Zigzag graphene nanoribbons: bandgap and midgap state modulation (original) (raw)
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Tuning charge and spin excitations in zigzag edge nanographene ribbons
Scientific Reports, 2012
Graphene and its quasi-one-dimensional counterpart, graphene nanoribbons, present an ideal platform for tweaking their unique electronic, magnetic and mechanical properties by various means for potential next-generation device applications. However, such tweaking requires knowledge of the electron-electron interactions that play a crucial role in these confined geometries. Here, we have investigated the magnetic and conducting properties of zigzag edge graphene nanoribbons (ZGNRs) using the many-body configuration interaction (CI) method on the basis of the Hubbard Hamiltonian. For the half-filled case, the many-body ground state shows a ferromagnetic spin-spin correlation along the zigzag edge, which supports the picture obtained from one-electron theory. However, hole doping reduces the spin and charge excitation gap, making the ground state conducting and magnetic. We also provide a two-state model that explains the low-lying charge and spin excitation spectrum of ZGNRs. An experimental setup to confirm the hole-mediated conducting and magnetic states is discussed. C arbon nanomaterials have gained sustained interest in recent times due to their fascinating electronic properties, which arise from electronic confinement in a reduced-length scale. The successful isolation of graphene by mechanical exfoliation 1-3 has provided further impetus and has allowed for the first time the understanding of various properties in a truly two-dimensional context 4-6. The ultrahigh charge carrier mobility, transparency and mechanically flexible properties of graphene provide an excellent platform for futuristic device applications 7. Another intriguing aspect of graphene is the strong nanoscale and edge effects on its electronic and magnetic properties. The quasi-one-dimensional ZGNRs have drawn particular attention because of their peculiar edge ferrimagnetism that arises from the edge-localised states near the zigzag edges 8-10. Intensive studies ranging from mean-field to density functional theory have been performed to understand the carbon-based magnetism in ZGNRs 11-14. A recent experiment using scanning tunnelling spectroscopy (STS) reports the splitting of the density of states due to the edge magnetism for chiral graphene nanoribbons 15. These theoretical and experimental studies have motivated the fabrication of spintronics devices based on graphene nanoribbons. Although its edge-state magnetism seems to contradict the Mermin-Wagner theorem, which rules out the possibility of long-range ordering in quasi-one-dimensional systems 16 , a recent theoretical report using quantum Monte Carlo supports the existence of such a long spin-spin correlation length along the zigzag edge and justifies the picture obtained from the one-electron theories 17. Moreover, many previous studies suggest various ways to tweak the electronic properties of ZGNRs to achieve magneto-transport by means of doping, chemical modifications or external perturbations 18-22. From the viewpoint of device applications, it is necessary to clarify the interplay between the edge magnetism and the hole-doping effect, as the electron density in the graphene system can be easily tuned using the back gate electrode 23-25. However, the hole-doping effect on edge magnetism has not been studied with the appropriate inclusion of electron-electron interactions. In this study, we theoretically investigate the magnetic and conducting properties of ZGNRs and their response to doping. The development of gapless charge and spin excitations with hole doping, i.e., holemediated metallic ferromagnetism, is also discussed for the first time in terms of its potential applications. Our theoretical analysis is based on a large-scale numerical simulation using the many-body configuration interaction (CI) method with the complete active space (CAS-CI) approximation, which correctly includes quantum fluctuations. We show that the microscopic origin of magnetism in ZGNRs can be well understood on the basis of a generic two-state model.
The Journal of Physical Chemistry C, 2008
Graphene nanoribbons with both armchair-and zigzag-shaped hydrogen-passivated edges (AGNR and ZGNR) have band gaps which depend on the width of the ribbon. In particular, a ZGNR has localized electronic states at the edge which decay exponentially toward the center of the ribbon. Interestingly, application of a uniform external electric field (E ext) in the direction perpendicular to the edge of a ZGNR is capable of reducing the band gap for one spin state () and opens the other spin state (R). Moreover, for a critical E ext the ZGNR becomes half-metallic. In the case of an 8-chain zigzag ribbon, the critical E ext is 2 V/nm within the local spin density approximation. Motivated by these findings, we study the influence on the gap of the electric field produced by a polar ad-molecule to the surface of an 8-zigzag ribbon. The formula units of the ad-molecules that we studied are NH 3 (CH) 6 CO 2 and NH 3 (CH) 10 CO 2. We show that within the generalized gradient approximation the band gap of 0.52 eV without ad-molecule is reduced to 0.27 eV for the-spin state and increased to 0.69 eV for the R-spin state. Also, combining the ad-molecule and E ext) 1 V/nm parallel to the dipole moment of the ad-molecule induces a reduction of the-spin band gap and an increase for the R-spin band gap. For E ext)-1 V/nm, antiparallel to the dipole moment of the ad-molecule, the band gap for both spin states is similar to the case without ad-molecule and E ext. These results suggest possible uses for the graphene nanoribbons as sensors or switching devices.
Graphene nanoribbons with mixed cove-cape-zigzag edge structure
Carbon, 2021
A recently developed bottom-up synthesis strategy enables the fabrication of graphene nanoribbons with well-defined width and non-trivial edge structures from dedicated molecular precursors. Here we discuss the synthesis and properties of zigzag nanoribbons (ZGNRs) modified with periodic cove-capecove units along their edges. Contrary to pristine ZGNRs, which show antiferromagnetic correlation of their edge states, the edge-modified ZGNRs exhibit a finite single particle band gap without localized edge states. We report the on-surface synthesis of such edge-modified ZGNRs and discuss tunneling conductance dI/dV spectra and dI/dV spatial maps that reveal a noticeable localization of electronic states at the cape units and the opening of a band gap without presence of edge states of magnetic origin. A thorough ab initio investigation of the electronic structure identifies the conditions under which antiferromagnetically coupled, edge-localized states reappear in the electronic structure. Further modifications of the ribbon structure are proposed that lead to an enhancement of such features, which could find application in nanoelectronics and spintronics.
Computational Materials Science, 2012
We investigate the electronic properties of symmetric zigzag-edged graphene nanoribbon (ZGNR) in the presence of nitrogen (N) substitutional doping by ab initio density functional theory. The transformation energies indicate that the impurity prefers to distribute near the edges. With N-doping moving from edge to center, the electronic transport properties are mainly governed by holes and carriers, respectively. The charge transfer induced by substitutional doping is analyzed in detail and the influences of doping on the electronic transport properties of the defective nanostructure have been discussed. Our results suggest that ZGNRs' transport properties can be tuned via tailoring the atomic structures in terms of selective doping profiles, which would be helpful for designing graphene nanoribbon (GNR)-based nanoelectronic devices in future.
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.
Electronic states of zigzag graphene nanoribbons with edges reconstructed with topological defects
Physica B: Condensed Matter
The energy spectrum and electronic density of states (DOS) of zigzag graphene nanoribbons with edges reconstructed with topological defects are investigated within the tight-binding method. In case of the Stone-Wales zz (57) edge the low-energy spectrum is markedly changed in comparison to the pristine zz edge. We found that the electronic DOS at the Fermi level is different from zero at any width of graphene nanoribbons. In contrast, for ribbons with heptagons only at one side and pentagons at another one the energy gap at the Fermi level is open and the DOS is equal to zero. The reason is the influence of uncompensated topological charges on the localized edge states, which are topological in nature. This behavior is similar to that found for the structured external electric potentials along the edges.
On-surface synthesis of graphene nanoribbons with zigzag edge topology
Nature, 2016
Graphene-based nanostructures exhibit a vast range of exciting electronic properties that are absent in extended graphene. For example, quantum confinement in carbon nanotubes and armchair graphene nanoribbons (AGNRs) leads to the opening of substantial electronic band gaps that are directly linked to their structural boundary conditions 1,2. Even more intriguing are nanostructures with zigzag edges, which are expected to host spin-polarized electronic edge states and can thus serve as key elements for graphene-based spintronics 3. The most prominent example is zigzag graphene nanoribbons (ZGNRs) for which the edge states are predicted to couple ferromagnetically along the edge and antiferromagnetically between them 4. So far, a direct observation of the spin-polarized edge states for specifically designed and controlled zigzag edge topologies has not been achieved. This is mainly due to the limited precision of current top-down approaches 5-10 , which results in poorly defined edge structures. Bottom-up fabrication approaches, on the other hand, were so far only