1,964 research outputs found
The unique chemical reactivity of a graphene nanoribbon's zigzag edge
The zigzag edge of a graphene nanoribbon possesses a unique electronic state
that is near the Fermi level and localized at the edge carbon atoms. We
investigate the chemical reactivity of these zigzag edge sites by examining
their reaction energetics with common radicals from first principles. A
"partial radical" concept for the edge carbon atoms is introduced to
characterize their chemical reactivity, and the validity of this concept is
verified by comparing the dissociation energies of edge-radical bonds with
similar bonds in molecules. In addition, the uniqueness of the zigzag-edged
graphene nanoribbon is further demonstrated by comparing it with other forms of
sp2 carbons, including a graphene sheet, nanotubes, and an armchair-edged
graphene nanoribbon.Comment: 24 pages, 9 figure
Electronic Ground State of Higher Acenes
We examine the electronic ground state of acenes with different number of
fused benzene rings (up to 40) by using first principles density functional
theory. Their properties are compared with those of infinite polyacene. We find
that the ground state of acenes that consist of more than seven fused benzene
rings is an antiferromagnetic (in other words, open-shell singlet) state, and
we show that this singlet is not necessarily a diradical, because the spatially
separated magnetizations for the spin-up and spin-down electrons increase with
the size of the acene. For example, our results indicate that there are about
four spin-up electrons localized at one zigzag edge of 20-acene. The reason
that both acenes and polyacene have the antiferromagnetic ground state is due
to the zigzag-shaped boundaries, which cause pi-electrons to localize and form
spin orders at the edges. Both wider graphene ribbons and large
rectangular-shaped polycyclic aromatic hydrocarbons have been shown to share
this antiferromagnetic ground state. Therefore, we demonstrate that the
pi-electronic structure of higher acenes and ployacene are still dictated by
the zigzag edges, and our results provide a consistent description of their
electronic ground state.Comment: revised: corrected some errors, rephrased some discussions, and added
a reference (Ref. 29); 19 pages, 6 figure
Au40: A Large Tetrahedral Magic Cluster
40 is a magic number for tetrahedral symmetry predicted in both nuclear
physics and the electronic jellium model. We show that Au40 could be such a a
magic cluster from density functional theory-based basin hopping for global
minimization. The putative global minimum found for Au40 has a twisted pyramid
structure, reminiscent of the famous tetrahedral Au20, and a sizable HOMO-LUMO
gap of 0.69 eV, indicating its molecular nature. Analysis of the electronic
states reveals that the gap is related to shell closings of the metallic
electrons in a tetrahedrally distorted effective potential.Comment: 5 pages, 5 figures, phys. rev. b, in pres
Simulating the Initial Stage of Phenolic Resin Carbonization via the ReaxFF Reactive Force Field
Pyrolysis of phenolic resins leads to carbon formation. Simulating this resin-to-carbon process atomistically is a daunting task. In this paper, we attempt to model the initial stage of this process by using the ReaxFF reactive force field, which bridges quantum mechanical and molecular mechanical methods. We run molecular dynamics simulations to examine the evolution of small molecules at different temperatures. The main small-molecule products found include H_2O, H_2, CO, and C_2H_2. We find multiple pathways leading to H_2O formation, including a frequent channel via β-H elimination, which has not been proposed before. We determine the reaction barrier for H_2O formation from the reaction rates obtained at different temperatures. We also discuss the relevance of our simulations to previous experimental observations. This work represents a first attempt to model the resin-to-carbon process atomistically
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