Phonon Group Velocity Research Articles

Quasi-one-dimensional thermal transport in trigonal selenium crystal

The lattice thermal conductivity in van der Waals crystal selenium is investigated by solving the phonon Boltzmann transport equation combined with the first-principles calculations. The lattice thermal conductivity of selenium along the perpendicular to the chain direction is extremely low, which is 5 times lower than the conductivity in the parallel direction, showing a clear quasi-one-dimensional heat transfer feature. The small phonon group velocity, acoustic-optical phonon branches mixing and bonding anharmonicity along the perpendicular to the chain direction contribute to the high anisotropy in thermal conductivity. Moreover, particles with several nanometers can introduce boundary scattering to a large portion of phonons modes in selenium, where the lattice thermal conductivity is greatly reduced. Both the unique one-dimensional thermal transport property and low thermal conductivity make selenium to be a potential thermal management in thermoelectric and other electronic technologies.

Unraveling the Impact of Graphene Addition to Thermoelectric SrTiO3 and La-Doped SrTiO3 Materials: A Density Functional Theory Study.

We present a detailed theoretical investigation of the interaction of graphene with the SrO-terminated (001) surface of pristine and La-doped SrTiO3. The adsorption of graphene is thermodynamically favorable with interfacial adsorption energies of −0.08 and −0.32 J/m2 to pristine SrTiO3 and La-doped SrTiO3 surfaces, respectively. We find that graphene introduces C 2p states at the Fermi level, rendering the composite semimetallic, and thus the electrical properties are predicted to be highly sensitive to the amount and quality of the graphene. An investigation of the lattice dynamics predicts that graphene adsorption may lead to a 60–90% reduction in the thermal conductivity due to a reduction in the phonon group velocities, accounting for the reduced thermal conductivity of the composite materials observed experimentally. This effect is enhanced by La doping. We also find evidence that both La dopant ions and adsorbed graphene introduce low-frequency modes that may scatter heat-carrying acoustic phonons, and that, if present, these effects likely arise from stronger phonon–phonon interactions.

Phonon propagation in isotopic diamond superlattices

The out-of-plane thermal conductivity and elastic constant of epitaxial [100] $^{12}\mathrm{C}/^{13}\mathrm{C}$ superlattice diamonds with layer thicknesses of 1, 30, and 100 nm are evaluated by picosecond ultrasound spectroscopy. The measured elastic constants of the superlattices are equivalent to those of single-layer diamond thin films. This result confirms our success in synthesizing superlattice specimens with few defects at the interfaces. Therefore, the phonon transport behavior is governed by the mass difference, not the interfacial defects. The measured thermal conductivity of the superlattices is lower than that of a pure $^{12}\mathrm{C}$ isotope diamond thin film. We estimated the lattice thermal conductivity using the lattice dynamics calculation, attributing the lowered thermal conductivity to the decrease in the phonon group velocity in superlattices. We further consider the effect of mini-umklapp scattering in the superlattice, which explains the dependence of the thermal conductivity on the layer thickness. We reveal that the mini-umklapp scattering effect becomes significant only for an isotope diamond superlattice because of the high Debye temperature and large relative mass difference.

Ultrahigh thermal conductivity in hexagonal BC6N- An efficient material for nanoscale thermal management- A first principles study

Engineering materials with high thermal conductivity are of fundamental interest for efficiently dissipating heat in micro/nanoelectronics. Using first principles computations we report an ultra-high thermal conductivity of 2090 Wm−1K−1 (1395 Wm−1K−1) for hexagonal pure (natural) BC6N(h-BC6N). This value is among the highest thermal conductivities known after diamond and cubic boron arsenide. This ultra-high lattice thermal conductivity (k) is mainly attributed with high phonon group velocities of both acoustic and optical phonons arising from strong CC and BN bonds as well as the light atomic mass of the constituent elements such as boron (B), carbon (C) and nitrogen (N). We also report size dependent thermal conductivity of h-BC6N nanostructures by including boundary scattering. At room temperature (300 K) and at nanoscale length (L) of 100 nm, a high k value of 175 Wm−1K−1 is observed (higher than the bulk k value of silicon). Optical phonons with large group velocities are mainly responsible for this high thermal conductivity in h-BC6N nanostructures. High thermal conductivity of h-BC6N makes it a candidate material for heat dissipation in micro/nano thermal management applications.

Extraordinary thermoelectric performance in 2D group III monolayer XP3 (X = Al, Ga, and In)

Recently, two-dimensional (2D) group III monolayer XP3 (X = Al, Ga, and In) have attracted great attention due to their excellent physical properties. We systematically investigate the thermoelectric (TE) properties of monolayer XP3 by combining first-principles calculations and Boltzmann transport theory. It can be confirmed that monolayer XP3 are dynamically stable structures and semiconductors with indirect band gaps. The carrier mobilities of monolayer XP3 are calculated by using the deformation potential theory. Besides, at 300 K, the power factors of n-type doping AlP3, GaP3, and InP3 are 35.51, 12.47, and 18.41 mW m−1 K−2, respectively. Moreover, the phononic transport properties of all monolayers are evaluated, and lattice thermal conductivities of AlP3, GaP3, and InP3 are 0.60, 4.47, and 1.93 W m−1 K−1 at 300 K, respectively, which can be explained by small phonon group velocity and ultra-low phonon lifetime. Finally, the outstanding TE figure of merits (ZT) of AlP3, GaP3, and InP3 with n-type doping are 8.33, 2.09, and 4.22 at 700 K, respectively, which establishes a new record of TE performance of 2D materials. Our work clearly shows the prominent properties of novel 2D material XP3 and will further promote technological development in the TE field.

Thickness dependent anisotropy of in-plane Raman modes under different temperatures in supported few-layer WTe2

Tungsten ditelluride (WTe2) has attracted extensive interest because of promising magnetic, electrical, and thermal properties, such as non-saturable large magnetoresistance, pressure-induced superconductivity, and low thermal conductivity, owing to the heavy atomic mass and low Debye temperature. Here, we report the thickness dependence of anisotropic in-plane Raman modes under different temperatures in supported few-layer WTe2. With angle-resolved Raman spectroscopy, supported few-layer WTe2 reveals a temperature-related anisotropy in Raman modes, and the highest degree of anisotropy is obtained between zigzag and armchair directions, which could be resulted from the different phonon group velocities along these directions. In addition, as the thickness of the WTe2 nanosheet decreases from bulk to 10 nm, the anisotropic ratio (χzigzag/χarmchair) increases from 1.09 to 1.45. Such enhancement in the anisotropic ratio could result from the thickness-dependent surface scattering in supported few-layer WTe2. Our study provides guidance to regulate the anisotropy ratio in thermoelectrical and optoelectronic devices, which might inspire the innovation of functional two-dimensional devices in the future.

Computational study of mechanical stability and phonon properties of MXenes Mo2ScC2T2 (T = O and F): 2D materials

The structural, electronic, elastic, and phonon properties of pristine Mo2ScC2 and surface terminated Mo2ScC2T2 (T = O and F) were investigated by employing density functional theory calculations. Generalized gradient approximation was used to model exchange–correlation effects. The electronic band structure was calculated using both Perdew–Burke–Ernzerhof and Heyd–Scuseria–Ernzerhof functional and found that all considered materials are metallic. Elastic constants, Young's moduli, shear moduli, in-plane stiffnesses, and Poisson's ratios were tabulated and showed that pristine Mo2ScC2 and surface terminated Mo2ScC2T2 satisfy the mechanical stability criteria. Furthermore, Mo2ScC2O2 has been found to be a significant candidate for ultrasensitive sensors due to its ultrahigh Young's modulus. The absence of the imaginary line in phonon band structures confirms the dynamic stability of Mo2ScC2, Mo2ScC2O2, and Mo2ScC2F2. Furthermore, Debye temperatures, phonon group velocities, thermodynamic properties, and Grüneisen parameters were calculated for Mo2ScC2 and Mo2ScC2O2. Finally, it is understood that Mo2ScC2 and Mo2ScC2O2 are suitable candidates for applications that require low thermal conductivity.

Phonon transport properties of two dimensional group-III nitrides (BN, AlN, and GaN)

In this work, the lattice thermal conductivity of two-dimensional honeycomb structures of BN, AlN, and GaN studied using first-principles density functional theory and solving the linearized Boltzmann transport equation by relaxation time approximation. Investigation of the influence of the phonon modes on the thermal conductivity revealed that ZA, and LA + TA + ZO modes have the most and TO + LO have the least contribution to the thermal conductivity of BN whereas in AlN, the contribution of ZA decreases while the effect of TO + LO increases. In GaN, the TO + LO branches Play the major role on the thermal conductivity. The observed differences in the lattice thermal conductivity of these monolayers were explained by considering the variations of their frequency-dependent phonon lifetimes, group velocities, and heat capacities. In BN monolayer, the group velocity and phonon lifetime of the acoustic phonons found to be higher whereas the heat capacity of it's LO and TO branches was lower than the monolayers of AlN and GaN and therefore, the contribution of the acoustical phonon branches to thermal conductivity has become more important. However in GaN, the LO and TO phonons revealed the highest phonon lifetime and specific heat resulting the dominance of the contribution of these phonon modes to the total thermal conductivity.

Effects of van der Waals interactions on the phonon transport properties of tetradymite compounds

Unlike tremendous works on the electronic structures of tetradymite compounds, studies of their thermal properties are relatively rare. Here, first-principles calculations and Boltzmann theory are combined to investigate the phonon transport of such kind of layered materials. Using four binary tetradymites as prototypical examples, it is interesting to find that the weak van der Waals (vdW) interactions play an important role in determining their lattice thermal conductivities, which are obviously higher than those without the consideration of vdW, especially for the out-of-plane direction. In principle, such enhanced phonon transport can be attributed to the decreased interlayer spacing caused by the presence of vdW, which effectively reduces the strong anharmonicity of the systems. Indeed, we observe relatively smaller Grüneisen parameter together with larger phonon group velocity and relaxation time. Our theoretical work demonstrates the vital importance of the seemingly weak vdW forces in predicting the phonon transport properties of various layered structures.

Ultra-low lattice thermal conductivity and high thermoelectric efficiency of K3AuO

Based on the combination of first-principles calculations and Boltzmann transport equation, we investigated the thermal transport properties of K3AuO and predicted a figure of merit ZT=2.01 at 700 K with p-type doping. Such a high thermoelectric efficiency can be attributed to the ultra-low lattice thermal conductivity with a value of 0.48 Wm−1 K−1 at 300 K, and detailed research shows that the low lattice thermal conductivity arises from the small phonon group velocity and high scattering rates; moreover, the figure of merit ZT of p-type doped K3AuO can be maintained at around 2 in a relatively wide carrier concentration of 5.5×1020–1.2×1021 cm−3, which demonstrates the stable thermoelectric properties of K3AuO.

Thermal conductivity of magnesium telluride (MgTe) - A first principles study

In this work, we report thermal conductivity(k) of magnesium telluride (MgTe) with various crystallographic phases such as rocksalt, zincblende, wurtzite and nickel arsenic (NiAs) using density functional theory and Boltzmann transport equation. Our first principles calculations result shows the low thermal conductivity of MgTe with kNiAs < krocksalt < kwurtzite < kzincblende. We systematically investigated the phonon group velocity, phonon scattering rate and mode contributed thermal conductivity of transverse acoustic (TA), longitudinal acoustic (LA) and optical phonons. Our first principles calculations shows that ultra-low thermal conductivity of 2.645 Wm−1K−1 for NiAs phase is due to the dominant scattering of TA and LA phonons by low frequency optical phonons. We also analyzed the length dependence thermal conductivity of MgTe at nanometer length-scales. At nanometer length scales such as 50 nm for NiAs phase, room temperature thermal conductivity of less than 1.4 Wm−1K−1 shows a promising nature of MgTe for thermoelectric applications.

Heat transport through propagon-phonon interaction in epitaxial amorphous-crystalline multilayers

Managing heat dissipation is a necessity for nanoscale electronic devices with high-density interfaces, but despite considerable effort, it has been difficult to establish the phonon transport physics at the interface due to a “complex” interface layer. In contrast, the amorphous/epitaxial interface is expected to have almost no “complex” interface layer due to the lack of lattice mismatch strain and less associated defects. Here, we experimentally observe the extremely-small interface thermal resistance per unit area at the interface of the amorphous-germanium sulfide/epitaxial-lead telluride superlattice (~0.8 ± 4.0 × 10‒9 m2KW−1). Ab initio lattice dynamics calculations demonstrate that high phonon transmission through this interface can be predicted, like electron transport physics, from large vibron-phonon density-of-states overlapping and phonon group velocity similarity between propagon in amorphous layer and “conventional” phonon in crystal. This indicates that controlling phonon (or vibron) density-of-states and phonon group velocity similarity can be a comprehensive guideline to manage heat conduction in nanoscale systems.

The lattice thermal conductivity in monolayers group-VA: from elements to binary compounds

The lattice thermal conductivity () of the monolayers of partial group-VA elements and binary compounds are systemically investigated by the first-principles calculations and phonon Boltzmann transport equation (PBTE), including aW-antimonene, α-arsenene, black phosphorus, α-SbAs, α-SbP and α-AsP. The values decrease with the increasing of atomic mass for these materials with similar geometry and valence structures. It is ascribed to phonon branches softening, low phonon group velocity, and large Grüneisen parameters. Due to the neutralization of phonon group velocity and phonon lifetime, of binary compounds is between their corresponding elements. As the atomic radius and mass increase, the bond strength and the phonon group velocity decreases. Furthermore, the dimensionless parameter , which comes from the Slack equation and only has the dependence of Grüneisen parameter, grows up with the atomic mass rising, which indicates that a larger anharmonicity is present in the heavier V-V monolayers. For SbAs and SbP compounds, the thermal conductivity anisotropy mainly results from the anisotropy of elastic coefficients along armchair and zigzag directions. Our results highlight the impact of atomic arrangement on the thermal conductivity of group VA binary compounds. This work paves a way to modulate the thermal conductivity of 2D VA elements by incorporation atoms with suitable mass and may guide to improve thermoelectrical performance via the alloying method.

A Colossal Enhancement of Thermoelectric Performance of Monolayer SbAs Using Strain Engineering

The effect of uniaxial strain on the thermoelectric (TE) conversion efficiency of puckered monolayer arsenic antimonide (SbAs) is explored based on the density function and Boltzmann transport equation (BTE) theories. The lattice thermal conductivity (κl) can be sharply lowered through tensile strain. Taking a + 3.25% tensile strain as an example, κl along the armchair (AC) direction at 300 K decreases almost three times than that without a strain, which is ascribed to the low phonon group velocity, large Grüneisen parameter, and high three‐phonon scattering phase space. The carrier effective mass decreases in imposing tensile strain monolayer SbAs and slightly suppresses the Seebeck coefficient; however, the electrical conductivity is improved due to the small effective mass and long relaxation time. Thus, power factor increases by 3.4 times under a + 3.25% tensile at cm−2 compared with a SbAs monolayer without strain. Thereafter, the figure of merit in an n‐type SbAs monolayer with a tensile strain of +3.25% is eight times as large as that in SbAs without strain at 300 K along the AC direction. Furthermore, the maximum TE figure of merit is 1.85 for an n‐type SbAs monolayer with +3.25% strain at 800 K. This study paves a way toward improving the TE performance through strain engineering in the TE field.

Tunable phononic thermal transport in two-dimensional C6CaC6 via guest atom intercalation

The graphite intercalation compounds have attracted wide interest due to the superconductivity. In this work, the thermal transport in bilayer graphene intercalated with Ca atoms (C6CaC6) at room temperature is studied by using non-equilibrium molecular dynamics simulations. Our simulation results show that the in-plane lattice thermal conductivity (κL) of C6CaC6 is significantly lower than that of the bilayer graphene. The detailed phonon mode analysis reveals that the reduction of κL is because of the mode hybridization and flatbands induced by the intercalated Ca atoms, leading to the decrease in phonon group velocity and the enhancement of phonon scattering. Unlike the role of van der Waals interactions in multilayer graphene and supported graphene, increasing coupling strength between intercalated Ca atoms and graphene brings an enhanced κL in C6CaC6. The spectral phonon analysis uncovers that such anomalous phenomenon is caused by the redistribution of phonon scattering phase space originated from the shift of the flatbands. This study indicates that atom intercalation is an effective way to regulate the heat transport in two-dimensional materials.

Physical Insights on the Lattice Softening Driven Mid‐Temperature Range Thermoelectrics of Ti/Zr‐Inserted SnTe—An Outlook Beyond the Horizons of Conventional Phonon Scattering and Excavation of Heikes’ Equation for Estimating Carrier Properties

AbstractMost of the best known SnTe‐based materials exhibit an attractive thermoelectric figure of merit (zT) only at the high‐temperature regime, but their performance at the low‐mid temperature ranges is quite uninspiring, and this discordance necessitates a large temperature gradient (∆T ≥ 550 K) to effectuate a reasonable efficiency, η. Here, the transition elements, Ti and Zr, that have not been used in the past are tried as dopants for SnTe and an enhanced device/average zT and/or η are reported with a lower ∆T ≈ 400 K and without the requisite for a stupendous peak/maximum zT. This notable performance emanates from—i) improved weighted mobility by optimally balancing between effective mass, carrier concentration, and mobility, ii) coupling of charge carriers with magnetic entropy, and the paramount factor being the iii) weakening of the chemical bonds (lattice softening). The thermal damping caused by lattice softening affects the phonon group velocity and the elastic properties, and the resultant increase in the degree of anharmonicity and the high density of internal strain‐fields, along with the phonon scattering effects, play an active role in tuning the overall thermoelectric performance. This work also excavates/opens up the discussion of applying the Heikes’ equation to qualitatively compare the trend of charge carriers for a given thermoelectric material system.

First principles study of the stability and thermal conductivity of novel Li-Be hybrid ceramics

Hybrid Li-Be ceramics, combining both tritium (T) breeder (Li) and neutron multiplier (Be) for use as part of the fuel cycle of future nuclear fusion reactors are proposed. The development of such hybrid materials may reduce thermal gradients through better matching of thermal properties, mitigating the detrimental effects that may accompany traditional breeder systems while maintaining acceptable neutron multiplication and T breeding. First-principles methods are used to investigate stability and thermal transport properties of a set of compounds containing both Li and Be. It is demonstrated that BeLi2O4Ge and BeLi2O4Si are mechanically stable and have formation energies comparable with leading candidates for solid state breeder materials, Li2TiO3, Li2ZrO3. It is also demonstrated that similar to the leading candidates, these compounds are insulators with thermal transport defined by phonons. The calculated thermal conductivity of BeLi2O4Ge is slightly higher compared to Li2TiO3 or Li2ZrO3 while in the BeLi2O4Si it is almost three times higher compared to the rest of compounds due to a higher phonon group velocities and increased phonon lifetimes. These results indicate that hybrid Li-Be ceramics offer a potential route towards better matching of thermal properties with minimal functional property degradation, thereby offering better overall fuel cycle performance.

Tunable anisotropic thermal transport in porous carbon foams: The role of phonon coupling

Carbon foams (CFs) possess high storage capacity, good electronic conductivity and superb mechanical strength, which demonstrate promising applications in many engineering fields. Understanding thermal transport in CFs is critical for the design and reliability of functional electronic devices based on them. In this work, we systematically study anisotropic thermal transport in the CFs composed of sixfold-wing graphene nanoribbons by using equilibrium molecular dynamics simulations. The results showed that the remarkable anisotropic behavior reflecting geometric anisotropy can be attributed to the orientation-dependent group velocity of long wavelength phonons. Moreover, it is found that the anisotropic ratio could be effectively regulated by compress/tensile strains. Detailed spectral analysis revealed that the loading of strain would significantly modify the coupling level between the transverse and longitudinal vibrational modes, resulting in a change to the anisotropic ratio. For thermal management application, the interfacial thermal conductance (TBC) of CFs/silicon substrate is predicted to be about 35 MW/m2 K−1, which is comparable to the TBC of the transferred metal films on silicon or SiO2 substrates. Furthermore, the TBC could be further enhanced by increasing ambient temperature or external stress. Our results might provide guidance for the development of thermal interfacial materials and thermal channeling devices.

Low lattice thermal conductivity of pentadiamond

The lattice thermal conductivity of carbon materials is particularly interesting because it can vary within a range spanning five orders of magnitude depending on the atomic configuration. Herein, we systematically study the lattice thermal conductivity and phonon transport properties of pentadiamond, a new three-dimensional carbon allotrope consisting of pentagonal carbon rings. Based on first-principles calculations and an iterative solution to the linearized Boltzmann transport equation, the intrinsic lattice thermal conductivity (kl) is found to be 490.88 W/mK at room temperature, much lower than 2664.93 W/mK of diamond. A detailed analysis of both harmonic and anharmonic properties reveals that the low kl of pentadiamond essentially originates from its large phonon phase space, short phonon relaxation time resulting from strong overlap between the acoustic and low-lying optical phonon branches, and the low phonon group velocity. The distinct thermal transport behavior exhibited in pentadiamond further shows the diversity and complexity in lattice thermal conductivity of carbon allotropes.

Uniform Strain-Dependent Thermal Conductivity of Pentagonal and Hexagonal Silicene

Two-dimensional (2D) pentagonal monolayer structures have shown promising characteristics and fascinating physical and chemical properties. The disparate strain-dependent thermal conductivity of two-dimensional penta-structures was reported, but the difference between the silicon-based pentagonal and hexagonal structures is barely researched. In this work, based on first-principles calculations, we studied the strain-modulated phonon transport behavior of two 2D pentagonal (penta-SiH and bilayer penta-Si) and one hexagonal silicene structures (H-silicene), of which the penta-SiH and H-silicene mean the structures are hydrogenated for the purpose of thermodynamical stability. We found that the silicon-based pentagonal structure also presented a different strain-dependent thermal conductivity from other pentagonal materials, such as penta-graphene, penta-SiC, or penta-SiN. Moreover, even with the similar strain-dependent thermal transport behavior in penta-SiH and bilayer penta-silicene, we find that the governing mechanism is still different. For both pentagonal silicene structures, the thermal conductivity presents a large improvement at first as the tensile strain increases from 0 to 10% and then stabilizes with a strain larger than 10%. A detailed analysis shows that the in-plane modes contributed the most part to the group velocity enhancement under strains in penta-SiH which is opposite from the bilayer penta-graphene, although the phonon group velocity and phonon lifetime of both structures increase with applied strain. On the other hand, a similarity was found in pentagonal silicene and hexagonal silicene despite the differences in geometry structures. Furthermore, based on the detailed analysis between the pentagonal (penta-SiH) and hexagonal silicene structures (H-silicene), the difference in out-of-plane phonon scattering cannot be ignored: different major scattering channels of the out-of-plane flexural modes result in different thermal conductivity sensitivity to strains, and the disparity in anharmonicity leads to different thermal conductivity under no strain.