Electron acceptor molecules deposited on epitaxial graphene studied by means of low temperature scanning tunneling microscopy/spectroscopy

Tesis doctoral inédita leída en la Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Física de la Materia Condensada. Fecha de lectura: 22-07-3013En esta tesis se presenta el estudio de dos moléculas aceptoras de electrones bien conocidas, TCNQ y F4-TCNQ, sobre grafeno epitaxial. El crecimiento y caracterización de sus propiedades electrónicas y magnéticas se realizó mediante Microscopía y Espectroscopia Túnel de Barrido (STM/STS) en Ultra Alto Vacío y a baja temperatura. El grafeno epitaxial se crece sobre Ru(0001) y Ir(111) mediante descomposición térmica de moléculas. A pesar de que en ambos casos la superficie de grafeno presenta un patrón de moiré, estos dos sistemas son complementarios en lo que se refiere a la interacción entre el grafeno y el metal. El balance entre las interacciones molécula-molécula y molécula-substrato es estudiada para ambas moléculas a diferentes recubrimientos mediante STM. El grafeno epitaxial nos permite la pasivación de la superficie altamente reactiva del metal permitiendo el estudio de moléculas individuales prácticamente inalteradas así como la formación de diferentes estructuras moleculares autoensambladas. La espectroscopia túnel de barrido a baja temperatura es utilizada para dar a conocer la estructura electrónica de estas capas moleculares. En el caso de las TCNQ sobre grafeno/Ir(111), las moléculas se mantienen prácticamente neutras debido a la débil interacción con el sustrato. Por otra parte, en el caso de las TCNQ adsorbidas en grafeno/Ru(0001), estos experimentos, en concordancia con cálculos DFT+D2, mostraron la ocupación parcial del LUMO de la molécula de TCNQ neutra. Este orbital se divide en dos orbitales con espines opuestos, SOMO y SUMO. El papel de la capa de grafeno es modular la hibridización entre el electrón desapareado transferido a la molécula y los electrones de conducción del Ru, dando lugar a un efecto Kondo dependiente de la posición de absorción. Esta dependencia es confirmada para recubrimientos mayores de TCNQ y para moléculas de F4-TCNQ en la misma superficie. Además, el efecto combinado de la interacción atractiva entre las moléculas de TCNQ y la ocupación parcial del LUMO da lugar a la formación de bandas intermoleculares extendidas espacialmente que permiten a las moléculas deslocalizar la carga adquirida. Estas bandas están divididas en espín, con sólo la banda de espín mayoritaria ocupada, como revelaron los cálculos DFT. La existencia del correspondiente orden magnético a largo alcance es detectado mediante experimentos de STM polarizados en espín a 4.6 K. Estos resultados son la primera evidencia experimental y teórica de la existencia de orden magnético a largoThis thesis presents a study of two well-known electron acceptor molecules, TCNQ and F4- TCNQ, deposited on epitaxial graphene. The growth and characterization of the electronic and magnetic properties of these systems have been performed by means of Low Temperature Scanning Tunneling Microscopy and Spectroscopy (STM/STS) in Ultra High Vacuum. A graphene layer is grown by chemical vapor deposition on Ru(0001) and Ir(111) surfaces. Although in both cases the graphene surface presents a moiré superstructure, these two systems are completely different from the graphene-metal interaction point of view. The relative strength between the molecule-molecule and molecule-substrate interactions is systematically studied for both molecules at different molecular coverages by means of STM. The epitaxial graphene passivates the highly reactive metallic surfaces allowing the study of almost unperturbed single molecules and the formation of self-assembly molecular structures. Low temperature scanning tunneling spectroscopy is used to unveil electronic structure of these molecular layers. In the case of TCNQ on graphene/Ir(111), the weak moleculesubstrate interaction keeps the molecules practically neutral. On the other hand, in the case of TCNQ deposited on graphene/Ru(0001), these experiments, in agreement with DFT+D2 calculations, reveal the partial occupation of the LUMO of the neutral TCNQ molecule. This orbital splits into a spin-up SOMO and a spin-down SUMO. We find that the graphene layer modulates the hybridization between the transferred unpaired electron and the Ru conduction electrons leading to a site dependent Kondo effect. This dependence is further confirmed for higher TCNQ coverages and for F4-TCNQ molecules on the same surface. In addition to that, the combined effect of the attractive interaction between TCNQ molecules and the partial occupation of the LUMO gives rise to the formation of spatially extended intermolecular bands, that allow the molecules to delocalize the charge acquired. The bands are spin split, with only the majority spin band being occupied as revealed in the DFT calculations. The existence of the corresponding long-range magnetic order is detected by spin polarized STM at 4.6 K. These findings are the first experimental and theoretical evidence for the existence of long-range magnetic order in delocalized bands in a purely organic molecular monolayer

Atomic-scale study of type-II Dirac semimetal PtTe2 surface

Dirac semimetals (DSM) host linear bulk bands and topologically protected surface states, giving rise to exotic and robust properties. Platinum ditelluride (PtTe2) belongs to this interesting group of topological materials. Here, we employ scanning tunneling microscopy (STM) in combination with first-principles calculations to visualize and identify the native defects at the surface of a freshly cleaved PtTe2 crystal. Around these defects, short-wavelength electron density oscillations are observed. Fourier transform analysis of the energy-dependent quasiparticle interference patterns is in good agreement with our calculated joint density of states, demonstrating the singular properties of the surface of this type-II DSM. Our results evidence the power of STM in understanding the surface of topological material

Familial hypercholesterolaemia in children and adolescents from 48 countries: a cross-sectional study

Background: Approximately 450 000 children are born with familial hypercholesterolaemia worldwide every year, yet only 2·1% of adults with familial hypercholesterolaemia were diagnosed before age 18 years via current diagnostic approaches, which are derived from observations in adults. We aimed to characterise children and adolescents with heterozygous familial hypercholesterolaemia (HeFH) and understand current approaches to the identification and management of familial hypercholesterolaemia to inform future public health strategies. Methods: For this cross-sectional study, we assessed children and adolescents younger than 18 years with a clinical or genetic diagnosis of HeFH at the time of entry into the Familial Hypercholesterolaemia Studies Collaboration (FHSC) registry between Oct 1, 2015, and Jan 31, 2021. Data in the registry were collected from 55 regional or national registries in 48 countries. Diagnoses relying on self-reported history of familial hypercholesterolaemia and suspected secondary hypercholesterolaemia were excluded from the registry; people with untreated LDL cholesterol (LDL-C) of at least 13·0 mmol/L were excluded from this study. Data were assessed overall and by WHO region, World Bank country income status, age, diagnostic criteria, and index-case status. The main outcome of this study was to assess current identification and management of children and adolescents with familial hypercholesterolaemia. Findings: Of 63 093 individuals in the FHSC registry, 11 848 (18·8%) were children or adolescents younger than 18 years with HeFH and were included in this study; 5756 (50·2%) of 11 476 included individuals were female and 5720 (49·8%) were male. Sex data were missing for 372 (3·1%) of 11 848 individuals. Median age at registry entry was 9·6 years (IQR 5·8-13·2). 10 099 (89·9%) of 11 235 included individuals had a final genetically confirmed diagnosis of familial hypercholesterolaemia and 1136 (10·1%) had a clinical diagnosis. Genetically confirmed diagnosis data or clinical diagnosis data were missing for 613 (5·2%) of 11 848 individuals. Genetic diagnosis was more common in children and adolescents from high-income countries (9427 [92·4%] of 10 202) than in children and adolescents from non-high-income countries (199 [48·0%] of 415). 3414 (31·6%) of 10 804 children or adolescents were index cases. Familial-hypercholesterolaemia-related physical signs, cardiovascular risk factors, and cardiovascular disease were uncommon, but were more common in non-high-income countries. 7557 (72·4%) of 10 428 included children or adolescents were not taking lipid-lowering medication (LLM) and had a median LDL-C of 5·00 mmol/L (IQR 4·05-6·08). Compared with genetic diagnosis, the use of unadapted clinical criteria intended for use in adults and reliant on more extreme phenotypes could result in 50-75% of children and adolescents with familial hypercholesterolaemia not being identified. Interpretation: Clinical characteristics observed in adults with familial hypercholesterolaemia are uncommon in children and adolescents with familial hypercholesterolaemia, hence detection in this age group relies on measurement of LDL-C and genetic confirmation. Where genetic testing is unavailable, increased availability and use of LDL-C measurements in the first few years of life could help reduce the current gap between prevalence and detection, enabling increased use of combination LLM to reach recommended LDL-C targets early in life