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The metal Al is commonly attached to external circuits as the source and drain in GaN-based field effect transistors, so profound comprehension of the energy transfer between electrons and phonons in Al/GaN is crucial for nanofabrication and thermal management of electronic devices. Time-domain thermoreflectance (TDTR) is an effective technique for measuring the strength of non-equilibrium electron-phonon (e-ph) coupling. The two-temperature model (TTM) is widely employed in conjunction with TDTR methods to determine e-ph coupling factors. However, TTM is a gray method and cannot take into account interactions between electrons and different phonon modes. Therefore, in this work, we use the TDTR technique to analyze the non-equilibrium transport properties of pure Al and the thickness dependence of the e-ph coupling with Al nanofilms, and the coupling strengths of high-energy electrons excited by femtosecond lasers with different modes of phonons are obtained in conjunction with MTM. The results show that the e-ph coupling coefficients of Al nanofilms on GaN substrates are larger than those of pure Al. In conjunction with the TTM, we determined the coupling strength between high-energy electrons excited by femtosecond laser pulses and various phonon modes. Compared to the transverse acoustic branch-1 (TA1) and transverse acoustic branch-2 (TA2) modes, the longitudinal acoustic (LA) phonon mode of Al exhibits a higher e-ph coupling factor. This suggests that the LA mode predominates in the electron relaxation process after ultrafast femtosecond laser excitation. This study provides experimental and theoretical guidance for laser processing and electronic device design.

Pressure is a powerful way to modulate physical properties. Understanding the effect of pressure on the thermal transport properties of thermoelectric materials is of great importance for the efficient design and optimization of thermoelectric performance. In this work, based on first-principles calculations and phonon Boltzmann transport theory, we find that the lattice thermal conductivities of Ag-based chalcopyrites AgXY 2 (X = Al, Ga, and In; Y = S, Se, and Te) are dramatically suppressed by applying pressure. The inherent distorted tetrahedral configuration together with highly delocalized p-orbital electrons promotes the formation of metavalent bonding. The fact of metavalent bonding with a single bonding electron and small electron transfer between neighboring atoms leads to soft low-frequency optical phonons. With the increase of pressure, the softening of acoustic and low-frequency optical phonons induces enhanced anharmonicity and scattering channels. Such strong acoustic-optical phonon coupling results in larger phonon scattering rates and thus lowers the lattice thermal conductivity. These findings not only help unveil the underlying physical mechanisms for the anomalous thermal transport behaviors under high pressure, but also pave the way for the pressure tuning of high-performance Ag-based thermoelectric materials.

Owing to nano-MoS 2 's excellent anti-friction and anti-wear properties, nano-MoS 2 , which can act as a nano-additive in lubricating oil or solid lubricants, is believed to have great potential in the lubrication of power machinery and moving parts of a spacecraft. The molecular dynamics method was used to construct a rough surface and a multi-level asperity structure to simulate starved lubrication before oil film breakdown, and the lubrication mechanism of MoS 2 as a nano-additive or directly coated on the textured surface could reduce the friction coefficient and wear was explained from the atomic perspective. Simulations showed that the multilayer MoS 2 played a role of load-bearing at light load or low velocity, and slipped into the grooves to repair the surface under heavy load or high velocity. Even if local asperity contact occurs, MoS 2 nanoparticles could accelerate the detachment of the initial asperity contact to prevent large-scale adhesion. The MoS 2 nanoparticles transformed the pure liquid oil film into a liquid-solid composite oil film, which was more suitable for lubrication under heavy load and high velocity because it increased the contact area, protected the friction surface and prevented asperity contact. The proposed lubrication mechanism contributes to understanding the frictional properties of layered nanomaterials under extreme conditions and provides a reference for further application of MoS 2 materials in the field of lubrication.

With excellent physical and chemical properties, 2D TMDC materials have been widely used in engineering applications, but they inevitably suffer from the dual effects of strain and device size. As typical 2D TMDCs, HfSe 2 and ZrS 2 are reported to have excellent thermoelectric properties. Thermal transport properties have great significance for exerting the performance of materials, ensuring device lifetime and stable operation, but current research is not detailed enough. Here, first-principles combined with the phonon Boltzmann transport equation are used to study the phonon transport inside monolayer HfSe 2 and ZrS 2 under tensile strain and finite size, and explore the band structure properties. Our research shows that they have similar phonon dispersion curve structures, and the band gap of HfSe 2 increases monotonically with the increase of tensile strain, while the bandgap of ZrS 2 increases and then decreases with the increase of tensile strain. Thermal conductivity has obvious strain dependence: with the increase of tensile strain, the thermal conductivity of HfSe 2 gradually decreases, while that of ZrS 2 increases slightly, and then gradually decreases. Reducing the system size can limit the contribution of phonons with a long mean free path, significantly decreasing thermal conductivity through the controlling effect of tensile strain. The mode contribution of thermal conductivity is systematically investigated, and anharmonic properties including mode and frequency-level scattering rates, group velocity and Grüneisen parameters are used to explain the associated mechanism. Phonon scattering processes and channels in various cases are discussed in detail. Our research provides a detailed understanding of the phonon transport and electronic structural properties of low thermal conductivity monolayers of HfSe 2 and ZrS 2 , and further completes the study of thermal transport of the two materials under strain and size tuning, which will provide a foundation for further popularization and application.

Zintl phase Mg 3 Sb 2 , which has ultra-low thermal conductivity, is a promising anisotropic thermoelectric material. It is worth noting that the prediction and experiment value of lattice thermal conductivity ( κ ) maintain a remarkable difference, troubling the development and application. Thus, we firstly included the four-phonon scattering processes effect and performed the Peierls-Boltzmann transport equation (PBTE) combined with the first-principles lattice dynamics to study the lattice thermal transport in Mg 3 Sb 2 . The results showed that our theoretically predicted κ is consistent with the experimentally measured, breaking through the limitations of the traditional calculation methods. The prominent four-phonon scatterings decreased phonon lifetime, leading to the κ of Mg 3 Sb 2 at 300 K from 2.45 (2.58) W m -1 K -1 to 1.94 (2.19) W m -1 K -1 along the in (cross)-plane directions, respectively, and calculation accuracy increased by 20%. This study successfully explains the lattice thermal transport behind mechanism in Mg 3 Sb 2 and implies guidance to advance the prediction accuracy of thermoelectric materials.

Thermoelectric materials with excellent performance can efficiently and directly convert waste heat into electrical energy. In today's era, finding thermoelectric materials with excellent performance and adjusting the thermoelectric parameters are essential for the sustainable development of energy in the context of the energy crisis and global warming. Through first-principles calculations, we notice that two-dimensional (2D) orthogonal ScX 2 (X = C and N) compounds show great potential in the field of thermoelectricity. Different from most materials containing C or N atoms, which are generally accompanied by high lattice thermal conductivity (TC), the 2D o -ScX 2 exhibited a rather low and anisotropic lattice TC. The κ 3L (the lattice thermal conductivity including the effect of three-phonon scattering and isotope scattering) of o -ScC 2 along the X and Y directions are 2.79 W m -1 K -1 and 1.55 W m -1 K -1 , and those of o -ScN 2 are 1.57 W m -1 K -1 and 0.56 W m -1 K -1 . By calculating the fourth-order interatomic force constants (IFCs), we obtain the κ 3+4L with the additional four-phonon scattering effect. Our results clearly show that four-phonon scattering plays an important role in the TC of the two materials, the κ 3+4L of o -ScC 2 is only half of its κ 3L. Furthermore, it can be noticed that the low lattice TCs of o -ScX 2 (X = C and N) are the result of many factors, e.g. , heavy atom doping, the strong anharmonicity caused by the vibration of Sc atoms in the out-of-plane direction and C(N) atoms in the in-plane direction, important four-phonon scattering and strongly polarized covalent bonds between X atoms and Sc atoms. Moreover, it is interesting to find that the thermal transport properties of o -ScX 2 are led by a different phonon mechanism, e.g. , the different TCs of o -ScC 2 and o -ScN 2 are determined by the anharmonic characteristic, and the harmonic characteristic plays a more important role in the anisotropy of o -ScX 2 (X = C and N). In general, our research can be expected to provide important guidance for the application of o -ScX 2 (X = C and N) in the thermoelectric field.

With the ability to alter the inherent interatomic electrostatic interactions, modulating external electric field strength is a promising approach to tune the phonon transport behavior and enhance the thermoelectric performance of two-dimensional (2D) materials. Here, by applying an electric field (Ez = 0.1 V Å-1), it is predicted that an ultralow value of the lattice thermal conductivity (0.016 W m-1 K-1) at 300 K of 2D indium selenide (InSe) is nearly three orders of magnitude lower than that under an electric field of 0 V Å-1 (27.49 W m-1 K-1). Meanwhile, we calculated the variations in the electrical conductivities, electronic thermal conductivities, Seebeck coefficients, and figure of merit (ZT) of 2D InSe along with the carrier (hole and electron doping) concentrations under some representative electric fields. Owing to the smaller total thermal conductivity along the armchair and zigzag directions, p-type doped 2D InSe at Ez = 0.1 V Å-1 exhibits a larger ZT value (∼1.6) compared to the ZT value (∼0.1) without an electric field at room temperature. The peak ZT value (∼0.53) of the n-type 2D InSe at Ez = 0.1 V Å-1 is much higher than that without an electric field (∼0.02) at the same temperature. Our results pave the way for applying an external electric field to modulate the phonon transport properties and greatly promote the thermoelectric performance of some specific 2D semiconductor materials without altering their crystal structure.

Through the first-principles density functional theory and the phonon Boltzmann transport equation, we investigated the phonon transport characteristics inside 1T-TiSe2. The calculation results of the lattice thermal conductivity (κl) show that the κl of TiSe2 is extremely low (1.28 W (m K)-1, 300 K) and decreases with the shrinkage of the sample size. Moreover, the results also prove the isotropic nature of thermal transport. By decomposing the contribution of the thermal conductivity according to the frequency, the κl of the single-layer TiSe2 is primarily attributed to the acoustic phonons and a small portion of optical phonons, with the frequency range of 0-4.5 THz. The calculation of the scattering rate further illustrates the competition of different scattering modes in this frequency range to verify the change in thermal conductivity of different sample sizes. The high scattering rate and low group velocity lead to the low thermal conductivity of the optical phonon mode in TiSe2. In addition, reducing the size of the system can significantly limit the thermal conductivity by eliminating the contribution of long mean free path phonons. When the characteristic length of the single-layer TiSe2 is about 14.92 nm, κl reduces to half. Our results also show that TiSe2 has an extremely high Grüneisen parameter (about 2.62). Further decomposition of the three-phonon scattering phase space and scattering rate demonstrates that in the range 0-4.5 THz, the absorption process is the main conversion form of phonons. We conclude that, due to the high Grüneisen parameter, the high anharmonicity in TiSe2 leads to the extremely low κl. This study provides κl related to the temperature, frequency, and MFP, and deeply discusses the phonon transport in TiSe2, which has great significance to further adjust the thermal conductivity to develop highly efficient thermoelectric materials and promote the application of devices based on TiSe2.

Manipulation of crystal symmetry is an important strategy to tune the thermoelectric performance. High-symmetry thermoelectric materials benefit from high band degeneracy. With first-principles calculations and Boltzmann transport theory, we systematically investigate the electronic and phononic transport properties of rhombohedral GeSe with higher symmetry. At optimized carrier concentrations, the maximum power factors are found to be 5.86 mW m-1 K-2 for the p-type and 4.45 mW m-1 K-2 for the n-type, respectively. The high p-type power factor originates from the highly degenerated L and Σ bands and small energy offset between them, while the n-type one results from the weak electron-phonon coupling. More importantly, rhombohedral GeSe possesses anisotropic and low lattice thermal conductivities of 3.58 W m-1 K-1 and 1.96 W m-1 K-1 at room temperature in the intralayer and interlayer directions, respectively, which is associated with the giant phonon anharmonicity driven by the resonant bonding. Combining the high power factor and low thermal conductivity, the predicted ZT values for p-type and n-type doping can reach 2.02 and 2.37 at 800 K. This study offers insights into the thermal and charge transport properties in rhombohedral GeSe, and demonstrates that both p-type and n-type GeSe are potential high-performance thermoelectric materials.

To reveal the effect of a striped superhydrophobic surface on frictional properties, molecular dynamics simulations were carried out to study the frictional properties of Couette flow. In particular, the influence of load on flow properties was considered in this work. The results showed that regions of gas in the groove and a low density region near the superhydrophobic surface were formed. Under a certain load, convex menisci appeared on top of the groove and some fluid atoms were trapped in it. Compared with the smooth hydrophobic surface, the striped superhydrophobic surface showed a reduction in friction owing to reduced liquid-solid contact area. With increasing load, the number of fluid atoms trapped in the groove increased prominently, which increased the friction force of the striped superhydrophobic surface more quickly. There was a critical load (Pcrit), such that the friction-reduction property of striped superhydrophobic surfaces appeared only when the load was smaller than it. By reducing the distance between adjacent stripes, the rate of increase in the number of fluid atoms trapped in grooves with load decreased significantly, which increased Pcrit. Under a large load, the friction force decreased with the distance between adjacent stripes. However, under a small load we observed the opposite trend.

In this study, strain modulation of the lattice thermal conductivity of monolayer and bilayer penta-graphene (PG) at room temperature was investigated using first-principles calculations combined with the phonon Boltzmann transport equation. The thermal conductivities of both the monolayer and the bilayer PG exhibit a robust nonmonotonic up-and-down behavior under strain despite the effect of van der Waals (vdW) interactions, and the thermal conductivities of bilayer PG under strain are significantly reduced by up to 87%. Using phonon-level systematic analysis, the variation of thermal conductivity with the increasing strain was determined by increasing the phonon lifetime in specific phonon modes, and that with the reduction of strain was determined by the decrease of both phonon group velocity and phonon lifetime. Moreover, bilayer PG shows an unexpectedly different response to strain when compared with monolayer PG, and a significantly larger reduction (>60%) in the thermal conductivity of bilayer PG is achieved when the strain reaches 10% because the interlayer interactions enhance the phonon anharmonicity of the phonon modes of ultra-low frequency. Our study shows that bilayer PG will have tremendous opportunities for application in thermal management and two-dimensional nanoscale electronic devices owing to its largely tunable thermal conductivity.

In this paper, the interfacial thermal transport properties at the interfaces between the cubic compounds of boron with selected group V elements (BP, BN, BAs and BSb) and various substrates (Si, 6H-SiC and 3C-SiC) were studied by first-principles calculations. Systematic analysis of the effect of crystal information on interfacial thermal transport is performed based on the study of phonon density of states, atomic mass, crystal structure and spectral heat flux, respectively. The results show that the overlap of the phonon density of states of the two interface materials is related to the interfacial thermal conductance. Other crystal information, the atomic mass and lattice constant, which cannot directly reflect the trend of interfacial thermal conductance, can only play a predictive role. Further deep insight suggests that the interfacial thermal conductance also depends strongly on the phonon thermal transport characteristics of different materials and the frequency-dependent spectral heat flux. The results from this work unveil the fact that Si and SiC as the substrate materials do not have absolute superiority or inferiority, depending on the matching rate of many factors of the two materials at the interfaces. This study explores the phonon-level mechanisms for interfacial thermal transport between compounds of boron with group V elements and Si/SiC substrates and provides effective ways to improve the interfacial thermal transport in silicon based modern micro/nano-electronic devices.

In this paper, first-principles calculations along with the phonon Boltzmann transport equation are used to study the strain- and size-dependent thermal conductivity of monolayer WSe2. The thermal conductivity of monolayer WSe2 is primarily contributed by the acoustic phonons and decreases with tensile strain due to the reduction in both the group velocity and phonon lifetime. Shrinking the system size also restricts the thermal conductivity significantly by ruling out the contributions of long mean free path phonons. The rate of decrease in thermal conductivity with tensile strain is found to be size dependent, which is attributed to the competition between the phonon-phonon scattering and the phonon-boundary scattering. The decreasing trend of the thermal conductivity of monolayer WSe2 through tensile strain paves the way for high-efficiency thermoelectric materials combining the strain-tuned electronic structure.

In this paper, the variation of the lattice thermal conductivity of GaAs under pressure within the range of 0-20 GPa at room temperature is investigated by combining first-principles calculations with an iterative solution of the phonon Boltzmann transport equation. Firstly, we calculated the lattice thermal conductivity of GaAs at 12 GPa, which increases by more than 37% in comparison with that under atmospheric pressure. The detailed analysis of phonon heat transport properties shows that the pressure contributes to increased phonon group velocity coupled with decreased phonon relaxation time, resulting in the pressure-induced nonlinear increase of the thermal conductivity of zinc blende GaAs. Besides, not only the structure but also the phonon heat transport properties of GaAs transform from isotropic to anisotropic beyond the phase transition pressure. This study provides a quantitative understanding of the thermal conductivity of GaAs considering pressure-induced phase transitions and highlights the importance of pressure in tuning lattice thermal conductivity, especially in pressure-induced phase change materials.