Graphene nanoflakes - structural and electronic properties

Physical Review B (Impact Factor: 3.74). 02/2013; 81:085430. DOI: 10.1103/PhysRevB.81.085430
Source: arXiv


The structures, cohesive energies and HOMO-LUMO gaps of graphene nanoflakes
and corresponding polycyclic aromatic hydrocarbons for a large variety of size
and topology are investigated at the density functional based tight-binding
level. Polyacene-like and honeycomb-like graphene nanoflakes were chosen as the
topological limit structures. The influence of unsaturated edge atoms and
dangling bonds on the stability is discussed. Our survey shows a linear trend
for the cohesive energy as function of Ns/N (N - total number of atoms and Ns
is number of edge atoms). For the HOMO-LUMO gap the trends are more complex and
include also the topology of the edges.

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    • "They are considered to be among the most stable topologies, while the least stable structures are the narrow graphene nanoribbons [5]. GNDs have been studied theoretically by classical molecular dynamics [37] and quantum mechanical methods [5] [38], and experimentally they were investigated by using transmission electron microscopy (TEM) and electron diffraction [39]. Carbon nanocones (CNCs) are curved graphene sheets with the addition of pentagons at the cone tips. "
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    ABSTRACT: We investigate the relative energetic stability of a variety of nanographene structures such as graphene nanoflakes, nanoribbons, nanodisks, and nanocones. We calculate the cohesive energies with respect to hydrogen passivation, edge nature (zigzag versus armchair) and shape (triangular, rectangular, hexagonal). The cohesive energy is confirmed to increase with size for all these structures. We pay particular attention to optimally-compact circular flakes and compare our theoretical results with round disks produced in a plasma torch atmosphere. We find in the calculations that round shape does not have preferred relative stability. This suggests that the observed disks are grown under conditions where carbon atoms are highly mobile. For graphene nanocones we obtain a similar result. Experimentally, the open base of a 19-degree-cone is observed perpendicular to the cone axis, but this does not correspond to the most stable configuration as obtained by the calculations. Instead, we find that both, disks and cones, prefer minimal length of the edge termination rather than a maximum in the cohesive energy. With respect to our results we discuss for polycyclic aromatic hydrocarbons (PAH) and atomic clusters, as models for graphene flakes, the significance of the cohesive energy for the observed abundances.
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    ABSTRACT: We investigate quantum transport properties of triangular graphene flakes with zigzag edges by using first principles calculations. Triangular graphene flakes have large magnetic moments which vary with the number of hydrogen atoms terminating its edge atoms and scale with its size. Electronic transmission and current-voltage characteristics of these flakes, when contacted with metallic electrodes, reveal spin valve and remarkable rectification features. The transition from ferromagnetic to antiferromagnetic state under bias voltage can, however, terminate the spin polarizing effects for specific flakes. Geometry and size dependent transport properties of graphene flakes may be crucial for spintronic nanodevice applications. Comment: 5 pages, 4 Figures
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    ABSTRACT: We investigate the effect of N/B doping on the electronic properties for a zero-dimensional zigzag-edged triangular graphene, wherein two sets of sublattices are unbalanced, using density functional theory (DFT). We find that the substitutional N/B atom energetically prefers to distribute in the major sublattice. After the N/B doping, the net spin for triangular graphene is reduced and full or partial depolarization occurs depending on doping sites. Our DFT calculations show that the triangular graphene with N/B doped in the major sublattice has a larger energy gap, and the electronic properties depend on the doping position. There is an impurity state below or above the Fermi level for the N/B-doped triangular graphene, depending on the sublattice at which the dopant locates. The dependence of the electronic properties on doping position is attributed to the competition between the Coulomb attraction of N(+) (B(-)) and the correlation with nonbonding states for the extra charge introduced by the N/B atom.
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