Characterization and decoupling of high-quality graphene grown by fullerene decomposition on Cu foilsby

Tesis Doctoral inédita leída en la Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Física de la Materia Condensada. Fecha de lectura: 25-06-201

Scalable High-Mobility Graphene/hBN Heterostructures

Graphene-hexagonal boron nitride (hBN) scalable heterostructures are pivotal for the development of graphene-based high-tech applications. In this work, we demonstrate the realization of high-quality graphene-hBN heterostructures entirely obtained with scalable approaches. hBN continuous films were grown via ion beam-assisted physical vapor deposition directly on commercially available $SiO_2/Si$ and used as receiving substrates for graphene single-crystal matrixes grown by chemical vapor deposition on copper. The structural, chemical, and electronic properties of the heterostructure were investigated by atomic force microscopy, Raman spectroscopy, and electrical transport measurements. We demonstrate graphene carrier mobilities exceeding $10,000 cm^2/Vs$ in ambient conditions, 30% higher than those directly measured on $SiO_{2}/Si$. We prove the scalability of our approach by measuring more than 100 transfer length method devices over a centimeter scale, which present an average carrier mobility of $7500 \pm 850 cm^{2}/Vs$. The reported high-quality all-scalable heterostructures are of relevance for the development of graphene-based high-performing electronic and optoelectronic applications

Band structure, superconductivity and polytypism in AuSn4_4

The orthorhombic compound AuSn4 is compositionally similar to the Dirac node arc semimetal PtSn$_4$. AuSn$_4$ is, contrary to PtSn$_4$, superconducting with a critical temperature of T$_c$ = 2.35 K. Recent measurements present indications for quasi two-dimensional superconducting behavior in AuSn$_4$. Here we present measurements of the superconducting density of states and the band structure of AuSn$_4$ through Scanning Tunneling Microscopy (STM) and Angular Resolved Photoemission Spectroscopy (ARPES). The superconducting gap values in different portions of the Fermi surface are spread around {\Delta}0 = 0.4 meV, which is close to but somewhat larger than $\Delta =$ 1.76kBT$_c$ expected from BCS theory. We observe superconducting features in the tunneling conductance at the surface up to temperatures about 20% larger than bulk Tc. The band structure calculated with Density Functional Theory (DFT) follows well the results of ARPES. The crystal structure presents two possible stackings of Sn layers, giving two nearly degenerate polytypes. This makes AuSn$_4$ a rather unique case with a three dimensional electronic band structure but properties ressembling those of low dimensional layered compounds

Observation of a gel of quantum vortices in a superconductor at very low magnetic fields

A gel consists of a network of particles or molecules formed for example using the sol-gel process, by which a solution transforms into a porous solid. Particles or molecules in a gel are mainly organized on a scaffold that makes up a porous system. Quantized vortices in type-II superconductors mostly form spatially homogeneous ordered or amorphous solids. Here we present high-resolution imaging of the vortex lattice displaying dense vortex clusters separated by sparse or entirely vortex-free regions in β-Bi2Pd superconductor. We find that the intervortex distance diverges upon decreasing the magnetic field and that vortex lattice images follow a multifractal behavior. These properties, characteristic of gels, establish the presence of a novel vortex distribution, distinctly different from the well-studied disordered and glassy phases observed in high-temperature and conventional superconductors. The observed behavior is caused by a scaffold of one-dimensional structural defects with enhanced stress close to the defects. The vortex gel might often occur in type-II superconductors at low magnetic fields. Such vortex distributions should allow to considerably simplify control over vortex positions and manipulation of quantum vortex states.Fil: Llorens, José Benito. Universidad Autónoma de Madrid; EspañaFil: Embon, Lior. Weizmann Institute Of Science.; IsraelFil: Correa, Alexandre. Consejo Superior de Investigaciones Científicas; España. Instituto de Ciencia de Materiales de Madrid; EspañaFil: González, Jesús David. Universidad del Magdalena; Colombia. Universiteit Antwerp; BélgicaFil: Herrera, Edwin. Universidad Autónoma de Madrid; España. Universidad Central; ColombiaFil: Guillamón, Isabel. Universidad Autónoma de Madrid; EspañaFil: Luccas, Roberto F.. Consejo Superior de Investigaciones Científicas; EspañaFil: Azpeitia, Jon. Consejo Superior de Investigaciones Científicas; EspañaFil: Mompeán, Federico J.. Consejo Superior de Investigaciones Científicas; EspañaFil: García Hernández, Mar. Consejo Superior de Investigaciones Científicas; EspañaFil: Munuera, Carmen. Consejo Superior de Investigaciones Científicas; EspañaFil: Aragón Sánchez, Jazmín. Comisión Nacional de Energía Atómica. Gerencia del Area de Investigación y Aplicaciones No Nucleares. Gerencia de Física (Centro Atómico Bariloche). División Bajas Temperaturas; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología - Nodo Bariloche | Comisión Nacional de Energía Atómica. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología - Nodo Bariloche; ArgentinaFil: Fasano, Yanina. Comisión Nacional de Energía Atómica. Gerencia del Area de Investigación y Aplicaciones No Nucleares. Gerencia de Física (Centro Atómico Bariloche). División Bajas Temperaturas; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología - Nodo Bariloche | Comisión Nacional de Energía Atómica. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología - Nodo Bariloche; ArgentinaFil: Milosevic, Milorad V.. Universiteit Antwerp; BélgicaFil: Suderow, Hermann. Universidad Autónoma de Madrid; EspañaFil: Anahory, Yonathan. The Hebrew University of Jerusalem; Israe

Charge density wave in layered La1-xCexSb2

The layered rare-earth diantimonides RSb2 are anisotropic metals with generally low electronic densities whose properties can be modified by substituting the rare earth. LaSb2 is a nonmagnetic metal with a low residual resistivity presenting a low-temperature magnetoresistance that does not saturate with the magnetic field. It has been proposed that the latter can be associated to a charge density wave (CDW), but no CDW has yet been found. Here we find a kink in the resistivity above room temperature in LaSb2 (at 355 K) and show that the kink becomes much more pronounced with substitution of La by Ce along the La1-xCexSb2 series. We find signatures of a CDW in x-ray scattering, specific heat, and scanning tunneling microscopy (STM) experiments in particular for x≈0.5. We observe a distortion of rare-earth-Sb bonds lying in-plane of the tetragonal crystal using x-ray scattering, an anomaly in the specific heat at the same temperature as the kink in resistivity and charge modulations in STM. We conclude that LaSb2 has a CDW which is stabilized in the La1-xCexSb2 series due to substitutional disorder.E.H. acknowledges the support of Departamento Administrativo de Ciencia, Tecnología e Innovación, COL-CIENCIAS (Colombia) Programa Doctorados en el Exterior Convocatoria 568-2012. This work was supported by the Spanish MINECO (FIS2014-54498-R, MAT2011-27470-C02-02, and CSD-2009-00013), by the European Union (Graphene Flagship Contract No. CNECT-ICT-604391 and COST MP1201 action), and by the Comunidad de Madrid through programs Nanofrontmag-CM (S2013/MIT-2850) and MAD2D-CM (S2013/MIT-3007). We acknowledge MINECO and CSIC for financial support and for provision of synchrotron radiation facilities and would like to thank the SpLine BM25 staff for assistance in using the beamline

Characterization and decoupling of high-quality graphene grown by fullerene decomposition on Cu foils

Memoria presentada para optar al grado de Doctor en Ciencias Físicas.-- Tutor: Hermann Suderow (Universidad Autónoma de Madrid).El objetivo de la presente tesis se basa en la optimización del crecimiento de grafeno por métodos novedosos con precursores en estado sólido (C60) y en una exhaustiva caracterización de dicho grafeno tanto morfológica como estructural y química. Para buscar aplicaciones tecnológicas del grafeno en cobre, se han llevado a cabo experimentos para la optimización del proceso de transferencia del grafeno a sustratos aislantes, tales como el SiO2, por métodos químicos. Para dicha optimización se han considerado dos aproximaciones: por un lado, la intercalación en la interfase grafeno/cobre de moléculas por métodos electroquímicos. Por otro lado, se han empleado técnicas de ultra alto vacío para la intercalación de elementos como el oxígeno en dicha interfase también con el propósito de debilitar interacciones que afectan directamente a su transferencia. Finalmente, se ha equipado el laboratorio con un imán vectorial superconductor con capacidad de aplicar campo magnético en todas las direcciones del espacio, ensamblado a un equipo criogénico y a un microscopio de fuerzas magnéticas (MFM) con capacidad de medir de manera local las propiedades magnéticas superficiales.Programa de FPI del MINECO, referencia BES-2012- 058600.S

Low temperature metal free growth of graphene on insulating substrates by plasma assisted chemical vapor deposition

Direct growth of graphene films on dielectric substrates (quartz and silica) is reported, by means of remote electron cyclotron resonance plasma assisted chemical vapor deposition r-(ECR-CVD) at low temperature (650 °C). Using a two step deposition process– nucleation and growth– by changing the partial pressure of the gas precursors at constant temperature, mostly monolayer continuous films, with grain sizes up to 500 nm are grown, exhibiting transmittance larger than 92% and sheet resistance as low as 900 Ω sq−1. The grain size and nucleation density of the resulting graphene sheets can be controlled varying the deposition time and pressure. In additon, first-principles DFT-based calculations have been carried out in order to rationalize the oxygen reduction in the quartz surface experimentally observed. This method is easily scalable and avoids damaging and expensive transfer steps of graphene films, improving compatibility with current fabrication technologies.This work has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 696656. JA acknowledges funding from FPI Program of MINECO (BES-2012-058600). JIM acknowledges funding from the ERC-Synergy Program (Grant ERC-2013-SYG-610256 NANOCOSMOS) and computing resources from CTI-CSIC. CM acknowledges the financial support by the 'Ramón y Cajal' Program of MINECO (RYC-2014-16626).Peer reviewe

Impact of Silver on the Structural and Wettability Properties of ZnO Films Grown by Oblique Angle Magnetron Sputtering

Un-doped (uZO) and silver-doped zinc oxide (SZO) films were prepared by oblique incidence sputtering deposition under different process parameters. The crystalline structure, chemical composition, and surface morphology were correlated with the optical properties, as well as with the wettability of the films. In the case of uZO films, the orientation, inclination, and morphology of the columnar structure determined the wettability of the layer, moving from a hydrophilic- to hydrophobic-like character. In the case of SZO films, although almost all of them displayed hydrophobic behavior, the hydrophobic character increased with the Ag content. The most hydrophobic surface was obtained when the Ag content in the layers was greater than 7 at.% and, in these cases, the structural results indicate that the layers were formed by a disordered mixture of Zn and Ag oxides.This research was funded by grant TED2021-129876B-I00 of the Ministerio de Ciencia, Innovación y Universidades (Spain).Peer reviewe

Electron-stimulated desorption kinetics of ultra-thin LiCl films on graphene

[EN] Ultra-thin alkali halide films are an important substrate for studying single quantum objects at the atomic scale, as they serve as decoupling scaffolds. However, their stability upon exposure to electrons and photons, the most common probes used to study nanomaterials, is not fully understood. Here we present a study of the evolution of the structure of ultra-thin LiCl films, grown on graphene, upon low-energy electron irradiation by means of microspot-low-energy electron diffraction and microscopy. We find that the intensity of the LiCl diffraction spots irradiated at various electron energies follows a bi-exponential decay function, which can be rationalized by two different desorption regimes. In addition, we detect a change in the work function caused by the electron irradiation, confirming desorption of the LiCl film from the graphene layer. We understand the underlying mechanisms for the electron-induced desorption of the salt film in terms of the evolution of the elementary quasiparticles involved in the process: holes, excitons and F and H center pairs. Moreover, a direct comparison of the electron-induced and the soft X-ray photon-induced processes reveal that, in addition to LiCl desorption, the intercalation of lithium into graphene reported for X-ray induced desorption does not take place in electron-stimulated desorption.This work was supported by the EU Graphene Flagship funding(Grant Graphene Core3 881603). The authors acknowledge financial support from projects RYC2020-029800-I, PID2021-125309OA-I00,PID2020-113142RB-C21, CNS2022-135658 TED2021-129999B-C31 and TED2021-129416A-I00 funded by MCIN/AEI/10.13039/ 501100011033 and EUR2021-122006.Peer reviewe