Built-up AFM tips by metal nanoclusters engineering

The ability to probe tip-sample interactions by Atomic Force Microscopy (AFM) has recently boosted our understanding of the matter at the atomic scale, enabling the study of properties of surfaces and adsorbates which were previously inaccessible. Nevertheless, this sensitivity to forces presents some drawbacks, as the requirement of a sharp tip apex to prevent the loss of spatial resolution due to the existence of long-range interactions. In this work, we have overcome this long-standing challenge by investigating the controlled extraction of single metallic nanoclusters, selectively grown on graphene. Our results show that the successive extraction of cluster allows to grow nanotips, which minimize the long-range tip-sample interactions and greatly enhance the topographic resolution. We have demonstrated that the created nanotips are very stable, which enables exchanging the sample and using the same nanotip to explore different surfaces without loss of resolution. Since metallic clusters of very different materials and sizes can be grown and selectively extracted by AFM, ours work paves also the way to the specific functionalization of AFM-tips to sense a large variety of interactionsFinancial support from the Spanish Ministerio de Economía y Competitividad (MINECO) and Fondo Europeo de Desarrollo Regional (FEDER) under grants No. MAT2016-77852-C2-2-R and MAT2016-80907-P and by the Comunidad de Madrid NMAT2D-CM program under grant S2018/NMT-4511 is gratefully acknowledged. Financial support from the Spanish Ministerio de Ciencia e Innovacion under grant Nº PID2019-106268GB-C31 is also gratefully acknowledged. We thank Rubén Pérez and Oscar Custance for helpful discussions and Antonio J. Martínez-Galera for helpful discussions and technical assistanc

Tip and Surface Determination from Experiments and Simulations of Scanning Tunneling Microscopy and Spectroscopy

We present a very efficient and accurate method to simulate scanning tunneling microscopy images and spectra from first-principles density functional calculations. The wave-functions of the tip and sample are calculated separately on the same footing, and propagated far from the surface using the vacuum Green's function. This allows to express the Bardeen matrix elements in terms of convolutions, and to obtain the tunneling current at all tip positions and bias voltages in a single calculation. The efficiency of the method opens the door to real time determination of both tip and surface composition and structure, by comparing experiments to simulated images for a variety of precomputed tips. Comparison with the experimental topography and spectra of the Si(111)-(7x7) surface show a much better agreement with Si than with W tips, implying that the metallic tip is terminated by silicon.Comment: 4 pages, 4 figure

Electronic and structural characterization of divacancies in irradiated graphene

We provide a thorough study of a carbon divacancy, a fundamental but almost unexplored point defect in graphene. Low temperature scanning tunneling microscopy (STM) imaging of irradiated graphene on different substrates enabled us to identify a common two-fold symmetry point defect. Our first principles calculations reveal that the structure of this type of defect accommodates two adjacent missing atoms in a rearranged atomic network formed by two pentagons and one octagon, with no dangling bonds. Scanning tunneling spectroscopy (STS) measurements on divacancies generated in nearly ideal graphene show an electronic spectrum dominated by an empty-states resonance, which is ascribed to a spin-degenerated nearly flat band of $\pi$-electron nature. While the calculated electronic structure rules out the formation of a magnetic moment around the divacancy, the generation of an electronic resonance near the Fermi level, reveals divacancies as key point defects for tuning electron transport properties in graphene systems.Comment: 5 page

Shaping graphene superconductivity with nanometer precision

Graphene holds great potential for superconductivity due to its pure 2D nature, the ability to tune its carrier density through electrostatic gating, and its unique, relativistic-like electronic properties. At present, still far from controlling and understanding graphene superconductivity, mainly because the selective introduction of superconducting properties to graphene is experimentally very challenging. Here, a method is developed that enables shaping at will graphene superconductivity through a precise control of graphene-superconductor junctions. The method combines the proximity effect with scanning tunnelling microscope (STM) manipulation capabilities. Pb nano-islands are first grown that locally induce superconductivity in graphene. Using a STM, Pb nano-islands can be selectively displaced, over different types of graphene surfaces, with nanometre scale precision, in any direction, over distances of hundreds of nanometres. This opens an exciting playground where a large number of predefined graphene-superconductor hybrid structures can be investigated with atomic scale precision. To illustrate the potential, a series of experiments are performed, rationalized by the quasi-classical theory of superconductivity, going from the fundamental understanding of superconductor-graphene-superconductor heterostructures to the construction of superconductor nanocorrals, further used as “portable” experimental probes of local magnetic moments in grapheneThe authors acknowledge funding from the Spanish Ministry of Science and Innovation MCIN/AEI/10.13039/297 501100011033 though grants # PID2020-115171GB-I00, PID2020-114880GB-I00, PID2019-107338RB-C61 and the “María de Maeztu” Programme for Units of Excellence in R&D (CEX2018-000805-M, CEX2020-001038-M), the Comunidad de Madrid NMAT2D-CM program under grant S2018/NMT-4511, the Comunidad de Madrid, the Spanish State and the European Union by the Recovery, Transformation and Resilience Plan “Materiales Disruptivos Bidimensionales (2D)” (MAD2D-CM)-UAM3 and the European Union through the Next Generation EU funds and the Horizon 2020 FET-Open project SPRING (No. 863098). J. C. C. thanks the German Science Foundation DFG and SFB 1432 for sponsoring his stay at the University of Konstanz as a Mercator Fello

Quantum Confinement of Dirac Quasiparticles in Graphene Patterned with Sub‐Nanometer Precision

Quantum confinement of graphene Dirac‐like electrons in artificially crafted nanometer structures is a long sought goal that would provide a strategy to selectively tune the electronic properties of graphene, including bandgap opening or quantization of energy levels. However, creating confining structures with nanometer precision in shape, size, and location remains an experimental challenge, both for top‐down and bottom‐up approaches. Moreover, Klein tunneling, offering an escape route to graphene electrons, limits the efficiency of electrostatic confinement. Here, a scanning tunneling microscope (STM) is used to create graphene nanopatterns, with sub‐nanometer precision, by the collective manipulation of a large number of H atoms. Individual graphene nanostructures are built at selected locations, with predetermined orientations and shapes, and with dimensions going all the way from 2 nm up to 1 µm. The method permits the patterns to be erased and rebuilt at will, and it can be implemented on different graphene substrates. STM experiments demonstrate that such graphene nanostructures confine very efficiently graphene Dirac quasiparticles, both in 0D and 1D structures. In graphene quantum dots, perfectly defined energy bandgaps up to 0.8 eV are found that scale as the inverse of the dot’s linear dimension, as expected for massless Dirac fermions.This work was supported by AEI and FEDER under projects MAT2016-80907-P and MAT2016-77852-C2-2-R (AEI/FEDER, UE) by the Fundación Ramón Areces, the Comunidad de Madrid NMAT2D-CM program under grant S2018/NMT-4511, and the Spanish Ministry of Science and Innovation, through the “María de Maeztu” Programme for Units of Excellence in R&D (CEX2018-000805-M). European Union through the FLAG-ERA program HiMagGraphene project PCIN-2015-030; No. ANR-15-GRFL-0004) and the Graphene Flagship program (Grant agreement 604391). J.L.L acknowledges financial support from the ETH Fellowship program; J.F.-R. acknowledges supported by Fundação para a Ciência e a Tecnologia grants P2020-PTDC/FIS-NAN/3668/2014 and TAPEXPL/NTec/0046/2017

Sistemas metal-semiconductor estudiados mediante microscopía de efecto túnel de temperatura variable : propiedades electrónicas, transiciones de fase y difusión superficial

Tesis doctoral inédita leída en la Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Física de la Materia Condensada. Fecha de lectura: 19-12-200

Graphene nanopatterning with 2.5 nm precision: combining bottom-up and top-down techniques

Presentation given at the APS March Meeting, held in San Antonio (Texas, United States) on March 2-6, 2015. Session W1: Focus Sesson: Graphene: Nanostructures.The selective modification of pristine graphene represents an essential step to fully exploit its potential. Here we merge bottom-up and top-down strategies to tailor graphene with nanometer accuracy. In a first step, graphene electronic properties are macroscopically modified using the periodic potential generated by the self assembly of metal cluster superlattices on a graphene/Ir(111) surface. Then, we show that individual metal clusters can be selectively removed at room temperature by a STM tip with perfect reproducibility, which enables one to nanopattern the system down to the 2.5 nm limit given by the distance between neighbouring clusters, i.e., the periodicity of the moire-pattern. The method can be carried out on micrometer sized regions, with clusters of different materials -which allows tuning the strength of the periodic potential- and the structures so created are stable even at room temperature. As a result, we can strategically combine graphene regions that should present large differences in their electronic structure to design graphene nanostructures with specific functionalities

Observation of Yu–Shiba–Rusinov States in Superconducting Graphene

When magnetic atoms are inserted inside a superconductor, the superconducting order is locally depleted as a result of the antagonistic nature of magnetism and superconductivity. Thereby, distinctive spectral features, known as Yu–Shiba–Rusinov states, appear inside the superconducting gap. The search for Yu–Shiba–Rusinov states in different materials is intense, as they can be used as building blocks to promote Majorana modes suitable for topological quantum computing. Here, the first observation of Yu–Shiba–Rusinov states in graphene, a non-superconducting 2D material, and without the participation of magnetic atoms, is reported. Superconductivity in graphene is induced by proximity effect brought by adsorbing nanometer-scale superconducting Pb islands. Using scanning tunneling microscopy and spectroscopy the superconducting proximity gap is measured in graphene, and Yu–Shiba–Rusinov states are visualized in graphene grain boundaries. The results reveal the very special nature of those Yu–Shiba–Rusinov states, which extends more than 20 nm away from the grain boundaries. These observations provide the long-sought experimental confirmation that graphene grain boundaries host local magnetic moments and constitute the first observation of Yu–Shiba–Rusinov states in a chemically pure system.This work was supported by AEI and FEDER under projects MAT2016-80907-P and MAT2016-77852-C2-2-R (AEI/FEDER, UE), by the Fundación Ramón Areces, and by the Comunidad de Madrid NMAT2D-CM program under grant S2018/NMT-4511. J.F.R. acknowledges financial support European Regional Development Fund Project No. NORTE-01-50145- FEDER-000019, and the UTAPEXPL/NTec/0046/2017 projects, as well as Generalitat Valenciana funding Prometeo2017/139 and MINECO Spain (Grant No. MAT2016-78625-C2). J.L.L is grateful for financial support from the Academy of Finland Projects Nos. 331342 and 336243

Shaping graphene superconductivity with nanometer precision

International audienceGraphene holds great potential for superconductivity due to its pure two-dimensional nature, the ability to tune its carrier density through electrostatic gating, and its unique, relativistic-like electronic properties. At present, we are still far from controlling and understanding graphene superconductivity, mainly because the selective introduction of superconducting properties to graphene is experimentally very challenging. Here, we have developed a method that enables shaping at will graphene superconductivity through a precise control of graphene-superconductor junctions. The method combines the proximity effect with scanning tunnelling microscope (STM) manipulation capabilities. We first grow Pb nano-islands that locally induce superconductivity in graphene. Using a STM, Pb nano-islands can be selectively displaced, over different types of graphene surfaces, with nanometre scale precision, in any direction, over distances of hundreds of nanometres. This opens an exciting playground where a large number of predefined graphene-superconductor hybrid structures can be investigated with atomic scale precision. To illustrate the potential, we perform a series of experiments, rationalized by the quasi-classical theory of superconductivity, going from the fundamental understanding of superconductor-graphene-superconductor heterostructures to the construction of superconductor nanocorrals, further used as "portable" experimental probes of local magnetic moments in graphene

Nanociencia: manipulación a escala atómica y molecular

El invento de la microscopía de proximidad ha permitido la manipulación individual de átomos y moléculas con el objetivo de conformar sistemas funcionales nanométricos que constituyen el núcleo de la Nanociencia. En este artículo presentamos ejemplos de manipulación atómica, partiendo del artículo pionero de Eigler para el Xe sobre Ni y continuando con experimentos recientes en superficies semiconductoras desarrollados por investigadores procedentes del Laboratorio de Nuevas Microscopías. Finalmente se muestra cómo la creación de unos pocos defectos mediante irradiación con Ar+ permite ajustar la conductancia de nanotubos de carbono metálicos