Phonon Relaxation Time Research Articles

Unexpected Enhanced Thermal Conductivity of GaxIn1–xSb Ternary Alloys

Alloying is one of the most common approaches to modulate the properties of semiconductors, such as indium antimonide (InSb) with practical applications in mid-infrared optoelectronics. Thermal conductivity plays a key role in governing the operating performance of devices, and the high-efficiency regulation is of great significance to desirable applications. In this study, we studied the thermal transport properties of InSb, GaSb, and their GaxIn1–xSb alloys by solving the Boltzmann transport equation based on first-principles calculations. The thermal conductivity of GaxIn1–xSb alloys (x = 0.25, κ = 9.21 Wm–1 K–1; x = 0.5, κ = 11.22 Wm–1 K–1; and x = 0.75, κ = 15.45 Wm–1 K–1) is enhanced compared with that of InSb (κ = 5.15 Wm–1 K–1), which is unlike the conventional belief that alloying usually reduces thermal conductivity. Through fundamental analysis of phonon transport properties, the higher phonon group velocity and phonon relaxation time caused by the weak phonon anharmonicity lead to higher thermal conductivity of the GaxIn1–xSb alloys. The weakened polarization of GaxIn1–xSb alloys occurring in the alloying process generates the increased thermal conductivity, which can be figured out by analyzing the orbital-projected electronic density of states and the electron localization functions. The underlying mechanism of unconventional high thermal conductivity of the GaxIn1–xSb alloys in this study would provide guidance for the device applications accompanied by thermal transport.

Atomic-Scale Surface Engineering for Giant Thermal Transport Enhancement Across 2D/3D van der Waals Interfaces.

Heat dissipation in two-dimensional (2D) material-based electronic devices is a critical issue for their applications. The bottleneck for this thermal issue is inefficient for heat removal across the van der Waals (vdW) interface between the 2D material and its supporting three-dimensional (3D) substrate. In this work, we demonstrate that an atomic-scale thin amorphous layer atop the substrate surface can remarkably enhance the interfacial thermal conductance (ITC) of the 2D-MoS2/3D-GaN vdW interface by a factor of 4 compared to that of the untreated crystalline substrate surface. Meanwhile, the ITC can be broadly manipulated through adjusting substrate surface roughness. Phonon dynamic and heat flux spectrum analyses show that this giant enhancement is attributed to the increased phonon densities and channels at the interfaces and enhanced phonon coupling. The slight surface fluctuation in MoS2 and the increased diffuse interfacial scattering facilitate energy transfer from MoS2's in-plane phonons to its out-of-plane phonons and then to the substrate. In addition, it is further found that the substrate and its surface topology can dramatically influence the thermal conductivity of MoS2 due to the reduction of phonon relaxation time, especially for low-frequency acoustic phonons. This study elucidates the effects of the amorphous surface of the substrate on thermal transport across 2D/3D vdW interfaces and provides a new dimension to aid in the heat dissipation of 2D-based electronic devices via atomic-scale surface engineering.

Enhancing phonon thermal transport in 2H-CrX2 (X = S and Se) monolayers through robust bonding interactions.

Inspired by the groundbreaking discovery of the 2H-MoS2 monolayer with outstanding physical properties, the electronic structure, structural stability, and thermal transport of 2H-CrX2 (X = S and Se) monolayers are theoretically evaluated using density functional theory (DFT) calculations and semiempirical Boltzmann transport theory. The 2H-CrX2 (X = S and Se) monolayers are direct semiconductors with the bandgaps of 0.91 and 0.69 eV. The elastic modulus and phonon dispersion curve analysis show that the 2H-CrX2 (X = S and Se) monolayers possess excellent mechanical and dynamic stabilities on account of elastic constants satisfying the Born-Huang criterion and the absence of negative frequencies. The thermal stabilities of the 2H-CrX2 (X = S and Se) monolayers at 300 K are proved by ab initio molecular dynamics (AIMD) simulations, as evidenced by the slight changes in the structural evolution and small fluctuation in total energy. High thermal conductivities of 131.7 and 88.6 W m-1 K-1 are discovered for 2H-CrS2 and 2H-CrSe2 monolayers at 300 K. Further analysis of the phonon group velocity, phonon relaxation time, and Grüneisen parameter shows that the high lattice thermal conductivities of 2H-CrX2 (X = S and Se) monolayers could be attributed to the great bond strength, large Young's modulus, relatively small atomic mass, high phonon group velocity, and long phonon relaxation time. In addition, the various scattering mechanisms are further considered in the calculations of phonon thermal transport to evaluate the effect of the scattering rates of the 2H-CrS2 and 2H-CrSe2 monolayers on the lattice thermal conductivity, and the determinative role is found for the phonon boundary scattering. Our present study would not only offer a fundamental understanding of the thermal transport properties of the 2H-CrX2 (X = S and Se) monolayers, but also provide theoretical guidelines for the experimental investigation of thermal management materials with 2H-phase.

First-principles study of the thermal transport properties of superconducting NbN

We investigate the thermal transport properties of NbN by considering three-phonon scattering (3ph), four-phonon scattering (4ph), electron-phonon (ep) scattering, and isotope (iso) scattering. By considering 4ph scattering, we find that 4ph scattering has less effect on the lattice thermal conductivity (κph) within 3-9 THz, while ep scattering has a greater effect on κph which is close to 3ph scattering in this frequency range. When the pressure is increased to 15 GPa, the κph by 148%, which is due to the decrease of the scattering phase space resulting in an increase of phonon relaxation time. We find that the κph of NbN is close to 100 W/mK, which is a potential high thermal conductivity material.

Suppressed thermal conductivity of bilayer SnS: A comparative study among the monolayer, bilayer and bulk SnS

The layer-stacking of 2D materials have been proven to be an effective tool to modulate the low-dimensional electronic structures and transport properties. In this study, the lattice thermal transport properties of bilayer SnS are investigated systematically by solving phonon Boltzmann transport equation based on first-principles calculations. The stacking effect on the in-plane thermal conductivity is further revealed through comparing phonon transport properties of the monolayer, bilayer and bulk SnS. A dramatic suppression of the thermal conductivity, even below 0.5 Wm −1 K −1 per layer at 300 K, can be observed for the fully relaxed bilayer SnS, which implies the great potential for SnS-based thermoelectric applications. The underlying phonon transport mechanisms are also uncovered, and the drop of phonon relaxation time, resulted from the enhanced interlayer anharmonic phonon scattering, is responsible for the suppression of lattice thermal transport. • The stacking stability of bilayer SnS is revealed, and the AA′-stacking is the most dynamically stable configuration for the bilayer SnS. • A dramatically suppressed thermal conductivity for the special stacking configuration of the bilayer SnS, e. g. 0.294 (0.112) Wm −1 K −1 per layer at 300 K along the zigzag (armchair) direction, can be predicted. • The underlying phonon transport mechanisms, and the stacking effects, are also revealed through a comparative study among the monolayer, bilayer and bulk SnS.

Phonon transport mechanism of HfO2 ultrathin film with temperature-correction full-band Monte Carlo simulation

Hafnium dioxide (HfO2) has been widely used in microelectronics nowadays and commonly withstands extremely high temperatures, so the investigation of its thermodynamic properties is particularly essential. This paper develops a temperature-correction full-band Monte Carlo (TFMC) method to investigate the HfO2 ultrathin film. The phonon dynamics parameters of HfO2 are calculated based on the first-principles method. TFMC can better describe the phonon cumulative distribution function in different temperatures by modifying the phonon relaxation time and heat capacity. The thermal conductivity of HfO2 ultrathin film is calculated based on the above method and is in good agreement with the literature. It is observed that the optical phonons in HfO2 ultrathin film are prominent in the phonon heat transport, which is quite different from the mechanism in common semiconductor materials. Combined with the full-band diffuse mismatch model, the Si-based HfO2 ultrathin film is studied. It is found that the existence of the interface with substrates improves the thermodynamic properties of the ultrathin film, which provides a reference for the selection of substrate materials.

Phonon transport in two-dimensional carbon-boron material and heterointerfaces

Developing semiconducting materials with suitable band gap has attracted increasing attention for next-generation intelligent electronic devices. Recently, the newly synthesized monolayer carbon boron (C3B) material exhibits outstanding electronic properties and processes with indirect bandgap, which is considered as the promising alternatives of graphene. The heat conduction capability plays a critical role in the performance of C3B-based electronic devices and thermocatalysis applications. Herein, the thermal conductivity of monolayer C3B and graphene, and interfacial phonon transport across heterointerface are systematically investigated utilizing non-equilibrium molecular dynamics simulation. Compared with graphene, the reduced thermal conductivity of C3B sheet mainly stems from the decreased phonon group velocity and phonon relaxation time when the periodical boron atoms are introduced in C3B sheet based on the lattice dynamics and spectral phonon transmission analysis. Meanwhile, the influences of temperature and strain on the thermal conductivity of C3B are also discussed. Moreover, the thermal transport across the graphene|C3B heterointerface with different temperatures is analyzed. Interestingly, it is found that the larger interfacial thermal conductance of zigzag heterointerfaces mainly originates from the stronger phonon-phonon coupling between the out-of-plane vibrational modes of C3B and graphene. Our work would provide more insights for fundamental understanding the thermal transport properties of two-dimensional materials and be beneficial to the design of thermal management and thermoelectric materials.

Extraordinary role of resonant dopant vanadium for improving thermoelectrics in n-type PbTe

Inspired by the bottleneck that the optimization of the dimensionless figure of merit zT is limited by the concept of amorphous limit, impurity-induced band structure distortion independent of phonon properties opens up an effective avenue to significantly enhance room-temperature thermoelectrics of narrow-bandgap materials. In this work, density functional theory calculations confirm the virtual bound state of vanadium at the bottom of the conduction band in n-type PbTe. The effectiveness of the resonant level in enhancing electrical properties not only depends on the electronic structure or scattering mechanism near the chemical potential, but also on the Fermi level. Therefore, a delicate manipulation of the Fermi level is achieved by trace copper substitution to optimize the thermoelectric performance. Furthermore, based on the single parabolic band model and the relaxation time approximation, the dominant term of the transition-metal localized d-electron resonant state in increasing the Seebeck coefficient is clarified in n-type PbTe, which is significantly different from that of the thallium resonant level. More importantly, combined with the mechanism of shortening phonon relaxation time through multi-scale defect sources, the lattice thermal conductivity is drastically reduced. Here, the n-type Pb0.967Cu0.003V0.03Te sample enables an increased peak zT of ∼1.3, as well as the average zTavg of ∼1.1 from 523 to 823 K. The current findings provide a paradigm for the transition-metal resonant states to regulate thermoelectrics.

Effects of electron-phonon coupling on the phonon transport properties of the Weyl semimetals NbAs and TaAs: A comparative study

It has now become recognized that the electron-phonon coupling (EPC) may play an important role in governing the phonon transport, especially for metallic and semiconducting systems at high carrier concentration. Here we focus on the Weyl semimetals TaAs and NbAs and give a comparative study on their phonon transport properties by explicitly including the EPC in first-principles calculations. It is found that the lattice thermal conductivities of both systems are significantly reduced by the EPC, which is more pronounced for the TaAs compared with the NbAs at the same carrier concentration. Detailed analysis indicates that the TaAs exhibits smaller EPC phonon relaxation time, as characterized by stronger EPC strength which is associated with larger deformation potential constant and Born effective charge. Moreover, we see that the TaAs exhibits obviously larger overlap between the EPC relaxation time and that from intrinsic phonon-phonon scattering, which could further reduce the lattice thermal conductivity. Our work not only highlights the vital importance of EPC in accurately predicting the phonon transport behaviors, but also offers a simple alternative to evaluate the EPC strength of various material systems.

Tunable thermal transport properties of bilayer GeS with stacking patterns

The stacking of 2D layered materials can be an effective tool to modulate low-dimensional electronic structures and transport properties. In this work, using first-principles calculations, the thermal transport properties of a GeS bilayer are systematically investigated by solving the phonon Boltzmann transport equation. Various stacking configurations for bilayer GeS are introduced, and two dynamically stable structures are confirmed. The results indicate that the thermal transport property of the GeS bilayer can be dramatically suppressed due to a decreased phonon relaxation time, which is dependent on the stacking patterns and interlayer distances. The underlying phonon transport mechanisms and the stacking effects on the lattice thermal conductivity for bilayer GeS are further revealed through a comparative study among monolayer, bilayer, and bulk GeS. In addition, the in-plane anisotropy of the thermal transport properties is also enhanced for the GeS bilayer, which is also found to be dependent on the stacking pattern. The significantly suppressed thermal conductivity for the GeS bilayer evaluated in this work implies great potential for 2D multilayer-based thermoelectric devices and applications.

High performance GeTe thermoelectrics enabled by lattice strain construction

Numerous intrinsic Ge vacancies in thermoelectric GeTe not only lead to overhigh carrier concentration but also seriously deteriorate carrier mobility, which shackles its thermoelectric performance. The efficient strategy and the related underlying mechanism in suppressing intrinsic Ge vacancy, however, are rarely researched yet. Herein, we demonstrated that lattice strain could be employed to regulate the defects concentration and then optimize electrical transport performance. Theoretically, the calculated results showed that lattice strain could efficiently raise the formation energy of Ge vacancies, weakening the carrier scattering and improving the carrier mobility. Calculated band structure revealed that Sb doping and Ge vacancy introduction could promote band convergence and thus efficiently decouple the electrical transport parameters. Experimentally, lattice strain was constructed through high-energy ball milling combined with spark plasma sintering to reduce the concentration of Ge vacancy and relaxation time of phonons, leading to high carrier mobility and low lattice thermal conductivity. Additionally, we carefully modulated the nominal content of Ge, and then a high ZT of 2.0 at 723 K in Ge0.90Sb0.08Te alloy was obtained. This work highlights that the lattice strain can be utilized to simultaneously optimize thermal and electrical transport properties.

Ultralow lattice thermal conductivity and promising thermoelectric properties of a new 2D MoW3Te8 membrane

Assembling new van der Waals (vdW) materials is challenging for the development of two-dimensional (2D) function devices. The MoW3X8 membrane (X = Se, Te) is a new 2D TMDs membrane molecule composed of one WX2 monolayer and one WX2-MoX2-WX2 sandwich trilayer. The presence of Mo/W atoms endows the new structure with the bridges between X atoms that connect pairs of MoX2/WX2 monolayers and the terminal sites that produce the van der Waals gap in these layers. The mirror symmetry is broken and the phonon dispersion is suppressed by reducing the dimensionality of the MoW3X8 membrane. In this work, the phonon transport and thermoelectric properties of the MoW3X8 membrane are investigated using first-principles method combined with the semi-classical Boltzmann transport and relaxation time approximation (RTA) theories. It is found that the larger gap between low-frequency and high-frequency optical branches in the membrane prevents atomic vibrations and drastically reduces the phonon velocity in a mid-frequency range below the gap. The low-lying optical and acoustic phonon modes are closely linked in MoW3Te8 membranes, enhancing the phonon–phonon scattering and thereby shortening the phonon relaxation time. These characteristics allow the MoW3Te8 membrane to achieve an extremely low lattice thermal conductivity of 0.49 Wm-1K−1 relative to that of the MoW3Se8 membrane (3.25 Wm-1K−1), which also leads to the improvement in thermoelectric performance of the former one. Besides, the maximum ZT values of 4 (4.5) at 900 K and the carrier concentration of 4 × 1011 cm−2 in the n-type (p-type) MoW3Te8 membrane could be enriched because of the low lattice thermal conductivity.

Lattice Boltzmann method with effective correction of phonon properties for nano/microscale heat transfer

The significant self-heating effect severely restricts the performance and reliability of nano-electronic devices. Accordingly, it is very important to understand the process and mechanism of nano/microscale heat transfer for thermal management and thermal design of devices. In this work, we propose a new Lattice Boltzmann Method (LBM) scheme with effective correction of phonon mean free path (MFP) and relaxation time to study phonon heat transfer in silicon thin films and silicon medium with defects, where the correction factor is dependent on the lattice structure of LBM. The transformation analysis of phonon transfer mechanism at different scales shows that the size effect of cross-plane thermal conductivity is more remarkable than that of in-plane thermal conductivity. And the thermal conductivity of silicon medium with defects decreases exponentially as defect density increases. The proposed new LBM scheme can generate more accurate results than the traditional ones in the heat conduction simulations of different nano/microscale structures.

Role of polarization-photon coupling in ultrafast terahertz excitation of ferroelectrics

We investigate the role of polarization-photon coupling (specifically, the polarization-oscillation-induced radiation electric field) in the excitation of ferroelectric thin films by an ultrafast terahertz electric field pulse. Analytical formulas are developed to predict how the frequencies and relaxation time of three-dimensional soft mode phonons (intrinsic polarization oscillation) are modulated by the radiation electric field and epitaxial strain. Ultrafast terahertz-pulse-driven excitation of harmonic polarization oscillation in strained single-domain ferroelectric thin film is then simulated using a dynamical phase-field model that considers the coupled strain-polarization-photon dynamics. The frequencies and relaxation time extracted from such numerical simulations agree well with analytical predictions. In relatively thin films, it is predicted that the radiation electric field slightly reduces the frequencies but significantly shortens the relaxation time. These results demonstrate the necessity of considering polarization-photon coupling in understanding and predicting the response of ferroelectric materials to ultrafast pulses of terahertz and higher frequencies.

Effect of electron–phonon coupling on transport properties of monolayer germanene: A thermoelectric perspective

The thermoelectric properties and the effect of electron–phonon coupling on scattering time of charge carriers in monolayer low-buckled germanene have been studied using a combination of first-principles calculation and semi classical Boltzmann transport theory. A systematic and precise estimation of thermoelectric transport properties for intrinsic and extrinsic monolayer germanene has been carried using hybrid functional HSE06. These thermoelectric properties significantly depend on scattering time of charge carriers. So we studied the effect of electron–phonon coupling on scattering time. The incorporation of optical phonons along with acoustic phonons in the calculation enhances the accuracy of scattering time and hence this increases the precision of thermoelectric transports properties. In the estimation of thermoelectric figure-of-merit, the constant scattering time approach shows an underestimation about two orders of magnitude than our approach. The lattice thermal conductivity is found 8.16 Wm−1K−1 at room temperature. The contribution of phonon relaxation time and phonon group velocity to the lattice thermal conductivity is also investigated. The maximum figure-of-merit obtained is 0.17 in n-type germanene at 1200 K.

Thermal Transport Properties of Na2X (X = O and S) Monolayers

Motivated by the excellent functional thin film devices made from two-dimensional materials, we investigated the thermal transport properties of Na2X (X = O and S) monolayers using first-principle calculations. The thermal conductivity at room temperature was 1.055 W/mK and 1.822 W/mK for the Na2O monolayers and Na2S monolayers, respectively. The high thermal conductivity for the Na2S monolayers is mainly contributed to by in-plane transverse acoustic (TA) phonons. The group velocity for the Na2S monolayers exhibits lower group velocity and a larger phonon relaxation time than the Na2O monolayers. Our results are helpful for functional thin film devices made using Na2X (X = O and S) monolayers.

Ultralow and anisotropic thermal conductivity in graphene phononic metamaterials

Phononic metamaterials based on the idea of phonon coherent resonance attract increasing research attention because of their unique properties. In this paper, we have studied the heat transport in graphene phononic metamaterials (GPMs, pillared graphene nanoribbons) using molecular dynamics simulations. The results show that, GPMs have lower thermal conductivities in the different in-plane directions and stronger anisotropy in thermal conductance, compared with graphene nanoribbons (GNRs). As the temperature decreases or the height and width of pillar increases, the GPM thermal conductivity in both directions decreases and the anisotropic ratio increases. The pillar-driven suppression of thermal conductivity along the periodic direction is attributed to the phonon localization and the local resonance leading to the flat bands and band gaps, while the reduction in thermal conductivity along the aperiodic direction is due to the phonon boundary scattering. The thermal anisotropy results from the anisotropic phonon relaxation times and the anisotropic contributions of low-frequency-phonons. Our findings reveal the phonon transport mechanisms of GPMs in different directions, and shed some light on the GPM potential for thermoelectric and directional heat management applications.

Effects of biaxial strain on thermal conductivity of novel puckered C2N2 monolayer: A first-principles study

Strain engineering is an effective way to tune thermal and electrical properties in two-dimensional (2D) material. In this work, we systematically investigated the strain dependent lattice thermal conductivity of novel puckered C2N2 monolayer by solving the phonon Boltzmann transportation equation (BTE) with interatomic force constants extracted from first-principles calculations. At 300 K, the unstrained puckered C2N2 monolayer exhibit a high thermal conductivity of 587 W m−1K−1 and 355 W m−1K−1 along the zigzag and armchair direction, respectively. At 8% biaxial tensile strain, the thermal conductivity along the zigzag direction is 61 W m−1K−1, which is reduced by 89.6%. The underlying mechanism for such a remarkable decline of thermal conductivity in puckered C2N2 was correlated to the fact that stretch reduces both group velocity and phonon relaxation time. Our findings offer insight into the influence of strain on 2D materials and will be helpful for applications of these materials in nanoelectronics devices.

Evolution of in-plane heat transport in tellurium from 2D to 3D

Tellurene (Te) has attracted tremendous interest due to its outstanding electronic, thermoelectric, and optoelectronic properties. Recently, it is found that 2-layer Te exhibits an abnormal anisotropy of in-plane thermal conductivity. This inspires the current investigation, where the thickness dependence of in-plane thermal conductivities of tellurium from 2D to 3D is obtained by first-principles calculations and Monte Carlo simulations. Two intriguing phenomena are discovered: (1) Thermal conductivity rebound (TCR), i.e., the thermal conductivity first reduces and then increases with the increasing thickness. The predicted lowest point emerges between 6-layer to 15-layer; (2) Thermal anisotropy reversal (TAR), i.e., the in-plane thermal conductivity anisotropy reverses from abnormal (κ⊥/κ∥ > 1) to normal (κ⊥/κ∥ < 1) with increasing thickness. The predicted reversal point occurs at around 7-layer. To understand these phenomena, the frequency- and mode-dependent analyses on phonon group velocity and relaxation time are performed. As the thickness increases, the relaxation time almost monotonically increases, whereas the velocity of low-frequency optical (LFO) phonons shows the same varying trend with thermal conductivity, making it the main factor accounting for the TCR and TAR. The trend of the group velocity of LFO phonons can be attributed to the lattice expansion, which diminishes the covalent-like quasi-bonding (CLQB) in the cross-chain direction. The layer-dependent thermal transport of Te revealed in this work is expected to provide guidance for Te-based functional devices, for instance, the thermoelectric system where the lowest thermal conductivity is favorable.

Thermal conductivity of SrTiO3 under high-pressure

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.