Resonant Photoemission Research Articles

Resonant photoemission spectroscopy of atomic layer Fe2N on Cu(111) with continuous angular rotation of linearly polarized light

Linearly polarized soft x-rays provide information about electronic or magnetic anisotropy through absorption into materials or generation of photoelectrons. In order to change the relative angle between linear polarization and sample crystalline axes, either x-ray polarization or the sample needs to be rotated. Due to difficulties of polarization control in the soft x-ray range, a conventional approach was to rotate the sample. However, this method is not compatible, for example, with operando measurements on non-uniform samples where sample size and rotational motion are severely restricted. At BL07LSU of SPring-8, we developed a new method to rotate the linear polarization angle using a segmented cross undulator. We report an application of this linear polarization rotation to resonant photoemission spectroscopy on an magnetic atomic layer Fe2N on Cu(111) to probe the electronic anisotropy of the 3d states in the vicinity of the Fermi level.

The Metal-Oxide Nanoparticle-Aqueous Solution Interface Studied by Liquid-Microjet Photoemission.

ConspectusThe liquid-microjet technique combined with soft X-ray photoelectron spectroscopy (PES) has become an exceptionally powerful experimental tool to investigate the electronic structure of liquid water and nonaqueous solvents and solutes, including nanoparticle (NP) suspensions, since its first implementation at the BESSY II synchrotron radiation facility 20 years ago. This Account focuses on NPs dispersed in water, offering a unique opportunity to access the solid-electrolyte interface for identifying interfacial species by their characteristic photoelectron spectral fingerprints. Generally, the applicability of PES to a solid-water interface is hampered due to the small mean free path of the photoelectrons in solution. Several approaches have been developed for the electrode-water system and will be reviewed briefly. The situation is different for the NP-water system. Our experiments imply that the transition-metal oxide (TMO) NPs used in our studies reside close enough to the solution-vacuum interface that electrons emitted from the NP-solution interface (and from the NP interior) can be detected.We were specifically exploring aqueous-phase TMO NPs that have a high potential for (photo)electrocatalytic applications, e.g., for solar fuel generation. The central question we address here is how H2O molecules interact with the respective TMO NP surface. Liquid-microjet PES experiments, performed from hematite (α-Fe2O3, iron(III) oxide) and anatase (TiO2, titanium(IV) oxide) NPs dispersed in aqueous solutions, exhibit sufficient sensitity to distinguish between free bulk-solution water molecules and those adsorbed at the NP surface. Moreover, hydroxyl species resulting from dissociative water adsorption can be identified in the photoemission spectra. An important aspect is that in the NP(aq) system the TMO surface is in contact with a true extended bulk electrolyte solution rather than with a few monolayers of water, as is the case in experiments using single-crystal samples. This has a decisive effect on the interfacial processes that can occur since NP-water interactions can be uniquely investigated as a function of pH and provides an environment allowing for unhindered proton migration. Our studies confirm that water is dissociatively adsorbed at the hematite surface and molecularly adsorbed at the TiO2 NP surface at low pH. In contrast, at near-basic pH the water interaction is dissociative at the TiO2 NP surface.The liquid-microjet measurements presented here also highlight the multiple aspects of photoemission necessary for a full characterization of TMO nanoparticle surfaces in aqueous environments. For instance, we exploit the ability to increase species-specific electron signals via resonant photoemission, so-called partial electron yield X-ray absorption (PEY-XA) spectra, and from valence photoelectron and resonant Auger-electron spectra. We also address the potential of these resonance processes and the associated ultrafast electronic relaxations for determining charge transfer or electron delocalization times, e.g., from Fe3+ located at the hematite nanoparticle interface into the aqueous-solution environment.

Electronic Structure of Orthorhombic Bi4V2O11 and Role of Gap States in Photoelectrochemical Water Splitting

A group of bismuth vanadium oxides shows promise as an excellent material for photoanodes in water splitting. However, the efficiency of Bi4V2O11 in water splitting is not satisfactory compared to BiVO4. In this study, we investigated the electronic structure of orthorhombic Bi4V2O11 using density functional theory (DFT) and synchrotron soft X-ray spectroscopy techniques to clarify the causes of inefficiency. Our DFT calculations of the two-fold superlattice, as confirmed by near-edge X-ray absorption fine structure measurements, revealed its indirect-band-gap nature. An angle-resolved photoelectron spectroscopy study identified the states within the band gap, and the resonant photoemission technique and DFT simulation clarified that the origin of the gap states was anisotropic localization of electrons around V–O tetrahedra. Water absorption enhances the gap states, which dramatically impedes the kinetics of water oxidation reactions. Our results reveal the role of gap states in water oxidation reactions and offer new insights into manipulating gap states in 3d transition metal oxides.

Oxygen on-site Coulomb energy in Pr1.3−xLa0.7CexCuO4 and Bi2Sr2CaCu2O8+δ and its relation with Heisenberg exchange

We study the electronic structure of electron-doped Pr$_{1.3-x}$La$_{0.7}$Ce$_{x}$CuO$_{4}$ (PLCCO ; $T_{c}$ = 27 K, x = 0.1) and hole-doped Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ (Bi2212 ; $T_{c}$ = 90 K) cuprate superconductors using x-ray absorption spectroscopy (XAS) and resonant photoemission spectroscopy (Res-PES). From Res-PES across the O K-edge and Cu L-edge, we identify the O 2p and Cu 3d partial density of states (PDOS) and their correlation satellites which originate in two-hole Auger final states. Using the Cini-Sawatzky method, analysis of the experimental O 2p PDOS shows an oxygen on-site Coulomb energy for PLCCO to be $U_{p}$ = 3.3$\pm$0.5 eV and for Bi2212, $U_{p}$ = 5.6$\pm$0.5 eV, while the copper on-site Coulomb correlation energy, $U_{d}$ = 6.5$\pm$0.5 eV for Bi2212. The expression for the Heisenberg exchange interaction $J$ in terms of the electronic parameters $U_{d}$, $U_{p}$, charge-transfer energy $\Delta$ and Cu-O hopping $t_{pd}$ obtained from a simple Cu$_2$O cluster model is used to carry out an optimization analysis consistent with $J$ known from scattering experiments. The analysis also provides the effective one band on-site Coulomb correlation energy $\tilde{U}$ and the effective hopping $\tilde{t}$. PLCCO and Bi2212 are shown to exhibit very similar values of $\tilde{U}$/$\tilde{t}$ $\sim$9-10, confirming the strongly correlated nature of the singlet ground state in the effective one-band model for both the materials.

Investigating the Intrinsic Anisotropy of VO2(101) Thin Films Using Linearly Polarized Resonant Photoemission Spectroscopy

VO2 is one of the most studied vanadium oxides because it undergoes a reversible metal-insulator transition (MIT) upon heating with a critical temperature of around 340 K. One of the most overlooked aspects of VO2 is the band’s anisotropy in the metallic phase when the Fermi level is crossed by two bands: π* and d||. They are oriented perpendicularly in one respect to the other, hence generating anisotropy. One of the parameters tuning MIT properties is the unbalance of the electron population of π* and d|| bands that arise from their different energy position with respect to the Fermi level. In systems with reduced dimensionality, the electron population disproportion is different with respect to the bulk leading to a different anisotropy. Investigating such a system with a band-selective spectroscopic tool is mandatory. In this manuscript, we show the results of the investigation of a single crystalline 8 nm VO2/TiO2(101) film. We report on the effectiveness of linearly polarized resonant photoemission (ResPES) as a band-selective technique probing the intrinsic anisotropy of VO2.

Investigation of electronic properties of Mn doped SnO2 thin film

Probing the electronic structure of Mn doped SnO2 is crucial due to its enormous applications including optoelectronics, superconductors, gas sensors etc. In the present study we have investigated the complete electronic structure of the 10% Mn doped SnO2 thin film on Si (001) substrate. X-ray diffraction confirms the single-phase growth with decreasing crystallite size with Mn doping. X-ray photoemission spectroscopic study reveals the presence of dominant Sn4+ with minor Sn2+ valence state owing to oxygen deficiency resulting in Mn4+ and Mn2+ state in our grown film. Our resonant photoemission study reveals the origin of different bands near Fermi level, which can be further tuned for various fundamental properties.

Theory of circular dichroism in angle-resolved resonant photoemission from magnetic surfaces

A theoretical method is presented for angle-resolved photoemission at the transition metal $L$-edge resonance. It combines atomic multiplet calculations for the second-order resonant photoemission amplitude on the core-level site and a single scattering calculation of the photoelectron final state. The theory is applied to a magnetized Ni(111) surface excited with circularly polarized x rays at the Ni ${L}_{2,3}$-edge resonance with a focus on the circular dichroism (CD) signal. Good agreement with available experimental data is achieved. It is shown that the CD pattern is composed of a slowly varying magnetic signal induced by the atomic resonant process and a signal of fast angular modulations that are due to the interference of primary and scattered waves, known as the Daimon effect. The two types of CD signals are found to be nearly additive. At the Ni ${L}_{2}$-edge resonance, the angular dependence of the magnetic CD is well described by a simple expression known from x-ray magnetic CD. At the ${L}_{3}$ edge, however, the angular dependence is more complex and shows a pronounced final state multiplet dependence. With the present theory, it becomes possible to extract element- and site-selective magnetic information of surfaces from the CD in angle-resolved resonant photoemission data.

The ALBA high-stability monochromator for VUV and soft X-rays

LOREA is ALBA’s beamline devoted to the investigation of solids electronic structure by means of Angle Resolved Photo-Emission Spectroscopy (ARPES). The beamline operates in the photon energy range 10-1000 eV with tuneable linear and circular polarizations produced by an APPLE II helical undulator. Thanks to its energy range and the high photon flux, LOREA is suitable for high resolution VUV ARPES investigations in the 10-200 eV range, while it is feasible to extend ARPES measurements to the 200-600 eV energy range (soft X-ray ARPES). Core level photoemission, resonant photoemission and X-ray absorption spectroscopies will be accessible in the whole energy range. The energy selection is obtained by an Hettrick-Underwood monochromator without entrance slit. The optical arrangement, with 3 spherical mirrors (SM) and 4 plane varied line-spaced (VLS) gratings, is able to cover the entire energy range of the beamline. The monochromator includes the motions to select and do the fine adjustments of the mirrors (pitch and height), and to scan the energy and select among the different gratings (grating pitch and grating horizontal translation). The monochromator has been designed by the ALBA team of engineers, and has been fully assembled and commissioned at the facility. Besides the required range and resolution performances, it has been designed to achieve high stability and reproducibility, and optimal performance of the optical surfaces under different heat loads and conditions. The cooling circuits of mirrors and gratings are mechanically decoupled from the optical elements. In the case of the gratings, heat load is removed by flexible copper straps connected from optics to rigid water lines, through temperature controller devices based on Peltier elements. The use of Peltier element allows stabilizing the temperature of the gratings to room temperature also under quickly varying heat loads. The gradients within the grating are well below the one degree, and the thermal equilibrium with the surrounding mechanics contributes the long-term stability of the system. The water circuit and the Peltiers, rest in an independent platform inside the vacuum chamber, that allows them drift freely with no effect on the position of optical elements. In the case of the mirrors, the water tubes and cooling pads are not pressed against the mirrors, but just in contact through a 0.1 mm thick pellicle of eutectic InGa. This allows for a very efficient heat transfer using a minimum contact surface sufficient to evacuate up to 60 W, and without any deformation of the mirrors. The mechanics are also designed so that no flexible loops are required, which contributes to a better vibration stability of the system. Mirrors and gratings can be removed from the monochromator inside their holders and with the cooling scheme installed on it. This is a mandatory goal of design, as it is necessary for a careful installation and control of surface deformations at the optics laboratory. In this contribution we describe the main features of the monochromator that allow reaching the target performances, especially those concerning the cooling scheme. And also, we provide details about the positioning mechanics of the optical elements, the energy scanning mechanism and the vacuum system. The monochromator has been already mounted and installed and it is already in operation. The first results of the He photoionization spectra shows an energy resolution better than 10meV at 60eV, with a strong ionization signal and very low noise.

Nonmonotonous temperature fluctuations of the orbital moment and spin-orbit coupling in multiferroic gallium ferrite thin films

Temperature dependent x-ray absorption spectroscopy and x-ray magnetic circular dichroism experiments were performed at the Fe ${L}_{2,3}$ edges to unveil the magnetic orbital moment and spin-orbit coupling (SOC) behavior in thin films of the multiferroic ${\mathrm{Ga}}_{0.6}{\mathrm{Fe}}_{1.4}{\mathrm{O}}_{3}$ (GFO) compound. A thorough analysis of the data evidenced a nonmonotonous behavior for both magnetic orbital moment and SOC with a minimum at approximately 120 K, accompanied with indications of disturbances of the magnetic exchange and electric field dipole. Resonant photoemission spectroscopy experiments performed at the Fe ${L}_{2,3}$ edges rendered an important modification of the crystal field configuration around this temperature value, pointing to a temperature-driven structural trigonal distortion. These instabilities can be compared to those observed in other iron oxides, in particular, during transitions such as the Verwey transition, with the difference that on GFO they do not have any impact on the macroscopic magnetic properties, such as the coercive field and the global magnetization. This work demonstrates that the absence of instabilities in magnetism at the macroscopic scale does not guarantee the absence of fluctuations at the atomic scale. Such atomic-scale fluctuations of the SOC are undoubtedly shown here and have an impact on the spin Hall magnetoresistance response of the films. This point is of importance for prospective applications of GFO in spin-orbitronics.

Influence of Excess Charge on Water Adsorption on the BiVO4(010) Surface.

We present a combined computational and experimental study of the adsorption of water on the Mo-doped BiVO4(010) surface, revealing how excess electrons influence the dissociation of water and lead to hydroxyl-induced alterations of the surface electronic structure. By comparing ambient pressure resonant photoemission spectroscopy (AP-ResPES) measurements with the results of first-principles calculations, we show that the dissociation of water on the stoichiometric Mo-doped BiVO4(010) surface stabilizes the formation of a small electron polaron on the VO4 tetrahedral site and leads to an enhanced concentration of localized electronic charge at the surface. Our calculations demonstrate that the dissociated water accounts for the enhanced V4+ signal observed in ambient pressure X-ray photoelectron spectroscopy and the enhanced signal of a small electron polaron inter-band state observed in AP-ResPES measurements. For ternary oxide surfaces, which may contain oxygen vacancies in addition to other electron-donating dopants, our study reveals the importance of defects in altering the surface reactivity toward water and the concomitant water-induced modifications to the electronic structure.

Study on the magnetic origin in p-type ferromagnetic semiconductor (Ga,Fe)Sb: ab initio calculations

The electronic and magnetic properties of Ga 1−x Fe x Sb (x=0.125, 0.25, 0.5) have been investigated using the first-principles calculations based on density functional theory (DFT) and DFT plus on-site repulsion scheme (DFT+U). We found that the Coulomb U parameter plays an important role in predicting the correct stable magnetic order. Using the DFT+U method, the ferromagnetic (FM) ordering is found to be the magnetic ground state for both concentrations 12.5% and 25%, where the DFT failed without on-site repulsion U-term correction. This failure of semi-local DFT functionals is the fingerprint of the correlated nature of Fe-3d electrons. This result is in good agreement with experiment. Increasing Fe concentration up to 50% changing the magnetic ground state from FM to antiferromagnetic (AF) phase. We have used the Heyd-Scuseria-Ernzerhof (HSE) method to calculate the unfolded band structure of supercell for Ga 1−x Fe x Sb for x=0.125, in both FM and AF phases. The location of Fe-3d states in the calculation of the unfolded band structure showed a good accordance with recent resonance photoemission spectroscopy (RPES) experimental results. We have found that hole carriers have an important role at Fermi level E F . The rules of thumb to predict the stability of the ferromagnetism in Fe doped GaSb FMS have been discussed.

An unusual Cu/Te hybridization in the Cu0.3ZrTe2 intercalation compound

The electronic structure of the novel intercalation compound Cu0.3ZrTe2 was analyzed using the X-ray photoelectron spectroscopy, resonant photoelectron spectroscopy and DFT calculations. The resonant photoemission enhancement of the valence band features of Cu0.3ZrTe2 at the Zr 3p→4d and Cu 3p→3d photo absorption thresholds was studied. The tetrahedral coordination of the Cu atoms by the Te atoms was shown to be responsible for the strong chemical bonding of the Cu atoms within the ZrTe2 lattice through the Cu 3d/Te 5p hybridization. These findings were confirmed by theoretical density of states (DOS) calculations performed for CuxZrTe2 (x = 0.25 and 0.5) with the Cu atoms tetrahedrally and octahedrally coordinated by Te atoms. It was concluded that the substitution of atoms in the CuxTiSe2 system with heavier and polarizable ones (Se→Te, Ti→Zr), i.e. an increase in the lattice polarizability, destabilizes the octahedral coordination of the Cu atoms by the Se/Te ones. This limits the stability of the phases characterized by quantum states with a charge density wave and superconductivity.

The Valence Band Structure of the [Ni(Salen)] Complex: An Ultraviolet, Soft X-ray and Resonant Photoemission Spectroscopy Study

The valence band photoemission (VB PE) spectra of the [Ni(Salen)] molecular complex were measured by ultraviolet, soft X-ray and resonant photoemission (ResPE) using photons with energies ranging from 21.2 eV to 860 eV. It was found that the Ni 3d atomic orbitals’ (AOs) contributions are most significant for molecular orbitals (MOs), which are responsible for the low-energy PE band at a binding energy of 3.8 eV in the VB PE spectra. In turn, the PE bands in the binding energies range of 8–16 eV are due to the photoionization of the MOs of the [Ni(Salen)] complex with dominant contributions from C 2p AOs. A detailed consideration was made for the ResPE spectra obtained using photons with absorption resonance energies in the Ni 2p3/2, N 1s, and O 1s Near-Edge X-ray Absorption Fine Structure (NEXAFS) spectra. A strong increase in the intensity of the PE band ab was found when using photons with an energy 854.4 eV in the Ni 2p3/2 NEXAFS spectrum. This finding is due to the high probability of the participator-Auger decay of the Ni 2p3/2−13d9 excitation and confirms the relationship between the PE band ab with the Ni 3d-derived MOs.

Development of magnetism in Fe-doped magnetic semiconductors: Resonant photoemission and x-ray magnetic circular dichroism studies of (Ga,Fe)As

Fe-doped III-V ferromagnetic semiconductors (FMSs) such as (In,Fe)As, (Ga,Fe)Sb, (In,Fe)Sb, and (Al,Fe)Sb are promising materials for spintronic device applications because of the availability of both $\mathrm{n}$- and $\mathrm{p}$-type materials and the high Curie temperatures. On the other hand, (Ga,Fe)As, which has the same zinc-blende crystal structure as the Fe-doped III-V FMSs, shows paramagnetism. The origin of the different magnetic properties in the Fe-doped III-V semiconductors remains to be elucidated. To address this issue, we use resonant photoemission spectroscopy (RPES) and x-ray magnetic circular dichroism (XMCD) to investigate the electronic and magnetic properties of the Fe ions in a paramagnetic $({\mathrm{Ga}}_{0.95},{\mathrm{Fe}}_{0.05})\mathrm{As}$ thin film. The observed Fe $2p\text{\ensuremath{-}}3d$ RPES spectra show that the Fe $3d$ states are similar to those of ferromagnetic (Ga,Fe)Sb. The estimated Fermi level is located in the middle of the band gap in (Ga,Fe)As. The Fe ${L}_{2,3}$ XMCD spectra of $({\mathrm{Ga}}_{0.95},{\mathrm{Fe}}_{0.05})\mathrm{As}$ show preedge structures, which are not observed in the Fe-doped FMSs, indicating that the minority-spin (\ensuremath{\downarrow}) ${e}_{\ensuremath{\downarrow}}$ states are vacant in $({\mathrm{Ga}}_{0.95},{\mathrm{Fe}}_{0.05})\mathrm{As}$. The XMCD results suggest that the carrier-induced ferromagnetic interaction in $({\mathrm{Ga}}_{0.95},{\mathrm{Fe}}_{0.05})\mathrm{As}$ is short ranged and weaker than that in the Fe-doped FMSs. The experimental findings suggest that the electron occupancy of the ${e}_{\ensuremath{\downarrow}}$ states contributes to the appearance of ferromagnetism in the Fe-doped III-V semiconductors, for $\mathrm{p}$-type as well as $\mathrm{n}$-type compounds.

Intervalence charge transfer and charge transport in the spinel ferrite ferromagnetic semiconductor Ru-doped CoFe2O4

Inter-valence charge transfer (IVCT) is electron transfer between two metal $M$ sites differing in oxidation states through a bridging ligand: $M^{n+1} + M'^{m} \rightarrow M^{n} + M'^{m+1}$. It is considered that IVCT is related to the hopping probability of electron (or the electron mobility) in solids. Since controlling the conductivity of ferromagnetic semiconductors (FMSs) is a key subject for the development of spintronic device applications, the manipulation of the conductivity through IVCT may become a new approach of band engineering in FMSs. In Ru-doped cobalt ferrite CoFe$_2$O$_4$ (CFO) that shows ferrimagnetism and semiconducting transport properties, the reduction of the electric resistivity is attributed to both the carrier doping caused by the Ru substitution for Co and the increase of the carrier mobility due to hybridization between the wide Ru $4d$ and the Fe $3d$ orbitals. The latter is the so-called IVCT mechanism that is charge transfer between the mixed valence Fe$^{2+}$/Fe$^{3+}$ states facilitated by bridging Ru $4d$ orbital: Fe$^{2+}$ + Ru$^{4+}$ $\leftrightarrow$ Fe$^{3+}$ + Ru$^{3+}$. To elucidate the emergence of the IVCT state, we have conducted x-ray absorption spectroscopy (XAS) and resonant photoemission spectroscopy (RPES) measurements on non-doped CFO and Co$_{0.5}$Ru$_{0.5}$Fe$_2$O$_4$ (CRFO) thin films. The observations of the XAS and RPES spectra indicate that the presence of the mixed valence Fe$^{2+}$/Fe$^{3+}$ state and the hybridization between the Fe $3d$ and Ru $4d$ states in the valence band. These results provide experimental evidence for the IVCT state in CRFO, demonstrating a novel mechanism that controls the electron mobility through hybridization between the $3d$ transition-metal cations with intervening $4d$ states.

Investigation of Electronic States and Magnetic Domain Structure of La1 – xSmxMn2Si2 (x = 0, 0.25) Layered Intermetallic Compounds by Resonant Photoemission Spectroscopy and Magnetic Force Microscopy

Investigation of Electronic States and Magnetic Domain Structure of La1 – xSmxMn2Si2 (x = 0, 0.25) Layered Intermetallic Compounds by Resonant Photoemission Spectroscopy and Magnetic Force Microscopy

Photoelectron diffraction, multi-atom resonant photoemission, pioneered by Fadley

We discuss two important subjects pioneered by Fadley. As the first topic, we discuss the energy dependence of the intensity ratio observed in Mn 3s multiplet splitting excited from antiferromagnetic MnO, MnF2 and KMnF3. We apply the many-body scattering theory developed by Hedin in order to incorporate the extrinsic effects. We find that the extrinsic effects play an important role to explain the energy dependence of the branching ratio. Preliminary calculations show the smaller energy dependence than that observed. As the second topic we briefly discuss some features in multi-atom resonant photoemission (MARPE) based on the first principle quantum theory. Importance of the local symmetry around an emitter is stressed.

Polarized dependence of soft X-ray absorption near edge structure of ZnO films implanted by Yb

Virgin and Yb-implanted epitaxial ZnO films grown using atomic layer deposition (ALD) were investigated by X-ray absorption spectroscopy (XAS). XAS study revealed a strong polarization dependence of films determined by the orientation of the polarization vector of the synchrotron radiation to the sample surface. It also indicated that the implantation and subsequent annealing have an important influence on the native point defect complexes in the ZnO. Comparison of experimental spectra with the modelled ones, which are computed based on the linear combination of model spectra corresponding to the selected point defects and their complexes, confirmed the presence of donor-acceptor complexes (mVZn - nVO, m = 1,4; n = 1,2) in the samples under study. The mechanism of vacancy complexes formation is unclear as it takes place under non-equilibrium conditions, for which any theoretical method has not been well established. Exploring the 3 d → 4 f absorption, it was found that oxidation state of Yb in ZnO is 3+, which is consistent with the XPS findings and previously conducted Resonant Photoemission Spectroscopy (RPES) investigations. The inversion of the polarization dependence for samples with different Yb fluences visible in Yb M5 spectra can be associated with a tilt of the oxygen pseudo octahedra or/and with their distortion. The analysis of the presented data suggests that the donor-acceptor complexes are present both in as grown and implanted films and may influence their electrical properties. This suggestion was confirmed by previous Hall measurements showing that the resistivity of annealed ZnO:Yb film with a fluence of 5e15 ions/cm2 decreases by about one order compared to the one with a fluence of 5e14 ions/cm2.

Redox-mediated C–C bond scission in alcohols adsorbed on CeO2− x thin films

The decomposition mechanisms of ethanol and ethylene glycol on well-ordered stoichiometric CeO2(111) and partially reduced CeO2−x (111) films were investigated by means of synchrotron radiation photoelectron spectroscopy, resonant photoemission spectroscopy, and temperature programmed desorption. Both alcohols partially deprotonate upon adsorption at 150 K and subsequent annealing yielding stable ethoxy and ethylenedioxy species. The C–C bond scission in both ethoxy and ethylenedioxy species on stoichiometric CeO2(111) involves formation of acetaldehyde-like intermediates and yields CO and CO2 accompanied by desorption of acetaldehyde, H2O, and H2. This decomposition pathway leads to the formation of oxygen vacancies. In the presence of oxygen vacancies, C–O bond scission in ethoxy species yields C2H4. In contrast, C–C bond scission in ethylenedioxy species on the partially reduced CeO2−x (111) is favored with respect to C–O bond scission and yields methanol, formaldehyde, and CO accompanied by the desorption of H2O and H2. Still, scission of C–O bonds on both sides of the ethylenedioxy species yields minor amounts of accompanying C2H4 and C2H2. C–O bond scission is coupled with a partial recovery of the lattice oxygen in competition with its removal in the form of water.

Electronic structure of rare-earth semiconducting ErN thin films determined with synchrotron radiation photoemission spectroscopy and first-principles analysis

Erbium nitride (ErN) is an emerging rocksalt rare-earth semiconducting pnictide and has attracted significant interest in recent years for its potential applications in thermoelectric energy conversion, spintronic devices, and for the Gifford-McMahon cryocoolers. Due to the Er intra-$4f$ electronic transition, Er-doped III-nitride semiconductors such as GaN, InGaN, etc. exhibit strong emission in the retina-safe and fiber optical communication wavelength window of 1.54 \ensuremath{\mu}m that is researched extensively for developing solid-state lasers, amplifiers, and light-emitting devices. However, due to ErN's propensity for oxidation in ambient, high-quality ErN thin film growth has been challenging and an in-depth understanding of its electronic structure remains unanswered. In this work, the valence band electronic structure of ErN thin films is measured with normal as well as with resonant synchrotron-radiation photoemission spectroscopy. Photoemission measurements show a valence band maximum and Fermi energy difference of \ensuremath{\sim}2.3 eV in ErN. First-principles density functional theory (DFT) calculations are performed not only to explain the valence band electronic structure but also to determine transport properties such as effective mass and deformation potentials. Strong localized Er-$4f$ states are observed \ensuremath{\sim}6--8 eV below the valence band maxima and the valence band edge is found to exhibit N-$2p$ character. Resonant photoemission data corroborates the DFT calculations. To accurately capture the electronic structure in modeling, beyond generalized gradient approximation (GGA) methods such as (a) Heyd-Scuseria-Ernzerhof hybrid functional and (b) GGA+U Hubbard correction schemes are utilized. Determination of the electronic structure of ErN marks significant progress in developing ErN-based electronic, optoelectronic, and thermoelectric devices.