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Two-dimensional transition metal carbides or nitrides, so-called MXenes, hold the prospect of a proactive emergence as innovative catalysts and device components owing to the specific qualities gained from the chemical species that functionalize the layers. Tuning the nature and the number of the surface terminations becomes the key factor for the effective use of MXenes in technology. This study explores the capability of H atoms to modify the surface composition of Ti3C2Txflakes. While exposing the sample at room temperature to H atoms, the change of its surface chemical state is followed by synchrotron radiation X-ray photoelectron spectroscopy. It turns out that halogen terminations are progressively and substantially removed. In parallel, the O terminations are partially converted into OH groups, the O/OH ratio being possibly controlled by the OH–OH repulsion. The dramatic surface composition change leaves the valence state of the Ti atoms almost unchanged. Density functional theory simulations of the valence band spectra for different Ti3C2Txmodel systems identify all spectral features and model the change of the electronic properties around the Fermi level. Heating the hydrogenated sample to 400 K removes the OH groups, leaving the MXene surface deprived of most of the pristine terminations, thus giving way to new, application-oriented functionalization.

High-density structures of subsurface phosphorus dopants in silicon continue to garner interest as a silicon-based quantum computer platform; however, a much-needed confirmation of their dopant arrangement has been lacking. In this work, we take advantage of the chemical specificity of X-ray photoelectron diffraction to obtain the precise structural configuration of P dopants in subsurface Si:P δ-layers. The growth of δ-layer systems with different levels of doping is carefully studied and verified using X-ray photoelectron spectroscopy and low-energy electron diffraction. Subsequent diffraction measurements reveal that in all cases, the subsurface dopants primarily substitute with Si atoms from the host material. Furthermore, no signs of carrier-inhibiting P–P dimerization can be observed. Our observations not only settle a nearly decade-long debate about the dopant arrangement but also demonstrate how X-ray photoelectron diffraction is surprisingly well suited for studying subsurface dopant structure. This work thus provides valuable input for an updated understanding of the behavior of Si:P δ-layers and the modeling of their derived quantum devices.

Ultralow-energy (ULE) ion implantation is increasingly being explored as a method to substitutionally dope graphene. However, complex implantation-related effects such as defect creation and surface contamination, and how they can be minimized by thermal annealing, remain poorly understood. Here, we address these open questions taking as the model case epitaxial graphene grown on Cu(111), which was subsequently ULE implanted with Mn at 40 eV and then studied as a function of annealing temperature under ultrahigh vacuum. While significant surface cleaning occurs at annealing temperatures as low as 200 °C, recovery from the implantation-induced disorder requires at least 525 °C. Upon high-temperature annealing, in the 600–700 °C range, the Mn atoms that were incorporated upon implantation as intercalated species (between graphene and the Cu surface) experience diffusion into the Cu layer, creating a subsurface alloy. Annealing at 700 °C restored implanted graphene to a nearly pristine state, with a well-ordered graphene lattice with substitutional Mn atoms and a well-defined Dirac cone. In addition to the insight into the complex physicochemical effects induced by thermal annealing, our results provide useful guidelines for future experimental studies on graphene that is modified (e.g., substitutionally doped) by using ULE ion implantation.

The vibrational features of carbon monoxide (CO) adsorbed on Ir(111) were studied by means of high-resolution core-level spectroscopy. By monitoring the Ir 4f7/2core level as a function of exposure, we proved that the CO adsorbs on the surface always in on-top sites, in agreement with the results of vibrational spectroscopy techniques and density functional theory studies. The C 1s vibrational splittings measured for the p(√3 × √3)R30° (233.4 ± 0.5 meV) and c(4 × 2√3)rect (231.4 ± 0.4 meV) structures were in good agreement with the Z+ 1 model. Despite the very small error bar of the measurements, it was not possibile to reveal any anharmonic contribution to the spectral lineshape. We speculate that the contribution of the unresolved vibrational mode of the frustrated translation or the effect of phonon-mediated interaction with the substrate can account for the observation of this outcome.

With the goal to delevop the fabrication of a new type of Pt-nanoparticle carbon–support electrocatalyst, we investigate the carbon embedding of Pt cluster superlattices grown on the moiré of a monolayer of hexagonal boron nitride (h-BN) on Ir(111). Our combined scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) study establishes conformal C embedding of the Pt clusters on h-BN/Ir(111) without deterioration of superlattice order, preferential and strong binding of the embedding carbon to the Pt clusters, and upon annealing the formation of a homogeneous amorphous carbon (a-C) matrix. There are indications that while the a-C matrix and the Pt clusters bind strongly to each other, upon annealing both weaken their binding to h-BN.

MXenes are a young family of two-dimensional transition metal carbides, nitrides, and carbonitrides with highly controllable structure, composition, and surface chemistry to adjust for target applications. Here, we demonstrate the modifications of two-dimensional MXenes by low-energy ion implantation, leading to the incorporation of Mn ions in Ti3C2Tx(where Txis a surface termination) thin films. Damage and structural defects caused by the implantation process are characterized at different depths by XPS on Ti 2p core-level spectra, by ToF-SIMS, and with electron energy loss spectroscopy analyses. Results show that the ion-induced alteration of the damage tolerant Ti3C2Txlayer is due to defect formation at both Ti and C sites, thereby promoting the functionalization of these sites with oxygen groups. This work contributes to the inspiring approach of tailoring 2D MXene structure and properties through doping and defect formation by low-energy ion implantation to expand their practical applications.

Motivated by the relevance of Pt clusters in heterogeneous catalysis, ordered Pt cluster superlattices are templated by the moiré of monolayer hexagonal boron nitride (h-BN) with Ir(111). Using X-ray photoelectron spectroscopy, scanning tunneling microscopy, and scanning tunneling spectroscopy, we provide a comprehensive picture for the mechanism of cluster binding, the thermal stability of clusters, cluster superlattice decay, the morphological evolution of cluster shape with size, and finally Pt cluster decoupling and internal electronic structure. The key findings are that the Pt cluster superlattices on h-BN on Ir(111) are thermally more stable than any other Pt cluster superlattice, are well decoupled from their substrate, and display a discrete cluster height-dependent electronic structure.

Cluster superlattice membranes consist of a two-dimensional hexagonal lattice of similar-sized nanoclusters sandwiched between single-crystal graphene and an amorphous carbon matrix. The fabrication process involves three main steps, the templated self-organization of a metal cluster superlattice on epitaxial graphene on Ir(111), conformal embedding in an amorphous carbon matrix, and subsequent lift-off from the Ir(111) substrate. The mechanical stability provided by the carbon-graphene matrix makes the membrane stable as a free-standing material and enables transfer to other substrates. The fabrication procedure can be applied to a wide variety of cluster materials and cluster sizes from the single-atom limit to clusters of a few hundred atoms, as well as other two-dimensional layer/host matrix combinations. The versatility of the membrane composition, its mechanical stability, and the simplicity of the transfer procedure make cluster superlattice membranes a promising material in catalysis, magnetism, energy conversion, and optoelectronics.

We investigated the epitaxial growth of single-layer molybdenum disulfide (MoS2) on graphene/Ir(111), aiming to understand its steps and mechanism and verify the stability of the heterostructure. By means of high-resolution X-ray photoelectron spectroscopy, we have revealed that accurate temperature control is crucial in allowing the formation and avoiding the degradation of single-layer MoS2on graphene. We observed that keeping the substrate temperature T>800 K during the growth promotes efficient sulfur intercalation under graphene and the dissolution of sulfur in the Ir bulk, two processes that have been targeted as ones mainly responsible for irreversible degradation of the MoS2single layer on graphene. We believe that these results could be instrumental in understanding and improving the epitaxial growth protocols for the growth of heterostructures combining epitaxial graphene and transition-metal dichalcogenides.

We present a comparative study of the room-temperature adsorption of p-aminophenol (p-AP) molecules on three metal surfaces, namely Cu(110), Cu(111), and Pt(111). We show that the chemical nature and the structural symmetry of the substrate control the activation of the terminal molecular groups, which result in different arrangements of the interfacial molecular layer. To this aim, we have used in situSTM images combined with synchrotron radiation high-resolution XPS and NEXAFS spectra, and the results were simulated by DFT calculations. On copper, the interaction between the molecules and the surface is weaker on the (111) surface crystal plane than on the (110) one, favoring molecular diffusion and leading to larger ordered domains. We demonstrate that the p-AP molecule undergoes spontaneous dehydrogenation of the alcohol group to form phenoxy species on all the studied surfaces; however, this process is not complete on the less reactive surface, Cu(111). The Pt(111) surface exhibits stronger molecule–surface interaction, inducing a short-range ordered molecular arrangement that increases over time. In addition, on the highly reactive Pt(111) surface other chemical processes are evidenced, such as the dehydrogenation of the amine group.

Graphene has been proposed to be either fully transparent to van der Waals interactions to the extent of allowing switching between hydrophobic and hydrophilic behavior, or partially transparent (translucent), yet there has been considerable debate on this topic, which is still ongoing. In a combined experimental and theoretical study we investigate the effects of different metal substrates on the adsorption energy of atomic (argon) and molecular (carbon monoxide) adsorbates on high-quality epitaxial graphene. We demonstrate that while the adsorption energy is certainly affected by the chemical composition of the supporting substrate and by the corrugation of the carbon lattice, the van der Waals interactions between adsorbates and the metal surfaces are partially screened by graphene. Our results indicate that the concept of graphene translucency, already introduced in the case of water droplets, is found to hold more generally also in the case of single polar molecules and atoms, which are apolar.

Despite its broad potential applications, substitution of carbon by transition metal atoms in graphene has so far been explored only to a limited extent. We report the realization of substitutional Mn doping of graphene to a record high atomic concentration of 0.5%, which was achieved using ultralow-energy ion implantation. By correlating the experimental data with the results of ab initio Born–Oppenheimer molecular dynamics calculations, we infer that direct substitution is the dominant mechanism of impurity incorporation. Thermal annealing in ultrahigh vacuum provides efficient removal of surface contaminants and additional implantation-induced disorder, resulting in Mn-doped graphene that, aside from the substitutional Mn impurities, is essentially as clean and defect-free as the as-grown layer. We further show that the Dirac character of graphene is preserved upon substitutional Mn doping, even in this high concentration regime, making this system ideal for studying the interaction between Dirac conduction electrons and localized magnetic moments. More generally, these results show that ultralow energy ion implantation can be used for controlled functionalization of graphene with substitutional transition-metal atoms, of relevance for a wide range of applications, from magnetism and spintronics to single-atom catalysis.

Nanostructured architectures based on graphene/metal interfaces might be efficiently exploited in hydrogen storage due to the attractive capability to provide adsorption sites both at the top side of graphene and at the metal substrate after intercalation. We combined in situhigh-resolution X-ray photoelectron spectroscopy and scanning tunneling microscopy with theoretical calculations to determine the arrangement of hydrogen atoms at the graphene/Ni(111) interface at room temperature. Our results show that at low coverage H atoms predominantly adsorb as monomers and that chemisorption saturates when ∼25% of the surface is hydrogenated. In parallel, with a much lower rate, H atoms intercalate below graphene and bind to Ni surface sites. Intercalation progressively destabilizes the C–H bonds and triggers the release of the hydrogen chemisorbed on graphene. Valence band and near-edge absorption spectroscopy demonstrate that the graphene layer is fully lifted when the Ni surface is saturated with H. Thermal programmed desorption was used to determine the stability of the hydrogenated interface. Whereas the H atoms chemisorbed on graphene remain unperturbed over a wide temperature range, the intercalated phase abruptly desorbs 50–100 K above room temperature.

By combining high-resolution photoelectron spectroscopy and ab initio calculations, we show that different carbon clusters can be formed on Ir(111) upon low temperature molecular beam epitaxy using a solid state carbon source. Besides carbon monomers, also dimers, trimers, and larger clusters are detected through C 1s core level measurements. The spectroscopic signal of carbon monomers is then used as a fingerprint to detect their presence during the early stages of graphene growth by ethylene chemical vapor deposition at high temperature. We demonstrate that our spectroscopic approach can be employed to investigate the role of carbon monomers and dimers in the nucleation and growth of graphene on different metal surfaces.

Combining density functional theory, nudged elastic band, and high-energy resolution X-ray photoelectron spectroscopy experiments, we study the early stages and reaction pathways whereby ethylene molecules decompose on Ni3Al(111) prior to graphene nucleation and growth. After characterizing stable configurations of ethylene on the surface, and all intermediate products leading to carbon species, we calculate energy barriers for all relevant processes, including dehydrogenation, isomerization, C–C cleavage, and their respective inverse reactions. This quantitative analysis helps in identifying the most probable reaction pathways. The combination of temperature dependent C 1s core-level photoelectron spectroscopy measurements and of core-level shift calculations for all the different species investigated, allow us to understand the temperature evolution of the surface species and to identify the whole reaction mechanism. A combined analysis of this kind is useful for understanding which species are present on the surface at various temperatures during chemical vapor deposition graphene growth experiments.

Ultrathin films of iron oxide form a two-dimensional (2D) FeO layer on Ir(111). Because of difference in lattice constant between 2D-FeO and Ir(111), a moiré superstructure is formed. The 2D-FeO/Ir(111) structure is examined by soft X-ray photoelectron spectroscopy, X-ray photoemission diffraction, scanning tunneling microscopy, and low-energy electron diffraction. A 2D-FeO layer may also be grown by iron intercalation and subsequent oxidation underneath a graphene layer on Ir(111). Thus, the graphene can be decoupled from the metal by the 2D-FeO layer. Changes in the graphene C 1s binding energy can be mainly explained by shifts in the Fermi level of graphene as a consequence of interface band alignment for weak interactions between graphene and the substrate. A shift of C 1s to lower binding energy, for graphene supported on FeO/Ir(111), is a consequence of the dipole moment in the 2D-FeO layer normal to the Ir(111) surface. Broadening of the C 1s peak is consistent with a locally varying 2D-FeO dipole within the moiré structure and thus implies a modulated charge doping of the graphene.

Iron impurities in the rutile-TiO2(011) surface can result in the formation of an ordered, ternary iron–titanium oxide monolayer. Here the FeTi2O5mixed oxide monolayer predicted by DFT simulations is confirmed by synchrotron based angle scanned X-ray photoelectron diffraction (XPD) studies. The ternary oxide monolayer has been synthesized on rutile-TiO2(011) substrate via two different experimental pathways: first, by segregation of Fe impurities from the bulk by annealing a clean TiO2(011) substrate in 1 × 10–7mbar of oxygen at ∼450 °C and, second, by physical vapor deposition of Fe on clean TiO2(011) in 1 × 10–7mbar of oxygen at ∼450 °C. For both preparation procedures an intermixed surface oxide is formed with Fe and Ti in 2+ and 4+ charge states, respectively. The surface is characterized by high-resolution and fast X-ray photoelectron spectroscopy (XPS), XPD, and low energy electron diffraction (LEED). Multiple electron scattering simulations implemented in the electron diffraction in atomic clusters (EDAC) package were performed for comparing experimental XPD patterns with structural models. The reliability of the approach was tested on XPD pattern of reconstructed clean TiO2(011)-2 × 1 surface, for which a structural model exists. The XPD pattern of the monolayer ternary iron titanium oxide is compared to structural models proposed by density functional theory (DFT)-based models.

Pristine single-walled carbon nanotubes (SWCNTs) are rather inert to O2and N2, which for low doses chemisorb only on defect sites or vacancies of the SWCNTs at the ppm level. However, very low doping has a major effect on the electronic properties and conductivity of the SWCNTs. Already at low O2doses (80 L), the X-ray photoelectron spectroscopy (XPS) O 1s signal becomes saturated, indicating nearly all of the SWCNT’s vacancies have been oxidized. As a result, probing vacancy oxidation on SWCNTs via XPS yields spectra with rather low signal-to-noise ratios, even for metallicity-sorted SWCNTs. We show that, even under these conditions, the first-principles density functional theory calculated Kohn–Sham O 1s binding energies may be used to assign the XPS O 1s spectra for oxidized vacancies on SWCNTs into its individual components. This allows one to determine the specific functional groups or bonding environments measured. We find the XPS O 1s signal is mostly due to three O-containing functional groups on SWCNT vacancies: epoxy (C2>O), carbonyl (C2>CO), and ketene (CCO), as ordered by abundance. Upon oxidation of nearly all of the SWCNT’s vacancies, the central peak’s intensity for the metallic SWCNT sample is 60% greater than that for the semiconducting SWCNT sample. This suggests a greater abundance of O-containing defect structures on the metallic SWCNT sample. For both metallic and semiconducting SWCNTs, we find O2does not contribute to the measured XPS O 1s spectra.

By means of a combination of surface-science spectroscopies and theory, we investigate the mechanisms ruling the catalytic role of epitaxial graphene (Gr) grown on transition-metal substrates for the production of hydrogen from water. Water decomposition at the Gr/metal interface at room temperature provides a hydrogenated Gr sheet, which is buckled and decoupled from the metal substrate. We evaluate the performance of Gr/metal interface as a hydrogen storage medium, with a storage density in the Gr sheet comparable with state-of-the-art materials (1.42 wt %). Moreover, thermal programmed reaction experiments show that molecular hydrogen can be released upon heating the water-exposed Gr/metal interface above 400 K. The Gr hydro/dehydrogenation process might be exploited for an effective and eco-friendly device to produce (and store) hydrogen from water, i.e., starting from an almost unlimited source.

In this experimental work we have studied and characterized a surface phase transition on a decagonal Al72Ni11Co17sample between a crystalline phase, which is induced by sputtering with energetic noble gas ions, and the bulklike decagonal phase. The surface transition to the stable, quasicrystalline phase was followed as a function of temperature by two complementary techniques: high-resolution X-ray photoelectron spectroscopy (HR-XPS) and low-energy electron diffraction (LEED). The observation of shifts in the core–electron binding energies of the constituent atoms and of other photoemission line shape parameters as the surface undergoes the phase transition and the comparison with the structural information obtained from the LEED data enabled us to determine the evolution of the surface from 5 twinned cubic crystalline domains to the decagonal phase. A quasicrystalline chemical environment with some degree of long-range order is observed for Al before the quasicrystalline surface is fully recovered, pointing to a two-step reordering process, where two sublattices order separately at different temperatures. Strong modifications in the valence band are reflected in the core level binding energies, and the observed reduction in density of states at the Fermi energy for the decagonal phase is consistent with a Hume–Rothery stabilization mechanism

The interaction between a Ni(111) substrate covered by a complete Gr monolayer and H atoms occurs through two parallel routes leading to the hydrogen chemisorption on graphene and, at much lower rate but still with some ease, to the intercalation of H atoms below it. This latter reaction determines a direct interaction of the H atoms with the metal surface and eventually the H diffusion into the Ni bulk under the Gr cover. In this study we have combined high-resolution X-ray photoelectron spectroscopy, thermal programmed desorption, and density functional theory calculations to establish how the chemisorption and intercalation yields and their interplay depend on temperature and to find out how graphene affects the amount and the evolution of the hydrogen diffused in the Ni bulk. We found that between 150 and 320 K, hydrogen chemisorption on Gr is independent of temperature and that Gr lifting, which signals the H intercalation below it, does not occur below 180–200 K, being limited by an energy barrier of the order of 150 meV. For the heavily hydrogenated samples, when H atoms diffuse also into the Ni bulk, the Gr cover plays a key role for H storage because it strongly enhances the amount of H loaded in the interface with respect to the bare Ni(111) substrate. This behavior, possibly exhibited also by other graphene/metal interfaces provided that intercalation of H below graphene can readily occur, might foster the design of innovative materials to be applied for H storage.

We 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.

The assembly and the orientation of functionalized pentacene at the interface with inorganics strongly influence both the electric contact and the charge transport in organic electronic devices. In this study electronic spectroscopies and theoretical modeling are combined to investigate the properties of the bis(triisopropylsilylethynyl)pentacene (TIPS-Pc)/Au(111) interface as a function of the molecular coverage to compare the molecular state in the gas phase and in the adsorbed phase and to determine the thermal stability of TIPS-Pc in contact with gold. Our results show that in the free molecule only the acene atoms directly bonded to the ligands are affected by the functionalization. Adsorption on Au(111) leads to a weak coupling which causes only modest binding energy shifts in the TIPS-Pc and substrate core level spectra. In the first monolayer the acene plane form an angle of 33 ± 2° with the Au(111) surface at variance with the vertical geometry reported for thicker solution-processed or evaporated films, whereas the presence of configurational disorder was observed in the multilayer. The thermal annealing of the TIPS-Pc/Au(111) interface reveals the ligand desorption at ∼470 K, which leaves the backbone of the decomposed molecule flat-lying on the metal surface as in the case of the unmodified pentacene. The weak interaction with the metal substrate causes the molecular dissociation to occur 60 K below the thermal decomposition taking place in thick drop-cast films.

Carbon nanotubes are a natural choice as gas sensor components given their high surface to volume ratio, electronic properties, and capability to mediate chemical reactions. However, a realistic assessment of the interaction of the tube wall and the adsorption processes during gas phase reactions has always been elusive. Making use of ultraclean single-walled carbon nanotubes, we have followed the adsorption kinetics of NO2and found a physisorption mechanism. Additionally, the adsorption reaction directly depends on the metallic character of the samples. Franck–Condon satellites, hitherto undetected in nanotube–NOxsystems, were resolved in the N 1sX-ray absorption signal, revealing a weak chemisorption, which is intrinsically related to NO dimer molecules. This has allowed us to identify that an additional signal observed in the higher binding energy region of the core level C 1sphotoemission signal is due to the CO species of ketene groups formed as reaction byproducts . This has been supported by density functional theory calculations. These results pave the way toward the optimization of nanotube-based sensors with tailored sensitivity and selectivity to different species at room temperature.

The charge transfer rates of a localized excited electron to graphene monolayers with variable substrate coupling have been investigated by the core hole clock method with adsorbed argon. Expressed as charge transfer times, we find strong variations between ∼3 fs (on graphene “valleys” on Ru(0001)) to ∼16 fs (quasi-free graphene on SiC, O/Ru(0001), or SiO2/Ru). The values for the “hills” on Gr/Ru and on Gr/Pt(111) are in between, with the ratio 1.7 between the charge transfer times measured on “hills” and “valleys” of Gr/Ru. We discuss the results for Gr on metals in terms of hybridized Ru–C orbitals, which change with the relative Gr–Ru alignment and distance. The charge transfer on the decoupled graphene layers must represent the intrinsic coupling to the graphene empty π* states. Its low rate may be influenced by processes retarding the spreading of charge after transfer.

Combined fast X-ray photoelectron spectroscopy and density functional theory calculations reveal the presence of two types of hydrogen adsorbate structures at the graphene/Ir(111) interface, namely, graphane-like islands and hydrogen dimer structures. While the former give rise to a periodic pattern, dimers tend to destroy the periodicity. Our data reveal distinctive growth rates and stability of both types of structures, thereby allowing one to obtain well-defined patterns of hydrogen clusters. The ability to control and manipulate the formation and size of hydrogen structures on graphene facilitates tailoring of its properties for a wide range of applications by means of covalent functionalization.

Iron-phthalocyanine molecules self-assemble on the moiré pattern of graphene/Ir(111) as a flat and weakly interacting layer, as determined by core-level photoemission and absorption spectroscopy. The graphene buffer layer decouples the FePc two-dimensional structure from the underlying metal; the electronic structure of the FePc molecular macrocycles is preserved; and the Fe-L2,3edges present narrower and slightly modified resonances at the FePc single-layer coverage with respect to a thin film. The FePc layer induces a slight electron doping to the Ir-supported graphene resulting in the Dirac cone position expected for an ideal free-standing-like graphene layer with the standard Fermi velocity.

Using photoemission spectroscopy techniques, we show that oxygen intercalation is achieved on an extended layer of epitaxial graphene on Ir(111), which results in the “lifting” of the graphene layer and in its decoupling from the metal substrate. The oxygen adsorption below graphene proceeds as on clean Ir(111), giving only a slightly higher oxygen coverage. Upon lifting, the C 1s signal shows a downshift in binding energy, due to the charge transfer to graphene from the oxygen-covered metal surface. Moreover, the characteristic spectral signatures of the graphene–substrate interaction in the valence band are removed, and the spectrum of strongly hole-doped, quasi free-standing graphene with a single Dirac cone around the K̅point is observed. The oxygen can be deintercalated by annealing, and this process takes place at around T= 600 K, in a rather abrupt way. A small amount of carbon atoms is lost, implying that graphene has been etched. After deintercalation graphene restores its interaction with the Ir(111) substrate. Additional intercalation/deintercalation cycles readily occur at lower oxygen doses and temperatures, consistently with an increasingly defective lattice. Our findings demonstrate that oxygen intercalation is an efficient method for fully decoupling an extended layer of graphene from a metal substrate, such as Ir(111). They pave the way for the fundamental research on graphene, where extended, ordered layers of free-standing graphene are important and, due to the stability of the intercalated system in a wide temperature range, also for the advancement of next-generation graphene-based electronics.

The chemisorption of O atoms on graphite and the thermal reduction of the oxidized surface were studied by means of high energy resolution photoelectron spectroscopy with synchrotron radiation. The C 1s and O 1s core levels and the valence band spectra were used to identify the different oxidizing surface species and to evaluate the extension of the sp2conjugation as a function of oxidation time and annealing temperature. We found that epoxy groups are the dominant species only at the low oxidation stage, and ethers and semiquinones form as oxidation proceeds. The evolution of the ether/epoxy ratio with increasing oxygen coverage provides evidence for the occurrence of C–C bond unzipping. Epoxy groups are the functionalities with the lowest thermal stability and start to desorb around 370 K, strongly affecting the desorption temperature of other functional groups. The ratio between ethers and epoxy groups determines the balance between epoxy–epoxy and epoxy–ether reactions, the latter promoting the removal of C atoms from the C backbone. Adsorbate spectroscopy during thermal annealing definitely proves the catalytic effect of the basal plane oxygen atoms on the desorption reactions.

The chemical and physical properties of nanoclusters largely depend on their sizes and shapes. This is partly due to finite size effects influencing the local electronic structure of the nanocluster atoms which are located on the nanofacets and on their edges. Here we present a thorough study on graphene-supported Rh nanocluster assemblies and their geometry-dependent electronic structure obtained by combining high-energy resolution core level photoelectron spectroscopy, scanning tunneling microscopy, and density functional theory. We demonstrate the possibility to finely control the morphology and the degree of structural order of Rh clusters grown in register with the template surface of graphene/Ir(111). By comparing measured and calculated core electron binding energies, we identify edge, facet, and bulk atoms of the nanoclusters. We describe how small interatomic distance changes occur while varying the nanocluster size, substantially modifying the properties of surface atoms. The properties of under-coordinated Rh atoms are discussed in view of their importance in heterogeneous catalysis and magnetism.

The formation of a hexagonal boron nitride (h-BN) layer through dissociation of borazine (B3N3H6) molecules on Ir(111) has been investigated by a combination of X-ray photoelectron spectroscopy, near-edge X-ray absorption fine structure, temperature-programmed desorption, and low-energy electron diffraction. At low temperature (T= 170 K), molecular borazine adsorption occurs with the plane of the benzene-like ring parallel to the substrate. Dehydrogenation is observed at temperatures higher than 250 K and extends up to 900 K, with a maximum H2desorption rate around 300 K. Besides dehydrogenation, room temperature adsorption of borazine leads to the formation of atomic and molecular fragments due to the break-up of part of the BN bonds. The epitaxial growth of h-BN starts at temperature higher than 1000 K where an extended and long-range ordered layer is obtained. The presence of a corrugation in the h-BN layer with moiré periodicity of (13 × 13)/(12 × 12) BN/Ir unit cell is reflected in the double component structure of the B 1s and N 1s core level spectra.

The electronic structure and chemical reactivity changes of highly under-coordinated Rh atoms on a Pt(111) surface versus coordination number were studied by a combination of synchrotron radiation core level photoemission spectroscopy and density functional theory. The properties of Rh adatoms are strongly modified by the Pt substrate, when compared with the equivalent atomic configuration in the homometallic environment. A remarkable linear relationship is found between core level shifted spectral components, originating from different atomic geometrical structures, and the corresponding induced d-band center shifts. This finding strongly underscores the relevance of core level shifts as reliable experimental descriptors of chemical reactivity, also for under-coordinated atoms in bimetallic transition metal systems. The availability of an experimental tool capable of detecting local surface alloy configurations with enhanced chemical reactivity opens up the possibility to investigate more complex systems with high density of under-coordinated atoms, such as nanostructured multicomponent alloy surfaces and bimetallic nanoclusters.

Oxygen-induced Rh3d5/2surface core level shifts were used to probe the local electronic structure of first layer Rh atoms in the (2 × 2)pg reconstructed phase formed upon oxygen adsorption. By comparison of the computed shifts for differently reconstructed geometries with the experimental shifts obtained from high-energy resolution photoemission measurements, we confirm that the reconstructed phase is formed by Rh atoms single- and double-bonded to oxygen, yielding a shift from the bulk component of −185 and +140 meV, respectively. We find that the core level shifts are affected by the local rhomboidal distortion of the surface lattice. Moreover, we show that, on Rh(100), the oxygen-induced surface core level shifts are dominated by initial state effects and proportional to the calculated shift of the d-band center. The latter is therefore correlated to the local surface chemical reactivity, as already found for other adsorbate systems on transition metals.

The interaction of hydrogen with Rh(110) was studied by means of high-resolution core level photoelectron spectroscopy, in combination with temperature-programmed desorption spectroscopy, low energy electron diffraction, and scanning tunneling microscopy measurements. By exploiting the H-induced Rh3 d5/2surface core level shift sensitivity to the local coordination environment, we propose here new structural models for the intermediate coverage 0.67 and 1.0 ML overlayers, while we confirm the previous results for the low coverage structures at 0.33 and 0.5 ML, and for the saturation phase at 2.0 ML. Surface core level shift values are interpreted on the basis of our previous data about the hydrogen adsorption on the (100) and (111) rhodium surfaces. The proposed structural models are supported by scanning tunneling images of the surface and provide a comprehensive description of all the available experimental data.

The local changes of the Rh(100) electronic structure induced by sulfur adsorption at different coverage have been investigated by combining high-energy resolution core level photoemission spectroscopy, low-energy electron diffraction, Monte Carlo simulations, and ab initio calculations. Our results show that upon adsorption the local density of states does not change appreciably beyond the next neighbors, thus supporting the conclusion that the well-known catalyst's sulfur poisoning effect cannot be related to electronic structure long-range modifications. We also find that the sulfur-induced Rh 3d52component originated by the second layer Rh atoms below the sulfur adsorbate shifts by as much as −235 meV with respect to deeper layer contributions. This result points out the importance of considering the contribution of subsurface atoms in the overall 3d52core-level line shape of transition metal surfaces. Ab initio calculations allow a detailed quantitative understanding of the measured core level shifts. Possible mechanisms that explain the observed core level shifts are discussed.

The possibility of depositing precisely mass-selected Ag clusters (Ag1, Ag3, and Ag7) on Ru(0001) was instrumental in determining the importance of the in-plane coordination number (CN) and allowed us to establish a linear dependence of the Ag 3d5/2core-level shift on CN. The fast cluster surface diffusion at room temperature, caused by the low interaction between silver and ruthenium, leads to the formation of islands with a low degree of ordering, as evidenced by the high density of low-coordinated atomic configurations, in particular CN = 4 and 5. On the contrary, islands formed upon Ag7deposition show a higher density of atoms with CN = 6, thus indicating the formation of islands with a close-packed atomic arrangement. This combined experimental and theoretical approach, when applied to clusters of different elements, offers the perspective to reveal nonequivalent local configurations in two-dimensional (2D) materials grown using different building blocks, with potential implications in understanding electronic and reactivity properties at the atomic level.