Metal phthalocyanines interaction with Co mediated by a moiré graphene superlattice.

The assembling of metal phthalocyanines on the rippled moiré superlattice of graphene/Ir(111) intercalated with one Co layer is driven by the site-dependent polarization field induced by the incommensurate graphene-Co interface. We have performed an X-ray absorption and photoemission study to unveil the role of the metallic centers and of the organic ligands in the molecule-Co interaction process mediated by graphene. Notably, we consider different electronic molecular orbitals, i.e. phthalocyanines with Cu and Mn metallic ions. The spectroscopic response suggests almost unaltered CuPc molecular states upon adsorption, and the rippled graphene carpet decouples completely the electronic interaction between the molecules and the Co layer, while a slight hybridization is present for MnPcs. MnPc molecules, trapped in the valleys of the moiré graphene superlattice, slightly intermix, through the orbitals protruding out of the molecular plane, with the underlying Co, while the organic ligands are almost unaltered. Graphene acts as an interlayer and mediates the interaction between metal phthalocyanines and the metallic substrate, preventing a strong chemical intermixing and enabling the assembly of almost unaltered molecules, preserving their electronic/magnetic state.

Spin Structure of K Valleys in Single-Layer WS_{2} on Au(111).

The spin structure of the valence and conduction bands at the K[over ¯] and K[over ¯]^{'} valleys of single-layer WS_{2} on Au(111) is determined by spin- and angle-resolved photoemission and inverse photoemission. The bands confining the direct band gap of 1.98 eV are out-of-plane spin polarized with spin-dependent energy splittings of 417 meV in the valence band and 16 meV in the conduction band. The sequence of the spin-split bands is the same in the valence and in the conduction bands and opposite at the K[over ¯] and the K[over ¯]^{'} high-symmetry points. The first observation explains "dark" excitons discussed in optical experiments; the latter points to coupled spin and valley physics in electron transport. The experimentally observed band dispersions are discussed along with band structure calculations for a freestanding single layer and for a single layer on Au(111).

A first-principles study of stability of surface confined mixed metal oxides with corundum structure (Fe2O3, Cr2O3, V2O3).

Surface-confined mixed metal oxides can have different chemical properties compared to their host metal oxide support. For this reason, mixed transition metal oxides can offer tunable redox properties. Herein, we use density functional theory to predict the stability of the (0001) surface termination for mixed metal oxides consisting of Fe2O3, Cr2O3 and V2O3. We show that the pure oxide surface stability can predict the surface segregation preference of the surface-confined mixed metal oxides. We focus on substitution of Fe in the V2O3(0001) surface, for which we observe that Fe substitution increases the reducibility of the resulting mixed metal oxide surface. Our results suggest Fe is only stable on the surface under very high temperature and/or low-pressure conditions. Using thermodynamic relationships, we predict the transition points for these surface-confined mixed metal oxides at which exchange between surface/subsurface and subsurface/surface metal atoms occur due to changes in the oxygen chemical potential.

Comparing graphene growth on Cu(111) versus oxidized Cu(111).

The epitaxial growth of graphene on catalytically active metallic surfaces via chemical vapor deposition (CVD) is known to be one of the most reliable routes toward high-quality large-area graphene. This CVD-grown graphene is generally coupled to its metallic support resulting in a modification of its intrinsic properties. Growth on oxides is a promising alternative that might lead to a decoupled graphene layer. Here, we compare graphene on a pure metallic to graphene on an oxidized copper surface in both cases grown by a single step CVD process under similar conditions. Remarkably, the growth on copper oxide, a high-k dielectric material, preserves the intrinsic properties of graphene; it is not doped and a linear dispersion is observed close to the Fermi energy. Density functional theory calculations give additional insight into the reaction processes and help explaining the catalytic activity of the copper oxide surface.

The highest oxidation state observed in graphene-supported sub-nanometer iron oxide clusters.

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 Fen clusters by means of high-resolution X-ray Photoelectron Spectroscopy. We show a dependency of the core electron Fe 2p3/2 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.

Correction: In situ observation of the on-surface thermal dehydrogenation of n-octane on Pt(111).

Correction for 'In situ observation of the on-surface thermal dehydrogenation of n-octane on Pt(111)' by Daniel Arribas et al., Nanoscale, 2023, https://doi.org/10.1039/d3nr02564k.

In situ observation of the on-surface thermal dehydrogenation of n-octane on Pt(111).

The catalytic dehydrogenation of alkanes constitutes a key step for the industrial conversion of these inert sp3-bonded carbon chains into other valuable unsaturated chemicals. To this end, platinum-based materials are among the most widely used catalysts. In this work, we characterize the thermal dehydrogenation of n-octane (n-C8H18) on Pt(111) under ultra-high vacuum using synchrotron-radiation X-ray photoelectron spectroscopy, temperature-programmed desorption and scanning tunneling microscopy, combined with ab initio calculations. At low activation temperatures, two different dehydrogenation stages are observed. At 330 K, n-C8H18 effectively undergoes a 100% regioselective single C-H bond cleavage at one methyl end. At 600 K, the chemisorbed molecules undergo a double dehydrogenation, yielding double bonds in their carbon skeletons. Diffusion of the dehydrogenated species leads to the formation of carbon molecular clusters, which represents the first step towards poisoning of the catalyst. Our results reveal the chemical mechanisms behind the first stages of alkane dehydrogenation on a platinum model surface at the atomic scale, paving the way for designing more efficient dehydrogenation catalysts.

Spotting Local Environments in Self-Assembled Monolayer-Protected Gold Nanoparticles.

Organic-inorganic (O-I) nanomaterials are versatile platforms for an incredible high number of applications, ranging from heterogeneous catalysis to molecular sensing, cell targeting, imaging, and cancer diagnosis and therapy, just to name a few. Much of their potential stems from the unique control of organic environments around inorganic sites within a single O-I nanomaterial, which allows for new properties that were inaccessible using purely organic or inorganic materials. Structural and mechanistic characterization plays a key role in understanding and rationally designing such hybrid nanoconstructs. Here, we introduce a general methodology to identify and classify local (supra)molecular environments in an archetypal class of O-I nanomaterials, i.e., self-assembled monolayer-protected gold nanoparticles (SAM-AuNPs). By using an atomistic machine-learning guided workflow based on the Smooth Overlap of Atomic Positions (SOAP) descriptor, we analyze a collection of chemically different SAM-AuNPs and detect and compare local environments in a way that is agnostic and automated, i.e., with no need of a priori information and minimal user intervention. In addition, the computational results coupled with experimental electron spin resonance measurements prove that is possible to have more than one local environment inside SAMs, being the thickness of the organic shell and solvation primary factors in the determining number and nature of multiple coexisting environments. These indications are extended to complex mixed hydrophilic-hydrophobic SAMs. This work demonstrates that it is possible to spot and compare local molecular environments in SAM-AuNPs exploiting atomistic machine-learning approaches, establishes ground rules to control them, and holds the potential for the rational design of O-I nanomaterials instructed from data.

Thiolate end-group regulates ligand arrangement, hydration and affinity for small compounds in monolayer-protected gold nanoparticles.

The ability to control the properties of monolayer protected gold nanoparticles (MPNPs) discloses unrevealed features stemming from collective properties of the ligands forming the monolayer and presents opportunities to design new materials. To date, the influence of ligand end-group size and capacity to form hydrogen bonds on structure and hydration of small MPNPs (<5 nm) has been poorly studied. Here, we show that both features determine ligands order, solvent accessibility, capacity to host hydrophobic compounds and interfacial properties of MPNPs. The polarity perceived by a radical probe and its binding constant with the monolayer investigated by electron spin resonance is rationalized by molecular dynamics simulations, which suggest that larger space-filling groups - trimethylammonium, zwitterionic and short polyethylene glycol - favor a radial organization of the thiolates, whereas smaller groups - as sulfonate - promote the formation of bundles. Zwitterionic ligands create a surface network of hydrogen bonds, which affects nanoparticle hydrophobicity and maximize the partition equilibrium constant of the probe. This study discloses the role of the chemistry of the end-group on monolayer features with effects that span from molecular- to nano-scale and opens the door to a shift in the conception of new MPNPs exploiting the end-group as a novel design motif.

Anisotropic strain in epitaxial single-layer molybdenum disulfide on Ag(110).

In this work we prove that ordered single-layer MoS2 can be grown epitaxially on Ag(110), despite the different crystalline geometry of adsorbate and substrate. A comprehensive investigation of electronic and structural features of this interface is carried out by combining several techniques. Photoelectron diffraction experiments show that only two mirror crystalline domains coexist in equal amount in the grown layer. Angle-resolved valence band photoelectron spectroscopy shows that MoS2 undergoes a semiconductor-to-metal transition. Low-energy electron diffraction and scanning-tunneling microscopy experiments reveal the formation of a commensurate moiré superlattice at the interface, which implies an anisotropic uniaxial strain of the MoS2 crystalline lattice of ca. 3% in the [11̄0] direction of the Ag(110) surface. These outcomes suggest that the epitaxial growth on anisotropic substrates might be an effective and scalable method to generate a controlled and homogeneous strain in MoS2 and possibly other transition-metal dichalcogenides.

Correction: Unusual reversibility in molecular break-up of PAHs: the case of pentacene dehydrogenation on Ir(111).

[This corrects the article DOI: 10.1039/D0SC03734F.].

Atomic Undercoordination in Ag Islands on Ru(0001) Grown via Size-Selected Cluster Deposition: An Experimental and Theoretical High-Resolution Core-Level Photoemission Study.

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 3d5/2 core-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 Ag7 deposition 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.

Unusual reversibility in molecular break-up of PAHs: the case of pentacene dehydrogenation on Ir(111).

In this work, we characterize the adsorption of pentacene molecules on Ir(111) and their behaviour as a function of temperature. While room temperature adsorption preserves the molecular structure of the five benzene rings and the bonds between carbon and hydrogen atoms, we find that complete C-H molecular break up takes place between 450 K and 550 K, eventually resulting in the formation of small graphene islands at temperatures larger than 800 K. Most importantly a reversible temperature-induced dehydrogenation process is found when the system is annealed/cooled in a hydrogen atmosphere with a pressure higher than 5 × 10-7 mbar. This novel process could have interesting implications for the synthesis of larger acenes and for the manipulation of graphene nanoribbon properties.

Dual-Route Hydrogenation of the Graphene/Ni Interface.

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 situ high-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.

Hydrogen interaction with graphene on Ir(1 1 1): a combined intercalation and functionalization study.

We demonstrate a procedure for obtaining a H-intercalated graphene layer that is found to be chemically decoupled from the underlying metal substrate. Using high-resolution x-ray photoelectron spectroscopy and scanning tunneling microscopy techniques, we reveal that the hydrogen intercalated graphene is p-doped by about 0.28 eV, but also identify structures of interfacial hydrogen. Furthermore, we investigate the reactivity of the decoupled layer towards atomic hydrogen and vibrationally excited molecular hydrogen and compare these results to the case of non-intercalated graphene. We find distinct differences between the two. Finally, we discuss the possibility to form graphane clusters on an iridium substrate by combined intercalation and H atom exposure experiments.

Comparison of surface structures of corundum Cr2O3(0 0 0 1) and V2O3(0 0 0 1) ultrathin films by x-ray photoelectron diffraction.

Thin 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 soft-x-ray photoemission spectroscopy and x-ray photoelectron diffraction are used to characterize the surface layers of these two corundum-structured 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 sub-surface sites.

Graphene growth by molecular beam epitaxy: an interplay between desorption, diffusion and intercalation of elemental C species on islands.

The growth of graphene by molecular beam epitaxy from an elemental carbon precursor is a very promising technique to overcome some of the main limitations of the chemical vapour deposition approach, such as the possibility to synthesize graphene directly on a wide variety of surfaces including semiconductors and insulators. However, while the individual steps of the chemical vapour deposition growth process have been extensively studied for several surfaces, such knowledge is still missing for the case of molecular beam epitaxy, even though it is a key ingredient to optimise its performance and effectiveness. In this work, we have performed a combined experimental and theoretical study comparing the growth rate of the molecular beam epitaxy and chemical vapour deposition processes on the prototypical Ir (111) surface. In particular, by employing high-resolution fast X-ray photoelectron spectroscopy, we were able to follow the growth of both single- and multi-layer graphene in real time, and to identify the spectroscopic fingerprints of the different C layers. Our experiments, supported by density functional theory calculations, highlight the role of the interaction between different C precursor species and the growing graphene flakes on the growth rate of graphene. These results provide an overview of the main differences between chemical vapour deposition and molecular beam epitaxy growth and thus on the main parameters which can be tuned to optimise growth conditions.

The adsorption of silicon on an iridium surface ruling out silicene growth.

The adsorption of Si atoms on a metal surface might proceed through complex surface processes, whose rate is determined differently by factors such as temperature, Si coverage, and metal cohesive energy. Among other transition metals, iridium is a special case since the Ir(111) surface was reported first, in addition to Ag(111), as being suitable for the epitaxy of silicene monolayers. In this study we followed the adsorption of Si on the Ir(111) surface via high resolution core level photoelectron spectroscopy, starting from the clean metal surface up to a coverage exceeding one monolayer, in a temperature range between 300 and 670 K. Density functional theory calculations were carried out in order to evaluate the stability of the different Si adsorption configurations as a function of the coverage. Results indicate that, at low coverage, the Si adatoms tend to occupy the hollow Ir sites, although a small fraction of them penetrates the first Ir layer. Si penetration of the Ir surface can take place if the energy gained upon Si adsorption is used to displace the Ir surface atoms, rather then being dissipated differently. At a Si coverage of ∼1 monolayer, the Ir 4f spectrum indicates that not only the metal surface but also the layers underneath are perturbed. Our results point out that the Si/Ir(111) interface is unstable towards Si-Ir intermixing, in agreement with the silicide phase formation reported in the literature for the reverted interface.

Exciting H2 Molecules for Graphene Functionalization.

Hydrogen functionalization of graphene by exposure to vibrationally excited H2 molecules is investigated by combined scanning tunneling microscopy, high-resolution electron energy loss spectroscopy, X-ray photoelectron spectroscopy measurements, and density functional theory calculations. The measurements reveal that vibrationally excited H2 molecules dissociatively adsorb on graphene on Ir(111) resulting in nanopatterned hydrogen functionalization structures. Calculations demonstrate that the presence of the Ir surface below the graphene lowers the H2 dissociative adsorption barrier and allows for the adsorption reaction at energies well below the dissociation threshold of the H-H bond. The first reacting H2 molecule must contain considerable vibrational energy to overcome the dissociative adsorption barrier. However, this initial adsorption further activates the surface resulting in reduced barriers for dissociative adsorption of subsequent H2 molecules. This enables functionalization by H2 molecules with lower vibrational energy, yielding an avalanche effect for the hydrogenation reaction. These results provide an example of a catalytically active graphene-coated surface and additionally set the stage for a re-interpretation of previous experimental work involving elevated H2 background gas pressures in the presence of hot filaments.

Thermolubricity of gas monolayers on graphene.

Nanofriction of Xe, Kr and N2 monolayers deposited on graphene was explored with a quartz crystal microbalance (QCM) at temperatures between 25 and 50 K. Graphene was grown by chemical vapour deposition and transferred to the QCM electrodes with a polymer stamp. It was found to strongly adhere to the gold electrodes at temperatures as low as 5 K and at frequencies up to 5 MHz. At low temperatures, the Xe monolayers are fully pinned to the graphene surface. Above 30 K, the Xe film slides and the depinning onset coverage beyond which the film starts sliding decreases with temperature. Similar measurements repeated on bare gold show an enhanced slippage of the Xe films and a decrease of the depinning temperature below 25 K. Nanofriction measurements of Kr and N2 confirm this scenario. This thermolubric behaviour is explained in terms of a recent theory of the size dependence of static friction between adsorbed islands and crystalline substrates.