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Momentum-resolved high-resolution electron energy loss spectroscopy has been used to probe the phonon dispersion of periodically rippled graphene grown on Ru(0001). Optical phonons in this system show anomalies with respect to other graphene/metal interfaces. Novel modes with weak intensity are observed at ∼86 and ∼150 meV. Moreover, the dispersion relation of the highest optical branches in the Γ-K direction strongly deviates from the expected behavior. These unusual features in the phonon dispersion should be related to the corrugated and chiral structure of the graphene overlayer on Ru(0001).

The Pt3Ni(111) alloy is characterized by the presence of an outermost surface layer only composed by Pt atoms, that is, the so-called Pt skin, which is at the basis of its outstanding performances ...

Investigations on plasmonic modes of graphene, topological insulators, black phosphorus, boron nitride and two-dimensional carbides by means of transmission electron microscopy are here reviewed. Electron energy loss spectroscopy with atomic resolution allows observing the enhancement of graphene plasmons at substitutional atoms. For topological insulators, the two existing modes in the ultraviolet range have different dispersion relation, thus evidencing band-structure effects on plasmon dispersion. Multiple plasmonic modes in the visible range have been reveled in topological-insulator nanoplatelets. Concerning black phosphorus, the plasmonic spectrum changes with thickness: while the bulk is characterized by a single plasmon mode around 20 eV, other low-energy interband plasmons emerge in few layers. Likewise, an additional mode at 7.5 eV is present in the plasmonic spectrum of few-layer hexagonal boron nitride ((h-BN)). Interband plasmons in graphene, (h-BN), or transition-metal dichalcogenides exhibit frequency dependence on thickness. Such a phenomenon is absent in two-dimensional carbides, for which the bulk plasmon does not change with thickness, while the surface plasmon can be tuned by chemical functionalization and/or thickness.

By means of high-resolution electron energy loss spectroscopy, we investigate the low-energy excitation spectrum of transition-metal monopnictides hosting Weyl fermions. We observe gapped plasmonic modes in (001)-oriented surfaces of single crystals of NbAs and TaAs at 66 and 68 meV, respectively. Our findings are consistent with theory and we estimate an effective Coulomb interaction strength $\alpha_{\rm eff}\approx0.41$ for both samples. We also demonstrate that the modification of the surface of transition-metal monopnictides by the adsorption of chemical species (in our case, oxygen and hydrocarbon fragments) changes the frequency of the plasmonic excitations, with a subsequent modification of the effective interaction strength in the 0.30-0.48 range. The remarkable dependence of plasmonic features on the presence of adsorbates paves the way for plasmonic sensors based on Weyl semimetals operating in the mid-infrared., Comment: 7 pages, 3 figures

By using high-resolution electron energy loss spectroscopy and density functional theory, we have studied the interaction of carbon monoxide with a defected graphene sheet. Both experiments and theory indicate that unsaturated C atoms at vacancies are able to capture carbon monoxide at room temperature. Definitely, the sequestration of carbon monoxide at carbon vacancies implies the formation of a weak C-O-C bond, evidenced by its peculiar vibrational energy of 150 meV (1209 cm-1). We also show that, by tuning the density of vacancies, it is possible to obtain a selective CO adsorption at vacancies without intercalation under the graphene cover. Captured CO molecules are stably adsorbed up to a temperature of 500 K. These findings represent the initial step toward the development of processes for the graphene-mediated partial sequestration and selective oxidation of CO.

Cadmium arsenide (Cd3As2) has recently attracted considerable interest for the presence of 3D massless Dirac fermions with ultrahigh mobility and magnetoresistance. However, its surface properties are currently largely unexplored both theoretically and experimentally, due to the very large unit cell and the challenging growth of single-crystal samples, respectively. Here, by combining ab initio calculations with surface-science spectroscopic experiments, the presence of a surface reconstruction is unveiled in centimeter-scale (112)-oriented Cd3As2 single-crystal foils produced by the self-selecting vapor growth. Outermost Cd atoms descend into the As-sublayer with a subsequent self-passivation of the dangling bonds with As atoms, forming the triangle lattice previously imaged by scanning tunneling microscopy. Moreover, the oxidation mechanism of this reconstructed surface, dominated by the formation of As?O?Cd bonds, is revealed. Interestingly, it is found that the band structure of the reconstructed surface of Cd3As2 is quite robust against surface oxidation. Both computational and experimental findings point to a successful exploitation in technology of Cd3As2 single crystals.

Materials exhibiting nodal-line fermions promise superb impact on technology for the prospect of dissipationless spintronic devices. Among nodal-line semimetals, the ZrSiX (X = S, Se, Te) class is the most suitable candidate for such applications. However, the surface chemical reactivity of ZrSiS and ZrSiSe has not been explored yet. Here, by combining different surface-science tools and density functional theory, it is demonstrated that the formation of ZrSiS and ZrSiSe surfaces by cleavage is accompanied by the washing up of the exotic topological bands, giving rise to the nodal line. Moreover, while the ZrSiS has a termination layer with both Zr and S atoms, in the ZrSiSe surface, reconstruction occurs with the appearance of Si surface atoms, which is particularly prone to oxidation. It is demonstrated that the chemical activity of ZrSiX compounds is mostly determined by the interaction of the Si layer with the ZrX sublayer. A suitable encapsulation for ZrSiX should not only preserve their surfaces from interaction with oxidative species, but also provide a saturation of dangling bonds with minimal distortion of the surface. © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim U.S. Department of Energy, USDOE: DE-SC0014208 A.P. thanks Elettra Sincrotrone Trieste S.C.p.A. for financial support. Z.M. thanks the support by the U.S. Department of Energy under grant DE-SC0014208 for material synthesis. This work was partly performed in the framework of the Nanoscience Foundry and Fine Analysis facility (NFFA-MIUR Italy Progetti Internazionali).

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.

We explore the broadband excitation spectrum of bulk PtTe$_2$ using electron energy loss spectroscopy and density functional theory. In addition to infrared modes related to intraband 3D Dirac plasmon and interband transitions between the 3D Dirac bands, we observe modes at 3.9, 7.5 and 19.0 eV in the ultraviolet region. The comparison of the excitation spectrum with the calculated orbital-resolved density of states allows us to ascribe spectral features to transitions between specific electronic states. Additionally, we study the thickness dependence of the high-energy plasmon in the PtTe2 thin films. We show that, unlike graphene, the high-energy plasmon in PtTe2 thin film gets red-shifted by 2.5 eV with increasing thickness., 9 pages with 7 figures

Transition-metal dichalcogenides showing type-II Dirac fermions are emerging as innovative materials for nanoelectronics. However, their excitation spectrum is mostly unexplored yet. By means of high-resolution electron energy loss spectroscopy and density functional theory, here, we identify the collective excitations of type-II Dirac fermions (3D Dirac plasmons) in PtTe_{2} single crystals. The observed plasmon energy in the long-wavelength limit is ∼0.5 eV, which makes PtTe_{2} suitable for near-infrared optoelectronic applications. We also demonstrate that interband transitions between the two Dirac bands in PtTe_{2} give rise to additional excitations at ∼1 and ∼1.4 eV. Our results are crucial to bringing to fruition type-II Dirac semimetals in optoelectronics.

By means of momentum-resolved electron energy loss spectroscopy (EELS) coupled with scanning transmission electron microscopy, we have studied the dispersion relation of interband plasmonic modes in the ultraviolet in black phosphorus. We find that the dispersion of the interband plasmons is anisotropic. Experimental results are reproduced by density functional theory, by taking into account both the anisotropy of the single-particle response function, arising from the anisotropic band structure, and the damping. Moreover, our theoretical model also indicates the presence of low-energy excitations in the near-infrared that are selectively active in the armchair direction, whose existence has been experimentally validated by high-resolution EELS (HREELS) in reflection mode.

The excitation spectrum of graphene on Cu(111), probed by momentum-resolved electron-energy-loss spectroscopy, exhibits multiple acoustic surface plasmons (ASP), arising from both the graphene overlayer and the Cu(111) substrate. We have investigated the mutual interaction between the ASP of the graphene overlayer and that of the underlying Cu(111) substrate. The weak interaction between graphene and Cu(111) implies that the ASP of Cu(111) survives, with only slight changes in its group velocity. Remarkably, we find that the damping processes of ASP are mitigated as compared to the cases of other graphene/metal interfaces. Therefore, this interface is an ideal graphene/metal contact for graphene-based plasmonics. Moreover, the presence of graphene coating protects the ASP of Cu(111) from quenching in an ambient atmosphere.

Here, we examine the influence of surface chemical reactivity toward ambient gases on the performance of nanodevices based on two-dimensional materials “beyond graphene” and novel topological phases of matter. While surface oxidation in ambient conditions was observed for silicene and phosphorene with subsequent reduction of the mobility of charge carriers, nanodevices with active channels of indium selenide, bismuth chalcogenides and transition-metal dichalcogenides are stable in air. However, air-exposed indium selenide suffers of p-type doping due to water decomposition on Se vacancies, whereas the low mobility of charge carriers in transition-metal dichalcogenides increases the response time of nanodevices. Conversely, bismuth chalcogenides require a control of crystalline quality, which could represent a serious hurdle for up scaling.

By analyzing phonon dispersion, we have evaluated the average Young’s modulus and Poisson’s ratio in graphite and in graphene grown on Ru(0001), Pt(111), Ir(111), Ni(111), and BC3/NbB2(0001). In both flat and corrugated graphene sheets and in graphite, we find a Poisson’s ratio of 0.19 and a Young’s modulus of 342 N/m. The unique exception is graphene/Ni(111), for which we find different values because of the stretching of C-C bonds occurring in the commensurate overstructure (0.36 and 310 N/m for the Poisson’s ratio and Young’s modulus, respectively). Such findings are in excellent agreement with calculations performed for a free-standing graphene membrane. The high crystalline quality of graphene grown on metal substrates leads to macroscopic samples with high tensile strength and bending flexibility for use in technological applications such as electromechanical devices and carbon-fiber reinforcements.

When the phonon spectrum of a material is measured in a scattering experiment, selection rules preclude the observation of phonons that are odd under reflection by the scattering plane. Understanding these rules is crucial to correctly interpret experiments and to detect broken symmetries. Taking graphene as a case study, in this work we derive the complete set of selection rules for the honeycomb lattice, showing that some of them have been missed or misinterpreted in the literature. Focusing on the technique of high-resolution electron energy loss spectroscopy (HREELS), we calculate the scattering intensity for a simple force constant model to illustrate these rules. In addition, we present HREELS measurements of the phonon dispersion for graphene on Ru(0001) and find excellent agreement with the theory. We also illustrate the effect of different symmetry breaking scenarios in the selection rules and discuss previous experiments in light of our results., Comment: 8 pages, 5 figures

High-resolution electron energy loss spectroscopy is a suitable tool for investigating phonons in graphene, due to its exceptional energy resolution in both the energy and momentum domains. In this chapter, we show that the experimental phonon dispersion of graphene can be used to estimate elastic properties and electron–phonon coupling. Novel coupling mechanisms of Dirac cone electrons in graphene with out-of-plane optical phonons of the graphene lattice, activated only whenever the graphene sheet is supported by a solid substrate, are also discussed.

Phonon dispersion in periodically rippled graphene/Ru(0001) has been studied by high-resolution electron energy loss spectroscopy and density functional theory. We show that, in addition to the usual phonon modes of the free-standing graphene lattice, the phonon spectrum of graphene/Ru(0001) exhibits additional modes arising from the peculiar nanostructured morphology of this system. In particular, the out-of-plane optical phonon has two components at 83 and 105 meV, related to the spectral contribution arising from carbon atoms in the valleys and in the hills of the ripples, respectively.

By means of angle-resolved electron energy loss spectroscopy, we have measured the interband it plasmon in high-quality graphene grown on peeled-off epitaxial Cu(111) foils. Experimental loss spectra have been reproduced by means of a hydrodynamic model. The dispersion relation of the plasmon frequency shows a nearly-flat dispersion up to a critical wave-vector of 0.3 angstrom(-1). We propose that the observed behavior could be originated by confinement effects in ripples in the strained graphene sheet. Strain also limits the dispersion at higher momenta, as a consequence of the increased effective mass of charge carriers. The analysis of momentum dependence of the line-width and of the inverse quality factor indicates that damping processes are dominated by decay in electron-hole pairs via indirect interband transitions. (C) 2016 Elsevier Ltd. All rights reserved.

High-resolution electron energy loss spectroscopy has been used to study the coadsorption of water and carbon monoxide at the Pt3Ni(111) surface. The analysis of the vibrational spectrum indicates the reaction between coadsorbates (CO and OH groups of ice) with the formation of HOCO. Di-σ-trans-HOCO species are stabilized on the surface at up to 165 K. For higher temperature, HOCO desorbs from the surface.

The electronic response of self-doped graphene supported on metal substrates shows two well-distinct plasmon modes: the acoustic plasmon, characterized by a linear dispersion, and a nonlinear plasmon. The nonlinear plasmon originates from the anisotropy of the electron bands of graphene around the K point. The existence of two plasmons with a different dispersion relation demonstrates that charge carriers with different group velocity may coexist within the same band in epitaxial graphene.

Plasmons in graphene have unusual properties and offer promising prospects for plasmonic applications covering a wide frequency range, ranging from terahertz up to the visible. Plasmon modes have been recently studied in both free-standing and supported graphene. Here, we review plasmons in graphene with particular emphasis on plasmonic excitations in epitaxial graphene and on the influence of the underlying substrate on the screening processes. Although the theoretical comprehension of plasmons in supported graphene is still incomplete, several experimental results provide clues regarding the nature of plasmonic excitations in graphene on metals and semiconductors. Plasmon in graphene can be tuned by chemical doping and gating potentials. We show through selected examples that the adsorbates can be used to tune the plasmon frequency, while the intercalation of chemical species allows the decoupling of the graphene sheet from the substrate to recover the plasmon dispersion of pristine graphene. Finally, we also report intriguing effects due to many-body interaction, such as the excitations generated by electron-electron coupling (magnetoplasmons) and the composite modes arising from the coupling of plasmons with phonons and with charge carriers.

Understanding the microscopic mechanism of oxygen interaction with the surfaces of Pt-based alloys is an important issue for applications in various fields such as corrosion and oxygen reaction reduction in fuel cells, for which Pt-based alloys are more efficient catalysts with respect to platinum. Herein, the interaction of oxygen with Pt3Ni(111) has been studied by X-ray photoemission, X-ray absorption, and high-resolution electron energy loss spectroscopies. We have found that the oxidation of Pt3Ni(111) at 600 K leads to the segregation of Ni atoms to the surface region. The presence of Ni atoms at the Pt3Ni(111) surface allows O-2 to dissociate to form NiOx surface species. High-resolution electron energy loss spectroscopy measurements performed on Pt(111), Ni(111), and Pt3Ni(111) directly show that, on the Pt3Ni(111) surface, oxygen binds selectively on Ni sites and, moreover, that the O-Ni bond formed on Pt3Ni is weaker than the O-Pt bond formed on Pt(111).

We have studied the interaction of CO with epitaxial graphene on Pt(1 1 1) and Ru(0 0 0 1) by means of high-resolution electron energy loss spectroscopy measurements. Our experiments unambiguously demonstrate that in ultra-high vacuum conditions CO does not intercalate underneath the graphene monolayer supported on Pt(1 1 1) and Ru(0 0 0 1). For submonolayer coverages of graphene on Pt(1 1 1), CO adsorption occurs only in Pt on-top sites while bridge sites, usually populated on the clean Pt(1 1 1) surface, are inhibited. Instead, we find that epitaxial graphene is rather reactive toward water molecules.

The interaction of water with periodically rippled graphene deposited on Ru(0 0 0 1) and nearly-flat graphene/Pt(1 1 1) has been investigated by using high-resolution electron energy loss spectroscopy. Graphene samples were exposed to ambient air humidity as well as to water molecules under controlled conditions in vacuum. In both cases, loss measurements show that water molecules dosed at room temperature dissociate giving rise to C–H bonds. We suggest that water molecules intercalate under graphene and are split by the underlying metal catalyst. On the lattice-mismatched graphene/Ru(0 0 0 1) interface, the corrugation of the graphene overlayer induces site selectivity for H adsorption in ortho and para dimers. On the other hand, no dimer formation occurs for the nearly-flat, multidomain graphene/Pt(1 1 1) interface.

The collective electronic excitations in thin Ag films deposited onto the Ni(111) surface were studied by high-resolution electron energy loss spectroscopy. A broad loss peak at 7.7 eV was assigned to the Ag multipole plasmon, in excellent agreement with calculations based on s-d polarization model. Ag multipole plasmon was excited only at grazing incidence. Furthermore, a strong dependence on the impinging energy exists. Multipole plasmon could be measured only for a very strict range of primary electron beam energies and it was excited by electrons scattered at a reflection plane located just underneath the jellium edge. Such mode was found to be dramatically more sensible to the state of the surface with respect to ordinary surface plasmon. Moreover, we report experimental evidence of interference effects in surface plasmon excitation.

This work reports the peculiar properties of a graphene film prepared by the chemical vapor deposition of ethylene in high vacuum on a well oriented and carefully cleaned Pt(111) crystal surface maintained at high temperature. In-situ and ex-situ characterization techniques (low-energy electron diffraction, high-resolution electron energy loss spectroscopy, scanning electron microscopy and Raman micro-spectroscopy) used here indicate the prevalence of single-layer regions and the presence of two different orientations of the graphene sheets with respect to the Pt(111) substrate. In most of the deposited area, evidence is found of a compressive stress for the graphene lattice, as a net result of the growth process on a metal substrate. This graphene film grown on Pt(111) exhibits a lower degree of order and of homogeneity with respect to the exfoliated graphene on Si/SiO2, as it is found generally for graphene on metals, but several characterization techniques indicate a better quality than in previous deposition experiments on the same metal substrate. Copyright © 2013 John Wiley & Sons, Ltd.

By means of vibrational spectroscopy and density functional theory (DFT), we investigate CO adsorption on phosphorene-based systems. We find stable CO adsorption at room temperature on both phosphorene and bulk black phosphorus. The adsorption energy and vibrational spectrum are calculated for several possible configurations of the CO overlayer. We find that the vibrational spectrum is characterized by two different C–O stretching energies. The experimental data are in good agreement with the prediction of the DFT model and reveal the unusual C–O vibrational band at 165–180 meV, activated by the lateral interactions in the CO overlayer. [Figure not available: see fulltext.] © 2016, Tsinghua University Press and Springer-Verlag Berlin Heidelberg.

We have studied the electronic excitation spectrum in periodically rippled graphene on Ru(0 0 0 1) and flat, commensurate graphene on Ni(1 1 1) by means of high-resolution electron energy loss spectroscopy and a combination of density functional theory and tight-binding approaches. We show that the periodic moiré superlattice originated by the lattice mismatch in graphene/Ru(0 0 0 1) induces the emergence of an extra mode, which is not present in graphene/Ni(1 1 1). Contrary to the ordinary intra-band plasmon of doped graphene, the extra mode is robust in charge-neutral graphene/metal contacts, having its origin in electron–hole inter-band transitions between van Hove singularities that emerge in the reconstructed band structure, due to the moiré pattern superlattice., The support by the Netherlands National Computing Facilities foundation (NCF) are acknowledged. SY and MIK thank financial support from the European Research Council Advanced Grant program (contract 338957). The research has also received funding from the European Union Seventh Framework Programme under Grant Agreement No. 604391 Graphene Flagship. RR acknowledges financial support from MINECO (Spain) through grant FIS2014-58445-JIN.

In this review, we present the applications of thermoplasmonics in membrane processes. We discuss the influence of the heat capacity of the solvent, the amount of plasmonic nanoparticles in the membrane, the intensity of the light source and the transmembrane flow rate on the increase of permeability. Remarkably, thermoplasmonic effects do not involve any noticeable loss of membrane rejection. Herein, we consider application feasibilities, including application fields, requirements of feed, alternatives of light sources, promising thermoplasmonic nanoparticles and scaling up issues.

We demonstrate that, in contrast to most two-dimensional materials, ultrathin flakes of InSe are stable under ambient conditions. Despite their ambient stability, InSe-based nanodevices show an environmental p-type doping, suppressed by capping InSe with hexagonal boron nitride. By means of transport experiments, density functional theory and vibrational spectroscopy, we attribute the p-type doping assumed by uncapped InSe under an ambient atmosphere to the decomposition of water at Se vacancies. We have estimated the site-dependent adsorption energy of O2, N2, H2O, CO and CO2 on InSe. A stable adsorption is found only for the case of H2O, with a charge transfer of only 0.01 electrons per water molecule.

We present a theoretical modeling of the energy-loss spectroscopy data for monolayer graphene (MLG) supported by Pt(111), Ru(0001) and Ni(111) substrates. To reproduce the experimental loss function, we have used a two-dimensional, two-fluid hydrodynamic model for interband transitions of graphenes pi and sigma electrons and an empirical Drude-Lorentz model in the local approximation for metal substrates. The electronic response from the visible to the ultraviolet frequency range has been nicely reproduced for MLG/Pt(111) and MLG/Ru(0001). For graphene nanodomes on Ru(0001), the loss function of valleys shows an additional mode at 8-9 eV. By contrast, this models fails for the case of MLG/Ni(111), presumably due to the strong hybridization between the pi states of graphene and the d bands of Ni, which is not accounted for in the model. (C) 2015 Elsevier Ltd. All rights reserved.

Pt3Ni(1 1 1) is the most efficient catalyst for the oxygen reduction reaction. Herein, we investigated the oxidation process of Pt3Ni(1 1 1) by time-resolved X-ray photoemission spectroscopy experiments with synchrotron radiation. For O2 exposure at room temperature, two well-distinct O species are present with a sticking coefficient of 0.23–0.24 in the temperature range of interest for fuel-cell technology. We show that the Pt skin of Pt3Ni(1 1 1) is selectively replaced by Ni2O3 and NiO surface-skin layers upon heating in an O2 environment at 700 and 900 K, respectively. By dosing with O atoms, the formation of PtO is possible at 293 K, whereas Ni2O3 is formed at 353 K.

The average elastic properties and the interatomic force constants of a quasi-freestanding graphene sample have been evaluated by analyzing the phonon dispersion measured by means of angle-resolved energy loss spectroscopy. We found a Poisson’s ratio of 0.19 and a Young’s modulus of 342 N/m. Such findings are in excellent agreement with calculations performed for a free-standing graphene membrane. Present results indicate that the high crystalline quality of graphene grown on metal substrates leads to macroscopic samples of high tensile strength and bending flexibility to be used for technological applications such as electromechanical devices and carbon-fiber reinforcements.

Vibrational spectroscopy has been used to study the interaction of water with quasi-freestanding graphene grown on Pt(1 1 1). A sharp O–H (O–D) vibrational band centered at 457 (337) meV is a direct evidence of the existence of non-H- (non-D-) bonded water molecules at the water/graphene interface. This finding is expected to play a significant role in understanding the behavior of water at hydrophobic surfaces. Such finding is in agreement with results reported for water adsorbed on other hydrophobic surfaces and with the behavior of water confined in carbon nanotubes and between graphene sheets.

The understanding of the elementary steps occurring in catalytic reactions in the heterogeneous phase is one of the foremost goals of surface science. Adsorption on solid surfaces is the first step in catalytic reactions. Therefore, the individuation of adsorption sites for reactive chemical species is essential information to tailor catalytic properties of surfaces and interfaces. As a matter of fact, the change in adsorption site often implies a different reactivity for chemisorbed adsorbates and a selective catalytic activity. In this Feature Article, we evidence how vibrational spectroscopy can be used for individuating adsorption sites in coadsorption systems on catalyst surfaces. In particular, we studied CO coadsorption with oxygen, nitrogen, and hydrogen on the Ni(111) and the Ni(100) surfaces. Our attention was focused on the determination of CO adsorption sites in the various investigated systems. For CO adsorbed alone on the substrate, the preferential adsorption sites are the 3-fold hollow on ...

High-resolution electron energy loss spectroscopy has been used to study the coadsorption of CO and O on Ni(1 1 1) and Ni(1 0 0) surfaces at 250 K. These vibrational measurements showed that with increasing surface coverage at 250 K, the preadsorbed O caused changes in the adsorption sites for the after-adsorbed CO molecules revealing, however, dissimilar adsite alterations for the two Ni single crystals. Also, a different behavior towards carbonate formation was found upon CO adsorption at 250 K on Ni(1 0 0) and Ni(1 1 1) surfaces precovered with O at 250 K.

High-resolution electron energy loss spectroscopy has been used to study low-energy bulk excitation modes of planar-bound electrons (bulk plasmons) in nickel. We observed for the first time a bulk plasmon at about 1.2 eV, in agreement with dielectric theory. The behavior of its amplitude with the off-specular angle ensures the dipolar nature of such mode. On the other hand, the intensity of the plasmon peak is vanishing upon ion bombardment due to the sputtering-induced modification of dielectric function.

High-resolution electron energy loss spectroscopy has been used to investigate the catalytic properties of Na-doped Au monolayer grown on Cu(111). The presence of Na atoms allows the dissociation of oxygen molecules and the stabilization of atomic oxygen. The strong reduction of the dissociation barrier for O2 promoted by Na could readily favor many surface chemical reactions, due to the key role of atomic oxygen in many oxidation processes catalyzed by gold.

Vibrational spectroscopy has been used to study the interaction at room temperature of H 2 O with ultrathin alkali films grown on Cu(1 1 1), Ni(1 1 1) and Ag/Ni(1 1 1). We find strongly enhanced dissociation efficiency in the monolayer regime of Na/Cu(1 1 1), indicative of an active role of the existing Na-derived quantum wells in hydrolysis. No water adsorption has been observed for alkalis (Na, K) adsorbed on Ni(1 1 1), very likely due to the absence of electron confinement in these systems. On the other hand, partial water dissociation has been revealed in K-doped silver thin layers on Ni(1 1 1), in the presence of Ag 5sp-derived quantum wells.

PtTe2 is a novel transition-metal dichalcogenide hosting type-II Dirac fermions that displays application capabilities in optoelectronics and hydrogen evolution reaction. Here it is shown, by combining surface science experiments and density functional theory, that the pristine surface of PtTe2 is chemically inert toward the most common ambient gases (oxygen and water) and even in air. It is demonstrated that the creation of Te vacancies leads to the appearance of tellurium-oxide phases upon exposing defected PtTe2 surfaces to oxygen or ambient atmosphere, which is detrimental for the ambient stability of uncapped PtTe2-based devices. On the contrary, in PtTe2 surfaces modified by the joint presence of Te vacancies and substitutional carbon atoms, the stable adsorption of hydroxyl groups is observed, an essential step for water splitting and the water–gas shift reaction. These results thus pave the way toward the exploitation of this class of Dirac materials in catalysis. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Horizon 2020 Framework Programme, H2020: 696656-GrapheneCore1 A.P. and G.C. thank Vito Fabio for technical support and Marilena Zappia for having participated in HREELS experiments. This work was partly performed in the framework of the Nanoscience Foundry and Fine Analysis (NFFA-MIUR Italy Progetti Internazionali) project under Proposal No. 2017004. This project received funding from the European Union’s Horizon 2020 research and innovation program under Grant Agreement No. 696656-GrapheneCore1.

High-resolution electron energy loss spectroscopy was used to investigate the electronic properties of (111)-oriented Au ultrathin films grown on Cu(111). The loss spectrum showed several features which were ascribed to both single and collective excitations. In particular we distinguished features assignable to dispersionless single-particle transitions and the dispersing electron-hole continuum as well as the ordinary and s-like surface plasmon, and the multipole surface plasmon.

We present high-resolution energy loss spectroscopy measurements on the dispersion and damping of collective electronic excitations in ultrathin Ag films grown on Cu(111) and Ni(111). For Ag/Cu(111) the surface plasmon was found to be confined within grains but confinement was removed by annealing. On the other hand, a purely quadratic dispersion was observed for the layer-by-layer Ag film deposited onto the Ni(111) surface.

The coadsorption of Na and CO on Ni(111) at room temperature was investigated using high-resolution electron energy loss spectroscopy. A notable softening of the Na–Ni bond was revealed in the Na+CO coadsorption phase. A lengthening of the Na–Ni bond was observed upon annealing the substrate. Oxygen adsorption on Na+CO/Ni(111) caused a significant blue-shift of the C–O stretching frequency. Exposing the Na/Ni(111) to oxygen, no Na–O bond was observed. On the contrary, the formation of the Na–O bond was easily achieved by exposing the Na+CO/Ni(111) phase to oxygen.

The surface plasmon in nanoscale thin Ag films deposited on Ni(111) was studied as a function of the impinging electron beam energy and of the scattering angle. The nonmonotonous behavior of the amplitude of the Ag surface plasmon suggests the occurrence of interference effects between the incoming and the outgoing field. Such interference phenomena occur whenever the scattering plane is in the close vicinities of the jellium edge. An alternative explanation involves diffraction effects.

The damping processes of electronic collective excitations of Ag/Ni(1 1 1) were studied by high-resolution electron energy spectroscopy. The FMHM of the Ag surface plasmon was reported as a function of Ag thickness, primary electron beam energy, Ag surface plasmon energy, and parallel momentum transfer. The broadening of the Ag surface plasmon was found to be related to 5sp–5sp transitions, for which a critical wave vector of 0.2 A −1 exists. Moreover, we provide a direct evidence of the occurrence of chemical

The effects of the annealing procedure at 400–450 K on the electronic properties of nanoscale thin films of Ca, Au and Ag grown on Cu(1 1 1) at room temperature were probed by high-resolution electron energy loss spectroscopy measurements. Ca surface plasmon underwent to a significant red-shift upon annealing, due to the oxidation of the topmost Ca layer. Water strongly interacted with the CaO interface at room temperature. Au surface plasmon disappeared upon annealing the gold film, as a consequence of the formation of an Au–Cu alloy. Ag surface plasmon red-shifted both in the annealed adlayer and with increasing temperature compared with the frequency recorded for the as-deposited silver film.