Low Phonon Group Velocities Research Articles

Thermal transport properties in monolayer GeS

Utilizing the first-principle techniques combined with the Boltzmann transport equation, we investigate the thermal transport properties of monolayer GeS. The calculated lattice thermal conductivity κL is fairly low, e.g., 11.76 (12.53) Wm−1K−1 at 300 K, along the x (y) axis, which is much lower than the values for phosphorene, monolayer MoS2, graphene, etc. Our analysis reveals that the low lattice thermal conductivity is resulted from the strong three-phonon scattering process and the low phonon group velocity. Due to the weak anisotropy of GeS, we study the contribution of each phonon branch to the heat transport along x and y axis, and find that the longitudinal acoustic (LA) mode contributes most of the total κL along x axis, while the transverse acoustic (TA) branch has the largest contribution to the total κL along y axis. Additionally, the size dependence of thermal conductivity with respect to the nanoribbon width is also studied for the design of nanometer devices.

Thermoelectric Properties of Hexagonal M₂C₃ (M = As, Sb, and Bi) Monolayers from First-Principles Calculations.

Hexagonal M2C3 compound is a new predicted functional material with desirable band gaps, a large optical absorption coefficient, and ultrahigh carrier mobility, implying its potential applications in photoelectricity and thermoelectric (TE) devices. Based on density-functional theory and Boltzmann transport equation, we systematically research the TE properties of M2C3. Results indicate that the Bi2C3 possesses low phonon group velocity (~2.07 km/s), low optical modes (~2.12 THz), large Grüneisen parameters (~4.46), and short phonon relaxation time. Based on these intrinsic properties, heat transport ability will be immensely restrained and therefore lead to a low thermal conductivity (~4.31 W/mK) for the Bi2C3 at 300 K. A twofold degeneracy is observed at conduction bands along Γ-M direction, which gives a high n-type electrical conductivity. Its low thermal conductivity and high Seebeck coefficient lead to an excellent TE response. The maximum thermoelectric figure of merit (ZT) of n-type can approach 1.41 for Bi2C3. This work shows a perspective for applications of TE and stimulate further experimental synthesis.

Electronic, phonon transport and thermoelectric properties of Cs2InAgCl6 from first-principles study

We have studied electronic, phonon transport and thermoelectric properties of Cs2InAgCl6 using first-principles calculations. The PBE functional severely underestimates the bandgap and calculated value for fully relaxed Cs2InAgCl6 is 1.01 eV. Our calculated direct bandgap using TB-mBJ potential of partially relaxed Cs2InAgCl6 is 3.3 eV which agree with the measured value 3.3 eV whereas it is 2.53 eV for fully relaxed Cs2InAgCl6 and underestimates the experimental value by 23.3%. The In-5s and Ag-4d mainly formulate the direct bandgap. We have found that the lattice thermal conductivity (κl) is quite low (0.2 Wm−1K−1) in Cs2InAgCl6. The quite low phonon group velocity in the large weighted phase space and high anharmonicity (large phonon scattering) are responsible for low κl value. This halide possesses high Seebeck coefficient (∼900 μV/K at 700 K) because of its large bandgap. The maximum ZT value obtained at 700 K is ∼0.94. Therefore, we predict that Cs2InAgCl6 is a promising material for thermoelectric device applications.

First-principles study of the layered thermoelectric material TiNBr.

Layer-structured materials are often considered to be good candidates for thermoelectric materials, because they tend to exhibit intrinsically low thermal conductivity as a result of atomic interlayer interactions. The electrical properties of layer-structured materials can be easily tuned using various methods, such as band modification and intercalation. We report TiNBr, as a member of the layer-structured metal nitride halide system MNX (M = Ti, Zr, Hf; X = Cl, Br, I), and it exhibits an ultrahigh Seebeck coefficient of 2215 μV K−1 at 300 K. The value of the dimensionless figure of merit, ZT, along the A axis can be as high as 0.661 at 800 K, corresponding to a lattice thermal conductivity as low as 1.34 W (m K)−1. The low κl of TiNBr is associated with a collectively low phonon group velocity (2.05 × 103 m s−1 on average) and large phonon anharmonicity that can be quantified using the Grüneisen parameter and three-phonon processes. Animation of the atomic motion in highly anharmonic modes mainly involves the motion of N atoms, and the charge density difference reveals that the N atoms become polarized with the merging of anharmonicity. Moreover, the fitting procedure of the energy–displacement curve verifies that in addition to the three-phonon processes, the fourth-order anharmonic effect is also important in the integral anharmonicity of TiNBr. Our work is the first study of the thermoelectric properties of TiNBr and may help establish a connection between the low lattice thermal conductivity and the behavior of phonon vibrational modes.

High Thermoelectric Figure of Merit via Tunable Valley Convergence Coupled Low Thermal Conductivity in AIIBIVC2V Chalcopyrites

Development of efficient thermoelectric materials requires a designing approach that leads to excellent electronic and phononic transport properties. Using first-principles density functional theory and semiclassical Boltzmann transport theory, we report unprecedented enhancement in electronic transport properties of AIIBIVC2V (group II = Be, Mg, Zn, and Cd; group IV = Si, Ge, and Sn; and group V = P and As) chalcopyrites via isoelectronic substitution. Multiple valleys in conduction bands, present in these compounds, are tuned to converge by substitution of group IV dopant. Additionally, this substitution improves the convergence of valence bands, which is found to have a direct correlation with the tetragonal distortion of these chalcopyrites. Furthermore, several chalcopyrite compounds with heavy elements such as Zn, Cd, and As possess low phonon group velocities and large Gruneisen parameters that lead to low lattice thermal conductivity. Combination of optimized electronic transport properties and lo...

Radial dependence of thermal transport in silicon nanowires

A radial decomposition of the heat flux in a silicon crystalline nanowire is studied with molecular dynamics (MD) and Monte Carlo (MC) simulations. Less heat flux is carried in the external layer of nanowires than in the center. The difference between the center and the surface is of the order of 50% and 30% with MD and MC simulations, respectively. As a result, a heat flux close to the surface is 30% and 15% lower than the total axial heat flux in the structure. The physical mechanism behind is analyzed from partial contribution of each atom in each phonon mode calculated from lattice dynamics. The reduction of the flux close to the surface is related to back-scattering, the amorphous-like DOS of the external layer and flattened dispersion curves, thus lower phonon group velocities. Our study points to the need for cautions analysis of experimental determination of the thermal conductivity involving contact measurements, such as scanning thermal microscopy.

Record-Low and Anisotropic Thermal Conductivity of a Quasi-One-Dimensional Bulk ZrTe5 Single Crystal.

Zirconium pentatelluride (ZrTe5) has recently attracted renewed interest owing to many of its newly discovered extraordinary physical properties, such as 2D and 3D topological-insulator behavior, pressure-induced superconductivity, Weyl semimetal behavior, Zeeman splitting, and resistivity anomaly. The quasi-one-dimensional structure of single-crystal ZrTe5 also promises large anisotropy in its thermal properties, which have not yet been studied. In this work, via time-domain thermoreflectance measurements, ZrTe5 single crystals are discovered to possess a record-low thermal conductivity along the b-axis (through-plane), as small as 0.33 ± 0.03 W m-1 K-1 at room temperature. This ultralow b-axis thermal conductivity is 12 times smaller than its a-axis thermal conductivity (4 ± 1 W m-1 K-1) owing to the material's asymmetrical crystalline structure. First-principles calculations are further conducted to reveal the physical origins of the ultralow b-axis thermal conductivity, which can be attributed to: (1) the resonant bonding and strong lattice anharmonicity induced by electron lone pairs, (2) the weak interlayer van der Waals interactions, and (3) the heavy mass of Te atoms, which results in low phonon group velocity. This work sheds light on the design and engineering of high-efficiency thermal insulators for applications such as thermal barrier coatings, thermoelectrics, thermal energy storage, and thermal management.

Theoretical exploration of the abnormal trend in lattice thermal conductivity for monosilicates RE2SiO5 (RE = Dy, Ho, Er, Tm, Yb and Lu)

Rare earth monosilicates RE2SiO5 have been considered as promising environmental barrier coating materials for silicon-based ceramics due to their low thermal conductivity and good high-temperature stability. We herein performed a systematic study of the lattice dynamics for RE2SiO5 (RE = Dy, Ho, Er, Tm, Yb and Lu) using first-principles calculations. The loosely bound rare earth atoms provide large Grüneisen parameters and low phonon group velocities, both of which determine the low thermal conductivity. Theoretical exploration predicts an anomalous increase of lattice thermal conductivity with increment of RE atomic number and the mechanism is explained by the stronger atomic bonding and weaker phonon anharmonicity. Although incorporating heavier atoms has long been considered as an effective way to reduce lattice thermal conductivity, this work addresses the importance of bonding heterogeneity and anharmonicity rather than atomic mass variation. This theoretical study suggests an alternative approach towards the design of new thermal insulating materials.

Origin of ultra-low lattice thermal conductivity in Cs2BiAgX6 (X=Cl, Br) and its impact on thermoelectric performance

We have explored electronic and thermoelectric properties of bismuth-based double-perovskite halides Cs2BiAgX6 (X = Cl, Br) by using first principles calculations. The calculated indirect bandgaps 2.85 eV and 1.99 eV for Cs2BiAgCl6 and Cs2BiAgBr6, respectively well agree with the measured values (2.77 eV of Cs2BiAgCl6 and 2.19 eV of Cs2BiAgBr6). We have calculated the relaxation time and lattice thermal conductivity by using relaxation time approximation (RTA) within the supercell approach. The lattice thermal conductivities for both compounds are remarkably low and the obtained values at 300 K for Cs2BiAgCl6 and Cs2BiAgBr6 are 0.078 and 0.065 Wm-1K-1, respectively. Such quite low lattice thermal conductivity arises due to low phonon group velocity in the large weighted phase space and large phonon scattering. The large Seebeck coefficient obtained for both halides at 400 K. We have obtained the maximum power factors at 700 K and the corresponding thermoelectric figure of merit for Cs2BiAgCl6 and Cs2BiAgBr6 are 0.775 and 0.774, respectively. The calculated results reveal that both halides are potential thermoelectric materials.

Phonon thermal transport in monolayer FeB2 from first principles

We investigate the phonon thermal transport in two-dimensional (2D) metal-boride material (FeB2) by combining the phonon Boltzmann transport equation with first principle calculations. Strikingly, a fairly low lattice thermal conductivity, such as 51.9 W/mK at room temperature, is discovered in this new 2D monolayer boron-based Dirac material. Our analyses indicate that the low lattice thermal conductivity is resulted from the strong three-phonon scattering and the low phonon group velocity. Due to relatively weak three-phonon scattering, the out-of-plane acoustic phonon modes play a significant role in the thermal transport of this monolayer FeB2. In addition, we also study the size dependence of thermal conductivity for applications in design of nanometer devices.

Interplay of Structural and Bonding Characters in Thermal Conductivity and Born-Effective Charge of Transition Metal Dichalcogenides

Thermal transport in a material is governed by anharmonicity of crystal potential, which depends on the type of interatomic interaction. Using first-principles calculations, we report that lattice thermal conductivity (κlatt) and its anisotropy (κx,y – κz) of transition metal dichalcogenides (TMDs) increase by orders of magnitude with the change of constituent metal atom from Zr/Hf to Mo/W. This unprecedented difference in κlatt is substantiated by lower phonon group velocity, and 4 times larger anharmonicity of Zr/Hf based TMDs compared to Mo/W based TMDs. The sign and the absolute value of the Born effective charges, which emerges from the ionicity of the bonds, are found to be different for these two classes of materials. This leads to a significant difference in their interlayer van der Waals (vdW) interaction strengths, which are shown to be inversely related to the anisotropy in κlatt.

Unusual phonon behavior and ultra-low thermal conductance of monolayer InSe.

Monolayer indium selenide (InSe) possesses numerous fascinating properties, such as high electron mobility, quantum Hall effect and anomalous optical response. However, its phonon properties, thermal transport properties and the origin of its structural stability remain unexplored. Using first-principles calculations, we show that the atoms in InSe are highly polarized and such polarization causes strong long-range dipole-dipole interaction (DDI). For acoustic modes, DDI is essential for maintaining its structural stability. For optical modes, DDI causes a significant frequency shift of its out-of-phase vibrations. Surprisingly, we observed that there were two isolated frequency regimes, which were completely separated from other frequency regimes with large frequency gaps. Within each frequency regime, only a single phonon mode exists. We further reveal that InSe possesses the lowest thermal conductance among the known two-dimensional materials due to the low cut-off frequency, low phonon group velocities and the presence of large frequency gaps. These unique behaviors of monolayer InSe can enable the fabrication of novel devices, such as thermoelectric module, single-mode phonon channel and phononic laser.

Low lattice thermal conductivity and good thermoelectric performance of cinnabar

Based on the combination of first-principles calculations, Boltzmann transport equation, and electron-phonon interaction (EPI), we investigate the thermal and electronic transport properties of crystalline cinnabar $(\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{HgS})$. The calculated lattice thermal conductivity ${\ensuremath{\kappa}}_{L}$ is remarkably low, e.g., $0.60\phantom{\rule{4pt}{0ex}}{\mathrm{Wm}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ at $300\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, which is about $30%$ of the value for the typical thermoelectric material PbTe. Via taking fully into account the k dependence of the electron relaxation time computed from the EPI matrix, the accurate numerical results of thermopower $S$, electrical conductivity $\ensuremath{\sigma}$, and electronic thermal conductivity ${\ensuremath{\kappa}}_{E}$ are obtained. The calculated power factor ${S}^{2}\ensuremath{\sigma}$ is relatively high while the value of ${\ensuremath{\kappa}}_{E}$ is negligible, which, together with the fairly low ${\ensuremath{\kappa}}_{L}$, leads to a good thermoelectric performance in the $n$-type doped $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{HgS}$, with the figure of merit $zT$ even exceeding 1.4. Our analyses reveal that (i) the large weighted phase space and the quite low phonon group velocity result in the low ${\ensuremath{\kappa}}_{L}$, (ii) the presence of flat band around the Fermi level combined with the large band gap causes the high $S$, and (iii) the small electron linewidths of the conduction band lead to a large relaxation time and thus a relatively high $\ensuremath{\sigma}$. These results support that $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{HgS}$ is a potential candidate for thermoelectric applications.

Arsenene and Antimonene: Two-Dimensional Materials with High Thermoelectric Figures of Merit

Graphene has a big head start, but other two-dimensional (2D) materials continue to pique interest---and not just for electronics. These systems also might be good, for example, in thermoelectric devices to scavenge wasted energy, and many basic questions need to be addressed. The authors use density functional theory plus semiclassical Boltzmann transport to study As and Sb monolayers, and find that both show excellent thermoelectric response. With low phonon group velocities and vibrational frequencies reducing lattice thermal conductivity, they outperform all known 2D materials, and are serious competition even for well-established thermoelectrics.

Phonon thermal transport in 2H, 4H and 6H silicon carbide from first principles

Silicon carbide (SiC) is a wide band gap semiconductor with a variety of industrial applications. Among its many useful properties is its high thermal conductivity, which makes it advantageous for thermal management applications. In this paper we present ab initio calculations of the in-plane and cross-plane thermal conductivities, κin and κout, of three common hexagonal polytypes of SiC: 2H, 4H and 6H. The phonon Boltzmann transport equation is solved iteratively using as input interatomic force constants determined from density functional theory. Both κin and κout decrease with increasing n in nH SiC because of additional low-lying optic phonon branches. These optic branches are characterized by low phonon group velocities, and they increase the phase space for phonon-phonon scattering of acoustic modes. Also, for all n, κin is found to be larger than κout in the temperature range considered. At electron concentrations present in experimental samples, scattering of phonons by electrons is shown to be negligible except well below room temperature where it can lead to a significant reduction of the lattice thermal conductivity. This work highlights the power of ab initio approaches in giving quantitative, predictive descriptions of thermal transport in materials. It helps explain the qualitative disagreement that exists among different sets of measured thermal conductivity data and provides information of the relative quality of samples from which measured data was obtained.

Native surface oxide turns alloyed silicon membranes into nanophononic metamaterials with ultralow thermal conductivity

A detailed understanding of the relation between microscopic structure and phonon propagation at the nan oscale is essential to design materials with desired phononic and thermal properties.Here we uncover a new mechanism of phonon interaction in surface oxidized membranes, i.e., native oxide layers interact with phonons in ultra-thin silicon membranes through local resonances. The local resonances reduce the low frequency phonon group velocities and shorten their mean free path. This effect opens up a new strategy for ultralow thermal conductivity design as it complements the scattering mechanism which scatters higher frequency modes effectively. The combination of native oxide layer and alloying with germanium in concentration as small as 5% reduces the thermal conductivity of silicon membranes to 100 time lower than the bulk. In addition, the resonance mechanism produced by native oxide surface layers is particularly effective for thermal condutivity reduction even at very low temperatures, at which only low frequency modes are populated.

Thermal Conductivity of Graphene Wrinkles: A Molecular Dynamics Simulation

Based on the nonequilibrium molecular dynamics simulations, the heat conduction in a novel deformation of graphene, named graphene wrinkle (GW), is investigated. Distinct from pristine graphene, the GW exhibits a relatively low thermal conductivity. We observe that the low thermal conductivity stems from the strong phonon localizations, which are concentrated on the joint regions between crests and troughs of wrinkles. The suppression in GW thermal conductivity could be further attributed to the enhanced phonon scatterings, as evidenced by the vibrational density of states (VDOS) attenuation in the low frequency region, the G-band redshift of VDOS due to the flattened phonon dispersion curves (low phonon group velocities), and the decreased phonon lifetime. In addition, we find that the thermal conductivity of GW is almost insensitive to temperature in the range between 200 and 600 K. It is induced by the significant contribution of low frequency phonon modes, which are more influential in the direction p...

Ab Initio-Based Bond Order Potential to Investigate Low Thermal Conductivity of Stanene Nanostructures.

We introduce a bond order potential (BOP) for stanene based on an ab initio derived training data set. The potential is optimized to accurately describe the energetics, as well as thermal and mechanical properties of a free-standing sheet, and used to study diverse nanostructures of stanene, including tubes and ribbons. As a representative case study, using the potential, we perform molecular dynamics simulations to study stanene's structure and temperature-dependent thermal conductivity. We find that the structure of stanene is highly rippled, far in excess of other 2-D materials (e.g., graphene), owing to its low in-plane stiffness (stanene: ∼ 25 N/m; graphene: ∼ 480 N/m). The extent of stanene's rippling also shows stronger temperature dependence compared to that in graphene. Furthermore, we find that stanene based nanostructures have significantly lower thermal conductivity compared to graphene based structures owing to their softness (i.e., low phonon group velocities) and high anharmonic response. Our newly developed BOP will facilitate the exploration of stanene based low dimensional heterostructures for thermoelectric and thermal management applications.

Thermal conductivity of penta-graphene from molecular dynamics study.

Using classical equilibrium molecular dynamics simulations and applying the original Tersoff interatomic potential, we study the thermal transport property of the latest two dimensional carbon allotrope, penta-graphene. It is predicted that its room-temperature thermal conductivity is about 167 W/mK, which is much lower than that of graphene. With normal mode decomposition, the accumulated thermal conductivity with respect to phonon frequency and mean free path is analyzed. It is found that the acoustic phonons make a contribution of about 90% to the thermal conductivity, and phonons with mean free paths larger than 100 nm make a contribution over 50%. We demonstrate that the remarkably lower thermal conductivity of penta-graphene compared with graphene results from the lower phonon group velocities and fewer collective phonon excitations. Our study highlights the importance of structure-property relationship and provides better understanding of thermal transport property and valuable insight into thermal management of penta-graphene.

Low thermal conductivity of graphyne nanotubes from molecular dynamics study

It is well known that carbon nanotubes (CNTs) possess ultrahigh thermal conductivity that is comparable to bulk diamond. However, no research has studied the possible low thermal conductivity of different CNTs so far. By performing nonequilibrium molecular dynamic simulations, we reveal that the perfect graphyne nanotube (GNT) exhibits an unprecedentedly low thermal conductivity (below 10 W/mK at room temperature), which is generally two orders of magnitude lower than that of ordinary CNTs and even lower than the values reported for defected, doped, and chemically functionalized CNTs. By performing phonon polarization and spectral energy density analysis, we observe that the ultralow thermal conductivity stems from the unique atomic structure of the GNT, consisting of the weak acetylenic linkage ($sp$ C-C bonds) and the strong hexagonal ring ($s{p}^{2}$ C-C bonds), which results in a large vibrational mismatch between these two components, and thus induces significantly inefficient heat transfer. Moreover, the thermal transport in GNT with a large number of acetylenic linkages is dominated by the low frequency longitudinal modes in the linkage. Such strong confinement of the low frequency thermal energy results in the extremely low thermal conductivity due to the flattened phonon dispersion curves (low phonon group velocities). The exploration of the abnormal thermal transport of GNTs paves the way for design and application of the relevant devices that could benefit from the ultralow thermal conductivity, such as thermoelectrics for energy conversion.