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Organic-inorganic halide perovskites have been used as light absorbers in photovoltaic cells. The energy band gap of these perovskites can be tuned by mixing various halides at different concentrations. In this work, mixed iodide-bromide perovskite films at various concentrations were deposited on fluorine-doped tin oxide glass substrates via a spin coating technique. The films were characterized using X-ray diffraction (XRD), ultraviolet-visible spectroscopy and synchrotron-radiation photoemission spectroscopy. The XRD results suggest that the lattice spacing decreased with increasing bromide concentration, which also relates to an increase in the energy band gap. The mixed halide films showed less crystallinity than that of pure halide films. The photoemission results showed that the iodide concentration at the surface was higher than expected. This could have been due to the higher solubility of iodide perovskite in the dimethylformamide solvent than that of bromide perovskite. These results suggest that the energy levels at the surface of the films can be different from those of the bulk material.

Author(s): Rattanachata, A; Nicolai, LC; Martins, HP; Conti, G; Verstraete, MJ; Gehlmann, M; Ueda, S; Kobayashi, K; Vishik, I; Schneider, CM; Fadley, CS; Gray, AX; Minar, J; Nemsak, S | Abstract: In the last decade rare-earth hexaborides have been investigated for their fundamental importance in condensed matter, and for their applications in advanced technological fields. Among these compounds, LaB6 has a special place, being a traditional d-band metal without additional f bands. In order to understand the bulk electronic structure of the more complex rare-earth hexaborides, in this paper we investigate the bulk electronic structure of LaB6 using tender/hard x-ray photoemission spectroscopy, measuring both core-level and angle-resolved valence-band spectra. Furthermore, we compare the La 3d core level spectrum to cluster model calculations in order to understand the bulklike core-hole screening effects. The results show that the La 3d well-screened peak is at a lower binding energy compared to the main poorly screened peak; the relative intensity between these peaks depends on how strong the hybridization is between La and B atoms. We show that the recoil effect, negligible in the soft x-ray regime, becomes prominent at higher kinetic energies for lighter elements, such as boron, but is still negligible for heavy elements, such as lanthanum. In addition, we report the bulklike band structure of LaB6 determined by tender/hard x-ray angle-resolved photoemission spectroscopy (HARPES). We compare HARPES experimental results to the free-electron final-state calculations and to the more precise one-step photoemission theory including matrix element and phonon excitation effects. The agreement between the features present in the experimental ARPES data and the theoretical calculations is very good. In addition, we consider the nature and the magnitude of phonon excitations in order to interpret HARPES experimental data measured at different temperatures and excitation energies. We demonstrate that the one-step theory of photoemission and HARPES experiments provides, at present, the only approach capable of probing, both experimentally and theoretically, true "bulklike"electronic band structure of rare-earth hexaborides and strongly correlated materials.

In the last decade rare-earth hexaborides have been investigated for their fundamental importance in condensed matter physics, and for their applications in advanced technological fields. Among these compounds, LaB$_6$ has a special place, being a traditional d-band metal without additional f- bands. In this paper we investigate the bulk electronic structure of LaB$_6$ using hard x-ray photoemission spectroscopy, measuring both core-level and angle-resolved valence-band spectra. By comparing La 3d core level spectra to cluster model calculations, we identify well-screened peak residing at a lower binding energy compared to the main poorly-screened peak; the relative intensity between these peaks depends on how strong the hybridization is between La and B atoms. We show that the recoil effect, negligible in the soft x-ray regime, becomes prominent at higher kinetic energies for lighter elements, such as boron, but is still negligible for heavy elements, such as lanthanum. In addition, we report the bulk-like band structure of LaB$_6$ determined by hard x-ray angle-resolved photoemission spectroscopy (HARPES). We interpret HARPES experimental results by the free-electron final-state calculations and by the more precise one-step photoemission theory including matrix element and phonon excitation effects. In addition, we consider the nature and the magnitude of phonon excitations in HARPES experimental data measured at different temperatures and excitation energies. We demonstrate that one step theory of photoemission and HARPES experiments provide, at present, the only approach capable of probing true bulk-like electronic band structure of rare-earth hexaborides and strongly correlated materials., Total 26 pages, Total 11 figures

International audience; The functionalization of graphene is a well-established route for modulating its optoelectronic properties for a wide range of applications. Here, we studied, using photoemission spectroscopies and synchrotron radiation, the band structure upon evaporation of a p-type dopant tetrafluoro-tetracyanoquinodimethane (F4-TCNQ) molecules and determined the work function (WF) shift over a large area of epitaxial graphene grown on a 4H-SiC (0001) silicon carbide substrate. This system exhibits peculiar nanostructures composed of mono and multilayers, notably at the step edges where the electronic properties differ from the terraces. We observed, owing to the high spatial resolution of photoemission electron microscopy (PEEM), that after the adsorption of F4-TCNQ, multilayer graphene on step edges was subjected to less charge transfer compared to the monolayer graphene on terraces, making their final WF smaller. We calculated the thermoelectric properties of this functionalized graphene system by using density functional theory and Boltzmann transport formalism within the range of the Fermi level (EF), and the carrier concentration, which was experimentally determined. We show that the Seebeck coefficient (S) on the nanofacets is 25% larger than on the monolayer terraces, and the maximum power factor (PF) is on the order of 10−2 W/K2m. This order of magnitude is comparable to the PF of commercial thermoelectric materials such as bulk bismuth telluride.

Two-dimensional transition metal carbides, carbonitrides, and nitrides, called MXenes, exhibit high metallic conductivity, ion intercalation capability, and reversible redox activity, prompting their applications in energy storage and conversion, electromagnetic interference (EMI) shielding, and electronics, among many other fields. It has been shown that replacement of ∼50% of carbon atoms in the most popular MXene family member, titanium carbide (Ti3C2Tx), by nitrogen atoms, forming titanium carbonitride (Ti3CNTx), leads to drastically different properties. Such properties include very high negative charge in solution and extreme EMI shielding effectiveness, exceeding all known materials, even metals at comparable thicknesses. Here, by using ultraviolet photoemission spectroscopy (UPS), the electronic structures of Ti3CNTx and Ti3C2Tx are systematically investigated and compared as a function of charge carrier density. We observe that, in contrast to Ti3C2Tx, the Ti 3p core-level of Ti3CNTx exhibits a counterintuitive shift to a lower binding energy of up to ∼250 meV upon increasing the electron density, which is a spectroscopic signature of negative electronic compressibility (NEC). These experimentally measured chemical potential shifts are well captured by the density functional theory (DFT) calculation. The DFT results also further suggest that the hybridization of titanium–nitrogen bonding in Ti3CNTx helps to promote the available states of Ti atoms for receiving more electrons above the fermi level and leads to the observed NEC. Our findings explain the differences in electronic properties between the two very important and widely studied MXenes and also suggest a new strategy to apply the NEC effect of Ti3CNTx in energy and charge storage applications.

LaNiO$_3$ (LNO) is an intriguing member of the rare-earth nickelates in exhibiting a metal-insulator transition for a critical film thickness of about 4 unit cells [Son et al., Appl. Phys. Lett. 96, 062114 (2010)]; however, such thin films also show a transition to a metallic state in superlattices with SrTiO$_3$ (STO) [Son et al., Appl. Phys. Lett. 97, 202109 (2010)]. In order to better understand this transition, we have studied a strained LNO/STO superlattice with 10 repeats of [4 unit-cell LNO/3 unit-cell STO] grown on an (LaAlO$_3$)$_{0.3}$(Sr$_2$AlTaO$_6$)$_{0.7}$ substrate using soft x-ray standing-wave-excited angle-resolved photoemission (SWARPES), together with soft- and hard- x-ray photoemission measurements of core levels and densities-of-states valence spectra. The experimental results are compared with state-of-the-art density functional theory (DFT) calculations of band structures and densities of states. Using core-level rocking curves and x-ray optical modeling to assess the position of the standing wave, SWARPES measurements are carried out for various incidence angles and used to determine interface-specific changes in momentum-resolved electronic structure. We further show that the momentum-resolved behavior of the Ni 3d eg and t2g states near the Fermi level, as well as those at the bottom of the valence bands, is very similar to recently published SWARPES results for a related La$_{0.7}$Sr$_{0.3}$MnO$_3$/SrTiO$_3$ superlattice that was studied using the same technique (Gray et al., Europhysics Letters 104, 17004 (2013)), which further validates this experimental approach and our conclusions. Our conclusions are also supported in several ways by comparison to DFT calculations for the parent materials and the superlattice, including layer-resolved density-of-states results.

We have investigated the electronic structure of the dilute magnetic semiconductor (DMS) Ga0.98Mn0.02P and compared it to that of an undoped GaP reference sample, using hard x-ray photoelectron spectroscopy (HXPS) and hard x-ray angle-resolved photoemission spectroscopy (HARPES) at energies of about 3 keV. We present experimentaldata,aswellastheoreticalcalculations,tounderstandtheroleoftheMndopantintheemergenceof ferromagnetism in this material. Both core-level spectra and angle-resolved or angle-integrated valence spectra are discussed. In particular, the HARPES experimental data are compared to free-electron final-state model calculations and to more accurate one-step photoemission theory. The experimental results show differences between Ga0.98Mn0.02P and GaP in both angle-resolved and angle-integrated valence spectra. The Ga0.98Mn0.02P bands are broadened due to the presence of Mn impurities that disturb the long-range translational order of the host GaP crystal. Mn-induced changes of the electronic structure are observed over the entire valence band range, including the presence of a distinct impurity band close to the valence-band maximum of the DMS. These experimental results are in good agreement with the one-step photoemission calculations and a prior HARPES study of Ga0.97Mn0.03As and GaAs [Gray et al., Nat. Mater. 11, 957 (2012)], demonstrating the strong similarity between these two materials. The Mn 2p and 3s core-level spectra also reveal an essentially identical state in doping both GaAs and GaP.

Byla zkoumána elektronická struktura zředěného magnetického polovodiče (DMS) Ga 0,98 Mn 0,02 P a ve srovnání s referenčním vzorkem neopjatého GaP, za použití rentgenové fotoelektronové spektroskopie (HXPS) a pevná rentgenová rentgenová spektroskopie (HARPES) při energiích přibližně 3 keV. Prezentujeme experimentální data, stejně jako teoretické výpočty, aby pochopili roli Mn dopantu při vzniku ferromagnetismu v tomto materiálu. Spektra na úrovni jádra a valenční spektra s úhlem rozděleným nebo úhlem jsou diskutovány. Konkrétně jsou experimentální data HARPESu porovnávána s volně elektronovým modelem konečného stavu výpočty a přesnější jednostupňovou teorii fotoemisí. Experimentální výsledky ukazují rozdíly mezi Ga 0,98 Mn 0,02 P a GaP v obou úhlech a v úhlu integrovaných valenčních spekter. Ga 0,98 Mn 0,02 P pásy jsou rozšířeny kvůli přítomnosti Mn nečistot, které narušují dlouhý rozsah translačního pořadí hostitelský GaP křišťál. Změny elektronové struktury vyvolané Mn jsou pozorovány v celém pásmu valence rozsah, včetně přítomnosti odlišného pásma nečistot blízkého maximálnímu pásmu valence DMS. Tyto experimentální výsledky jsou v dobré shodě s jednokrokovým výpočtem emisí a předcházejícím HARPESem studie Ga 0,97 Mn 0,03 As a GaAs [Gray a kol., Nat. Mater. 11, 957 (2012)], což demonstruje silnou podobnost mezi těmito dvěma materiály. Spektra na úrovni jádra Mn 2p a 3s také odhalují v podstatě stejný stav doping jak GaAs, tak GaP. We have investigated the electronic structure of the dilute magnetic semiconductor (DMS) Ga0.98Mn0.02P and compared it to that of an undoped GaP reference sample, using hard x-ray photoelectron spectroscopy (HXPS) and hard x-ray angle-resolved photoemission spectroscopy (HARPES) at energies of about 3 keV. We present experimentaldata,aswellastheoreticalcalculations,tounderstandtheroleoftheMndopantintheemergenceof ferromagnetism in this material. Both core-level spectra and angle-resolved or angle-integrated valence spectra are discussed. In particular, the HARPES experimental data are compared to free-electron final-state model calculations and to more accurate one-step photoemission theory. The experimental results show differences between Ga0.98Mn0.02P and GaP in both angle-resolved and angle-integrated valence spectra. The Ga0.98Mn0.02P bands are broadened due to the presence of Mn impurities that disturb the long-range translational order of the host GaP crystal. Mn-induced changes of the electronic structure are observed over the entire valence band range, including the presence of a distinct impurity band close to the valence-band maximum of the DMS. These experimental results are in good agreement with the one-step photoemission calculations and a prior HARPES study of Ga0.97Mn0.03As and GaAs [Gray et al., Nat. Mater. 11, 957 (2012)], demonstrating the strong similarity between these two materials. The Mn 2p and 3s core-level spectra also reveal an essentially identical state in doping both GaAs and GaP.

We have investigated the electronic structure of the dilute magnetic semiconductor (DMS) Ga0.98Mn0.02P and compared it to that of an undoped GaP reference sample, using hard x-ray photoelectron spectroscopy (HXPS) and hard x-ray angle-resolved photoemission spectroscopy (HARPES) at energies of about 3 keV. We present experimental data, as well as theoretical calculations, to understand the role of the Mn dopant in the emergence of ferromagnetism in this material. Both core-level spectra and angle-resolved or angle-integrated valence spectra are discussed. In particular, the HARPES experimental data are compared to free-electron final-state model calculations and to more accurate one-step photoemission theory. The experimental results show differences between Ga0.98Mn0.02P and GaP in both angle-resolved and angle-integrated valence spectra. The Ga0.98Mn0.02P bands are broadened due to the presence of Mn impurities that disturb the long-range translational order of the host GaP crystal. Mn-induced changes of the electronic structure are observed over the entire valence band range, including the presence of a distinct impurity band close to the valence-band maximum of the DMS. These experimental results are in good agreement with the one-step photoemission calculations and a prior HARPES study of Ga0.97Mn0.03As and GaAs [Gray et al., Nat. Mater. 11, 957 (2012)], demonstrating the strong similarity between these two materials. The Mn 2p and 3s core-level spectra also reveal an essentially identical state in doping both GaAs and GaP.

The Fe/MgO magnetic tunnel junction is a classic spintronic system, with current importance technologically, and interest for future innovation. The key magnetic properties are linked directly to the structure of hard-to-access buried interfaces, and the Fe and MgO components near the surface are unstable when exposed to air, making a deeper probing, non-destructive, in-situ measurement ideal for this system. We have thus applied hard x-ray photoemission spectroscopy (HXPS) and standing-wave (SW) HXPS in the few keV energy range to probe the structure of an epitaxially-grown MgO/Fe superlattice. The superlattice consists of 9 repeats of MgO grown on Fe by magnetron sputtering on an MgO (001) substrate, with a protective Al2O3 capping layer. We determine through SW-HXPS that 8 of the 9 repeats are similar and ordered, with a period of 33 $\pm$ 4 angstrom, with minor presence of FeO at the interfaces and a significantly distorted top bilayer with ca. 3 times the oxidation of the lower layers at the top MgO/Fe interface. There is evidence of asymmetrical oxidation on the top and bottom of the Fe layers. We find agreement with dark-field scanning transmission electron microscope (STEM) and x-ray reflectivity measurements. Through the STEM measurements we confirm an overall epitaxial stack with dislocations and warping at the interfaces of ca. 5 angstrom. We also note a distinct difference in the top bilayer, especially MgO, with possible Fe inclusions. We thus demonstrate that SW-HXPS can be used to probe deep buried interfaces of novel magnetic devices with few angstrom precision., Comment: 37 pages, 13 figures

Control and manipulation of synthesis parameters of thin film coatings is of critical concern in determination of material properties and performance. Structural and morphological properties of rf-sputtered WC-Co thin films deposited under varying deposition parameters namely, substrate temperature and rf power are presented in this data article. The surface morphology, crystallite size and nature were acquired using x-ray photoelectron spectroscopy (XPS) and Grazing Incidence X-ray absorption spectroscopy (GI-XAS). Furthermore, Synchrotron findings are correlated with complimentary data acquired from Scanning electron microscopy (SEM), Raman spectroscopy and surface profilometry to predict and point out optimum synthesis parameters for best properties of the film. Keywords: WC-Co thin films, X-ray photoelectron spectroscopy (XPS), Grazing incidence X-ray absorption spectroscopy (GI-XAS), Synchrotron radiation, SEM

The interfaces between two condensed phases often exhibit emergent physical properties that can lead to new physics and novel device applications and are the subject of intense study in many disciplines. We here apply experimental and theoretical techniques to the characterization of one such interesting interface system: the two-dimensional electron gas (2DEG) formed in multilayers consisting of SrTiO3 (STO) and GdTiO3 (GTO). This system has been the subject of multiple studies recently and shown to exhibit very high carrier charge densities and ferromagnetic effects, among other intriguing properties. We have studied a 2DEG-forming multilayer of the form [6 unit cells (u.c.) STO/3 u.c. of GTO](20) using a unique array of photoemission techniques including soft and hard x-ray excitation, soft x-ray angle-resolved photoemission, core-level spectroscopy, resonant excitation, and standing-wave effects, as well as theoretical calculations of the electronic structure at several levels and of the actual photoemission process. Standing-wave measurements below and above a strong resonance have been exploited as a powerful method for studying the 2DEG depth distribution. We have thus characterized the spatial and momentum properties of this 2DEG in detail, determining via depth-distribution measurements that it is spread throughout the 6 u.c. layer of STO and measuring the momentum dispersion of its states. The experimental results are supported in several ways by theory, leading to a much more complete picture of the nature of this 2DEG and suggesting that oxygen vacancies are not the origin of it. Similar multitechnique photoemission studies of such states at buried interfaces, combined with comparable theory, will be a very fruitful future approach for exploring and modifying the fascinating world of buried-interface physics and chemistry.

© 2018 Author(s). Free-standing nanoribbons of InAs quantum membranes (QMs) transferred onto a (Si/Mo) multilayer mirror substrate are characterized by hard x-ray photoemission spectroscopy (HXPS) and by standing-wave HXPS (SW-HXPS). Information on the chemical composition and on the chemical states of the elements within the nanoribbons was obtained by HXPS and on the quantitative depth profiles by SW-HXPS. By comparing the experimental SW-HXPS rocking curves to x-ray optical calculations, the chemical depth profile of the InAs(QM) and its interfaces were quantitatively derived with ångström precision. We determined that (i) the exposure to air induced the formation of an InAsO4 layer on top of the stoichiometric InAs(QM); (ii) the top interface between the air-side InAsO4 and the InAs(QM) is not sharp, indicating that interdiffusion occurs between these two layers; (iii) the bottom interface between the InAs(QM) and the native oxide SiO2 on top of the (Si/Mo) substrate is abrupt. In addition, the valence band offset (VBO) between the InAs(QM) and the SiO2/(Si/Mo) substrate was determined by HXPS. The value of VBO = 0.2 ± 0.04 eV is in good agreement with literature results obtained by electrical characterization, giving a clear indication of the formation of a well-defined and abrupt InAs/SiO2 heterojunction. We have demonstrated that HXPS and SW-HXPS are non-destructive, powerful methods for characterizing interfaces and for providing chemical depth profiles of nanostructures, quantum membranes, and 2D layered materials.