59 research outputs found
Tuning the Dirac Cone of Bilayer and Bulk Structure Graphene by Intercalating First Row Transition Metals using First Principles Calculations
Modern nanoscience has focused on two-dimensional (2D) layer structure
materials which have garnered tremendous attention due to their unique
physical, chemical and electronic properties since the discovery of graphene in
2004. Recent advancement in graphene nanotechnology opens a new avenue of
creating 2D bilayer graphene (BLG) intercalates. Using first-principles DFT
techniques, we have designed 20 new materials \textit{in-silico} by
intercalating first row transition metals (TMs) with BLG, i.e. 10 layered
structure and 10 bulk crystal structures of TM intercalated in BLG. We
investigated the equilibrium structure and electronic properties of layered and
bulk structure BLG intercalated with first row TMs (Sc-Zn). The present DFT
calculations show that the 2 sub-shells of C atoms in graphene and the
3 sub-shells of the TM atoms provide the electron density near the
Fermi level controlling the material properties of the BLG-intercalated
materials. This article highlights how the Dirac point moves in both the BLG
and bulk-BLG given a different TM intercalated materials. The implications of
controllable electronic structure and properties of intercalated BLG-TM for
future device applications are discussed. This work opens up new avenues for
the efficient production of two-dimensional and three-dimensional carbon-based
intercalated materials with promising future applications in nanomaterial
science.Comment: 60 pages, 9 figures. arXiv admin note: text overlap with
arXiv:1701.03936 by other author
Iron Intercalation in Covalent-Organic Frameworks: A Promising Approach for Semiconductors
Covalent-organic frameworks (COFs) are intriguing platforms for designing
functional molecular materials. Here, we present a computational study based on
van der Waals dispersion-corrected hybrid density functional theory (DFT-D) to
design boroxine-linked and triazine-linked COFs intercalated with Fe. Keeping
the original symmetry of the pristine COF (COF-Fe-0), we have
computationally designed seven new COFs by intercalating Fe atoms between two
organic layers. The equilibrium structures and electronic properties of both
the pristine and Fe-intercalated COF materials are investigated here. We
predict that the electronic properties of COFs can be fine tuned by adding Fe
atoms between two organic layers in their structures. Our calculations show
that these new intercalated-COFs are promising semiconductors. The effect of Fe
atoms on the electronic band structures and density of states (DOSs) has also
been investigated using the aforementioned DFT-D method. The contribution of
the -subshell electron density of the Fe atoms plays an important role in
improving the semiconductor properties of these new materials. These
intercalated-COFs provide a new strategy to create semi-conducting materials
within a rigid porous network in a highly controlled and predictable manner.Comment: 39 pages. arXiv admin note: text overlap with arXiv:1703.0261
MultiBinding Sites United in Covalent-Organic Frameworks (MSUCOF) for H Storage and Delivery at Room Temperature
The storage of hydrogen gas (H) has presented a significant challenge
that has hindered its use as a fuel source for transportation. To meet the
Department of Energy's ambitious goals of achieving g L volumetric
and wt \% gravimetric uptake targets, materials-based approaches are
essential. Designing materials that can efficiently store hydrogen gas requires
careful tuning of the interactions between the gaseous H and the surface of
the material. Metal-Organic Frameworks (MOFs) and Covalent-Organic Frameworks
(COFs) have emerged as promising materials due to their exceptionally high
surface areas and tunable structures that can improve gas-framework
interactions. However, weak binding enthalpies have limited the success of many
current candidates, which fail to achieve even g L volumetric
uptake at ambient temperatures. To overcome this challenge, We utilized quantum
mechanical (QM) based force fields (FF) to investigate the uptake and binding
enthalpies of 3 linkers chelated with 7 different transition metals (TM),
including both precious metals (Pd and Pt) and first row TM (Co, Cu, Fe, Ni,
Mn), to design 24 different COFs in-silico. By applying QM-based FF with grand
canonical Monte Carlo (GCMC) from 0-700 bar and 298 K, We demonstrated that
Co-, Ni-, Mn-, Fe-, Pd-, and Pt-based MSUCOFs can already achieve the
Department of Energy's hydrogen storage targets for 2025. Surprisingly, the
COFs that incorporated the more affordable and abundant first-row TM often
outperformed the precious metals. This promising development brings us one step
closer to realizing a hydrogen-based energy economy
Exploring Low Internal Reorganization Energies for Silicene Nanoclusters
High-performance materials rely on small reorganization energies to
facilitate both charge separation and charge transport. Here, we performed DFT
calculations to predict small reorganization energies of rectangular silicene
nanoclusters with hydrogen-passivated edges denoted by H-SiNC. We observe that
across all geometries, H-SiNCs feature large electron affinities and highly
stabilized anionic states, indicating their potential as n-type materials. Our
findings suggest that fine-tuning the size of H-SiNCs along the zigzag and
armchair directions may permit the design of novel n-type electronic materials
and spinctronics devices that incorporate both high electron affinities and
very low internal reorganization energies.Comment: 25 pages, 6 figure
Reaction Mechanism of the Selective Reduction of CO to CO by a Tetraaza [CoNH] Complex in the Presence of Protons
The tetraaza [CoNH] complex (\textbf{1}) is remarkable
for its ability to selectively reduce CO to CO with 45\% Faradaic
efficiency and a CO to H ratio of 3:2. We employ density functional theory
(DFT) to determine the reasons behind the unusual catalytic properties of
\textbf{1} and the most likely mechanism for CO reduction. The selectivity
for CO over proton reduction is explained by analyzing the catalyst's
affinity for the possible ligands present under typical reaction conditions:
acetonitrile, water, CO, and bicarbonate. After reduction of the catalyst
by two electrons, formation of [CoNH]-CO is
strongly favored. Based on thermodynamic and kinetic data, we establish that
the only likely route for producing CO from here consists of a protonation step
to yield [CoNH]-COH, followed by reaction with
CO to form [CoNH]-CO and bicarbonate. This
conclusion corroborates the idea of a direct role of CO as a Lewis acid to
assist in {C-O} bond dissociation, a conjecture put forward by other authors to
explain recent experimental observations. The pathway to formic acid is
predicted to be forbidden by high activation barriers, in accordance with the
products that are known to be generated by \textbf{1}. Calculated physical
observables such as standard reduction potentials and the turnover frequency
for our proposed catalytic cycle are in agreement with available experimental
data reported in the literature. The mechanism also makes a prediction that may
be experimentally verified: that the rate of CO formation should increase
linearly with the partial pressure of CO.Comment: 8 pages, 2 figure
Polychrony as Chinampas
We study the flow of signals through paths with the following condition: a
node emits a signal if two incoming signals from other nodes arrive
coincidentally or if it receives an external stimuli. We apply our study to
count and describe families of polychrony groups on a line, and we introduce
triangular sequences.Comment: 32 pages. We refocus our study on nonlinear signal-flow graphs. We
add possible generalizations of our wor
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