We show that thermal rectification (TR) in asymmetric graphene nanoribbons (GNRs) is originated from phonon confinement in the lateral dimension, which is a fundamentally new mechanism different from that in macroscopic heterojunctions. Our molecular dynamics simulations reveal that, though TR is significant in nanosized asymmetric GNRs, it diminishes at larger width. By solving the heat diffusion equation, we prove that TR is indeed absent in both the total heat transfer rate and local heat flux for bulk-size asymmetric single materials, regardless of the device geometry or the anisotropy of the thermal conductivity. For a deeper understanding of why lateral confinement is needed, we have performed phonon spectra analysis and shown that phonon lateral confinement can enable three possible mechanisms for TR: phonon spectra overlap, inseparable dependence of thermal conductivity on temperature and space, and phonon edge localization, which are essentially related to each other in a complicated manner. Under such guidance, we demonstrate that other asymmetric nanostructures, such as asymmetric nanowires, thin films, and quantum dots, of a single material are potentially high-performance thermal rectifiers. KEYWORDS: Thermal rectification, phonon lateral confinement, phonon localization, edge/surface effect, molecular dynamics, phonon spectra I nspired by the impact of electric diodes on the electronics industry, extensive attention has been given to the search of rectification of various other transport processes.1−3 Thermal rectification (TR) is a diode-like behavior where the heat current changes in magnitude when the applied temperature (T) bias is reversed in direction. A perfect thermal rectifier would be one that is highly thermal conductive in one direction while insulating in the other, and it is expected to work as a promising thermal management component of electronics as chip size continues decreasing or as a stand-alone thermally driven computing system replacing the electronic ones in certain conditions.Numerous studies have predicted or demonstrated the existence of TR in bulk or nanosized systems, most of which are heterojunctions (HJ) or graded systems.3−11 For twosegment systems, TR was usually attributed to the different Tdependence of the thermal conductivity (κ), 5,7 and for interfaces TR has been interpreted as the different phonon spectra mismatch before and after reversing the applied T bias. Phonon localization was suggested to play a role as well. 12,13 Recently, TR was also predicted to occur in asymmetric pristine carbon nanostructures, 13−17 which are composed of a single material and are attractive for their simple structure and high thermal conductance. 18 However, the origin of TR in such homogeneous nanostructures remains unclear. In this work, we have observed, using molecular dynamics and analytical derivations, that phonon confinement in the lateral dimension is required for TR to occur in asymmetric homogeneous structures made of a single material. We further show that...

The mean-free-paths (MFPs) of energy carriers are of critical importance to the nanoengineering of better thermoelectric materials. Despite significant progress in the first-principlesbased understanding of the spectral distribution of phonon MFPs in recent years, the spectral distribution of electron MFPs remains unclear. In this work, we compute the energy dependent electron scatterings and MFPs in silicon from first-principles. The electrical conductivity accumulation with respect to electron MFPs is compared to that of the phonon thermal conductivity accumulation to illustrate the quantitative impact of nanostructuring on electron and phonon transport. By combining all electron and phonon transport properties from first-principles, we predict the thermoelectric properties of the bulk and nanostructured silicon, and find that silicon with 20 nm nanograins can result in more than five times enhancement in their thermoelectric figure of merit as the grain boundaries scatter phonons more significantly than that of electrons due to their disparate MFP distributions.Nanostructuring has proven to be an effective strategy to improve the figure of merit of thermoelectric materials [1][2][3][4][5][6][7][8][9][10][11]. The figure of merit is proportional to the electrical conductivity (), the square of the Seebeck coefficient (S) and inversely proportional to the thermal conductivity consisting of both phonon (k p ) and electron (k e ) contributions. The most effective nanostructuring approach so far has been reducing the phonon thermal conductivity while maintaining the electronic performance. For this strategy to be effective, the nanostructures should be smaller than the phonon mean free path (MFP) but larger than the electron MFP so that phonons are more strongly scattered than electrons. It is understood that both electron and phonon MFPs have a distribution over certain energy range. There has been good progress in predicting the spectral phonon MFPs for a range of bulk single crystals and alloys [12][13][14][15][16][17][18][19][20][21][22][23][24]. However, there has been no discussion on the spectral electron MFPs from first-principles. Surprisingly, this status exists even for silicon, one of the most important materials. Existing knowledge on electron scattering, relaxation time, and MFP, is mostly based on analytical models derived from Fermi's golden rule assuming ideal electron and phonon dispersions [25,26]. Past work on the phonon MFP distributions based on first-principles simulations, however, shows that such semi-empirical treatments on scattering lead to large error [13,21,23,27].In this work, we compute the electron scattering rates and MFPs in silicon from first-principles and examine their dependence on energy, doping concentration, and their contributions to electronic conductivity and Seebeck coefficient. We demonstrate quantitatively the large disparity in the electron and phonon MFP distributions in silicon, and use the information obtained to predict that nanostructures with size of 20 nm can...

Raman spectroscopy has been widely used to measure thermal conductivity (κ) of 2D materials such as graphene. This method is based on a well-accepted assumption that different phonon polarizations are in near thermal equilibrium. However, in this work we show that in laser irradiated single layer graphene, different phonon polarizations are in strong non-equilibrium, using predictive simulations based on first principles density functional perturbation theory (DFPT) and a multi-temperature model. We first calculate the electron cooling rate due to phonon scattering as a function of the electron and phonon temperatures, and the results clearly illustrate that optical phonons dominate the hot electron relaxation process. We then use these results in conjunction with the phonon scattering rates computed using perturbation theory to develop a multi-temperature model, and resolve the spatial temperature distributions of the energy carriers in graphene under steady state laser irradiation. Our results show that electrons, optical phonons, and acoustic phonons are in strong non-equilibrium, with the ZA phonons showing the largest non-equilibrium to other phonon modes, mainly due to their weak coupling to other carriers in suspended graphene. Since ZA phonons are the main heat carriers in graphene, we estimate that neglecting this non-equilibrium leads to underestimation of thermal conductivity in experiments at room temperature by a factor of 1.35 to 2.6, depending on experimental conditions and assumptions used. Underestimation is also expected in Raman measurements of other 2D materials when the opticalacoustic phonon coupling is weak.

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.Graphene 1,2 is a monolayer of graphite with a honeycomb lattice structure. It exhibits many unique properties and has drawn intense attention in the past few years. The unusual electronic properties of graphene are promising in many fundamental studies and applications, e.g., the ultrahigh electron mobility 2 and the tunable band gap and magnetic properties by the size and edge chirality of GNRs. [3][4][5][6] Graphene also has remarkable thermal properties. The measured value of thermal conductivity of graphene reaches as high as several thousand of W/m-K, 7-10 among the highest values of known materials. Previous studies [11][12][13] show that the thermal transport in GNRs depends on the edge chirality of GNRs. In realistic graphene samples, the edges are often passivated [14][15][16] and the isotope composition can be controlled. 17 Motivated by these, we study the effect of the edge H-passivation and various isotope distributions on the thermal transport in GNRs. We find that the thermal conductivity can be reduced by the edge H-passivation and tuned by the isotope distributions. Our study is useful in nanoscale control and management of thermal transport by engineering the chemical composition of GNRs.In this work, we employ the classical molecular dynamics (MD, similar to the method in Ref. 11) to calculate the thermal conductivities of GNRs. We use the Brenner potential, 18 which incorporates the many-body carbon-carbon and carbon-hydrogen interactions by introducing a fractional number of covalent bonds. This method has been successfully applied to many carbonbased systems, 19,20 especially to graphene. 11,21,22 The structures of GNRs are shown in Fig. 1 (with edge H-

Electron-phonon (e-p) interaction and transport are important for laser-matter interactions, hot-electron relaxation, and metal-nonmetal interfacial thermal transport. A widely used approach is the two-temperature model (TTM), where e-p coupling is treated with a gray approach with a lumped coupling factor G ep and the assumption that all phonons are in local thermal equilibrium. However, in many applications, different phonon branches can be driven into strong nonequilibrium due to selective e-p coupling, and a TTM analysis can lead to misleading or wrong results. Here, we extend the original TTM into a general multitemperature model (MTM), by using phonon branch-resolved e-p coupling factors and assigning a separate temperature for each phonon branch. The steady-state thermal transport and transient hot electron relaxation processes in constant and pulse laser-irradiated single-layer graphene (SLG) are investigated using our MTM respectively. Results show that different phonon branches are in strong nonequilibrium, with the largest temperature rise being more than six times larger than the smallest one. A comparison with TTM reveals that under steady state, MTM predicts 50% and 80% higher temperature rises for electrons and phonons respectively, due to the "hot phonon bottleneck" effect. Further analysis shows that MTM will increase the predicted thermal conductivity of SLG by 67% and its hot electron relaxation time by 60 times. We expect that our MTM will prove advantageous over TTM and gain use among experimentalists and engineers to predict or explain a wide ranges of processes involving laser-matter interactions.