Thermal stability of corrugated epitaxial graphene grown on Re(0001)

Velocity-Dependent Friction of Graphene at Electrical Contact Interfaces

Graphene has enormous potential as a solid lubricant at sliding electrical contact interfaces of micro-/nanoelectromechanical systems that suffer severe wear. Understanding the velocity-dependent friction of graphene under different applied voltages contributes to the application of graphene in sliding electrical contact scenarios. The friction of graphene, measured by conductive atomic force microscopy, under low applied voltage increases logarithmically with sliding velocity─the same as when no voltage is applied but at a faster rate of increase. The variation of friction was explained by the thermally activated Prandtl-Tomlinson model with increased potential barrier and temperature because of the applied voltage. An opposite trend in which friction decreases with sliding velocity is observed under high applied voltage. Topography, adhesion measurements, and SEM characterization demonstrate the wear of the tip. Moreover, the tip wears more severely at low sliding velocity compared to a high sliding velocity. It was interpreted that the excessive Joule heat at the electrical contact interface under high applied voltage weakens the mechanical properties of the tip, resulting in wear and high friction. The increase in the sliding velocity could accelerate the Joule heat transfer and reduce wear and friction. The studies provide beneficial guidelines for the design of graphene-lubricated sliding electrical contact interfaces.

Highly Efficient Self-Encapsulated Flexible Semitransparent Perovskite Solar Cells via Bifacial Cation Exchange

Flexible semitransparent perovskite solar cells (ST-PSCs) have great potential for use in high-density energy systems, such as building or vehicle integrated photovoltaics, considering the great features of PSC devices, including high performance, light weight, thin-film processability, and high near-infrared transmittance. Despite numerous efforts toward achieving efficiency and flexibility in ST-PSCs, the realization of high-performance and operational stability in ST-PSCs still require further development. Herein, we demonstrated the development of highly efficient, stable, and flexible ST-PSCs using polyimide-integrated graphene electrodes via a lamination-assisted bifacial cation exchange strategy. A high-quality perovskite layer was obtained through the cation exchange reaction using the lamination process, and ST-PSCs with 15.1% efficiency were developed. The proposed ST-PSC device also demonstrated excellent operational stability, mechanical durability, and moisture stability owing to the chemically inert and mechanically robust graphene electrodes. This study provides an effective strategy for developing highly functional ST-perovskite optoelectronic devices with high-performance and long-term operational stability.

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 3d_5/2 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.

Method for the Manual Analysis of Moiré Structures in STM images

A method is presented to manually determine the lattice parameters of commensurate hexagonal moiré structures resolved by STM. It solves the problem that lattice parameters of moiré structures usually cannot be determined by inspection of an STM image, so that computer-based analyses are required. The lattice vector of a commensurate moiré structure is a sum of integer multiples both of the two basis vectors of the substrate and of the adsorbed layer. The method extracts the two factors with respect to the adsorbed layer from an analysis of the Fourier transform of an STM image. These two factors are related to the two factors with respect to the substrate layer. Using the cell augmentation method, six possible moiré structures are identified by algebra. When the orientation and lattice constant of the substrate are roughly known, this information is usually sufficient to determine a unique moiré structure and its lattice parameters.

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.

Three-dimensional microporous graphene decorated with lithium

Three-dimensional (3D) graphene-based architectures can combine the two-dimensional properties of graphene with the high surface-to-volume ratio required for a large variety of technological applications. We present a spectro-microscopy study of stable microporous 3D few-layer graphene structures with a very low density of defects/edges and of unsaturated bonds, as deduced by Raman and core level photoemission spectroscopy. These qualities make these interconnected graphene networks ideal candidates to accommodate lithium adatoms, with a high density of Li per unit volume and a Li uptake per C atom higher than the value observed for graphite, as confirmed by core level photoemission spectroscopy.

Size-Selective Carbon Clusters as Obstacles to Graphene Growth on a Metal

Chemical vapor deposition (CVD) on metals is so far the best suited method to produce high-quality, large-area graphene. We discovered an unprecedentedly large family of small size-selective carbon clusters that form together with graphene during CVD. Using scanning tunneling microscopy (STM) and density functional theory (DFT), we unambiguously determine their atomic structure. For that purpose, we use grids based on a graphene moiré and a dilute atomic lattice that unambiguously reveal the binding geometry of the clusters. We find that the observed clusters bind in metastable configurations on the substrate, while the thermodynamically stable configurations are not observed. We argue that the clusters are formed under kinetic control and establish that the evolution of the smallest clusters is blocked. They are hence products of surface reactions in competition with graphene growth, rather than intermediary species to the formation of extended graphene, as often assumed in the literature. We expect such obstacles to the synthesis of perfect graphene to be ubiquitous on a variety of metallic surfaces.

Substrate-induced enhancement of the chemical reactivity in metal-supported graphene

Graphene is commonly regarded as an inert material. However, it is well known that the presence of defects or substitutional hetero-atoms confers graphene promising catalytic properties. In this work, we use first-principles calculations to show that it is also possible to enhance the chemical reactivity of a graphene layer by simply growing it on an appropriate substrate. Our comprehensive study demonstrates that, in strongly interacting substrates like Rh(111), graphene adopts highly rippled structures that exhibit areas with distinctive chemical behaviors. According to the local coupling with the substrate, we find areas with markedly different adsorption, dissociation and diffusion pathways for both molecular and atomic oxygen, including a significant change in the nature of the adsorbed molecular and dissociated states, and a dramatic reduction (∼60%) of the O2 dissociation energy barrier with respect to free-standing graphene. Our results show that the graphene-metal interaction represents an additional and powerful handle to tailor the graphene chemical properties with potential applications to nano patterning, graphene functionalization and sensing devices.

Hydrocarbon decomposition kinetics on the Ir(111) surface

The kinetics of the thermal decomposition of hydrocarbons on the Ir(111) surface is determined using kinetic Monte Carlo (kMC) and rate equations simulations, both based on the density functional theory (DFT) calculated energy barriers of the involved reaction processes. This decomposition process is important for understanding the early stages of epitaxial graphene growth where the deposited hydrocarbon acts as a carbon feedstock for graphene formation. The methodology of the kMC simulations and the rate equation approaches is discussed and a comparison between the results obtained from both approaches is made in the case of the temperature programmed decomposition of ethylene for different initial coverages. The theoretical results are verified against experimental data from in situ X-ray photoelectron spectroscopy (XPS) experiments. Both theoretical approaches give reasonable results; however we find that, as expected, rate equations are less reliable at high coverages. We find that the agreement between experiment and theory can be improved in all cases if slight adjustments are made to the energy barriers in order to account for the intrinsic errors in DFT. Finally we extend our approach to the case where hydrocarbon species are dosed onto the substrate continuously, as in the chemical vapour deposition (CVD) graphene growth method. For ethylene and methane the thermal decomposition mechanism is determined, and it is found that in both cases the formation of C monomers is to be expected, which is limited by the presence of hydrogen atoms.

Unveiling the atomistic mechanisms for oxygen intercalation in a strongly interacting graphene–metal interface

The atomistic mechanisms involved in the oxygen intercalation in the strongly interacting G/Rh(111) system are characterized in a comprehensive experimental and theoretical study, combining scanning tunneling microscopy and DFT calculations.

Graphene-based materials supported advanced oxidation processes for water and wastewater treatment: a review

Advanced oxidation processes (AOPs) received much attention in the field of water and wastewater treatment due to its ability to mineralize persistent organic pollutants from water medium. The addition of graphene-based materials increased the efficiency of all AOPs significantly. The present review analyzes the performance of graphene-based materials that supported AOPs in detail. Recent developments in this field are highlighted. A special focus has been awarded for the performance enhancement mechanism of AOPs in the presence of graphene-based materials.

Two-Dimensional Hallmark of Highly Interconnected Three-Dimensional Nanoporous Graphene

Scaling graphene from a two-dimensional (2D) ideal structure to a three-dimensional (3D) millimeter-sized architecture without compromising its remarkable electrical, optical, and thermal properties is currently a great challenge to overcome the limitations of integrating single graphene flakes into 3D devices. Herewith, highly connected and continuous nanoporous graphene (NPG) samples, with electronic and vibrational properties very similar to those of suspended graphene layers, are presented. We pinpoint the hallmarks of 2D ideal graphene scaled in these 3D porous architectures by combining the state-of-the-art spectromicroscopy and imaging techniques. The connected and bicontinuous topology, without frayed borders and edges and with low density of crystalline defects, has been unveiled via helium ion, Raman, and transmission electron microscopies down to the atomic scale. Most importantly, nanoscanning photoemission unravels a 3D NPG structure with preserved 2D electronic density of states (Dirac cone like) throughout the porous sample. Furthermore, the high spatial resolution brings to light the interrelationship between the topology and the morphology in the wrinkled and highly bent regions, where distorted sp² C bonds, associated with sp³-like hybridization state, induce small energy gaps. This highly connected graphene structure with a 3D skeleton overcomes the limitations of small-sized individual graphene sheets and opens a new route for a plethora of applications of the 2D graphene properties in 3D devices.

Strong Adlayer-Substrate Interactions "Break" the Patching Growth of h-BN onto Graphene on Re(0001)

Hetero-epitaxial growth of hexagonal boron nitride (h-BN) from the edges of graphene domains or vice versa has been widely observed during synthesis of in-plane heterostructures of h-BN-G on Rh(111), Ir(111), and even Cu foil. We report that on a strongly coupled Re(0001) substrate via a similar two-step sequential growth strategy, h-BN preferably nucleated on the edges of Re(0001) steps rather than on the edges of existing graphene domains. Statistically, one-third of the domain boundaries of graphene and h-BN were patched seamlessly, and the others were characterized by obvious "defect lines" when the total coverage approached a full monolayer. This imperfect merging behavior can be explained by translational misalignment and lattice mismatch of the resulting separated component domains. According to density functional theory calculations, this coexisting patching and non-patching growth behavior was radically mediated by the strong adlayer-substrate (A-S) interactions, as well as the disparate formation energies of the attachment of B-N pairs or B-N lines along the edges of the Re(0001) steps versus the graphene domains. This work will be of fundamental significance for the controllable synthesis of in-plane heterostructures constructed from two-dimensional layered materials with consideration of A-S interactions.

The physics and chemistry of graphene-on-surfaces

This review describes the major “graphene-on-surface” structures and examines the roles of their properties in governing the overall performance for specific applications.

Influence of interface interaction on the moiré superstructures of graphene on transition-metal substrates

The formation of moiré superstructures between graphene and its underlying substrate has attracted significant attention because it significantly influences the morphology and properties of graphene.

Universal classification of twisted, strained and sheared graphene moiré superlattices

Moiré superlattices in graphene supported on various substrates have opened a new avenue to engineer graphene's electronic properties. Yet, the exact crystallographic structure on which their band structure depends remains highly debated. In this scanning tunneling microscopy and density functional theory study, we have analysed graphene samples grown on multilayer graphene prepared onto SiC and on the close-packed surfaces of Re and Ir with ultra-high precision. We resolve small-angle twists and shears in graphene, and identify large unit cells comprising more than 1,000 carbon atoms and exhibiting non-trivial nanopatterns for moiré superlattices, which are commensurate to the graphene lattice. Finally, a general formalism applicable to any hexagonal moiré is presented to classify all reported structures.

Switchable graphene-substrate coupling through formation/dissolution of an intercalated Ni-carbide layer

Control over the film-substrate interaction is key to the exploitation of graphene’s unique electronic properties. Typically, a buffer layer is irreversibly intercalated “from above” to ensure decoupling. For graphene/Ni(111) we instead tune the film interaction “from below”. By temperature controlling the formation/dissolution of a carbide layer under rotated graphene domains, we reversibly switch graphene’s electronic structure from semi-metallic to metallic. Our results are relevant for the design of controllable graphene/metal interfaces in functional devices.

Ethylene decomposition on Ir(111): initial path to graphene formation

The complete mechanism behind the thermal decomposition of ethylene (C₂H₄) on Ir(111), which is the first step of graphene growth, is established for the first time employing a combination of experimental and theoretical methods.

Growth of carbon nanoshells on tungsten carbide for loading Pt with enhanced electrocatalytic activity and stable anti-poisoning performance

Nearly transparent carbon nanoshells precipitated on WC were prepared by deoxidization approach. After Pt were loaded on the composite, an electrocatalyst with excellent electrochemical activity and stable anti-poisoning properties was obtained.

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.

Approaching truly freestanding graphene: the structure of hydrogen-intercalated graphene on 6H-SiC(0001)

We measure the adsorption height of hydrogen-intercalated quasifreestanding monolayer graphene on the (0001) face of 6H silicon carbide by the normal incidence x-ray standing wave technique. A density functional calculation for the full (6√3×6√3)-R30° unit cell, based on a van der Waals corrected exchange correlation functional, finds a purely physisorptive adsorption height in excellent agreement with experiments, a very low buckling of the graphene layer, a very homogeneous electron density at the interface, and the lowest known adsorption energy per atom for graphene on any substrate. A structural comparison to other graphenes suggests that hydrogen-intercalated graphene on 6H-SiC(0001) approaches ideal graphene.

Insights into carbon nanotube and graphene formation mechanisms from molecular simulations: a review

The discovery of carbon nanotubes (CNTs) and graphene over the last two decades has heralded a new era in physics, chemistry and nanotechnology. During this time, intense efforts have been made towards understanding the atomic-scale mechanisms by which these remarkable nanostructures grow. Molecular simulations have made significant contributions in this regard; indeed, they are responsible for many of the key discoveries and advancements towards this goal. Here we review molecular simulations of CNT and graphene growth, and in doing so we highlight the many invaluable insights gained from molecular simulations into these complex nanoscale self-assembly processes. This review highlights an often-overlooked aspect of CNT and graphene formation-that the two processes, although seldom discussed in the same terms, are in fact remarkably similar. Both can be viewed as a 0D → 1D → 2D transformation, which converts carbon atoms (0D) to polyyne chains (1D) to a complete sp(2)-carbon network (2D). The difference in the final structure (CNT or graphene) is determined only by the curvature of the catalyst and the strength of the carbon-metal interaction. We conclude our review by summarizing the present shortcomings of CNT/graphene growth simulations, and future challenges to this important area.

Cathode lens spectromicroscopy: methodology and applications

The implementation of imaging techniques with low-energy electrons at synchrotron laboratories allowed for significant advancement in the field of spectromicroscopy. The spectroscopic photoemission and low energy electron microscope, SPELEEM, is a notable example. We summarize the multitechnique capabilities of the SPELEEM instrument, reporting on the instrumental aspects and the latest developments on the technical side. We briefly review applications, which are grouped into two main scientific fields. The first one covers different aspects of graphene physics. In particular, we highlight the recent work on graphene/Ir(100). Here, SPELEEM was employed to monitor the changes in the electronic structure that occur for different film morphologies and during the intercalation of Au. The Au monolayer, which creeps under graphene from the film edges, efficiently decouples the graphene from the substrate lowering the Dirac energy from 0.42 eV to 0.1 eV. The second field combines magnetism studies at the mesoscopic length scale with self-organized systems featuring ordered nanostructures. This example highlights the possibility to monitor growth processes in real time and combine chemical characterization with X-ray magnetic circular dichroism-photoemission electron microscopy (XMCD-PEEM) magnetic imaging by using the variable photon polarization and energy available at the synchrotron source.

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.

Induced superconductivity in graphene grown on rhenium

We report a new way to strongly couple graphene to a superconductor. The graphene monolayer has been grown directly on top of a superconducting Re(0001) thin film and characterized by scanning tunneling microscopy and spectroscopy. We observed a moiré pattern due to the mismatch between Re and graphene lattice parameters that we have simulated with ab initio calculations. The density of states around the Fermi energy appears to be position dependent on this moiré pattern. Tunneling spectroscopy performed at 50 mK shows that the superconducting behavior of graphene on Re is well described by the Bardeen-Cooper-Schrieffer theory and stands for a very good interface between the graphene and its metallic substrate.

In situ observations of the atomistic mechanisms of Ni catalyzed low temperature graphene growth

The key atomistic mechanisms of graphene formation on Ni for technologically relevant hydrocarbon exposures below 600 °C are directly revealed via complementary in situ scanning tunneling microscopy and X-ray photoelectron spectroscopy. For clean Ni(111) below 500 °C, two different surface carbide (Ni2C) conversion mechanisms are dominant which both yield epitaxial graphene, whereas above 500 °C, graphene predominantly grows directly on Ni(111) via replacement mechanisms leading to embedded epitaxial and/or rotated graphene domains. Upon cooling, additional carbon structures form exclusively underneath rotated graphene domains. The dominant graphene growth mechanism also critically depends on the near-surface carbon concentration and hence is intimately linked to the full history of the catalyst and all possible sources of contamination. The detailed XPS fingerprinting of these processes allows a direct link to high pressure XPS measurements of a wide range of growth conditions, including polycrystalline Ni catalysts and recipes commonly used in industrial reactors for graphene and carbon nanotube CVD. This enables an unambiguous and consistent interpretation of prior literature and an assessment of how the quality/structure of as-grown carbon nanostructures relates to the growth modes.

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.

Fine tuning of graphene-metal adhesion by surface alloying

We show that bimetallic surface alloying provides a viable route for governing the interaction between graphene and metal through the selective choice of the elemental composition of the surface alloy. This concept is illustrated by an experimental and theoretical characterization of the properties of graphene on a model PtRu surface alloy on Ru(0001), with a concentration of Pt atoms in the first layer between 0 and 50%. The progressive increase of the Pt content determines the gradual detachment of graphene from the substrate, which results from the modification of the carbon orbital hybridization promoted by Pt. Alloying is also found to affect the morphology of graphene, which is strongly corrugated on bare Ru, but becomes flat at a Pt coverage of 50%. The method here proposed can be readily extended to several supports, thus opening the way to the conformal growth of graphene on metals and to a full tunability of the graphene-substrate interaction.

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.

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.

Phonon dispersion of quasi-freestanding graphene on Pt(111)

High-resolution electron energy loss spectroscopy has been used to probe phonon dispersion in quasi-freestanding graphene epitaxially grown on Pt(111). Loss spectra clearly show different dispersing features related to both acoustic and optical phonons. The present results have been compared with graphene systems which strongly interact with the substrate, i.e. the nearly-flat monolayer graphene (MLG)/Ni(111) and the corrugated MLG/Ru(0001). We found that the phonon dispersion of graphene/Pt(111) reproduces well the behavior of pristine graphite. This could be taken as an indication of the negligible interaction between the graphene sheet and the underlying Pt substrate. The softening of out-of-plane modes observed for interacting graphene/metal interfaces does not occur for the nearly-free-standing graphene/Pt(111).

Size-selected epitaxial nanoislands underneath graphene moiré on Rh(111)

We use in situ scanning tunneling microscopy (STM) to investigate intercalation of the ferromagnetic 3d metals Ni and Fe underneath a graphene monolayer on Rh(111). Upon thermal annealing of graphene/Rh(111) with the deposited metal on top, we observe the formation of epitaxial monatomic nanoislands grown pseudomorphically on Rh(111) and covered by graphene. The size and shape of intercalated nanoislands is strongly influenced by the local spatial variation of the graphene-Rh bonding strength. In particular, the side length of the intercalated nanoislands shows maxima around discrete values imposed by the periodicity of the graphene moiré. Intercalation can be performed efficiently and without any visible damage of the graphene overlayer in the studied temperature range between 670 and 870 K. We identify the main intercalation path to be via diffusion through pre-existing lattice defects in graphene, accompanied by the second mechanism which is based on the material diffusion via metal-generated defects followed by the defect healing of the graphene lattice. We deem these graphene-capped and sharply confined ferromagnetic nanoislands interesting in the fields of spintronics and nanomagnetism.