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Herein, (14,14,14)‐graphyne (GY) supported by silicon substrate is chosen to be the research object. The results demonstrate that the increasing distance between substrate and supported materials (dsub‐sup) results in the enhancement of thermal conductivity (TC) of supported GY, and the TC of supported GY is even higher than that of free‐standing GY when dsub‐sup exceeds a certain value, which means substrate plays an abnormal promoting role in the thermal transport in supported materials (SM). This phenomenon breaks the traditional cognition that the increasing dsub‐sup can only lead to the TC of SM approaching that of free‐standing model. The related mechanism can be seen as the combined impact of weak interaction of long‐dsub‐sup substrate and tensile effect led by lattice mismatch between substrate and GY. Combining with phonon analysis, it can be observed that the influence of substrate shows a closer relationship with phonon scattering, i.e., the anharmonicity, especially the anharmonicity of out‐of‐plane direction. The anomalous promoting effect of long‐dsub‐sup can be also attributed to the weaker scattering of out‐of‐plane phonon, especially the reduced four‐order phonon scattering. This research provides a new idea to suppress the negative effect of substrate on heat dissipation in electronic devices. [ABSTRACT FROM AUTHOR]

Based on the first-principles calculations of the phonon Boltzmann transport equation (pBTE) under the framework of the three-phonon scattering theory, we characterized the temperature and size dependence of the lattice thermal conductivity of monolayer silicene under the tensile strain. Our research shows that the lattice thermal conductivity of silicene has obvious strain dependence, and demonstrates the great potential of thermal management by applying strain in silicene. The TA phonon mode contributes the most to the thermal conductivity of silicene, while the contribution of the ZA phonon is suppressed. With the increase in tensile strain, the contribution of LA mode phonons to the thermal conductivity increases rapidly, and eventually become the dominant phonon mode of the silicene lattice thermal conductivity. We suspect that this phenomenon is caused by the reduction of the warpage of the silicene and the restoration of the crystal symmetry due to the tensile strain. When the characteristic size is less than 10 nm, the lattice thermal conductivity of silicene is no longer sensitive to temperature, and with the increase in tensile strain, the effective phonon mean free path (MFP) of silicene also increases, and the size effect is more obvious. The characterization of the scattering channel reveals its significant influence on the characteristics of thermal transport capacity of different phonon modes. These findings deepen the understanding of the phonon dynamics of the monolayer silicene-like structure, and provide the reference and theoretical basis for the research on the heat management of the corresponding material combined with strain and size and the development of thermal management technology. [ABSTRACT FROM AUTHOR]

Pressure is an effective way to regulate physical properties of ABO3 perovskites, such as thermal conductivity κ of SrTiO3, which can enhance fundamental understanding of structure–property relationships. In this Letter, κ of SrTiO3 was investigated up to ∼20 GPa using high-pressure time domain thermoreflectance together with Raman spectroscopy and first-principles calculations. Our theoretical predictions effectively explain the measured results. In both cubic and tetragonal phases, κ increased with compression, and optical phonons are the dominant heat carriers. The phonon group velocity and relaxation time make prominent contributions to κ with compression in the cubic phase, while the reduction in the anharmonicity of phonon modes and the phonon scattering channels dominates the increase in κ in the tetragonal phase. Especially, during the transition from cubic to tetragonal phases, there is a significant drop in κ, which originally results from the TiO6 octahedral distortion induced by the soft-phonon-mode, which markedly reduces the phonon group velocity. Our results not only help reveal the pressure effect on κ of complex oxides but also pave their way for applications on high-temperature superconductors and spin devices. [ABSTRACT FROM AUTHOR]

The van der Waals (vdW) heterostructures of bilayer transition metal dichalcogenide obtained by vertically stacking have drawn increasing attention for their enormous potential applications in semiconductors and insulators. Here, by using the first-principles calculations and the phonon Boltzmann transport equation (BTE), we studied the phonon transport properties of WS2/WSe2 bilayer heterostructures (WS2/WSe2-BHs). The lattice thermal conductivity of the ideal WS2/WSe2-BHs crystals at room temperature (RT) was 62.98 W/mK, which was clearly lower than the average lattice thermal conductivity of WS2 and WSe2 single layers. Another interesting finding is that the optical branches below 4.73 THz and acoustic branches have powerful coupling, mainly dominating the lattice thermal conductivity. Further, we also noticed that the phonon mean free path (MFP) of the WS2/WSe2-BHs (233 nm) was remarkably attenuated by the free-standing monolayer WS2 (526 nm) and WSe2 (1720 nm), leading to a small significant size effect of the WS2/WSe2-BHs. Our results systematically demonstrate the low optical and acoustic phonon modes-dominated phonon thermal transport in heterostructures and give a few important guidelines for the synthesis of van der Waals heterostructures with excellent phonon transport properties. [ABSTRACT FROM AUTHOR]

Generally speaking, for materials with the same structure, the thermal conductivity is higher for lighter atomic masses. However, we found that the thermal conductivity of CaO is lower than that of CaS, despite the lighter atomic mass of O than S. To uncover the underlying physical mechanisms, the thermal conductivity of CaM (M = O, S, Se, Te) and the corresponding response to strain is investigated by performing first-principles calculations along with the phonon Boltzmann transport equation. For unstrained system, the order of thermal conductivity is CaS > CaO > CaSe > CaTe. This order remains unchanged in the strain range of −2% to 5%. When the compressive strain is larger than 2%, the thermal conductivity of CaO surpasses that of CaS and becomes the highest thermal conductivity material among the four compounds. By analyzing the mode-dependent phonon properties, the phonon lifetime is found to be dominant over other influential factors and leads to the disparate response of thermal conductivity under strain. Moreover, the changing trend of three-phonon scattering phase space is consistent with that of phonon lifetime, which is directly correlated to the phonon frequency gap induced by the LO-TO splitting. The variation of Born effective charge is found to be opposite for CaM. The Born effective charge of CaO decreases with tensile strain increasing, demonstrating stronger charge delocalization and lower ionicity, while the Born effective charges of CaS, CaSe, and CaTe show a dramatic increase. Such variation indicates that the bonding nature can be effectively tuned by external strain, thus affecting the phonon anharmonic properties and thermal conductivity. The difference of bonding nature is further confirmed by the band structure. Our results show that the bonding nature of CaM can be modulated by external strain and leads to disparate strain dependent thermal conductivity. [ABSTRACT FROM AUTHOR]

• The interfacial thermal conductance of GaN/AlN is investigated by both MD simulations and Monte Carlo simulations based on first-principles calculations. • Interfacial phonon modes are the additional channels for interfacial thermal transport. • Modal temperature and wave-resolved spectral heat flux provide modal-level and frequency-level acknowledge. Interfacial thermal conductance (ITC) is of critical importance for GaN-based device performance. GaN/AlN is a key component in GaN-based devices. Therefore, systematically investigating the interfacial thermal transport between GaN and AlN is of great significance for the thermal management of GaN-based devices. The temperature dependence of ITC across GaN/AlN interfaces are investigated using nonequilibrium molecular dynamics (NEMD) and Monte Carlo simulations based on first-principles calculations. Interestingly, the calculated ITC is much larger than that of GaN/metal, GaN/Si and GaN/SiC. Structural similarity and interfacial phonon modes are used to reveal the underlying mechanism. Moreover, the modal temperature and wave-resolved spectral heat flux are calculated to understand the interfacial thermal nonequilibrium and the dominant thermal transport channel, respectively. This study explores the modal-level and frequency-level mechanisms for interfacial thermal transport between GaN and AlN, which provides effective understanding in thermal management of GaN-based devices. [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]

[Display omitted] The miniaturization of devices makes interfacial resistance to heat dissipation more and more obvious. Therefore, it is important to study interfacial thermal transport systematically. A modified higher harmonic inelastic model is proposed to describe the inelastic scattering at the interface. Combined with this interface model, the thermal transport of interface structures is studied by the Boltzmann transport equation. The results show that the introduction of inelastic scattering enhances the heat flux contribution of low-frequency phonons, changes the phonon distribution near the interface, and enhances the phonon non-equilibrium. This demonstrates the importance of inelastic interfacial scattering in simulations. By analyzing the phonon distribution, it is found that temperature and thickness affect the interfacial heat transport through the redistribution of phonons. The redistribution has the opposite effect on the contribution of inelastic scattering. By studying ballistic phonons in the interface structure, it is found that the ballistic phonons are mainly concentrated in the low-frequency region. Due to the longer mean free path, the phonons with frequencies below 2.5 THz are more likely to maintain a ballistic transport state. Similar to nanoscale hotspots, strong quasi-ballistic transport hinders heat diffusion. The energy fraction of ballistic phonons increases by 49%, and the local effective thermal conductivity drops by 52%. This work studied the mechanism of interfacial thermal transport, and will be useful for future interface simulation. [ABSTRACT FROM AUTHOR]

The weakly van der Waals interactions make monolayer TMDCs straightforward to vertically stack to bilayer systems, which have attracted enormous attention because of their potential applications in optoelectronics and electronic devices. In particular, MoS 2 /WS 2 heterostructure exhibits excellent performance in photodetectors. The thermal transport behavior plays a vital role in life and performance of devices, which have been rarely reported. Here, the thermal properties and thermal transport mechanism of monolayer MoS 2 and WS 2 , bilayer MoS 2 and WS 2 and MoS 2 /WS 2 heterostructure are systematically investigated by first-principles calculations and Boltzmann transport theory. The results indicate that the thermal conductivity of bilayer systems is considerably higher than that of correlated monolayer systems. Most interesting, the thermal conductivity of MoS 2 /WS 2 heterostructure is not only much lower than the thermal conductivity of 2L-WS 2 and 2L-MoS 2 , but also much lower than that of monolayer WS 2 and MoS 2. The harmonic properties mainly including phonon dispersion and anharmonic properties mainly including scattering rate and Grüneisen parameters are used to elucidate the corresponding mechanism. Moreover, the underlying electronic properties mainly including interlayer charge transfer and interlayer coupling play an essential role in thermal transport of MoS 2 /WS 2 heterostructure. The results suggest that the thermal conductivity of TMDCs can be largely tunes by various combinations, such as WS 2 /WS 2 or WS 2 /MoS 2 , which provides an effective way for manipulating the thermal transport properties of MoS 2 and WS 2 for future emerging applications. [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]

Abstract: The thermal boundary conductance between multilayer structures including Al film, semiconductors with high Debye temperatures (GaN, AlN, Si, diamond) and dielectric substrates (sapphire) has been measured using a two-color femtosecond laser pump-probe system (a variation of transient time-domain thermoreflectance, TDTR). The thermal boundary conductance for the combinations of semiconductors and dielectrics falls within a relatively narrow range, 10–20 MW m−2 K−1, at room temperature. The measured thermal boundary conductance between Al film and semiconductor or dielectric substrates is one order of magnitude larger than that between semiconductor and dielectric substrates. A modified diffuse mismatch model (DMM) is used to interpret the data and extract the phonon transmissivity at the interface. The predicted results of the DMM corrected by attenuation constant agree well with the experimental values. Over a wide phonon velocity, both the measured and predicted results decrease with the increasing average phonon velocity. Both the vibration mismatch and changes in the localized phonon transport near the interface contribute to the reduction in thermal boundary conductance. Other scattering mechanisms are discussed which may explain the failure of the DMM at room temperature. [Copyright &y& Elsevier]