Mechanisms governing phonon scattering by topological defects in graphene nanoribbons (original) (raw)
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Applied Physics Letters, 2012
Equilibrium molecular dynamics simulations show that graphene nanoribbons (GNRs) with zigzag edges have higher thermal conductivity (j) than armchair-edged ones, and the difference diminishes with increasing temperature or ribbon width. The dominant phonon wavelength for thermal transport can be much longer (by orders of magnitude) than the difference between the "roughness" of smooth zigzag and armchair edges. Therefore, the roughness scattering theory is not sufficient to explain the largely different j of GNRs with different edge chiralities. Crosssectional decomposition of the steady-state heat flux shows significant suppression of thermal transport at edges, especially in armchair ones. This behavior is explored by phonon spectra analysis. Considerable phonon localization at edges is concluded to underlie the edge-chirality dependent j of GNRs. V C 2012 American Institute of Physics. [http://dx.Graphene has been recognized as a potential substitute for silicon in the electronics industry, mainly owing to its outstanding electronic and thermal properties. 1-5 The tunable band-gap opening and edge-chirality dependent electronic property of graphene nanoribbon (GNR), a narrow strip of graphene, makes the vast application of graphene-based devices even more promising. 1,6 GNR has also been predicted to have edge-chirality dependent thermal conductivity (j), which was mostly predicted to be higher in zigzagedged GNRs (zGNR) than armchair-edged ones (aGNR), i.e., Dj ¼ j zGNR À j aGNR > 0, though the underlying mechanism remains to be an open question. The notable difference between the topology of zigzag and armchair edges can lead people to attribute Dj to the surface/edge roughness scattering as usually seen in nanostructures. 10-13 As for thermal transport, the consideration of the surface/edge roughness scattering is usually meaningful only when the RMS height of the surface/edge variation (d) is comparable to the dominant phonon wavelength (k dom ) for carrying heat. For narrow GNRs, the thermal conductance (G) can be estimated by integrating the Landauer formula over the entire first Brillouin zone (FBZ) as 9,14
Bimodal Phonon Scattering in Graphene Grain Boundaries
Graphene has served as the model 2D system for over a decade, and the effects of grain boundaries (GBs) on its electrical and mechanical properties are very well investigated. However, no direct measurement of the correlation between thermal transport and graphene GBs has been reported. Here, we report a simultaneous comparison of thermal transport in supported single crystalline graphene to thermal transport across an individual graphene GB. Our experiments show that thermal conductance (per unit area) through an isolated GB can be up to an order of magnitude lower than the theoretically anticipated values. Our measurements are supported by Boltzmann transport modeling which uncovers a new bimodal phonon scattering phenomenon initiated by the GB structure. In this novel scattering mechanism, boundary roughness scattering dominates the phonon transport in low-mismatch GBs, while for higher mismatch angles there is an additional resistance caused by the formation of a disordered region at the GB. Nonequilibrium molecular dynamics simulations verify that the amount of disorder in the GB region is the determining factor in impeding thermal transport across GBs.
Physical Review B, 2014
Quantum thermal transport in armchair and zigzag graphene nanoribbons are investigated in the presence of single atomic vacancies and subject to different boundary conditions. We start with a full comparison of the phonon polarizations and energy dispersions as given by a fifth-nearest-neighbor force-constant model (5NNFCM) and by elasticity theory of continuum membranes (ETCM). For free-edges ribbons we discuss the behavior of an additional acoustic edge-localized flexural mode, known as fourth acoustic branch (4ZA), which has a small gap when it is obtained by the 5NNFCM. Then, we show that ribbons with supported-edges have a sample-size dependent energy gap in the phonon spectrum which is particularly large for in-plane modes. Irrespective to the calculation method and the boundary condition, the dependence of the energy gap for the low-energy optical phonon modes against the ribbon width W is found to be proportional to 1/W for in-plane, and 1/W 2 for out-of-plane phonon modes. Using the 5NNFCM, the ballistic thermal conductance and its contributions from every single phonon mode are then obtained by the non equilibrium Green's function technique. We found that, while edge and central localized single atomic vacancies do not affect the low-energy transmission function of in-plane phonon modes, they reduce considerably the contributions of the flexural modes. On the other hand, in-plane modes contributions are strongly dependent on the boundary conditions and at low temperatures can be highly reduced in supportededges samples. These findings could open a route to engineer graphene based devices where it is possible to discriminate the relative contribution of polarized phonons and to tune the thermal transport on the nanoscale.
Control of thermal and electronic transport in defect-engineered graphene nanoribbons
2011
ABSTRACT The influence of the structural detail and defects on the thermal and electronic transport properties of graphene nanoribbons (GNRs) is explored by molecular dynamics and nonequilibrium Green's function methods. A variety of randomly oriented and distributed defects, single and double vacancies, StoneÀWales defects, as well as two types of edge form (armchair and zigzag) and different edge roughnesses are studied for model systems similar in sizes to experiments (> 100 nm long and> 15 nm wide).
Carbon, 2012
Classical molecular dynamics with the AIREBO potential is used to investigate the thermal conductivity of both zigzag and armchair graphene nanoribbons possessing different densities of Stone-Thrower-Wales (STW) defects. Our results indicate that the presence of the defects can decrease thermal conductivity by more than 50%. The larger the defect density, the lower the conductivity, with the decrease significantly higher in zigzag than in armchair nanoribbons for all defect densities. The effect of STW defects in the temperature range 100-600 K was also determined. Our results showed the same trends in thermal conductivity decreases at all temperatures. However, for higher defect densities there was less variation in thermal conductivity at different temperatures.
Effects of a grain boundary loop on the thermal conductivity of graphene: A molecular dynamics study
Computational Materials Science, 2013
Thermal transport in graphene with one type of grain boundary loop was investigated using non-equilibrium molecular dynamics simulation method. The results showed that thermal conductivity is very sensitive to defect concentration. It rapidly decreases in the presence of a defect. This is attributed the phonon defects scattering which shorten the phonon mean free paths leading to the reduction in thermal conductivity. Furthermore, temperature dependency of thermal conductivity of pristine and defected graphene was determined. The results indicated that thermal conductivity of defect-free graphene varies significantly with temperature, while thermal conductivity of graphene with defect remains nearly invariant with the temperature of the system. This implies the possibility of phonon-defect scattering domination over Umklapp phonon-phonon scattering in graphene with defect.
Lattice thermal conductivity of graphene nanoribbons: Anisotropy and edge roughness scattering
Applied Physics Letters, 2011
We present a calculation of the thermal conductivity of graphene nanoribbons ͑GNRs͒, 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.
Parity conservation in electron-phonon scattering in zigzag graphene nanoribbon
Mode space approach for tight-binding transport simulations in graphene nanoribbon field-effect transistors including phonon scattering J. Appl. Phys. 113, 144506 (2013); 10.1063/1.4800900 Chiral graphene nanoribbons: Objective molecular dynamics simulations and phase-transition modeling J. Chem. Phys. 137, 234702 (2012); 10.1063/1.4770002 Phonon limited transport in graphene nanoribbon field effect transistors using full three dimensional quantum mechanical simulation J. Appl. Phys. 112, 094505 (2012); 10.1063/1.4764318 Engineering enhanced thermoelectric properties in zigzag graphene nanoribbons