1,720,977 research outputs found
Mechanism of alcohol chemical vapor deposition growth of carbon nanotubes: Catalyst oxidation
No full textAlcohol chemical vapor deposition (ACVD) was established as one of the most promising methods for single-walled carbon nanotube (SWCNT) growth almost two decades ago however the mechanisms behind its success remain elusive. To unveil the mechanism of SWCNT growth via ACVD, we employed density functional tight binding molecular dynamics simulations, supplying ethanol to a Fe nanoparticle. Here we demonstrate the oxidation of the Fe catalyst with varying supply rates of ethanol and how the catalyst composition is controlled by the reaction pathways mediated by the hydroxyl OH radical. Following ethanol dissociation on Fe and subsequent O dissolution, the catalyst becomes oxidized and the mobility and availability of Fe to bond with C are reduced. However, SWCNT growth is promoted via the key reaction pathways of the hydroxyl H; controlling the catalyst composition through the formation and release of H2O and H-2. These reaction pathways also demonstrate how active growth species such as ethylene can be formed preferentially to ethane from ethanol dissociation. This work provides important insight into the mechanism of how the catalyst composition changes during ACVD and can be extended to understand the catalyst nature during other O-assisted SWCNT growth processes such as H2O-assisted supergrowth and CO/CO2-promoted growth. (C) 2022 Elsevier Ltd. All rights reserved
High Temperature Accelerated StoneWales Transformation and the Threshold Temperature of IPRC60 Formation
No full textThe Stone-Wales bond rotation isomerization of nonicosahedral C-60 (C-2v-C-60) into isolated-pentagon rule following icosahedral C-60 (I-h-C-60 or IPR-C-60) is a limiting step in the synthesis of I-h-C-60. However, extensive previous studies indicate that the potential energy barrier of the Stone-Wales bond rotation is between 6 and 8 eV, extremely high to allow for bond rotation at the temperatures used to produce fullerenes conventionally. This is also despite data indicating a possible fullerene road mechanism that necessitates low-temperature annealing. However, these previous investigations often have limiting factors, such as using the harmonic approximation to determine free energies at high temperatures or considering only the reverse I-h-C-60 to C-2v-C-60 transition as a basis. Indeed, when the difference in energy between I-h-C-60 and C-2v-C-60 is accounted for, this barrier is generally reduced by similar to 1.5 eV. Thus, utilizing the recently developed density functional tight binding metadynamics (DFTB-MTD) interface, the effects of temperature on the bond rotation in the conversion of C-2v-C-60 to I-h-C-60 have been investigated. We found that Stone-Wales bond rotations are complex processes with both in-plane and out-of-plane transition states, and which transition path dominates depends on temperature. Our results clearly show that at temperatures of 2000 K, the free energy for a C-2v-C-60 to I-h-C-60 transition is only similar to 4.21 eV and further reduces to similar to 3.77 eV at 3000 K. This translates to transition times of similar to 971 mu s at 2000 K and similar to 34 ns at 3000 K, indicating that defect healing is a fast process at temperatures typical of arc jet or laser ablation experiments. Conversely, below similar to 2000 K, bond rotation becomes prohibitively slow, putting a lower threshold limit on the temperature of fullerene formation and subsequent annealing
Multilayer graphene sunk growth on Cu(111) surface
No full textThe controllable growth of multilayer graphene is a challenging research topic. Prior results show graphene adlayers can grow beneath pre-existing graphene layers on a Cu(111). The conventional inverted-wedding-cake (IWC) model used to describe this incurs an energy disadvantage due to deformation in the overlying graphene. We propose an alternative theoretical model, the sunk growth mode, for understanding multilayer graphene growth on Cu substrates. Extensive density functional theory (DFT) calculations show that multilayer graphene grown via this sunk mode is energetically favourable compared to the on-terrace growth mode for Cu(111). These results reveal that graphene underlayers tend to grow in a sunk growth mode, minimizing deformation in the overlayers, reducing deformation energy. Further density functional tight binding-molecular dynamic (DFTBMD) simulations on Cu(111) substrates yield sunken structures consistent with our sunk growth mode. Moreover, AFM investigations of experimentally grown multilayer graphene on polycrystaline Cu show that while friction data indicates multiple graphene edges in the sample, the topological height measurement indicates flat graphitic sheets, further confirming our sunk growth mode. This discovery provides a novel and more reasonable model for the "underlayer" growth of multilayer graphene and can be extended to a general theory for the multilayer graphene growth on various substrates
Role of Graphitic Bowls in Temperature Dependent Fullerene Formation
No full textFullerenes are used extensively in organic electronics as electron acceptors among other uses; however, there are still several key mysteries regarding their formation such as the importance of graphitic intermediates and the thermokinetics of initial cage formation. To this end, we have conducted density functional tight binding molecular dynamics (DFTB-MD) calculations on disintegrated Ih-C60 to investigate the formation mechanisms of fullerenes at high temperature conditions. From the results of these DFTB-MD calculations we were able to develop a thermokinetic model to describe the free energies and kinetics of fullerene formation at a range of temperatures. Direct observation of the mechanism revealed fullerenes readily forming in nanosecond times between 2000 and 3000 K but were hindered above this temperature window. Analysis revealed temperature dependent formation mechanisms where at low temperatures (<2750K) flat graphitic bowls play an important part as metastable intermediates while highly curved bowls follow a direct fast transformation. Meanwhile at higher temperatures (>2750 K), flat bowls become the transitory structure between chains and fullerene. Free energy analysis from our thermokinetic model shows this change in graphitic bowls to being transitory hinders fullerene formation at high temperatures compared to lower temperatures, essentially kinetically trapping C60 as chain networks. This investigation gives new key insights into the formation mechanisms of C60 fullerenes and highlights important intermediates while also illuminating the temperature window for fullerene formation, facilitating better optimization of experimental methods
Catalytic growth of ultralong graphene nanoribbons on insulating substrates
Full text linkGraphene nanoribbons (GNRs) with widths of a few nanometres are promising
candidates for future nano-electronic applications due to their structurally
tunable bandgaps, ultrahigh carrier mobilities, and exceptional stability.
However, the direct growth of micrometre-long GNRs on insulating substrates,
which is essential for the fabrication of nano-electronic devices, remains an
immense challenge. Here, we report the epitaxial growth of GNRs on an
insulating hexagonal boron nitride (h-BN) substrate through
nanoparticle-catalysed chemical vapor deposition (CVD). Ultra-narrow GNRs with
lengths of up to 10 {\mu}m are synthesized. Remarkably, the as-grown GNRs are
crystallographically aligned with the h-BN substrate, forming one-dimensional
(1D) moir\'e superlattices. Scanning tunnelling microscopy reveals an average
width of 2 nm and a typical bandgap of ~1 eV for similar GNRs grown on
conducting graphite substrates. Fully atomistic computational simulations
support the experimental results and reveal a competition between the formation
of GNRs and carbon nanotubes (CNTs) during the nucleation stage, and van der
Waals sliding of the GNRs on the h-BN substrate throughout the growth stage.
Our study provides a scalable, single-step method for growing micrometre-long
narrow GNRs on insulating substrates, thus opening a route to explore the
performance of high-quality GNR devices and the fundamental physics of 1D
moir\'e superlattices
Theory of sigma bond resonance in flat boron materials
Full text linkHere, the authors present a resonance theory to describe the bonding configuration of flat boron materials without quantum calculation. Like aromaticity theory in carbon, it allows to intuitively understand the stability and properties of boron-related material
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
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