Disparate Strain Dependent Thermal Conductivity of Two-dimensional Penta-Structures

Unexpected thermal transport properties of MgSiO3monolayer at extreme conditions

The thermal transport properties of mantle minerals are of paramount importance to understand the thermal evolution processes of the Earth. Here, we perform extensively structural searches of two-dimensional MgSiO3monolayer by CALYPSO method and first-principles calculations. A stable MgSiO3monolayer withPmm2 symmetry is uncovered, which possesses a wide indirect band gap of 4.39 eV. The calculations indicate the lattice thermal conductivities of MgSiO3monolayer are 49.86 W (mK)-1and 9.09 W (mK)-1inxandydirections at room temperature. Our findings suggest that MgSiO3monolayer is an excellent low-dimensional thermoelectric material with highZTvalue of 4.58 from n-type doping in theydirection at 2000 K. The unexpected anisotropic thermal transport of MgSiO3monolayer is due to the puckered crystal structure and the asymmetric phonon dispersion as well as the distinct electron states around the Fermi level. These results offer a detailed description of structural and thermal transport properties of MgSiO3monolayer at extreme conditions.

Thermal Transport in Pentagonal CX2 (X = N, P, As, and Sb)

Two-dimensional (2D) materials with a pentagonal structure have many unique physical properties and great potential for applications in electrical, thermal, and optical fields. In this paper, the intrinsic thermal transport properties of 2D pentagonal CX2 (X = N, P, As, and Sb) are comparatively investigated. The results show that penta-CN2 has a high thermal conductivity (302.7 W/mK), while penta-CP2, penta-CAs2, and penta-CSb2 have relatively low thermal conductivities of 60.0, 36.9, and 11.8 W/mK, respectively. The main reason for the high thermal conductivity of penta-CN2 is that the small atomic mass of the N atom is comparable to that of the C atom, resulting in a preferable pentagonal structure with stronger bonds and thus a higher phonon group velocity. The reduction in the thermal conductivity of the other three materials is mainly due to the gradually increased atomic mass from P to Sb, which reduces the phonon group velocity. In addition, the large atomic mass difference does not result in a huge enhancement of the anharmonicity or weakening of the phonon relaxation time. The present work is expected to deepen the understanding of the thermal transport of main group V 2D pentagonal carbons and pave the way for their future applications, also, providing ideas for finding potential thermal management materials.

The effect of four-phonon interaction on phonon thermal conductivity of hexagonal VTe2 and puckered pentagonal VTe2

The traditional view is that complex structures have lower lattice thermal conductivity. However, it is observed that complex structures have higher lattice thermal conductivity than simple atomic structures in VTe2 systems after considering the four-phonon scattering effect. In this work, we calculate the lattice thermal conductivity of an H-VTe2 monolayer with a simple atomic structure and that of a PP-VTe2 monolayer with a complex atomic arrangement using first-principles calculations combined with the Boltzmann transport theory under the conditions of with and without the four-phonon scattering process. Our findings reveal that the lattice thermal conductivity of the PP-VTe2 monolayer along the x or y direction is 3-4 times lower than that of the H-VTe2 monolayer when only considering the three-phonon scattering process. After taking into account the four-phonon scattering process, the lattice thermal conductivity of both monolayers decreases. For the H-VTe2 monolayer, the lattice thermal conductivity decreases by 88.7% (from 1.33 to 0.15 W m-1 K-1) compared to only considering the three-phonon scattering process, mainly due to strong four-phonon scattering. In addition, the PP-VTe2 monolayer experiences a lower decrease in lattice thermal conductivity, with reductions of 12.5% (from 0.4 to 0.35 W m-1 K-1) and 11.7% (from 0.34 to 0.3 W m-1 K-1) in the x and y directions, respectively, because of the weak four-phonon scattering. Notably, the lattice thermal conductivity with the four-phonon scattering process of the H-VTe2 monolayer is twice as low as that of the PP-VTe2 monolayer. Hence, our findings suggest that even simple atomic structures can exhibit lower lattice thermal conductivity than complex structures when considering four-phonon interaction.

Type-1 Pentagonal Tiling Realized in 2D Penta-SrP2 Sheet

According to the systematic classification of pentagon-based two-dimensional (2D) materials [ Phys. Rep. 2022, 964, 1], only type-2 and type-4 out of the 15 pentagonal tiling patterns have been realized in 2D materials so far. Here, we propose the first stable pentagon-based 2D material characterized by the type-1 pentagonal tiling pattern named penta-SrP2. We find that penta-SrP2 is not only thermally and mechanically stable but also dynamically stable when the temperature is above 200 K derived from the calculations by taking both phonon renormalization and thermal expansion into consideration. Moreover, the penta-SrP2 sheet is semiconducting with an indirect band gap of 0.96 eV. These findings expand the family of pentagon-based 2D materials in morphology and provide a new perspective to explore the dynamical stability of high-temperature phases.

Reply to the 'Comment on "Two-dimensional penta-like PdPSe with a puckered pentagonal structure: a first-principles study"' by S. Chowdhury, F. Shojaei and B. Mortazavi, Phys. Chem. Chem. Phys., 2023, , DOI: 10.1039/D2CP01587K

We respond to the recent criticism of our paper [Phys. Chem. Chem. Phys., 2022, 24, 9990–9997] and provide further discussion on the analysis of the PdPSe monolayer.

Pentagonal CmXnY6-m-n (m = 2, 3; n = 1, 2; X, Y = B, N, Al, Si, P) Monolayers: Janus Ternaries Combine Omnidirectional Negative Poisson Ratios with Giant Piezoelectric Effects

Two-dimensional (2D) materials composed of pentagon and Janus motifs usually exhibit unique mechanical and electronic properties. In this work, a class of ternary carbon-based 2D materials, C_mX_nY_6-m-n (m = 2, 3; n = 1, 2; X, Y = B, N, Al, Si, P), are systematically studied by first-principles calculations. Six of 21 Janus penta-C_mX_nY_6-m-n monolayers are dynamically and thermally stable. The Janus penta-C₂B₂Al₂ and Janus penta-Si₂C₂N₂ exhibit auxeticity. More strikingly, Janus penta-Si₂C₂N₂ exhibits an omnidirectional negative Poisson ratio (NPR) with values ranging from -0.13 to -0.15; in other words, it is auxetic under stretch in any direction. The calculations of piezoelectricity reveal that the out-of-plane piezoelectric strain coefficient (d₃₂) of Janus panta-C₂B₂Al₂ is up to 0.63 pm/V and increases to 1 pm/V after a strain engineering. These omnidirectional NPR, giant piezoelectric coefficients endow the Janus pentagonal ternary carbon-based monolayers as potential candidates in the future nanoelectronics, especially in the electromechanical devices.

Tensile strain and finite size modulation of low lattice thermal conductivity in monolayer TMDCs (HfSe2 and ZrS2) from first-principles: a comparative study

With excellent physical and chemical properties, 2D TMDC materials have been widely used in engineering applications, but they inevitably suffer from the dual effects of strain and device size. As typical 2D TMDCs, HfSe₂ and ZrS₂ are reported to have excellent thermoelectric properties. Thermal transport properties have great significance for exerting the performance of materials, ensuring device lifetime and stable operation, but current research is not detailed enough. Here, first-principles combined with the phonon Boltzmann transport equation are used to study the phonon transport inside monolayer HfSe₂ and ZrS₂ under tensile strain and finite size, and explore the band structure properties. Our research shows that they have similar phonon dispersion curve structures, and the band gap of HfSe₂ increases monotonically with the increase of tensile strain, while the bandgap of ZrS₂ increases and then decreases with the increase of tensile strain. Thermal conductivity has obvious strain dependence: with the increase of tensile strain, the thermal conductivity of HfSe₂ gradually decreases, while that of ZrS₂ increases slightly, and then gradually decreases. Reducing the system size can limit the contribution of phonons with a long mean free path, significantly decreasing thermal conductivity through the controlling effect of tensile strain. The mode contribution of thermal conductivity is systematically investigated, and anharmonic properties including mode and frequency-level scattering rates, group velocity and Grüneisen parameters are used to explain the associated mechanism. Phonon scattering processes and channels in various cases are discussed in detail. Our research provides a detailed understanding of the phonon transport and electronic structural properties of low thermal conductivity monolayers of HfSe₂ and ZrS₂, and further completes the study of thermal transport of the two materials under strain and size tuning, which will provide a foundation for further popularization and application.

Ultra-high thermal conductivity of two-dimensional C23

High thermal conductivity is of great interest due to the novel applications in high-performance heat dissipation for microelectronic devices. Two-dimensional (2D) materials with graphene as a representative have attracted tremendous interest due to the excellent properties, where C₂₃is an emerging 2D allotrope of carbon with a large bandgap. In this paper, by solving the Boltzmann transport equation based onstate-of-the-artfirst-principles calculations, the C₂₃is predicted to have an ultrahigh thermal conductivity of 2051.47 Wm^-1K^-1, which is on the same order of magnitude as graphene. Based on the comparative analysis among C₂₃, graphene, and penta-graphene, it is shown that the unique spatial structure and the orbital hybridization of C₂₃lead to weak anharmonicity, which results in the large relaxation time of phonons and finally results in ultrahigh thermal conductivity. Our study is expected to promote the comprehensive understanding of thermal transport in C₂₃and shed light on future exploration of novel materials with high thermal conductivity.

Enhancement in photocatalytic water splitting using van der Waals heterostructure materials based on penta-layers

Recently, van der Waals heterostructures (vdWHs) have been used to improve the performance of 2D materials, enabling more applications. By using first-principles calculations, we have studied the electronic and optical properties of vdWHs composed of penta-siligraphene and other penta-layers (p-Si2C4/p-X; X = Si₂N₄, ZnO₂, Ge₂C₄ or SiGeC₄). The stability of the vdWHs is verified by computing their binding energy, vibrational phonon spectra and ab initio molecular dynamics simulations. By assessing the electronic properties, we have found that the p-Si2C4/p-ZnO2, p-Si2C4/p-Ge2C4 and p-Si2C4/p-SiGeC4 vdWHs are semiconductors with an indirect band gap characterized by type-I band alignment. Meanwhile, the p-Si2C4/p-Si2N4 vdWH is a quasi-direct band gap semiconductor characterized by type-II band alignment. Bader charge analysis and charge density of p-Si2C4/p-Si2N4 vdWHs showed that photogenerated electrons move from the p-Si2N4 monolayer to the p-Si2C4 monolayer limiting the recombination of photogenerated charges and improving the photocatalytic efficiency. Furthermore, the p-Si2C4/p-Si2N4 vdWH exhibits suitable band edge positions compared to isolated monolayers suggesting its potential applicability in photocatalytic water splitting. The calculated optical absorption revealed that the p-Si2N4 monolayer exhibits substantial optical absorption in the ultraviolet (UV) range, while the p-Si2C4 monolayer and the p-Si2C4/p-Si2N4 vdWH show outstanding optical absorption on the order of 10⁵ cm^-1 in the visible and UV ranges. More importantly, the p-Si2C4/p-Si2N4 vdWH can greatly improve the optical absorption in these regions, which leads to high-efficiency usage of solar energy. Our study provides a route to design new vdWHs based on pentagonal monolayers, as well as an efficient photocatalyst for photocatalytic water splitting and optical devices.

Efficient modulation of thermal transport in two-dimensional materials for thermal management in device applications

With the development of chip technology, the density of transistors on integrated circuits is increasing and the size is gradually shrinking to the micro-/nanoscale, with the consequent problem of heat dissipation on chips becoming increasingly serious. For device applications, efficient heat dissipation and thermal management play a key role in ensuring device operation reliability. In this review, we summarize the thermal management applications based on 2D materials from both theoretical and experimental perspectives. The regulation approaches of thermal transport can be divided into two main types: intrinsic structure engineering (acting on the intrinsic structure) and non-structure engineering (applying external fields). On one hand, the thermal transport properties of 2D materials can be modulated by defects and disorders, size effect (including length, width, and the number of layers), heterostructures, structure regulation, doping, alloy, functionalizing, and isotope purity. On the other hand, strain engineering, electric field, and substrate can also modulate thermal transport efficiently without changing the intrinsic structure of the materials. Furthermore, we propose a perspective on the topic of using magnetism and light field to modulate the thermal transport properties of 2D materials. In short, we comprehensively review the existing thermal management modulation applications as well as the latest research progress, and conclude with a discussion and perspective on the applications of 2D materials in thermal management, which will be of great significance to the development of next-generation nanoelectronic devices.

Piezoelectric ferromagnetism in Janus monolayer YBrI: a first-principles prediction

Coexistence of intrinsic ferromagnetism and piezoelectricity, namely piezoelectric ferromagnetism (PFM), is crucial to advance multifunctional spintronic technologies. In this work, we demonstrate that Janus monolayer YBrI is a PFM, which is dynamically, mechanically and thermally stable. The electronic correlation effects on the physical properties of YBrI are investigated by using generalized gradient approximation plus U (GGA+U) approach. For out-of-plane magnetic anisotropy, YBrI is a ferrovalley (FV) material, and its valley splitting is larger than 82 meV within the considered U range. The anomalous valley Hall effect (AVHE) can be achieved under an in-plane electric field. However, for in-plane magnetic anisotropy, YBrI is a common ferromagnetic (FM) semiconductor. When considering intrinsic magnetic anisotropy, the easy axis of YBrI is always in-plane, and its magnetic anisotropy energy (MAE) varies from 0.309 meV to 0.237 meV (U = 0.0 eV to 3.0 eV). However, the magnetization can be adjusted from the in-plane to out-of-plane direction by an external magnetic field, and then lead to the occurrence of valley polarization. Moreover, the missing centrosymmetry along with broken mirror symmetry results in both in-plane and out-of-plane piezoelectricity in the YBrI monolayer. At a typical U = 2.0 eV, the piezoelectric strain coefficient d₁₁ is predicted to be -5.61 pm V^-1, which is higher than or comparable with the ones of other known two-dimensional (2D) materials. The electronic and piezoelectric properties of YBrI can be effectively tuned by applying biaxial strain. For example, tensile strain can enhance valley splitting and d₁₁ (absolute value). The predicted magnetic transition temperature of YBrI is higher than those of experimentally synthesized 2D FM materials CrI₃ and Cr₂Ge₂Te₆. Our findings of these distinctive properties could pave the way for designing multifunctional spintronic devices, and bring forward a new perspective for constructing 2D materials.

First-Principles Prediction of New 2D p-SiPN: A Wide Bandgap Semiconductor

Pentagonal two-dimensional ternary sheets are an emerging class of materials because of their novel characteristic and wide range of applications. In this work, we use first-principles density functional theory (DFT) calculations to identify a new pentagonal SiPN, p-SiPN, which is geometrically, thermodynamically, dynamically, and mechanically stable, and has promising experimental potential. The new p-SiPN shows an indirect bandgap semiconducting behavior that is highly tunable with applied equ-biaxial strain. It is mechanically isotropic, along the x-y in-plane direction, and is a soft material possessing high elasticity and ultimate strain. In addition, its exceptional anisotropic optical response with strong UV light absorbance, and small reflectivity and electron energy loss make it a potential material for optoelectronics and nanomechanics.

Prediction of Two-Dimensional Group IV Nitrides AxNy (A = Sn, Ge, or Si): Diverse Stoichiometric Ratios, Ferromagnetism, and Auxetic Mechanical Property

In this work, we unveiled a new class of two-dimensional (2D) group IV nitride A_xN_y (A = Sn, Ge, or Si) prototypes, C2/m A₄N, P3̅m1 A₃N, P3m1 A₂N, P3̅m1 A₃N₂, P6̅m2 AN, P3̅m1 AN, P6̅2m A₃N₄, P3m1 A₂N₃, P4̅2₁m AN₂, and P3̅m1 AN₃, by using evolutionary algorithms combined with first-principles calculations. Using HSE06 functional calculations, a wide range of band gaps from metal to semiconductor (0.405-5.050 eV) and ultrahigh carrier mobilities (1-24 × 10³ cm² V^-1 s^-1) were evidenced in these 2D structures. We found that 2D P3m1 Sn₂N₃, Ge₂N₃, and Si₂N₃ are intrinsic ferromagnetic semiconductors with gaps of 0.677, 1.285, and 2.321 eV, respectively. The lattice symmetry and Si-to-N₂ charge transfer upon strain lead to large anisotropic negative Poisson's ratios (-0.281 to -0.146) along whole in-plane directions in 2D P4̅2₁m SiN₂. Our findings not only enrich the family of 2D nitrides but also highlight the promising optoelectronic and nanoauxetic applications of 2D group IV nitrides.

Effect of High Order Phonon Scattering on the Thermal Conductivity and Its Response to Strain of a Penta-NiN2 Sheet

Motivated by the recent synthesis of penta-NiN₂, a new two-dimensional (2D) planar material entirely composed of pentagons [ ACS Nano 2021, 15, 13539], we study its thermal transport properties based on first-principles calculations and solving the Boltzmann transport equation within the self-consistent phonon theory and four-phonon scattering formalism. We find that the intrinsic lattice thermal conductivity of penta-NiN₂ is 11.67 W/mK at room temperature, which is reduced by 89.32% as compared to the value obtained by only considering three-phonon scattering processes. More interestingly, different from the general response of thermal conductivity to external strain in most 2D materials, an oscillatory decrease of the thermal conductivity with increasing biaxial tensile strain is observed, which can be attributed to the renormalization of vibrational frequencies and the nonmonotonic variation of phonon scattering rates. This work provides an accurate intrinsic thermal conductivity of penta-NiN₂ and elucidates the effects of the strain-tuned vibrational modes and phonon band gap on the four-phonon scattering processes, shedding light on a better understanding of the physical mechanisms of thermal transport properties in 2D pentagon-based materials.

Efficient mechanical modulation of the phonon thermal conductivity of Mo6S6 nanowires

Mo₆S₆ nanowires are emerging as key building blocks for flexible devices and are competitive with carbon nanotubes due to easier separation and functionalization. Here, it is reported the phonon thermal conductivity (κ) of Mo₆S₆ nanowires via molecular dynamics simulations. It shows a large tunability of low-frequency phonon thermal conductivity (κ_lf)Amax from 27.2-191 W (m K)^-1, an increase of around 702% via mechanical strain. Below critical tension/torsion strain, their phonon thermal conductivity monotonically reduces/enlarges; whereas above this value, an inverse trend is identified. On the other hand, Mo₆S₆ nanowires show unusual auxetic behavior. The transitions involved in phonon thermal conductivity are molecularly illustrated by a strain-induced crossover in bond configurations and are explained based on a competition mechanism between phonon scattering and group velocity. This study provides insights into the thermal transport and auxetic properties of low-dimensional structures and the thermal management of Mo₆S₆ nanowire-based systems.

Two-dimensional layered MSi2N4 (M = Mo, W) as promising thermal management materials: a comparative study

With the miniaturization and integration of nanoelectronic devices, efficient heat removal becomes a key factor affecting their reliable operation. Two-dimensional (2D) materials, with high intrinsic thermal conductivity, good mechanical flexibility, and precisely controllable growth, are widely accepted as ideal candidates for thermal management materials. In this work, by solving the phonon Boltzmann transport equation (BTE) based on first-principles calculations, we investigated the thermal conductivity of novel 2D layered MSi₂N₄ (M = Mo, W). Our results point to a competitive thermal conductivity as large as 162 W m^-1 K^-1 of monolayer MoSi₂N₄, which is around two times larger than that of WSi₂N₄ and seven times larger than that of monolayer MoS₂ despite their similar non-planar structures. It is revealed that the high thermal conductivity arises mainly from its large group velocity and low anharmonicity. Our result suggests that MoSi₂N₄ could be a potential candidate for 2D thermal management materials.

Strain- and electric field-enhanced optical properties of the penta-siligraphene monolayer

The effect of biaxial strain and an external electric field on the optical properties of the penta-siligraphene monolayer are reported.

The disparate effect of strain on thermal conductivity of 2-D materials

Thermal transport in 2-D (dimensional) structures is highly susceptible to external perturbations such as strain, owing to their high surface-to-volume ratio. In this study, we investigate the influence of strain on the thermal conductivity of flat (graphene and hexagonal boron nitride), buckled and puckered (molybdenum disulfide and black phosphorous) 2-D materials. Unlike bulk materials where the thermal conductivity reduces with strain, the thermal conductivity of 2-D materials under strain is observed to be unique and dependent on the material considered. To understand such diverse strain-dependent thermal conductivity in 2-D materials, the phonon mode properties are calculated. It was observed that the strain softens the longitudinal mode (LA), whereas the out-of-plane acoustic mode (ZA) undergoes stiffening albeit various extents. In flat 2-D materials, the dispersion of ZA mode is linearized under strain while it tends to linearize in buckled and puckered structures. The variation in the phonon group velocity of ZA mode coupled with the anomalous behavior of the phonon lifetime of acoustic modes results in a diverse strain dependence of the thermal conductivity of 2-D materials. Our findings offer insight into the influence of strain of 2-D materials and will be helpful in tailoring the thermal properties of these materials for various applications such as nanoelectronics and thermoelectric devices.

Predicted intrinsic piezoelectric ferromagnetism in Janus monolayer MnSbBiTe4: a first principles study

Two-dimensional (2D) piezoelectric ferromagnetism (PFM) is essential for the development of the next-generation multifunctional spintronic technologies. Recently, the layered van der Waals (vdW) compound MnBi₂Te₄ as a platform to realize the quantum anomalous Hall effect (QAHE) has attracted great interest. In this work, the Janus monolayer MnSbBiTe₄ with dynamic, mechanical and thermal stabilities is constructed from a synthesized non-piezoelectric MnBi₂Te₄ monolayer by replacing the top Bi atomic layer with Sb atoms. The calculated results show that monolayer MnSbBiTe₄ is an intrinsic ferromagnetic (FM) semiconductor with a gap value of 0.25 eV, whose easy magnetization axis is out-of-plane direction with magnetic anisotropy energy (MAE) of 158 μeV per Mn. The predicted Curie temperature T_C is about 20.3 K, which is close to that of monolayer MnBi₂Te₄. The calculated results show that the in-plane d₁₁ is about 5.56 pm V^-1, which is higher than or comparable to those of other 2D known materials. Moreover, it is found that strain engineering can effectively tune the piezoelectric properties of Janus monolayer MnSbBiTe₄. The calculated results show that tensile strain can improve the d₁₁, which is improved to be 21.16 pm V^-1 at only 1.04 strain. It is proved that the ferromagnetic order, semiconducting properties, out-of-plane easy axis and a large d₁₁ are robust against electronic correlations. Our work provides a possible way to achieve PFM with a large d₁₁ in well-explored vdW compound MnBi₂Te₄, which makes it possible to use the piezoelectric effect to tune the quantum transport process.

Ultralow lattice thermal conductivity and dramatically enhanced thermoelectric properties of monolayer InSe induced by an external electric field

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

Two-Dimensional Tetrahex-GeC2: A Material with Tunable Electronic and Optical Properties Combined with Ultrahigh Carrier Mobility

Based on first-principles calculations, we propose a novel two-dimensional (2D) germanium carbide, tetrahex-GeC₂, and determine its electronic and optical properties. Each Ge atom binds to four C atoms, in contrast to the known 2D hexagonal germanium carbides. Monolayer tetrahex-GeC₂ possesses a narrow direct band gap of 0.89 eV, which can be effectively tuned by applying strain and increasing the thickness. Its electron mobility is extraordinarily high (9.5 × 10⁴ cm²/(V s)), about 80 times that of monolayer black phosphorus. The optical absorption coefficient is ∼10⁶ cm^-1 in a wide spectral range from near-infrared to near-ultraviolet, comparable to perovskite solar cell materials. We obtain high cohesive energy (5.50 eV/atom), excellent stability, and small electron/hole effective mass (0.19/0.10 m₀). Tetrahex-GeC₂ turns out to be a very promising semiconductor for nanoelectronic, optoelectronic, and photovoltaic applications.

Two-dimensional Weyl points and nodal lines in pentagonal materials and their optical response

Two-dimensional pentagonal structures based on the Cairo tiling are the basis of a family of layered materials with appealing physical properties. In this work we present a theoretical study of the symmetry-based electronic and optical properties of these pentagonal materials. We provide a complete classification of the space groups that support pentagonal structures for binary and ternary systems. By means of first-principles calculations, the electronic band structures and the local spin textures in momentum space are analyzed for four examples of these materials, namely, PdSeTe, PdSeS, InP₅ and GeBi₂, all of which are dynamically stable. Our results show that pentagonal structures can be realized in chiral and achiral lattices with Weyl nodes pinned at high-symmetry points and nodal lines along the Brillouin zone boundary; these degeneracies are protected by the combined action of crystalline and time-reversal symmetries. Additionally, we computed the linear and nonlinear optical features of the proposed pentagonal materials and discuss some particular features such as the shift current, which shows an enhancement due to the presence of nodal lines and points, and their possible applications.

Thermo-mechanical correlation in two-dimensional materials

Two-dimensional (2D) materials have received tremendous attention from the research community in the past decades, because of their numerous striking physical, chemical and mechanical properties and promising potential in a wide range of applications. This field is strongly interdisciplinary, requiring efficient integration of knowledge with different insights. In this review, we summarize the up-to-date research on the thermal and mechanical properties and thermo-mechanical correlation in 2D materials, including both theoretical and experimental insight. Firstly, the mechanical properties of 2D nanomaterials are discussed, in which the underlying physics is summarized. Then, we discuss the impacts of thermal fluctuation on the mechanical properties. Next, from experimental points of view, we present the methods to introduce strain in 2D materials experimentally and the experimental tools to measure the degree of strain. Finally, we discuss the fundamental phonon and thermal properties of 2D materials, including the strain effects on phonon dispersion, phonon hydrodynamic behavior, phonon topological feature, ballistic thermal conductance and diffusive thermal conductivity. This article presents an advanced understanding of the mechanical and thermal properties of 2D materials, which provides new opportunities for promoting their applications in nanoscale electronic, optoelectronic, and thermal functional devices.

Recent progress in the development of thermal interface materials: a review

Modern electronic devices are characterized by high-power and high-frequency with excessive heat accumulation. Thermal interface materials (TIMs) are of crucial importance for efficient heat dissipation to maintain proper functions and lifetime for these devices. The most promising TIMs are those polymer-based nanocomposites consisting of polymers and low-dimensional materials with high thermal conductivity (TC). This perspective summarizes the recent progress on the thermal transport properties of newly discovered one-dimensional (1D) nanomaterials and two-dimensional (2D) nanomaterials as well as three-dimensional (3D) nanostructures consisting of these 1D and 2D nanomaterials. Moreover, the applications of various nanomaterials in polymer nanocomposites for advanced TIMs are critically reviewed and the mechanism of TC enhancement is analysed. It is hoped that the present review could provide better understanding of the thermal transport properties of recently developed 2D nanomaterials and various 3D nanostructures as well as relevant polymer-based TIMs, shedding more light on the thermal management research.

Hydrogen-induced tunable electronic and optical properties of a two-dimensional penta-Pt2N4 monolayer

Most known two-dimensional materials lack a suitable wide-bandgap, and hydrogenation can be effectively utilized to tune the bandgap of some 2D materials. By employing density functional theory calculations, we investigate the effect of hydrogenation on the electronic and optical properties of a recently reported anisotropic penta-Pt2N4 monolayer. The results show that penta-Pt2N4 is thermally and mechanically stable after hydrogenation and also possesses anisotropic Young's modulus and Poisson's ratio. The electronic property analysis using the hybrid functional reveals that penta-Pt2N4 exhibits a bandgap of 1.10 eV, and the hydrogenation significantly enhances the bandgap to 2.70 eV. Furthermore, the hydrogenated Pt2N4 displays a strong optical absorption of up to 6.45 × 105 cm-1 in the ultraviolet region, and low absorption and low reflectivity in the visible region. Our results strongly suggest that the hydrogenated Pt2N4 has tunable electronic and optical properties for applications as a hole-transport material layer in solar cells in the visible region, and as an ultraviolet detector in the ultraviolet region of the electromagnetic spectrum.

The Thermoelectric Properties of Monolayer MAs2 (M = Ni, Pd and Pt) from First-Principles Calculations

The thermoelectric property of the monolayer MAs₂ (M = Ni, Pd and Pt) is predicted based on first principles calculations, while combining with the Boltzmann transport theory to confirm the influence of phonon and electricity transport property on the thermoelectric performance. More specifically, on the basis of stable geometry structure, the lower lattice thermal conductivity of the monolayer NiAs₂, PdAs₂ and PtAs₂ is obtained corresponding to 5.9, 2.9 and 3.6 W/mK. Furthermore, the results indicate that the monolayer MAs₂ have moderate direct bang-gap, in which the monolayer PdAs₂ can reach 0.8 eV. The Seebeck coefficient, power factor and thermoelectric figure of merit (ZT) were calculated at 300, 500 and 700 K by performing the Boltzmann transport equation and the relaxation time approximation. Among them, we can affirm that the monolayer PdAs₂ possesses the maximum ZT of about 2.1, which is derived from a very large power factor of 3.9 × 10¹¹ W/K²ms and lower thermal conductivity of 1.4 W/mK at 700 K. The monolayer MAs₂ can be a promising candidate for application at thermoelectric materials.

Strain-induced band modulation and excellent stability, transport and optical properties of penta-MP2 (M = Ni, Pd, and Pt) monolayers

First principle calculations utilizing density functional theory were carried out to investigate the electronic, transport and optical properties of penta-MP₂ (M = Ni, Pd and Pt) monolayer compounds under applied uniaxial and biaxial tensile strains. With an optimum magnitude of applied strain, we found band gap transitions in penta-MP₂ monolayers from zero/narrow to the semiconductor regime, wherein band gaps were noticed to be firmly dependent on the applied uniaxial and biaxial tensile strains. In this study, the PBE approach was used primarily to evaluate electronic properties, from where the identified architectures of penta-MP₂ with maximum obtained bandgaps under respective optimum strains were assessed through the HSE06 method of calculation for better estimation of band gaps and optical properties. Prior to HSE calculations, we affirmed our assessment for the stability and reliability of the compounds under uniaxial and biaxial strains of up to 15% through phonon spectrum and elastic calculations. A distinct transition was also noted from semiconductor to metal for all compounds after the applied optimum uniaxial and biaxial strains. The optical absorption spectra in all the stretched penta-MP₂ compounds reached the order of 10⁶ cm^-1, with significant peaks belonging to the IR and visible regions; this indicates promising applications of these materials in high-performance solar energy and good hot mirror materials. The enhanced I-V responses under uniaxial and biaxial tensile strains using the non-equilibrium Green's function (NEGF) approach confirm the usefulness of the strained state of the considered penta-MP₂ monolayers. The results show that tuning electronic properties, I-V characteristics and optical properties of stretched penta-MP₂ compounds under tensile strain merits significant future applications in optoelectronic devices and as good hot mirror materials.

Anharmonic lattice dynamics and thermal transport of monolayer InSe under equibiaxial tensile strains

Two-dimensional (2D) InSe, which exhibits high electron mobility and a wide band gap has emerged as a promising material for photoelectric and thermoelectric applications. The inadequate understanding of the lattice thermal conductivity (κ), however, hampers the advancement of 2D InSe. Herein, by taking into account anharmonicity up to the fourth order and introducing the equibiaxial tensile strain (ϵ), we have performed an in-depth study on the lattice dynamics of 2D InSe. Interestingly, theκexhibits a non-monotonic behaviour as a function of equibiaxial tensile strain, which can be attributed to the changes in acoustic phonon lifetimes. At the Γ point, a blue-shift of the lowest optical mode and a red-shift of the uppermost optical mode are reported for the first time. More strikingly, the blue-shift can be largely suppressed by equibiaxial tensile strain. Further study indicates that the unique transition of the potential energy surface is responsible for the disappearance of the blue-shift. Our work may enlighten the future research on phonon engineering and management of the lattice thermal conductivity of 2D InSe.

Thermoelectric Penta-Silicene with a High Room-Temperature Figure of Merit

Silicon is one of the most frequently used chemical elements of the periodic table in nanotechnology (Goodilin et al., ACS Nano 2019, 13, 10879-10886). Two-dimensional silicene, a silicon analogue of graphene, has been readily obtained to make field-effect transistors since 2015 (Tao et al., Nat. Nanotechnol. 2015, 10, 227; Tsai et al., Nat. Commun. 2013, 4, 1500). Recently, as new members of the silicene family, penta-silicene and its nanoribbon have been experimentally grown on a Ag(110) surface with exotic electronic properties (Cerdá et al., Nat. Commun. 2016, 7, 13076; Sheng et al., Nano Lett. 2018, 18, 2937-2942). However, the thermoelectric performance of penta-silicene has not been so far studied, which would hinder its potential applications of electric generation from waste heat and solid-state Peltier coolers. Based on the Boltzmann transport theory and ab initio calculations, we find that penta-silicene shows remarkable room-temperature figures of merit ZT of 3.4 and 3.0 at the reachable hole and electron concentrations, respectively. We attribute this high ZT to the superior "pudding-mold" electronic band structure and ultralow lattice thermal conductivity. The discovery provides new insight into the transport property of pentagonal nanostructures and highlights the potential applications of thermoelectric materials at room temperature.

Ultra-low thermal conductivity and high thermoelectric performance of two-dimensional triphosphides (InP3, GaP3, SbP3 and SnP3): a comprehensive first-principles study

By performing first-principles calculations combined with the Boltzmann transport equation, we report a comprehensive study of the thermal and thermoelectric properties of monolayer triphosphides InP3, GaP3, SbP3 and SnP3. Firstly, we studied the structure and phonon dispersion, and discussed the long-range atomic interactions by analyzing the second-order interatomic force constants (IFCs). Next, we predicted the corresponding thermal conductivities of monolayer InP3, GaP3, SbP3 and SnP3 at 300 K to be 0.64 W m-1 K-1, 3.02 W m-1 K-1, 1.04 W m-1 K-1 and 0.48 W m-1 K-1, respectively. To study the thermoelectric properties, the carrier mobility and electron relaxation time of the four materials were predicted by the deformation potential theory method and explained by analyzing their energy band structures. Then, the Seebeck coefficient, electrical conductivity and thermoelectric figure of merit (ZT) at different temperatures were calculated by using the Boltzmann transport equation with relaxation time approximation. Finally, we predicted the maximum ZT values of InP3, GaP3, SbP3 and SnP3 to be up to 2.6, 0.9, 1.9 and 3.7 at 300 K and up to 4.6, 1.6, 3.5 and 6.1 at 500 K, respectively. With ultra-low thermal conductivity and high thermoelectric performance, monolayer triphosphides are considered as potential candidates for thermoelectric materials.

A unique pentagonal network structure of the NiS2 monolayer with high stability and a tunable bandgap

The NiS₂ monolayer with an intriguing pentagonal ring network is stable up to 500 K based on density functional theory calculations.

Coexistence of intrinsic piezoelectricity and ferromagnetism induced by small biaxial strain in septuple-atomic-layer VSi2P4

The VSi₂P₄ spans a wide range of properties upon the increasing strain from ferromagnetic metal (FMM) to spin-gapless semiconductor (SGS) to ferromagnetic semiconductor (FMS) to SGS to ferromagnetic half-metal (FMHM).

Penta-Graphene as a Potential Gas Sensor for NOx Detection

Two-dimensional (2D) penta-graphene (PG) with unique properties that can even outperform graphene is attracting extensive attention because of its promising application in nanoelectronics. Herein, we investigate the electronic and transport properties of monolayer PG with typical small gas molecules, such as CO, CO₂, NH₃, NO and NO₂, to explore the sensing capabilities of this monolayer by using first-principles and non-equilibrium Green's function (NEGF) calculations. The optimal position and mode of adsorbed molecules are determined, and the important role of charge transfer in adsorption stability and the influence of chemical bond formation on the electronic structure of the adsorption system are explored. It is demonstrated that monolayer PG is most preferred for the NO_x (x = 1, 2) molecules with suitable adsorption strength and apparent charge transfer. Moreover, the current-voltage (I-V) curves of PG display a tremendous reduction of 88% (90%) in current after NO₂ (NO) adsorption. The superior sensing performance of PG rivals or even surpasses that of other 2D materials such as graphene and phosphorene. Such ultrahigh sensitivity and selectivity to nitrogen oxides make PG a superior gas sensor that promises wide-ranging applications.

Symmetry-protected metallic and topological phases in penta-materials

We analyze the symmetry and topological features of a family of materials closely related to penta-graphene, derived from it by adsorption or substitution of different atoms. Our description is based on a novel approach, called topological quantum chemistry, that allows to characterize the topology of the electronic bands, based on the mapping between real and reciprocal space. In particular, by adsorption of alkaline (Li or Na) atoms we obtain a nodal line metal at room temperature, with a continuum of Dirac points around the perimeter of the Brillouin zone. This behavior is also observed in some substitutional derivatives of penta-graphene, such as penta-PC₂. Breaking of time-reversal symmetry can be achieved by the use of magnetic atoms; we study penta-MnC₂, which also presents spin-orbit coupling and reveals a Chern insulator phase. We find that for this family of materials, symmetry is the source of protection for metallic and nontrivial topological phases that can be associated to the presence of fractional band filling, spin-orbit coupling and time-reversal symmetry breaking.

Intrinsic Thermal Shock Behavior of Common Rutile Oxides

Rutile TiO2, VO2, CrO2, MnO2, NbO2, RuO2, RhO2, TaO2, OsO2, IrO2, SnO2, PbO2, SiO2, and GeO2 (space group P42/mnm) were explored for thermal shock resistance applications using density functional theory in conjunction with acoustic phonon models. Four relevant thermomechanical properties were calculated, namely thermal conductivity, Poisson’s ratio, the linear coefficient of thermal expansion, and elastic modulus. The thermal conductivity exhibited a parabolic relationship with the linear coefficient of thermal expansion and the extremes were delineated by SiO2 (the smallest linear coefficient of thermal expansion and the largest thermal conductivity) and PbO2 (vice versa). It is suggested that stronger bonding in SiO2 than PbO2 is responsible for such behavior. This also gave rise to the largest elastic modulus of SiO2 in this group of rutile oxides. Finally, the intrinsic thermal shock resistance was the largest for SiO2, exceeding some of the competitive phases such as Al2O3 and nanolaminated Ti3SiC2.

First Principles Study of Penta-siligraphene as High-Performance Anode Material for Li-Ion Batteries

From first principles calculations, a novel pentagonal Si/C complexity is predicted to have potential applications as a promising anode material for Li-ion batteries. It is found that the structural and thermal stability of the penta-siligraphene (P-Si2C4) is better than penta-graphene that is composed of C atoms only. Electronic band structure analysis shows that the empty C-2pz state in the P-Si2C4 provides space to accommodate and stabilize electrons from Li, which makes Li storage energetically favorable. As a result, four Li atoms can be stored by one formula unit of the P-Si2C4, corresponding to a theoretical gravimetric Li storage capacity of 1028.7 mAhg-1. The metallic electronic structures of the Li-adsorbed P-LixSi2C4 as well as very small Li migration energy barriers are beneficial for fast charge/discharge performance of the battery. The mechanism on the Li adsorption interaction on the P-Si2C4 is discussed. These results demonstrate a novel strategy to design two-dimensional Si/C complex anode materials for high-performance Li-ion batteries.

Thermal Conductivity Enhancement in MoS_{2} under Extreme Strain

Because of their weak interlayer bonding, van der Waals (vdW) solids are very sensitive to external stimuli such as strain. Experimental studies of strain tuning of thermal properties in vdW solids have not yet been reported. Under ∼9% cross-plane compressive strain created by hydrostatic pressure in a diamond anvil cell, we observed an increase of cross-plane thermal conductivity in bulk MoS_{2} from 3.5 to about 25  W m^{-1} K^{-1}, measured with a picosecond transient thermoreflectance technique. First-principles calculations and coherent phonon spectroscopy experiments reveal that this drastic change arises from the strain-enhanced interlayer interaction, heavily modified phonon dispersions, and decrease in phonon lifetimes due to the unbundling effect along the cross-plane direction. The contribution from the change of electronic thermal conductivity is negligible. Our results suggest possible parallel tuning of structural, thermal, and electrical properties of vdW solids with strain in multiphysics devices.

Born effective charge removed anomalous temperature dependence of lattice thermal conductivity in monolayer GeC

Due to potential applications in nano- and opto-electronics, two-dimensional (2D) materials have attracted tremendous interest. Their thermal transport properties are closely related to the performance of 2D materials-based devices. Here, the phonon transports of monolayer GeC with a perfect planar hexagonal honeycomb structure are investigated by solving the linearized phonon Boltzmann equation within the single-mode relaxation time approximation (RTA). Without inclusion of Born effective charges (Z ^*) and dielectric constants ([Formula: see text]), the lattice thermal conductivity ([Formula: see text]) decreases almost linearly above 350 K, deviating from the usual [Formula: see text] law. The underlying mechanism is because the contribution to [Formula: see text] from high-frequency optical phonon modes increases with increasing temperature, and the contribution exceeds one from acoustic branches at high temperature. These can be understood by huge phonon band gap caused by large difference in atom mass between Ge and C atoms, which produces important effects on scattering process involving high-frequency optical phonon. When considering Z ^* and [Formula: see text], the phonon group velocities and phonon lifetimes of high-frequency optical phonon modes are obviously reduced with respect to ones without Z ^* and [Formula: see text]. The reduced group velocities and phonon lifetimes give rise to small contribution to [Formula: see text] from high-frequency optical phonon modes, which produces the the traditional [Formula: see text] relation in monolayer GeC. Our works highlight the importance of Z ^* and [Formula: see text] to investigate phonon transports of monolayer GeC, and motivate further theoretical or experimental efforts to investigate thermal transports of other 2D materials.

Ultra-low lattice thermal conductivity of monolayer penta-silicene and penta-germanene

We study the lattice thermal conductivity of two-dimensional (2D) pentagonal systems, such as penta-silicene and penta-germanene.

Stabilization of two-dimensional penta-silicene for flexible lithium-ion battery anodes via surface chemistry reconfiguration

Surface chemistry reconfiguration is employed to acquire stable penta-silicene with tunable properties for use in flexible lithium-ion battery anodes.

Disparate strain response of the thermal transport properties of bilayer penta-graphene as compared to that of monolayer penta-graphene

In this study, strain modulation of the lattice thermal conductivity of monolayer and bilayer penta-graphene (PG) at room temperature was investigated using first-principles calculations combined with the phonon Boltzmann transport equation.

Effects of tensile strain and finite size on thermal conductivity in monolayer WSe2

The strain- and size-dependent lattice thermal conductivity of monolayer WSe₂ has been investigated using the first-principles based Boltzmann transport equation.

Phase transformation in two-dimensional covalent organic frameworks under compressive loading

As a new class of two-dimensional (2D) materials, 2D covalent organic frameworks (COFs) are proven to possess remarkable electronic and magnetic properties. However, their mechanical behaviours remain almost unexplored. In this work, taking the recently synthesised dimethylmethylene-bridged triphenylamine (DTPA) sheet as an example, we investigate the mechanical behaviours of 2D COFs based on molecular dynamics simulations together with density functional theory calculations. A novel phase transformation is observed in DTPA sheets when a relatively large in-plane compressive strain is applied to them. Specifically, the crystal structures of the transformed phases are topographically different when the compressive loading is applied in different directions. The compression-induced phase transformation in DTPA sheets is attributed to the buckling of their kagome lattice structures and is found to have significant impacts on their material properties. After the phase transformation, Young's modulus, band gap and thermal conductivity of DTPA sheets are greatly reduced and become strongly anisotropic. Moreover, a large in-plane negative Poisson's ratio is found in the transformed phases of DTPA sheets. It is expected that the results of the compression-induced phase transformation and its influence on the material properties observed in the present DTPA sheets can be further extended to other 2D COFs, since most 2D COFs are found to possess a similar kagome lattice structure.

van der Waals Graphene Kirigami Heterostructure for Strain-Controlled Thermal Transparency

Programming thermal transport across interfaces by engineering strain is of critical importance for exploring mechanical controllable and thermal manageable devices with multifunctionalities. Here, we report a van der Waals heterostructure that is composed of bilayer graphene kirigami with diverse layer cut patterns and assembly organizations and show that the thermal flow intensity across the van der Waals interfaces, named as thermal transparency, could be continuously regulated by applying an external in-plane tensile strain. The density of atomic interactions across the interfaces and the distribution of delocalized phonon modes in each graphene kirigami are elucidated to understand the underlying thermal transport mechanism and are also incorporated into a theoretical model for quantitative predictions of thermal conductance under mechanical strain. A proof-of-conceptual van der Waals graphene kirigami heterostructure by design is proposed and validated through extensive full-scale atomistic simulations on the feasibility and reliability of regulating the transparency ratio of thermal transport by mechanical strain, demonstrating its potential applications in thermal and electronic devices.

Ab initio quantum transport in AB-stacked bilayer penta-silicene using atomic orbitals

The current carried by a material subject to an electric field is microscopically inhomogeneous and can be modelled using scattering theory, in which electrons undergo collisions with the microscopic objects they encounter. We herein present a methodology for parameter-free calculations of the current density from first-principles using density functional theory, Wannier functions and scattering matrices. The methodology is used on free-standing AB-stacked bilayer penta-silicene. This new Si allotrope has been proposed to have a higher stability than any of its hexagonal bilayer counterparts. Furthermore, its semiconducting properties make it ideal for use in electronic components. We unveil the role of the p_z orbitals in the transport through a three-dimensional quantum wire and present current density streamlines that reveal the locations of the highest charge flow. The present methodology can be expanded to accommodate many electron degrees of freedom, the application of electromagnetic fields and many other physical phenomena involved in device operation.

A methodology for parameter-free calculations of current density from first-principles using density functional theory, Wannier functions and scattering matrices is presented. The methodology is used on free-standing AB-stacked bilayer penta-silicene.

Tight-binding model for opto-electronic properties of penta-graphene nanostructures

We present a tight-binding parametrization for penta-graphene that correctly describes its electronic band structure and linear optical response. The set of parameters is validated by comparing to ab-initio density functional theory calculations for single-layer penta-graphene, showing a very good global agreement. We apply this parameterization to penta-graphene nanoribbons, achieving an adequate description of quantum-size effects. Additionally, a symmetry-based analysis of the energy band structure and the optical transitions involved in the absorption spectra is introduced, allowing for the interpretation of the optoelectronic features of these systems.

Lone-Pair Electrons Do Not Necessarily Lead to Low Lattice Thermal Conductivity: An Exception of Two-Dimensional Penta-CN2

It has long been documented in the literature that lone-pair electrons (LPE) are generally thought to lead to low lattice thermal conductivity (κ_L) of bulk materials by inducing strong phonon anharmonicity. Herein, we show an exceptional case of two-dimensional (2D) penta-CN₂ that possesses LPE but exhibits more than doubled κ_L (660.71 W m^-1 K^-1) than the LPE free counterpart of penta-graphene (252.95 W m^-1 K^-1), which is unexpected and contradictory to the traditional theory of LPE leading to low κ_L. Based on the comparative study of four 2D systems possessing LPE and their respective LPE free counterparts (planar C₃N vs graphene and penta-CN₂ vs penta-graphene), the underlying mechanism is found lying in the bonds homogenization in penta-CN₂ due to the wide spatial extension of the nonsymmetrically distributed LPE, which compensates the lattice anharmonicity due to LPE and is responsible for the opposite tendency of LPE-affected κ_L in the four 2D systems.

A C20 fullerene-based sheet with ultrahigh thermal conductivity

A new two-dimensional (2D) carbon allotrope, Hexa-C20, composed of C20 fullerene is proposed. State-of-the-art first principles calculations combined with solving the linearized phonon Boltzmann transport equation confirm that the new carbon structure is not only dynamically and thermally stable, but also can withstand temperatures as high as 1500 K. Hexa-C20 possesses a quasi-direct band gap of 3.28 eV, close to that of bulk ZnO and GaN. The intrinsic lattice thermal conductivity κlat of Hexa-C20 is 1132 W m-1 K-1 at room temperature, which is much larger than those of most carbon materials such as graphyne (82.3 W m-1 K-1) and penta-graphene (533 W m-1 K-1). Further analysis of its phonons uncovers that the main contribution to κlat is from the three-phonon scattering, while the three acoustic branches are the main heat carriers, and strongly coupled with optical phonon branches via an absorption process. The ultrahigh lattice thermal conductivity and an intrinsic wide band gap make the Hexa-C20 sheet attractive for potential thermal management applications.