Intercalation of sulfur in epitaxial graphene on ruthenium(0001) studied by means of scanning tunneling microsocopy and spectroscopy

Tesis Doctoral inédita leída en la Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Física de la Materia Condensada. Fecha de lectura: 26 de febrero de 2016En esta tesis se estudia la interacción del sulfuro de hidrógeno (H2S) con el grafeno crecido epitaxialmente sobre el plano basal del rutenio (Ru(0001)). El crecimiento de las muestras y su caracterización se realizó mediante microscopía y espectroscopia de efecto túnel (STM/STS) en ultra alto vacío (UHV). Parte de la caracterización se realizó a baja temperatura y se utilizaron de manera complementaria otras técnicas de caracterización en UHV. El grafeno epitaxial sobre Ru(0001) (grafeno/Ru(0001)) se crece mediante descomposición catalítica de etileno en condiciones de UHV. El grafeno/Ru(0001) muestra una interacción intensa entre el grafeno y el sustrato que resulta en una modulación topográfica y electrónica en forma de patrón de moiré debido a la pequeña diferencia en el parámetro de red de ambos. El grafeno así crecido está fuertemente anclado al Ru(00001) y altamente dopado (tipo-n), perdiendo su carácter semimetálico y su característica relación de dispersión lineal. Las imágenes de STM demuestran que la exposición de grafeno/Ru(0001) a H2S resulta en la intercalación de azufre entre el grafeno y el sustrato. Los experimentos sugieren que se produce mediante la formación de defectos puntuales en la superficie del grafeno. Los átomos de azufre intercalados se estructuran formando diferentes reconstrucciones que son descritas y analizadas. Para complementar la descripción geométrica de las estructuras que forma el azufre intercalado, se han realizado experimentos de adsorción de azufre en Ru(0001) sin la capa de grafeno epitaxial. Las diferentes reconstrucciones de la capa de azufre intercalado en el sistema estudiado alteran las propiedades del grafeno que hay sobre ella. La densidad superficial de átomos de azufre y sus características geométricas reducen la interacción del grafeno con el Ru(0001) en diferente medida, como evidencia la reducción en la corrugación del patrón de moiré en los experimentos de STM. Para analizar en más detalle la influencia de las diferentes configuraciones del azufre sobre las propiedades del sistema se realizaron experimentos de STS a baja temperatura que muestran la aparición de una serie de resonancias equiespaciadas en torno al nivel de Fermi tras la exposición a H2S. El desplazamiento de estos picos a mayores energías con el aumento de la densidad de azufre sugiere la reducción del dopaje del grafeno intercalado con respecto al grafeno/Ru(0001). Su origen no ha podido ser establecido, pero se proponen posibles explicaciones para su aparición. Por último, se llevaron a cabo experimentos de manipulación de la superficie de grafeno/S/Ru(0001) utilizando la punta del STM. Cuando la densidad de la capa intercalada permite la movilidad del azufre se ha conseguido crear patrones geométricos desplazando los átomos intercalados de manera que se recupera la interacción entre el grafeno y el Ru(0001) en zonas concretas. Las estructuras así creadas tienen tamaños nanométricos y son estables a temperatura ambiente.This thesis studies the interaction of hydrogen sulfide (H2S) with graphene epitaxially grown on the basal plane of ruthenium (Ru(0001)). Sample growth and characterization is carried out by means of scanning tunneling microscopy and spectroscopy (STM/STS) in ultra-high vacuum (UHV) conditions. Part of the characterization was done at low temperature, and complementary UHV characterization techniques were also used. Epitaxial graphene on Ru(0001) (graphene/Ru(0001)) is grown by catalytic decomposition of ethylene in UHV conditions. Graphene/Ru(0001) shows a strong interaction between graphene and its substrate that results in a topographic and electronic modulation emerging as a moiré pattern due to the lattice mismatch between both materials. The resulting graphene layer is strongly bound to Ru(0001) and highly doped (n-type), losing its semi-metallic character and its characteristic linear dispersion relation. STM images show that exposing graphene/Ru(0001) to H2S results in the intercalation of sulfur between graphene and its substrate. The experiments suggest that intercalation occurs via the formation of local defects in the graphene’s surface. The intercalated sulfur atoms are structured forming different reconstructions which are described and analyzed. In order to complement the geometrical description of the intercalated sulfur layer, experiments on the adsorption of sulfur on Ru(0001) have been carried out. The different reconstructions of the intercalated system alter the properties of the graphene layer on top. The surface density of sulfur atoms and their geometrical characteristics reduce the interaction between graphene and Ru(0001) in different ways, as it is evident from the corrugation reduction of the moiré in STM experiments. In order to get a deeper insight on the influence of the geometrical configurations of sulfur on the system’s properties we performed low-temperature STS experiments. They show the emergence of a series of evenly spaced resonances close to the Fermi level after H2S exposure. The shift of these resonances towards higher energies with increasing sulfur density suggest the reduction of the doping of the intercalated graphene with respect to graphene/Ru(0001). Their origin is not clear, but some possible explanations are suggested. Lastly, manipulation experiments were carried out using the STM tip. When the density of the intercalated layer is low enough to allow for the sulfur mobility, geometric patterns have been drawn on the surface by moving the intercalated atoms in such a way that the strong graphene-Ru interaction is recovered at specific places. The so-created structures have lateral sizes in the nanometer range and are stable at room temperature

Hands-on quantum sensing with NV- centers in diamonds

The physical properties of diamond crystals, such as color or electrical conductivity, can be controlled via impurities. In particular, when doped with nitrogen, optically active nitrogen-vacancy centers (NV), can be induced. The center is an outstanding quantum spin system that enables, under ambient conditions, optical initialization, readout, and coherent microwave control with applications in sensing and quantum information. Under optical and radio frequency excitation, the Zeeman splitting of the degenerate states allows the quantitative measurement of external magnetic fields with high sensitivity. This study provides a pedagogical introduction to the properties of the NV centers as well as a step-by-step process to develop and test a simple magnetic quantum sensor based on color centers with significant potential for the development of highly compact multisensor systemsThis research was funded by MICIN-AEI: Grants DETECTAc and EQC2018-005134-P Comunidad de Madrid: Grant TEC2SPACE-CM P2018/NMT-4291, ONR-G: G#N62909-19-1-2053 (DEFROST), MADE-MICINN: PID2019-105552RB-C44. Garantía Juvenil nº201701520868, R.B.-G. would like to thank Comunidad de Madrid for the funding through the grant 2019-T2/IND-1336

Resonant-Tunnelling Diodes as PUF building blocks

Resonant-Tunnelling Diodes (RTDs) have been proposed as building blocks for Physical Unclonable Functions (PUFs). In this paper we show how the unique RTD current-voltage (I-V) spectrum can be translated into a robust digital representation. We analyse 130 devices and show that RTDs are a viable PUF building block

Optical identification using imperfections in 2D materials

The ability to uniquely identify an object or device is important for authentication [1]. Imperfections, locked into structures during fabrication, can be used to provide a fingerprint that is challenging to reproduce. In this paper, we propose a simple optical technique to read unique information from nanometer-scale defects in 2D materials. Imperfections created during crystal growth or fabrication lead to spatial variations in the bandgap of 2D materials that can be characterized through photoluminescence measurements. We show a simple setup involving an angle- adjustable transmission filter, simple optics and a CCD camera can capture spatially- dependent photoluminescence to produce complex maps of unique information from 2D monolayers. Atomic force microscopy is used to verify the origin of the optical signature measured, demonstrating that it results from nanometer-scale imperfections. This solution to optical identification with 2D materials could be employed as a robust security measure to prevent counterfeiting

Organic covalent patterning of nanostructured graphene with selectivity at the atomic level

Organic covalent functionalization of graphene with long-range periodicity is highly desirable-it is anticipated to provide control over its electronic, optical, or magnetic properties-and remarkably challenging. In this work we describe a method for the covalent modification of graphene with strict spatial periodicity at the nanometer scale. The periodic landscape is provided by a single monolayer of graphene grown on Ru(0001) that presents a moiré pattern due to the mismatch between the carbon and ruthenium hexagonal lattices. The moiré contains periodically arranged areas where the graphene-ruthenium interaction is enhanced and shows higher chemical reactivity. This phenomenon is demonstrated by the attachment of cyanomethyl radicals (CH2CN•) produced by homolytic breaking of acetonitrile (CH3CN), which is shown to present a nearly complete selectivity (>98%) binding covalently to graphene on specific atomic sites. This method can be extended to other organic nitriles, paving the way for the attachment of functional molecules

Inorganically coated colloidal quantum dots in polar solvents using a microemulsion-assisted method

The dielectric nature of organic ligands capping semiconductor colloidal nanocrystals (NCs) makes them incompatible with optoelectronic applications. For this reason, these ligands are regularly substituted through ligand-exchange processes by shorter (even atomic) or inorganic ones. In this work, an alternative path is proposed to obtain inorganically coated NCs. Differently to regular ligand exchange processes, the method reported here produces core–shell NCs and the removal of the original organic shell in a single step. This procedure leads to the formation of connected NCs resembling 1D worm-like networks with improved optical properties and polar solubility, in comparison with the initial CdSe NCs. The nature of the inorganic shell has been elucidated by X-ray Absorption Near Edge Structure (XANES), Extended X-ray Absorption Fine Structure (EXAFS) and X-ray Photoelectron Spectroscopy (XPS). The 1D morphology along with the lack of long insulating organic ligands and the higher solubility in polar media turns these structures very attractive for their further integration into optoelectronic devices.Facultad de Ciencias ExactasInstituto de Investigaciones Fisicoquímicas Teóricas y Aplicada

N-state random switching based on quantum tunnelling

In this work, we show how the hysteretic behaviour of resonant tunnelling diodes (RTDs) can be exploited for new functionalities. In particular, the RTDs exhibit a stochastic 2-state switching mechanism that could be useful for random number generation and cryptographic applications. This behaviour can be scaled to N-bit switching, by connecting various RTDs in series. The InGaAs/AlAs RTDs used in our experiments display very sharp negative differential resistance (NDR) peaks at room temperature which show hysteresis cycles that, rather than having a fixed switching threshold, show a probability distribution about a central value. We propose to use this intrinsic uncertainty emerging from the quantum nature of the RTDs as a source of randomness. We show that a combination of two RTDs in series results in devices with three-state outputs and discuss the possibility of scaling to N-state devices by subsequent series connections of RTDs, which we demonstrate for the up to the 4-state case. In this work, we suggest using that the intrinsic uncertainty in the conduction paths of resonant tunnelling diodes can behave as a source of randomness that can be integrated into current electronics to produce on-chip true random number generators. The N-shaped I-V characteristic of RTDs results in a two-level random voltage output when driven with current pulse trains. Electrical characterisation and randomness testing of the devices was conducted in order to determine the validity of the true randomness assumption. Based on the results obtained for the single RTD case, we suggest the possibility of using multi-well devices to generate N-state random switching devices for their use in random number generation or multi-valued logic devices.</p