Tuning the polarized quantum phonon transmission in graphene nanoribbons (original) (raw)
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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.
Parity conservation in electron-phonon scattering in zigzag graphene nanoribbon
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Phonon transport effects in one-dimensional width-modulated graphene nanoribbons
Journal of Applied Physics, 2016
We investigate the thermal conductance of one-dimensional periodic width-modulated graphene nanoribbons using lattice dynamics for the phonon spectrum and the Landauer formalism for phonon transport. We conduct a full investigation considering all relevant geometrical features, i.e., the various lengths and widths of the narrow and wide regions that form the channel. In all cases that we examine, we find that width-modulation suppresses the thermal conductance at values even up to ∼70% below those of the corresponding uniform narrow nanoribbon. We show that this can be explained by the fact that the phonon spectrum of the width-modulated channels acquires less dispersive bands with lower group velocities and several narrow bandgaps, which reduce the phonon transmission function significantly. The largest degradation in thermal conductance is determined by the geometry of the narrow regions. The geometry of the wider regions also influences thermal conductance, although modestly. Our...
Journal of Applied Physics, 2012
This paper present a study of carrier transport in graphene nanoribbon (GNR) transistors using three-dimensional quantum mechanical simulations based on a real-space approach of the non-equilibrium Green's function (NEGF) formalism in the ballistic and dissipative limit. The carrier transport parameters are determined in the presence of electron-phonon scattering, and its influence on carrier mobility including both optical phonons (OP) and acoustic phonons (AP). The performances of GNRFETs are investigated in detail considering the third nearest neighbour tight-binding approximation. The low-field mobility is extracted in the presence of AP and OP as a function of nanoribbon width and length, from which the diffusive/ballistic limit of operation in GNRFETs is determined.
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
Resonant states in heterostructures of graphene nanoribbons
Physica B: Condensed Matter, 2009
We study the transport properties of heterostructures of armchair graphene nanoribbons (AGNR) forming a double symmetrical barrier configuration. The systems are described by a single-band tightbinding Hamiltonian and Green's functions formalism, based on real-space renormalization techniques. We present results for the quantum conductance and the current for distinct configurations, focusing our analysis on the dependence of the transport with geometrical effects such as separation, width and transverse dimension of the barriers. Our results show the apparition of a series of resonant peaks in the conductance, showing a clear evidence of the presence of resonant states in the conductor. Changes in the barrier dimensions allow the modulation of the resonances in the conductance, making possible to obtain a complete suppression of electron transmission for determined values of the Fermi energy. The current-voltage curves show the presence of a negative differential resistance effect with a threshold voltage that can be controlled by varying the separation between the barriers and by modulating its confinement potential.
Spectrally-resolved thermal transport in graphene nanoribbons
Journal of Applied Physics, 2019
Thermal transport properties of graphene nanoribbons are investigated using first principles phonon transport studies. Ribbons of varying widths are considered to investigate the transition of key thermal properties with width. The lattice structure of the ribbons is fully resolved and phonon transport is modeled by accounting for all three-phonon scattering processes using a solution of the linearized Boltzmann transport equation. A 3x reduction in intrinsic thermal conductivity is observed compared to bulk graphene arising from increased strength of three-phonon scattering due to the additional non-degenerate phonon modes that appear due to finite edges of confined nanoribbons. Strong dependence of thermal conductivity on ribbon width is also observed. The underlying mechanisms for thermal conductivity reduction and width dependence are presented by analyzing frequency-and polarization-resolved phonon transport. The additional scattering pathways present in 1D GNRs lead to a significant reduction in the thermal conductivity of otherwise highly-conducting flexural phonons in bulk graphene. In contrast, confinement-induced changes to the density of states, specific heat or group velocity, and the subsequent impact on lattice thermal conductivity, are found to be relatively small.
Mechanisms governing phonon scattering by topological defects in graphene nanoribbons
Nanotechnology, 2015
Understanding phonon scattering by topological defects in graphene is of particular interest for thermal management in graphene-based devices. We present a study that quantifies the roles of the different mechanisms governing defect phonon scattering by comparing the effects of ten different defect structures using molecular dynamics. Our results show that phonon scattering is mainly influenced by mass density difference, with general trends governed by the defect formation energy and typical softening behaviors in the phonon density of state. The phonon scattering cross-section is found to be far larger than that geometrically occupied by the defects. We also show that the lattice thermal conductivity can be reduced by a factor of up to ~30 in the presence of the grain boundaries formed by these defects.
Low-dimensional phonon transport effects in ultranarrow disordered graphene nanoribbons
Physical Review B, 2015
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 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.
Applied Physics Letters, 2010
Using classical molecular dynamics simulation, we have studied the effect of edge-passivation by hydrogen (Hpassivation) and isotope mixture (with random or supperlattice distributions) on the thermal conductivity of rectangular graphene nanoribbons (GNRs) (of several nanometers in size). We find that the thermal conductivity is considerably reduced by the edge H-passivation. We also find that the isotope mixing can reduce the thermal conductivities, with the supperlattice distribution giving rise to more reduction than the random distribution. These results can be useful in nanoscale engineering of thermal transport and heat management using GNRs.