1,720,958 research outputs found
Selective Self-Assembly and Modification of Herringbone Reconstructions at a Solid–Liquid Interface of Au(111)
No full textThe precise control of molecular self-assembly on surfaces presents many opportunities for the creation of complex nanostructures. Within this endeavor, selective patterning by exploiting molecular interactions at the solid–liquid interface would be a beneficial capability. Using scanning tunneling microscopy at the 1,2,4-trichlorobenzene/Au(111) interface, we observed selective self-assembly of 1,3,5-tris(4-methoxyphenyl)benzene (TMPB) molecules in the face-centered cubic (FCC) regions of Au(111). Density functional theory calculations suggest higher adsorption energy of TMPB molecules at FCC regions, explaining the preference for self-assembly. The molecular coverage is found to increase with the concentration of the applied solution, eventually yielding a full monolayer. Moreover, the adsorption of TMPB molecules induces a concentration-dependent lifting of the herringbone reconstruction, observed as an increase in the area of the FCC regions at higher concentrations. Our results represent a simple and cost-effective selective nanoscale patterning method on Au(111), providing a possible avenue to guide the co-adsorption of other functional molecules
Probing the Thermodynamics of Moiré Patterns in Molecular Self-Assembly at the Liquid–Solid Interface
No full textThree types of moiré patterns, denoted as α-, β-, and γ-patterns, were observed in the molecular self-assembly of 1,3,5-tris(4-cyanophenyl)benzene (TCPB) on HOPG. Their relative stability was studied through thermally induced phase transitions monitored in situ by scanning tunneling microscopy. The incommensurate γ-pattern is thermodynamically more stable than the commensurate α- and β-patterns. The preference in the γ-pattern was explained by a static distortion wave, which was exclusively observed in this pattern. Through the lateral relaxation of TCPB lattice, the free energy of the γ-pattern can be reduced with respect to that of the α- and β-moiré patterns. This investigation provides insights into the thermodynamics of surface-confined supramolecular systems where the molecular lattice is incongruent with the substrate
Tandem Desulfurization/C–C Coupling Reaction of Tetrathienylbenzenes on Cu(111): Synthesis of Pentacene and an Exotic Ladder Polymer
No full textSurface-confined reactions represent a powerful approach for the precise synthesis of low-dimensional organic materials. A complete understanding of the pathways of surface reactions would enable the rational synthesis of a wide range of molecules and polymers. Here, we report different reaction pathways of tetrathienylbenzene (T1TB) and its extended congener tetrakis(dithienyl)benzene (T2TB) on Cu(111), investigated using scanning tunneling microscopy, X-ray photoelectron spectroscopy, and density functional theory calculations. Both T1TB and T2TB undergo desulfurization when deposited on Cu(111) at room temperature. Deposition of T1TB at 453 K yields pentacene through desulfurization, hydrogen transfer, and a cascade of intramolecular cyclization. In contrast, for T2TB the intramolecular cyclization stops at anthracene and the following intermolecular C–C coupling produces a conjugated ladder polymer. We show that tandem desulfurization/C–C coupling provides a versatile approach for growing carbon-based nanostructures on metal surfaces
Molecular self-assembly at the solid-liquid interface by scanning tunneling microscopy and density functional theory.
Get PDFMolecular self-assembly on surfaces is a versatile bottom-up approach to producing complex nanostructures with desirable optoelectronic properties and device applications. However, the precise fabrication of nanostructures through self-assembly is challenging due to complex factors that determine the final assembly formed. In this thesis, we study the self-assembly of organic molecules at solid-liquid interfaces using scanning tunneling microscopy (STM) and density functional theory (DFT) to gain a deeper understanding of the factors that affect the nanostructures produced.
Molecular self-assembly at solid-liquid interfaces has gained interest as a straightforward and low-cost method to produce molecular nanostructures. At a solution-Au(111) interface, we observed selective self-assembly of 1,3,5-tris(4-methoxyphenyl)benzene (TMPB) molecules in the FCC regions of Au(111) using STM. Higher adsorption energy of TMPB molecules at FCC regions is indicated by DFT calculations, which rationalizes the preference for self-assembly. The molecular coverage is observed to increase with the concentration of the applied solution. In addition, the adsorption of TMPB molecules results in a concentration-dependent lifting of the herringbone reconstruction. Our results show a straightforward and cost-effective selective nanoscale patterning method on Au(111), which could enable an avenue to direct the co-adsorption of other functional molecules.
Controlling the self-assembly of hydrogen-bonded (H-bonded) N-unsubstituted diketopyrrolopyrroles (DPP) molecules, a commonly used organic semiconductor, on the surface is pivotal for their applications in devices. We studied the self-assembly of diselenophenylDPP (DSeDPP), dithiazolylDPP (DTzDPP), and dithienothiophenylDPP (DTTDPP) at solid-liquid interfaces using STM and DFT. We observed that the three DPP molecules either co-assemble with the solvent or form homoasemblies at solution-highly ordered pyrolytic graphite interfaces, and these observations were rationalized using DFT simulations. Homoassemblies are formed by planar molecules or molecules with out-of-plane twists, depending on the strain in obtaining the twisted conformers. Our results show that the (hetero)atoms in the aromatic ring and the number of aromatic rings have a decisive effect on the DPP self-assembly, which could provide insights into tuning their film morphology and its properties (e.g. transport properties).
In summary, the molecular self-assembly studies at solid-liquid interfaces demonstrated the role substrate reconstructions and molecular structures play in the final assembly formed. Our fundamental studies provided strategies to tailor molecular self-assemblies on surfaces, which are relevant in optimizing the device performance. L'auto-assemblage moléculaire sur des surfaces est une approche ascendante polyvalente pour produire des nanostructures complexes avec les propriétés optoélectroniques désirées pour le développement de dispositifs, par exemple. Cependant, la fabrication précise de nanostructures par auto-assemblage est un défi en raison des facteurs complexes qui déterminent la forme de l'assemblage final. Dans cette thèse, nous étudions l'auto-assemblage de molécules organiques aux interfaces solide-liquide en utilisant la microscopie à effet tunnel (STM) et la théorie de la fonctionnelle de la densité (DFT) pour mieux comprendre les facteurs qui affectent les nanostructures produites. L'auto-assemblage moléculaire aux interfaces solide-liquide suscite un intérêt croissant comme méthode simple et peu coûteuse pour produire des nanostructures moléculaires. À l'interface solution-Au(111), nous avons observé un auto-assemblage sélectif de molécules de 1,3,5-tris(4-méthoxyphényl)benzène (TMPB) dans les régions cubiques à faces centrées (FCC) de Au(111) à l'aide de la STM. L'énergie d'adsorption plus élevée des molécules de TMPB dans les régions FCC est indiquée par les calculs de DFT, ce qui justifie la préférence pour l'auto-assemblage. On observe que la couverture moléculaire augmente avec la concentration de la solution appliquée. En outre, l'adsorption de molécules de TMPB induit un soulèvement de la reconstruction en chevrons qui dépend de la concentration. Nos résultats montrent une méthode simple et rentable de structuration sélective à l'échelle nanométrique sur Au(111), qui pourrait permettre de diriger la co-adsorption d'autres molécules fonctionnelles. Le contrôle de l'auto-assemblage de molécules de dicétopyrrolopyrroles (DPP) N-non substituées avec liason hydrogène (liaison H), un semi-conducteur organique couramment utilisé, sur la surface est essentiel pour leurs applications dans des dispositifs. Nous avons étudié l'auto-assemblage du disélénophénylDPP (DSeDPP), du dithiazolylDPP (DTzDPP) et du dithiénothiophénylDPP (DTTDPP) aux interfaces solide-liquide à l'aide de la STM et de la DFT. Nous avons observé que les trois molécules de DPP s'assemblent avec le solvant ou forment des homo-assemblages aux interfaces entre la solution et le graphite pyrolytique hautement ordonné. Ces observations ont été rationalisées à l'aide de simulations DFT. Les homo-assemblages sont formés de molécules planaires ou de molécules présentant des torsions hors plan, dépendemment de la contrainte exercée pour obtenir les conformères tordus. Nos résultats montrent que les (hétéro)atomes de l'anneau aromatique et le nombre d'anneaux aromatiques ont un effet important sur l'auto-assemblage des DPP, ce qui pourrait permettre d'ajuster la morphologie du film et ses propriétés (par exemple, les propriétés de transport).En résumé, les études d'auto-assemblage moléculaire aux interfaces solide-liquide ont permis de démontrer le rôle que jouent les reconstructions du substrat et les structures moléculaires dans l'assemblage final formé. Nos études fondamentales ont fourni des stratégies pour adapter les auto-assemblages moléculaires sur les surfaces, ce qui est important pour optimiser les performances des dispositifs. <br /
Dynamical evolution of Ge quantum dots on Si(111): from island formation to high temperature decay
Get PDFHeteroepitaxial growth is a process of profound fundamental importance as well as an avenue to realize nanostructures such as Ge/Si quantum dots (QDs), with appealing properties for applications in opto- and nanoelectronics. However, controlling the Ge/Si QD size, shape, and composition remains a major obstacle to their practical implementation. Here, Ge nanostructures on Si(111) were investigated in situ and in real-time by low energy electron microscopy (LEEM), enabling the observation of the transition from wetting layer formation to 3D island growth and decay. The island size, shape, and distribution depend strongly on the growth temperature. As the deposition temperature increases, the islands become larger and sparser, consistent with Brownian nucleation and capture dynamics. At 550 degrees C, two distinct Ge/Si nanostructures are formed with bright and dark appearances that correspond to flat, atoll-like and tall, faceted islands, respectively. During annealing, the faceted islands increase in size at the expense of the flat ones, indicating that the faceted islands are thermodynamically more stable. In contrast, triangular islands with uniform morphology are obtained from deposition at 600 degrees C, suggesting that the growth more closely follows the ideal shape. During annealing, the islands formed at 600 degrees C initially show no change in morphology and size and then rupture simultaneously, signaling a homogeneous chemical potential of the islands. These observations reveal the role of dynamics and energetics in the evolution of Ge/Si QDs, which can serve as a step towards the precise control over the Ge nanostructure size, shape, composition, and distribution on Si(111)
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Get PDF“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
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe 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
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe 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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