Investigate Phonon Transport Research Articles

Bulk-like phonon transport in multilayer graphene nanostructures with consecutive twist angles

We investigate phonon transport in consecutively twisted multilayer graphene (CTMG) via atomistic simulations, assuming the same twist angle between any adjacent layers. Intriguingly, the calculated temperature drop between each two layers along the cross-plane direction in CTMG appears nearly identical, indicating an ill-defined Kapitza length. Moreover, the bulk-like thermal resistance increases with rising temperature. Both phenomena suggest that heat conduction in CTMG more closely resembles that in bulk materials as compared to layered materials with an individual mismatched interface, such as twisted multilayer graphene (TMG). This peculiar behavior originates from the unique nanostructure of CTMG. First, the common twist angle creates a consistent charge distribution at each interface, leading to a higher binding energy and reduced mismatch of the phonon density of states across the interface, and therefore a more uniform temperature drop. Further, the continuum of interfaces induces strong scattering of low-energy acoustic phonons generated by emission processes at the interfaces, which results in a temperature dependence opposite to that of TMG. Our findings enable a better understanding of phonon transport in twisted multilayer graphene, and may also facilitate the exploration of other interface-engineered van der Waals nanostructures.

Quantitative Analysis of Nonequilibrium Phonon Transport Near a Nanoscale Hotspot

Understanding thermal transport by phonons near nanoscale hotspots is critical for many engineering applications, especially for nanoscale thermal measurements and nanoelectronics. Previous studies usually adopted the gray phonon Boltzmann transport equation to consider ballistic phonon transport or the multitemperature model to consider selective phonon excitation. However, nonequilibrium phonon transport near a nanoscale hotspot cannot be fully captured by either method. In this work, we employ the more rigorous nongray phonon Boltzmann transport equation to investigate phonon transport near nanoscale hotspots. We first consider hotspots in a one-dimensional system with two phonon modes to extract the underlying physics. Thermal transport is found to be less efficient near these hotspots, leading to significantly smaller effective thermal conductivity. The mechanism behind this is that different phonon modes have different temperatures and are therefore not in equilibrium. Nonequilibrium is caused by both the selective electron-phonon interaction and ballistic phonon transport. For relatively large hotspots, nonequilibrium is primarily contributed to by the selective excitation effect. When the hotspot size further reduces, the contribution from ballistic phonon transport becomes increasingly more important. Furthermore, we quantitatively study the thermal transport of laser-heated hotspots in Raman experiments and Joule heating in fin field-effect transistors (FETs) using the nongray phonon Boltzmann transport equation. We find that the measured thermal conductivity of single-layer graphene can be significantly underestimated if the nonequilibrium effect is ignored. Additionally, the peak temperature rise in a silicon fin FET is much larger than that calculated by the heat-diffusion equation.

(Invited) Thermal Transport in Multi-component Nanoparticle Superstructures

This works presents a computational study of thermal properties of nanoparticle assemblies. Thermal management in micro- and nano-scale structures is of great significance in applications such as electronic devices. We investigate phonon transport and thermal properties of hybrid organic-inorganic nanocrystal assemblies using Molecular Dynamics simulations. The results demonstrate the dependence of thermal conductivity and phonon transport on various features of nanocrystalline structures and provide insight for future investigation of phononic spectrum of superstructures with complex, multi-component building blocks.

Probing the phonon mean free paths in dislocation core by molecular dynamics simulation

Thermal management is extremely important for designing high-performance devices. The lattice thermal conductivity of materials is strongly dependent on detailed structural defects at different length scales, particularly point defects like vacancies, line defects like dislocations, and planar defects such as grain boundaries. Traditionally, the McKelvey–Shockley phonon Boltzmann’s transport equation (BTE) method, combined with molecular dynamics simulations, has been widely used to evaluate the phonon mean free paths (MFPs) in defective systems. However, this method can only provide the aggregate MFPs of the whole sample, as it is challenging to extract the MFPs in different regions with varying thermal conductivities. In this study, the 1D McKelvey–Shockley phonon BTE method was extended to model inhomogeneous materials, where the contributions of defects to the phonon MFPs are explicitly obtained. Then, the method was used to study the phonon scattering with the core structure of an edge dislocation. The phonon MFPs in the dislocation core were obtained and were found to be consistent with the analytical model in a way that high frequency phonons are likely to be scattered in this area. This method not only advances the knowledge of phonon–dislocation scattering but also shows the potential to investigate phonon transport behaviors in more complicated materials.

Full counting statistics of phonon transport in disordered systems

The coherent potential approximation (CPA) within full counting statistics (FCS) formalism is shown to be a suitable method to investigate average electric conductance, shot noise as well as higher order cumulants in disordered systems. We develop a similar FCS-CPA formalism for phonon transport through disordered systems. As a byproduct, we derive relations among coefficients of different phonon current cumulants. We apply the FCS-CPA method to investigate phonon transport properties of graphene systems in the presence of disorders. For binary disorders as well as Anderson disorders, we calculate up to the $8$-th phonon transmission moments and demonstrate that the numerical results of the FCS-CPA method agree very well with that of the brute force method. The benchmark shows that the FCS-CPA method achieves $20$ times more speedup ratio. Collective features of phonon current cumulants are also revealed.

Thermal conductivity of single-layer MoS2(1−x)Se2x alloys from molecular dynamics simulations with a machine-learning-based interatomic potential

Single-layer transition metal dichalcogenides (TMDs) and their alloys have attracted intensive attention due to their potential in optoelectronics and energy conversion. Understanding the thermal transport in these two-dimensional materials is crucial for designing reliable devices where these materials are integrated. Molecular dynamics simulation is a commonly employed approach to investigate phonon transport and thermal conductivity in solids, but interatomic potentials that could satisfactorily characterize the phonon properties of multiple TMDs simultaneously are not available at present. In this paper, a machine-learning-driven interatomic potential for MoS2-MoSe2 system based on the spectral neighbor analysis approach is parameterized by learning from a large amount of data generated from first-principles. The phonon dispersion and mode-specific Gruneisen parameters of MoS2 and MoSe2, as well as the phonon dispersion of the MoS2-MoSe2 superlattice, obtained from first-principles are well reproduced by the potential parameterized. By performing equilibrium molecular dynamics simulations, the lattice thermal conductivity of MoS2(1−x)Se2x alloys are calculated and a ten-fold reduction compared with MoS2 is found when x = 50%. Moreover, the roles of mass disorder and force-field disorder on the low thermal conductivity is identified. The parameterized interatomic potential could be utilized to study phonon transport in other MoS2-MoSe2 nanostructures.

Phonon transport properties of two-dimensional electride Ca2N—A first-principles study

We investigate phonon transport in dicalcium nitride (Ca2N), an electride with two-dimensional confined electron layers, using first-principles density functional theory and the phonon Boltzmann transport equation. The in-plane (κ100) and out-of-plane (κ001) lattice thermal conductivities at 300 K are found to be 11.72 W m−1 K−1 and 2.50 W m−1 K−1, respectively. Spectral analysis of lattice thermal conductivity shows that ∼85% of κ100 and κ001 is accumulated by phonons with frequencies less than 5.5 THz and 2.5 THz, respectively. Modal decomposition of lattice thermal conductivity further reveals that the optical phonons contribute to ∼68% and ∼55% of overall κ100 and κ001, respectively. Phonon dispersion suggests that the large optical phonon contribution is a result of low frequency optical phonons with high group velocities and the lack of phonon bandgap between the acoustic and optical phonon branches. We find that the optical phonons with frequencies below ∼5.5 THz have similar three-phonon phase space and scattering rates as acoustic phonons. Comparison of the contributions from emission and absorption processes reveals that the three-phonon phase space and scattering rates of phonons—optical or acoustic—with frequencies below 5.5 THz are largely dominated by absorption processes. We conclude that the large contribution to lattice thermal conductivity by optical phonons is due to the presence of multiple low frequency optical phonon modes with high group velocities and similar phase space and scattering rates as the acoustic phonons. This study provides the frequency and temperature dependent lattice thermal conductivity and insights into phonon transport in Ca2N, both of which have important implications for the development of Ca2N based devices.

Coherent acoustic phonons in van der Waals nanolayers and heterostructures

Terahertz (THz) and sub-THz coherent acoustic phonons have been successfully used as probes of various quantum systems. Since their wavelength is in the nanometer range, they can probe nanostructures buried below a surface with nanometer resolution and enable control of electrical and optical properties on a picosecond time scale. However, coherent acoustic phonons have not yet been widely used to study van der Waals (vdW) two-dimensional (2D) materials and heterostructures. This class of 2D systems features strong covalent bonding of atoms in the layer planes and weak van der Waals attraction between the layers. The dynamical properties of the interface between the layers or between a layer and its supporting substrate are often omitted as they are difficult to probe. On the other hand, these play a crucial role in interpreting experiments and/or designing new device structures. Here we use picosecond ultrasonic techniques to investigate phonon transport in vdW InSe nanolayers and InSe/hBN heterostructures. Coherent acoustic phonons are generated and detected in these 2D systems and allow us to probe elastic parameters of different layers and their interfaces. In particular, our study of the elastic properties of the interface between vdW layers reveals a strong coupling over a wide range of frequencies up to 0.1 THz, offering prospects for high-frequency electronics and technologies that require control over the charge and phonon transport across an interface. In contrast, we reveal a weak coupling between the InSe nanolayers and sapphire substrates, relevant in thermoelectrics and sensing applications, which can require quasi-suspended layers.

Transmission function and thermal conductivity of Si phononic crystals

We investigate phonon transport properties of Si phononic crystals by using an atomistic Green function method. By varying form factors such as thickness, orientation, and pore size of unit cells, mean-free path and associated thermal conductivity of phononic crystals are obtained. We present the empirical formula for thermal conductivity as a function of cell size and, by extrapolating results from it for realistic cell sizes, the thermal conductivity is compared with experimental ones. The formula is found to predict a nearly equal amount of the suppression in thermal conductivity to the experimental values, implying a feasible calculation method for predicting thermal properties of phononic crystals.

Phonon Transport Simulations in Low-Dimensional Disordered Graphene Nanoribbons

In this paper, we investigate phonon transport in low-dimensional disordered armchair graphene nanoribbons (GNRs) in order to illuminate phonon transport effects in 1-D disordered systems. We use the nonequilibrium Green's function simulation technique for transport, coupled to the force constant method for obtaining the phonon bandstructure. We focus on how different parts of the phonon spectrum are influenced by geometrical confinement and line edge roughness. In the ballistic case, phonons throughout the entire phonon energy spectrum contribute to thermal transport. With the introduction of line-edge roughness, the phonon transmission is reduced, but quantitatively and qualitatively in different ways throughout the energy spectrum. We identify how each region of the spectrum reacts to low-dimensionality and disorder, and elaborate on how phonon transport is affected by that. We explain how and when phonons in different energies within the spectrum flow either ballistically, diffusively, or become localized depending on features of the channel geometry. Finally, we derive exponents related to the length and width dependence of the thermal transport in the GNRs under the influence of line-etch roughness. Our results could provide generic features of thermal transport in different classes of low-dimensional materials beyond GNRs, and could help to better understand the heat transport at the nanoscale.

Phonon transport in perovskite SrTiO3 from first principles

We investigate phonon transport in perovskite strontium titanate (SrTiO3), which is stable above its phase transition temperature (∼105 K), by using first-principles molecular dynamics and anharmonic lattice dynamics. Unlike conventional ground-state-based perturbation methods that give imaginary phonon frequencies, the current calculation reproduces stable phonon dispersion relations observed in experiments. We find that the contribution of optical phonons to the overall lattice thermal conductivity is larger than 60%, which is markedly different from the usual picture with the dominant contribution from acoustic phonons. Mode-dependent and pseudopotential-dependent analyses suggest the strong attenuation of acoustic phonon transport originating from strong anharmonic coupling with the transversely polarized ferroelectric modes.

Thermal conductivity reduction in Si-isotope-multilayers

We employ the atomistic Greenʼs function (AGF) formalism to investigate phonon transport in silicon isotope multilayers. Since we use the AGF method, we can distinguish between coherent scattering at isotope multilayers and incoherent phonon–phonon scattering. Our investigations show that there are two different regimes for coherent scattering at isotope multilayers: the periodic regime, with periodically arranged layers, and a random regime, with layers of different thicknesses. The phonon scattering is stronger in the random regime, whereas in the periodic regime the phonon scattering rapidly saturates with the number of layers. Even though the phonon scattering in the random regime is stronger than in the periodic regime, the resulting thermal resistance in both regimes is much lower than the one obtained by combining the interfaces of the multilayer incoherently. We also found that the periodic regime is sensitive to small deviations. A small deviation from the perfect periodic arrangement leads to the behavior of the random regime. These deviations are so small that it is extremely difficult to actually grow perfect periodic isotope multilayers and observe the periodic regime experimentally.

Temperature Enhancement Through Interaction of Thermal Waves for Phonon Transport in Silicon Thin Films

Recently, Siemens et al. found that the Fourier law may overestimate the energy transported away from one hot spot in the thin film. In this work, the lattice Boltzmann method is employed to investigate phonon transport in silicon thin films. It is found that in the transitional or ballistic regime when the thermal waves initiated from temperature disturbances on two surfaces of the thin film meet in the inner region, the temperature rises significantly, which is different from the case when the heat conduction is induced by the temperature disturbance on one surface of the thin film. Therefore, the interaction of thermal waves induced by the thermal transport from two nanoscale hot spots separated by a nanoscale distance inside the thin film may temporarily cause a higher temperature in some regions between the two hot spots than that predicted by the Fourier law.

Ballistic thermal conductance in graphene nanoribbon with double-cavity structure

We investigate phonon transport and thermal conductance in a Graphene Nanoribbon modulated with a double-cavity quantum structure at low temperatures. Two methods are compared: the force-constant and elastic wave continuum models. Calculations show that both the models show the similar thermal conductance property at low temperatures despite the excited theory of the discrete phonon modes in quantum structure being not the same. However, in the higher temperature region, the thermal conductance in the force-constant model is bigger than that in the elastic wave continuum model. The difference originates from the inequable cutoff frequencies of the phonon modes. A brief analysis of these results is given.

Thermal transport in graphene and effects of vacancy defects

We have performed molecular dynamics simulations to investigate phonon transport in graphene at 300 K with the Green-Kubo method. We show that the thermal conductivity (TC) of pristine graphene is $2903\ifmmode\pm\else\textpm\fi{}93$ W/mK, and the out-of-plane phonon mode contribution is $1202\ifmmode\pm\else\textpm\fi{}32$ W/mK. We further investigate thermal transport in graphene with different vacancy defect concentrations, and the TC shows that it is possible to achieve a maximum of a thousandfold reduction with a high defect density.

Fabrication of Microdevices with Integrated Nanowires for Investigating Low-Dimensional Phonon Transport

Phonons in low-dimensional structures with feature sizes on the order of the phonon wavelength may be coherently scattered by the boundary. This may give rise to a new regime of heat conduction, which can impact thermal energy transport and conversion. Traditional methods used to investigate phonon transport in one-dimensional structures suffer from uncertainty due to contact resistance, defects, and limited control over sample dimensions. We have developed a new batch-fabrication technique for suspended microdevices with integrated silicon nanowires from silicon-on-insulator (SOI) wafers. The nanowires are defect-free and have extremely high aspect ratios (length/critical dimension >2000). The nanowire dimensions (length and critical dimension) can be precisely controlled during fabrication. With these novel devices, phonon transport in silicon nanowires is systematically investigated. The room temperature thermal conductivity of nanowires with critical width around 80 nm is about 20 W/(m K) and much lower than that in smooth VLS wires. This suggests that the surface morphology of the structures has a significant effect on the thermal conductivity, but this phenomenon is not currently understood. This fabrication technique can also be used for thermal transport investigation in a wide-range of low-dimensional structures.

Monte Carlo Simulation of Silicon Nanowire Thermal Conductivity

Monte Carlo simulation is applied to investigate phonon transport in single crystalline Si nanowires. Phonon-phonon normal (N) and Umklapp (U) scattering processes are modeled with a genetic algorithm to satisfy energy and momentum conservation. The scattering rates of N and U scattering processes are found from first-order perturbation theory. The thermal conductivity of Si nanowires is simulated and good agreement is achieved with recent experimental data. In order to study the confinement effects on phonon transport in nanowires, two different phonon dispersions, one from experimental measurements on bulk Si and the other solved from elastic wave theory, are adopted in the simulation. The discrepancy between simulations using different phonon dispersions increases as the nanowire diameter decreases, which suggests that the confinement effect is significant when the nanowire diameter approaches tens of nanometers. It is found that the U scattering probability in Si nanowires is higher than that in bulk Si due to the decrease of the frequency gap between different modes and the reduced phonon group velocity. Simulation results suggest that the dispersion relation for nanowires obtained from elasticity theory should be used to evaluate nanowire thermal conductivity as the nanowire diameter is reduced to the sub-100 nm scale.