We theoretically investigate the thermoelectric properties of zigzag graphene nanoribbons in the presence of extended line defects, substrate impurities and edge roughness along the nanoribbon's length. A nearest-neighbor tight-binding model for the electronic structure and a fourth nearestneighbor force constant model for the phonon bandstructure are used. For transport we employ quantum mechanical non-equilibrium Green's function simulations. Starting from the pristine zigzag nanoribbon structure that exhibits very poor thermoelectric performance, we demonstrate how after a series of engineering design steps the performance can be largely enhanced. Our results could be useful in the design of highly efficient nanostructured graphene nanoribbon based thermoelectric devices.
The thermoelectric properties of graphene-based antidot lattices are theoretically investigated. A third nearest-neighbor tight-binding model and a fourth nearest-neighbor force constant model are employed to study the electronic and phononic band structures of graphene antidot lattices with circular, rectangular, hexagonal, and triangular antidot shapes. Ballistic transport models are used to evaluate transport coefficients. Methods to reduce the thermal conductance and to increase the thermoelectric power factor of such structures are studied. Our results indicate that triangular antidot lattices have the smallest thermal conductance due to longer boundaries and the smallest distance between the neighboring antidots. Among them, iso-triangular antidot lattices have also a large power factor and as a result a large figure of merit. V
Following our recent study on the electronic properties of rough nanoribbons [1], in this paper the role of geometrical and roughness parameters on the thermal properties of armchair graphene nanoribbons is studied. Employing a fourth nearest-neighbor force constant model in conjuction with the nonequilibrium Green's function method the effect of lineedge-roughness on the lattice thermal conductivity of rough nanoribbons is investigated. The results show that a reduction of about three orders of magnitude of the thermal conductivity can occur for ribbons narrower than 10 nm. The results indicate that the diffusive thermal conductivity and the effective mean free path are directly proportional to the ribbon's width and the roughness correlation length, but inversely proportional to the roughness amplitude. Based on the numerical results an analytical model for the thermal conductivity of narrow armchair graphene nanoribbons is proposed in this paper. The developed model can be used in the analysis of graphene-based nano transistors and thermoelectric devices, where the appropriate selection of geometrical and roughness parameters are essential for optimizing the thermal properties.
We investigate the influence of low-dimensionality and disorder in phonon transport in ultra-narrow armchair graphene nanoribbons (GNRs) using non-equilibrium Green's function (NEGF) simulation techniques. We specifically focus on how different parts of the phonon spectrum are influenced by geometrical confinement and line edge roughness. Under ballistic conditions, phonons throughout the entire phonon energy spectrum contribute to thermal transport. With the introduction of line edge roughness, the phonon transmission is reduced, but in a manner which is significantly non-uniform throughout the spectrum. We identify four distinct behaviors within the phonon spectrum in the presence of disorder: i) the low-energy, low-wavevector acoustic branches have very long mean-free-paths and are affected the least by edge disorder, even in the case of ultra-narrow W=1nm wide GNRs; ii) energy regions that consist of a dense population of relatively 'flat' phonon modes (including the optical branches) are also not significantly affected, except in the case of the ultra-narrow W=1nm GNRs, in which case the transmission is reduced because of band mismatch along the phonon transport path; iii) 'quasi-acoustic' bands that lie within the intermediate region of the spectrum are strongly affected by disorder as this part of the spectrum is depleted of propagating phonon modes upon both confinement and disorder (resulting in sparse E(q) phononic bandstructure), especially as the channel length increases; iv) the strongest reduction in phonon transmission is observed in energy regions that are composed of a small density of phonon modes, in which case roughness can introduce transport gaps that greatly increase with channel length. We show that in GNRs of widths as small as W=3nm, under moderate roughness, both the low-energy acoustic modes and dense regions of optical modes can retain semi-ballistic transport properties, even for channel lengths up to 2 L=1μm. These modes tend to completely dominate thermal transport. Modes in the sparse regions of the spectrum, however, tend to fall into the localization regime, even for channel lengths as short as 10s of nanometers, despite their relatively high phonon group velocities.
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