86,657 research outputs found
Interplay between local structure and supramolecular order in the reactivity of surface-supported molecular aggregates
No full textSPM tools have been extensively used in recent years to prepare, characterize and test the structural and functional properties of self-assembled molecular networks on surfaces. In many cases the molecular positional and conformational degrees of freedom determine a complex interfacial energy landscape, where subtle equilibria lead to the formation of various 2D ordered phases, often related to each other by phase transitions able to trigger the molecular reactivity, which therefore depends on subtle interrelations between local adsorption sites, intramolecular structure and long range supramolecular order. This talk will address some aspects of the structure-reactivity relationships in several surface-supported molecular networks, studied by means of STM complemented by other surface science and computational tools: e.g. the dependence of the catalytic activity toward oxygen reduction and of the magnetic properties of an Ag-supported iron phthalocyanine monolayer on the supramolecular arrangement of the molecules [1]; the requirements for either thermal [2] or photo-excited [3] covalent stabilization of single- and bicomponent supramolecular networks containing functionalized porphyrins on Ag low-index surfaces; the bottom up synthesis of long range ordered 1D poly-para-phenylene nanowires and 2D graphene nanoribbons on gold substrates [4]. [1] Casarin et al., J. Phys. Chem. C, 2010 ,114, 2144; Sedona, F. et al. Nat. Mater, 2012, 11, 970, Bartolomé et al. J. Phys. Chem. C, 2015 ,119, 12488. [2] Di Marino et al., Langmuir, 2010, 26, 2466; Sedona et al., ACS Nano, 2010, 4, 5147; Sedona et al. J. Phys. Chem. C, 2014 ,118, 1587. [3] Basagni et al., Chem. Eur. J. 2014, 20, 14296. [4] Basagni et al., J. Am. Chem. Soc. 2015, 137, 1802; Basagni et al., ACS Nano 2016, 10, 2644
Heat or light? Tools of choice for on-surface synthesis
No full textIn recent years, direct on-surface synthesis in UHV has been exploited as a promising strategy to obtain thermally and chemically stable structures by covalent bonding of suitable precursors. So far, covalent linking of organic molecules onto metal, semiconducting and bulk insulator surfaces has been mostly carried out thermally. Heat supplied to the system promotes the formation of covalent bonds between the monomeric building units either simultaneously with the surface diffusion phenomena it promotes (i.e. under dynamic bond-forming conditions) or as a trigger, after a pre-assembly step into a surface-supported supramolecular, non-covalent network. However, heat as a tool for on-surface synthesis has the ambivalent status of a pharmakon, since it has the capacity to be beneficial and detrimental to the production and to the structural quality of the covalently-linked network at the same time. The close interplay between molecular surface diffusion, chemical reactivity and temperature, in fact, often hampers the possibility to independently control reaction initiation and surface mobility, ultimately leading to highly defective covalent networks, poorly ordered on the long range. Photochemically activated reactions are a potentially powerful tool to stabilize self-organized structures without disrupting the long-range order. Yet, to date photo-initiated on-surface reactions are still rather uncommon, since the processes following light absorption are not completely understood and, in particular, the role of the substrate is still poorly characterized in quantitative terms: the high quenching rate of electronically excited species on metal surfaces, which inhibits the photophysical processes commonly observed in the gas phase and in solution; the presence of new, surface-related excitation and relaxation pathways; the occurrence of charge-transfer-mediated photochemistry are but a few important issues. Based on the recent experience of our group in this field [1–6], in this talk I will provide some hints on the necessary conditions to be fulfilled to make either heat or light – or a combination of both – the tool of choice for the successful on-surface synthesis of long range-ordered covalent networks. References [1] F. Sedona, M. Di Marino, M. Sambi, T. Carofiglio, E. Lubian, M. Casarin, and E. Tondello. ACS Nano 4, 5147 (2010) [2] A. Basagni, L. Colazzo, F. Sedona, M. Di Marino, T. Carofiglio, E. Lubian, D. Forrer, A. Vittadini, M. Casarin, A. Verdini, A. Cossaro, L. Floreano and M. Sambi. Chem. Eur. J. 20, 14296 (2014) [3] A. Basagni, F. Sedona, C. A. Pignedoli, M. Cattelan, L. Nicolas, M. Casarin, and M. Sambi. J. Am. Chem. Soc. 137, 1802 (2015) [4] A. Basagni, L. Ferrighi, M. Cattelan, L. Nicolas, K. Handrup, L. Vaghi, A. Papagni, F. Sedona, C. Di Valentin, S. Agnoli and M. Sambi. Chem. Commun. 51, 12593 (2015) [5] S. Tognolini, S. Ponzoni, F. Sedona, M. Sambi and S. Pagliara. Phys. Chem. Lett. 6, 3632 (2015) [6] A. Basagni, G. Vasseur, C. A. Pignedoli, M. Vilas-Varela, D. Peña, L. Nicolas, L. Vitali, J. Lobo-Checa, D. G. de Oteyza, F. Sedona, M. Casarin, J. E. Ortega, and M. Sambi. ACS Nano 10, 2644 (2016
On-surface Photochemistry of Pre-ordered 1-Methyl-2-phenyl-acetylenes: C-H Bond Activation and Intermolecular Coupling on Highly Oriented Pyrolytic Graphite
No full textIn thiscontributionwereporton light-inducedmetal-freecouplingof propynylbenzenemolecularunitsonhighlyorientedpyrolyticgraphite.The reactionoccurswithinthe self-assembledmonolayerandleadsto the generationof covalentlycoupled1,5-hexadiyneandpara terphenylderivativesundertopologicalcontrol. Such photochemical uncatalysed pathway represents an original approachin the fieldof topological C-C coupling at the solid/liquid interface and provides new insight into the low temperature formation of aromatic compounds at the surface of carbonaceous supports
A theoretical study of the occupied and unoccupied electronic structure of high-and intermediate-spin transition metal phthalocyaninato (Pc) complexes: VPc, CrPc, MnPc, and FePc
No full textThe structural, electronic, and spectroscopic properties of high-and intermediate-spin transition metal phthalocyaninato complexes (MPc; M = V, Cr, Mn and Fe) have been theoretically investigated to look into the origin, symmetry and strength of the M–Pc bonding. DFT calculations coupled to the Ziegler’s extended transition state method and to an advanced charge density and bond order analysis allowed us to assess that the M–Pc bonding is dominated by σ interactions, with FePc having the strongest and most covalent M–Pc bond. According to experimental evidence, the lightest MPcs (VPc and CrPc) have a high-spin ground state (GS), while the MnPc and FePc GS spin is intermediate. Insights into the MPc unoccupied electronic structure have been gained by modelling M L2,3-edges X-ray absorption spectroscopy data from the literature through the exploitation of the current Density Functional Theory variant of the Restricted Open-Shell Configuration Interaction Singles (DFT/ROCIS) method. Besides the overall agreement between theory and experiment, the DFT/ROCIS results indicate that spectral features lying at the lowest excitation energies (EEs) are systematically generated by electronic states having the same GS spin multiplicity and involving M-based single electronic excitations; just as systematically, the L3-edge higher EE region of all the MPcs herein considered includes electronic states generated by metal-to-ligand-charge-transfer transitions involving the lowest-lying π* orbital (7eg) of the phthalocyaninato ligand
Enhancing Sedona (formerly GeoSpark) with Efficient k Nearest Neighbor Join Processing
No full textSedona (formerly GeoSpark) is an in-memory cluster computing system for processing large-scale spatial data, which extends the core of Apache Spark to support spatial datatypes, partitioning techniques, indexes, and operations (e.g., spatial range, k Nearest Neighbor (kNN) and spatial join queries). k Nearest Neighbor Join Query (kNNJQ) finds for each object in one dataset P, k nearest neighbors of this object in another dataset Q. It is a common operation used in numerous spatial applications (e.g., GISs, location-based systems, continuous monitoring, etc.). kNNJQ is a time-consuming spatial operation, since it can be considered a hybrid of spatial join and nearest neighbor search. Given that Sedona outperforms other Spark-based spatial analytics systems in most cases and, it does not support kNN joins, including kNNJQ is a worthwhile challenge. Therefore, in this paper, we investigate how to design and implement an efficient kNNJQ algorithm in Sedona, using the most appropriate spatial partitioning technique and other improvements. Finally, the results of an extensive set of experiments with real-world datasets are presented, demonstrating that the proposed kNNJQ algorithm is efficient, scalable and robust in Sedona. © 2021, Springer Nature Switzerland AG
The role of acetonitrile in the direct synthesis of hydrogen peroxide over palladium supported by ion-exchange resins
No full textAcetonitrile is used as an effective, non-corrosive selectivity enhancer for the direct synthesis of hydrogen peroxide (HP). Palladium catalysts supported by activated carbon or poly-divinylbenzene are poisoned by acetonitrile, which makes them very little productive. Conversely the catalysts supported by strongly acidic ion-exchange resin are much more selective and productive in the presence of acetonitrile, and for relatively long time. TEM shows that the size of their metal nanoparticles can be decreased by a reconstruction process, possibly sustained by leached palladium(II) species. HP decomposition experiments show that acetonitrile inhibits the O-O bonds splitting
Pseudomorphic-to-bulk fcc phase transition of thin Ni films on Pd(100)
No full textWe have measured the transformation of pseudomorphic Ni films on Pds100d into their bulk fcc phase as a
function of the film thickness. We made use of x-ray diffraction and x-ray induced photoemission to study the
evolution of the Ni film and its interface with the substrate. The growth of a film with tetragonally strained face
centered symmetry (fct) has been observed by out-of-plane x-ray diffraction up to a limit thickness of 10 Ni
pseudomorphic layers (some of them partially filled and intermixed with the substrate), where a new fcc
bulklike phase is formed. After the formation of the bulklike Ni domains, we observed the pseudomorphic fct
domains to disappear preserving the number of layers and their spacing. The phase transition thus proceeds via
lateral growth of the bulklike phase within the pseudomorphic one, i.e., the bulklike fcc domains penetrate
down to the substrate when formed. This large depth of the walls separating the domains of different phases is
also indicated by the increase of the intermixing at the substrate–film interface, which starts at the onset of the
transition and continues at even larger thickness. The bulklike fcc phase is also slightly strained; its relaxation
towards the orthomorphic lattice structure proceeds slowly with the film thickness, being not yet completed at
the maximum thickness presently studied of 30 Å ( ca 17 layers)
Variations on the Author
No full text“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Large freestanding 2D covalent organic framework nanofilms exhibiting high strength and stiffness
No full textTwo-dimensional covalent organic frameworks (2D COFs) represent an ideal platform to develop novel technological applications. The integration of 2D COFs into thin-film device architectures requires a deep knowledge of their mechanical performance, especially as large-area ultrathin films. Here, we report the synthesis, transfer, and mechanical characterization of large-area freestanding 2D COF films with nanometer thicknesses. Imine-linked COF nanofilms are prepared by condensation reaction at air-water interface, which provides freestanding, uniform centimeter-scale 2D COF films with controlled thickness. The developed procedure enables the direct transfer of the synthetized large-area COF nanofilm onto patterned substrates for mechanical characterization. Tensile tests are performed on freestanding 2D COF films with 85 nm thickness and with a testing area as large as 0.3 mm(2). The measured strength of the COF nanofilms is 188 +/- 57 MPa, while the Young's modulus is 37 +/- 15 GPa. Our findings not only demonstrate the high stiffness and strength of COF nanofilms over a large-area, which make them suitable for applications where high mechanical performance is required, but also pave the way for a fundamental understanding of the relationship between the structures and macroscopic mechanical properties of 2D COFs. Thus, the method that we developed herein will enable a broad exploration of the properties of large-area 2D COFs that will guide their engineering design toward the development of novel COF-based devices. (C) 2022 Elsevier Ltd. All rights reserved
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