1,721,042 research outputs found
The Elettra 2.0 Beamlines
Full text linkThe Elettra 2.0 project was approved by the Italian Government in 2017, with plans for the new machine to commence serving external users in 2027. The design phase lead to a final version of Elettra 2.0 a fully transversely coherent source up to 0.5 keV-photon energy, more than doubling the total average current and increasing brightness by more than two orders of magnitude as compared to the current source, and maintaining a diversified beamline portfolio to allow experiments across a broad spectrum of photon energies, from a few tens of eV to several tens of keV, while substantially increasing the number of beamlines operating in the hard-X-ray range. In perspective, the possibility of producing picosecond-long light pulses at a MHz repetition rate across multiple beamlines simultaneously, without interference to standard multi-bunch operation is also being considered. Another important aspect of Elettra 2.0 is the high degree of transverse coherence in both the horizontal and vertical directions, projected to improve by a factor of 60 at 1 keV as compared to the current source
Comparison of surface structures of corundum Cr2O3(0 0 0 1) and V2O3(0 0 0 1) ultrathin films by x-ray photoelectron diffraction
No full textThin Cr2O3(0 0 0 1) layers are formed by oxidation of a Cr(1 1 0) single crystal. This surface
is further modified by growing an epitaxial ultrathin V2O3(0 0 0 1) film by reactive vapor deposition. Synchrotron based softxray photoemission spectroscopy and xray photoelectron diffraction are used to characterize the surface layers of these two corundumstructured oxides. By comparison of experimental XPD patterns with simulated electron multiple scattering calculations, two distinctively different surface terminations are extracted for the two oxides. While for V2O3 we confirm the previously proposed vanadyl-terminated surface structure, we propose a new surface structure for Cr2O3 that consists of excess chromium atoms occupying interstitial subsurface sites
Synthesis of nitrogen-doped epitaxial graphene via plasma-assisted method: Role of the graphene-substrate interaction
Full text linkFunctionalization of graphene by substitution of carbon with nitrogen atoms is a promising way to tailor its electronic properties, but a good control over the heteroatomic configuration in the graphene network is most often a difficult task. In this paper, the synthesis of N-doped graphene by nitrogen plasma treatment of graphene/Ir(111) is presented. The formation of substitutional, pyrrolic and pyridinic nitrogen is analyzed by means of X-ray photoelectron spectroscopy (XPS) and X-ray photoelectron diffraction (XPD). The graphene–Ir interaction is suggested to control the variation in the relative concentration of the nitrogen species. Annealing of the sample also leads to modifications of the nitrogen species incorporated in the graphene layer. Furthermore, the connection of the substitutional nitrogen arrangement with its corresponding spectroscopic fingerprint is unequivocally confirmed by XPD measurements, which give also a direct insight on the local geometry of the nitrogen atoms incorporated in the carbon network
A high-resolution core level spectroscopy study of Ir: From flat to reconstructed and stepped surfaces
No full textThe ability to distinguish surface atoms with different coordination numbers is of fundamental importance in many fields of materials science, ranging from magnetism to heterogeneous catalysis. In this study, we exploited the capability of high-resolution core-level photoelectron spectroscopy in combination with density functional theory-based calculations to investigate this kind of atomic configurations. The measurement of the 4f7/2 core level of the (111), (100), (110), (311), and (510) surfaces allowed us to highlight the differences between surface atoms with different coordination, also related to surface reconstruction processes, atomic diffusion, and morphological changes. The shifts in core levels were correlated with the modification of the effective coordi- nation number, which takes into account the role of the variability in the Ir-Ir interatomic distances, and with the variations in the centroids of the projected d-band ΔBd, a quantity in turn related to chemical reactivity. Besides solid surfaces, the results may aid in understanding the properties of Ir nanoparticles whose active sites on nanofacets, such as edges, corners, and steps, are of paramount relevance in heterogeneous catalysis, especially in the electrocatalytic oxygen evolution reaction
Epitaxial Growth of a Single-Domain Hexagonal Boron Nitride Monolayer
No full textWe investigate the structure of epitaxially grown hexagonal boron nitride (h-BN) on Ir(111) by chemical vapor deposition of borazine. Using photoelectron diffraction spectroscopy, we unambiguously show that a single-domain h-BN monolayer can be synthesized by a cyclic dose of high-purity borazine onto the metal substrate at room temperature followed by annealing at T = 1270 K, this method giving rise to a diffraction pattern with 3-fold symmetry. In contrast, high-temperature borazine deposition (T = 1070 K) results in a h-BN monolayer formed by domains with opposite orientation and characterized by a 6-fold symmetric diffraction pattern. We identify the thermal energy and the binding energy difference between fcc and hcp seeds as key parameters in controlling the alignment of the growing h-BN clusters during the first stage of the growth, and we further propose structural models for the h-BN monolayer on the Ir(111) surface
Chemical gating of epitaxial graphene through ultrathin oxide layers
No full textWe achieved a controllable chemical gating of epitaxial graphene grown on metal substrates by exploiting the electrostatic polarization of ultrathin SiO2 layers synthesized below it. Intercalated oxygen diffusing through the SiO2 layer modifies the metal–oxide work function and hole dopes graphene. The graphene/oxide/metal heterostructure behaves as a gated plane capacitor with the in situ grown SiO2 layer acting as a homogeneous dielectric spacer, whose high capacity allows the Fermi level of graphene to be shifted by a few hundreds of meV when the oxygen coverage at the metal substrate is of the order of 0.5 monolayers. The hole doping can be finely tuned by controlling the amount of interfacial oxygen, as well as by adjusting the thickness of the oxide layer. After complete thermal desorption of oxygen the intrinsic doping of SiO2 supported graphene is evaluated in the absence of contaminants and adventitious adsorbates. The demonstration that the charge state of graphene can be changed by chemically modifying the buried oxide/metal interface hints at the possibility of tuning the level and sign of doping by the use of other intercalants capable of diffusing through the ultrathin porous dielectric and reach the interface with the metal
Probing the chemical nature of surface oxides during coal char oxidation by high-resolution XPS
No full textCoal chars, like most solid carbons, have a pronounced tendency to chemisorb oxygen at low and moderate temperatures. Characterization by XPS of surface oxides on carbon has been accomplished with the aim of providing a better atomistic insight into: (a) reactions involved in molecular oxygen adsorption on coals and (b) the relations between the nanostructure of solid carbons and the chemistry of oxidation. High-resolution C 1s and O 1s core level and valence band XPS spectra effectively reflected the oxidative functionalization of different types of coals and synthetic carbons upon oxidation in air at moderate temperatures (300 and 500 °C). More specifically, analysis of C 1s and valence band spectra could be directed to monitor the structural evolution of the carbons in terms of extension of sp2 versus sp3 conjugation, carbon vacancies and oxidized carbon. Comparison of the O 1s spectra, on the other hand, provided a tool to characterize the nature of oxygen bonding on carbon and to determine the relative abundance of carbon-oxygen species. Results underline the important role of epoxy groups in the early stages of oxidation, providing a mechanistic framework for the identification of the stable and metastable intermediates in the heterogeneous oxidation of coal by molecular oxygen. © 2015 Elsevier Ltd. All rights reserved
Determining the atomic coordination number in the structure of β12 borophene on Ag(111) via X-ray photoelectron diffraction analysis
Full text linkThis study investigates the electronic properties of the borophene β12 phase on Ag(111) and correlates them with specific structural features by combining high-resolution core-level photoelectron spectroscopy, X-ray photoelectron diffraction, and density functional theory-based calculations. We establish a link between the atomic coordination number of the non-equivalent B atoms in the β12 unit cell and the observed spectroscopic signatures in the B 1s spectrum. This finding is conclusively proven by photoelectron diffraction, which confirms that this polymorph exhibits minimal corrugation on Ag(111). These results contribute to a deeper understanding of the properties of various borophene structures on metallic substrates and may stimulate further studies in realizing nanoscaled structures where the atomic coordination number plays a central role
The highest oxidation state observed in graphene-supported sub-nanometer iron oxide clusters
Full text linkIron oxide nanoclusters are of interest for a broad range of applications, but limited experimental information on their oxidation mechanism is available outside of the gas phase. Here, the oxidation of graphene-supported size-selected Fe-n clusters is studied using high-resolution X-ray Photoelectron Spectroscopy.Size-selected iron oxide nanoclusters are outstanding candidates for technological-oriented applications due to their high efficiency-to-cost ratio. However, despite many theoretical studies, experimental works on their oxidation mechanism are still limited to gas-phase clusters. Herein we investigate the oxidation of graphene-supported size-selected Fe-n clusters by means of high-resolution X-ray Photoelectron Spectroscopy. We show a dependency of the core electron Fe 2p(3/2) binding energy of metallic and oxidized clusters on the cluster size. Binding energies are also linked to chemical reactivity through the asymmetry parameter which is related to electron density of states at the Fermi energy. Upon oxidation, iron atoms in clusters reach the oxidation state Fe(II) and the absence of other oxidation states indicates a Fe-to-O ratio close to 1:1, in agreement with previous theoretical calculations and gas-phase experiments. Such knowledge can provide a basis for a better understanding of the behavior of iron oxide nanoclusters as supported catalysts
Ultra-Low Atomic Diffusion Barrier on Two-Dimensional Materials: The Case of Pt on Epitaxial Graphene
No full textUnderstanding the energetics of atomic diffusion on graphene and two-dimensional (2D) materials is critical for advancing ultraminiaturized nanodevices, where even single-atom dynamics can significantly impact their functionality and performance, and for designing next-generation catalysts with superior activity and selectivity. In this work, we demonstrate that the combination of fast, high-resolution X-ray photoelectron spectroscopy (HR-XPS) and density functional theory (DFT) simulations provides a powerful approach to probe Pt atoms diffusion on epitaxial graphene. HR-XPS with its high chemical sensitivity and temporal resolution allows in situ tracking of Pt 4f7/2spectral components associated with monomers, dimers, and larger clusters at low temperature. This capability enabled us to monitor the rapid decay of monomer coverage and the subsequent aggregation into larger clusters. By fitting the time evolution of the different Pt species using a kinetic model, we extracted a diffusion barrier of 128 ± 6 meV, in excellent agreement with the 130 meV value obtained by nudged elastic band (NEB) calculations. These findings establish fast HR-XPS as a noninvasive, high surface-sensitive, and chemically specific technique for quantifying ultralow diffusion barriers of atoms on weakly interacting two-dimensional materials. This approach provides a practical framework for exploring surface dynamics and for guiding the controlled assembly of small atomic clusters or ordered superlattices on 2D templates
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