1,720,965 research outputs found
Reactivity of porphyrin- and phthalocyanine-based assemblies at surfaces from UHV to near-ambient pressure
Full text linkMetal-organic materials represent a promising alternative to traditional technologies, minimizing the consumption of rare metals and production costs. Furthermore, their properties can be tuned through chemical functionalization or by tailoring the electronic structure, making them extremely versatile.
This thesis is dedicated to the growth and experimental characterization of porphyrin- and phthalocynine-based biomimetic materials, focusing on their reactivity at surfaces.
Two different Pd-heterostructures are investigated in the first part of this work, PdTPP on Cu(100) and 2HPc on Pd(001). We focus on Pd-based molecular tectons since they demonstrated drug delivery capabilities, relevant for cancer therapy research. The electronic and conformational properties of PdTPP/Cu(100) are studied through a combination of multiple spectroscopic and microscopic techniques, revealing their instability upon deposition on the metallic copper termination. The strong interaction with the surface induces a thermally-enhanced redox metal exchange between Pd and Cu surface atoms, paralleled by dehydrogenation reactions, ultimately leading to a reconstruction of the organic structure. We finally demonstrate that PdTPP can be decoupled and stabilized by exploiting an oxygen buffer layer. Starting from the non-metalated precursor, we show instead that PdPcs are easily obtained by self-metalation of 2HPcs deposited on Pd(001), promoted by a mild annealing.
The second part of this work presents the growth and characterization of MnTPyP and MnTPyP-Co on inert substrates, Gr/Ir(111) and Au(111). Previous studies predicted and demonstrated the catalytic abilities of Co and Mn organic compounds towards oxygen evolution and reduction reactions, regulating the operation of metal-air batteries. Our investigation of these model catalysts, with stronger focus on the Gr/Ir(111) support, extends from ultrahigh vacuum to near-ambient pressure conditions in order to tackle their catalytic properties in situ. The coordination of Co to a MnTPyP layer leads to a dramatic non-local electronic and vibronic structure modification, allowing to tune the oxidation state of the metal sites and to modify the frontier molecular orbitals. Additionally, the peripheral coordination of a second metal induces a rearrangement of the tectons supramolecular ordering. The comparison between the properties of these systems on Gr/Ir(111) and Au(111) reveals some differences associated to the surface trans-effect originating from the different molecule-substrate interaction.
The chemical activity of the MnTPyP-Co/Gr/Ir(111) system towards CO and O2 is investigated by complementary in situ spectrocopies. CO reversibly ligates to Co, where the adsorbate coverage is determined by the equilibrium between the adsorption and desorption processes. We also find that anti-cooperative mechanisms arise between neighbouring Co sites, mediated by the organic matrix, revealed by means of a pressure uptake from ultra-high vacuum to near-ambient pressure.
Finally, the pressure-dependent results of the O2 reactivity study are presented. At low pressure, molecular oxygen reversibly ligates to Co in equilibrium the O2 background. At high pressure instead the activated O-O bond is cleaved, leading to a non-reversible oxidation of the framework and graphene. Oxygen dissociation is the preliminary step of oxygen reduction reactions, thus its activation at room temperature is an extremely promising result. Indeed, this property can be exploited to oxidise other gases, preventing the catalyst poisoning.Metal-organic materials represent a promising alternative to traditional technologies, minimizing the consumption of rare metals and production costs. Furthermore, their properties can be tuned through chemical functionalization or by tailoring the electronic structure, making them extremely versatile.
This thesis is dedicated to the growth and experimental characterization of porphyrin- and phthalocynine-based biomimetic materials, focusing on their reactivity at surfaces.
Two different Pd-heterostructures are investigated in the first part of this work, PdTPP on Cu(100) and 2HPc on Pd(001). We focus on Pd-based molecular tectons since they demonstrated drug delivery capabilities, relevant for cancer therapy research. The electronic and conformational properties of PdTPP/Cu(100) are studied through a combination of multiple spectroscopic and microscopic techniques, revealing their instability upon deposition on the metallic copper termination. The strong interaction with the surface induces a thermally-enhanced redox metal exchange between Pd and Cu surface atoms, paralleled by dehydrogenation reactions, ultimately leading to a reconstruction of the organic structure. We finally demonstrate that PdTPP can be decoupled and stabilized by exploiting an oxygen buffer layer. Starting from the non-metalated precursor, we show instead that PdPcs are easily obtained by self-metalation of 2HPcs deposited on Pd(001), promoted by a mild annealing.
The second part of this work presents the growth and characterization of MnTPyP and MnTPyP-Co on inert substrates, Gr/Ir(111) and Au(111). Previous studies predicted and demonstrated the catalytic abilities of Co and Mn organic compounds towards oxygen evolution and reduction reactions, regulating the operation of metal-air batteries. Our investigation of these model catalysts, with stronger focus on the Gr/Ir(111) support, extends from ultrahigh vacuum to near-ambient pressure conditions in order to tackle their catalytic properties in situ. The coordination of Co to a MnTPyP layer leads to a dramatic non-local electronic and vibronic structure modification, allowing to tune the oxidation state of the metal sites and to modify the frontier molecular orbitals. Additionally, the peripheral coordination of a second metal induces a rearrangement of the tectons supramolecular ordering. The comparison between the properties of these systems on Gr/Ir(111) and Au(111) reveals some differences associated to the surface trans-effect originating from the different molecule-substrate interaction.
The chemical activity of the MnTPyP-Co/Gr/Ir(111) system towards CO and O2 is investigated by complementary in situ spectrocopies. CO reversibly ligates to Co, where the adsorbate coverage is determined by the equilibrium between the adsorption and desorption processes. We also find that anti-cooperative mechanisms arise between neighbouring Co sites, mediated by the organic matrix, revealed by means of a pressure uptake from ultra-high vacuum to near-ambient pressure.
Finally, the pressure-dependent results of the O2 reactivity study are presented. At low pressure, molecular oxygen reversibly ligates to Co in equilibrium the O2 background. At high pressure instead the activated O-O bond is cleaved, leading to a non-reversible oxidation of the framework and graphene. Oxygen dissociation is the preliminary step of oxygen reduction reactions, thus its activation at room temperature is an extremely promising result. Indeed, this property can be exploited to oxidise other gases, preventing the catalyst poisoning
Thermally Activated on‐Surface Self‐Metalation of Pd‐Phthalocyanines
No full textPhthalocyanines (Pcs) are essential organic molecules with wide-ranging applications in fields such as catalysis and optoelectronics, owing to their stable aromatic structure and exceptional thermal stability. Equally important is their versatility, which can be achieved by modifying the central metal atom. In this context, metalation—the process by which a central metal atom is incorporated into the macrocycle—plays a critical role in tailoring both the electronic and structural properties of Pcs. In this work, we investigate the thermally activated on-surface synthesis of palladium phthalocyanine (PdPc) by depositing metal-free phthalocyanine ((Formula presented.)) onto a Pd(001) surface in ultra-high vacuum (UHV) conditions. Using temperature-dependent X-ray photoemission spectroscopy (XPS), we analyze the evolution of the N (Formula presented.) core level signal, confirming the successful formation of PdPc. The appearance of a secondary peak in the Pd (Formula presented.) core level, attributed to Pd atoms coordinated within the phthalocyanine molecule after metalation, provides further evidence for the PdPc synthesis. Additionally, near-edge X-ray absorption fine structure (NEXAFS) reveals that no desorption or chemical degradation occurs during the process. We believe this study represents a significant step forward in the scalable synthesis of PdPc, which, like other Pd-containing organic molecules, holds great potential for applications in catalysis and cancer therapy
Local Geometry, Structure and Electronic Resonances Enhancing the SFG Signal from CO on Ir Surfaces
Full text linkSum frequency generation (SFG) spectroscopy was used to study CO adsorption on smooth and rough Ir(111) single crystal surfaces, the cleanliness, composition, order and morphology of which were comprehensively characterized by Auger electron spectroscopy (AES), low energy ion scattering (LEIS), low energy electron diffraction (LEED), and scanning tunneling microscopy (STM). For CO adsorbed on Ir(111), the resonant SFG signal intensity associated with the internal C-O stretch mode was about eight times stronger on a rough termination than on a smooth surface. Herein, we thoroughly discuss the origin of this phenomenon and consider several possible contributing factors, including coverage and lateral interactions, molecular hyperpolarizability (IR dipole moment and Raman polarizability), adsorption geometry (tilt angle), Fermi resonances, adsorbate hot vibrational bands, and surface plasmons and electronic structure. It is concluded that the sputter-induced local roughness of the Ir surface (grains evidenced by STM) facilitates the light-induced excitation of localized surface plasmon resonances (LSPR), accounting for the observed signal enhancement
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“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
Stabilization versus competing de-metalation, trans-metalation and (cyclo)-dehydrogenation of Pd porphyrins at a copper surface
Full text linkMetal-porphyrins are studied intensively due their potential applications, deriving from the variety of electronic and chemical properties, tunable by selecting metal centers and functional groups. Metalation, de- and trans-metalation processes are fundamental in this sense to investigate both the synthesis and the stability of these molecular building blocks. More specifically, Pd coordination in tetrapyrroles revealed to be potentially interesting in the fields of cancer therapy, drug delivery and light harvesting. Thus, we focused on the stability of palladium tetraphenyl porphyrins (PdTPPs) on a copper surface by means of combined spectroscopy and microscopy approaches. We find that PdTPPs undergo coverage-dependent trans-metalation accompanied by steric rearrangements already at room temperature, and fully trans-metalate to CuTPPs upon mild annealing. Side reactions such as (cyclo)-dehydrogenation and structural reorganization affect the molecular layer, with Pd-Cu alloying and segregation occurring at higher temperature. Instead, oxygen passivation of the Cu support prevents the metal-involving reactions, thus preserving the layer and increasing the chemical and temperature stability of the Pd porphyrins.LN
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
From borophene polymorphs towards a single honeycomb borophane phase : reduction of hexagonal boron layers on Al(111)
No full textThe recent interest in characterizing 2D boron polymorphs has led to claims of the first stabilization of a honeycomb phase with conical Dirac-like electron dispersion. However, the synthesis of chemically stable, single, and homogeneous 2D boron phases still represents a significant experimental challenge. This is ascribed to the intrinsic boron electronic configuration that, at variance with carbon, leads to the formation of multi-center covalent bonds. External charge compensation by substrate-induced doping can steer the geometry of the layer, both in the buckling and in the density of B vacancies, like in the case of the recently achieved stabilization of honeycomb boron layers on Al(111). The price to pay is however a strong boron-support interaction, resulting in general in a limiting kinetic hindrance with respect to the synthesis of homogenous single phases. In the specific case of Al(111) an AlB2 layer is known to form at the surface, quite far from a desirable quasi-freestanding borophene monolayer and at variance with graphene, which can be easily synthesized in an almost freestanding configuration e.g. on Ir(111). We provide here evidence for the (reversible) formation of well-ordered honeycomb borophane upon hydrogenation of the honeycomb boron phase on Al(111)
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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