1,721,008 research outputs found
Self-passivation leads to semiconducting edges of black phosphorene
No full textThe edges of black phosphorene (BP) have been extensively explored. The previous experimental observations that all the BP edges are semiconducting implies that the as-cut edges of BP tend to be reconstructed. Here we present a global structural search of three typical BP edges, namely armchair, zigzag and zigzag-1 edges. It is found that all the three pristine edges are metastable, and all of them can be quickly self-passivated by (i) forming P=P double bonds (one r and one p bond), (ii) reconstructing new polygonal rings will all P atoms bonded with three sp3 bonds or (iii) forming a special P(2)-P(4) configuration with a two-coordinated P atom accommodating two lone pair electrons and one four-coordinated P atom without lone pair electrons. Highly different from the pristine edges, all these highly stable reconstructed edges are semiconducting. This study showed that the reconstruction of the edges of a 2D material, just like the surfaces of a 3D crystal, must be considered for both fundamental studies and practical applications. Besides BP, this study also sheds light on the structures and properties of the edges of many other 2D materials.11Nsciescopu
Mechanism of 2D Materials??? Seamless Coalescence on a Liquid Substrate
No full textThe seamless coalescence of parallelly aligned 2D materials is the primary route toward the synthesis of wafer-scale single crystals (WSSCs) of 2D materials. The epitaxial growth of various 2D materials on a single-crystal substrate, which is an essential condition of the seamless coalescence approach, has been extensively explored in previous studies. Here, by using hexagonal boron nitride (hBN) growth on a liquid gold surface as an example, we demonstrate that growth of WSSCs of 2D materials via the seamless coalescence of self-aligned 2D islands on a liquid substrate is possible. Here we show that, in the presence of hydrogen, all the hBN edges tend to be hydrogen terminated and the coalescence of hBN islands occurs only if their crystallographic lattices of neighboring hBN islands are aligned parallelly. The mechanism of hBN self-alignment revealed in this study implies that, under the optimum experimental condition, the seamless coalescence of 2D materials on a liquid substrate is possible and thus provides guidance for synthesizing WSSCs of various 2D materials by using liquid phase substrates
Mechanism of MoS2 Growth on a Au(111) Surface: An Ab Initio Molecular Dynamics Study
No full textChemical vapor deposition (CVD) of transition-metal dichalcogenide (TMD) thin films, such as MoS2, on a gold (Au) surface has been regarded as one of the most promising approaches for the mass production of high-quality TMD thin films. However, the mechanism of TMD CVD growth on a gold surface remains a mystery, and many experimental observations, such as the surface chemistry during the initial stage of TMD growth and the formation of T-phase MoS2 on a Au surface, remain unclear. In this study, we systematically explored the initial stage of MoS2CVD growth on a Au(111) surface by using density functional theory-based molecular dynamics simulations. Some critical steps of MoS2 growth, such as the sulfidation of MoO3, the passivation of the Au(111) surface in the S-rich environment, and the lifting of Mo atoms from the Au substrate to form stable MoS2 nuclei, have been revealed in our atomic simulations. The theoretically predicted most stable T-phase small MoS2 clusters agree well with the previous experimental observations. Therefore, with an increase in the size of MoS2, a phase transition from the T phase to the H phase is essential for the growth of highly stable H-phase MoS2 films. This study greatly deepens our understanding of the mechanism of TMD CVD growth on a Au surface and provides guidance for the controllable CVD synthesis of various TMDs
Atomically Sharp, Closed Bilayer Phosphorene Edges by Self-Passivation
Full text linkTwo-dimensional (2D) crystals' edge structures not only influence their
overall properties but also dictate their formation due to edge-mediated
synthesis and etching processes. Edges must be carefully examined because they
often display complex, unexpected features at the atomic scale, such as
reconstruction, functionalization, and uncontrolled contamination. Here, we
examine atomic-scale edge structures and uncover reconstruction behavior in
bilayer phosphorene. We use in situ transmission electron microscopy (TEM) of
phosphorene/graphene specimens at elevated temperatures to minimize surface
contamination and reduce e-beam damage, allowing us to observe intrinsic edge
configurations. Bilayer zigzag (ZZ) edge was found the most stable edge
configuration under e-beam irradiation. Through first-principles calculations
and TEM image analysis under various tilting and defocus conditions, we find
that bilayer ZZ edges undergo edge reconstruction and so acquire closed,
self-passivated edge configurations. The extremely low formation energy of the
closed bilayer ZZ edge and its high stability against e-beam irradiation are
confirmed by first-principles calculations. Moreover, we fabricate bilayer
phosphorene nanoribbons with atomically-sharp closed ZZ edges. The identified
bilayer ZZ edges will aid in the fundamental understanding of the synthesis,
degradation, reconstruction, and applications of phosphorene and related
structures.Comment: 22 pages, 5 figure
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
Why Carbon Nanotubes Grow
No full text© 2022 American Chemical Society.Despite three decades of intense research efforts, the most fundamental question "why do carbon nanotubes grow?"remains unanswered. In fact, carbon nanotubes (CNTs) should not grow since the encapsulation of a catalyst with graphitic carbon is energetically more favorable than CNT growth in every aspect. Here, we answer this question using a theoretical model based on extensive first-principles and molecular dynamics calculations. We reveal a historically overlooked yet fundamental aspect of the CNT-catalyst interface, viz., that the interfacial energy of the CNT-catalyst edge is contact angle-dependent. The contact angle increases via graphitic cap lift-off, drastically decreasing the interfacial formation energy by up to 6-9 eV/nm, overcoming van der Waals cap-catalyst adhesion, and driving CNT growth. Mapping this remarkable and simple interplay allows us to understand, for the first time, why CNTs grow.11Nsciescopu
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
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
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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