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The presence of an interlayer at the interface can significantly influence thermal transport, which is crucial for nanoscale materials and devices. As the structural scale approaches the mean-free-path or wavelength of phonons, the mode-resolved quantitative description of multiple reflections and transmissions of phonons between interfaces becomes challenging, thereby limiting the understanding of interfacial thermal transport. Considering the characteristic size of the structure, we examined the phonon propagation within the nanoscale interlayer from the perspective of phonon wave behaviors. A transfer matrix method is utilized to compute the energy transmission spectra of phonons across interfaces and directly determine the effective interfacial thermal conductance of the 3-layer structure. The model is validated through theoretical calculations involving typical heterostructures. Additionally, we suggest thermal optimization strategies for GaN-on-diamond structures incorporating SiN and AlN barrier layers to harness the potential performance of GaN-based electronic devices. As the interlayer thickness approaches sub-nanometers, we observe weakened phonon scattering, leading to a substantial enhancement in thermal transport. Energy transmission spectra reveal that this enhancement is driven by the dominance of tunneling phonons. • Wave behaviors of phonon propagation within nanoscale interlayer are examined. • A transfer matrix method is proposed to compute the phonon transmission spectra. • The model is validated through calculations involving typical heterostructures. • Weakened phonon scattering leads to a substantial enhancement in thermal transport. [ABSTRACT FROM AUTHOR]

Understanding the microscopic dynamics of the fundamental energy carriers in condensed matter under extreme pressure conditions can reveal insights into unique physical phenomena that are otherwise not easily discernible at ambient conditions. Here, by utilizing a combination of machine learning interatomic potential (MLP)-based molecular dynamics simulations to calculate phonon thermal conductivity and parameter-free first-principles calculations of electron–phonon coupling and electronic thermal conductivity, we show that (contrary to typical elemental metals) the phonon contribution to the total thermal conductivity in iridium can be increased from 18% at ambient conditions to 40% at 60 GPa hydrostatic pressure conditions. The strength of electron–phonon scattering, as quantified by the mass enhancement parameter, decreases monotonically with pressure resulting in an increase in the lifetime of electrons around the Fermi energy. This consequently leads to a monotonic increase in the electron thermal conductivity from 134 W m−1 K−1 at ambient to 177 W m−1 K−1 at 60 GPa. Similarly, the phonon thermal conductivity has a relatively higher increase from 30 W m−1 K−1 at ambient to 120 W m−1 K−1 at 60 GPa. This is attributed to considerable phonon hardening and increase in the group velocity of the heat carrying phonons with pressure. Along with the pronounced phonon contributions to the total thermal conductivity, we also show that the temperature dependence of the phonon thermal conductivity at ambient pressure is dictated by higher-order phonon interactions (beyond the typical three-phonon processes) in iridium. • Lattice contribution to thermal conductivity in iridium increases from 18% at 0 GPa to 40% at 60 GPa. • Higher-order phonon scattering (beyond three-phonons) dictates the lattice contribution. • Electron-phonon coupling decreases with increasing pressure in iridium. [ABSTRACT FROM AUTHOR]

• Gas adsorbates exhibit varying vibrational dynamics in different porous ZIF phases. • Thermal conductivities and heat conduction mechanisms of various ZIF-adsorbate systems are evaluated. • Gas dynamics are "fingerprints" to govern the influences of adsorbates on heat transport of MOFs. Understanding the thermal conductivity (κ) of metal-organic frameworks (MOFs) with adsorbates is critical for tailoring their heat dissipation in practical applications. However, a detailed description of how adsorbates influence the heat transport of MOFs is still lacking. In this work, we employ molecular dynamics simulations combined with lattice dynamics calculations to investigate the effect of gas adsorbates on κ of representative crystalline and glassy zeolitic imidazolate frameworks (ZIFs). We identify that the diffusive adsorbate motions in the crystalline ZIFs with large pores can enhance lattice anharmonicity, serving as a source of phonon scattering and significantly suppressing κ. Conversely, the adsorbates with restricted dynamics in limited space (i.e., in the dense ZIF crystals or glasses) behave more like a heat transfer medium and facilitate heat transport. Besides pore size, gas dynamics are also proved to vary upon gas-lattice interaction strength, and dictate if gas adsorbates serve as a heat conduction suppressor or promoter. We anticipate that this governing role of gas dynamics is applicable to a diverse range of MOF-adsorbate systems. As such, our work offers guidance for the practical thermal management of MOF porous materials to ensure their desired storage or separation performance. [Display omitted] [ABSTRACT FROM AUTHOR]

• The unusual phonon transport mechanism of methane hydrate under different strains was investigated by classical molecular dynamics simulation. • The thermal conductivity of methane hydrate attains a maximum value at a strain of −0.04. • The preferential phonon channels were opened due to the overlapping of the phonon vibration modes between methane and water molecules. Unveiling the thermal transport mechanisms of natural gas hydrate under strain conditions is fundamental to the heat transfer of the hydrate reservoir during drilling and production. This work investigated the structural stability and phonon transport mechanisms of methane hydrate using equilibrium molecular dynamics simulations subjected to uniaxial compression strain. The results indicate that the stability of large cages within methane hydrates weakens under strain. However, the presence of methane molecules effectively stabilizes the cage structure, enabling it to remain intact even under significant deformation. Furthermore, the thermal conductivity of methane hydrate demonstrates non-linear behavior, characterized by an initial increase followed by a subsequent decrease with increasing uniaxial compression strain. Particularly, the thermal conductivity of methane hydrate attains a maximum value of 0.87±0.018 W⋅m−1⋅K−1 at a strain of -0.04. The results suggest that the elevated spectral energy density and the prolonged phonon lifetime contribute to the enhanced thermal conductivity of methane hydrate. Interestingly, it was observed that the overlapping of phonon vibration modes between water and methane molecules results in the suppression of the peaks and valleys when the strain reaches -0.04. This overlap eliminates the gap and opens up an advantageous channel for phonon transport, thereby enhancing the heat transport capability of methane hydrate. The opening of an advantageous channel for phonon transport results in an extended phonon lifetime and a subsequent increase in thermal conductivity. This study presents a novel perspective for comprehending the unusual thermal conductivity of methane hydrate under strain conditions. [ABSTRACT FROM AUTHOR]

Since the landmark paper by Hicks and Dresselhaus [Phys. Rev. B 47 , 16631(R) (1993)], there has been a general consensus that one-dimensional nanoscale conductors, i.e. nanowires, provide the long sought paradigm to implement the so-called phonon-glass electron-crystal material, which results in large improvements in the thermoelectric figure of merit ZT. Despite some encouraging—though isolated—experimental results, this idea has never been subjected to a rigorous scrutiny and the effect of the coupled dynamics of electrons and phonons has usually been oversimplified. To bypass these limitations, we have calculated the effective thermoelectric parameters for silicon nanowires (SiNWs) by iteratively solving the coupled electron-phonon Boltzmann transport equation (EPBTE) supplied with first-principles data. This allows for an unprecedented precision in determining the correct dependence of the thermoelectric parameters with system size; including, but not limited to, the figure of merit and its enhancement or degradation due to nanostructuring. Indeed, we demonstrate that the commonly used relaxation time approximation (RTA), or the uncoupled beyond the RTA (iterative) solution fail to describe the correct effect of nanostructuring on the thermoelectric properties and efficiency in SiNWs due to the strong contribution of phonon drag to the Seebeck coefficient, so that the use of fully coupled solution of the EPBTE is essential to obtain the correct effect of nanostructuring. Most importantly, we show that, contrarily to what commonly argued, resorting to NWs is not necessarily beneficial for ZT. Indeed, in a wide range of diameters nanostructuring diminishes the Seebeck coefficient faster than the decrease in thermal conductivity, due to the suppression of very long wavelength phonons responsible for the largest contribution to the phonon drag component of the Seebeck coefficient. This penalty to ZT can be mitigated if the NWs have a very rough surface, providing additional reduction to the thermal conductivity. Additionally, we demonstrate that our methodology provides improved data sets for an accurate determination of doping concentration in NWs through electrical-based inference and excellent agreement with the available experimental data. • Proper inclusion of electron-phonon coupling is required for accurate computational prediction of thermoelectric properties. • Large- and medium-thickness (R ≳ 30 nm) smooth Si nanowires may have a ZT lower than bulk. • Above is due to the suppression of the phonon drag contribution to S , illustrating that nanostructuring not always benefits ZT. • Large surface roughness diminishes the reduction of ZT when going to the nanowire form. • Expressions are provided to extract doping from the measurement of the radius and Seebeck coefficient of a Si nanowire. [ABSTRACT FROM AUTHOR]

• A new two-dimensional gallium oxide Ga 2 O 2 is predicted and systematically investigated. • Two-dimensional Ga 2 O 2 monolayer has an indirect band gap of 2.77 eV and a high room-temperature electron mobility up to 12,800 cm2 V−1 s−1. • Two-dimensional Ga 2 O 2 has extremely low thermal conductivity, attributed to the relatively high group velocity of optical phonons and the strong anharmonicity of acoustic phonons. • Two-dimensional Ga 2 O 2 has a high ZT value of beyond 3 at room temperature, and up to 6.5 at 500 K. Gallium oxide such as two-dimensional (2D) Ga 2 O 3 monolayers has been widely used in lasers, dielectric coatings for solar cells, low-dimensional electronic and optoelectronic device applications due to their excellent electrical optical and thermal properties. In this work, we predicted a new 2D gallium oxide Ga 2 O 2 and systematically investigated its stability, electronic properties, and thermoelectric performance. The phonon dispersion, elastic tensor and ab initio molecular dynamics simulations show that the Ga 2 O 2 monolayer is dynamically, mechanics and thermally stable. We find the Ga 2 O 2 monolayer is an indirect semiconductor with a wide band gap of 2.77 eV and high electron mobility. In particular, 2D Ga 2 O 2 has extremely low thermal conductivity and the optical phonons dominate the thermal conductivity. By analyzing its harmonic and anharmonic properties, we find that the weak interaction between Ga atoms is the main reason for this phenomenon. At last, we predict that the maximum ZT values of 2D Ga 2 O 2 are evaluated up to 6.5 at 500 K and 3.3 at 300 K, respectively. With ultra-low thermal conductivity and high thermoelectric performance, 2D Ga 2 O 2 is considered a potential candidate for thermoelectric materials. [ABSTRACT FROM AUTHOR]

The first-principles calculations have been widely used to predict the thermoelectric figure of merit (ZT) for the single crystal, but such precise calculations are confined to phonon-limited intrinsic system, and the defects present in practical samples cannot be easily modeled. So, a question arises as to whether the phonon-limited ZT can represent the thermoelectric potential of the material. In this work, we demonstrate that the exclusion of defect scattering could provide ZT close to the actual values in single crystal by analyses of the ZT involved parameters. This is also illustrated in the case of SnSe by performing first-principles calculations of phonon-limited ZT compared with available experimental data. The calculated ZT can reproduce the measured values, indicating that the calculations possess predictive capability. On the basis, strong and anisotropic thermoelectric enhancement of SnSe induced by pressure is found. The pressure significantly increases the thermoelectric power factor of n -type SnSe along its a crystallographic axis and that of p -type SnSe along the b axis, while the increase of lattice thermal conductivity is relatively small. As a result, a large enhancement of ZT is obtained. By applying pressure up to 3.2 GPa, at room temperature, the a -axis ZT of n -type SnSe is as high as 1.2, while the b -axis ZT of p -type SnSe reaches up to 0.8, which are more than twice larger than the values at ambient pressure. This study provides strong confidence and support for the application of first-principles calculations to search high-performance thermoelectric materials and thus would be an important reference for future researches. • The ZT of defected single crystal is reasonably obtained by first-principles calculations without defect scattering. • The calculated ZT for SnSe reproduces the experimental values. • The pressure significantly increases the ZT of n - and p -type SnSe along its a and b axis, respectively. [ABSTRACT FROM AUTHOR]

• Heat conduction in solids exhibits fluid-like, convective energy transport, diverging from the classical theory of diffusive heat transfer. • Employing the Monte Carlo algorithm integrated with ab initio approaches to phonon scatterings, Boltzmann transport equations beyond the relaxation time approximation uncovers a novel hydrodynamic phonon transport regime in graphite. • Phonon vortex and phonon jet flow are visualized in hydrodynamic phonon transport of graphite. In classical theory, heat conduction in solids is regarded as a diffusion process driven by a temperature gradient, whereas fluid transport is understood as convection process involving the bulk motion of the liquid or gas. In the framework of ab initio theory, which is directly built upon quantum mechanics without relying on measured parameters or phenomenological models, we observed and investigated the fluid-like convective transport of energy carriers in solid heat conduction. Thermal transport, carried by phonons, is simulated in graphite by solving the Boltzmann transport equation using a Monte Carlo algorithm. To capture convective transport, with phonon distributions deviating significantly from equilibrium Bose-Einstein distribution, we determined phonon interactions using ab initio approaches that go beyond relaxation time approximations. The presence of strong momentum-conserved Normal scatterings in graphite introduces a regime for hydrodynamic phonon transport. Fluid-like features, such as vortex and jet flow, are visualized and compared with classical theories on heat diffusion and fluid convection. Our study on phonon convection enhances fundamental understandings of heat conduction in solids from both atomic scale and quantum aspects, innovating thermal designs for future microelectronic devices and other thermal management applications. This potentially offers solutions for heat dissipation challenges in the post-Moore era. [ABSTRACT FROM AUTHOR]

• Thermal conductivity of Si nano films is reduced due to resonant pillars. • The thickness of nanofilm impacts thermal transport more than resonant pillars. • Void defects on pillars can disrupt phonon resonance. Nanophononic materials have shown significant potential for advancing future thermoelectrics due to their ability to achieve low thermal conductivity. Recent studies have unveiled that thermal conductivity can be further diminished through the phenomenon of phonon resonance, which emerges from the incorporation of additional pillar structures. In this investigation, we delve into the realm of phonon resonance within silicon nanofilms (SiNFs) outfitted with resonant pillars, by employing classical molecular dynamics simulations. Our study reveals that SiNFs featuring resonant pillars exhibit a variable reduction in thermal conductivity contingent upon the thickness of the SiNFs. This reduction is notably influenced by the geometry of the pillars. Remarkably, thermal conductivity can be diminished by up to 20 % for SiNFs endowed with pillars on both sides, characterized by a pillar height of 0.8 nm and a duty cycle of 50 %. This reduction significantly surpasses the corresponding decrease observed in SiNFs equipped with unilateral pillars (13 %). Furthermore, the choice of an appropriate pillar height contributes to a more substantial drop in thermal conductivity. Specifically, a reduction of 14 % is observed for the model with a pillar height of 0.8 nm, while a height of 3.1 nm leads to a more pronounced reduction of 21 %. Intriguingly, we also identified that void defects present on the pillars have the potential to disrupt phonon resonance, thereby leading to an increase in thermal conductivity. The insights garnered from our findings offer valuable guidance for the strategic design of low-dimensional materials, capitalizing on the phenomenon of phonon resonance. [ABSTRACT FROM AUTHOR]

• The N-face GaN optical phonon branch is localized more strongly than Ga-face. • The interface phonon of GaN/Al x Ga 1- x N is significantly localized. • The interface phonon dispersion curve is elevated. • Lattice distortion leads to a drift of the interface phonon branch. The phonon transport processes within a few atomic layers near heterointerfaces play a crucial role in thermoelectric applications such as nitride-based high electron mobility transistors (HEMTs) and light-emitting diodes (LEDs). However, a systematic investigation of their localized transport characteristics remains unexplored. This article employs the Interfacial Phonons method based on first-principles calculations to extract interface phonon information for GaN/Al x Ga 1- x N heterostructures. Molecular dynamics simulations are utilized to compare the phonon behaviors between Ga-face and N-face GaN-based heterointerfaces. The research extensively investigates the dependence of interfacial thermal transport on the concentration of Al atoms under ordered substitutional doping and the strength of uniaxial compressive strain. The research revealed that both Ga-face and N-face GaN/Al x Ga 1- x N heterointerfaces exhibit phonon branch drift. The differences in charge distribution at the polar surfaces lead to varying degrees of Brillouin zone distortion. The vibrational modes of the atoms at the N-face GaN interface are more compact and localized than those of Ga-face GaN, resulting in a stronger degree of localization for the optical phonon branches. When varying the doping concentration of Al atoms, some phonon branches at the GaN interface appear outside the band structure of the intrinsic bulk and surface states, exhibiting strong localized characteristics. The introduction of Al atoms at the heterointerface induces additional phonon scattering, and the lattice vibration mismatch between Al and Ga atoms results in a non-uniform residual stress field at the interface, further enhancing phonon-interface scattering. Moderate compressive strain-induced lattice distortion promotes the overlapping of phonon modes at the interface, allowing some localized phonons to serve as interface transport modes. This optimization enhances phonon transport at the heterointerface. This research provides valuable insights into a comprehensive understanding of interfacial thermal transport in GaN-based semiconductors. Moreover, it offers useful theoretical guidance for thermal management through the manipulation of phonon wave properties. [ABSTRACT FROM AUTHOR]

• Ultra-low lattice thermal conductivity of BaAg 2 SnSe 4 with the inclusion of the temperature-induced phonon coherent resonance effect is firstly captured. • Coupled with particle-like and wave-like propagation of phonon excitations successfully resolve the discrepancy between the theoretical and experimental κ L. • The results unequivocally verify the authenticity of the two-channel transport phenomenon driven by temperature in the case of superior thermoelectric materials. Phonon heat transfer by thermoelectric modules undoubtedly plays a pivotal role for the future waste heat recovery and utilization. BaAg 2 SnSe 4 , a prototypical glasslike compound among quaternary non-centrosymmetric chalcogenides, demonstrates remarkable thermoelectric performance due to its exceptionally low lattice thermal conductivity (κ L). Even with a comprehensive grasp of the underlying cause behind ultra-low κ L and the subsequent heightened thermoelectric conversion efficiency, the temperature-dependent lattice dynamics and glass-like attributes governing κ L continue to pose unresolved questions. In this study, we investigate the phonon thermal transport properties of quaternary BaAg 2 SnSe 4 by incorporating the influence of phonon coherent resonance, thereby departing from the conventional Peierls's framework. Based on the temperature dependence of the interatomic force constants and coherences' contribution to total lattice thermal conductivity (κ L (C)), we intriguingly observe that the particle-like driven lattice thermal conductivity (κ L (P)) and κ L (C) have the quite significant and dominant role for describing ultralow phonon transport behaviors in the middle-high-temperature regions. Specially, enhanced phonon anharmonicity coupled with excited coherent phonons due to increasing temperature yields two close values, with 700 K values of 0.16 and 0.13 W/mK predicted for BaAg 2 SnSe 4 , respectively, accelerating its application in thermoelectric field. Furthermore, we also find that the off-diagonal terms of heat flux operators in κ L exist distinction for different phonons, with zero contribution modes exhibiting temperature-independent phonon coherence effect. Temperature response of the force constants tensors and coherent resonance proposed in our work can be employed as a powerful tool, which would substantially be expected to inspire a broad revisiting phonon transport study in various device applications, including advanced thermal barrier coatings and superior thermoelectrics. [Display omitted] [ABSTRACT FROM AUTHOR]

• For the first time, first-principles calculations are used to predict the lattice thermal conductivity of free-standing monolayer XS2 (X = mo and W) and monolayer XS2 supported on SiC (0001) and ZnO (0001) substrates. • The lattice thermal conductivity of monolayer XS2 supported on SiC (0001) and ZnO (0001) substrates show a significant decrease, and the decrease is even greater for the monolayer XS2 supported on ZnO (0001) substrates, reaching about 65–70 %. • The substrate drives softening of the acoustic branch, making the low-frequency phonon group velocity decrease and the three-phonon scattering enhance. • From the perspective of atomic bonding, the substrate breaks the symmetry of bonding of monolayer XS2, which leads to the increase of phonon anharmonicity and ultimately to a decrease in thermal conductivity of monolayer XS2. Thermoelectric materials have properties such as direct conversion of waste heat into electricity and environmental friendliness, which make them promising for renewable energy utilization, however, the lower conversion efficiency limits the large-scale application of thermoelectric materials. Monolayer transition metal dichalcogenides (TMDs) are candidates for thermoelectric materials due to their high power factor, but their higher thermal conductivity (TC) limits the further improvement of the figure of merit (ZT). Here, we employ first-principles calculations and solve the Boltzmann transport equation (BTE) to compare the lattice TC of free-standing monolayer XS 2 (X = Mo and W) and monolayer XS 2 supported on SiC (0001) and ZnO (0001) substrates. We find that the lattice TC of monolayer XS 2 supported on SiC (0001) and ZnO (0001) substrates are sharply reduced. Especially supported on the ZnO (0001) substrate, the lattice TC of XS 2 reduce by 65 %–70 %, which is expected to improve the thermoelectric properties of monolayer XS 2. In addition, we report the underlying cause of this significant reduction in TC. Firstly, the substrate drives softening of the acoustic branch, which leads to a decrease of phonon group velocity in the low-frequency and an enhancement of the three-phonon scattering. Then, the asymmetric bonding of XS 2 is induced by the substrates, which enhance phonon anharmonicity and reduce the phonon lifetime. Our results demonstrate that the lattice TC of monolayer XS 2 can be tuned by adjusting the substrate, which has certain reference values for the thermoelectric design of XS 2. SiC (0001) and ZnO (0001) substrates break the symmetry of the monolayer XS 2 (X = Mo and W) bonding, resulting in an increase in phonon anharmonicity, which reduces the contribution of the low-frequency acoustic branch of monolayer XS 2 to the thermal conductivity. [Display omitted] [ABSTRACT FROM AUTHOR]

• A new type of double-layer vdWH (Ti 2 C/BP vdWH) was constructed by stacking pristine monolayer black phosphorus (BP) and Ti 2 C vertically. • Stacking BP and Ti 2 C achieves great thermoelectric properties, owing to these two materials have complementary thermoelectric properties. • The ZT value of Ti 2 C/BP vdWHs at room temperature (300 K) is approximately 103 and 104 times that of Ti 2 C an BP, respectively. • Ti 2 C/BP vdWHs have higher electron conductance and lower phonon conductance than both BP and Ti 2 C. • The mutual compensation of the electronic energy bands and interfacial phonon scattering enhance the thermoelectric properties. Vertical van der Waals heterostructures (vdWHs) exhibit considerable freedom in integrating two-dimensional layered materials for achieving excellent performance. In this study, we construct a double-layer Ti 2 C/BP vdWH by stacking pristine monolayer black phosphorus (BP) and Ti 2 C vertically. The in-plane thermoelectric properties of BP, Ti 2 C and Ti 2 C/BP vdWHs at different temperatures are investigated using density functional theory combined with the non-equilibrium Green's function method. Results showed that the thermoelectric figure of merit (ZT) of Ti 2 C/BP vdWHs at room temperature (300 K) was approximately 103 and 104 times that of Ti 2 C and BP, respectively. Because of the mutual compensation of the electronic energy bands and interfacial phonon scattering, Ti 2 C/BP vdWHs have higher electron conductance and lower phonon conductance than both BP and Ti 2 C, which significantly enhances the ZT value of these structures. More importantly, the prominent thermoelectric properties of Ti 2 C/BP vdWHs maintain greater stability as the temperature rises (300–900 K), which leads to a high thermoelectric power factor and ZT value without relying on high temperatures. These results provide a new idea for designing two-dimensional functionalised nanoscale devices with excellent thermoelectric performance. [ABSTRACT FROM AUTHOR]

Bismuth-based compounds have been identified as a class of promising candidates for the realization of high-performance thermoelectrics, derived from their exceptional electrical conductivity and intrinsic low lattice thermal conductivity κ L. Here, we employed first-principles calculations, self-consistent phonon theory, and compressive sensing technique in conjunction with the Boltzmann transport equation to investigate the anharmonic thermoelectric properties of full-heusler bismuth compounds Cs x Rb 3 − x Bi. As the Cs content increases, the κ L of Cs x Rb 3 − x Bi decreases due to impurity scattering effects, and then increases. Among them, at 300 K, Rb 3 Bi has the largest κ L of 0.69 Wm−1K−1, while Cs 2 RbBi has the smallest κ L of 0.45 Wm−1K−1, both of which are lower than the κ L of traditional bismuth-based thermoelectric materials. The ultralow κ L originates from the strong lattice anharmonicity. In addition, the coexistence of high dispersion and flat band edges leads to high thermoelectric power factor at optimal doping concentration. As a result, the largest thermoelectric figure of merit ZT values of 2.59 (4.19) at 300 (500) K were obtained for CsRb 2 Bi at optimal hole doping level. These findings suggest that Cs x Rb 3 − x Bi compounds are superior for thermal management and thermoelectric applications. • At finite temperatures, Cs x Rb 3 − x Bi compounds are not only mechanically stable but also dynamically stable. • The κ L of Rb 3 Bi, CsRb 2 Bi, Cs 2 RbBi, and Cs 3 Bi are 0.69, 0.47, 0.45, and 0.56 Wm−1K−1, respectively, at 300 K. • The p-type doped CsRb 2 Bi exhibits a ZT value of 2.59, while at 500K, the ZT value reaches 4.19. • The ultra-low κ L of CsRb 2 Bi originates from the rattling vibration of Bi atoms, which manifests as an 'avoided crossing' phenomenon. [ABSTRACT FROM AUTHOR]

• The lattice thermal conductivity (κ) of uniaxial diamond has been calculated from first-principles. • The sensitivity of the κ of uniaxially strained diamond to size effects differed according to the scale. • The phonon group velocities of the TA1 and LA branches under different strains led to the relative change of the two κ at different temperatures. The miniaturization of high-frequency electronic devices has led to heat accumulation in their components. The ultrahigh thermal conductivity of diamond makes it an ideal material for heat dissipation, but factors such as temperature changes during the working process, special structures, thermal stress, and lattice mismatch can affect performance. This study investigated the effects of stress, temperature, isotope purity, and feature size on the thermal conductivity of diamond using a first-principles method. The sensitivity to size effects of the thermal conductivity of uniaxially strained diamond differed according to the scale. Under a low temperature (100 K), and with a feature size of 1 µm, the thermal conductivity of diamond with large-scale tensile strain exceeded that of a diamond with small-scale strain, which was an anomalous phenomenon. Isotope purification did not significantly affect the variation rule of the thermal conductivity of strained diamond. [Display omitted] [ABSTRACT FROM AUTHOR]

• A differential electro-thermal bridge measurement is established to enhance the signal-tonoise ratio and obtain over 40% reduction in the uncertainty in the measured thermal resistance of individual multi-walled carbon nanotubes (MWCNTs). • The measured interface thermal resistance is used to obtain the sample temperature profile at the contact and eliminate the contact thermal resistance error in the measured thermal conductivity of the suspended sample segment. • The measured thermal resistances of multiple suspended segments of the same MWCNT increase linearly with length, revealing diffusive phonon transport due to phonon-defect scattering. • Theoretical analysis of the measurement results suggests submicron phonon mean free paths due to extrinsic phonon scattering by extended defects such as grain boundaries. • Electron beam damage and surface amorphous coating of the nanotube decreases the obtained thermal conductivity by a factor of 3. Carbon nanotubes (CNTs) are quasi-one dimensional nanostructures that display both high thermal conductivity for potential thermal management applications and intriguing low-dimensional phonon transport phenomena. In comparison to the advances made in the theoretical calculation of the lattice thermal conductivity of CNTs, thermal transport measurements of CNTs have been limited by either the poor temperature sensitivity of Raman thermometry technique or the presence of contact thermal resistance errors in sensitive two-probe resistance thermometry measurements. Here we report advances in a multi-probe measurement of the intrinsic thermal conductivity of individual multi-walled CNT samples that are transferred from the growth substrate onto the measurement device. The sample-thermometer thermal interface resistance is directly measured by this multi-probe method and used to model the temperature distribution along the contacted sample segment. The detailed temperature profile helps to eliminate the contact thermal resistance error in the obtained thermal conductivity of the suspended sample segment. A differential electro-thermal bridge measurement method is established to enhance the signal-to-noise ratio and reduce the measurement uncertainty by over 40%. The obtained thermal resistances of multiple suspended segments of the same MWCNT samples increase nearly linearly with increasing length, revealing diffusive phonon transport as a result of phonon-defect scattering in these MWCNT samples. The measured thermal conductivity increases with temperature and reaches up to 390 ± 20 W m−1 K−1 at room temperature for a 9-walled MWCNT. Theoretical analysis of the measurement results suggests submicron phonon mean free paths due to extrinsic phonon scattering by extended defects such as grain boundaries. The obtained thermal conductivity is decreased by a factor of 3 upon electron beam damage and surface contamination of the CNT sample. [ABSTRACT FROM AUTHOR]

• Four-phonon scattering is significant in 2D hexagonal structures. • Atomic mass ratio's raise enhances four-phonon scattering and weakens N scattering process. • N scattering process is stronger in three-phonon process. Comprehensive understanding of phonon transport will facilitate the exploration of materials. Four-phonon scattering is find to be important to determine thermal conductivity in many materials, and normal scattering (N process) could lead to some unique phonon transport behaviors, especially in 2D materials. In this work, we studied four-phonon and normal scattering in hexagonal structures, both of which were found to be significant. With the increase of atomic mass ratio, the relative four-phonon scattering is gradually enhanced, whereas N process is weakened. Callaway model and Allen's modified models were used to approximate the thermal conductivity, and we find that the models are applicable in some cases with relative weak N scattering intensity. [ABSTRACT FROM AUTHOR]

• Smooth GaN-diamond interface has a low thermal resistance (1.5m2·K/GW), consistent with the diffusion mismatch model. • Ga/N vacancy vacancies increase the interfacial thermal resistance by up to 67% at a concentration of 30%. • Vacancy-phonon scattering boosts anharmonic phonon scattering, redistributing energy and disturbing collective vibration. • Thermal conductivity reduction of defected GaN significantly increases the total resistance of a junction. Diamond substrate with superior thermal conductivity has been the most promising heat sink to solve the heat dissipation issues in GaN-based power electronics. The phonon transport behaviors across the GaN-diamond interface crucially determine the thermal performance and still remain largely unexplored especially considering the presence of pre-interface vacancies. Herein, the influence of localized high-concentration Ga/N atomic vacancies on phonon transport across the GaN-diamond interface is fully investigated using non-equilibrium molecular dynamics. The calculated interface thermal resistance (ITR) of the perfect GaN-diamond interface at room temperature is approximately 1.5 m2·K/GW, consistent with prediction of the diffusion mismatch model. After introducing the Ga/N vacancies at a concentration of 30%, the existence of vacancy-phonon scattering increases the interfacial thermal resistance (ITR) by up to 67%. Analyzing the phonon density of states and the spectral heat current, the heat flux contributed by GaN high-frequency phonons decreases after introducing vacancy defects. This is attributed that the vacancy-phonon scattering in GaN enhances anharmonic phonon scattering, and the phonon energy is redistributed among different phonon modes. Moreover, the collective vibration of propagation phonons is disrupted randomly by the vacancy-phonon scattering. This work aims to understand the influence of vacancy-phonon scattering on the phonon transport across GaN-diamond interface and provide helpful guidance to engineering such interface for better heat dissipation performance. [ABSTRACT FROM AUTHOR]

• The functionalization will deteriorate the thermal transport in filler. • The functionalization will enhance the interface thermal conductance. • The functionalization is helpful below the critical value. • The C-doping is the most effective type of functionalizations. Functionalizations of boron nitride nanosheets (BNNS) are found to be useful for improving the thermal conductivity of materials. However, the mechanism responsible for the enhancement is still open. In this paper, classic molecular dynamic simulations and theoretical analysis model based on the effective medium theory were adopted to investigate the effect mechanism of three types of functionalizations, including C-doping, CH 3 -grafting and the single-vacancy, on the thermal conductivity of BNNS/paraffin composites. Research results showed that the functionalizations would produce the phonon impurity scattering, which deteriorated the thermal transport of BNNS. However, it could promote the phonon coupling between in-plane and out-of-plane of BNNS, which will boost the energy propagation between paraffin and BNNS and eventually enhance the interface thermal conductance. Under the interaction of the two opposite phenomena, the C-doping functionalization shows to be the most effective method to enhance the thermal conductivity of composites. Comparing with pristine BNNS (p-BNNS), there was a critical size for the thermal conductivity enhancement of functionalized BNNS/paraffin composites. The critical size of the C-doping could reach to 9.1 μm for the functionalized ratio of 2.5%. With the size of 2 μm of the C-doping BNNS, the thermal conductivity of composites could be enhanced by 30% compared with the p-BNNS/paraffin and 10 times compared with paraffin. The current research provides fundamental suggestions for the selections of fillers to enhance the thermal conductivity of matrix. [ABSTRACT FROM AUTHOR]

Highlights • Developed novel FET-Raman for measuring the k of suspended nm-thick materials. • The FET-Raman does not need laser absorption data and temperature rise. • The FET-Raman can measure martials of various thickness: nm to bulk. • The FET-Raman is insensitive to optical properties of nm-thick materials. • The FET-Raman frequency can be adjusted to reach the optimum accuracy. Abstract Temperature-dependent Raman properties calibration and laser absorption evaluation are two significant sources of error in measurement of nm-thick materials thermal conductivity based on steady state Raman spectroscopy. A new Raman probing technique, frequency-domain energy transport state-resolved Raman (FET-Raman), is developed to resolve these issues and improve the measurement precision significantly. The FET-Raman uses a steady-state laser and an amplitude-modulated square wave laser for heating the material and simultaneous exciting Raman signals. Under these two energy transport states, Raman-shift power coefficients for both states are determined, and their ratio is used to determine the in-plane thermal conductivity of nm-thick material. Four MoSe 2 samples with different thicknesses (5–80 nm) suspended on a circular hole are used to explore the capability of this new technique. The in-plane thermal conductivity of these samples increases from 6.2 ± 0.9 to 25.7 ± 7.7 W·m−1·K−1 with increased thickness. This is attributed to the increment of surface phonon scattering effect for thinner samples. The FET-Raman technique provides a novel way to measuring thermal conductivity of nm-thick materials with high accuracy and great ease of implementation. [ABSTRACT FROM AUTHOR]

Highlights • Up to 8-fold conductivity reduction below Casimir limit via phonon backscattering. • Theoretically-obtained ultra-low silicon thermal conductivity of less than 2 W/m K. • Novel parametric design for studying trapping, backscatter, and intrinsic scattering. • Development of phonon filters using backscatter. • Backscatter increases participation of thermal carriers with higher energy. Abstract At nanometer to micron length scales, there exists a strong competition between intrinsic and extrinsic scattering mechanisms that can curtail the free flight of phonons and ultimately affect the thermal transport. Despite significant progress in showing the ability to reach behaviors significantly below the Casimir limit, little appears to be understood about the competition between these scattering sources. In this investigation, we propose a simple one-parameter geometry that simultaneously modulates backscattering and trapping effects to enable directed study of these different means of controlling phonons. The geometry is a simple sequence of chambers offset from one another by a defined distance. We use the geometry to study the effects of phonon backscatter, trapping, and corner-turning on the thermal conductance in Si nanowires (NWs). We employ a full Brillouin zone Boltzmann Transport Equation (BTE) method to determine spatially-varying phonon densities in the geometry. Significantly greater impact is seen due to backscatter than any other means of arresting phonon flow. By creating a geometry that maximizes backscatter, a roughly 8-fold reduction in thermal conductance below the Casimir limit can be achieved at room temperature which is a factor of four smaller than the nearest reported value in the literature. The geometry is also useful for systematic investigation of other means of controlling phonons and affecting thermal transport; particularly, we investigate diffuse versus specular boundary scattering and the induced misalignment between the phonon flow and thermal flux due to the shape of the geometry. These effects combine to offer new insights into fundamental phonon behaviors and possible routes to phonon control. [ABSTRACT FROM AUTHOR]

• The effects of phonon renormalization induced by the fourth-order anharmonicity and 4-phonon scattering are fully considered when calculating the KXY thermal transport properties. • We verified the microscopic origin of heat transport from the microscopic point of view of the bonding strength between atoms and combined with the results of phonon group velocity, scattering rate, Grüneisen parameter, and mean free path. • We capture heat-carrying high-frequency optical phonons that make an anomalous contribution to heat transport. • We verified and predicted the mechanical properties of KXY. [Display omitted] In this work, we systematically studied the thermal transport properties of ternary zintl compound KXY using self-consistent phonon theory combined with compressive sensing technology and the Boltzmann transport equation. The effects of quartic anharmonic induced phonon renormalization and 4 - phonon scattering on the results are fully considered. Vibrational mode visualization reveals that the anomalous contribution of optical phonons to lattice thermal conductivity in KCaBi originates from high-speed out-of-phase vibrations of Ca atoms. We analyzed the microscopic mechanism of heat transport from the perspective of atomic bonding combined with the results of group velocity, scattering rate, Gr u ¨ neisen parameter, mean free path, etc. In addition, we verified and predicted the mechanical properties in KXY by calculation. [ABSTRACT FROM AUTHOR]

• Phonon thermal transport properties in polycrystalline graphene are studied. • Grain size and grain orientation play important role in the phonon thermal conductivity of polycrystalline graphene. • The introduction of strain and vacancy defects can decrease the phonon thermal conductivity of polycrystalline graphene. The phonon thermal conductivity of polycrystalline graphene nanoribbons has been calculated by non-equilibrium molecular dynamics method. The effects of grain size and boundary orientation, vacancy defect and strain on the phonon thermal conductivity of polycrystalline graphene nanoribbons have been studied. It was found that for polycrystalline graphene nanoribbons with the same average grain size, the phonon thermal conductivity increased with the increase of nanoribbon length. At the same size of nanoribbons, the phonon thermal conductivity increases with the increase of grain size in polycrystalline graphene nanoribbons. Grain boundary orientation has also an important impact on the phonon thermal conductivity. In addition, the phonon thermal conductivity of polycrystalline graphene decreases with the increase of tensile strain/compressive strain because the strain induces the change of structure and mechanical properties, which affects the phonon transport behavior. Compared with the pristine graphene, the vacancy defects lead to the suppression of some phonon modes of polycrystalline graphene and thus the reduction of phonon thermal conductivity due to the enhancement of phonon boundary scattering and phonon defect scattering. Our results are helpful to understand the physical mechanism of phonon thermal transport in polycrystalline graphene, and provides an important guidance for the regulation of phonon thermal conductivity of polycrystalline graphene. The effects of grain boundary, vacansy defect and strain on phonon thermal conductivity of polycrystalline graphene nanoribbons are studied theoretically. [Display omitted] [ABSTRACT FROM AUTHOR]

• Pressure-enhanced phonon conduction was studied for four isotopic c -BNs. • The suppression of the aao process is mainly responsible for the enhanced κ. • When pressure is applied, the contribution of TA phonons to κ can increase more. • The thermal transport of isotopically enriched c -BN is less sensitive to pressure. Phonon transport in cubic boron nitride (c -BN) at elevated pressure in several tens GPa is potentially of interest for cooling high-power electronics. In this work, we study the pressure-dependent thermal conductivity (up to 20 GPa) of isotope-engineered c -BN by solving the Boltzmann transport equation with inputs only from ab initio calculations. Four boron isotope compositions are analyzed, including natural abundance, enriched 10B and 11B, and a roughly equal mixture of the two. In all cases, the thermal conductivity of c -BN increases linearly with pressure. Compared to isotope-enriched c -BN, the thermal conductivity of isotopically mixed c -BN is less sensitive to pressure variations since phonon-isotope scattering causes substantial thermal resistance yet is pressure independent. Based on mode-resolved analysis, the thermal conductivity enhancement is mainly attributed to the suppression of the aao absorption processes with transverse-acoustic phonons in the frequency range from 300 to 600 cm–1, due to the synergistic effect between decreased coupling strength and phonon hardening. In addition, suppression of the aaa processes is also responsible for a slight increase in thermal conductivity. Our results are further supported by the frequency- and mean-free-path-dependent spectra of the thermal conductivity. This work provided a fundamental interpretation of pressure-dependent phonon transport in c -BN from a microscopic perspective. [ABSTRACT FROM AUTHOR]

• The thermal conductivity of Nafion membranes is dominant affected by water content. • The hydrogen bonds network is the dominant factor affecting thermal conductivity. • There are two significant ways to improve interchain heat transport by hydrogen bonds. • A well-established thermal network is formed when water content is more than 15. • Hydration level influences the effect of temperature on the thermal conductivity. The thermal conductivity of the proton exchange membrane significantly affects the operating performance of the fuel cell. In this work, the impact of water content and temperature on thermal conductivity of hydrated Nafion membranes was investigated via molecular dynamics simulations and the influencing mechanism was explained from the perspective of interchain and intrachain heat transfer. The study shows that the thermal conductivity is mainly positively affected by water content, which is a result of the triple effect of hydrogen bonding acting as interchain heat transfer bridge, the stronger microphase separation allowing water to behave more like bulk water, and the decrease in the degree of chain torsion resulting in less phonon scattering along the chain. Particularly, we emphasized two crucial paths in which hydrogen bonding improves interchain heat transport. Furthermore, the temperature has little and slightly negative effects on thermal conductivity at low and high hydration levels, respectively. That is because, in low hydration levels, the incomplete hydrogen bond network is hardly influenced, but the bridging effect is suppressed and intrachain heat transfer capacity is reduced in high hydration levels. We believe this study can broaden the design basis of high temperature polymer electrolyte membranes. [Display omitted] [ABSTRACT FROM AUTHOR]

In order to develop a better understanding of the thermal conduction of the intermetallic compounds NiAl and Ni 3 Al, detailed electron-phonon scattering was investigated by first-principles calculations. The phonon thermal conductivity of NiAl and Ni 3 Al was calculated by considering phonon-phonon and electron-phonon scatterings. It is found that electron-phonon coupling has a strong effect on the phonon thermal conductivity. At a temperature of 100 K, the electron-phonon coupling will induce 55% and 75% reduction in phonon thermal conductivity of NiAl and Ni 3 Al, respectively. Such an effect decreases with increasing temperature. The electron thermal conductivity was also predicted by considering electron-phonon scattering. We find that though the electrons dominate the thermal transport in NiAl and Ni 3 Al at high temperatures, the phonons have significant contribution to the total thermal conductivity in the sub-room temperature range. Moreover, we find that at room temperature phonons with mean free paths between 1 and 100 nm are the dominant contributors to the thermal conductivity, while the mean free paths of dominant electrons are 1–20 nm. [ABSTRACT FROM AUTHOR]

• The multidimensional phonon Boltzmann Transport Equation (BTE) is solved. • A frequency domain thermo-reflectance experiment is simulated. • Phase lag is computed with and without optical phonons and compared to experiments. • Thermal conductivity is extracted and shows suppression. The multidimensional phonon Boltzmann Transport Equation (BTE) was solved numerically in cylindrical coordinates and in time domain to simulate a Frequency Domain Thermo-Reflectance (FDTR) experimental setup. The phase lag between the pump and probe laser signals were computed for a pump laser modulation frequency ranging from 20 to 200 MHz. Results were obtained both with and without the inclusion of optical phonons, as well as with two different relaxation time-scale expressions (Holland versus Broido) for scattering, obtained from the literature for silicon. It was found that inclusion of optical phonons significantly improved the agreement between the measured and the computed phase lag. Subsequently, the thermal conductivity was extracted by fitting the Fourier heat conduction equation results—also solved numerically in time domain—to the measured and computed (using BTE) phase lag values. While both relaxation time-scales exhibited thermal conductivity suppression, a clear superiority of one time-scale expression over the other could not be established. With the Broido time-scale, the thermal conductivity extracted from BTE calculations overpredicted the value extracted from experiments regardless of whether optical phonons were included in the calculations. In contrast, with the Holland time-scale, the extracted thermal conductivity value overpredicted the value extracted from experiments when optical phonons were included, but underpredicted the value when optical phonons were excluded. [ABSTRACT FROM AUTHOR]

• Nanoparticles are embedded to further reduce the thermal conductivity of the superlattice. • Effect of particle number and size on the thermal conductivity are studied. • Characteristics of the thermal conductivity are deeply revealed by calculating the phonons transport parameters. Utilizing different nanostructures can effectively control phonons transport of their corresponding macrostructures, and this will contribute to the development of thermoelectric energy harvesting and microelectronic thermal management. This work tends to decrease the thermal conductivity (TC) of Si/Ge superlattices (SLs) by embedding germanium nanoparticles (GNPs). The thermal transport property in such structure was investigated by non-equilibrium molecular dynamics (NEMD) method. Results showed that embedding NPs can significantly reduce the TC of SLs, but only when the particle number is less than half of the SL's periods, above which the reduction effect will be greatly decreased. In addition, we also investigated the effects of temperature, system length, particle size and number on the TC. The thermal transport properties of the system are deeply revealed by calculating and analyzing phonons parameters such as density of states, participation ratio, mean free paths (MFPs) and spectral heat flow. [ABSTRACT FROM AUTHOR]

• Reports a two-temperature time-fractional model of electron-phonon heat transfer. • It is an accurate and convenient tool for modeling ultrafast laser processing. • Its accuracy can reach that of Boltzmann transport equation but much more efficient. • Its accuracy greatly outperforms that of the conventional two-temperature model. This research investigates electron-phonon coupled thermal transport in heterogeneous systems under femtosecond laser pulses. A two-temperature time-fractional (2T-TF) model based on the Caputo fractional derivative is presented, which is validated against experimental data and two-temperature Boltzmann transport equation (2T-BTE) results. The 2T-TF model is demonstrated to be more accurate than the diffusive two-temperature (2T) model based on Fourier's law, while its complexity can be much lower than 2T-BTE simulations. Moreover, various forms of thermal resistances can be readily implemented to the 2T-TF model. Using multi-layer metal-nonmetal thin films as model systems, we demonstrate that our 2T-TF model can reliably predict electron-phonon coupled thermal transport across metal-metal and metal-nonmetal interfaces as well as electron cooling in the top metallic layer after ultrafast laser irradiation. The 2T-TF model can serve as a convenient and reliable tool for simulating electron-phonon coupled thermal transport in heterogeneous systems that are vastly seen in laser manufacturing and micro-/nano-electronic devices. [ABSTRACT FROM AUTHOR]

• Ultrahigh thermal conductance is found at the atomical level interface • The anharmonic phonon process contributes largely to the thermal transport • Anharmonic process originates from phonon occupation and inelastic scatterings • The extended vibrational modes dominate interfacial thermal transfer The scatterings among the heat carriers at metal/semiconductor heterojunctions are important for the heat dissipation process in quantum information science systems. Phonon-phonon coupling at interfaces is accepted to be one of the main channels for thermal energy across the metal/semiconductor interface. Here, we show the thermal conductance across the metal/semiconductor interface, i.e., Al/Si interface, at room temperature is around 350 MW/m2K, which is 1.43 times higher than the harmonic limit and stems from the strong anharmonic phonon-phonon scatterings at the interface. The anharmonic phonon-phonon scatterings are found to benefit the interfacial thermal conductance from two mechanisms, i.e., the increase of the phonon occupation and inelastic phonon scattering channels. Our modal analysis suggests that the thermal conductance contributed by the interfacial vibrational modes is negligible, and the interfacial thermal transfer is dominated by the extended vibrational modes. By comparing our calculated thermal conductance with the experimental measurements, we find that electron-phonon coupling contributes ∼9% to the total interfacial thermal conductance, and the phonon-phonon coupling dominates the thermal transport across the interface. Our findings here have provided guidance for the thermal management design in the related quantum information electronics. [ABSTRACT FROM AUTHOR]

• A hybrid MC-DOM method has been proposed for interfacial phonon tranpsort. • The computational efficiency can be improved over a hundred times. • The in-plane phonon transport with rough interfaces can be well captured by the hybrid method. In recent years, the rapid development of micro/nanosystems raises the demand of thermal optimization of the new designs, and thus the accurate and efficient prediction of phonon transport in mesoscopic systems with complex interfaces is required. This work proposes a hybrid Monte Carlo-discrete ordinates method (MC-DOM) for steady-state phonon transport in such systems by developing an algorithm to exchange information between MC and DOM subdomains. Based on the phonon Boltzmann transport equation which is physically compatible with the system scale, our formulation exploits the computational efficiency of DOM in the bulk region with flexibility of MC near complex interfaces. The cross-plane phonon transport through the thin films is considered first as the benchmark for method verification, whose results agree well with those calculated by numerical and analytical solutions. It is noteworthy that compared to MC as a pure particle method, the hybrid method runs faster over a hundred of times for the temperature calculation when high-precision results are needed. The hybrid method is then applied to simulate the in-plane phonon transport through the double-layer thin films with rough interfaces. It is found that for rectangular interfaces with different roughness, the interfaces perpendicular to the heat flux will cause a larger thermal resistance than the parallel ones. Our hybrid method enriches numerical tools for mesoscopic phonon transport simulation, and is helpful to reveal the mechanisms behind and optimize the heat transport in micro/nanosystems with complex geometries. [ABSTRACT FROM AUTHOR]

• Wave-particle crossover phonon thermal transport was discovered in graphene/C 3 N lateral superlattices. • The density of the hetero-interface has the dual effects on phonon thermal transport. • Thermal conductivity depends on the phonon mean free path of the and is independent of the group velocity. [Display omitted] Two-dimensional (2D) lateral superlattices, a typical artificial nano-phononic crystal, have stimulated widespread interests and potential application prospects in terms of their physically interesting features. Herein, we have found wave-particle crossover of phonon transport in the graphene (Gr)/2D polyaniline (C 3 N) lateral superlattices, which is an indication of a transition in the phonon transport mechanism from the incoherent to coherent regime. Due to the high structural similarity of C 3 N to Gr, the thermal conductivity of the Gr/C 3 N lateral superlattice can achieve an ultra-wide modulation range of about 500 ∼ 1100 W m − 1 K − 1 , which is far beyond that of other superlattices. The analysis shows that an increase of the interface density will on the one hand weaken the thermal conductivity by increasing phonon-interface scattering, and on the other hand increase it by lowering the phonon transport barriers and allowing more long-wavelength phonons to participate in the transport. This determines the parabolic trend of thermal conductivity containing the minimum, and also reflects the dual effects of hetero-interfaces on phonon thermal transport. In addition, phonon calculations show that the above variation in thermal conductivity is entirely attributable to differences in the phonon mean free path for different interface densities and is not related to their group velocity. [ABSTRACT FROM AUTHOR]

• This work simulates the thermal transport in metal/semiconductor bilayer films, including Au/Si, Cu/Si, and Al/Si. • The calculated thermal conductivity of the Au/Si multilayers films agrees with the experimental measurements. • By considering the energy exchange between electrons and phonons, the phonon temperature in metal gradually displays a nonlinear relation. • The heat flux distribution shows that electrons carrying heat in metal transfer all heat to the phonons near the interface, followed by phonon transmission or reflection at the interface. Thermal transport across the interface between metals and semiconductors is ubiquitous in micro- and nano-manufacturing devices. The direct simulation of thermal transport in such multilayers is rarely studied due to different main carriers involved, such as electrons in metals and phonons in semiconductors. This study investigates thermal transport in metal/semiconductor multilayer films using the coupled electron and phonon Boltzmann transport equations combined with the phonon diffuse mismatch model. The calculated overall thermal conductivity demonstrates the importance of electron-phonon coupling transport and then the present work gives a critical thickness of the metal layer for considering electron-phonon coupling transport. If only one side of the metal layer is in contact with the semiconductor, the electron-phonon coupling transport in metal layer should be considered when the metal layer thickness is larger than 12.5 nm, 7.5 nm and 2 nm for Au/Si, Cu/Si and Al/Si bilayer films, respectively. This critical thickness will be approximately double if two sides of the metal layer are both in contact with the semiconductor due to the non-equilibrium between electrons and phonons at both sides, such as the super-lattice with infinity periods. Additionally, there exist a minimum thermal conductivity in metal/semiconductor multilayers when changing the thickness of the metal layer. This work will promote a deeper understanding of the thermal transport in metal/semiconductor multilayers at the micro and nanoscale and provide the insightful indication for the manipulation of thermal conductivity in multilayers. [ABSTRACT FROM AUTHOR]

• Heat dissipation issues at near-junction region of GaN HEMT are reviewed comprehensively. • Phonon transport properties in GaN and GaN nanostructures are discussed based on both computations and experiments. • Phonon transport tuning mechanisms in perspectives of particle, wave, and topological nature of phonons for GaN and GaN nanostructures are introduced. The heat dissipation issue has now become one of the most important bottlenecks for power electronics due to the rapid increase in power density and working frequency. Towards the wide bandgap semiconductor GaN high electron mobility transistors (HEMTs), near-junction thermal management is the breakthrough that mainly includes the accurate thermal modeling and effective thermal design for GaN and GaN HEMTs. In this review, we first offer a comprehensive understanding of phonon thermal transport in GaN and GaN HEMTs, including non-equilibrium transport of electrons and phonons in the heat generation process, phonon thermal conductivity, heat spreading, and interfacial thermal transport. Then, we review the current tuning mechanisms and methods for thermal transport in GaN and GaN HEMTs which are classified into three categories according to the particle nature, wave nature, and topological nature of phonons. At last, we conclude by providing our perspectives on challenges and opportunities in the research of phonon thermal transport and its tuning mechanisms in GaN HEMTs. [ABSTRACT FROM AUTHOR]

• A mode- and space-resolved phonon transport model of alloy nanostructures is presented to find materials with ultra-low thermal conductivity • Opposite trends in the effectiveness of porosity on thermal conductivity reduction are explained • It is shown that alloy scattering in materials such as SiGe pushes medium MFP phonons to lower MFPs without hindering long MFP phonons. • It is shown that in contrast to SiGe in materials such as AlInAs the entire phonon spectrum scales to lower MFP • The opposite trends in the effect of porosity on thermal conductivity reduction in alloys is further explained using ballistic correction model. Nanostructured semiconducting alloys obtain ultra-low thermal conductivity as a result of the scattering of phonons with a wide range of mean-free-paths (MFPs). In these materials, long-MFP phonons are scattered at the nanoscale boundaries whereas short-MFP high-frequency phonons are impeded by disordered point defects introduced by alloying. While this trend has been validated by simplified analytical and numerical methods, an ab-initio space-resolved approach remains elusive. To fill this gap, we calculate the thermal conductivity reduction in porous alloys by solving the mode-resolved Boltzmann transport equation for phonons using the finite-volume approach. We analyze different alloys, length scales, concentrations, and temperatures, obtaining a very large reduction in the thermal conductivity over the entire configuration space. For example, a ∼ 97% reduction is found for Al 0.8 In 0.2 As with 25% porosity. Furthermore, we employ these simulations to validate our recently introduced "Ballistic Correction Model" (BCM), an approach that estimates the effective thermal conductivity using the characteristic MFP of the bulk alloy and the length-scale of the material. The BCM is then used to provide guiding principles in designing alloy-based nanostructures. Notably, it elucidates how porous alloys such as Si x Ge 1 − x obtain larger thermal conductivity reduction compared to porous Si or Ge , while also explaining why we should not expect similar behavior in alloys such as Al x In 1 − x As. By taking into account the synergy from scattering at different scales, we provide a route for the design of materials with ultra-low thermal conductivity. [ABSTRACT FROM AUTHOR]

Unlike phonon-boundary resistance at the interface of two dissimilar lattices, the phonon grain-boundary resistance AR p , gb is over an ultrathin atomic restructured region bounded by two identical lattices. Using nonequilibrium, classical molecular dynamic simulations on bicrystal UO 2 over 300–1200 K, we predict that AR p , gb (i.e., phonon, grain mean free path) is independent of temperature and the grain boundary type (e.g., tilt, twist). We compare these predictions with existing analytical models and identify those which include the proper grain-boundary phonon scattering mechanisms. Also, using the same embedded-atom interatomic potential models, we predict the phonon dispersion, density of states and bulk thermal conductivity of UO 2 , and verify the predictions (comparing with available ab initio molecular dynamics and experimental results), under equilibrium and nonequilibrium simulations. [ABSTRACT FROM AUTHOR]

• The thermal conductivities of Ti 3 C 2 T x nanosheets with different surface terminations were investigated by molecular dynamics simulation. • The interfacial thermal resistance between Ti 3 C 2 T x and epoxy matrix was examined. • The thermal conductivities of Ti 3 C 2 T x /epoxy composites with different filler parameters were predicted through effective medium theory. • This work provides a view on the thermal property on Ti 3 C 2 T x MXene and its polymer composites. Ti-based MXene has been widely investigated for its excellent electronic properties but the thermal performance of Ti 3 C 2 T x still remains uncharted on the molecular scale, which could significantly affect the thermal conductivity of their composites. This work adopted molecular dynamics method and effective medium theory (EMT) to analyze the thermal conductivity of 4 types of MXenes, including Ti 3 C 2 , Ti 3 C 2 F 2 , Ti 3 C 2 O 2 and Ti 3 C 2 (OH) 2 , as well as their epoxy composites. The results showed that the terminal groups could reduce the phonon scattering in MXenes which could increase the thermal conductivity and also the interfacial affinity between MXene and epoxy molecules by improving interfacial binding energy. The Ti 3 C 2 O 2 has the highest thermal conductivity of 140.25 Wm−1K−1, while Ti 3 C 2 (OH) 2 has the lowest interfacial thermal resistance, which could lead a higher thermal conductivity in composite when its flake was under the critical size. The EMT results showed that Ti 3 C 2 O 2 could significantly improve the TC of epoxy under a lower filler content; and the MD simulation parameters were validated by the comparison between other works and EMT prediction in this work. The current study provides suggestions for selecting Ti 3 C 2 T x as fillers in composites that need fast heat dissipation. [ABSTRACT FROM AUTHOR]

• Twist angle is introduced as a new degree of freedom to tune thermal conductivity. • Thermal conductivity of bilayer sample exhibits "W" shape as twist angle increase. • Lowest value of bilayer MoS 2 when twist angle is odd multiple of 15°. • Lowest value of trilayer MoS 2 when sum of twist angles is odd multiple of 15°. Exploring approaches to reduce lattice thermal conductivity is effective to improve the performance of molybdenum disulfide thermoelectric materials, and tuning lattice thermal conductivity via twist angles attracts increasing attention. However, few studies focused on the effects of twist angle on lattice thermal conductivity of molybdenum disulfide thermoelectric materials, particularly, the coupling effects of two twist angles on lattice thermal conductivity of tri-layer molybdenum disulfide. In this work, the effects of twist angles on lattice thermal conductivity of twisted bilayer and trilayer molybdenum disulfide were investigated by molecular dynamics simulation. Phonon dispersion was calculated through density functional theory to provide a deeper understanding of thermal conduction. The results indicated that for the twisted bilayer sample, the relationship between thermal conductivity and twist angle exhibits "W-shaped", and the minimum thermal conductivity can be obtained with the twist angle of 15° The phonon analyses indicated that the twist angle has a significant effect on ZA branches, and the effects of the twist angle on group velocities and phonon lifetime of LA and TA branches contribute to the change of thermal conductivity. As for the twisted tri-layer sample, the minimum thermal conductivity can be obtained when the sum of two twist angles is an odd multiple of 15°, which is recommended for thermoelectric materials design. [Display omitted] [ABSTRACT FROM AUTHOR]

• An abnormal in-plane thermal conductivity anisotropy of bilayer α-phase tellurene is found. • κ⊥ / κ∥ of 2L-aTe is 1.174, in sharp contrast to bulk Te for which κ// / κ⊥ is 0.435. • Phonon transport properties and lattice structure change are analyzed to explain the anomalous thermal conductivity anisotropy. • Covalent-like quasi-bonding and its electronic origins are discussed to uncover the physical mechanism. In layered materials, it is well known that the cross-plane or cross-chain (CC) thermal conductivity is lower than that of the in-plane or along-chain direction due to weaker bonding in the cross-plane or CC directions. In this work, surprisingly, we predict using first-principles that the CC thermal conductivity κ ⊥ is higher than the along-chain thermal conductivity κ ∥ with a κ ⊥ / κ ∥ ratio of 1.174 in bilayer α -phase tellurene (2L-αTe), an emerging 2D material that has recently drawn extensive interest due to its intriguing electronic, thermoelectronic, and piezoelectronic properties. Also, the room-temperature κ ⊥ and κ ∥ of 2L-αTe are 5.69 W/mK and 4.85 W/mK, which are 251% and 31% enhanced from that of bulk Te, respectively. A detailed analysis of lattice structure, phonon spectra, and electron properties of 2L-αTe and bulk Te is provided. We find that the larger lattice shrink in CC direction and the change in band angle lead to stiffened phonons and weaker anharmonicity, which in turn yield higher phonon group velocity and longer phonon lifetime in CC direction for 2L-αTe, resulting in the enhanced thermal conductivity and the anomolus thermal anisotropy. Crystal orbital Hamiltan population (COHP) and electron localization function (ELF) analysis demonstrate the enhancement of the strength of "covalent-like quasi-bonding" (CLQB) in CC direction of 2L-αTe, which is accompanied by the lattice shrink in CC direction and the change of band angle. Our study identifies a case of unexpected anisotropic thermal transport in 2D materials, and sheds light on the improvement of thermal conductivity isotropy in 2D materials for the thermal management of electronic devices. [ABSTRACT FROM AUTHOR]

• Hydrostatic pressure plays hugely positive role in the thermal conductivities of XC (X=Si, Ge) with zinc blende (ZB) and rock salt (RS) phases, and it also results in an enormous reduction of thermal conductivity when phase transition occurs. • The joint growth of phonon group velocity and phonon lifetime results in the obvious enhancement of thermal conductivity of SiC during the processes before and after phase transition. • During the process before / after phase transition, the abnormal increase of phonon scattering rate / little variation of phonon group velocity offsets the increase of thermal conductivity of GeC caused by phonon group velocity / phonon lifetime, which results in the smaller enhancement of thermal conductivity of GeC. • The thermal transport characteristic of GeC before phase transition is determined by harmonic characteristic and that after phase transition shows a closer relationship with the anharmonic characteristic. • The tremendous reduction of thermal conductivity of SiC during phase transition is caused by the enhancement of the phonon scattering intensity, and that of GeC is leaded by the increase of the number of phonons scattering channels. In this paper, the effect of hydrostatic pressure on the thermal transport properties of silicon carbide (SiC) and germanium carbide (GeC) with zinc blende (ZB) and rock salt (RS) phases at room temperature is investigated by solving the phonon Boltzmann heat transport equation. Studies show that the thermal conductivities of the two materials increase with the raising pressure before the phase transition, however, the enhancement of GeC (40%) is much smaller than that of SiC (100%). Combined with phonon analysis, it is found that this phenomenon originates from the abnormal enhanced phonon scattering rate under high pressure, which partly offsets the increase of phonon group velocity and further suppresses the increase of thermal conductivity. Next, when the phase transition occurs, the thermal conductivities of SiC and GeC decrease sharply by 76% and 86%, respectively, which is mainly caused by the increase of the anharmonic characteristic related to phonon scattering. Finally, after the phase transition, the thermal conductivities of the two materials still show increasing trends with the raising pressure, however, the phonon group velocity of GeC does not change significantly and the variation of its thermal conductivity is mainly determined by the phonon lifetime, which is opposite to the case before phase transition, whose enhancement of thermal conductivity is mainly caused by the increase of phonon group velocity. The results indicate that GeC has a more complex phonon transport mechanism compared with SiC, i.e., its thermal conductivity before phase transition is determined by harmonic characteristic and that after phase transition shows a closer relationship with the anharmonic characteristic. This study provides data and mechanism support for the regulation of thermal conductivity from the perspective of pressure-induced phase transition, and as well as supplies some guiding suggestions for engineering applications in the field of thermal transport regulation. [Display omitted] Hydrostatic pressure plays positive role in the thermal conductivities of XC (X=Si, Ge) during the processes before and after phase transition, and it also plays enormous negative role in the thermal conductivity when phase transition occurs. There are a variety of mutually restrictive phonon mechanisms existed in GeC, which can boost its application in the regulation of TC [ABSTRACT FROM AUTHOR]

The fractal tree-shaped network is introduced to reconstruct the Si/Ge nanocomposites in this paper with a focus on reducing the thermal conductivity of thermo-electrical materials by enhancement of phonon scattering at the interfaces of nanostructures. The molecular dynamics simulation with the use of the empirical Tersoff potential is applied to study the thermal conductivity of Si/Ge nanocomposites with fractal tree-shaped networks. The effects of fractal dimensions and branch level on the thermal conductivity of nanocomposites are evaluated and analyzed. The results indicate that, compared with conventional rectangular core, the fractal tree-shaped network is more superior in the reduction of thermal conductivity of nanocomposites owing to stronger phonon interface scattering for larger interface density. In particular, the larger length and width fractal dimensions as well as more branch levels are preferred for the Si/Ge nanocomposites with fractal tree-shaped networks. [ABSTRACT FROM AUTHOR]

• The effect of twin interface on the lattice thermal conductivity of superlattice was studied. • Twin interface can destroy coherent phonon transport, and cause phonons localization. • Thermal conductivity dependence on period length is strongly affected by twin interface. Twin interfaces are easily formed in superlattices due to their lower interfacial energy. However, there are relatively few studies on their effect on the thermal conductivity of superlattices, and the conclusions are unclear. In particular, the degree of influence of the presence of twin interfaces on the thermal conductivity is inconsistent. Therefore, the thermal conductivities of silicon/germanium superlattices with twin interfaces were studied by non-equilibrium molecular dynamics simulations. It was found that the twin interface destroys coherent phonon transport, causes phonon localization, and leads a decrease in the thermal conductivity. The degree of influence of the twin interface on the thermal conductivity is strongly dependent on the period length, the system length, and temperature. Furthermore, phonon density of states, phonon participation rate, and spectral heat flow calculations were employed to deduce the phonon transport mechanisms in superlattices with twin interfaces. Twin interfaces are easily formed in superlattices due to their lower interfacial energy. However, there are relatively few studies on their effect on the thermal conductivity of superlattices, and the conclusions are unclear. In particular, the degree of influence of the presence of twin interfaces on the thermal conductivity is inconsistent. Therefore, the thermal conductivities of silicon/germanium superlattices with twin interfaces were studied by non-equilibrium molecular dynamics simulations. It was found that the twin interface destroys coherent phonon transport, causes phonon localization, and leads a decrease in the thermal conductivity. The degree of influence of the twin interface on the thermal conductivity is strongly dependent on the period length, the system length, and temperature. Furthermore, phonon density of states, phonon participation rate, and spectral heat flow calculations were employed to deduce the phonon transport mechanisms in superlattices with twin interfaces. [ABSTRACT FROM AUTHOR]

• The thermal conductance across Si/4H-SiC is modulated broadly from ∼300 to ∼1000 MW/m2K via confining nanopatterns in the interfacial region. • The thermal conductance across Si/4H-SiC interface shows a nonmonotonic nanopattern section dependence. • The thermal conductance is enhanced by ∼11% compared to that of the bare Si/4H-SiC interface once the enhance of the phonon channels is dominant. Manipulating the thermal transport across the interfaces via nanostructuring is critical for thermal management in electronics and energy conversion in thermoelectrics. Recent experiments have enabled the fabrication of controllable nanopatterns at interfaces, and therefore provide a new perspective to design and manipulate interfacial heat transfer. By performing nonequilibrium molecular dynamics simulations, we reported that the interfacial thermal conductance of Si/4H-SiC interfaces can be modulated broadly from ∼ 300 MW/m2K to ∼ 1000 MW/m2K by confining nanopatterns with a thickness on the order of nanometers, i.e., smaller than 30 nm. Based on the spectral heat current and participation ratio analysis, the nonmonotonic nanopattern section-dependent thermal conductance as observed in our simulations was found to be originated from two competing mechanisms, i.e., phonon-boundary scattering and interfacial phonon transport channels. The corresponding interfacial thermal conductance initially decreased with the nanopattern section when the phonon-boundary scattering is dominant and became stronger, and then increased when there were many more possible interfacial phonons transport channels. The thermal conductance was enhanced by ∼11% compared to that of the bare Si/4H-SiC interface once the latter mechanism was dominant. Besides, the thermal resistance induced by the pattern itself becomes evident and must be considered when the height of the nanopattern becomes greater. Our work here provides a comprehensive investigation on manipulating the thermal transport across the interfaces via controllable nanopatterns, which is important and meaningful for designing and optimizing the advanced thermal interface materials. [ABSTRACT FROM AUTHOR]

Abstract: We report a molecular dynamics study of heat conduction in carbon nanotube (CNT)/polyethylene (PE) composites. Particular attention was focused on two key interfacial effects: the thermal boundary resistance across the CNT/PE interface, and the reduction of CNT thermal conductivity due to boundary phonon scattering at the interface with amorphous PE matrix. The calculations reveal a relatively high thermal boundary resistance across the CNT/polyethylene interfaces (∼1×10−7 m2 KW−1) and a moderate but non-negligible reduction (∼28%) of CNT thermal conductivity. By a mode-dependent phonon transport analysis, the phonon boundary scattering rate was found to increase with decreasing frequency, which suggests a size effect in the thermal conductivity reduction. Finally, the likely impact of the thermal resistances on the effective thermal conductivity of a macroscopic CNT/PE composite is quantified based on an effective medium approximation model. [Copyright &y& Elsevier]

Abstract: A hybrid phonon gas model was developed to describe heat pulse propagation in dielectric crystals. In this model, heat energy is described as a mixture of longitudinal and transverse phonon gases. The novel aspects of this model include: (1) the distinction between longitudinal and transverse phonon excitations is taken into account; (2) transitions between ballistic and second sound transport, ballistic and diffusive transport, and second sound and diffusive transport are successfully captured. For the first time, benchmark cases of heat-pulse experiments in NaF at low temperatures in ballistic, ballistic–diffusive, and diffusive regions have been completely reconstructed in numerical simulations, which demonstrate the validity of the new model. It was elucidated how heat pulses are transmitted by longitudinal, ballistic transverse and dispersive transverse phonons at low temperatures. The numerical results not only yield new insight in physics of transient ballistic–diffusive heat conduction with effects from different phonon scattering processes, but also provide numerical tools to study similar transient ballistic–diffusive heat conduction in nanoelectronics and modern optoelectronics. [Copyright &y& Elsevier]

• In the graphene/h-BN vdW superlattice, the wave-particle crossover of phonon transport was found, which is reflected in the transition from incoherent to coherent regime as its interface density increases. • The thermal conductivities of the superlattices with higher interface density are more sensitive to changes in temperature and interface coupling strength. • Based on GPU-accelerated extensive molecular dynamics simulations, this study conducted a comprehensive and in-depth discussion on the related phonon thermal transport properties. [Display omitted] The van der Waals (vdW) superlattice, obtained by applying the concept of the periodic superlattice to two-dimensional materials using low-energy vdW physical assembly, is undoubtedly an instrumental avenue for the modulation of material properties. In the field of nanoscale thermal transport, the influence of the periodic structure of superlattice on the wave-particle phonon transport regime arouses substantial interests from the standpoint of basic physics and applied science. In the Graphene/h-BN vdW superlattice, we have found the wave-particle crossover of phonon transport, which is reflected in the transition from incoherent to coherent regime as the interface density increases. The analysis reveals that the increased thermal conductivity owing to coherent transport effects will amply compensate for the progressively increasing interface phonon scattering throughout this process. In addition, due to the stronger effects of the above two aspects, the superlattices with higher interface density are more sensitive to changes in temperature and interface coupling strength, which are manifested in the rate of change in thermal conductivity caused by their alteration, respectively. These results establish an in-depth understanding of coherent phonon transport while exploring the possibility of phonon wave-particle crossover in vdW superlattices, providing guidance for related thermal management based on phonon engineering. [ABSTRACT FROM AUTHOR]

• The phonon resonant strength is strongly dependent on surface orientations of the base membranes. • The introduction of resonant pillars can effectively break the ballistic transport characteristics at low-frequencies. • Vacancy defects between the pillars and the base material lead to the abnormal increase of thermal conductivity in the resonant structures. • the thermal transport at intermediate frequency range is enhanced by the interface vacancy defects. Engineering low-frequency phonon transport in nanostructures with the phonon resonant mechanism has become an important research direction. On the basis of non-equilibrium molecular dynamics simulations, the thermal transport in pristine and resonant Si-membranes bounded with {100}, {110} and {111} facets is investigated. It is found that the creation of surfaces can introduce anisotropic thermal transport due to the lattice symmetry breaking. Besides, ballistic phonon transport is found in pristine membranes with lengths up to 500 nm at low-frequencies with a critical frequency mainly dependent on the crystallographic orientation. Moreover, although surface resonances can dramatically reduce the thermal conductivity of all membranes, the resonant effect strongly relies on membrane orientation. Among the three studied membrane orientations, the resonant effect is maximized in the {111}-membrane, where the thermal conductivity is tuned from the largest one to the smallest one among the three membrane types by resonant pillars. The large thermal conductivity reduction in the {111}-membranes by resonances originated from the reduced spectral heat flux between 3 and 12 THz. Furthermore, the resonant coupling strength can be tuned by the interface vacancy between resonant pillars and the base material, which can enhance phonon transport at an intermediate frequency range. Our work provides further insights on thermal transport engineering by phonon resonances and could be useful for thermal conductivity engineering with surface orientations and resonances. [ABSTRACT FROM AUTHOR]

• A novel Janus structure is constructed by modifying the symmetry of t -PdTe 2 to investigate phonon transport properties. • The thermal conductivities of t -PdTe 2 and its Janus structures exhibit obvious length and layer number dependence. • A synergy effect of phonon lifetime and phonon group velocity is found in Janus t -Te32PdTe monolayer due to the mass disorder, which suppress the thermal conductivity. Recently, monolayer hexagonal PdTe 2 (t -PdTe 2) has increasingly promised a great potential in both thermoelectric and thermal management application due to its high Seebeck coefficient and intrinsic large band gap. Here, we investigate the thermal conductivity of monolayer t -PdTe 2 and its Janus structures utilizing nonequilibrium molecular dynamics simulations (NEMD). The dimensional parameters and temperature dependence of their thermal conductivities are discussed and simulations results indicate that the thermal conductivity of t -PdTe 2 increase with the increasing length, while decrease with the increasing layers due to the existence of van der Waals force between layers. Interestingly, it was found that the lowest thermal conductivity of Janus structure is attributed to the large phonon scattering from mass disorders and the breakdown of inversion symmetry compared with pristine one. Moreover, the effect of temperature on the thermal conductivity of t -PdTe 2 and its Janus structures are discussed. Our study is beneficial for thermal management of nanodevices and optimizing the thermoelectric properties of t -PdTe 2 based materials. [ABSTRACT FROM AUTHOR]