Ullmann coupling reaction in unconventional surfaces

131 p.En el presente trabajo estudiamos el crecimiento de nano-estructuras de grafeno (GNR) mediante la reacción de Ullmann en superficies tecnológicamente relevantes.Desde su redescubrimiento allá por el año 2004, el grafeno es conocido como el material del futuro debido, entre otras cosas, a sus excelentes propiedades tanto químicas como electrónicas. Sin embargo, para su implementación en la industria de la nano electrónica y/o nana óptica en forma de dispositivo, necesita poder ser apagado y encendido, es decir, tener un ratio ¿on/off¿. El grafeno es un semimetal por lo que no se puede encender ni apagar intrínsecamente. De las muchas estrategias para conseguir dicho objetivo, uno de los más efectivos se basa en el confinamiento del grafeno, es decir, en disminuir de dos dimensiones a cuasi una dimensión el tamaño del grafeno. En consecuencia, se generan las estructuras llamadas nano grafenos o GNR.Sus excelentes propiedades electrónicas, similares a las del grafeno, así como el gap intrínseco (lo que permite encender o apagarlos) de las estructuras de GNR, hace de ellos componentes idóneos para su implementación en dispositivos electrónicos como por ejemplo los transistores de potencia. Una de las maneras más atractivas de sintetizar estas estructuras de GNR es mediante la reacción de Ullmann, que consiste en ensamblar moléculas previamente prediseñadas en una superficie catalizadora. Sin embargo, por ahora, la reacción de Ullmann solo se ha podido realizar con éxito en superficies metálicas, donde, intrínsecamente, las propiedades del GNR se desvanecen o hibridan. Por ello, una vez sintetizados los GNR, tienen que ser transferidos a superficies modelo, como un semiconductor en el caso del transistor, para poder ser implementados en futuros nano dispositivos, y normalmente la eficacia de dicha transferencia es muy pobre. El siguiente paso en el campo, y el objetivo del presente trabajo, es crecer estructuras de GNR mediante la reacción de Ullmann directamente en superficies tecnológicamente más relevantes donde a su vez, las propiedades de dichas estructuras es preservada.En nuestro trabajo demostramos la viabilidad de crecer GNR mediante la reacción de Ullmann en tres tipos de superficies tecnológicamente atractivas : (i) en un cristal escalonado llamado c-Au(111), (ii) en la superficie magnética del GdAu2 y (iii) en un material semiconductor como el como el TiO2 . Los trabajos de investigación se han desarrollado en una campana de Ultra-alto Vacío, y para la consecución de los resultados descritos arriba, hemos utilizado las siguientes técnicas experimentales: (i) el microscopio de efecto túnel (STM), para discernir localmente las geometrías de los nano grafenos (ii) difracción de electrones (LEED), para obtener un promedio de las geometrías del nano grafeno y (iii) técnicas de fotoemisión, tanto para establecer las propiedades electrónicas de los nano grafenos como estudiar los mecanismos químicos de la reacción de Ullmann

Ullmann coupling reaction in unconventional surfaces

131 p.En el presente trabajo estudiamos el crecimiento de nano-estructuras de grafeno (GNR) mediante la reacción de Ullmann en superficies tecnológicamente relevantes.Desde su redescubrimiento allá por el año 2004, el grafeno es conocido como el material del futuro debido, entre otras cosas, a sus excelentes propiedades tanto químicas como electrónicas. Sin embargo, para su implementación en la industria de la nano electrónica y/o nana óptica en forma de dispositivo, necesita poder ser apagado y encendido, es decir, tener un ratio ¿on/off¿. El grafeno es un semimetal por lo que no se puede encender ni apagar intrínsecamente. De las muchas estrategias para conseguir dicho objetivo, uno de los más efectivos se basa en el confinamiento del grafeno, es decir, en disminuir de dos dimensiones a cuasi una dimensión el tamaño del grafeno. En consecuencia, se generan las estructuras llamadas nano grafenos o GNR.Sus excelentes propiedades electrónicas, similares a las del grafeno, así como el gap intrínseco (lo que permite encender o apagarlos) de las estructuras de GNR, hace de ellos componentes idóneos para su implementación en dispositivos electrónicos como por ejemplo los transistores de potencia. Una de las maneras más atractivas de sintetizar estas estructuras de GNR es mediante la reacción de Ullmann, que consiste en ensamblar moléculas previamente prediseñadas en una superficie catalizadora. Sin embargo, por ahora, la reacción de Ullmann solo se ha podido realizar con éxito en superficies metálicas, donde, intrínsecamente, las propiedades del GNR se desvanecen o hibridan. Por ello, una vez sintetizados los GNR, tienen que ser transferidos a superficies modelo, como un semiconductor en el caso del transistor, para poder ser implementados en futuros nano dispositivos, y normalmente la eficacia de dicha transferencia es muy pobre. El siguiente paso en el campo, y el objetivo del presente trabajo, es crecer estructuras de GNR mediante la reacción de Ullmann directamente en superficies tecnológicamente más relevantes donde a su vez, las propiedades de dichas estructuras es preservada.En nuestro trabajo demostramos la viabilidad de crecer GNR mediante la reacción de Ullmann en tres tipos de superficies tecnológicamente atractivas : (i) en un cristal escalonado llamado c-Au(111), (ii) en la superficie magnética del GdAu2 y (iii) en un material semiconductor como el como el TiO2 . Los trabajos de investigación se han desarrollado en una campana de Ultra-alto Vacío, y para la consecución de los resultados descritos arriba, hemos utilizado las siguientes técnicas experimentales: (i) el microscopio de efecto túnel (STM), para discernir localmente las geometrías de los nano grafenos (ii) difracción de electrones (LEED), para obtener un promedio de las geometrías del nano grafeno y (iii) técnicas de fotoemisión, tanto para establecer las propiedades electrónicas de los nano grafenos como estudiar los mecanismos químicos de la reacción de Ullmann

Doping of graphene nanoribbons via functional group edge modification

We report the on-surface synthesis of 7-armchair graphene nanoribbons (7-AGNRs) substituted with nitrile (CN) functional groups. The CN groups are attached to the GNR backbone by modifying the 7-AGNR precursor. Whereas many of these groups survive the on-surface synthesis, the reaction process causes the cleavage of some CN from the ribbon backbone and the on-surface cycloisomerization of few nitriles onto pyridine rings. Scanning tunneling spectroscopy and density functional theory reveal that CN groups behave as very efficient n-dopants, significantly downshifting the bands of the ribbon and introducing deep impurity levels associated with the nitrogen electron lone pairs.This work was supported by FP7 FET-ICT “Planar Atomic and Molecular Scale devices” (PAMS) project (funded by the European Commission under Contract No. 610446), by the Agencia Estatal de Investigacion (Cooperative Grant No. MAT2016-78293 and Grant FIS2015-62538-ERC), the Basque Government (Dep. de Educacion and UPV/EHU, Grant No. IT-756-13, and Dep. Industry, Grant PI_2015_1_42), the Xunta de Galicia (Centro singular de investigacion de Galicia accreditation 2016−2019, ED431G/09), and the European Regional Development Fund (ERDF).Peer Reviewe

Interplay between steps and oxygen vacancies on curved TiO2(110)

et al.A vicinal rutile TiO(110) crystal with a smooth variation of atomic steps parallel to the [1-10] direction was analyzed locally with STM and ARPES. The step edge morphology changes across the samples, from [1-11] zigzag faceting to straight [1-10] steps. A step-bunching phase is attributed to an optimal (110) terrace width, where all bridge-bonded O atom vacancies (O vacs) vanish. The [1-10] steps terminate with a pair of 2-fold coordinated O atoms, which give rise to bright, triangular protrusions (S) in STM. The intensity of the Ti 3d-derived gap state correlates with the sum of O vacs plus S protrusions at steps, suggesting that both O vacs and steps contribute a similar effective charge to sample doping. The binding energy of the gap state shifts when going from the flat (110) surface toward densely stepped planes, pointing to differences in the Ti polaron near steps and at terraces.We acknowledge financial support from the Spanish Ministry of Economy (Grants MAT2013-46593-C6-4-P and MAT2013-46593-C6-2-P) and the Basque Government (Grant IT621-13 and IT756-13). M.S. and U.D. acknowledge support from the ERC Advanced Grant “OxideSurfaces”. D.S.P. and M.M. acknowledge support from the Marie Curie ITN “THINFACE” and financial support by the Deutsche Forschungsgemeinschaft. through SFB 1083 “Structure and Dynamics of Internal Interfaces”.Peer Reviewe

Hanle Magnetoresistance in Thin Metal Films with Strong Spin-Orbit Coupling

We report measurements of a new type of magnetoresistance in Pt and Ta thin films. The spin accumulation created at the surfaces of the film by the spin Hall effect decreases in a magnetic field because of the Hanle effect, resulting in an increase of the electrical resistance as predicted by Dyakonov [PRL 99, 126601 (2007)]. The angular dependence of this magnetoresistance resembles the recently discovered spin Hall magnetoresistance in Pt/Y3Fe5O12 bilayers, although the presence of a ferromagnetic insulator is not required. We show that this Hanle magnetoresistance is an alternative, simple way to quantitatively study the coupling between charge and spin currents in metals with strong spin-orbit coupling.Comment: 13 pages, 3 figure

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

Optimal Manufacturing Configuration Selection: Sequential Decision Making and Optimization using Reinforcement Learning

In manufacturing, different costs must be considered when selecting the optimal manufacturing configuration. Costs include manufacturing costs, material costs, labor costs, and overhead costs. Optimal manufacturing configurations are those that minimize production criteria, such as costs, production speed, and flexibility, while still meeting the required production levels and quality standards. To find the optimal manufacturing configuration, manufacturers often use a combination of traditional techniques, e.g., mathematical modeling, simulation, and optimization, to evaluate the tradeoffs between different cost factors and identify configurations that provide the best balance between cost and performance. However, these techniques may require long development and simulation time, and/or may require expert knowledge. This paper presents a method for selecting the optimal manufacturing configuration, focusing on cost optimization, using a reinforcement learning (RL) approach for sequential decision-making. The proposed method involves developing a RL environment, requiring lower development and simulation times than traditional techniques, that captures the incurred costs, recurring costs, production rates, and setup times of manufacturing configurations. The problem is then solved using the Proximal Policy Optimization algorithm to identify the configuration that minimizes costs while still meeting the required production levels and quality standards. The effectiveness of the proposed method is validated through a machining process planning case study with multiple cost factors and production constraints. In particular, the machining process plan was developed for an industry-relevant product prototype. The results show that the proposed method can find solutions that are robust to stochastic noise, providing valuable insights for manufacturers looking to optimize manufacturing operations

Seismic performance of existing hollow reinforced concrete bridge columns

Highway bridges can be considered as crucial civil structures for economic and social progress of urban areas. The damages to highway bridges due to earthquake events may have dramatic impact on the interested area, with or without life threatening consequences, since bridges are essential for relief operations. For these reasons, the assessment of seismic performance of existing bridge structures is a paramount issue, especially in those countries, such as Italy, where most of existing bridges was constructed before the advancement in earthquake engineering principles and seismic design codes. Several major earthquakes occurred throughout the world highlighted the seismic vulnerability of the bridge piers, due to obsolete design. If, for ordinary shaped reinforced concrete (RC) bridge columns the seismic assessment issue can be considered as almost solved, due to several analytical assessment formulations available in literature, and adopted by codes, the same cannot be said for columns with hollow-core cross section, despite their widespread use. None of the current codes addresses specialized attention to RC hollow core members, and only quite recently, attention has been paid to experimental cyclic response of hollow columns. Some critical issues for hollow RC columns are related to the assessment of their shear capacity, special focusing on degradation mechanisms, and the high shear deformation characterizing the seismic response of such elements. In the above outlined contest, a contribution in the seismic assessment of hollow bridges piers is provided by the present work: the investigation of cyclic lateral response of RC existing bridge piers with hollow rectangular and hollow circular cross-section is performed. Special attention has been focused on failure mode prediction and shear capacity assessment. A critical review of the state-of-the-art and of the theoretical background is firstly carried out: the review process has been focused on the past experimental and analytical research on seismic performance of hollow reinforced concrete bridge piers, both for hollow rectangular and hollow circular cross sections. The experimental campaign, conducted at Laboratory of the Department of Structures for Engineering and Architecture, University of Naples “Federico II”, is presented. The experimental program comprised tests on six reduced-scale RC bridge piers with hollow cross-section (four rectangular shaped and two circular shaped). The specimens were ad hoc designed in order to be representative of the existing bridge columns typical of the Italian transport infrastructures realized before 1980, by using a scaling factor equal to 1:4. The construction procedure is detailed, too. All the tests were performed in quasi-static way by applying increasing horizontal displacement cycles with constant axial load (equal to 5% of the axial compressive capacity) until collapse. The monitoring system is accurately explained: it was composed of two sub-systems, one used for global measures (forces and displacement), and the other to deeply investigate about local deformation. Experimental results for both hollow rectangular and hollow circular specimens are reported: for each specimen the results in terms of lateral load versus drift are shown and the evolution of observed damage with increasing displacement is described and related to the lateral load-drift response. An experimental analysis of deformability contributions to the top displacement is performed, mainly in order to better understand the relevance of taking into account shear deformations for bridge piers assessment. The energy dissipation capacity is also analyzed, evaluating the equivalent damping ratio and its evolution with ductility. For hollow rectangular specimens, the global response is modelled through a three-component numerical model, in which flexure, shear and bar slip are considered separately. The main goal of the numerical analysis is to reproduce the experimental deformability contributions. The last part of the work focuses on the definition of proper shear strength models for both hollow rectangular and hollow circular cross sections, and the definition of a deformability capacity model for hollow rectangular cross section. To this aim, two different experimental databases are collected and critically analyzed. The effectiveness in shear capacity and failure mode prediction of main existing shear capacity models is investigated, by applying these models to the database columns. Based on the obtained results, some modifications to existing shear strength models are discussed and proposed in order to improve their reliability for hollow columns. Finally, a new drift capacity model is developed and proposed to assess drift at shear failure of hollow rectangular columns

Interplay between steps and oxygen vacancies on curved TiO2(110)

Trabajo presentado al Symposium on Surface Science (3S), celebrado en St. Christoph am Arlberg (Austria) del 21 al 27 de febrero de 2016.We acknowledge financial support from the Spanish Ministry of Economy (grant MAT2013-46593-C6-4-P and MAT2013-46593-C6-2-P) and the Basque Government (grant IT621-13 and IT756-13), the ERC Advanced Grant “OxideSurfaces”., and the Marie Curie ITN “THINFACE”.Peer reviewe