Single Crystal Metal Surfaces Research Articles

Theoretical Investigation on the Initial Reaction Mechanism of Hexaethynylbenzene on Au(111) Surface.

Graphyne has attracted considerable interest and attention since its successful synthesis, due to its enormous potential for applications in the fields of electronics, energy, catalysis, information technology, etc. Although various methods for synthesizing graphyne have been explored, single-layer graphynes have not been successfully developed. Hexaethynylbenzene (HEB) is considered an ideal precursor molecule because it can undergo Glaser coupling reactions between molecules to synthesize single layer graphdiyne on single crystal metal surfaces via on-surface reactions. Unfortunately, this method fails to achieve the expected results, and the underlying mechanism is not clear. In this work, we employed a combination of ab initio molecular dynamics (AIMD) and quantum mechanics (QM) methods to investigate the initial reaction mechanism of HEB molecules on a Au(111) surface. We revealed that HEB molecules undergo both intermolecular coupling and intramolecular cyclization on the Au(111) surface. The favorable pathways of these two types of reactions were then distinguished, confirming that the distance between the terminal carbon atoms of the ethynyl groups plays an important role in C-C coupling. The insights revealed from this work could facilitate the rational design of precursor molecules and deepen the understanding of the reaction processes.

An approach for patterned molecular adsorption on ferromagnets, achieved via Moiré superstructures.

We have used a scanning tunneling microscope operated under ultrahigh vacuum conditions to investigate an oxo-vanadium-salen complex V(O)salen, that has potential applications as qubits in future quantum-based technologies. The adsorption and self-assembly of V(O)salen on a range of single crystal metal surfaces and nanoislands and the influence of substrate morphology and reactivity has been measured. On the close-packed flat Ag(111) and Cu(111) surfaces, the molecules adsorb isolated or form small clusters arranged randomly on the surface, whereas structured adsorption occurs on two types of Co nanoislands; Co grown on Ag(111) and Ag capped Co islands grown on Cu(111), both forming a Moiré pattern at the surface. The adsorption configuration can by scanning tunneling spectroscopy be linked to the geometric and electronic properties of the substrates and traced back to a Co d-related surface state, illustrating how the modulated reactivity can be used to engineer a pattern of adsorbed molecules on the nanoscale.

Mixed Germania‐Silica Films on Ru(0001): A Combined Experimental and Theoretical Study

AbstractTwo‐dimensional oxide films are potentially useful for future technological applications, but also important objects to study model catalyst systems on the more fundamental side. Here we study silica, germania, and mixed silica‐germania films supported on a metal single crystal surface Ru(0001). Those mixed films are interesting objects to systematically modify the properties of silica films, which may be used as membranes or covers for model studies in confined space, due to the modification of the rather stiff silica layers by incorporating germanium atoms replacing silicon atoms. Here we report a combined experimental and theoretical study of such layers, where we show how X‐ray photoelectron spectroscopy in combination with LEED and I/V LEEM measurements allow us to judge the formation of such mixed films.

Predicting adsorption energies and the physical properties of H, N, and O adsorbed on transition metal surfaces: A machine learning study

We present a machine learning study utilizing the Hierarchically Interacting Particle Neural Network to predict the adsorption properties of atomic H, N, and O on various single metal and bimetallic single crystal FCC surfaces. Our trained HIP-NN models directly predict, from atomic species and coordinates of the clean surface, and initial adsorption site, adsorption energies with a MAE of 0.08 eV to 0.16 eV, adsorption heights with a MAE of 0.08 Å to 0.09 Å, and a buckling of the first layer of the substrate with a MAE of 0.07 Å to 0.12 Å, illustrating that a neural network can calculate the physical properties of a surface/adsorbate system along with adsorption energy. Our MAEs for adsorption energy also compare favorably with MAEs presented by other works predicting the adsorption energies of H, N, and O. We also demonstrate that while HIP-NN can predict on these systems without explicit periodic boundary conditions, the inclusion of explicit periodic boundary conditions drastically improves the results. Our results also reveal that the buckling of the first layer of the substrate can be utilized to prune datasets, removing unstable surfaces.

Measurements of Ion Transfer Kinetics for the Study of Electrocatalyst Corrosion in Gas Diffusion Electrode Microenvironments

Nanoparticulate Au, Ag, and Cu films with percolating ion-conducting polymers are common within devices for the electroreduction of carbon dioxide (CO2) to carbon monoxide, hydrocarbons, and alcohols. The kinetics of ion-transfer are influential on the rate of corrosion and catalyst sintering, but direct measurements of ion-transfer rates are challenging at bulk metal electrodes. We present data on the corrosion kinetics of metal nanoclusters and monolayers deposited via underpotential deposition on well-oriented and/or single-crystal metal surfaces within suspended ionomer electrolytes. Our results support the important role of nucleation and growth in corrosion processes as well as the influence of structural anions on reactive intermediates present during corrosion reactions. Figure 1

Slip-Line-Guided Growth of Graphene.

Manipulating the crystal orientation of emerging 2D materials via chemical vapor deposition (CVD) is a key premise for obtaining single-crystalline films and designing specific grain-boundary (GB) structures. Herein, the controllable crystal orientation of graphene during the CVD process is demonstrated on a single-crystal metal surface with preexisting atomic-scale stair steps resulting from dislocation slip lines. The slip-line-guided growth principle is established to explain and predict the crystal orientation distribution of graphene on a variety of metal facets, especially for the multidirectional growth cases on Cu(hk0) and Cu(hkl) substrates. Not only large-area single-crystal graphene, but also bicrystal graphene with controllable GB misorientations, are successfully synthesized by rationally employing tailored metal substrate facets. As a demonstration, bicrystal graphenes with misorientations of ≈21° and ≈11° are constructed on Cu(410) and Cu(430) foils, respectively. This guideline builds a bridge linking the crystal orientation of graphene and the substrate facet, thereby opening a new avenue for constructing bicrystals with the desired GB structures or manipulating 2D superlattice twist angles in a bottom-up manner.

Surface Structure-Sensitive Enantioselectivity: Aspartic Acid Reaction Kinetics on All Surfaces Vicinal to Cu(111)

Enantioselective reactions on chiral, single-crystal metal surfaces can provide novel insight into the origins of enantioselectivity in chiral molecular adsorption and catalysis. Using a high-throughput approach to study the surface of a spherical Cu single crystal, we have measured the surface reaction kinetics of aspartic acid decomposition on 169 different Cu(hkl) crystal planes vicinal to the Cu(111) orientation. These span both R- and S-surface chiralities comprehensively. The decomposition kinetics of d- and l-aspartic acids were measured and their decomposition rate constants were estimated on all 169 different surface orientations. The half-times, t1/2(hkl), for the decomposition of aspartic acid reveal that the reactivity for both enantiomers is lower along achiral directions exposing surfaces with (100) step edges than the reactivity along directions exposing surfaces with (110) step edges. The orientation-resolved rate constants, k(hkl), clearly reveal structure sensitivity on surfaces vicinal to Cu(111) but enantiospecificity is only observed for the initiation rate constant, ki(hkl).

Anomalous Light-Induced Spin-State Switching for Iron(II) Spin-Crossover Molecules in Direct Contact with Metal Surfaces.

Light-induced spin-state switching is one of the most attractive properties of spin-crossover materials. In bulk, low-spin (LS) to high-spin (HS) conversion via the light-induced excited spin-state trapping (LIESST) effect may be achieved with a visible light, while the HS-to-LS one (reverse-LIESST) requires an excitation in the near-infrared range. Now, it is shown that those phenomena are strongly modified at the interface with a metal. Indeed, an anomalous spin conversion is presented from HS state to LS state under blue light illumination for FeII spin-crossover molecules that are in direct contact with metallic (111) single-crystal surfaces (copper, silver, and gold). To interpret this anomalous spin-state switching, a new mechanism is proposed for the spin conversion based on the light absorption by the substrate that can generate low energy valence photoelectrons promoting molecular vibrational excitations and subsequent spin-state switching at the molecule-metal interface.

Photochemistry Highlights on On‐Surface Synthesis

On-surface chemistry is a promising way to achieve the bottom-up construction of covalently-bonded molecular precursors into extended atomically-precise polymers adsorbed on surfaces. These polymers exhibit unprecedented physical or chemical properties which are of great interest for various potential applications. These nanostructures were mainly obtained in ultra-high vacuum (UHV) on noble metal single-crystal surfaces by thermal annealing as stimulus to provoke the polymerization with a catalytic role of the surface adatoms. Nevertheless, photons are also a powerful source of energy to induce the formation of covalent architectures, even if it is less-used on surfaces than in solution. In this minireview, we discuss the photo-induced on-surface polymerization from the basic mechanisms of photochemistry to the formation of extended polymers on different kinds of surfaces, which are characterized by scanning probe microscopies.

On Controllability and Applicability of Surface Molecular Self-Assemblies.

Molecular self-assembly (MSA) refers to spontaneous arrangement of molecular building blocks into ordered structures governed by weak interactions. Due to its high versatility and reversibility, MSA has been widely employed as a robust bottom-up approach to fabricating low-dimensional functional nanostructures, which are used in various applications in nanoscience and technology. To date, tremendous effort has been devoted to constructing various MSAs at surfaces, ranging from self-assembled monolayers and two-dimensional (2D) nanoporous networks to complex 2D quasicrystals and Sierpiński triangle fractals. However, precise control of the assembled structures and efficient achievement of their full applicability remain two major challenges in the MSA field. As another widely employed bottom-up approach to fabricating nanostructures, on-surface reaction (OSR) refers to a reaction that occurs on the surface and is two-dimensionally confined. OSR offers the possibility to synthesize compounds that may not be feasibly achieved in solution chemistry. Compared with MSA based on weak intermolecular interactions, OSR-based structures possess high thermal and chemical stabilities due to internal strong covalent bonds. In this Account, we briefly overview recent achievements of MSAs on single crystal metal surfaces with a focus on their controllability and applicability in tweaking the properties of the molecular building blocks involved. Emphasis will be particularly placed upon mediation of OSRs with the MSA strategy. To explore surface MSAs, on the one hand, scanning tunneling microscopy and spectroscopy have been routinely employed as the experimental tools to probe the intermolecular interactions as well as geometric and electronic structures of the assemblies at the atomic and molecular levels. On the other hand, density functional theory and molecular dynamics have been theoretically applied to model and calculate the assembling systems, furthering our understanding of the experimental results. In principle, MSA is primarily balanced by molecule-molecule and molecule-substrate interactions under vacuum conditions. In terms of the assembling methodologies, people have been attempting to achieve rational design, accurate prediction, and controllable construction of assembled molecular nanostructures, namely, tentative design of specific backbones and functional groups of the molecular building blocks, and careful control of the assembling parameters including substrate lattice, temperature, coverage, and external environment as well. An obvious goal for the development of these methodologies lies in the ultimate applications of these MSAs. MSA can retrospectively affect the properties of the assembling molecules. For instance, self-assembled structures not only can serve as secondary templates to host guest molecules but also can stabilize surface metal adatoms. In fact, the electronics, magnetism, and optics of MSAs have been successfully explored. In surface chemistry, the MSA strategy can be further applied to mediate OSRs in at least three aspects: tweaking reaction selectivity, changing reaction pathway, and restricting reaction site. The governing principle lies in that the self-assembled molecules are confined in the assemblies so that the pre-exponential factors and the energy barriers in the Arrhenius equation of the involved reactions could be substantially varied because the subtle reaction mechanisms may change upon assembling. In this sense, the MSA strategy can be efficiently exploited to tune the properties of the assembling molecules and mediate OSRs in surface chemistry.

Nano-optical elements for surface plasmon waves: (50th anniversary of the Institute of Spectroscopy, Russian Academy of Sciences)

The main results obtained recently at the Laboratory of Laser Spectroscopy, Institute of Spectroscopy of the Russian Academy of Sciences in researching and developing various 2D optical elements for surface plasmon waves and their characterization using near- and far-field methods are presented. They include an optical medium for plasmon waves, a plasmon interferometer, a parabolic mirror for focusing plasmon waves, a dielectrically loaded plasmon-polariton waveguide, a plasmon nanolaser based on a plasmonic crystal, and a plasmon sensor of fluorescent biomarkers. lenses, mirrors, detectors, interferometers, waveguides, and lasers. We show that state-of-the-art techniques for the development of plasmon optics elements, which are based on utilizing single-crystal ultrahigh quality metal surfaces, enable us to come closer to the theoretically predicted characteristics of these elements.

CO methanation on ruthenium flat and stepped surfaces: Key role of H-transfers and entropy revealed by ab initio molecular dynamics

The methanation reaction converting CO into CH4 at ca. 400 °C is proposed to take place on uncoordinated surface sites (e.g. step-edges) of Ru catalysts because of the higher measured reactivity of stepped vs. flat single crystal metal surfaces. However, the reaction mechanisms occurring at step-edges remain unclear due to the limitations in modelling high temperatures processes by the common evaluation of potential energy surfaces by density functional theory calculations. Here, we show using ab initio molecular dynamics simulations on CO-covered Ru flat and stepped surfaces models at 400 °C that hydrogen-transfers favoured at elevated temperatures are crucial for both CO activation and CHx (x = 1, 2, 3) hydrogenation steps of the methanation reaction, especially at step-edges.

Fluorine-programmed nanozipping to tailored nanographenes on rutile TiO2 surfaces.

The rational synthesis of nanographenes and carbon nanoribbons directly on nonmetallic surfaces has been an elusive goal for a long time. We report that activation of the carbon (C)-fluorine (F) bond is a reliable and versatile tool enabling intramolecular aryl-aryl coupling directly on metal oxide surfaces. A challenging multistep transformation enabled by C-F bond activation led to a dominolike coupling that yielded tailored nanographenes directly on the rutile titania surface. Because of efficient regioselective zipping, we obtained the target nanographenes from flexible precursors. Fluorine positions in the precursor structure unambiguously dictated the running of the "zipping program," resulting in the rolling up of oligophenylene chains. The high efficiency of the hydrogen fluoride zipping makes our approach attractive for the rational synthesis of nanographenes and nanoribbons directly on insulating and semiconducting surfaces.

Symmetry-Induced Structuring of Ultrathin FeO and Fe₃O₄ Films on Pt(111) and Ru(0001).

Iron oxide films epitaxially grown on close-packed metal single crystal substrates exhibit nearly-perfect structural order, high catalytic activity (FeO) and room-temperature magnetism (Fe3O4). However, the morphology of the films, especially in the ultrathin regime, can be significantly influenced by the crystalline structure of the used support. This work reports an ultra-high vacuum (UHV) low energy electron/synchrotron light-based X-ray photoemission electron microscopy (LEEM/XPEEM) and electron diffraction (µLEED) study of the growth of FeO and Fe3O4 on two closed-packed metal single crystal surfaces: Pt(111) and Ru(0001). The results reveal the influence of the mutual orientation of adjacent substrate terraces on the morphology of iron oxide films epitaxially grown on top of them. On fcc Pt(111), which has the same mutual orientation of adjacent monoatomic terraces, FeO(111) grows with the same in-plane orientation on all substrate terraces. For Fe3O4(111), one or two orientations are observed depending on the growth conditions. On hcp Ru(0001), the adjacent terraces of which are ‘rotated’ by 180° with respect to each other, the in-plane orientation of initial FeO(111) and Fe3O4(111) crystallites is determined by the orientation of the substrate terrace on which they nucleated. The adaptation of three-fold symmetric iron oxides to three-fold symmetric substrate terraces leads to natural structuring of iron oxide films, i.e., the formation of patch-like magnetite layers on Pt(111) and stripe-like FeO and Fe3O4 structures on Ru(0001).

Graphene on nickel (100) micrograins: Modulating the interface interaction by extended moiré superstructures

Interaction with the substrate strongly affects the electronic/chemical properties of supported graphene. So far, graphene grown by chemical vapor deposition (CVD) on catalytic single crystal transition metal surfaces - mostly 3-fold close-packed - has mainly been studied. Herein, we investigated CVD graphene on a polycrystalline nickel (Ni) substrate, focusing in particular on (100) micrograins and comparing the observed behavior with that on single crystal Ni(100) substrate. The symmetry-mismatch leads to moiré superstructures with stripe-like or rhombic-network morphology, which were characterized by atomically-resolved scanning tunneling microscopy (STM). Density functional theory (DFT) simulations shed light on spatial corrugation and interfacial interactions: depending on the misorientation angle, graphene is either alternately physi- and chemisorbed or uniformly chemisorbed, the interaction being modulated by the (sub)nanometer-sized moiré superstructures. Ni(100) micrograins appear to be a promising substrate to finely tailor the electronic properties of graphene at the nanoscale, with relevant perspective applications in electronics and catalysis.

Application of work function measurements in the study of surface catalyzed reactions on Rh(1 0 0)

The present article aims to show how work function measurements (WF) can be applied in the study of elementary surface reaction steps on metallic single crystal surfaces. The work function itself can in many cases not be interpreted directly, as it lacks direct information on structural and chemical nature of the surface and adsorbates, but it can be a powerful tool when used together with other surface science techniques which provide information on the chemical nature of the adsorbed species. We here, illustrate the usefulness of work function measurements using Rh(100) as our model catalyst. The examples presented include work function measurements during adsorption, surface reaction, and desorption of a variety of molecules relevant for heterogeneous catalysis. Surface coverage of adsorbates, isosteric heat of adsorption, and kinetic parameters for desorption, desorption/decomposition temperatures of surface species, different reaction regimes were determined by WF with the aid of other surface science techniques.

Energy landscapes and dynamics of glycine on Cu(110).

Amino acids adsorbed on single-crystal metal surfaces have emerged as prototypical systems for exploring the properties that govern the development of long-range chirality in self-assembled monolayers (SAM) and supramolecular 2D networks. In this study, we characterise the self-assembly mechanism for glycine on the Cu(110) surface. This process occurs on a time scale that is too fast for most atomically resolved microscopic techniques, so the mechanism we propose here provides new insight for an important unexplored surface phenomenon.

Tuning high-harmonic generation by controlled deposition of ultrathin ionic layers on metal surfaces

High harmonic generation (HHG) from semiconductors and insulators has become a very active area of research due to its great potential for developing compact HHG devices. Here we show that by growing monolayers (ML) of insulators on single-crystal metal surfaces, one can tune the harmonic spectrum by just varying the thickness of the ultrathin layer, not the laser properties. This is shown from numerical solutions of the time-dependent Schr\"odinger equation for $n$ML NaCl/Cu(111) systems ($n=1-50$) based on realistic potentials available in the literature. Remarkably, the harmonic cutoff increases linearly with $n$ and as much as an order of magnitude when going from $n$ $=$ 1 to 30, while keeping the laser intensity low and the wavelength in the near-infrared range. Furthermore, the degree of control that can be achieved in this way is much higher than by varying the laser intensity. The origin of this behavior is the reduction of electronic "friction" when moving from the essentially discrete energy spectrum associated with a few-ML system to the continuous energy spectrum (bands) inherent to an extended periodic system.

Adsorption Properties of Xe on Ag/Cu(111) System: Real-Time Photoemission Investigation

We discuss here the adsorption process of rare gas atoms on a heterogeneous surface, Xe on Ag submonolayer partially covering Cu(111) surface. As a function of the Xe dosing, we have monitored in real-time the evolution of the Xe 4d core levels and of the LEED patterns, as well as the modification of the Shockley state band. We confirmed that the reduction in the density of states near the Fermi level strongly promotes the adsorption of Xe atoms on the surface. More interesting, it seems that the Xe overlayer on the Ag monolayer exhibits a disordered structure, which differs substantially from those observed for Xe on most of the single-crystal metal surfaces, where the Xe atoms form generally an epitaxial layer with hexagonally close-packed structure. Additionally, the commensurate (9 × 9) reconstruction characterizing the 1 ML Ag/Cu(111) is preserved after adsorption of Xe, and consequently leads to a band back-folding of shifted interface state on the 1 ML Ag islands covered by Xe. This finding is esse...

Room Temperature Carbonylation of Iron–Phthalocyanines Adsorbed on a Single Crystal Metal Surface: An in Situ SFG Investigation at Near-Ambient Pressure

We report the experimental spectroscopic evidence for the room temperature carbonylation, at equilibrium with the carbon monoxide gas phase, of iron phthalocyanines adsorbed on the (111) termination of the iridium single crystal. The adsorption process occurs at CO pressures above the mbar yield. Interestingly, heme and heme-like catalytic, carrier, and enzymatic biomolecules interact with the gaseous reactants at partial pressures that are similar to the values that we observed on this model system, a 2D layer of single-atom metallorganic catalysts preassembled under ultrahigh vacuum conditions. A simple Langmuir description of the adsorption mechanism yields a CO-Fe binding energy of 0.1–0.3 eV, depending on the assumptions about the pre-exponential factor. The internal vibrational structure of the adsorbed iron phthalocyanines is also investigated.