Three-phonon Scattering Phase Space Research Articles

First-principles calculations of thermoelectric transport properties in WSe2/SnS2 bilayer heterostructure

Recently, the experimentally fabricated van der Waals bilayer heterostructure of WSe2/SnS2 was found to possess excellent electronic and optoelectronic applications in p-n diode, photodetector and transistor (Yang et al., Nat. Commun., 2017, 8, 1906; Zhou et al., Adv. Mater., 2018, 30, 1703286). However, the thermoelectric properties have not been studied. Here, using the first-principles calculations and Boltzmann transport theory, we present a study on the thermoelectric transport properties for the WSe2/SnS2 bilayer heterostructure. The results show that the n-type power factor of WSe2/SnS2 bilayer heterostructure is greatly improved compared to that of WSe2 monolayer. The calculated phonon relaxation time and three-phonon scattering phase space indicate that the low-frequency optical branches overlapping with the acoustic modes in WSe2/SnS2 bilayer heterostructure provides more scattering channels, resulting in a lower lattice thermal conductivity, 22.69 W m−1 K−1 at room temperature, which is smaller than that of monolayer WSe2. The optimized n-type thermoelectric ZT value at 800 K for WSe2/SnS2 can reach 1.16, which indicate the bilayer heterostructure WSe2/SnS2 is predicted to be promising for thermoelectric applications.

Theoretical upper limits of the thermal conductivity of Si3N4

Silicon nitride (Si3N4) is a promising substrate for high-power electronics due to its superior mechanical properties and potential outstanding thermal conductivity (κ). As experiments keep pushing the upper limit of κ of Si3N4, it is believed that it can reach 450 W/mK, similar to SiC, based on classical models and molecular dynamics simulations. In this work, we reveal from first principles that the theoretical κ upper limits of β-Si3N4 are only 169 and 57 W/mK along the c and a axes at room temperature, respectively. Those of α-Si3N4 are about 116 and 87 W/mK, respectively. The predicted temperature-dependent κ matches well with the highest available experimental data, which supports the accuracy of our calculations, and suggests that the κ upper limit of Si3N4 has already been reached in the experiment. Compared to other promising semiconductors (e.g., SiC, AlN, and GaN), Si3N4 has a much lower κ than expected even though the chemical bonding and mechanical strengths are close or even stronger. We find the underlying reason is that Si3N4 has much lower phonon lifetimes and mean free paths (<0.5 μm) due to the larger three-phonon scattering phase space and stronger anharmonicity. Interestingly, we find that the larger unit cell (with more basis atoms) that leads to a smaller fraction of acoustic phonons is not the reason for lower κ. Grain size-dependent κ indicates that the grain boundary scattering plays a negligible role in most experimental samples. This work clarifies the theoretical κ upper limits of Si3N4 and can guide experimental research.

Finite-temperature force constants are essential for accurately predicting the thermal conductivity of rutile TiO2

Phonon transport in rutile $\mathrm{Ti}{\mathrm{O}}_{2}$ between temperatures of 100 and 900 K is investigated using lattice dynamics calculations and the Boltzmann transport equation. Zero- and finite-temperature force constants are extracted from density functional theory calculations to examine the effects of finite temperature on phonon properties and thermal conductivity. Using the zero-temperature force constants leads to thermal conductivity predictions along the $a$ and $c$ axes that are 50% lower than experimental measurements at a temperature of 300 K. The underprediction increases as the temperature is further increased. Using temperature-specific force constants, however, leads to thermal conductivity predictions that fall within the uncertainty bounds of multiple sets of experimental measurements. Phonon mode analysis reveals that the thermal conductivity underprediction from the zero-temperature force constants is a result of an underprediction of phonon lifetimes. The lifetime underprediction results from the temperature dependence of (i) the second-order force constants, which set the phonon frequencies and thus the three-phonon scattering phase space, and (ii) the third-order force constants, which influence the intrinsic scattering rates. Our results emphasize the importance of including finite-temperature effects when studying phonon transport in oxides and inform how nanostructuring impacts the thermal conductivity of rutile $\mathrm{Ti}{\mathrm{O}}_{2}$.

Lattice thermodynamic behavior in CeO2 from first principles

By combining the first principles with Boltzmann's transport equation, we have carried out a detailed calculation and analysis of the lattice thermal transport properties of CeO2. We found that as the pressure increases, the acoustic mode and optical mode of CeO2 gradually separate and an obvious bandgap appears. In addition, the lattice thermal conductivity increases significantly with increasing pressure, and can be increased from 16.99 W/mK (0 GPa) to 237.73 W/mK (160 GPa) at 300 K. By studying the contribution of phonons to the lattice thermal conductivity at different frequencies, we found that the lattice thermal conductivity of CeO2 is mainly provided by low-frequency phonons (<400 cm−1). Based on this we calculated and studied the phonon group velocity of low-frequency phonons, scattering rate, three-phonon scattering phase space and Grüneisen parameters. The results show that an increase in pressure significantly reduces the three-phonon scattering rate of CeO2 and the anharmonicity of the system, which in turn affect the lattice thermal conductivity seriously. Our work provides a theoretical basis for the subsequent study of CeO2 under high pressure.

Probing the intrinsic phonon transport properties of the SnTe thermoelectric material

Based on the Boltzmann transport theory combined with first-principles calculations, we analyze the lattice anharmonicity of SnTe and the respective contributions of the lattice thermal conductivity (κL) from six different phonon modes. The Grüneisen parameter (γ), three-phonon scattering phase space (SPS), phonon lifetime (τ), and phonon velocity (v) were obtained quantitatively. These calculations indicate that large τ with maximum value of 104 ps and high v of 4800 m/s for acoustic branches make the acoustic modes a major contributor to the high κL of SnTe (～78% for acoustic branches and ～22% for Optical branches). Further, the dependence of κL on phonon mean free path shows that designing nanostructures with characteristic length of less than 14.5 nm could be an effective way to decrease the κL. In addition, we employed high-temperature gradient directional solidification technique to grow the SnTe single crystal and the measured κL vaules of the single crystal match reasonably with the theoretical calculation results.

Why thermal conductivity of CaO is lower than that of CaS: a study from the perspective of phonon splitting of optical mode

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.

Titanium nitride halides monolayers: promising 2D anisotropic thermoelectric materials

2D graphene-like thermoelectric materials have been extensively explored, however, the studies have mostly focused on the binary compounds and most of them exhibit isotropic electron and phonon transport properties. Here, we investigate the thermoelectric transport properties of ternary TiNX (X = F, Cl, Br) monolayers by using first-principles combined with the Boltzmann transport theory. Both electron and phonon anisotropic transport properties are found. The large p -type power factor and the low lattice thermal conductivity along the y direction give rise to better thermoelectric performance along the y direction than the x direction, and the highest ZT values at 500 K reach 1.00, 0.89 and 1.17 along the y direction in p -type doping for TiNF, TiNCl, and TiNBr monolayer, respectively. The anisotropy and the difference of lattice thermal conductivities among TiNX monolayers are discussed in terms of the group velocities, the phonon relaxation time and the three-phonon scattering phase space. These results indicate that TiNX monolayers are promising candidates for 2D anisotropic thermoelectric materials.

The ultralow thermal conductivity and ultrahigh thermoelectric performance of fluorinated Sn2Bi sheet in room temperature

It has been a grand challenge to develop efficient thermoelectric materials for room temperature applications. Here, motivated by the recent experimental synthesis of Sn2Bi sheet on silicon wafer [Phys. Rev. Lett. 121, 126801 (2018)], for the first time we find that fluorination not only can stabilize Sn2Bi sheet in free-standing condition but also can significantly enhance the thermoelectric performance. The fluorinated Sn2Bi sheet exhibits an enlarged three-phonon scattering phase space, reduced group velocity and enhanced anharmonicity, leading to an ultralow lattice thermal conductivity of 0.19 W m−1 K−1 and an ultrahigh ZT value of 2.45 (1.70) at 300 K for n- (p-) doping, which are much higher than the reported values for n-type Bi2Te2.79Se0.21 polycrystalline alloy (ZT~1.2 at 357 K), and p-type nanocomposites Bi0.48Sb1.52Te3 (ZT~1.4 at 475 K), thus the fluorinated Sn2Bi sheet can be a promising candidate for room-temperature thermoelectric applications.

Thermal conductivity of bulk and porous ThO2: Atomistic and experimental study

Thorium dioxide (ThO2) is proposed to play a vital role in the world's future energy needs and is considered a better and safer alternative to the currently used nuclear fuel, uranium dioxide (UO2). Thermo-physical properties of ThO2 are superior to UO2, but the fundamental physics governing the heat transport in ThO2 is still ambiguous, and the available data for the thermal conductivity (k) of ThO2 was scattered. In this article, a systematic investigation regarding the lattice thermal conductivity (kL) of the bulk and porous ThO2 is carried out theoretically and validated with experiments. The phonon transport calculations were done using two different methods; ab-initio calculations combined with the Boltzmann transport equation (BTE) and the equilibrium molecular dynamics (EMD) simulations using Green Kubo (GK) approach. An extensive examination of the phonon mode contribution, available three-phonon scattering phase space modes, Grüneisen parameter, and mean free path (MFP) distributions were analyzed to understand the underlying physics in the thermal transport of ThO2. The effect of porosity on the kL by measurements and molecular dynamics (MD) simulations was explored. The measurements were performed on specimens with different porosity, that were prepared by spark plasma sintering (SPS) using the laser flash (LFA) technique. The results obtained demonstrated that the kL values predicted by both the BTE and the EMD simulations were in excellent agreement with our experimental measurements. Moreover, the model to simulate the 95% theoretical density (TD) using MD simulations also captured the decrease in thermal conductivity with porosity and agreed well with the measured results for 95% TD dense sintered pellets.

Atomistic and experimental study on thermal conductivity of bulk and porous cerium dioxide

Cerium dioxide (CeO2) is a surrogate material for traditional nuclear fuels and an essential material for a wide variety of industrial applications both in its bulk and nanometer length scale. Despite this fact, the underlying physics of thermal conductivity (kL), a crucial design parameter in industrial applications, has not received enough attention. In this article, a systematic investigation of the phonon transport properties was performed using ab initio calculations unified with the Boltzmann transport equation. An extensive examination of the phonon mode contribution, available three-phonon scattering phase space, mode Grüneisen parameter and mean free path (MFP) distributions were also conducted. To further augment theoretical predictions of the kL, measurements were made on specimens prepared by spark plasma sintering using the laser flash technique. Since the sample porosity plays a vital role in the value of measured kL, the effect of porosity on kL by molecular dynamics (MD) simulations were investigated. Finally, we also determined the nanostructuring effect on the thermal properties of CeO2. Since CeO2 films find application in various industries, the dependence of thickness on the in-plane and cross-plane kL for an infinite CeO2 thin film was also reported.

Theoretical Investigation on the Microscopic Mechanism of Lattice Thermal Conductivity of ZnXP2 (X = Si, Ge, and Sn).

Thermal conductivity is an important physical parameter for the application of nonlinear optical single crystal materials. The underlying science of thermal transport behavior is not well established both experimentally and theoretically. In the present work, we have studied the microscopic picture of lattice thermal conductivity of ZnXP2 (X = Si, Ge, Sn), chalcopyrite ABC2 type infrared optical crystals, by using a harmonic and anharmonic lattice dynamic method and phonon Boltzmann transport equation based on first-principle calculations. With the mass of atom X increased, the phonon frequencies and phonon group velocities of ZnXP2 (X = Si, Ge, Sn) are shown not surprisingly to be decreased. Nevertheless, the phonon lifetime of ZnXP2 is unexpectedly increased, which is the governing mechanism for the increased thermal conductivity as 12.5 W/(m·k), 31.6 W/(m·k), and 35.4 W/(m·k), for ZnSiP2, ZnGeP2, and ZnSnP2, respectively, at 300 K. The contributions of optical phonons (with the frequency below 150 cm-1) to the total thermal conductivity are remarkable, reaching 18%, 31%, and 34% for three compounds, due to the significantly increased phonon lifetime in the frequency range 50-150 cm-1. To explore the physical insights of phonon lifetime and phonon anharmonicity, three-phonon scattering phase space and electronic localization function analysis of the X-P bond are provided. The results show that the covalent nature of X-P bonds is enhanced with the increased mass of atom X = Si, Ge, Sn, which induces the reduction of three-phonon scattering phase space in the frequency range 50-150 cm-1, leading to the enhancement of the phonon lifetime and thermal conductivity of ZnXP2.

First-principles study of thermoelectric properties of blue phosphorene

Thermoelectric behavior involving coupled electron and phonon transport properties of monolayer blue phosphorene is studied using the Boltzmann transport equation and first-principles calculations based on the density functional perturbation theory and Wannier interpolation. We find that the maximum figure of merit, ZT, of 0.016 is achievable in n-type blue phosphorene at 500 K with a positive gate voltage of 0.948 V. The relatively low ZT value is due to the large lattice thermal conductivity, which is around 130 W/mK at 300 K. This large lattice thermal conductivity is attributed to the small three-phonon scattering phase space due to the presence of a large phonon frequency gap in the dispersion. The relatively large power factor coupled with large thermal conductivity suggests that blue phosphorene can be ideal for active cooling applications. Nanostructuring with a characteristic length of 10 nm, which should not impact electron transport, is promising to reduce lattice thermal conductivity significantly and help increase ZT to the order of magnitude of 1. Our findings may offer perspectives on the potential applications of blue phosphorene in nanoelectronics and thermoelectrics.

Influence of defects and doping on phonon transport properties of monolayer MoSe2

The doping of monolayer MoSe2 by tungsten (W) can suppress the Se vacancy concentration, but how doping and resulting change in defect concentration can tune its thermal properties is not understood yet. We use first-principles density functional theory (DFT) along with the phonon Boltzmann transport equation (BTE) to study the phonon transport properties of pristine MoSe2 and W doped MoSe2 with and without the presence of Se vacancies. We found that for samples without Se vacancy, the W doping could enhance the thermal transport of monolayer MoSe2 due to reduced three-phonon scattering phase space. For example, we observed that the 16.7% W doping increases the thermal conductivity of the monolayer MoSe2 with 2% Se vacancy by 80% if all vacancies can be suppressed by W-doping. However, the W doping in the defective MoSe2 amplifies the influence of the phonon scattering caused by the Se vacancies, which results in a further decrease in thermal conductivity of monolayer MoSe2 with defects. This is found to be related with higher phonon density of states of Mo0.83W0.17Se2 and larger mass difference between W and Se atoms compared to Mo and Se atoms. This study deciphers the effect of defects and doping on the thermal conductivity of monolayer MoSe2, which helps us understand the mechanism of defect-induced phonon transport, and provides insights into enhancing the heat dissipation in MoSe2-based electronic devices.

The impact of hydrogenation on the thermal transport of silicene

Silicene, the silicon counterpart of graphene, has been identified as a promising 2D material for electronics applications. The reported very low thermal conductivity of silicene can potentially pose challenges on the thermal management of such nanoelectronics, which can in turn influence the device performance and reliability. Although the thermal conductivity of silicene has been studied, the impact of hydrogenation of silicene, which can happen spontaneously due to the resultant lower energy state, on its thermal transport ability is not clear. In this paper, we use first-principles calculations and iterative solution of phonon Boltzmann transport equation (BTE) to investigate and compare the thermal transport property of silicene and hydrogenated silicene. Surprisingly, we predict that the hydrogenation can lead to a large increase in thermal conductivity (from 22.5 W m−1 K−1 for silicene to 78.0 W m−1 K−1 for hydrogenated silicene at 300 K). We also find that the main contributor for such an improvement is the transverse acoustic phonon modes, and the reasons are the reduced anharmonicity as well as the reduced three-phonon scattering phase space after hydrogenation. This research may help better understand thermal transport in functionalized 2D materials and inspire new strategies to manipulate their thermal properties, which is of critical importance for designing high performance and reliable nanoelectronic devices.

Thermal Conductivity of Wurtzite Zinc-Oxide from First-Principles Lattice Dynamics--a Comparative Study with Gallium Nitride.

Wurtzite Zinc-Oxide (w-ZnO) is a wide bandgap semiconductor that holds promise in power electronics applications, where heat dissipation is of critical importance. However, large discrepancies exist in the literature on the thermal conductivity of w-ZnO. In this paper, we determine the thermal conductivity of w-ZnO using first-principles lattice dynamics and compare it to that of wurtzite Gallium-Nitride (w-GaN) – another important wide bandgap semiconductor with the same crystal structure and similar atomic masses as w-ZnO. However, the thermal conductivity values show large differences (400 W/mK of w-GaN vs. 50 W/mK of w-ZnO at room temperature). It is found that the much lower thermal conductivity of ZnO originates from the smaller phonon group velocities, larger three-phonon scattering phase space and larger anharmonicity. Compared to w-GaN, w-ZnO has a smaller frequency gap in phonon dispersion, which is responsible for the stronger anharmonic phonon scattering, and the weaker interatomic bonds in w-ZnO leads to smaller phonon group velocities. The thermal conductivity of w-ZnO also shows strong size effect with nano-sized grains or structures. The results from this work help identify the cause of large discrepancies in w-ZnO thermal conductivity and will provide in-depth understanding of phonon dynamics for the design of w-ZnO-based electronics.

Anisotropic lattice thermal conductivity in chiral tellurium from first principles

Using ab initio based calculations, we have calculated the intrinsic lattice thermal conductivity of chiral tellurium. We show that the interplay between the strong covalent intrachain and weak van der Waals interchain interactions gives rise to the phonon band gap between the lower and higher optical phonon branches. The underlying mechanism of the large anisotropy of the thermal conductivity is the anisotropy of the phonon group velocities and of the anharmonic interatomic force constants (IFCs), where large interchain anharmonic IFCs are associated with the lone electron pairs. We predict that tellurium has a large three-phonon scattering phase space that results in low thermal conductivity. The thermal conductivity anisotropy decreases under applied hydrostatic pressure.