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Lattice thermal conductivity of graphene nanoribbons: Anisotropy and edge roughness scattering
| Content Provider | Semantic Scholar |
|---|---|
| Author | Aksamija, Zlatan Knezevic, Irena |
| Copyright Year | 2011 |
| Abstract | We present a calculation of the thermal conductivity of graphene nanoribbons GNRs, based on solving the Boltzmann transport equation with the full phonon dispersions, a momentum-dependent model for edge roughness scattering, as well as three-phonon and isotope scattering. The interplay between edge roughness scattering and the anisotropy of the phonon dispersions results in thermal conduction that depends on the chiral angle of the nanoribbon. Lowest thermal conductivity occurs in the armchair direction and highest in zig-zag nanoribbons. Both the thermal conductivity and the degree of armchair/zig-zag anisotropy depend strongly on the width of the nanoribbon and the rms height of the edge roughness, with the smallest and most anisotropic thermal conductivities occurring in narrow GNRs with rough edges. © 2011 American Institute of Physics. doi:10.1063/1.3569721 Single-layer graphene is a unique material made up of a monolayer of sp 2 -hybridized carbon atoms that is capable of purely two-dimensional electrical 1 and thermal transport. 2 It can be fashioned into a broad range of shapes, from millimeter-sized flakes down to very narrow nanoribbons. Despite single-layer graphene possessing superior thermal conductivity, 3,4 graphene nanoribbons GNRs have been shown to have the potential to be excellent thermoelectrics with very high values of the thermoelectric figure-of-merit ZT. 5 The enhancement of ZT has been explained by the fact that the presence of line edge roughness in narrow GNRs affects thermal transport very strongly 6 while leaving electronic transport relatively unchanged. 5 Previous studies of the effect of width, line edge roughness, and anisotropy on thermal conductivity largely relied either on the ballistic approximation 7 or a simplified treatment of edge roughness scattering. 8 Studies based on molecular dynamics also demonstrated the anisotropy of thermal conductivity and sensitivity to width and edge roughness; 9 however, such studies were limited in the range of sizes that could be examined. In this letter, we study the lattice thermal conductivity in GNRs over a wide range of widths, edge roughness values, and chiral angles. We calculate thermal conductivity by solving the phonon Boltzmann transport equation in the relaxation time approximation, and account for phonon-phonon, phonon-isotope, and edge roughness scattering. We assume a thermal gradient is applied along the nanoribbon and show that thermal conductivity varies with the chiral angle of the ribbon, with a minimum in armchair and a maximum in zigzag GNRs. The angular variation becomes stronger as the width of the ribbon decreases because of the increased role of phonon scattering with the rough edges of the narrow nanoribbon. Edge roughness scattering also causes the overall value of the thermal conductivity to decrease with decreasing width. In order to obtain accurate thermal conductivities in an arbitrary transport direction, we employ the full phonon dispersion, shown in Fig. 1. Phonon dispersion has been previously measured by Raman spectroscopy 10 and x-ray scattering 11 and compared to calculations. Based on experimental results, the empirical fourth-nearest-neighbor 4NNR model of Saito 12 was reparametrized to include new experimental findings 10 and first-principles numerical calculations, 11 while also including off-diagonal terms of the force constant matrices 13 and rotational invariance conditions. 14 The reparametrized 4NNR model has been shown to offer an excellent fit to both experiments and firstprinciples calculations. 13 We use the 4NNR model with parameters obtained by fitting first-principles results with a small correction to the in- and out-of-plane tangential force constants to satisfy the rotational invariance condition. 14 Scattering from the rough edges of the nanoribbon is partially diffuse and can be accurately described by a |
| Starting Page | 141919 |
| Ending Page | 141919 |
| Page Count | 1 |
| File Format | PDF HTM / HTML |
| DOI | 10.1063/1.3569721 |
| Alternate Webpage(s) | http://homepages.cae.wisc.edu/~knezevic/pdfs/AksamijaAPL2011.pdf |
| Alternate Webpage(s) | https://doi.org/10.1063/1.3569721 |
| Volume Number | 98 |
| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
