1,721,019 research outputs found
Synthesis of Large-Area Single-Crystal Graphene
Full text linkThere have been breakthroughs in the mass production of graphene by chemical vapor deposition (CVD) and its practical applications have also been identified. Grain boundaries are typically present in 'CVD graphene' and adversely impact its properties. We summarize recent progress in growing large-area single crystal graphene. Centimeter-scale single-crystal, truly single-layer graphene (SLG) films have been reportedly achieved on single-crystal Cu(111) foils by CVD growth, while meter-scale single-crystal SLG films have been reportedly produced with assistance of a roll-to-roll technique. The growth of uniform single crystals of bilayer or multilayer graphene over a large area remains an exciting challenge. Layer-by-layer transfer and the stacking of single-crystal SLG is considered a promising route to making new types of 'single' crystals or quasicrystals with specific numbers of layers and different stacking angles
Repeatability and Reproducibility in the Chemical Vapor Deposition of 2D Films:A Physics-Driven Exploration of the Reactor Black Box
No full textAlthough chemical vapor deposition (CVD) remains the method of choice for synthesizing defect-free and high-quality 2D films (such as graphene and h-BN), the method has serious issues with process repeatability and reproducibility. This makes it difficult to build up from the literature, test a hypothesis quickly, or scale up a process. The primary reason for this is that the CVD reactor, to this day, remains a black box with a reaction environment that is poorly understood and cannot be measured or monitored directly. Consequently, it is also difficult to study process kinetics and growth mechanisms and correlate experimental results to atomic-level simulations. A possible way to overcome this problem is to use Computational Fluid Dynamics (CFD), both to identify the measurable external (process and reactor) parameters that control the reaction environment and to simulate this reaction environment and understand how it changes when these controllable external parameters are varied. This paper describes how this may be done in practice using the growth of single-layer graphene in a hot-wall tube reactor as the representative case and the CFD toolbox OpenFOAM. Based on our findings, we have shown why it is critical (1) to understand the flow properties inside the reactor and combine it with experimental results to study the growth process for graphene and other 2D films and (2) to measure, monitor, and report all relevant external parameters to ensure process repeatability and reproducibility
Encoding Enantiomeric Molecular Chiralities on Graphene Basal Planes
No full textGraphene has demonstrated broad applications due to its prominent properties. Its molecular structure makes graphene achiral. Here, we propose a direct way to prepare chiral graphene by transferring chiral structural conformation from chiral conjugated amino acids onto graphene basal plane through π–π interaction followed by thermal fusion. Using atomic resolution transmission electron microscopy, we estimated an areal coverage of the molecular imprints (chiral regions) up to 64 % on the basal plane of graphene (grown by chemical vapor deposition). The high concentration of molecular imprints in their single layer points to a close packing of the deposited amino acid molecules prior to “thermal fusion”. Such “molecular chirality-encoded graphene” was tested as an electrode in electrochemical enantioselective recognition. The chirality-encoded graphene might find use for other chirality-related studies and the encoding procedure might be extended to other two-dimensional materials. © 2022 Wiley-VCH GmbH.11Nsciescopu
Reference-free THz-TDS conductivity analysis of thin conducting films
Full text linkWe present a reference-free method to determine electrical parameters of thin conducting films by steady state transmission-mode terahertz time-domain spectroscopy (THzTDS). We demonstrate that the frequency-dependent AC conductivity of graphene can be acquired by comparing the directly transmitted THz pulse with a transient internal reflection within the substrate which avoids the need for a standard reference scan. The DC sheet conductivity, scattering time, carrier density, mobility, and Fermi velocity of graphene are retrieved subsequently by fitting the AC conductivity with the Drude model. This reference-free method was investigated with two complementary THz setups: one commercial fibre-coupled THz spectrometer with fast scanning rate (0.2-1.5 THz) and one air-plasma based ultra-broadband THz spectrometer for greatly extended frequency range (2-10 THz). Certain propagation correction terms for more accurate retrieval of electrical parameters are discussed
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
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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