Role of Vacancies in Zigzag Graphene Nanoribbons: An Ab Initio Study (original) (raw)
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Effects of vacancies on spin-dependent behavior of monolayer and bilayer graphene nanoribbons
Journal of Magnetism and Magnetic Materials, 2017
In this work, the effect of vacancies on magnetic properties and spin-dependent behaviors of monolayer and bilayer armchair and zigzag graphene nanoribbons is investigated using first principles calculations based on density functional theory (DFT). The armchair and zigzag graphene nanoribbons are composed of 6 rows and 4 rows of carbon atoms with the edges closed by the hydrogen atoms, respectively. Our results show that vacancies affect the magnetic properties and spin polarization of the graphene nanoribbons. It is seen that the monolayer armchair graphene nanoribbon with one vacancy in its supercell (24 carbon sites + 8 hydrogen sites) gives the magnetic moment of 0.79 µ B , while magnetic moment in the monolayer zigzag graphene nanoribbon with one vacancy in its supercell (24 carbon sites + 6 hydrogen sites) is 1.72 µ B (for site α) and 1.84 µ B (for site β). The highest and lowest values of magnetic moment in different configurations of the bilayer armchair (zigzag) graphene nanoribbons with one vacancy in each layer of the supercell give 1.54 µ B and 1.29 µ B (3.51 µ B and 2.72 µ B), respectively. Numerical values of the magnetic moment in different configurations depended on the distance of vacancies from each other and from nanoribbon's edge as well as their orientations.
Electronic Properties of Zigzag Graphene Nanoribbons Studied by TAO-DFT
Accurate prediction of the electronic properties of zigzag graphene nanoribbons (ZGNRs) has been very challenging for conventional electronic structure methods due to the presence of strong static correlation effects. To meet the challenge, we study the singlet-triplet energy gaps, vertical ionization potentials, vertical electron affinities, fundamental gaps, and symmetrized von Neumann entropy (i.e., a measure of polyradical character) of hydrogen-terminated ZGNRs with different widths and lengths using our recently developed thermally-assisted-occupation density functional theory (TAO-DFT) [Chai, J.-D. J. Chem. Phys. 2012, 136, 154104], a very efficient method for the study of large strongly correlated systems. Our results are in good agreement with the available experimental and high-accuracy ab initio data. The ground states of ZGNRs are shown to be singlets for all the widths and lengths investigated. With the increase of ribbon length, the singlet-triplet energy gaps, vertical ionization potentials, and fundamental gaps decrease monotonically, while the vertical electron affinities and symmetrized von Neumann entropy increase monotonically. On the basis of the calculated orbitals and their occupation numbers, the longer ZGNRs are shown to possess increasing polyradical character in their ground states, where the active orbitals are mainly localized at the zigzag edges.
Electronic and magnetic structure of graphene nanoribbons
This review covers some of the basic theoretical aspects of the electronic and magnetic structure of graphene nanoribbons, starting from the simplest tight-binding models to the more sophisticated ones where the electron-electron interactions are considered at various levels of approximation. Nanoribbons can be classified into two basic categories, armchair and zigzag, according to their edge termination, which determines profoundly their electronic structure. Magnetism, as a result of the interactions, appears in perfect zigzag ribbons as well as in armchair ribbons with vacancies and defects of different types.
Carrier density and magnetism in graphene zigzag nanoribbons
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The influence of carrier density on magnetism in a zigzag graphene nanoribbon is studied in a pi\pipi-orbital Hubbard-model mean-field approximation. Departures from half-filling alter the magnetism, leading to states with charge density variation across the ribbon and parallel spin-alignment on opposite edges. Finite carrier densities cause the spin-density near the edges to decrease steadily, leading eventually to the absence of magnetism. At low doping densities the system shows a tendency to multiferroic order in which edge charges and spins are simultaneously polarized.
The effects of vacancies on the transport properties of zigzag graphene nanoribbons
2010 IEEE Nanotechnology Materials and Devices Conference, 2010
The transport properties of zigzag graphene nanoribbons (ZGNRs) with different patterns of vacancies are investigated by using density functional theory and nonequilibrium Green's function (NEGF) formalism. It is found that the transport properties are different with a different lattice type vacancy (A-type or B-type vacancy). The conductance of ZGNRs is more sensitive to an interior vacancy than an edge vacancy. More importantly, the pattern of interior vacancies has enormous influence on the electron transport around the Femi energy. As hexagon carbons are removed, the ZGNRs transform from metallic to semiconducting. Thus one can tune the electron properties of ZGNRs by patterning vacancies.
Acta Physica Polonica A
Using first-principles calculations we have demonstrated that electronic and magnetic properties of armchair graphene nanoribbons are modified by introducing vacancies defects. The equilibrium geometries, electronic, charge spin density distributions, electronic band structures, and magnetic moments were examined in the presence of vacancies. We have found that introducing vacancies into armchair graphene nanoribbons changes the spatial distribution of neighbor atoms, particularly those located around the vacancies. Our calculations showed that the vacancies have significant effect on the magnetization of armchair graphene nanoribbons. Magnetic moment values and electronic behavior in different configurations depend on the number of vacancies. These results suggest that vacancy defects can be used to modify the electronic and the magnetic properties of armchair graphene nanoribbons.
Edge reconstructions induce magnetic and metallic behavior in zigzag graphene nanoribbons
Carbon, 2010
The edge reconstructions of zigzag graphene nanoribbons with one and two lines of alternating fused five and seven membered rings along one edge with hydrogen passivation are studied using first principles density functional theory. Reconstructions on one edge stabilize the systems in a metallic ground state with finite magnetic moment. The reconstructed edge suppresses the local spin density of atoms and contributes a finite density of states at the Fermi energy. Our study shows the possibilities of fabricating the metallic electrodes for semiconducting graphene devices with full control over their magnetic behavior without any lattice mismatch between leads and the channel.
Edge-functionalized and substitutionally doped graphene nanoribbons: Electronic and spin properties
Physical Review B, 2008
Graphene nanoribbons are the counterpart of carbon nanotubes in graphene-based nanoelectronics. We investigate the electronic properties of chemically modified ribbons by means of density functional theory. We observe that chemical modifications of zigzag ribbons can break the spin degeneracy. This promotes the onset of a semiconducting-metal transition, or of a half-semiconducting state, with the two spin channels having a different band gap, or of a spin-polarized half-semiconducting state, where the spins in the valence and conduction bands are oppositely polarized. Edge functionalization of armchair ribbons gives electronic states a few eV away from the Fermi level and does not significantly affect their band gap. N and B produce different effects, depending on the position of the substitutional site. In particular, edge substitutions at low density do not significantly alter the band gap, while bulk substitution promotes the onset of semiconducting-metal transitions. Pyridinelike defects induce a semiconducting-metal transition.
A theoretical study of chemical doping and width effect on zigzag graphene nanoribbons
Physica E: Low-dimensional Systems and Nanostructures, 2009
The energetics and the electronic properties of nitrogen-and boron-doped graphene nanoribbons with zigzag edges have been investigated using density functional theory calculations. For the optimized geometry configurations, vibrational frequency analysis and wavefunction stability tests have been carried out. Different doping site optimizations for a model nanoribbon have been performed and formation energy values of these sites revealed that zigzag edgesite for both of the dopants were the most favorable one. The effect of doping on the molecular orbital energies, HOMO-LUMO distributions, and density of states have been studied as well. It is found that molecular orbital distributions for pure zigzag nanoribbons are located in the zigzag edges while they exhibit different behaviors for the doped cases.
Applied Physics A, 2013
Electronic and transport properties of 11-zigzag graphene nanoribbons (11-z-GNRs) with two types of 3D paired pentagon-heptagon defects (3D-PPHD) are studied by using density functional theory combined with non-equilibrium Green's function method. The C ad-dimmers that have been introduced to z-GNRs to form these 3D-PPHDs, have induced local strains forcing the C-bonds in the ad-dimmers to hybridized in sp 3 -like rather than sp 2 -like orbitals.