Dirac cones and minigaps for graphene on Ir(111)

Tug-of-war between corrugation and binding energy: revealing the formation of multiple moiré patterns on a strongly interacting graphene-metal system

The formation of multidomain epitaxial graphene on Rh(111) under ultra-high vacuum (UHV) conditions has been characterized by scanning tunnelling microscopy (STM) measurements and density functional theory (DFT) calculations. At variance with the accepted view for strongly interacting graphene-metal systems, we clearly demonstrate the formation of different rotational domains leading to multiple moiré structures with a wide distribution of surface periodicities. Experiments reveal a correlation between the STM apparent corrugation and the lattice parameter of the moiré unit cell, with corrugations of just 30-40 pm for the smallest moirés. DFT calculations for a relevant selection of these moiré patterns show much larger height differences and a non-monotonic behaviour with the moiré size. Simulations based on non-equilibrium Green's function (NEGF) methods reproduce quantitatively the experimental trend and provide a detailed understanding of the interplay between electronic and geometric contributions in the STM contrast of graphene systems. Our study sheds light on the subtle energy balance among strain, corrugation and binding that drives the formation of the moiré patterns in all graphene/metal systems and suggests an explanation for the success of an effective model only based on the lattice mismatch. Although low values of the strain energy are a necessary condition, it is the ability of graphene to corrugate in order to maximize the areas of favourable graphene-metal interactions that finally selects the stable configurations.

Adatom bond-induced geometric and electronic properties of passivated armchair graphene nanoribbons

The geometric and electronic properties of passivated armchair graphene nanoribbons, enriched by strong chemical bonding between edge-carbons and various adatoms, are investigated by first-principle calculations. Adatom arrangements, bond lengths, charge distributions, and energy dispersions are dramatically changed by edge passivation. Elements with an atomic number of less than 20 are classified into three types depending on the optimal geometric structures: planar and non-planar structures, the latter of which are associated with specific arrangements and stacked configurations of adatoms. Especially, the nitrogen passivated nanoribbon is the most stable one with a heptagon-pentagon structure at the edges. The low-lying band structures are drastically varied, exhibiting non-monotonous energy dispersions and adatom-dominated bands. A relationship between energy gaps and ribbon widths no longer exists, and some adatoms further induce a semiconductor-metal transition. All the main characteristics are directly reflected in the density of states, revealing dip structures, plateaus, symmetric peaks, and square-root divergent asymmetric peaks.

Multifunctional Carbon Nanostructures for Advanced Energy Storage Applications

Carbon nanostructures-including graphene, fullerenes, etc.-have found applications in a number of areas synergistically with a number of other materials. These multifunctional carbon nanostructures have recently attracted tremendous interest for energy storage applications due to their large aspect ratios, specific surface areas, and electrical conductivity. This succinct review aims to report on the recent advances in energy storage applications involving these multifunctional carbon nanostructures. The advanced design and testing of multifunctional carbon nanostructures for energy storage applications-specifically, electrochemical capacitors, lithium ion batteries, and fuel cells-are emphasized with comprehensive examples.

Ultrafast electron dynamics in epitaxial graphene investigated with time- and angle-resolved photoemission spectroscopy

In order to exploit the intriguing optical properties of graphene it is essential to gain a better understanding of the light-matter interaction in the material on ultrashort timescales. Exciting the Dirac fermions with intense ultrafast laser pulses triggers a series of processes involving interactions between electrons, phonons and impurities. Here we study these interactions in epitaxial graphene supported on silicon carbide (semiconducting) and iridium (metallic) substrates using ultrafast time- and angle-resolved photoemission spectroscopy (TR-ARPES) based on high harmonic generation. For the semiconducting substrate we reveal a complex hot carrier dynamics that manifests itself in an elevated electronic temperature and an increase in linewidth of the π band. By analyzing these effects we are able to disentangle electron relaxation channels in graphene. On the metal substrate this hot carrier dynamics is found to be severely perturbed by the presence of the metal, and we find that the electronic system is much harder to heat up than on the semiconductor due to screening of the laser field by the metal.

Observation of single-spin Dirac fermions at the graphene/ferromagnet interface

With the discovery and first characterization of graphene, its potential for spintronic applications was recognized immediately. Since then, an active field of research has developed trying to overcome the practical hurdles. One of the most severe challenges is to find appropriate interfaces between graphene and ferromagnetic layers, which are granting efficient injection of spin-polarized electrons. Here, we show that graphene grown under appropriate conditions on Co(0001) demonstrates perfect structural properties and simultaneously exhibits highly spin-polarized charge carriers. The latter was conclusively proven by observation of a single-spin Dirac cone near the Fermi level. This was accomplished experimentally using spin- and angle-resolved photoelectron spectroscopy, and theoretically with density functional calculations. Our results demonstrate that the graphene/Co(0001) system represents an interesting candidate for applications in devices using the spin degree of freedom.

Electronic structure of epitaxial single-layer MoS2

The electronic structure of epitaxial single-layer MoS2 on Au(111) is investigated by angle-resolved photoemission spectroscopy. Pristine and potassium-doped layers are studied in order to gain access to the conduction band. The potassium-doped layer is found to have a (1.39±0.05)  eV direct band gap at K[over ¯] with the valence band top at Γ[over ¯] having a significantly higher binding energy than at K[over ¯]. The moiré superstructure of the epitaxial system does not lead to the presence of observable replica bands or minigaps. The degeneracy of the upper valence band at K[over ¯] is found to be lifted by the spin-orbit interaction, leading to a splitting of (145±4)  meV. This splitting is anisotropic and in excellent agreement with recent calculations. Finally, it is shown that the potassium doping does not only give rise to a rigid shift of the band structure but also to a distortion, leading to the possibility of band structure engineering in single-layers of transition metal dichalcogenides.

Towards scalable nano-engineering of graphene

By merging bottom-up and top-down strategies we tailor graphene's electronic properties within nanometer accuracy, which opens up the possibility to design optical and plasmonic circuitries at will. In a first step, graphene electronic properties are macroscopically modified exploiting the periodic potential generated by the self assembly of metal cluster superlattices on a graphene/Ir(111) surface. We then demonstrate that individual metal clusters can be selectively removed by a STM tip with perfect reproducibility and that the structures so created are stable even at room temperature. This enables one to nanopattern circuits down to the 2.5 nm only limited by the periodicity of the Moiré-pattern, i.e., by the distance between neighbouring clusters, and different electronic and optical properties should prevail in the covered and uncovered regions. The method can be carried out on micro-meter-sized regions with clusters of different materials permitting to tune the strength of the periodic potential.

Chemical control of graphene architecture: tailoring shape and properties

Single layer graphene and graphene oxide feature useful and occasionally unique properties by virtue of their two-dimensional structure. Given that there is a strong correlation between graphene architecture and its conductive, mechanical, chemical, and sorptive properties, which lead to useful technologies, the ability to systematically deform graphene into three-dimensional structures, therefore, provides a controllable, scalable route toward tailoring such properties in the final system. However, the advent of chemical methods to control graphene architecture is still coming to fruition and requires focused attention. The flexibility of the graphene system and the direct and indirect methods available to induce morphology changes of graphene sheets are first discussed in this review. Focus is then given toward chemical reactions that influence the shape of presynthesized graphene and graphene oxide sheets, from which a toolbox can be extrapolated and used in controlling the spatial arrangement of graphene sheets within composite materials and ultimately tailoring graphene-based device performance. Finally, the properties of three-dimensionally controlled graphene-based systems are highlighted for their use as batteries, strengthening additives, gas or liquid sorbents, chemical reactor platforms, and supercapacitors.

Tailoring the magnetism of Co atoms on graphene through substrate hybridization

We determine the magnetic properties of individual Co atoms adsorbed on graphene (G) with x-ray absorption spectroscopy and magnetic circular dichroism. The magnetic ground state of Co adatoms strongly depends on the choice of the metal substrate on which graphene is grown. Cobalt atoms on G/Ru(0001) feature exceptionally large orbital and spin moments, as well as an out-of-plane easy axis with large magnetic anisotropy. Conversely, the magnetic moments are strongly reduced for Co/G/Ir(111), and the magnetization is of the easy-plane type. We demonstrate how the Co magnetic properties, which ultimately depend on the degree of hybridization between the Co 3d orbitals and graphene π bands, can be tailored through the strength of the graphene-substrate coupling.

Image-potential states and work function of graphene

Image-potential states of graphene on various substrates have been investigated by two-photon photoemission and scanning tunneling spectroscopy. They are used as a probe for the graphene-substrate interaction and resulting changes in the (local) work function. The latter is driven by the work function difference between graphene and the substrate. This results in a charge transfer which also contributes to core-level shifts in x-ray photoemission. In this review article, we give an overview over the theoretical models and the experimental data for image-potential states and work function of graphene on various substrates.

Graphene on hexagonal boron nitride

The field of graphene research has developed rapidly since its first isolation by mechanical exfoliation in 2004. Due to the relativistic Dirac nature of its charge carriers, graphene is both a promising material for next-generation electronic devices and a convenient low-energy testbed for intrinsically high-energy physical phenomena. Both of these research branches require the facile fabrication of clean graphene devices so as not to obscure its intrinsic physical properties. Hexagonal boron nitride has emerged as a promising substrate for graphene devices as it is insulating, atomically flat and provides a clean charge environment for the graphene. Additionally, the interaction between graphene and boron nitride provides a path for the study of new physical phenomena not present in bare graphene devices. This review focuses on recent advancements in the study of graphene on hexagonal boron nitride devices from the perspective of scanning tunneling microscopy with highlights of some important results from electrical transport measurements.

Understanding the origin of band gap formation in graphene on metals: graphene on Cu/Ir(111)

Understanding the nature of the interaction at the graphene/metal interfaces is the basis for graphene-based electron- and spin-transport devices. Here we investigate the hybridization between graphene- and metal-derived electronic states by studying the changes induced through intercalation of a pseudomorphic monolayer of Cu in between graphene and Ir(111), using scanning tunnelling microscopy and photoelectron spectroscopy in combination with density functional theory calculations. We observe the modifications in the band structure by the intercalation process and its concomitant changes in the charge distribution at the interface. Through a state-selective analysis of band hybridization, we are able to determine their contributions to the valence band of graphene giving rise to the gap opening. Our methodology reveals the mechanisms that are responsible for the modification of the electronic structure of graphene at the Dirac point, and permits to predict the electronic structure of other graphene-metal interfaces.

The mechanism of caesium intercalation of graphene

Properties of many layered materials, including copper- and iron-based superconductors, topological insulators, graphite and epitaxial graphene, can be manipulated by the inclusion of different atomic and molecular species between the layers via a process known as intercalation. For example, intercalation in graphite can lead to superconductivity and is crucial in the working cycle of modern batteries and supercapacitors. Intercalation involves complex diffusion processes along and across the layers; however, the microscopic mechanisms and dynamics of these processes are not well understood. Here we report on a novel mechanism for intercalation and entrapment of alkali atoms under epitaxial graphene. We find that the intercalation is adjusted by the van der Waals interaction, with the dynamics governed by defects anchored to graphene wrinkles. Our findings are relevant for the future design and application of graphene-based nano-structures. Similar mechanisms can also have a role for intercalation of layered materials.

Exploring electronic structure of one-atom thick polycrystalline graphene films: a nano angle resolved photoemission study

The ability to produce large, continuous and defect free films of graphene is presently a major challenge for multiple applications. Even though the scalability of graphene films is closely associated to a manifest polycrystalline character, only a few numbers of experiments have explored so far the electronic structure down to single graphene grains. Here we report a high resolution angle and lateral resolved photoelectron spectroscopy (nano-ARPES) study of one-atom thick graphene films on thin copper foils synthesized by chemical vapor deposition. Our results show the robustness of the Dirac relativistic-like electronic spectrum as a function of the size, shape and orientation of the single-crystal pristine grains in the graphene films investigated. Moreover, by mapping grain by grain the electronic dynamics of this unique Dirac system, we show that the single-grain gap-size is 80% smaller than the multi-grain gap recently reported by classical ARPES.

An electronic structure perspective of graphene interfaces

The unusual electronic structure of graphene characterized by linear energy dispersion of bands adjacent to the Fermi level underpins its remarkable transport properties. However, for practical device integration, graphene will need to be interfaced with other materials: 2D layered structures, metals (as ad-atoms, nanoparticles, extended surfaces, and patterned metamaterial geometries), dielectrics, organics, or hybrid structures that in turn are constituted from various inorganic or organic components. The structural complexity at these nanoscale interfaces holds much promise for manifestation of novel emergent phenomena and provides a means to modulate the electronic structure of graphene. In this feature article, we review the modifications to the electronic structure of graphene induced upon interfacing with disparate types of materials with an emphasis on iterative learnings from theoretical calculations and electronic spectroscopy (X-ray absorption fine structure (XAFS) spectroscopy, scanning transmission X-ray microscopy (STXM), angle-resolved photoemission spectroscopy (ARPES), and X-ray magnetic circular dichroism (XMCD)). We discuss approaches for engineering and modulating a bandgap in graphene through interfacial hybridization, outline experimental methods for examining electronic structure at interfaces, and overview device implications of engineered interfaces. A unified view of how geometric and electronic structure are correlated at interfaces will provide a rational means for designing heterostructures exhibiting emergent physical phenomena with implications for plasmonics, photonics, spintronics, and engineered polymer and metal matrix composites.

Sample corrugation affects the apparent bond lengths in atomic force microscopy

Frequency modulation atomic force microscopy (AFM) allows the chemical structure of planar molecules to be determined with atomic resolution. Typically, these measurements are carried out in constant-height mode using carbon monoxide (CO) terminated tips. Such tips exhibit considerable flexibility, i.e., the CO molecule can bend laterally due to the tip-sample interaction. Using epitaxial graphene as a model system, we demonstrate experimentally that the apparent atomic positions measured by AFM depend on the sample corrugation. Using molecular mechanics simulations, we explain these observations by the interplay of the CO bending and the nonlinear background signal arising from the neighboring atoms. These effects depend nontrivially on the tip-sample distance and limit the achievable accuracy on the bond length determination based on AFM experiments.

Graphene-nickel interfaces: a review

Graphene on nickel is a prototypical example of an interface between graphene and a strongly interacting metal, as well as a special case of a lattice matched system. The chemical interaction between graphene and nickel is due to hybridization of the metal d-electrons with the π-orbitals of graphene. This interaction causes a smaller separation between the nickel surface and graphene (0.21 nm) than the typical van der Waals gap-distance between graphitic layers (0.33 nm). Furthermore, the physical properties of graphene are significantly altered. Main differences are the opening of a band gap in the electronic structure and a shifting of the π-band by ∼2 eV below the Fermi-level. Experimental evidence suggests that the ferromagnetic nickel induces a magnetic moment in the carbon. Substrate induced geometric and electronic changes alter the phonon dispersion. As a consequence, monolayer graphene on nickel does not exhibit a Raman spectrum. In addition to reviewing these fundamental physical properties of graphene on Ni(111), we also discuss the formation and thermal stability of graphene and a surface-confined nickel-carbide. The fundamental growth mechanisms of graphene by chemical vapor deposition are also described. Different growth modes depending on the sample temperature have been identified in ultra high vacuum surface science studies. Finally, we give a brief summary for the synthesis of more complex graphene and graphitic structures using nickel as catalyst and point out some potential applications for graphene-nickel interfaces.

Tailoring molecular self-assembly of magnetic phthalocyanine molecules on Fe- and Co-intercalated graphene

We investigate molecule-molecule, as well as molecule-substrate, interactions of phthalocyanine molecules deposited on graphene. In particular, we show how to tune the self-assembly of molecular lattices in two dimensions by intercalation of transition metals between graphene and Ir(111): modifying the surface potential of the graphene layer via intercalation leads to the formation of square, honeycomb, or Kagome lattices. Finally, we demonstrate that such surface induced molecular lattices are stable even at room temperature.

The backside of graphene: manipulating adsorption by intercalation

The ease by which graphene is affected through contact with other materials is one of its unique features and defines an integral part of its potential for applications. Here, it will be demonstrated that intercalation, the insertion of atomic layers in between the backside of graphene and the supporting substrate, is an efficient tool to change its interaction with the environment on the frontside. By partial intercalation of graphene on Ir(111) with Eu or Cs we induce strongly n-doped graphene patches through the contact with these intercalants. They coexist with nonintercalated, slightly p-doped graphene patches. We employ these backside doping patterns to directly visualize doping induced binding energy differences of ionic adsorbates to graphene through low-temperature scanning tunneling microscopy. Density functional theory confirms these binding energy differences and shows that they are related to the graphene doping level.

Functional hybrid systems based on large-area high-quality graphene

The properties of sp² carbon allotropes can be tuned and enriched by their interaction with other materials. The large interface to the outside world in these forms of carbon is ideally suited for combining in an optimal manner several functionalities thanks to this interaction. A wide range of novel materials holding strong promise in energy, optoelectronics, microelectronics, mechanics, or medical applications have been designed accordingly. Graphene, the last representative of this family of sp² carbon materials, has already yielded a wealth of hybrid systems. A new class of these hybrids is emerging, which allows researchers to exploit the properties of truly single-layer graphene. These systems rely on high-quality graphene. In this Account, we describe our recent efforts to develop hybrid systems through various approaches and with various scopes. Depending on the interaction between graphene and molecules, metal clusters, layers, and substrates, either graphene may essentially preserve the electronic properties that make it a unique platform for electronic transport, or new organization and properties in the materials may arise due to the graphene contact at the expense of deep modification of graphene's properties. We prepare our graphene samples by both mechanical exfoliation of graphite and chemical vapor deposition on metals. We use this to study graphene in contact with various species, which either decorate graphene or are intercalated between it and its substrate. We first address the electronic and magnetic properties in systems where graphene is in epitaxy with a metal and discuss the potential to manipulate the properties of both materials, highlighting graphene's role as a protective capping layer in magnetic functional systems. We then present graphene/metal dot hybrids, which can utilize the two-dimensional gas properties of Dirac fermions in graphene. These hybrids allow one to tune the coupling between clusters hosting electronically ordered states such as superconductivity and explore quantum phase transitions controlled by electrostatic back gates. We finally discuss the optical properties of hybrids in which graphene is decorated with optically active molecules. Depending on how close these molecules are to the graphene's electromechanical systems, the interaction of the system with light can be changed. Fields such as spintronics and catalysis could benefit from high-quality graphene based hybrid systems, which have not been fully explored.

The quasiparticle band dispersion in epitaxial multilayer silicene

The growth of multilayer silicene is an exciting challenge for the future of silicon nano-electronics. Here, we use angle-resolved photoemission spectroscopy to map the entire Brillouin zone (BZ) of (√3 × √3)R30° reconstructed epitaxial multilayer silicene islands, growing on top of the first (3 × 3) reconstructed silicene wetting layer, on Ag(111) substrates. We found Λ- and V-shape linear dispersions, which we relate to the π and π* bands of massless quasiparticles in multilayer silicene, at the BZ centre [Formula: see text] and at all the [Formula: see text] centres of the (√3 × √3)R30° Brillouin zones in the extended scheme, due to folding of the Dirac cones at the [Formula: see text] and [Formula: see text] points of the (1 × 1) silicene BZ. The Fermi velocity of ∼0.3 × 10(6) m s(-1) obtained is highly promising for potential silicene-based devices.

Tunable superlattice in graphene to control the number of Dirac points

Superlattice in graphene generates extra Dirac points in the band structure and their number depends on the superlattice potential strength. Here, we have created a lateral superlattice in a graphene device with a tunable barrier height using a combination of two gates. In this Letter, we demonstrate the use of lateral superlattice to modify the band structure of graphene leading to the emergence of new Dirac cones. This controlled modification of the band structure persists up to 100 K.

Temperature-driven reversible rippling and bonding of a graphene superlattice

In order to unravel the complex interplay between substrate interactions and film configuration, we investigate and characterize graphene on a support with non-three-fold symmetry, the square Ir(100). Below 500 °C, distinct physisorbed and chemisorbed graphene phases coexist on the surface, respectively characterized by flat and buckled morphology. They organize into alternating domains that extend on mesoscopic lengths, relieving the strain due to the different thermal expansion of film and substrate. The chemisorbed phase exhibits exceptionally large one-dimensional ripples with regular nanometer periodicity and can be reversibly transformed into physisorbed graphene in a temperature-controlled process that involves surprisingly few C-Ir bonds. The formation and rupture of these bonds, rather than ripples or strain, are found to profoundly alter the local electronic structure, changing graphene behavior from semimetal to metallic type. The exploitation of such subtle interfacial changes opens new possibilities for tuning the properties of this unique material.

Energetics and hierarchical interactions of metal-phthalocyanines adsorbed on graphene/Ir(111)

The adsorption of metal-phthalocyanine (MPc) layers (M = Fe, Co, Cu) assembled on graphene/Ir(111) is studied by means of temperature-programmed X-ray photoemission spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS). The balance between interaction forces among the organometallic molecules and the underlying graphene gives rise to flat-lying molecular layers, weakly interacting with the underlying graphene. Further MPc layers pile up face-on onto the first layer, up to a few nanometers thickness, as deduced by NEXAFS. The FePc, CoPc, and CuPc multilayers present comparable desorption temperatures, compatible with molecule-molecule interactions dominated by van der Waals forces between the π-conjugated macrocycles. The MPc single layers desorb from graphene/Ir at higher temperatures. The CuPc single layer desorbs at lower temperature than the FePc and CoPc single layers, suggesting a higher adsorption energy of the FePc and CoPc single layers on graphene/Ir with respect to CuPc, with increasing molecule-substrate interaction in the order E(CuPc) < E(FePc) ~ E(CoPc).

Controlling hydrogenation of graphene on Ir(111)

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.

Electronic and magnetic properties of zigzag graphene nanoribbons on the (111) surface of Cu, Ag, and Au

We carry out an ab initio study of the structural, electronic, and magnetic properties of zigzag graphene nanoribbons on Cu(111), Ag(111), and Au(111). Both, H-free and H-terminated nanoribbons are considered revealing that the nanoribbons invariably possess edge states when deposited on these surfaces. In spite of this, they do not exhibit a significant magnetization at the edge, with the exception of H-terminated nanoribbons on Au(111), whose zero-temperature magnetic properties are comparable to those of free-standing nanoribbons. These results are explained by the different hybridization between the graphene 2p orbitals and those of the substrates and, for some models, also by the charge transfer between the surface and the nanoribbon. Interestingly, H-free nanoribbons on Au(111) and Ag(111) exhibit two main peaks in the local density of states around the Fermi energy, which originate from different states and, thus, do not indicate edge magnetism.

Absence of edge states in covalently bonded zigzag edges of graphene on Ir(111)

The zigzag edges of graphene on Ir(111) are studied by ab initio simulations and low-temperature scanning tunneling spectroscopy, providing information about their structural, electronic, and magnetic properties. No edge state is found to exist, which is explained in terms of the interplay between a strong geometrical relaxation at the edge and a hybridization of the d orbitals of Ir atoms with the graphene orbitals at the edge.

Electron-phonon coupling in quasi-free-standing graphene

Quasi-free-standing monolayer graphene can be produced by intercalating species like oxygen or hydrogen between epitaxial graphene and the substrate crystal. If the graphene was indeed decoupled from the substrate, one would expect the observation of a similar electronic dispersion and many-body effects, irrespective of the substrate and the material used to achieve the decoupling. Here we investigate the electron-phonon coupling in two different types of quasi-free-standing monolayer graphene: decoupled from SiC via hydrogen intercalation and decoupled from Ir via oxygen intercalation. The two systems show similar overall behaviours of the self-energy and a weak renormalization of the bands near the Fermi energy. The electron-phonon coupling is found to be so weak that it renders the precise determination of the coupling constant λ through renormalization difficult. The estimated value of λ is 0.05(3) for both systems.

Electronic structure and imaging contrast of graphene moiré on metals

Realization of graphene moiré superstructures on the surface of 4d and 5d transition metals offers templates with periodically modulated electron density, which is responsible for a number of fascinating effects, including the formation of quantum dots and the site selective adsorption of organic molecules or metal clusters on graphene. Here, applying the combination of scanning probe microscopy/spectroscopy and the density functional theory calculations, we gain a profound insight into the electronic and topographic contributions to the imaging contrast of the epitaxial graphene/Ir(111) system. We show directly that in STM imaging the electronic contribution is prevailing compared to the topographic one. In the force microscopy and spectroscopy experiments we observe a variation of the interaction strength between the tip and high-symmetry places within the graphene moiré supercell, which determine the adsorption sites for molecules or metal clusters on graphene/Ir(111).

Interfacial properties of bilayer and trilayer graphene on metal substrates

One popular approach to prepare graphene is to grow them on transition metal substrates via chemical vapor deposition. By using the density functional theory with dispersion correction, we systematically investigate for the first time the interfacial properties of bilayer (BLG) and trilayer graphene (TLG) on metal substrates. Three categories of interfacial structures are revealed. The adsorption of B(T)LG on Al, Ag, Cu, Au, and Pt substrates is a weak physisorption, but a band gap can be opened. The adsorption of B(T)LG on Ti, Ni, and Co substrates is a strong chemisorption, and a stacking-insensitive band gap is opened for the two uncontacted layers of TLG. The adsorption of B(T)LG on Pd substrate is a weaker chemisorption, with a band gap opened for the uncontacted layers. This fundamental study also helps for B(T)LG device study due to inevitable graphene/metal contact.

Probing local hydrogen impurities in quasi-free-standing graphene

We report high-resolution scanning tunneling microscopy and spectroscopy of hydrogenated, quasi-free-standing graphene. For this material, theory has predicted the appearance of a midgap state at the Fermi level, and first angle-resolved photoemission spectroscopy (ARPES) studies have provided evidence for the existence of this state in the long-range electronic structure. However, the spatial extension of H defects, their preferential adsorption patterns on graphene, or local electronic structure are experimentally still largely unexplored. Here, we investigate the shapes and local electronic structure of H impurities that go with the aforementioned midgap state observed in ARPES. Our measurements of the local density of states at hydrogenated patches of graphene reveal a hydrogen impurity state near the Fermi level whose shape depends on the tip position with respect to the center of a patch. In the low H concentration regime, we further observe predominantly single hydrogenation sites as well as extended multiple C-H sites in parallel orientation to the lattice vectors, indicating an adsorption at the same graphene sublattice. This is corroborated by ARPES measurements showing the formation of a dispersionless hydrogen impurity state which is extended over the whole Brillouin zone.

Oxygen switching of the epitaxial graphene-metal interaction

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 K point 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.

Rationale for switching to nonlocal functionals in density functional theory

Density functional theory (DFT) has been steadily improving over the past few decades, becoming the standard tool for electronic structure calculations. The early local functionals (LDA) were eventually replaced by more accurate semilocal functionals (GGA) which are in use today. A major persisting drawback is the lack of the nonlocal correlation which is at the core of dispersive (van der Waals) forces, so that a large and important class of systems remains outside the scope of DFT. The vdW-DF correlation functional of Langreth and Lundqvist, published in 2004, was the first nonlocal functional which could be easily implemented. Beyond expectations, the nonlocal functional has brought significant improvement to systems that were believed not to be sensitive to nonlocal correlations. In this paper, we use the example of graphene nanodomes growing on the Ir(111) surface, where with an increase of the size of the graphene islands the character of the bonding changes from strong chemisorption towards almost pure physisorption. We demonstrate how the seamless character of the vdW-DF functionals makes it possible to treat all regimes self-consistently, proving to be a systematic and consistent improvement of DFT regardless of the nature of bonding. We also discuss the typical surface science example of CO adsorption on (111) surfaces of metals, which shows that the nonlocal correlation may also be crucial for strongly chemisorbed systems. We briefly discuss open questions, in particular the choice of the most appropriate exchange part of the functional. As the vdW-DF begins to appear implemented self-consistently in a number of popular DFT codes, with numerical costs close to the GGA calculations, we draw the attention of the DFT community to the advantages and benefits of the adoption of this new class of functionals.

Two distinct phases of bilayer graphene films on Ru(0001)

By combining angle-resolved photoemission spectroscopy and scanning tunneling microscopy we reveal the structural and electronic properties of multilayer graphene on Ru(0001). We prove that large ethylene exposure allows the synthesis of two distinct phases of bilayer graphene with different properties. The first phase has Bernal AB stacking with respect to the first graphene layer and displays weak vertical interaction and electron doping. The long-range ordered moiré pattern modulates the crystal potential and induces replicas of the Dirac cone and minigaps. The second phase has an AA stacking sequence with respect to the first layer and displays weak structural and electronic modulation and p-doping. The linearly dispersing Dirac state reveals the nearly freestanding character of this novel second-layer phase.

Patterning of graphene

Two-dimensional atomic sheets of carbon (graphene, graphane, etc.) are amenable to unique patterning schemes such as cutting, bending, folding and fusion that are predicted to lead to interesting properties. In this review, we present theoretical understanding and processing routes for patterning graphene and highlight potential applications. With more precise and scalable patterning, the prospects of integrating flat carbon (graphene) with curved carbon (nanotubes and half nanotubes) and programmable graphene folding are envisioned.

Optimizing long-range order, band gap, and group velocities for graphene on close-packed metal surfaces

We compare different growth methods with the aim of optimizing the long-range order of a graphene layer grown on Ru(0001). Combining chemical vapor deposition with carbon loading and segregation of the surface layer leads to autocorrelation lengths of 240 Å. We present several routes to band gap and charge carrier mobility engineering for the example of graphene on Ir(111). Ir cluster superlattices self-assembled onto the graphene moiré pattern produce a strong renormalization of the electron group velocity close to the Dirac point, leading to highly anisotropic Dirac cones and the enlargement of the gap from 140 to 340 meV. This gap can further be enhanced to 740 meV by Na co-adsorption onto the Ir cluster superlattice at room temperature. This value is close to that of Ge, and the high group velocity of the charge carriers is fully preserved. We also present data for Na adsorbed without the Ir clusters. In both cases we find that the Na is on top of the graphene layer.

Low energy electron microscopy and photoemission electron microscopy investigation of graphene

Low energy electron microscopy (LEEM) and photoemission electron microscopy (PEEM) are two powerful techniques for the investigation of surfaces, thin films and surface supported nanostructures. In this review, we examine the contributions of these microscopy techniques to our understanding of graphene in recent years. These contributions have been made in studies of graphene on various metal and SiC surfaces and free-standing graphene. We discuss how the real-time imaging capability of LEEM facilitates a deeper understanding of the mechanisms of dynamic processes, such as growth and intercalation. Numerous examples also demonstrate how imaging and the various available complementary measurement capabilities, such as selected area or micro low energy electron diffraction (μLEED) and micro angle resolved photoelectron spectroscopy (μARPES), allow the investigation of local properties in spatially inhomogeneous graphene samples.

Epitaxial growth of graphene on transition metal surfaces: chemical vapor deposition versus liquid phase deposition

The epitaxial growth of graphene on transition metal surfaces by ex situ deposition of liquid precursors (LPD, liquid phase deposition) is compared to the standard method of chemical vapor deposition (CVD). The performance of LPD strongly depends on the particular transition metal surface. For Pt(111), Ir(111) and Rh(111), the formation of a graphene monolayer is hardly affected by the way the precursor is provided. In the case of Ni(111), the growth of graphene strongly depends on the applied synthesis method. For CVD of propene on Ni(111), a 1 × 1 structure as expected from the vanishing lattice mismatch is observed. However, in spite of the nearly perfect lattice match, a multi-domain structure with 1 × 1 and two additional rotated domains is obtained when an oxygen-containing precursor (acetone) is provided ex situ.

Controllable p-doping of graphene on Ir(111) by chlorination with FeCl3

The in situ chlorination of graphene on Ir(111) has been achieved by depositing FeCl(3) followed by its thermal decomposition on the surface into FeCl(2) and Cl. This process is accompanied by an intercalation of Cl under graphene and formation of an epitaxial FeCl(2) film on top, which can be removed upon further annealing. A pronounced hole doping of graphene has been observed as a consequence of the annealing-assisted intercalation of Cl. This effect has been studied by a combination of core-level and angle-resolved photoelectron spectroscopies (CL PES and ARPES, respectively), near-edge x-ray absorption fine structure (NEXAFS) spectroscopy and low-energy electron diffraction (LEED). The ease of preparation, the remarkable reproducibility of the doping level and the reversibility of the doping upon annealing are the key factors making chlorination with FeCl(3) a promising route for tuning the electronic properties in graphene.

Moiré pattern induced by the electronic coupling between 1-octanol self-assembled monolayers and graphite surface

Two-dimensional self-assembly of 1-octanol molecules on a graphite surface is investigated using scanning tunneling microscopy (STM) at the solid/liquid interface. STM images reveal that this molecule self-assembles into a compact hydrogen-bonded herringbone nanoarchitecture. Molecules are preferentially arranged in a head-to-head and tail-to-tail fashion. A Moiré pattern appears in the STM images when the 1-octanol layer is covering the graphite surface. The large Moiré stripes are perpendicular to the 1-octanol lamellae. Interpretation of the STM images suggests that the Moiré periodicity is governed by the electronic properties of the graphite surface and the 1-octanol layer periodicity.

Local gating of an Ir(111) surface resonance by graphene islands

The influence of graphene islands on the electronic structure of the Ir(111) surface is investigated. Scanning tunneling spectroscopy (STS) indicates the presence of a two-dimensional electron gas with a binding energy of -160  meV and an effective mass of -0.18me underneath single-layer graphene on the Ir(111) surface. Density functional calculations reveal that the STS features are predominantly due to a holelike surface resonance of the Ir(111) substrate. Nanometer-sized graphene islands act as local gates, which shift and confine the surface resonance.