Survival of spin state in magnetic porphyrins contacted by graphene nanoribbons

We report on the construction and magnetic characterization of a fully functional hybrid molecular system composed of a single magnetic porphyrin molecule bonded to graphene nanoribbons with atomically precise contacts. We use on-surface synthesis to direct the hybrid creation by combining two molecular precursors on a gold surface. High-resolution imaging with a scanning tunneling microscope finds that the porphyrin core fuses into the graphene nanoribbons through the formation of new carbon rings at chemically predefined positions. These ensure the stability of the hybrid and the extension of the conjugated character of the ribbon into the molecule. By means of inelastic tunneling spectroscopy, we prove the survival of the magnetic functionality of the contacted porphyrin. The molecular spin appears unaffected by the graphenoid electrodes, and we simply observe that the magnetic anisotropy appears modified depending on the precise structure of the contacts.We acknowledge the financial support from Spanish Agencia Estatal de Investigación (AEI) (project nos. MAT2016-78293-C6 and FIS2015-62538-ERC, and the Maria de Maeztu Units of Excellence Programme MDM-2016-0618), the Basque Government (Department Industry, grant no. PI-2015-1-42), the European project PAMS (610446), the Xunta de Galicia (Centro singular de investigación de Galicia accreditation 2016 to 2019, ED431G/09), the European Research Council (grant agreement no. 635919), and the European Regional Development FundS

Band Depopulation of Graphene Nanoribbons Induced by Chemical Gating with Amino Groups

Altres ajuts: Xunta de Galicia (ED431G/09); Gobierno Vasco (IT1246-19, IT-1255-19); Diputación Foral de Gipuzkoa (RED 2019-096); CERCA Program/Generalitat de Catalunya; Program Interreg V-A España-Francia-Andorra (194/16 TNI)The electronic properties of graphene nanoribbons (GNRs) can be precisely tuned by chemical doping. Here we demonstrate that amino (NH) functional groups attached at the edges of chiral GNRs (chGNRs) can efficiently gate the chGNRs and lead to the valence band (VB) depopulation on a metallic surface. The NH-doped chGNRs are grown by on-surface synthesis on Au(111) using functionalized bianthracene precursors. Scanning tunneling spectroscopy resolves that the NH groups significantly upshift the bands of chGNRs, causing the Fermi level crossing of the VB onset of chGNRs. Through density functional theory simulations we confirm that the hole-doping behavior is due to an upward shift of the bands induced by the edge NH groups

Electronic consequences of chemical doping of 7-Armchair Graphene Nanoribbons

Resumen del trabajo presentado a la International Conference on Nanoscience + Technology (ICN+T), celebrada en Brno (Czech Republic) del 22 al 27 de julio de 2018.The tunable electronic structure of Graphene Nanoribbons (GNRs) with different edge types has provoked great interest due to potential applications in electronic devices as molecular diodes or transistors. Thanks to the on-surface synthesis of chemically customized molecular precursors, nanoribbons with atomically defined structure can be grown. This high precision in their bottom-up growth allows to tune their electronic structure via width control or chemical doping. Here we use two different strategies to chemically modify 7-armchair GNRs (7-AGNRs) to clarify how the chemical modifications on the nanoribbons’ structure affect their electronic properties. By means of Scanning Tunneling Spectroscopy we tackle with atomic precision this issue on 7-AGNRs with substitutional nitrile functional groups at the ribbons’ edges and on 7-AGNRs with substitutional boron atoms within the ribbons’ backbone. We find that in the first case the CN groups lead to an efficient n-like doping of the ribbon, while in the second case B atoms induce the formation of a new acceptor band and bandgap renormalization.Peer Reviewe

Hierarchy in the halogen activation during surface-promoted ullmann coupling

Within the collection of surface-supported reactions currently accessible for the production of extended molecular nanostructures under ultra-high vacuum, Ullmann coupling has been the most successful in the controlled formation of covalent single C−C bonds. Particularly advanced control of this synthetic tool has been obtained by means of hierarchical reactivity, commonly achieved by the use of different halogen atoms that consequently display distinct activation temperatures. Here we report on the site-selective reactivity of certain carbon-halogen bonds. We use precursor molecules halogenated with bromine atoms at two non-equivalent carbon atoms and found that the Ullmann coupling occurs on Au(111) with a remarkable predilection for one of the positions. Experimental evidence is provided by means of scanning tunneling microscopy and core level photoemission spectroscopy, and a rationalized understanding of the observed preference is obtained from density functional theory calculations.The project leading to this publication has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreement No 635919), from the Spanish Ministry of Economy, Industry and Competitiveness (MINECO, Grant Nos. MAT2016‐78293‐C6‐R) and María de Maeztu Units of Excellence Program MDM‐2026‐0618. We also acknowledge financial support from the Xunta de Galicia (Centro singular de investigación de Galicia, accreditation 2016–2019, ED431G/09) and Fondo Europeo de Desarrollo Regional (FEDER). This work used the “Imbabura” computer cluster of Yachay Tech University, which was purchased under contract No. 2017‐024 (SIE‐UITEY‐007‐2017).Peer reviewe

On-surface synthesis and electronic structure characterization of grafphene nanoribbons

[ES]: Esta tesis expone un estudio detallado sobre la síntesis en superficie de nanotiras de grafeno (en inglés 'Graphene Nanoribbons', GNRs) y la posterior caracterización de su estructura electrónica, principalmente empleando como técnica analítica, microscopía y espectroscopía de barrido de efecto túnel de baja temperatura (en inglés 'Low temperature-Scanning Tunnneling Microscopy/Spectrosocopy', LT-STM/STS) en condiciones de ultra alto vacío (en inglés 'Ultra High Vacuum', UHV). Para la obtención de los resultados detallados en esta tesis, se han empleado dos equipos de microscopía independientes, ambos ubicados en San Sebastián (Guipúzcoa). En primer lugar, 'Milano', un microscopio ensamblado a partir de elementos de distintios equipos y ubicado en el centro de investigación cooperativo 'CIC nanoGUNE'. En segundo lugar, 'Apollo', un equipo comercial Sigma-Omicron y situado en el instituto de investigación 'Centro de Física de Materiales' (CFM). Paralelamente al uso de estos microscopios, distintas técnicas de análisis de superficies, en concreto, espectroscopía de fotoemisión de rayos x (en ingles 'X-ray Photoemission Spectroscopy', XPS) y de ángulo resuelto (en inglés 'Angle-Resolved Photoemission Spectroscopy'), han complementado el estudio de las distintas reacciones de química de superficie que dan pie a la formación de las nanotiras de grafeno, así como el posterior estudio de sus propiedades electrónicas. El trabajo realizado durante esta tesis se enmarca dentro del campo científico conocido como 'nanociencia', definiendose esta como la rama de la ciencia centrada en el estudio de los fenómenos físico-químicos que se suceden en sistemas cuyas propiedades están gobernadas por las dimensiones que las definen, comprendiendose estas entre 1 y 100 nanometros.[EN]: This thesis presents a comprenhensive study on the on-surface synthesis and characterization of the electronic structure of di erent types of graphene nanoribbons (GNRs) formed on coinage metallic surfaces, being gold the most present substrate. Graphene nanoribbons (GNRs) are a new emergent material which is gaining considerable attention within the scientific community due to its wide-range potential applications derived from its exotic physical and chemical properties. Since GNRs derive from graphene, they preserve many of its interesting properties, such as the highest electron conductivity. In adittion, the reduced dimensionality of GNRs provide them with a tunable non-zero electrical band gap, no present in graphene, and required for its implementation into electronic devices. Moreover, and again in contrast with graphene, the presence of edges in GNRs brings the emergence of magnetic edge states with promising applications in spintronics. We employ on-surface chemistry startegies based on the use of aromatic molecular precursors for synthesizing the different GNRs studied here. By thermal annealing, we induce a controlled two-step reaction pathway to form these nanostructures onto different coinage metallic substrates, since their catalytic activity is needed for the product formation. The two chemical reaction accounted involves firstly, the merging of the molecular precursors into commonly linear polymric chains thorugh the well-known and controlled Ullmann coupling. In a second stage, higher temperatures induce the ciclodehydrogenation of the former polymers, giving rise to the formation of planar graphene nanoribbons.This thesis has been carried out at Donostia International Physics Center (DIPC), Centro de Física de Materiales - Materials Physics Center (CFM-MPC), and CIC nanoGUNE: nanoscience cooperative research center.Peer reviewe

Magnetic transport properties of magnetic porphyrins connected to graphene nanoribbon electrodes

Resumen del trabajo presentado a la International conference on: Novel 2D materials explored via scanning probe microscopy & spectroscopy; celebrada en Donostia.San Sebastián (País Vasco, España) del 25 al 29 de junio de 2018.Graphene and graphene nanoribbons (GNRs) are ideal systems for contacting functional molecules due to their extraordinary electron mobility and structural stability under high currents. In our previous work, we have shown the construction and magnetic characterization of a fully functional hybrid molecular system composed of a single magnetic porphyrin molecule covalently bonded to GNRs on a gold substrate. To better exploit the functionality of the devices, the magnetic transport measurement of the device is needed. Here in this work, by modifying the molecular precursors, the magnetic porphyrins were fused into two GRNs electrodes in one-dimensional manner. The STM tip was used to contact one GNRs electrode and lift the functional hybrid devices from the surface to form transport junctions. By means of inelastic tunneling spectroscopy, we identify the presence of the spin states of the magnetic porphyrin in the transport junctions although the GNRs electrodes are semiconductor. And we find that the magnetic anisotropy energy does not depends on length of the GNRs electrodes, but it is related to the d orbital of the magnetic porphyrin due to the e-e interactions.Peer reviewe

Quantum well edge states in graphene nanoribbons

Resumen del póster presentado a la International conference on: Novel 2D materials explored via scanning probe microscopy & spectroscopy; celebrada en Donostia.San Sebastián (País Vasco, España) del 25 al 29 de junio de 2018.Graphene nanoribbons (GNRs) can be synthesized on metal surfaces with atomic precision using on surface synthesis techniques. Their precise size and shape can be tuned finely by selecting appropriate precursor molecules. The incorporation of additional functional molecules during the on-surface synthesis allows the creation of hybrid systems. In earlier work, we demonstrated the creation of such hybrid systems by contacting magnetic porphyrin molecules with chiral (3,1)-GNRs on Au(111). However, in that work it was not analyzed in depth, to what extend the electronic bandstructure of the connecting GNRs is affected. Here, we present results from Fourier-transformed tunneling spectroscopy performed along the axis of a GNR segment enclosed by two porphyrins. We show that the presence of porphyrin creates a quantum well system, resulting in discrete resonant edge-states in the electronic bandstructure. We find a quadratic dispersion relation and extract from that an effective electron mass close to the one found in pristine ribbons.Peer reviewe

Electronic structure characterization of atomically-precise chiral graphene nanoribbons on gold surfaces

Resumen del póster presentado a la 13th European Conference on Surface Crystallography and Dynamics, celebrada en Donostia-San Sebastián (España) del 19 al 21 de junio de 2017.Graphene nanoribbons (GNRs) are narrow stripes of graphene that have attracted great attention because of their interest for both fundamental physics and promising applications. While sharing many of the appealing properties of their predecessor material graphene, such as high mobility charge-carriers and high specific surface area, they overcome some of its limitations as is the lack of a band gap. These nanostructures can display different edge orientations with respect to graphene´s lattice vectors that largely determine their main properties. Therefore three types of ribbons can be synthesized: armchair, zigzag or chiral GNRs, the latter ones presenting a periodic combination of both armchair- and zigzag-like segments. A recently established bottom-up synthesis method based on the use of molecular precursors as building blocks allows for the synthesis of these three types of GNRs with atomic precision. However, the limited experimental results on these structures are mostly focused on the electronic structure of armchair and, to a lesser extent, on zigzag GNRs, thus letting chiral GNRs (cGNRs) hardly explored. The growth of these ribbons on different noble metals was recently reported, hence we focus on the electronic structure of these nanomaterials. Here, using the same molecular precursor-based methodology, we report on the electronic structure of (3,1)-cGNRs on Au(111) by Scanning Tunneling Spectroscopy (STS). Moreover, the use of Au(322) vicinal substrate as template promotes the aligned growth of these ribbons along the terrace length thus enabling us to characterize the valence band by means of ARPES. Our results reveal a semiconducting bandgap on these chiral nanoribbons therefore confirming its potential applications for nanoelectronics.Peer reviewe

Structure-property relation in atomically precise graphene nanoribbons on Au(111)

Resumen del póster presentado a la 10th Conferencia Fuerzas y Túnel, celebrada en Girona (España) del 5 al 7 de septiembre de 2016.Graphene nanoribbons (GNRs) are narrow stripes of graphene, whose electronic properties strongly depend on their detailed structure. As an example, GNRs with armchair shaped edges are predicted to be semiconducting, with a width-dependent band gap, while those with zigzag shaped edges are predicted to be semi-metallic and present localized spin-polarized edge-states. Such variety of electronic behaviors places them as promising building blocks in next-generation nanoelectronic and optoelectronic devices. However, since the electronic properties of GNRs are highly susceptible to minimum changes in their structure, their precise synthesis and consequently the experimental confirmation of the predictions has remained a key challenge. Recent advances in bottom-up synthesis have shown that the growth of atomically precise GNRs can be produced by pre-designing the chemical structure of molecular precursors. By means of low-temperature Scanning Tunneling Microscopy (STM) and Spectroscopy (STS), we report the study of different GNRs synthesized on Au(111) from two organic precursors: 4,4’’-dibromo-p-terphenyl (DBTP) and 2,2’-dibromo-9.9’-bianthracene (DBBA). In the case of DBTP molecules, carefully annealing-triggered molecular reactions result in the formation of GNRs with different width on the same metallic substrate, enabling us to study the predicted band gap dependence of armchair nanoribbons on their width. Our results confirm this size dependence in armchair shaped GNRs and are in good agreement with calculations. In turn, the use of DBBA molecules leads to the synthesis of well-defined, chiral nanoribbons with a consequently lower bandgap. Fourier-transform STS analysis provides additional insight into the GNR’s band dispersion.Peer reviewe

Inducing magnetism in graphene nanoribbons on surfaces

Resumen del trabajo presentado al International workshop On-Surface Synthesis (OSS), celebrado en Sant Feliu de Guíxol (España) del 23 al 28 de septiembre de 2018.Large aromatic carbon nanostructures are cornerstone materials due to their increasingly role in functional devices. Among the many predicted applications, magnetism is the most unexpected one, but still an attractive challenge for its active role in spintronic devices. In our laboratories we aimed at exploring different methods for turning graphene nanoribbons (GNRs) magnetic using low temperature scanning tunneling microscopy (STM). The production of GNR can be realized with atomic precision on a metal surface using chemical strategies of on-surface synthesis, resulting in mostly defect-free ribbons and with a customized shape according to the utilized precursor. Magnetism can be induced by doping the carbon network with magnetic species. We incorporated magnetic molecular species into a ribbon using on-surface synthesis routes (see included image of a Fe porphyrin contacted to chiral nanoribbons) and proved that the molecular spin survives in the ribbon by using spin-excitation inelastic spectroscopy. Numerous predictions state that graphene can also spontaneously develop magnetism from the Coulomb repulsion of its pi-electrons. Crucial examples are the magnetization of zig-zag edges in graphene, or the emergence of paramagnetism in open shell graphenoid nanostructures. In this presentation, we will show the emergence of zero-energy edge modes in nanoribbons with a large density of zig-zag edges when a certain width is reached. In certain circumstances, the nanostructures exhibit spectroscopic fingerprints of spin localization, which allows us to identify and localize the sources of pi-paramagnetism with an STM.Peer reviewe