High-energy Photoelectron Spectroscopy Research Articles

HAXPES reference spectra of bulk Mo and MoSe2 with Cr Kα excitation

Monochromatic Cr Kα radiation (5414.8 eV) was used to acquire high-energy photoelectron spectroscopy (HAXPES) data on pure Mo and bulk MoSe2 compound with special attention paid to binding energy scale correction and quantification. The reported spectra include a survey scan and high-resolution Mo 2s, Mo 2p1/2, Mo 2p3/2, Mo 3d, Mo 3p1/2, Mo 3p3/2, Mo 3s, Mo 4s, Mo4p, Se2s, Se 2p1/2, Se 2p3/2, Se 3s, Se 3p1/2, Se 3p3/2, and Se 3d core-levels. The data will be useful as reference core-level spectra for HAXPES studies on molybdenum and its compounds.

HAXPES reference spectra of bulk W and WSe2 with Cr Kα excitation

Monochromatic Cr Kα radiation (5414.8 eV) was used to acquire high-energy photoelectron spectroscopy data on pure W and bulk WSe2 compound. The reported spectra include a survey scan and high-resolution W 3p1/2, W 3p3/2, W 3d3/2, W 3d5/2, W 4s, W 4p1/2, W 4p3/2, W 4d, W 4f, Se 2s, Se 2p1/2, Se 2p3/2, Se 3s, and Se 3p core-levels. The data will be useful as reference core-level spectra for HAXPES studies on tungsten and its compounds.

High-energy photoelectron spectroscopy of AlN with Cr Kα excitation

Aluminum nitride was analyzed by high-energy photoelectron spectroscopy using monochromatic Cr Kα (5414.8 eV) radiation. The reported data include a survey spectrum and high-resolution spectra of Al 1s, Al 2s, Al 2p, N 1s, O 1s, and C 1s core levels.

High-energy photoelectron spectroscopy of Si(100) with Cr Kα excitation

Si (100) wafer was analyzed by high-energy photoelectron spectroscopy (HAXPES) using monochromatic Cr Kα radiation (5414.8 eV). The HAXPES spectra reported here include a survey scan and high-resolution spectra of Si 1s, Si 2s, and Si 2p core-levels.

High-energy photoelectron spectroscopy of Si3N4 thin film on Si with Cr Kα excitation

Si3N4 thin film grown by low-pressure chemical vapor deposition was measured by high-energy photoelectron spectroscopy using monochromatic Cr Kα (5414.8 eV) radiation. A survey scan and high-resolution spectra of Si 1s, Si 2s, Si 2p, and N 1s are reported.

High-energy photoelectron spectroscopy of 6H-SiC wafer with Cr Kα excitation

A C-terminated SiC wafer was analyzed by high-energy photoelectron spectroscopy using monochromatic Cr Kα (5414.8 eV) radiation. The data include a survey scan and high-resolution spectra of Si 1s, Si 2s, Si 2p, C 1s, and O 1s core levels.

High-energy photoelectron spectroscopy of Al with Cr Kα excitation

Monochromatic Cr Kα radiation (5414.8 eV) was used to acquire high-energy photoelectron spectroscopy data on pure Al. The reported spectra include a survey scan and high-resolution Al 1s, Al 2s, Al 2p, and O 1s core-levels. The data will be useful as a comparison for the study in this field.

Fabrication and Study of the Electronic Structure of МоO3/Мо Nanofilms

The composition, the surface morphology, and the crystalline and electronic structure of МоО3 nanofilms are studied using a set of methods, namely, scanning electron spectroscopy and high energy electron diffraction, photoelectron spectroscopy, and secondary electron emission. These nanofilms are obtained by implanting $${\text{O}}_{{\text{2}}}^{{\text{ + }}}$$ ions into a Mo single crystal heated to Т = 850 K. Films with different thicknesses (~30, 60, and 90 A) are obtained at an ion energy of 1–5 keV and a dose of D = (4–8) × 1017 cm–2. It is shown that a continuous and homogeneous polycrystalline МоО3 film with a surface roughness of at most 1.5 nm is formed. The band gaps of these films are ~3.4 eV, and the widths of the conduction bands are 4.5 eV. It is discovered that there are four maxima of the density of electronic states in the valence band; probably, they are due to hybridization of the N5, N45, N4 energy levels of Mo with the L2, L23, and L3 energy levels of oxygen.

Identifying the nature of surface chemical modification for directed self-assembly of block copolymers.

In recent years, block copolymer lithography has emerged as a viable alternative technology for advanced lithography. In chemical-epitaxy-directed self-assembly, the interfacial energy between the substrate and each block copolymer domain plays a key role on the final ordering. Here, we focus on the experimental characterization of the chemical interactions that occur at the interface built between different chemical guiding patterns and the domains of the block copolymers. We have chosen hard X-ray high kinetic energy photoelectron spectroscopy as an exploration technique because it provides information on the electronic structure of buried interfaces. The outcome of the characterization sheds light onto key aspects of directed self-assembly: grafted brush layer, chemical pattern creation and brush/block co-polymer interface.

The crystal monochromator beamline KMC-1 at BESSY II

The KMC-1 is a soft x-ray double crystal monochromator beamline for the energy range between 2 and 12 keV. The bending magnet beamline as well as the experiment are under UHV-condition. It incorporates high indexed Si-crystals for high resolution and it is primarily used for HAXPES experiments employing the HIKE (High Kinetic Energy Photoelectron Spectroscopy) chamber.

Characterization of Platinum and Iridium Oxyhydrate Surface Layers from Platinum and Iridium Foils.

Platinum and iridium polycrystalline foils were oxidized electrochemically through anodization to create thin platinum and iridium hydrous oxide layers, which were analyzed through laboratory photoelectron spectroscopy during heating and time series (temperature-programmed spectroscopy). The films contain oxygen in the form of bound oxides, water, and hydroxides and were investigated by depth profiling with high-energy photoelectron spectroscopy. The Pt films are unstable and begin to degrade immediately after removal from the electrolyte to form core-shell structures with a metallic inner core and a hydrous oxide outer shell almost devoid of Pt. However, evidence was found for metastable intermediate states of degradation; therefore, it may be possible to manufacture PtOx phases with increased stability. Heating the film to even 100 °C causes accelerated degradation, which shows that stoichiometric oxides such as PtO2 or PtO are not the active species in the electrolyte. The Ir films exhibit increased stability and higher surface Ir content, and gentle heating at low temperatures leads to a decrease in defect density. Although both layers are based on noble metals, their surface structures are markedly different. The complexity of such hydrous oxide systems is discussed in detail with the goal of identifying the film composition more precisely.

Electron trajectory simulations of time-of-flight spectrometers for core level high-energy photoelectron spectroscopy at pulsed X-ray sources

The advent of Free Electron Lasers (FELs), able to provide short (2–100fs) and intense (1033 photons/s/mm2/mrad2/0.1%bandwidth) pulses of light also in the hard X-ray regime (ℏω> 2000eV), opens new possibilities to study the ultrafast dynamics of processes, exploiting the capability of Hard X-ray Photoelectron Spectroscopy (HAXPES) to measure core-level spectra of elements with bulk sensitivity. In order to detect the intense bursts of high kinetic energy electrons generated by the X-ray pulses with an energy resolution comparable to the existing category of electron analyzers, a new class of spectrometers must be designed. We present a characterization of two different TOF spectrometers, namely one based on a retarding cylindrical lens and another one based on the spherical reflector geometry. SIMION® software has been used in order to evaluate electron trajectories of high kinetic energy electrons (5000–10,000eV) and extract transmission properties, angular acceptance and energy resolution. It resulted that while the linear system is able to accept a larger solid angle (∼50msr), the spherical mirror offers a better resolving power (around 71,000). Both analyzers are capable of a transmission above 90% within range of kinetic energies wide enough to measure the full line-shape of a core photoionization peak. Furthermore, we proved that both instruments are able to discriminate between two consecutive electron bunches having a temporal separation inferior than 220ns, which is the distance between two consecutive photon pulses at the European X-ray Free Electron Laser (EXFEL), which is currently under construction in Hamburg.

Distinct electronic structure of the electrolyte gate-induced conducting phase in vanadium dioxide revealed by high-energy photoelectron spectroscopy.

The development of new phases of matter at oxide interfaces and surfaces by extrinsic electric fields is of considerable significance both scientifically and technologically. Vanadium dioxide (VO2), a strongly correlated material, exhibits a temperature-driven metal-to-insulator transition, which is accompanied by a structural transformation from rutile (high-temperature metallic phase) to monoclinic (low-temperature insulator phase). Recently, it was discovered that a low-temperature conducting state emerges in VO2 thin films upon gating with a liquid electrolyte. Using photoemission spectroscopy measurements of the core and valence band states of electrolyte-gated VO2 thin films, we show that electronic features in the gate-induced conducting phase are distinct from those of the temperature-induced rutile metallic phase. Moreover, polarization-dependent measurements reveal that the V 3d orbital ordering, which is characteristic of the monoclinic insulating phase, is partially preserved in the gate-induced metallic phase, whereas the thermally induced metallic phase displays no such orbital ordering. Angle-dependent measurements show that the electronic structure of the gate-induced metallic phase persists to a depth of at least ∼40 Å, the escape depth of the high-energy photoexcited electrons used here. The distinct electronic structures of the gate-induced and thermally induced metallic phases in VO2 thin films reflect the distinct mechanisms by which these states originate. The electronic characteristics of the gate-induced metallic state are consistent with the formation of oxygen vacancies from electrolyte gating.

Bulk electronic structure studied by hard X-ray photoelectron spectroscopy of the valence band: The case of intermetallic compounds

Photoelectron spectroscopy (PES) has evolved into the most relevant, powerful, and nondestructive method for investigating atoms, molecules, and solids. In particular, hard X-ray photoelectron spectroscopy (HAXPES) has emerged as a powerful tool for investigating the bulk electronic structure of materials in a variety of applied fields such as chemistry, physics, and materials science. In addition, PES was used for investigating the symmetries of various materials’ electronic structures. However, thus far, such studies have been restricted to atoms, molecules, adsorbates, and surfaces because low-energy (<1keV) electrons have limited probing depths. This is disadvantageous because three-dimensional (3D) bulk states cannot be studied. The present work demonstrates that this drawback can be eliminated by using hard X-rays with variable polarization for excitation. In the current study, this issue was investigated using several Heusler compounds, which have been attracting increasing levels of interest. There are more than 2000 Heusler compounds in total. Owing to their tunable electronic structures, Heusler compounds exhibit multifarious properties useful for spintronic, optoelectronic, shape memory, and thermoelectric applications. Herein, we report the results of bulk-sensitive, high energy photoelectron spectroscopy of the valence bands of several Heusler compounds for various applications. It is shown that the measured valence band spectra are clearly resolved and are in good agreement with the first-principles calculations of the compounds’ electronic structures.

Calculations of mean escape depths of photoelectrons in elemental solids excited by linearly polarized X-rays for high-energy photoelectron spectroscopy

We have calculated mean escape depths (MEDs, D) of photoelectrons from Si, Cu, and Au excited by linearly polarized X-rays over the 50–10,000eV energy range for high-energy photoelectron spectroscopy. These calculations were done with Monte Carlo methods using the values of electron inelastic mean free paths λ that were calculated by the relativistic full Penn algorithm and the Dirac–Hartree–Fock atomic potential for elastic scattering of electrons. In order to know the dependence of MED on the asymmetry parameter β and the emission angle θ, we calculated MEDs for β=−1 to β=2 (by 0.5 steps) and θ=0° to θ=80° (with respect to the surface normal). The resulting MED values increase as the β values decrease over the 50–10,000eV energy range. We also found that the values of D/λcosθ are approximately constant to within 10% for a given value of θ and for energies over 1000eV (except for the condition of β=−1) over a range of emission angles (typically 0–40° or 60°). We also found that the energy dependence of the ratios D/λcosθ at emission angle θ=0° can be expressed by the Jablonski–Powell predictive equation over the 50–10,000eV energy range.Finally, we have obtained a predictive MED equation that could be applied to the wide range of experimental conditions that are used in high-energy XPS with synchrotron radiation.

High energy photoelectron spectroscopy in basic and applied science: Bulk and interface electronic structure

With the access of new high-performance electron spectrometers capable of analyzing electron energies up to the order of 10keV, the interest for photoelectron spectroscopy has grown and many new applications of the technique in areas where electron spectroscopies were considered to have limited use have been demonstrated over the last few decades. The technique, often denoted hard X-ray photoelectron spectroscopy (HX-PES or HAXPES), to distinguish the experiment from X-ray photoelectron spectroscopy performed at lower energies, has resulted in an increasing interest in photoelectron spectroscopy in many areas. The much increased mean free path at higher kinetic energies, in combination with the elemental selectivity of the core level spectroscopies in general has led to this fact. It is thus now possible to investigate the electronic structure of materials with a substantially enhanced bulk sensitivity. In this review we provide examples from our own research using HAXPES which to date has been performed mainly at the HIKE facility at the KMC-1 beamline at HZB, Berlin. The review exemplifies the new opportunities using HAXPES to address both bulk and interface electronic properties in systems relevant for applications in magnetic storage, energy related research, but also in purely curiosity driven problems.

Relative sub-shell photoionization cross-sections of nickel metal determined by hard X-ray high kinetic energy photoemission

Recently, hard X-ray high kinetic energy photoelectron spectroscopy has lead to a break-through due to its non destructive way of investigating the bulk electronic properties of materials. However, due to the relatively new development of this technique there is a lack of information concerning the photoionization cross sections at high energies. Whenever compound materials are investigated or when estimating signal levels and the feasibility of an electron spectroscopy experiment the knowledge of cross sections is essential. In the present work the experimentally determined relative sub-shell photoionization cross sections of shallow levels of nickel metal in the energy range of 2–9keV will be shown. The data are compared with calculated sub-shell photoionization cross sections.

Electronic structures of the FeSe superconductor studied by high-energy photoelectron spectroscopy

The bulk electronic structures of an iron-based superconductor FeSe have been investigated by photoelectron and photoabsorption spectroscopies (PES and XAS) with use of high-energy photons (hν ≊ 0.7–0.9 and 8 keV). The core-level PES and XAS combined with the configuration interaction cluster model calculation reveal the highly coherent nature of the Fe 3d electrons which is realized by the hybridization between Fe 3d and Se 4p states.

Energy level alignment in TiO 2/dipole-molecule/P3HT interfaces

Controlling the energy levels at the interface between an inorganic and an organic material is of importance to improve the properties in devices based on such hybrid interfaces, and can be obtained by the incorporation of dipole molecules between the materials. In this report interfaces containing TiO 2, a dipole molecule (benzoic acid or 4-nitrobenzoic acid) and a polymer, poly(3-hexylthiophene) (P3HT) were investigated using high kinetic energy photoelectron spectroscopy. We could successfully measure through all materials in the fully assembled systems, and thereby experimentally quantify the dipole induced change in the energy level alignment of the polymer and the TiO 2.

Internal Structure of InP/ZnS Nanocrystals Unraveled by High-Resolution Soft X-ray Photoelectron Spectroscopy

High-energy resolution photoelectron spectroscopy (DeltaE < 200 meV) is used to investigate the internal structure of semiconductor quantum dots containing low Z-contrast elements. In InP/ZnS core/shell nanocrystals synthesized using a single-step procedure (core and shell precursors added at the same time), a homogeneously alloyed InPZnS core structure is evidenced by quantitative analysis of their In3d(5/2) spectra recorded at variable excitation energy. When using a two-step method (core InP nanocrystal synthesis followed by subsequent ZnS shell growth), XPS analysis reveals a graded core/shell interface. We demonstrate the existence of In-S and S(x)-In-P(1-x) bonding states in both types of InP/ZnS nanocrystals, which allows a refined view on the underlying reaction mechanisms.