88 research outputs found
DFT screened-exchange approach for investigating electronical properties of graphene-related materials
We present ab initio calculations of the bandstructure of graphene and of
short zigzag graphene nanoribbons by the screened-exchange-LDA method (sX-LDA)
within the framework of density functional theory (DFT). The inclusion of
non-local electron-electron interactions in this approach results in a
renormalization of the electronic bandstructure and the Fermi velocity compared
to calculations within local density approximation (LDA) gives good agreement
with experiment. Similarly, the band gaps in zigzag nanoribbons (ZGNR) are
widened by more than 200%, being of similar magnitude than bandgaps from past
studies based on quasiparticle bandstructures. We found a noticeable effect of
non-local exchange on the spin-polarization of the electronic ground state of
ZGNRs, compared to LDA and GGA-PW91 calculations.Comment: 5 pages, 3 figure
Family behavior and Dirac bands in armchair nanoribbons with 4β8 defect lines
Bottom-up synthesis from molecular precursors is a powerful route for the creation of novel synthetic carbon-based low-dimensional materials, such as planar carbon lattices. The wealth of conceivable precursor molecules introduces a significant number of degrees-of-freedom for the design of materials with defined physical properties. In this context, a priori knowledge of the electronic, vibrational and optical properties provided by modern ab initio simulation methods can act as a valuable guide for the design of novel synthetic carbon-based building blocks. Using density functional theory, we performed simulations of the electronic properties of armchair-edged graphene nanoribbons (AGNR) with a bisecting 4β8 ring defect line. We show that the electronic structures of the defective nanoribbons of increasing width can be classified into three distinct families of semiconductors, similar to the case of pristine AGNR. In contrast to the latter, we find that every third nanoribbon is a zero-gap semiconductor with Dirac-type crossing of linear bands at the Fermi energy. By employing tight-binding models including interactions up to third-nearest neighbors, we show that the family behavior, the formation of direct and indirect band gaps and of linear band crossings in the defective nanoribbons is rooted in the electronic properties of the individual nanoribbon halves on either side of the defect lines, and can be effectively through introduction of additional 'interhalf' coupling terms
Symmetry properties of vibrational modes in graphene nanoribbons
We present symmetry properties of the lattice vibrations of graphene
nanoribbons with pure armchair (AGNR) and zigzag edges (ZGNR). In
non-symmorphic nanoribbons the phonon modes at the edge of the Brillouin zone
are twofold degenerate, whereas the phonon modes in symmorphic nanoribbons are
non-degenerate. We identified the Raman-active and infrared-active modes. We
predict 3N and 3(N+1) Raman-active modes for N-ZGNRs and N-AGNRs, respectively
(N is the number of dimers per unit cell). These modes can be used for the
experimental characterization of graphene nanoribbons. Calculations based on
density functional theory suggest that the frequency splitting of the LO and TO
in AGNRs (corresponding to the E2g mode in graphene) exhibits characteristic
width and family dependence. Further, all graphene nanoribbons have a
Raman-active breathing-like mode, the frequency of which is inversely
proportional to the nanoribbon width and thus might be used for experimental
determination of the width of graphene nanoribbons.Comment: 10 pages, 5 figure
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