On the forbidden gap of finite graphene nanoribbons

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Title: On the forbidden gap of finite graphene nanoribbons
Authors: Vergés Brotons, José Antonio | Chiappe, Guillermo | Louis, Enrique
Research Group/s: Física de la Materia Condensada | Materiales Avanzados
Center, Department or Service: Universidad de Alicante. Departamento de Física Aplicada | Universidad de Alicante. Instituto Universitario de Materiales
Keywords: Solid state and materials | Finite graphene nanoribbons | Forbidden gap
Knowledge Area: Física de la Materia Condensada | Física Aplicada
Issue Date: Aug-2015
Publisher: EDP Sciences | Società Italiana di Fisica | Springer Berlin Heidelberg
Citation: The European Physical Journal B. 2015, 88: 200. doi:10.1140/epjb/e2015-60389-5
Abstract: The electronic structure of isolated finite graphene nanoribbons is investigated by solving, at the Hartree-Fock (HF) level, the Pariser, Parr and Pople (PPP) many-body Hamiltonian. The study is mainly focused on 7-AGNR and 13-AGNR (Armchair Graphene Nano-Ribbons), whose electronic structures have been recently experimentally investigated. Only paramagnetic solutions are considered. The characteristics of the forbidden gap are studied as a function of the ribbon length. For a 7-AGNR, the gap monotonically decreases from a maximum value of ~6.5 eV for short nanoribbons to a very small value of ~0.12 eV for the longer calculated systems. Gap edges are defined by molecular orbitals that are spatially localized near the nanoribbon extremes, that is, near both zig-zag edges. On the other hand, two delocalized orbitals define a much larger gap of about 5 eV. Conductance measurements report a somewhat smaller gap of ~3 eV. The small real gap lies in the middle of the one given by extended states and has been observed by STM and reproduced by DFT calculations. On the other hand, the length dependence of the gap is not monotonous for a 13-AGNR. It decreases initially but sharply increases for lengths beyond 30 Å remaining almost constant thereafter at a value of ~2.1 eV. Two additional states localized at the nanoribbon extremes show up at energies 0.31 eV below the HOMO (Highest Occupied Molecular Orbital) and above the LUMO (Lowest Unoccupied Molecular Orbital). These numbers compare favorably with those recently obtained by means of STS for a 13-AGNR sustained by a gold surface, namely 1.4 eV for the energy gap and 0.4 eV for the position of localized band edges. We show that the important differences between 7- and 13-AGNR should be ascribed to the charge rearrangement near the zig-zag edges obtained in our calculations for ribbons longer than 30 Å, a feature that does not show up for a 7-AGNR no matter its length.
Sponsor: Financial support by the Spanish ”Ministerio de Ciencia e Innovación MICINN” (Grants FIS2009-10325, FIS2012-35880 and FIS2012-33521) and the Universidad de Alicante is gratefully acknowledged. We also acknowledge support from the DGUI of the Comunidad de Madrid under the R&D Program of activities MODELICO-CM/S2009ESP-1691.
URI: http://hdl.handle.net/10045/53248
ISSN: 1434-6028 (Print) | 1434-6036 (Online)
DOI: 10.1140/epjb/e2015-60389-5
Language: eng
Type: info:eu-repo/semantics/article
Rights: © EDP Sciences, Società Italiana di Fisica, Springer-Verlag 2015. The final publication is available at Springer via http://dx.doi.org/10.1140/epjb/e2015-60389-5
Peer Review: si
Publisher version: http://dx.doi.org/10.1140/epjb/e2015-60389-5
Appears in Collections:INV - Física de la Materia Condensada - Artículos de Revistas
INV - LMA - Artículos de Revistas

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