Phonon Thermal Transport Research Articles

Lattice thermal conductivity and phonon transport properties of monolayer fluorographene

Lattice thermal conductivity and phonon transport properties of monolayer fluorographene

Electronic vs phononic thermal transport in Cr-doped V2O3 thin films across the Mott transition

Understanding the thermal conductivity of chromium-doped V2O3 is crucial for optimizing the design of selectors for memory and neuromorphic devices. We utilized the time-domain thermoreflectance technique to measure the thermal conductivity of chromium-doped V2O3 across varying concentrations, spanning the doping-induced metal–insulator transition. In addition, different oxygen stoichiometries and film thicknesses were investigated in their crystalline and amorphous phases. Chromium doping concentration (0%–30%) and the degree of crystallinity emerged as the predominant factors influencing the thermal properties, while the effect of oxygen flow (600–1400 ppm) during deposition proved to be negligible. Our observations indicate that even in the metallic phase of V2O3, the lattice contribution is the dominant factor in thermal transport with no observable impact from the electrons on heat transport. Finally, the thermal conductivity of both amorphous and crystalline V2O3 was measured at cryogenic temperatures (80–450 K). Our thermal conductivity measurements as a function of temperature reveal that both phases exhibit behavior similar to amorphous materials, indicating pronounced phonon scattering effects in the crystalline phase of V2O3.

Advancing high thermal conductivity: novel theories, innovative materials, and applications in thermal management technologies.

Effective thermal management is crucial for the performance and stability of modern electronics, emphasizing the demand for high thermal conductivity (κ). This review summarizes the latest development in highκ, discussing the emerging theories, innovative materials and practical applications for interfacial heat dissipation. Unique phononic thermal transport behaviors are discussed, including four phonon-phonon scattering, hydrodynamic phonons, surface phonon-polaritons, and more. The review also highlights innovative materials with highκ, such as two-dimensional pentagonal structures, boron carbon nitrogen structures, hexagonal boron arsenide andθ-phase tantalum nitride. In addition, the potential of polymer composites reinforced with highκfillers and surface engineering for advanced electronic applications are also discussed. By integrating these theoretical approaches and material innovations, this review offers comprehensive strategies for enhancing thermal management in modern electronic devices.

Thermal Rectification in Graphene-Boron Nitride Nanotube Hybrid Structures: An Independent Control Mechanism for Forward and Backward Heat Flux.

The weak van der Waals interactions in the out-of-plane direction result in markedly low thermal conductivity in one-dimensional (1D) and two-dimensional (2D) materials, which substantially restricts their applications. Developing three-dimensional (3D) columnar hybrid structures, featuring high thermal conductivity both within and beyond the plane, effectively addresses this challenge. This study investigated a 3D hybrid structure composed of graphene and boron nitride nanotubes (GR-BNNTs) using non-equilibrium molecular dynamics simulations. This approach allowed the examination of the formation mechanisms and key factors influencing thermal rectification (TR) in these materials. Our findings reveal a novel mechanism for independently regulating forward and backward heat fluxes in GR-BNNTs. By manipulating the thermal properties of the BNNTs and the graphene layer, the TR ratio can be controlled flexibly. Additionally, we identify specific strategies for independently adjusting the heat flux, such as altering the intercolumn distance of BNNTs, which impacts the backward flux merely, while applying strain to affect the forward flux merely. This research introduces a novel concept of independent regulation of forward and backward heat fluxes, providing significant insights into phonon thermal transport in 3D hybrid structures.

Revisiting thermal transport in CuInTe2: Role of temperature-dependent anharmonicity and higher-order phonon–phonon interactions

The temperature-dependent phonon thermal transport in CuInTe2 is investigated by considering lattice thermal expansion, temperature-dependent anharmonicity, higher-order phonon–phonon interactions, and phonon renormalization with input from first-principles based density functional theory calculations. Incorporating these higher-order temperature-dependent effects reveals that the thermal conductivity varies with temperature as T−1.59 consistent with experimental measurements ranging from T−1.62 to T−1.78. Using the lowest-order theory or only temperature-dependent interatomic force constants or four-phonon scattering resulted in a wrong description of thermal transport physics.

The amplification effect of four-phonon scattering in CdX (X=Se, Te): The role of mid-frequency phonons

Recently, CdX (X = Se, Te) materials have garnered notable interest owing to their great application potential for high-efficiency solar cells, fiber optic communication systems, and nanoelectronic devices. The performance of optoelectric devices is significantly influenced by the thermal conductivity. In this study, a combination of machine-learning methods and first-principles calculations is utilized to investigate the phonon thermal transport properties of CdSe and CdTe, as well as to give deep insights into the phonon behavior in heat transfer. It is found that acoustic phonons dominate the lattice thermal conductivity (κ) in CdSe, while optical phonons contribute around 50 % to the κ value in CdTe. There are strikingly strong four-phonon scattering phenomena in both materials, which result in 55 % and 66 % reduction in κ values for CdSe and CdTe, respectively. Further analysis shows that the mid-frequency phonons play an important role in amplifying the four-phonon scattering. Its special frequency positions permit an unusually large number of four-phonon scattering processes to occur and cause large scattering of low-frequency phonons. This mechanism differs from earlier proposed features regarding large acoustic-optical gaps, bunching of acoustic branches and four-phonon Fermi resonance, which implies strong four-phonon anharmonicity. Thus, the present work opens up new prospects in the exploration and design of innovative thermoelectric material, which provides a more thorough understanding of the four-phonon scattering mechanisms in crystalline solids.

Isotope interface engineering for thermal transport suppression in cryogenic graphene

The development of emerging technologies, such as quantum computing and semiconductor electronics, emphasizes the growing significance of thermal management at cryogenic temperatures. Herein, by designing isotope interfaces based on the Golomb ruler, we achieved effective suppression of the phonon thermal transport of cryogenic graphene. The pronounced disordering of the Golomb ruler sequence results in the stronger suppression of thermal transport compared to other sequences with the same isotope doping ratio. We demonstrated that the Golomb ruler-based isotope interfaces have strong scattering and confinement effects on phonon transport via extensive molecular dynamics simulations combined with wave packet analysis, with a proper correction for the missing quantum statistics. This work provides a new stream for the design of thermal transport suppression under cryogenic conditions and is expected to expand to other fields.

Raman spectrum and phonon thermal transport in van der Waals semiconductor GaPS4

The emergent van der Waals semiconductor GaPS4 is heralding frontiers for gallium-based semiconductors. Despite its potential, the intricacies of its Raman spectrum and phonon heat transport remain elusive. In this research, experimental and theoretical methods are employed to give a comprehensive portrayal. The Raman spectra and phonon calculations obtained were cross-validated, affirming the study's credibility. A total of 28 Raman peaks were identified, with all phonon irreducible representations delineated. Advanced calculations unveiled notable shifts in the transition of GaPS4 from bulk to monolayer. During this process, phonons undergo a red shift, and the vibration contributions of different atoms change. The lifetime and group velocity of low wavenumber phonons are markedly reduced, suppressing the thermal conductivity in the monolayer. The thermal conductivity of GaPS4 bulk at 300 K is 0.5 W/m K, and 0.13 W/m K for monolayer, while the thermal conductivity in the cleavage direction is lower. These findings offer a detailed account of the complex Raman spectra and phonon thermal transport properties of GaPS4, setting the stage for its subsequent exploration and prospective applications in electronic and thermal devices, and contributing to enriching condensed matter theory of phonon thermal transport in van der Waals materials.

Topological phonons and thermal conductivity of two-dimensional Dirac semimetal PtN4C2

PtN4C2 is a recently predicted two-dimensional (2D) Dirac semimetal exhibiting significant topological quantum spin and valley Hall effects. Herein, we explore its topological phonon states and thermal transport properties from first-principles calculations. In terms of symmetry arguments, we predict the existence of multiple topologically protected phononic Dirac points in the frequency range of 0–20 THz, which are evidenced by the relevant irreducible representations and calculated nontrivial edge states on the (100) surface. In addition, anharmonic phonon renormalization is found to play a significant role in determining the phonon spectrum, especially for the out-of-plane flexural acoustic (ZA) branch. Moreover, we explicitly consider three-phonon scattering, four-phonon scattering, and phonon renormalization to predict the lattice thermal conductivity κl of PtN4C2, by solving the Boltzmann transport equation. With the incorporation of four-phonon scattering, we predict that the intrinsic κl is 68 W/mK at room temperature, which is reduced by about 45% as compared to the value obtained by only including three-phonon scattering. This reduction is found to arise mainly from the ZA phonons, whose contribution to κl is significantly suppressed by four-phonon scattering, due to the restriction of the mirror symmetry-induced selection rules on three-phonon processes. We also unveil that the presence of Dirac points steepens the surrounding phonon dispersion and thus greatly increases the phonon group velocities, thereby making a considerable contribution to κl. This work establishes a thorough understanding of intrinsic topological phonons and thermal transport in PtN4C2 and highlights the importance of phonon renormalization and higher-order anharmonicity in determining the phonon transport properties of 2D materials.

Phonon thermal transport between two in-plane, two-dimensional nanoribbons in the extreme near-field regime

Phonon thermal transport between two in-plane, two-dimensional nanoribbons in the extreme near-field regime

Unveiling the magic of twist angle: Thermal transport in Two-dimensional Graphene/MoS2/Graphene Heterostructures

Thermal transport properties of two-dimensional vdW heterostructures have attracted considerable research interests in electron-related devices due to their powerful capability of multi-functional integration. Herein, we investigated the in-plane and out-of-plane thermal conductivity of graphene/MoS2 heterostructures under various twist angles utilizing molecular dynamics simulations. It was found that the in-plane thermal conductivity of MoS2 layer in heterostructures decreases with twist angle. Notably, the lowest thermal conductivity occurs at a twist angle of 10.893°, just 47.3 % of that in untwisted MoS2 layers in heterostructure. In addition, the interlayer interfacial thermal resistance between graphene and MoS2 layer increases with the twist angle but decreases with the layer numbers of graphene. The atomic position deviation, potential energy surface, phonon state of density are calculated to reveal the regulation mechanism of thermal conductivity by twist angle. Interestingly, it is found that the twist angles and the layer numbers of graphene can control the atomic vibration arising from potential energy surfaces and phonon transport through the overlap of phonon density of states between graphene and MoS2 layers. This study can provide valuable insights into the thermal conductivity modulation and phonon transport mechanism of heterostructures.

In-plane thermal conductivity of hexagonal boron nitride from 2D to 3D

The in-plane thermal conductivity of hexagonal boron nitride (h-BN) with varying thicknesses is a key property that affects the performance of various applications from electronics to optoelectronics. However, the transition of the thermal conductivity from two-dimensional (2D) to three-dimensional (3D) h-BN remains elusive. To answer this question, we have developed a machine learning interatomic potential within the neuroevolution potential (NEP) framework for h-BN, achieving a high accuracy akin to ab initio calculations in predicting its thermal conductivity and phonon transport from monolayer to multilayers and bulk. Utilizing molecular dynamics simulations based on the NEP, we predict the thermal conductivity of h-BN with a thickness up to ∼100 nm, demonstrating that its thermal conductivity quickly decreases from the monolayer and saturates to the bulk value above four layers. The saturation of its thermal conductivity is attributed to the little change in phonon group velocity and lifetime as the thickness increases beyond four layers. In particular, the weak thickness dependence of phonon lifetime in h-BN with a nanoscale thickness results from its extremely high phonon focusing along the in-plane direction. This research bridges the knowledge gap of phonon transport between 2D and 3D h-BN and will benefit the thermal design and performance optimization of relevant applications.

Thermal transport in fullerene-based molecular junctions: molecular dynamics simulations

We investigate phonon thermal transport of fullerene-based single-molecule junctions by employing classical molecular dynamics (MD) simulations. We compute the thermal conductances of C60 fullerene monomers, dimers, and trimers utilizing three distinct MD methods. We observe the equilibration dynamics in one approach, and employ two other nonequilibrium steady state simulation methods. We discuss technical aspects of each simulation technique, and show that their predictions for the thermal conductance agree. Our simulations reveal that while the thermal conductance of fullerene monomer and dimer junctions remains similar, that of trimer junctions experiences a significant reduction. This study could assist in the design of high-performing thermoelectric junctions, where low thermal conductance is desired.

Electron–phonon coupling and non-equilibrium thermal conduction in ultra-fast heating systems

A discrete unified gas kinetic scheme is developed to solve the Boltzmann transport equation (BTE) with coupled electron and phonon thermal transport, whose time step is not limited by the relaxation time. The key of the present scheme is that the electron/phonon advection, scattering and electron–phonon interactions are coupled together within one time step by solving the BTE again at the cell interface. Numerical results show that the present scheme can correctly predict the electron–phonon coupling effects, and is in agreement with typical two-temperature model and experimental results in existing literatures and our performed time-domain thermoreflectance technique. It can also capture the ballistic effects when the characteristic length is comparable to or smaller than the mean free path where the two-temperature model fails. In addition, two anomalous heat conduction phenomena are predicted by the present scheme, namely, heat flows from phonon to electron in transient thermal grating geometry and re-rise reflected signal appears in Au/Pt bilayer metal structure. The present work can provide theoretical guidance for the study of electron–phonon coupling and offer a useful tool for multi-scale thermal management.

Four-Phonon Enhanced the Thermoelectric Properties of ScSX (X = Cl, Br, and I) Monolayers.

Recently, the FeOCl-type two-dimensional materials have attracted significant attention owing to their versatile applications in fields such as thermoelectricity and photocatalysis. This study aims to systematically investigate the thermoelectric properties of ScSX (X = Cl, Br, and I) monolayers by a combination of the first-principles calculations and the machine-learning interatomic potential approach. These monolayers are indirect semiconductors with band gaps of 3.22 (ScSCl), 3.27 (ScSBr), and 2.87 eV (ScSI), respectively. The lattice thermal conductivity is decreased by 25.72% (20.90%), 44.05% (40.00%), and 30.96% (34.76%) for ScSCl, ScSBr, and ScSI along the x-axis (y-axis) when the four-phonon scattering is introduced, indicating its important role in phonon transport. Anharmonic phonon scattering yields high Grüneisen parameter and scattering rate values, hence causing these low lattice thermal conductivities. Additionally, the large Seebeck coefficients and electrical conductivities of n-type doped ScSX monolayers contribute to their excellent power factors (24.69, 25.66, and 24.99 mW/K2·m for ScSCl, ScSBr and ScSI at 300 K, respectively). Based on the excellent power factor and low thermal conductivity, the maximum values of the figure of merit are calculated to be 2.68, 3.39, and 3.21 for ScSCl, ScSBr, and ScSI monolayers at 700 K, respectively. Our research provides valuable insights into the phonon thermal transport of ScSX monolayers and suggests a promising approach to address high-order anharmonicity.

Unleashing the power of artificial intelligence in phonon thermal transport: Current challenges and prospects

The discovery of advanced thermal materials with exceptional phonon properties drives technological advancements, impacting innovations from electronics to superconductors. Understanding the intricate relationship between composition, structure, and phonon thermal transport properties is crucial for speeding up such discovery. Exploring innovative materials involves navigating vast design spaces and considering chemical and structural factors on multiple scales and modalities. Artificial intelligence (AI) is transforming science and engineering and poised to transform discovery and innovation. This era offers a unique opportunity to establish a new paradigm for the discovery of advanced materials by leveraging databases, simulations, and accumulated knowledge, venturing into experimental frontiers, and incorporating cutting-edge AI technologies. In this perspective, first, the general approach of density functional theory (DFT) coupled with phonon Boltzmann transport equation (BTE) for predicting comprehensive phonon properties will be reviewed. Then, to circumvent the extremely computationally demanding DFT + BTE approach, some early studies and progress of deploying AI/machine learning (ML) models to phonon thermal transport in the context of structure–phonon property relationship prediction will be presented, and their limitations will also be discussed. Finally, a summary of current challenges and an outlook of future trends will be given. Further development of incorporating AI/ML algorithms for phonon thermal transport could range from phonon database construction to universal machine learning potential training, to inverse design of materials with target phonon properties and to extend ML models beyond traditional phonons.

Glass-Like Phonon Dynamics and Thermal Transport in a GeTe Nano-Composite at Low Temperature.

In this work, the experimental evidence of glass-like phonon dynamics and thermal conductivity in a nanocomposite made of GeTe and amorphous carbon is reported, which is of interest for microelectronics, and specifically phase change memories. It is shown that, the total thermal conductivity is reduced by a factor of three at room temperature with respect to pure GeTe, due to the reduction of both electronic and phononic contributions. This latter, similarly to glasses, is small and weakly increasing with temperature between 100 and 300 K, indicating a mostly diffusive thermal transport and reaching a value of 0.86(7)Wm-1K-1 at room temperature. A thorough investigation of the nanocomposite's phonon dynamics reveals the appearance of an excess intensity in the low energy vibrational density of states, reminiscent of the Boson peak in glasses. These features can be understood in terms of an enhanced phonon scattering at the interfaces, due to the presence of elastic heterogeneities, at wavelengths in the 2-20 nm range. The findings confirm recent simulation results on crystalline/amorphous nanocomposites and open new perspectives in phonon and thermal engineering through the direct manipulation of elastic heterogeneities.

Dual-channel phonon transport leads to low thermal conductivity in pyrochlore La2Hf2O7

The pyrochlore oxide La2Hf2O7 (LHO) has attracted extensive attention in thermal management applications. Accurate determination and comprehensive understanding of thermal conductivity (κ) stand as imperative prerequisites for the effective utilization and manipulation of LHO in various applications. In this study, with the assistance of machine learning interatomic potential, we investigate the microscopic mechanisms of phonon thermal transport in LHO crystals. Our findings show that the thermal transport in LHO defies a precise explication through the conventional framework of the Boltzmann transport equation (BTE) due to the emergence of diffusive phonon modes, engendered by strong anharmonicity. Using the state-of-the-art dual-phonon model incorporating both propagating and diffusive modes, we successfully reproduce the experimentally measured results in both magnitude and temperature dependence. It is also shown that the thermal conduction in LHO crystals is dominated by the propagating modes at low temperatures while by the diffusive modes at high temperatures, which synergistically lead to a low κ for LHO crystals. The novel insights into phonon thermal transport provide a stepping stone for regulating the thermal transport properties of LHO crystals.

Direct observation of strong momentum-dependent electron-phonon coupling in a metal.

Phonon scattering in metals is one of the most fundamental processes in materials science. However, understanding such processes has remained challenging and requires detailed information on interactions between phonons and electrons. We use an ultrafast electron diffuse scattering technique to resolve the nonequilibrium phonon dynamics in femtosecond-laser-excited tungsten in both time and momentum. We determine transient populations of phonon modes which show strong momentum dependence initiated by electron-phonon coupling. For phonons near Brillouin zone border, we observe a transient rise in their population on a timescale of approximately 1 picosecond driven by the strong electron-phonon coupling, followed by a slow decay on a timescale of approximately 8 picosecond governed by the weaker phonon-phonon relaxation process. We find that the exceptional harmonicity of tungsten is needed for isolating the two processes, resulting in long-lived nonequilibrium phonons in a pure metal. Our finding highlights that electron-phonon scattering can be the determinant factor in the phonon thermal transport of metals.

Phonon Thermal Transport in UO_{2} via Self-Consistent Perturbation Theory.

Computing thermal transport from first-principles in UO_{2} is complicated due to the challenges associated with Mott physics. Here, we use irreducible derivative approaches to compute the cubic and quartic phonon interactions in UO_{2} from first principles, and we perform enhanced thermal transport computations by evaluating the phonon Green's function via self-consistent diagrammatic perturbation theory. Our predicted phonon lifetimes at T=600 K agree well with our inelastic neutron scattering measurements across the entire Brillouin zone, and our thermal conductivity predictions agree well with previous measurements. Both the changes due to thermal expansion and self-consistent contributions are nontrivial at high temperatures, though the effects tend to cancel, and interband transitions yield a substantial contribution.