Graphene nanoribbons with mixed cove-cape-zigzag edge structure (original) (raw)
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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
New Approaches to Edge-Doping Graphene Nanoribbons
Bulletin of the American Physical Society, 2016
TEAM 1 , FISCHER TEAM 2 , LOUIE TEAM 3-Graphene nanoribbons (GNRs) are narrow semiconducting strips of graphene that exhibit novel electronic and magnetic properties. New bottom-up fabrication techniques enable atomic-scale precision in GNR synthesis. The use of these techniques to reliably tune the position and size of GNR band gaps is an important challenge that also has relevance for the question of whether GNRs are viable for future nanotechnologies. We have used scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) to investigate how the geometry of heteroatom incorporation alters the electronic structure of bottom-up fabricated chevron-type GNRs. We find that the addition of nitrogen into the GNR edge via a five-membered ring yields a reduced band gap compared to the behavior of pristine, undoped chevron GNRs.
Role of Vacancies in Zigzag Graphene Nanoribbons: An Ab Initio Study
Journal of Nano Research, 2014
We have studied the effects of vacancies on the structural, electronic and magnetic properties of zigzag-edged graphene nanoribbons (ZGNRs). Our calculations were carried out using an abinitio density functional pseudopotential computational method combined with the generalized gradient approximation for the exchange-correlation functional. The equilibrium geometries, electronic charge spin density distributions, electronic band structures, and magnetic moments were examined in the presence of single vacancy and double vacancies. Structural optimization showed that vacancies induce substantial structural changes in ZGNRs. We found that introducing vacancies into ZGNR 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 ZGNR. The calculations showed that the changes in the structural geometry, the electronic structure and the magnetization of ZGNR...
Nature Materials, 2009
Graphene shows promise as a future material for nanoelectronics owing to its compatibility with industry-standard lithographic processing, electron mobilities up to 150 times greater than Si and a thermal conductivity twice that of diamond. The electronic structure of graphene nanoribbons (GNRs) and quantum dots (GQDs) has been predicted to depend sensitively on the crystallographic orientation of their edges; however, the influence of edge structure has not been verified experimentally. Here, we use tunnelling spectroscopy to show that the electronic structure of GNRs and GQDs with 2-20 nm lateral dimensions varies on the basis of the graphene edge lattice symmetry. Predominantly zigzag-edge GQDs with 7-8 nm average dimensions are metallic owing to the presence of zigzag edge states. GNRs with a higher fraction of zigzag edges exhibit a smaller energy gap than a predominantly armchair-edge ribbon of similar width, and the magnitudes of the measured GNR energy gaps agree with recent theoretical calculations.
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.
Towards Cove-Edged Low Band Gap Graphene Nanoribbons
Journal of the American Chemical Society, 2015
Graphene nanoribbons (GNRs), defined as nanometer-wide strips of graphene, have attracted increasing attention as promising candidates for next-generation semiconductors. Here, we demonstrate a bottom-up strategy toward novel low band gap GNRs (Eg=1.70 eV) with a well-defined cove-type periphery both in solution and on a solid substrate surface with chrysene as the key monomer. Corresponding cyclized chrysene-based oligomers consisting of the dimer and tetramer are obtained via an Ullmann coupling followed by oxidative intramolecular cyclodehydrogenation in solution, and much higher GNR homologues via on-surface synthesis. These oligomers adopt non-planar structures due to the steric repulsion between the two C-H bonds at the inner cove position. Characterizations by single crystal X-ray analysis, UV-vis absorption spectroscopy, NMR spectroscopy, and scanning tunneling microscopy (STM) are described. The interpretation is assisted by density functional theory (DFT) calculations.
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.
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.