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We studied the effect of alkali metal intercalation (Cs and Li) on the geometry of graphene on Ir(111) using the X-ray standing waves technique. For both alkali metals, the increase in the mean hei...

Collective electronic states such as the charge density wave (CDW) order and superconductivity (SC) respond sensitively to external perturbations. Such sensitivity is dramatically enhanced in two dimensions (2D), where 2D materials hosting such electronic states are largely exposed to the environment. In this regard, the ineludible presence of supporting substrates triggers various proximity effects on 2D materials that may ultimately compromise the stability and properties of the electronic ground state. In this work, we investigate the impact of proximity effects on the CDW and superconducting states in single-layer (SL) NbSe2 on four substrates of diverse nature, namely, bilayer graphene (BLG), SL-boron nitride (h-BN), Au(111), and bulk WSe2. By combining low-temperature (340 mK) scanning tunneling microscopy/spectroscopy and angle-resolved photoemission spectroscopy, we compare the electronic structure of this prototypical 2D superconductor on each substrate. We find that, even when the electronic band structure of SL-NbSe2 remains largely unaffected by the substrate except when placed on Au(111), where a charge transfer occurs, both the CDW and SC show disparate behaviors. On the insulating h-BN/Ir(111) substrate and the metallic BLG/SiC(0001) substrate, both the 3 × 3 CDW and superconducting phases persist in SL-NbSe2 with very similar properties, which reveals the negligible impact of graphene on these electronic phases. In contrast, these collective electronic phases are severely weakened and even absent on the bulk insulating WSe2 substrate and the metallic single-crystal Au(111) substrate. Our results provide valuable insights into the fragile stability of such electronic ground states in 2D materials., M.M.U. acknowledges support by the ERC Starting grant LINKSPM (grant 758558) and by the Spanish MINECO under grant no. PID2020-116619GB-C21. J.A.M. acknowledges financial support from the Danish Council for Independent Research, Natural Sciences under the Sapere Aude program (grant no. DFF-6108-00409). A.C, M.B., J.A.M., and P.H. acknowledge support from the VILLUM FONDEN via the Centre of Excellence for Dirac Materials (grant no. 11744). A.J.M.-G. acknowledges funding by the Spanish MINECO through project no. PID2020-116619GA-C22. E.C. and S.M.-V. acknowledge the Spanish MICINN (project PID2020-117152RB-I00 cofinanced by FEDER and the Unit of Excellence “Maria de Maeztu” CEX2019-000919-M) and the Generalitat Valenciana (Prometeo Programme and PO FEDER Program IDIFEDER/2018/061 and IDFEDER/2020/063). R.H. acknowledges support from Marie Skłodowska-Curie Individual Fellowships under HORIZON 2020 program for project MAGTMD (101033538).

Predicting the properties of two-dimensional (2D) materials as graphene and hexagonal boron nitride (h-BN) monolayers after their growth on any given substrate is a major challenge. While the influence of the electron configuration of the atoms of the underlying surface is well-understood, the effect of substrate geometry still remains unclear. The structural properties of h-BN monolayers grown on a rectangularly packed Rh(110) surface were characterized in situ by ultrahigh vacuum scanning tunneling microscopy and were compared to those that this material exhibits when grown on substrates showing different crystallographic orientations. Although the h-BN monolayer grown on Rh(110) was dominated by a unique quasiunidimensional moire pattern, suggesting considerable interface interaction, the moire corrugation was unexpectedly smaller than those reported for strongly interacting interfaces with hexagonal-terminated substrates, owing to differences in the possible binding landscapes at interfaces with differently oriented substrates. Moreover, a rule was derived for predicting how interface corrugation and the existence and extent of subregions within moire supercells containing favorable sites for orbital mixing between h-BN monolayers and their supports depend on substrate symmetry. These general symmetry considerations can be applied to numerous 2D materials, including graphene, thereby enabling the prediction of how substrate choice determines the properties of these materials. Furthermore, they could also provide new routes for tuning 2D material properties and for developing nanotemplates showing different geometries for growing adsorbate superlattices.

Metal nanoclusters, supported on inert substrates, exhibiting well-defined shapes and sizes in a broad range of temperatures are a major object of desire in nanotechnology. Here, a technique is presented that improves the thermal stability of monodisperse and crystalline transition metal nanoclusters grown in a regular array on metal-supported graphene. To stabilize the clusters after growth under ultrahigh vacuum the system composed of the aggregates and the graphene/metal interface is exposed to radicals resulting from the dissociation of diatomic gases. As a model system we have used Pt as the metal element for cluster growth and the template consisting of the moiré pattern resulting from the lattice mismatch between graphene and the Ir(111) surface. The study has been performed for deuterium and oxygen radicals, which interact very differently with graphene. Our results reveal that after radical exposure the thermally activated motion of Pt nanoclusters to adjacent moiré cells and the subsequent sintering of neighbor aggregates are avoided, most pronounced for the case of atomic O. For the case of D the limits of the improvement are given by radical desorption, whereas for the case of O they are defined by an interplay between coalescence and graphene etching followed by Pt intercalation, which can be controlled by the amount of exposure. Finally, we determined the mechanism of how radical adsorption improves the thermal stability of the aggregates.

Our scanning tunneling microscopy and X-ray photoelectron spectroscopy experiments along with first-principles calculations uncover the rich phenomenology and enable a coherent understanding of carbon vapor interaction with graphene on Ir(111). At high temperatures, carbon vapor not only permeates to the metal surface but also densifies the graphene cover. Thereby, in addition to underlayer graphene growth, upon cool down also severe wrinkling of the densified graphene cover is observed. In contrast, at low temperatures the adsorbed carbon largely remains on top and self-organizes into a regular array of fullerene-like, thermally highly stable clusters that are covalently bonded to the underlying graphene sheet. Thus, a new type of predominantly sp2-hybridized nanostructured and ultrathin carbon material emerges, which may be useful to encage or stably bind metal in finely dispersed form.

A 2D material consisting of a pseudo-ordered distribution of Ir nanocrystals supported on a h-BN/Rh(111) surface is presented here. The particular spatial distribution of the Ir nanoparticles is achieved thanks to the existence of a large variety of adsorption positions within the pores of the h-BN/Rh(111) nanomesh template with hexagonal symmetry. The resulting deviations of nanoparticle positions with respect to a perfect hexagonal lattice, which make this material of special interest in the field of optics, can be tuned by the temperature and the amount of Ir. Upon annealing, this material undergoes slight structural changes in the temperature range of 370–570 K and much more drastic ones, due to cluster coalescence, between 670 and 770 K. This relatively high onset of coalescence is encouraging for using this 2D material as a catalyst for reactions such as the oxidation of carbon monoxide or of nitrogen monoxide, which are especially relevant in the field of environmental science. Finally, metal nanostructures exhibiting regular geometries have been created from this material using a scanning tunneling microscope tip. Because of the insulating character of h-BN, these nanostructures could be very promising to use in the design of conductive nanotracks.

Scanning tunneling microscopy, low-energy electron diffraction, and helium atom scattering show a transition to a dimerizationlike reconstruction in the one-dimensional atomic chains on Bi(114) at low temperatures. One-dimensional metals are generally unstable against such a Peierls-like distortion, but neither the shape nor the spin texture of the Bi(114) Fermi contour favors the transition: Although the Fermi contour is one dimensional and thus perfectly nested, the very short nesting vector $2{k}_{F}$ is inconsistent with the periodicity of the distortion. Moreover, the nesting occurs between two Fermi contour branches of opposite spin, which is also expected to prevent the formation of a Peierls phase. Indeed, angle-resolved photoemission spectroscopy does not reveal any change in the electronic structure near the Fermi energy around the phase transition. On the other hand, distinct changes at higher binding energies are found to accompany the structural phase transition. This suggests that the transition of a strong-coupling type and that it is driven by phonon entropy rather than electronic entropy. This picture is supported by the observed short correlation length of the pairing distortion, the second-order-like character of the phase transition, and pronounced differences between the surface phonon spectra of the high- and low-temperature phases.

The mechanisms for oxygen etching of graphene on Ir(111) are uncovered through a systematic variation of the graphene morphology - ranging from an impermeable graphene layer to graphene nanoflakes - and the application of complementary experimental methods, including scanning tunneling microscopy, X-ray photoelectron spectroscopy, and temperature programmed desorption. Associated with a strong variation in the onset temperature for etching, we find a fundamental difference in the onset of etching for an impermeable layer and for graphene flakes. For the impermeable graphene layer etching is shown to nucleate at graphene pentagon heptagon point defects through molecules impinging from the gas phase. For graphene flakes the nucleation problem is absent due to the existence of edges in contact with the metallic substrate. The substrate enables dissociative chemisorption of oxygen, which can then diffuse as atomic oxygen to the graphene edge. Our results show that intercalation of oxygen is neither a necessary condition nor of specific relevance for etching. Based on our analysis, a quantitative estimate for the activation energy and attempt frequency of the elementary etch process in flake etching on Ir(111) is provided. (C) 2015 Elsevier Ltd. All rights reserved. (Less)

We create an interface of graphene with a metallic and magnetic support that leaves its electronic structure largely intact. This is achieved by exposing epitaxial graphene on ferromagnetic thin films of Co and Ni to vapor of the rare earth metal Eu at elevated temperatures, resulting in the intercalation of an Eu monolayer in between graphene and its substrate. The system is atomically well defined, with the Eu monolayer forming a (√3×√3)R30∘ superstructure with respect to the graphene lattice. Thereby, we avoid the strong hybridization with the (Ni,Co) substrate 3d states that otherwise drastically modify the electronic structure of graphene. This picture is suggested by our x-ray absorption spectroscopy measurements which show that after Eu intercalation the empty 2p states of C atoms resemble more the ones measured for graphite in contrast to graphene directly bound to 3d ferromagnetic substrates. We use x-ray magnetic circular dichroism at the Co and Ni L2,3 and Eu M4,5 as an element-specific probe to investigate magnetism in these systems. An antiferromagnetic coupling between Eu and Co/Ni moments is found, which is so strong that a magnetic moment of the Eu layer can be detected at room temperature. Density functional theory calculations confirm the antiferromagnetic coupling and provide an atomic insight into the magnetic coupling mechanism.

Through intercalation of metals and gases the Dirac cone of graphene on Ir(111) can be shifted with respect to the Fermi level without becoming destroyed by strong hybridization. Here, we use x- ray photoelectron spectroscopy to measure the C 1s core level shift (CLS) of graphene in contact with a number of structurally well-defined intercalation layers (O, H, Eu, and Cs). By analysis of our own and additional literature data for decoupled graphene, the C 1s CLS is found to be a non- monotonic function of the doping level. For small doping levels the shifts are well described by a rigid band model. However, at larger doping levels, a second effect comes into play which is proportional to the transferred charge and counteracts the rigid band shift. Moreover, not only the position, but also the C 1s peak shape displays a unique evolution as a function of doping level. Our conclusions are supported by intercalation experiments with Li, with which, due to the absence of phase separation, the doping level of graphene can be continuously tuned.

The local interaction between graphene and a host substrate strongly determines the actual properties of the graphene layer. Here we show that scanning tunneling microscopy (STM) can selectively help to visualize either the graphene layer or the substrate underneath, or even both at the same time, providing a comprehensive picture of this coupling with atomic precision and high energy resolution. We demonstrate this for graphene on Cu(111). Our spectroscopic data show that, in the vicinity of the Fermi level, graphene π bands are well preserved presenting a small n-doping induced by Cu(111) surface state electrons. Such results are corroborated by Angle-Resolved Photoemission Spectra (ARPES) and Density Functional Theory with van der Waals (DFT + vdW) calculations. Graphene tunable transparency also allows the investigation of the interaction between the substrate and foreign species (such as atomic H or C vacancies) on the graphene layer. Our calculations explain graphene tunable transparency in terms of the rather different decay lengths of the graphene Dirac π states and the metal surface state, suggesting that it should apply to a good number of graphene/substrate systems., We acknowledge financial support from Spanish Grant MAT2013-41636-P, PCIN-2015-030, MAT2011-23627, MAT2011-26534, MAT2013-6593-C6-4-P, MAT2014-54484-P, MDM-2014-0377, CSD2010-00024 (MINECO, Spain) and S2009/MAT-1467 (CAM, Spain), IT621-13 (Basque Government) the European Union structural funds and the Comunidad de Madrid MAD2D-CM Program (S2013/MIT-3007) and the European Union FLAG-ERA program. P.P. was supported by the Ramón y Cajal program. A.J.M.-G. was supported by the Marie Curie program.

Using scanning tunneling microscopy, the oxygen adsorbate superstructures on bare Ir(111) are identified and compared to the ones formed by intercalation in between graphene and the Ir(111) substrate. For bare Ir(111) we observe O-(2 × 2) and O-(2 × 1) structures, thereby clarifying a persistent uncertainty about the existence of these structures and the role of defects for their stability. For the case of graphene-covered Ir(111), oxygen intercalation superstructures can be imaged through the graphene monolayer by choosing proper tunneling conditions. Depending on the pressure, temperature and duration of O2 exposure as well as on the graphene morphology, O-(2 × 2), O-(√3×√3)-R30°, O-(2 × 1) and O-(2√3 × 2√3)-R30° superstructures with respect to Ir(111) are observed under the graphene cover. Two of these structures, the O-(√3 × √3)-R30° and the (2√3 × 2√3)-R30° structure are only observed when the graphene layer is on top. Phase coexistence and formation conditions of the intercalation structures between graphene and Ir(111) are analyzed. The experimental results are compared to density functional theory calculations including dispersive forces. The existence of these phases under graphene and their absence on bare Ir(111) are discussed in terms of possible changes in the adsorbate-substrate interaction due to the presence of the graphene cover.

The adsorption and growth of 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) on graphene monolayers epitaxially grown on Pt(111) surfaces is studied by a combination of experimental scanning tunneling microscopy (STM) and spectroscopy (STS) measurements and first-principles density functional theory (DFT) calculations. For submonolayer coverage, until the completion of the first layer, PTCDA molecules form a well-ordered herringbone structure with molecules lying flat on the graphene surface weakly coupled to the Pt(111) substrate. High-resolution STM imaging at different sample biases has allowed the identification of intramolecular features that can be related to the original PTCDA frontier orbitals. Theoretical STM calculations, based on local-orbital DFT, have been carried out on the full PTCDA/graphene/Pt(111) system. The comparison of theoretical and experimental STM images has allowed us to ascribe the origin of intramolecular features to the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO) of the free PTCDA molecules. Moreover, the experimental STS spectra display well-resolved peaks centered at -2.2 and +1.2 eV in excellent agreement with DFT calculations. This study reveals that the growth and electronic structure of PTCDA retain all of the essential electronic features of the molecular layer upon adsorption on this weakly coupled graphene on Pt(111) surface., Financial support from the CAM, under contract nos. CPI/0256/2007 and S2009/MAT-1467, and from the Spanish MICIIN under Grant nos. MAT2010-14902, CSD2010-00024, and FIS2010-16046, is gratefully acknowledged. J.I.M. acknowledges funding from Spanish MICIIN and CSIC through “Juan de la Cierva” and “JaeDoc” Programs.

The intercalation of Eu underneath Gr on Ir(111) is comprehensively investigated by microscopic, magnetic, and spectroscopic measurements, as well as by density functional theory. Depending on the coverage, the intercalated Eu atoms form either a $(2 \times 2)$ or a $(\sqrt{3} \times \sqrt{3})$R$30^{\circ}$ superstructure with respect to Gr. We investigate the mechanisms of Eu penetration through a nominally closed Gr sheet and measure the electronic structures and magnetic properties of the two intercalation systems. Their electronic structures are rather similar. Compared to Gr on Ir(111), the Gr bands in both systems are essentially rigidly shifted to larger binding energies resulting in n-doping. The hybridization of the Ir surface state $S_1$ with Gr states is lifted, and the moire superperiodic potential is strongly reduced. In contrast, the magnetic behavior of the two intercalation systems differs substantially as found by X-ray magnetic circular dichroism. The $(2 \times 2)$ Eu structure displays plain paramagnetic behavior, whereas for the $(\sqrt{3} \times \sqrt{3})$R$30^{\circ}$ structure the large zero-field susceptibility indicates ferromagnetic coupling, despite the absence of hysteresis at 10 K. For the latter structure, a considerable easy-plane magnetic anisotropy is observed and interpreted as shape anisotropy., Comment: 18 pages with 14 figures, including Supplemental Material

The initial stages of growth of PTCDA (3,4,9,10 perylene tetracarboxylic dianhydride) at room temperature (RT) on Ge(111)-[Formula: see text] surfaces have been studied by means of scanning tunneling microscopy (STM) under ultrahigh vacuum conditions. The results show that PTCDA molecules have a high mobility at RT on the well ordered areas of the semiconductor substrate, since nucleation is only observed in domain walls, steps and surface defects. However, no molecular ordering has been detected at submonolayer coverage. For higher coverages, the formation of three-dimensional (3D) molecular islands has been observed. These 3D islands present a crystalline nature as demostrated by molecularly resolved STM images. According to these STM measurements, PTCDA molecules are ordered in a herringbone structure, similar to the one observed in PTCDA bulk crystals. Moreover, the 3D crystallites are grown on top of a disordered molecular layer, which acts as a passivating layer.

Upon cooling, branched line defects develop in epitaxial graphene grown at high temperature on Pt(111) and Ir(111). Using atomically resolved scanning tunneling microscopy we demonstrate that these defects are wrinkles in the graphene layer, i.e. stripes of partially delaminated graphene. With low energy electron microscopy (LEEM) we investigate the wrinkling phenomenon in situ. Upon temperature cycling we observe hysteresis in the appearance and disappearance of the wrinkles. Simultaneously with wrinkle formation a change in bright field imaging intensity of adjacent areas and a shift in the moire spot positions for micro diffraction of such areas takes place. The stress relieved by wrinkle formation results from the mismatch in thermal expansion coefficients of graphene and the substrate. A simple one-dimensional model taking into account the energies related to strain, delamination and bending of graphene is in qualitative agreement with our observations., Supplementary information: S1: Photo electron emission microscopy and LEEM measurements of rotational domains, STM data of a delaminated bulge around a dislocation. S2: Movie with increasing brightness upon wrinkle formation as in figure 4. v2: Major revision including new experimental data