Point defects on graphene on metals

Understanding the coupling of graphene with its local environment is critical to be able to integrate it in tomorrow's electronic devices. Here we show how the presence of a metallic substrate affects the properties of an atomically tailored graphene layer. We have deliberately introduced single carbon vacancies on a graphene monolayer grown on a Pt(111) surface and investigated its impact in the electronic, structural and magnetic properties of the graphene layer. Our low temperature scanning tunneling microscopy studies, complemented by density functional theory, show the existence of a broad electronic resonance above the Fermi energy associated with the vacancies. Vacancy sites become reactive leading to an increase of the coupling between the graphene layer and the metal substrate at these points; this gives rise to a rapid decay of the localized state and the quenching of the magnetic moment associated with carbon vacancies in free-standing graphene layers

Molecular properties of PTCDA on graphene grown on a rectangular symmetry substrate

The chemical modulation associated with moiré patterns, arising at the interface of metal-supported 2D material systems, affects the interaction between molecules and 2D materials on surfaces. Since the crystallography of the support influences the interfacial chemistry of the moiré modulation, this parameter could also play a role in the graphene-molecule interaction, although studies using non-hexagonal metal supports are needed to investigate this effect. It is a key issue since graphene appears combined with organic films in most technological advances related to this material. Here, we have characterized the properties of PTCDA molecules on graphene grown on Rh(110) substrates, which exhibit a rectangular atomic packing, using scanning tunneling microscopy and spectroscopy. The results showed that PTCDA molecules are arranged on the surface into a herringbone structure exhibiting a long-range ordering, which grows continuously across substrate atomic steps edge and dislocations. The quasi-1D moiré patterns of the Gr/Rh(110) surfaces are found to provide an inert chemical landscape for the molecular arrangement. Bias voltage-dependent imaging of the orbital structure of PTCDA molecules and differential conductance spectra back up a weak molecule–substrate interaction scheme. Finally, the α-polymorph of bulk crystal PTCDA has been determined as the favored stacking configuration for bilayer molecules on Gr/Rh(110)Financial support from the Spanish Ministerio de Economía y Competitividad (MINECO) and Fondo Europeo de Desarrollo Regional (FEDER) under grant No. MAT2016-77852-C2-2-R, as well as, from the Spanish Ministerio de Ciencia e Innovacion ´ (MICINN) through the “María de Maetzu” program for units of excellence in R&D (grant No. CEX2018-000805-M) is gratefully acknowledged. A. J. M.-G. acknowledges funding by the Spanish Ministerio de Ciencia e Innovacion ´ (MICINN) through Project No. PID2020-116619GA-C22, as well as, from the Comunidad de Madrid and the Universidad Autonoma ´ de Madrid under project SI3/PJI/2021- 0050

Lateral Heterostructures of Graphene and h-BN with Atomic Lattice Coherence and Tunable Rotational Order

In-plane heterostructures of graphene and hexagonal boron nitride (h-BN) exhibit exceptional properties, which are highly sensitive to the structure of the alternating domains. Nevertheless, achieving accurate control over their structural properties, while keeping a high perfection at the graphene-h-BN boundaries, still remains a challenge. Here, the growth of lateral heterostructures of graphene and h-BN on Rh(110) surfaces is reported. The choice of the 2D material, grown firstly, determines the structural properties of the whole heterostructure layer, allowing to have control over the rotational order of the domains. The atomic-scale observation of the boundaries demonstrates a perfect lateral matching. In-plane heterostructures floating over an oxygen layer have been successfully obtained, enabling to observe intervalley scattering processes in graphene regions. The high tuning capabilities of these heterostructures, along with their good structural quality, even around the boundaries, suggest their usage as test beds for fundamental studies aiming at the development of novel nanomaterials with tailored properties.Financial support from the Spanish Ministerio de Economía y Competitividad (MINECO) and Fondo Europeo de Desarrollo Regional (FEDER) under grant No. MAT2016-77852-C2-2-R, as well as, from the Spanish Ministerio de Ciencia e Innovación through the “María de Maetzu” program for units of excellence in R&D (grant No. CEX2018-000805-M) was gratefully acknowledged. A. J. M.-G. acknowledged funding by the Spanish Ministerio de Ciencia e Innovación (MICINN through Project No. PID2020-116619GA-C22, as well as, by the Comunidad de Madrid and Universidad Autónoma de Madrid under Project No. SI3/PJI/2021-00500. Funding sources: Spanish MINECO (Ref: MAT2016-77852-C2-2-R). Spanish MICINN (Ref: PID2020-116619GA-C22). Comunidad de Madrid (CAM) and Universidad Autónoma de Madrid (UAM) (Ref: SI3/PJI/2021-00500

Unraveling the Highly Complex Nature of Antimony on Pt(111)

Understanding the growth behavior of group-V elements on metal surfacesprovides valuable information that can shed light on the feasibility of tailoringatomically thin monoelemental 2D polymorphs composed of pnictogens onthese metallic substrates. Here, by combining scanning tunneling micro-scopy (STM), low energy electron diffraction and Auger electron spectroscopymeasurements under ultra-high vacuum conditions, a wide variety of Sbreconstructions on single-crystal Pt(111) are identified and characterized. AtSb coverage of ≈0.2 ML, STM data are compatible with a scenario in which Sbatoms are randomly embedded into the topmost layer of Pt, in a substitutionalconfiguration and without establishing a periodic structure. This disorderis robust against thermal annealing and quenching. Increasing the surfacecoverage and whether or not the sample is annealed, different well-orderedSb phases are formed. The Sb structures synthesized at room temperaturewithout any heating process are best interpreted as surface alloys that involveonly the first atomic layer. In contrast, experimental evidence points towardsthe development of multilayer alloy phases for the annealed samplesFinancial support from the Spanish Ministerio de Economía y Competitividad (MINECO)and Fondo Europeo de Desarrollo Regional (FEDER) under grant No. MAT2016-77852-C2-2-R, as well as from the Spanish Ministerio de Ciencia e Innovación through the “María de Maetzu” program forunits of excellence in R&D (grant No. CEX2018-000805-M) is gratefully acknowledged. A.J.M.-G. acknowledges funding by the Spanish MINECO through Project No. PID2020-116619GA-C2

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 configurationsWe acknowledge financial support from Spanish grants MAT2013-41636-P, MAT2011-23627, MAT2011-26534, CSD2010-00024 (MINECO, Spain) and S2009/MAT-1467 (CAM, Spain). A.J.M.G. was supported by a Marie Curie action under the Seventh Framework Programme. P.P. was supported by the Ramón y Cajal Progra

Unconventional superconductivity mediated by spin fluctuations in single-layer NbSe2

Van der Waals materials provide an ideal platform to explore superconductivity in the presence of strong electronic correlations, which are detrimental of the conventional phonon-mediated Cooper pairing in the BCS-Eliashberg theory1 and, simultaneously, promote magnetic fluctuations. Despite recent progress in understanding superconductivity in layered materials, the glue pairing mechanism remains largely unexplored in the single-layer limit, where electron-electron interactions are dramatically enhanced. Here we report experimental evidence of unconventional Cooper pairing mediated by magnetic excitations in single-layer NbSe2, a model strongly correlated 2D material. Our high-resolution spectroscopic measurements reveal a characteristic spin resonance excitation in the density of states that emerges from the quasiparticle coupling to a collective bosonic mode. This resonance, observed along with higher harmonics, gradually vanishes by increasing the temperature and upon applying a magnetic field up to the critical values (TC and HC2), which sets an unambiguous link to the superconducting state. Furthermore, we find clear anticorrelation between the energy of the spin resonance and its harmonics and the local superconducting gap({\Delta}), which invokes a pairing of electronic origin associated with spin fluctuations. Our findings demonstrate the fundamental role that electronic correlations play in the development of superconductivity in 2D transition metal dichalcogenides, and open the tantalizing possibility to explore unconventional superconductivity in simple, scalable and transferable 2D superconductors.Comment: Manuscript and SI. Comment are welcom

Application of the Doehlert Design to Optimize the Signal Obtained in Photochemically Induced Fluorescence for the Determination of Eight Phenylureas

Abstract This work describes the optimization of a photochemically induced method for the detection of eight phenylureas has been developed by response surface methodology (RSM). These pesticides do not show native fluorescence but they were photolyzed into strongly fluorescent photoproducts under UV irradiation. The effect of the main variables affecting the yield of the photoderivatization reaction, and hence the fluorescence intensity, such as solvent, UV irradiation time and pH were optimized for each pesticide. A Doehlert design was applied in order to obtain maximum intensity fluorescence using response surface methodology. In general, a maximum was found for all pesticides using MeOH as organic solvent, except for diuron, whereas the effect of pH and irradiation time was different, according to each pesticide. Finally, the addition of β-cyclodextrin upon the photochemically induced fluorescence intensity was investigate. The fluorescence intensity was only improved for monolinuron at a concentration of 4×10 −3 M of β-cyclodextrin