordered as the thermal energy is too high and hence the hydrogen bonds break. The result is a structure with isolated monomers. Partial dissociation is observed for all three growths, with the molecular state only slightly favored in energy (20–40 meV) over the dissociated state. Heating of a thick film leads to more dissociation compared to a bilayer, when formed at 100 K. Thus, extending the water network facilitates proton transport and hence dissociation. The results reconcile apparent conflicting experimental results previously obtained by scanning tunneling microscopy (STM) and core level photoelectron spectroscopy.

Peculiar bonding associated with atomic doping and hidden honeycombs in borophene

Engineering atomic-scale structures allows great manipulation of physical properties and chemical processes for advanced technology. We show that the B atoms deployed at the centers of honeycombs in boron sheets, borophene, behave as nearly perfect electron donors for filling the graphitic $\sigma$ bonding states without forming additional in-plane bonds by first-principles calculations. The dilute electron density distribution owing to the weak bonding surrounding the center atoms provides easier atomic-scale engineering and is highly tunable via in-plane strain, promising for practical applications, such as modulating the extraordinarily high thermal conductance that exceeds the reported value in graphene. The hidden honeycomb bonding structure suggests an unusual energy sequence of core electrons that has been verified by our high-resolution core-level photoelectron spectroscopy measurements. With the experimental and theoretical evidence, we demonstrate that borophene exhibits a peculiar bonding structure and is distinctive among two-dimensional materials.

Quasi free-standing epitaxial graphene fabrication on 3C–SiC/Si(111)

Growing graphene on SiC thin films on Si is a cheaper alternative to the growth on bulk SiC, and for this reason it has been recently intensively investigated. Here we study the effect of hydrogen intercalation on epitaxial graphene obtained by high temperature annealing on 3C–SiC/Si(111) in ultra-high vacuum. By using a combination of core-level photoelectron spectroscopy, low energy electron diffraction, and near-edge x-ray absorption fine structure (NEXAFS) we find that hydrogen saturates the Si atoms at the topmost layer of the substrate, leading to free-standing graphene on 3C–SiC/Si(111). The intercalated hydrogen fully desorbs after heating the sample at 850 °C and the buffer layer appears again, similar to what has been reported for bulk SiC. However, the NEXAFS analysis sheds new light on the effect of hydrogen intercalation, showing an improvement of graphene’s flatness after annealing in atomic H at 600 °C. These results provide new insight into free-standing graphene fabrication on SiC/Si thin films.

The adsorption of silicon on an iridium surface ruling out silicene growth.

The adsorption of Si atoms on a metal surface might proceed through complex surface processes, whose rate is determined differently by factors such as temperature, Si coverage, and metal cohesive energy. Among other transition metals, iridium is a special case since the Ir(111) surface was reported first, in addition to Ag(111), as being suitable for the epitaxy of silicene monolayers. In this study we followed the adsorption of Si on the Ir(111) surface via high resolution core level photoelectron spectroscopy, starting from the clean metal surface up to a coverage exceeding one monolayer, in a temperature range between 300 and 670 K. Density functional theory calculations were carried out in order to evaluate the stability of the different Si adsorption configurations as a function of the coverage. Results indicate that, at low coverage, the Si adatoms tend to occupy the hollow Ir sites, although a small fraction of them penetrates the first Ir layer. Si penetration of the Ir surface can take place if the energy gained upon Si adsorption is used to displace the Ir surface atoms, rather then being dissipated differently. At a Si coverage of ∼1 monolayer, the Ir 4f spectrum indicates that not only the metal surface but also the layers underneath are perturbed. Our results point out that the Si/Ir(111) interface is unstable towards Si-Ir intermixing, in agreement with the silicide phase formation reported in the literature for the reverted interface.

Reference binding energies of transition metal carbides by core-level x-ray photoelectron spectroscopy free from Ar+ etching artefacts

We report x-ray photoelectron spectroscopy (XPS) core level binding energies (BE’s) for the widely-applicable groups IVb-VIb transition metal carbides (TMCs) TiC, VC, CrC, ZrC, NbC, MoC, HfC, TaC, and WC. Thin film samples are grown in the same deposition system, by dc magnetron co-sputtering from graphite and respective elemental metal targets in Ar atmosphere. To remove surface contaminations resulting from exposure to air during sample transfer from the growth chamber into the XPS system, layers are either (i) Ar+ ion-etched or (ii) UHV-annealed in situ prior to XPS analyses. High resolution XPS spectra reveal that even gentle etching affects the shape of core level signals, as well as BE values, which are systematically offset by 0.2–0.5 eV towards lower BE. These destructive effects of Ar+ ion etch become more pronounced with increasing the metal atom mass due to an increasing carbon-to-metal sputter yield ratio. Systematic analysis reveals that for each row in the periodic table (3d, 4d, and 5d) C 1s BE increases from left to right indicative of a decreased charge transfer from TM to C atoms, hence bond weakening. Moreover, C 1s BE decreases linearly with increasing carbide/metal melting point ratio. Spectra reported here, acquired from a consistent set of samples in the same instrument, should serve as a reference for true deconvolution of complex XPS cases, including multinary carbides, nitrides, and carbonitrides.

Individual Atomic Imaging of Multiple Dopant Sites in As-Doped Si Using Spectro-Photoelectron Holography.

The atomic scale characterization of dopant atoms in semiconductor devices to establish correlations with the electrical activation of these atoms is essential to the advancement of contemporary semiconductor process technology. Spectro-photoelectron holography combined with first-principles simulations can determine the local three-dimensional atomic structures of dopant elements, which in turn affect their electronic states. In the work reported herein, this technique was used to examine arsenic (As) atoms doped into a silicon (Si) crystal. As 3d core level photoelectron spectroscopy demonstrated the presence of three types of As atoms at a total concentration of approximately 1020 cm-3, denoted as BEH, BEM, and BEL. On the basis of Hall effect measurements, the BEH atoms corresponded to electrically active As occupying substitutional sites and exhibiting larger thermal fluctuations than the Si atoms, while the BEM atoms corresponded to electrically inactive As embedded in the AsnV (n = 2-4) type clusters. Finally, the BEL atoms were assigned to electrically inactive As in locally disordered structures.

Amorphous carbon nanocomposite films doped by titanium: Surface and sub-surface composition and bonding

Hydrogen free Ti-doped amorphous carbon layers were prepared by dual beam pulsed laser deposition using two excimer lasers. The air-exposed surfaces were analyzed by high-energy resolved and angular-resolved core-level photoelectron spectroscopy, and were then step-by-step sputtered with an argon gas cluster ion beam (ArGCIB), which is known to be a very gentle technique with respect to changes in surface chemistry. The results show that the top surface of the sample and its sub-surface region differ in composition and in bonding. The top surface is enriched by oxygen-bearing species. Carbon-bearing species located on the surface are mostly in sp3 hybridization. Titanium carbide clusters, TiC, are not directly exposed at the surface. They are embedded in a carbon network with dominating C sp2 hybridization. Their interface is formed by a distinct carbon-rich titanium carbide with stoichiometry close to TiC3. The surface damage induced by ArGCIB was found to be minimal, verifiably affecting carbon atoms in the carbon network.

Electronic properties and bonding in ZrHx thin films investigated by valence-band x-ray photoelectron spectroscopy

The electronic structure and chemical bonding in reactively magnetron sputtered ZrHx (x=0.15, 0.30, 1.16) thin films with oxygen content as low as 0.2 at% are investigated by 4d valence band, shallow 4p core-level and 3d core-level X-ray photoelectron spectroscopy. With increasing hydrogen content, we observe significant reduction of the 4d valence states close to the Fermi level as a result of redistribution of intensity towards the H 1s - Zr 4d hybridization region at about 6 eV below the Fermi level. For low hydrogen content (x=0.15, 0.30), the films consist of a superposition of hexagonal closest packed metal (alpha-phase)and understoichiometric delta-ZrHx (CaF2-type structure) phases, while for x=1.16, the film form single phase ZrHx that largely resembles that of stoichiometric delta-ZrH2 phase. We show that the cubic delta-ZrHx phase is metastable as thin film up to x=1.16 while for higher H-contents, the structure is predicted to be tetragonally distorted. For the investigated ZrH1.16 film, we find chemical shifts of 0.68 and 0.51 eV towards higher binding energies for the Zr 4p3/2 and 3d5/2 peak positions, respectively. Compared to the Zr metal binding energies of 27.26 and 178.87 eV, this signifies a charge-transfer from Zr to H atoms. The change in the electronic structure, spectral line shapes, and chemical shifts as function of hydrogen content is discussed in relation to the charge-transfer from Zr to H that affects the conductivity by charge redistribution in the valence band.

Density Functional Theory Modeling the Interfacial Chemistry of the LiNO3 Additive for Lithium–Sulfur Batteries by Means of Simulated Photoelectron Spectroscopy

Lithium–sulfur (Li–S) batteries are considered candidates for next-generation energy storage systems due to their high theoretical specific energy. There exist, however, some shortcomings of these batteries, not least the solubility of intermediate polysulfides into the electrolyte generating a so-called “redox shuttle”, which gives rise to self-discharge. LiNO3 is therefore frequently used as an electrolyte additive to help suppress this mechanism, but the exact nature of the LiNO3 functionality is still unclear. Here, density functional theory calculations are used to investigate the electronic structure of LiNO3 and a number of likely species (N2, N2O, LiNO2, Li3N, and Li2N2O2) resulting from the reduction of this additive on the surface of Li metal anode. The N 1s X-ray photoelectron spectroscopy core level binding energies of these molecules on the surface are calculated in order to compare the results with experimentally reported values. The core level shifts (CLS) of the binding energies are studie...

Partially Reversible Photoinduced Chemical Changes in a Mixed-Ion Perovskite Material for Solar Cells.

Metal halide perovskites have emerged as materials of high interest for solar energy-to-electricity conversion, and in particular, the use of mixed-ion structures has led to high power conversion efficiencies and improved stability. For this reason, it is important to develop means to obtain atomic level understanding of the photoinduced behavior of these materials including processes such as photoinduced phase separation and ion migration. In this paper, we implement a new methodology combining visible laser illumination of a mixed-ion perovskite ((FAPbI3)0.85(MAPbBr3)0.15) with the element specificity and chemical sensitivity of core-level photoelectron spectroscopy. By carrying out measurements at a synchrotron beamline optimized for low X-ray fluxes, we are able to avoid sample changes due to X-ray illumination and are therefore able to monitor what sample changes are induced by visible illumination only. We find that laser illumination causes partially reversible chemistry in the surface region, including enrichment of bromide at the surface, which could be related to a phase separation into bromide- and iodide-rich phases. We also observe a partially reversible formation of metallic lead in the perovskite structure. These processes occur on the time scale of minutes during illumination. The presented methodology has a large potential for understanding light-induced chemistry in photoactive materials and could specifically be extended to systematically study the impact of morphology and composition on the photostability of metal halide perovskites.

Metallic Cobalt to Spinel Co3O4—Electronic Structure Evolution by Near-Ambient Pressure Photoelectron Spectroscopy

In the present study, valence band (VB) and core level photoelectron spectroscopy was carried out to investigate the electronic structural changes from polycrystalline Co to spinel Co3O4, via CoO at near ambient pressures (NAP; ∼0.1 mbar). O2–Co and H2–CoOx gas–solid oxidative and reductive interactions, respectively, have been explored with UV photons (He–I) or low kinetic energy electrons (≤16 eV) under NAP conditions. Typical VB features of Co metal, CoO, Co3O4, and a mixed phase between any two adjacent features were observed and well corroborated with core level changes. Very significant and characteristic changes were observed with Co 3d features in the VB for each stage from Co oxidation to Co3O4 as well as Co3O4 reduction to CoO. Co3O4 and CoO can be reversibly obtained by alternating the conditions between 0.1 mbar of H2 at 650 K and 0.1 mbar of O2 at 400 K, respectively. A meaningful correlation is observed between the changes in work function with cation oxidation state; small changes in the st...

Selenidation of epitaxial silicene on ZrB2

Abstract The deposition of elemental Se on epitaxial silicene on ZrB 2 thin films was investigated with synchrotron-based core-level photoelectron spectroscopy and low-energy electron diffraction. The deposition of Se at room temperature caused the appearance of Si 2 p peaks with chemical shifts of n × 0.51 ± 0.04 eV (n = 1-4), suggesting the formation of SiSe 2 . This shows that capping the silicene monolayer, without affecting its structural and electronic properties, is not possible with Se. The annealing treatments that followed caused the desorption of Se and Si, resulting in the etching of the Si atoms formerly part of the silicene layer, and the formation of bare ZrB 2 (0001) surface area. In addition, a ZrB 2 (0001)-(√7 × 3)R40.9° surface reconstruction was observed, attributed to a Se-termination of the surface of the transition metal diboride thin film.