Orbitally driven low thermal conductivity of monolayer gallium nitride (GaN) with planar honeycomb structure: a comparative study

Synergistically Tuning Thermoelectric Properties of BaCdPF via Strain Engineering

The layered BaCdPF compound with the ZrSiCuAs-type structure emerges as a promising thermoelectric (TE) material due to the excellent electronic transport properties under the p-type doping circumstance. However, the fundamental mechanisms that regulate its thermal and electronic transport under strain engineering remain largely unexplored, hindering its practical applications. In current work, the crystal structure, thermal transport, and electronic transport properties of layered BaCdPF under strain engineering are systematically investigated by first-principles calculations in combination with a machine-learning interatomic potential approach. The BaCdPF maintains mechanical robustness, high dynamic and thermal stabilities across a wide range of strain conditions. The coexistence of band degeneracy and anisotropic band dispersions (heavy and light bands) at the valence band maximum plays a critical role in enhancing electronic transport properties. Under tensile strain, the chemical bond softening and four-phonon scattering favors two-dimensional (2D) phonon transport characteristics, yielding a low lattice thermal conductivity of 1.08 W m-1K-1 at 300 K under a -4% tensile strain. Conversely, compressive strain significantly enhances the carrier mobility, promoting three-dimensional (3D) electronic transport. The current work not only provides effective strategies for optimizing the TE performance of layered BaCdPF but also sheds light on the distinct interplay of 3D charge and 2D phonon transport under strain engineering, which offers broader implications for the design of strain-modulated TE materials.

Theoretical investigation of the ultralow thermal conductivity of 2D PbTe via a strain regulation method

To systematically develop an efficient computational protocol for discovering high-performance materials with desirable thermal conductivity, it is essential to theoretically understand the key factors influencing their heat transport capacity. Recent advancements in first-principles based calculations have significantly facilitated the exploration of structural and electronic properties of nanoscale devices. Efficient identification of the dominant factors impacting the materials' inner heat transport is crucial for their applications in real practice. In this study, we optimized our computation protocol originally proposed for thermal conductivity investigations and extended it to explore heat transport within a 2-dimensional (2D) alloying material, PbTe. We delved into the structural and electronic properties of PbTe in detail. Additionally, to assess the thermodynamic stability and describe the bonding network of this 2D alloying material, we calculated its phonon dispersion at different strains. The heat transport mechanism within PbTe has been systematically investigated. The insights gained from this work are instructive for future studies, particularly those focusing on the evaluation of 2D materials' thermoelectric performance. This research contributes to a broader understanding of how structural and electronic properties influence the inner heat transport of 2D materials, establishing a theoretical foundation for the development of high-performance thermal devices.

Thermoelectric Properties of a Light Compound Fe2S2: the Role of Electron Correlation Strengthened Spin-Orbital Coupling

Spin-orbit coupling (SOC) induced nontrivial bandgap and complex Fermi surface has been considered to be profitable for thermoelectrics, which, however, is generally appreciable only in heavy elements, thereby detrimental to practical application. In this study, the SOC-driven extraordinary thermoelectric performance in a light 2D material Fe₂S₂ is demonstrated via first-principles calculations. The abnormally strong SOC, induced by electron correlation through 3d orbitals polarization, significantly renormalizes the band structures, which opens the bandgap via Fe 3d orbitals inversion, exposes the second conduction valley with weak electron-phonon coupling, and aligns the energy of Fe 3d and S 3p orbitals with divergent momentum in valence band. Such topological band renormalization triggers improvement of both p- and n-type power factors by more than 200%. Combining with the low lattice thermal conductivity caused by lone pair electrons and intense high-order phonon scattering, the peak zT can reach 1.6 and 1.8 for p- and n-type Fe₂S₂ at 400 K, respectively. This work unravels the mechanism of SOC-provoked high zT in electron correlation systems, which inspires the development of high-performance thermoelectric materials without heavy and scarce elements.

Two-Dimensional MoSi2N4 Family: Progress and Perspectives Form Theory

Recently, a new two-dimensional (2D) layered MoSi2N4 has been successfully synthesized by chemical vapor deposition without knowing the 3D counterparts [ Science 2020, 369, 670-674]. The unique septuple-atomic-layer structure and diverse composition of MoSi2N4 have drawn tremendous interest in studying 2D MA2Z4 systems based on the MoSi2N4 structure. As an emerging family of 2D materials, MA2Z4 materials exhibit a wide range of properties and excellent tunability, making them highly promising for various applications. Herein, we summarize recent significant progress in property characterization of the MA2Z4 family. The electronic, magnetic, thermal transport, and superconducting properties, including their tunability through strain engineering and elemental substitution, are presented and elaborated in detail. Further perspectives and new opportunities of the emerging MA2Z4 family are presented at the end of this Perspective.

Study on the effects of strain and electrostatic doping on the magnetic anisotropy of GaN/VTe2van der waals heterostructure

Using a first-principles approach, this study delves into the effects of strain and electrostatic doping on the electronic and magnetic properties of the GaN/VTe2van der Waals (vdW) heterostructure. The results reveal that when the GaN/VTe2vdW heterostructure is doped with 0.1h/0.2hof electrostatic charge, its magnetization direction undergoes a remarkable reversal, shifting from out-of-plane orientation to in-plane direction. Therefore, we conduct a thorough investigation into the influence of electron orbitals on magnetic anisotropy energy. In addition, as the strain changes from -1% to 1%, the 100% spin polarization region of the GaN/VTe2vdW heterostructure becomes smaller. It is worth noting that at a doping concentration of 0.1h, the GaN/VTe2vdW heterostructure has a Curie temperature of 30 K above room temperature. This comprehensive study provides valuable insights and provides a reference for analyzing the electronic and magnetic properties of low-dimensional systems.

Strain and Substrate-Induced Electronic Properties of Novel Mixed Anion-Based 2D ScHX2 (X = I/Br) Semiconductors

Exploration of compounds featuring multiple anions beyond the single-oxide ion, such as oxyhalides and oxyhydrides, offers an avenue for developing materials with the prospect of novel functionality. In this paper, we present the results for a mixed anion layered material, ScHX2 (X: Br, I) based on density functional theory. The result predicted the ScHX2 (X: Br, I) monolayers to be stable and semiconducting. Notably, the electronic and mechanical properties of the ScHX2 monolayers are comparable to well-established 2D materials like graphene and MoS2, rendering them highly suitable for electronic devices. Additionally, these monolayers exhibit an ability to adjust their band gaps and band edges in response to strain and substrate engineering, thereby influencing their photocatalytic applications.

Monolayer 1T-Ag6S2 with Excellent Thermoelectric Properties

We obtained a new material called monolayer 1T-Ag6S2 by replacing metal atoms in 1T phase transition-metal dichalcogenide sulfides (TMDs) with octahedral Ag6 clusters. Subsequently, the thermoelectric transport properties of monolayer 1T-Ag6S2 were systematically investigated using first-principles calculations and the generalized gradient approximation (GGA-PBE) exchange correlation functional. The findings demonstrate that monolayer 1T-Ag6S2 displays characteristics of a wide-bandgap semiconductor, with a bandgap of 2.48 eV. Notably, the incorporation of Ag6 clusters disrupts the structural symmetry, effectively enhancing the electronic structure and phonon properties of the material. Due to the flat valence band near the Fermi level, the extended relaxation time of the hole results in a greater effective mass compared to the electron, leading to a significant increase in the Seebeck coefficient. Under optimal doping conditions, the power factor of monolayer 1T-Ag6S2 can achieve 14.9 mW/mK2 at 500 K. The intricate crystal structure induces phonon path bending, reduces the overall frequency of phonon vibrations (<10 THz), and causes hybridization of low-frequency optical and acoustic branches, resulting in remarkably low lattice thermal conductivity (0.20 and 0.17 W/mK along the x and y axes at 500 K, respectively). The monolayer 1T-Ag6S2 demonstrates a remarkably high figure of merit ZT of 3.14 (3.15) on the x (y) axis at 500 K, significantly higher than those of conventional TMD materials. Such excellent thermoelectric properties suggest that monolayer 1T-Ag6S2 is a promising thermoelectric (TE) material. Our work reveals the deep mechanism of cluster substitution to optimize the thermoelectric properties of materials and provides a useful reference for subsequent research.

Investigation of the lattice thermal transport properties of Janus XClO (X = Cr, Ir) monolayers by first-principles calculations

In the context of the global energy crisis, the development of high-performance heat transport devices within nano scales has become increasingly important. Theoretical discovery and evaluation of novel structures with high performance in thermal conductivity by affordable calculations could provide significant instructions for experimental studies focusing on thermoelectric device development. For 2-dimensional (2D) functional materials, their heat transport efficiency is correlated with their electronic properties and structural features. In this study, we computationally investigated the heat transport within Janus XClO (X = Cr, Ir); its structural and electronic properties were well solved by first-principles calculations. Furthermore, to evaluate thermodynamics stability and applicability, ab initio molecular dynamics (AIMD) simulations are conducted. Through a benchmarking study upon these XClO monolayers with different compositions, we noticed that their heat transport efficiency is associated with the percentage of doped magnetic atoms. The theoretical insights provided by this study are highly instructive for future experimental studies focusing on thermal device development.

Biaxial strain modulated electronic structures of layered two-dimensional MoSiGeN4 Rashba systems

The two-dimensional (2D) MA2Z4 family has received extensive attention in manipulating its electronic structure and achieving intriguing physical properties. However, engineering the electronic properties remains a challenge. Herein, based on first-principles calculations, we systematically investigate the effect of biaxial strains on the electronic structure of 2D Rashba MoSiGeN4 (MSGN), and further explore how the interlayer interactions affect the Rashba spin splitting (RSS) in such strained layered MSGN systems. After applying biaxial strains, the band gap decreases monotonically with increasing tensile strains but increases when the compressive strains are applied. An indirect-direct-indirect band gap transition is induced by applying a moderate compressive strain (<5%) in the MSGN systems. Due to the symmetry breaking and moderate spin-orbit coupling (SOC), the monolayer MSGN possesses an isolated RSS near the Fermi level, which could be effectively regulated to the Lifshitz-type spin splitting (LSS) by biaxial strain. For instance, the LSS ← RSS → LSS transformation of the Fermi surface is presented in the monolayer and a more complex and changeable LSS ← RSS → LSS → RSS evolution is observed in bilayer and trilayer MSGN systems as the biaxial strain varies from -8% to 12%, which actually depends on the appearance, variation, and vanish of the Mexican hat band in the absence of SOC under different strains. The contribution of the Mo-dz2 orbital hybridized with the N-pz orbital in the highest valence band plays a dominant role in band evolution under biaxial strains, where the RSS → LSS evolution corresponds to the decreased Mo-dz2 orbital contribution. Our study highlights the biaxial strain controllable RSS, in particular the introduction and even the evolution of LSS near the Fermi surface, which makes the strained MSGN systems promising candidates for future applications in spintronic devices.

The intrinsically low lattice thermal conductivity of monolayer T-Au6X2 (X = S, Se and Te)

Thermal conductivity (κ, which consists of electronic thermal conductivity κe and lattice thermal conductivity κl), as an essential parameter in thermal management applications, is a critical physical quantity to measure the heat transfer performance of materials. To seek low-κ materials for heat-related applications, such as thermoelectric materials and thermal barrier coatings. In this study, based on a complex cluster design, we report a new class of two-dimensional (2D) transition metal dichalcogenides (TMDs): T-Au6X2 (X = S, Se, and Te) with record ultralow κl values. At room temperature, the κl values of T-Au6S2, T-Au6Se2, and T-Au6Te2 are 0.25 (0.23), 0.30 (0.21), and 0.12 (0.10) W m-1 K-1 along the x-axis (y-axis) direction, respectively, exhibiting good thermal insulation. The ultralow κl originates from strong phonon softening and suppression, especially for the phonon with frequency 0-1 THz. In addition, T-Au6Te2 holds the lowest group velocity and phonon relaxation time among the three T-Au6X2 monolayers. Our study provides an alternative approach for achieving ultralow κl through complex cluster replacement. Meanwhile, this new class of TMDs is expected to shine in thermal insulation and thermoelectricity due to their ultralow κl values.

Alloying Reversed Anisotropy of Thermal Transport in Bulk Al0.5Ga0.5N

Anisotropic heat transfer is crucial for advanced thermal management in nanoelectronics, optoelectronics, thermoelectrics, etc. Traditional approaches modifying thermal conductivity (κ) mostly adjust the magnitude but disregard anisotropy. Herein, by solving the Boltzmann transport equation from first principles, we report κ anisotropy modulation by alloying gallium nitride (GaN) and aluminum nitride (AlN). The alloyed Al0.5Ga0.5N demonstrates reversed κ anisotropy compared to the parent materials, where the preferred thermal transport direction shifts from cross-plane to in-plane. Moreover, the κ anisotropy (κin-plane/κcross-plane) in the Al0.5Ga0.5N alloy is enhanced to 1.63 and 1.51 times that in bulk GaN and AlN, respectively, which can be further enhanced by increased temperature. Deep analysis attributes the alloying reversed κ anisotropy of Al0.5Ga0.5N to the structure distortion-driven phonon group velocity, as well as phonon anharmonicity. The alloying reversed κ anisotropy as reported in this study sheds light on future studies in advanced heat dissipation and intelligent thermal management.

Thermal transport and phonon localization in periodich-GaN/h-AlN superlattices

The widely observed non-diffusive phonon thermal transport phenomenon in nanostructures is largely attributed to classical size effects, which ignore the characteristic of phonon wave. In this context, the crossover transition process from incoherent to coherent phonon transport in two-dimensional heterogeneous periodich-GaN/h-AlN superlattices is demonstrated using a non-equilibrium molecular dynamics approach, where the localization behavior of thermal phonons is particularly significant. The results show that the thermal transport of the superlattice structure is affected by a combination of structural parameters and temperature. The thermal conductivity (TC) of the superlattice decreases and then increases as the interface density increases. Phonon-interface scattering dominates the incoherent phonon transport, while local phonons modulate the transport in the coherent region. Thus, the competition between phonon wave and particle properties causes the transition from incoherent to coherent phonon transport. In addition, as the TC valley depth slows down with increasing system temperature, the scattering of medium and high frequency phonons is enhanced and the phonon lifetime decreases. Research on localized phonons in superlattices provides theoretical support for thermal transport regulation in basal low-dimensional materials.

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.

Revealing the electronic, optical and photocatalytic properties of PN-M2CO2 (P = Al, Ga; M = Ti, Zr, Hf) heterostructures

Using DFT, the electronic structure, optical, and photocatalytic properties of PN (P = Ga, Al) and M2CO2 (M = Ti, Zr, Hf) monolayers and their PN-M2CO2 van der Waals heterostructures (vdWHs) are investigated. Optimized lattice parameters, bond length, bandgap, conduction and valence band edges show the potential of PN (P = Ga, Al) and M2CO2 (M = Ti, Zr, Hf) monolayers in photocatalytic applications, and the application of the present approach to combine these monolayers and form vdWHs for efficient electronic, optoelectronic and photocatalytic applications is shown. Based on the same hexagonal symmetry and experimentally achievable lattice mismatch of PN (P = Ga, Al) with M2CO2 (M = Ti, Zr, Hf) monolayers, we have fabricated PN-M2CO2 vdWHs. Binding energies, interlayer distance and AIMD calculations show the stability of PN-M2CO2 vdWHs and demonstrate that these materials can be easily fabricated experimentally. The calculated electronic band structures show that all the PN-M2CO2 vdWHs are indirect bandgap semiconductors. Type-II[-I] band alignment is obtained for GaN(AlN)-Ti2CO2[GaN(AlN)-Zr2CO2 and GaN(AlN)-Hf2CO2] vdWHs. PN-Ti2CO2 (PN-Zr2CO2) vdWHs with a PN(Zr2CO2) monolayer have greater potential than a Ti2CO2(PN) monolayer, indicating that charge is transfer from the Ti2CO2(PN) to PN(Zr2CO2) monolayer, while the potential drop separates charge carriers (electron and holes) at the interface. The work function and effective mass of the carriers of PN-M2CO2 vdWHs are also calculated and presented. A red (blue) shift is observed in the position of excitonic peaks from AlN to GaN in PN-Ti2CO2 and PN-Hf2CO2 (PN-Zr2CO2) vdWHs, while significant absorption for photon energies above 2 eV for AlN-Zr2CO2, GaN-Ti2CO2 and PN-Hf2CO2, give them good optical profiles. The calculated photocatalytic properties demonstrate that PN-M2CO2 (P = Al, Ga; M = Ti, Zr, Hf) vdWHs are the best candidates for photocatalytic water splitting.

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.

Activated Lone-Pair Electrons Lead to Low Lattice Thermal Conductivity: A Case Study of Boron Arsenide

Reducing thermal conductivity (κ) is of great significance to lots of applications, such as thermal insulation, thermoelectrics, etc. In this study, we propose an effective approach for realizing low κ by introducing lone-pair electrons or making the lone-pair electrons stereochemically active through bond nanodesigning. By cutting at the (111) cross section of the three-dimensional cubic boron arsenide (c-BAs), the κ is lowered by more than 1 order of magnitude in the resultant two-dimensional graphene-like BAs (g-BAs). The underlying mechanism of activating lone-pair electrons is analyzed based on the comparative study on the thermal transport properties and electronic structures of g-BAs, c-BAs, graphene, and diamond (c-BAs → g-BAs vs diamond → graphene). The proposed approach for realizing low κ and the underlying mechanism uncovered in this study would largely benefit the design of advanced thermal functional materials, especially in future research involving novel materials for energy applications.

Thermal Transport in 2D Materials

In recent decades, two-dimensional materials (2D) such as graphene, black and blue phosphorenes, transition metal dichalcogenides (e.g., WS₂ and MoS₂), and h-BN have received illustrious consideration due to their promising properties. Increasingly, nanomaterial thermal properties have become a topic of research. Since nanodevices have to constantly be further miniaturized, thermal dissipation at the nanoscale has become one of the key issues in the nanotechnology field. Different techniques have been developed to measure the thermal conductivity of nanomaterials. A brief review of 2D material developments, thermal conductivity concepts, simulation methods, and recent research in heat conduction measurements is presented. Finally, recent research progress is summarized in this article.

The record low thermal conductivity of monolayer cuprous iodide (CuI) with a direct wide bandgap

Two-dimensional materials have attracted significant research interest due to the fantastic properties that are unique to their bulk counterparts. In this paper, from the state-of-the-art first-principles, we predicted the stable structure of a monolayer counterpart of γ-CuI (cuprous iodide) that is a p-type wide bandgap semiconductor. The monolayer CuI presents multifunctional superiority in terms of electronic, optical, and thermal transport properties. Specifically, the ultralow thermal conductivity of 0.116 W m^-1 K^-1 is predicted for monolayer CuI, which is much lower than those of γ-CuI (0.997 W m^-1 K^-1) and other typical semiconductors. Moreover, an ultrawide direct bandgap of 3.57 eV is found in monolayer CuI, which is even larger than that of γ-CuI (2.95-3.1 eV), promising for applications in nano-/optoelectronics with better optical performance. The ultralow thermal conductivity and direct wide bandgap of monolayer CuI as reported in this study would promise its potential applications in transparent and wearable electronics.

Significantly suppressed thermal transport by doping In and Al atoms in gallium nitride

Thermal transport plays a key role in the working stability of gallium nitride (GaN) based optoelectronic devices, where doping has been widely employed for practical applications. However, it remains unclear how doping affects thermal transport. In this study, based on first-principles calculations, we studied the doping effect on the thermal transport properties of GaN by substituting Ga with In/Al atoms. The thermal conductivities at 300 K along the in-plane(out-of-plane) directions of In- and Al-doped GaN are calculated to be 7.3(8.62) and 12.45(11.80) W m^-1 K^-1, respectively, which are more than one order of magnitude lower compared to that of GaN [242(239) W m^-1 K^-1]. From the analysis of phonon transport properties, we find that the low phonon group velocity and small phonon relaxation time dominate the degenerated thermal conductivity, which originated from the strong phonon anharmonicity of In/Al-doped GaN. Furthermore, by examining the crystal structure and electronic properties, the lowered thermal conductivity is revealed lying in the strong polarization of In-N and Al-N bonds, which is due to the large difference in electronegativity of In/Al and N atoms. The results achieved in this study have guiding significance to the thermal transport design of GaN-based optoelectronic devices.

Nanostructure engineering of two-dimensional diamonds toward high thermal conductivity and approaching zero Poisson's ratio

Two-dimensional diamond, also called diamane, has attracted great research attention for its novel physical properties and potential applications in nanoelectronics, ultrasensitive resonators and thermal management. Compared with the hexagonal diamane, the physical properties of the rectangular diamane are less explored. In this work, using first-principles calculations, we conducted a comprehensive study on the electronic, phononic, thermal and mechanical properties of three types of rectangular diamanes. We found that rectangular diamanes possess a high Debye temperature (722-788 K) and a strong in-plane Young's modulus (405.9-575.9 N m^-1). We further show close to zero Poisson's ratio in the rectangular Pmma diamane. Moreover, based on the phonon Boltzmann transport equation, high room temperature lattice thermal conductivity (910-1807 W m^-1 K^-1) and strong configuration and orientation dependence are demonstrated. Phonon group velocity, relaxation time and characteristic square velocity are explored and it is demonstrated that phonon harmonic behavior is responsible for the remarkable configuration dependent thermal conductivity in rectangular diamanes. The present work underscores the use of nanostructure engineering to manipulate thermal conductivity of 2D diamond, which provides opportunities for developing effective thermal channeling devices.

Tensile Mechanical Behavior and the Fracture Mechanism in Monolayer Group-III Nitrides XN (X= Ga, In): Effect of Temperature and Point Vacancies

In this study, we have thoroughly investigated the tensile mechanical behavior of monolayer XN (X = Ga, In) using molecular dynamics simulations. The effects of temperature (100 to 800 K) and point vacancies (PVs, 0.1 to 1%) on fracture stress, strain, and elastic modulus of GaN and InN are studied. The effects of edge chiralities on the tensile mechanical behavior of monolayer XN are also explored. We find that the elastic modulus, tensile strength, and fracture strain reduce with increasing temperature. The point defects cause the stress to be condensed in the vicinity of the vacancies, resulting in straightforward damage. On the other hand, all the mechanical behaviors such as fracture stress, elastic modulus, and fracture strain show substantial anisotropic nature in these materials. To explain the influence of temperature and PVs, the radial distribution function (RDF) at diverse temperatures and potential energy/atom at different vacancy concentrations are calculated. The intensity of the RDF peaks decreases with increasing temperature, and the presence of PVs leads to an increase in potential energy/atom. The current work provides an insight into adjusting the tensile mechanical behaviors by making vacancy defects in XN (X = Ga, In) and provides a guideline for the applications of XN (X = Ga, In) in flexible nanoelectronic and nanoelectromechanical devices.

Infinite possibilities of ultrathin III-V semiconductors: Starting from synthesis

Ultrathin III-V semiconductors have been receiving tremendous research interest over the past few years. Owing to their exotic structures, excellent physical and chemical properties, ultrathin III-V semiconductors are widely applied in the field of electronics, optoelectronics, and solar energy. However, the strong chemical bonds in layers are the bottleneck of the two-dimensionalization preparation process, which hinders the further development of ultrathin III-V semiconductors. Some effective methods to synthesize ultrathin III-V semiconductors have been reported recently. In this perspective, we briefly introduce the structures and properties of ultrathin III-V semiconductors firstly. Then, we comprehensively summarize the synthetic strategies of ultrathin III-V semiconductors, mainly focusing on space confinement, atomic substitution, adhesion energy regulation, and epitaxial growth. Finally, we summarize the current challenges and propose the development directions of ultrathin III-V semiconductors in the future.

Vacancy-Induced Thermal Transport and Tensile Mechanical Behavior of Monolayer Honeycomb BeO

Because of the rapid shrinking trend of integrated circuits, the performances of nanodevices and nanomechanical systems are greatly affected by the joule heating and mechanical failure dilemma. In addition, structural defects are inevitable during experimental synthesis of nanomaterials, which may alter their physical properties significantly. Investigation of the thermal transport and mechanical behavior of nanostructured materials with structural defects is thus a crucial requirement. In this study, the thermal conductivity (TC) and tensile mechanical behavior of monolayer honeycomb BeO are systematically explored using molecular dynamics simulations. An infinite length bulk TC of ∼277.77 ± 8.93 W/mK was found for the pristine monolayer BeO. However, the insertion of 1% single vacancy (SV) and double vacancy (DV) defects reduces the TC by ∼36.98 and ∼33.52%, respectively. On the other hand, the uniaxial tensile loading produces asymmetrical fracture stress, elastic modulus, and fracture strain behaviors in the armchair and zigzag directions. The elastic modulus was reduced by ∼4.7 and ∼6.6% for 1% SV defects along the armchair and zigzag directions, respectively, whereas the reduction was ∼2.7 and ∼ 5.1% for 1% DV defects. Moreover, because of the strong symmetry-breaking effect, both the TC and mechanical strength were significantly lower for the SV defects than those for the DV defects. The highly softening and decreasing trends of the phonon modes with increasing vacancy concentration and temperature, respectively, were noticed for both types of defects, resulting in a reduction of the TC of the defected structures. These findings will be helpful for the understanding of the heat transport and mechanical characteristics of monolayer BeO as well as provide guidance for the design and control of BeO-based nanoelectronic and nanoelectromechanical devices.

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.

Large exciton binding energy, superior mechanical flexibility, and ultra-low lattice thermal conductivity in BiI3monolayer

The exciton binding energy, mechanical properties, and lattice thermal conductivity of monolayer BiI₃are investigated on the basis of first principle calculation. The excitation energy of monolayer BiI₃is predicted to be 1.02 eV, which is larger than that of bulk BiI₃(0.224 eV). This condition is due to the reduced dielectric screening in systems. The monolayer can withstand biaxial tensile strain up to 30% with ideal tensile strength of 2.60 GPa. Compared with graphene and MoS₂, BiI₃possesses superior flexibility and ductility due to its large Poisson's ratio and smaller Young's modulus by two orders of magnitude. The predicted lattice thermal conductivityk_Lof monolayer BiI₃is 0.247 W m^-1 K^-1at room temperature, which is lower than most reported values for other 2D materials. Such ultralowk_Lresults from the scattering between acoustic and optical phonon modes, heavy atomic mass, and relatively weak chemical bond.

2D GaN for Highly Reproducible Surface Enhanced Raman Scattering

Surface-enhanced Raman scattering (SERS) based on 2D semiconductors has been rapidly developed due to their chemical stability and molecule-specific SERS activity. High signal reproducibility is urgently required towards practical SERS applications. 2D gallium nitride (GaN) with highly polar Ga-N bonds enables strong dipole-dipole interactions with the probe molecules, and abundant DOS (density of states) near its Fermi level increases the intermolecular charge transfer probability, making it a suitable SERS substrate. Herein, 2D micrometer-sized GaN crystals are demonstrated to be sensitive SERS platforms with excellent signal reproducibility and stability. Strong dipole-dipole interaction between the dye molecule and 2D GaN enhances the molecular polarizability. Furthermore, 2D GaN benefits its SERS enhancement by the combination of increased DOS and more efficient charge transfer resonances when compared with its bulk counterpart.

Unique Arrangement of Atoms Leads to Low Thermal Conductivity: A Comparative Study of Monolayer Mg2C

Two-dimensional Mg₂C, one of the typical representative MXene materials, is attracting lots of attention due to its outstanding properties. In this study, we find the thermal conductivity of monolayer Mg₂C is more than 2 orders of magnitude lower than graphene and is even lower than MoS₂ despite the relatively lighter atoms of Mg and C. Based on the comparative analysis with graphene, silicene, and MoS₂, the underlying mechanism is found lying in the unique arrangement of atoms (lighter atoms in the middle plane) and large electronegativity difference in Mg₂C. The phonon anharmonicity is strong due to the resonant bonding. In addition, dual band gaps emerge in the phonon dispersion of Mg₂C, which limit the phonon-phonon scattering and reduce the phonon relaxation time. This study reveals a new mechanism responsible for low thermal conductivity, which would be helpful for designing thermal functional materials and pave the way for applications in thermoelectrics.

Potential thermoelectric materials: first-principles prediction of low lattice thermal conductivity of two-dimensional (2D) orthogonal ScX2 (X = C and N) compounds

Thermoelectric materials with excellent performance can efficiently and directly convert waste heat into electrical energy. In today's era, finding thermoelectric materials with excellent performance and adjusting the thermoelectric parameters are essential for the sustainable development of energy in the context of the energy crisis and global warming. Through first-principles calculations, we notice that two-dimensional (2D) orthogonal ScX₂ (X = C and N) compounds show great potential in the field of thermoelectricity. Different from most materials containing C or N atoms, which are generally accompanied by high lattice thermal conductivity (TC), the 2D o-ScX₂ exhibited a rather low and anisotropic lattice TC. The κ3L (the lattice thermal conductivity including the effect of three-phonon scattering and isotope scattering) of o-ScC₂ along the X and Y directions are 2.79 W m^-1 K^-1 and 1.55 W m^-1 K^-1, and those of o-ScN₂ are 1.57 W m^-1 K^-1 and 0.56 W m^-1 K^-1. By calculating the fourth-order interatomic force constants (IFCs), we obtain the κ3+4L with the additional four-phonon scattering effect. Our results clearly show that four-phonon scattering plays an important role in the TC of the two materials, the κ3+4L of o-ScC₂ is only half of its κ3L. Furthermore, it can be noticed that the low lattice TCs of o-ScX₂ (X = C and N) are the result of many factors, e.g., heavy atom doping, the strong anharmonicity caused by the vibration of Sc atoms in the out-of-plane direction and C(N) atoms in the in-plane direction, important four-phonon scattering and strongly polarized covalent bonds between X atoms and Sc atoms. Moreover, it is interesting to find that the thermal transport properties of o-ScX₂ are led by a different phonon mechanism, e.g., the different TCs of o-ScC₂ and o-ScN₂ are determined by the anharmonic characteristic, and the harmonic characteristic plays a more important role in the anisotropy of o-ScX₂ (X = C and N). In general, our research can be expected to provide important guidance for the application of o-ScX₂ (X = C and N) in the thermoelectric field.

Effect of hydrogenation on the thermal conductivity of 2D gallium nitride

The indirect bandgap of two-dimensional GaN hinders its application in the optical field. Hydrogenation can convert the bandgap type of the GaN monolayer from an indirect to a direct one and also tune the bandgap size. The thermal transport, an important property in the application of two-dimensional materials, is also influenced by hydrogenation. By performing first-principles calculations and solving the phonon Boltzmann equation, we investigate the effect of hydrogenation on the thermal conductivity of the GaN monolayer. The results show that hydrogenation will slightly increase the thermal conductivity of the GaN monolayer from 70.62 Wm^-1 K^-1 to 76.23 Wm^-1 K^-1 at 300 K. The little effect of hydrogenation on thermal conductivity is mainly dominated by two competing factors: (1) the reduction of ZA mode lifetime due to the breaking of reflection symmetry after hydrogenation and (2) the increased contribution from TA and LA modes due to the reduction of anharmonic scattering caused by the enlarged phonon bandgap after hydrogenation. The results are compared with other two-dimensional materials with hexagonal monolayer structures.

Tunable electronic and optical properties in buckling a non-lamellar B3S monolayer

We propose a novel polymorph of a hexagonal B₃S monolayer by combing structure swarm intelligence and first-principles calculations. Phonon spectrum analysis and ab initio molecular dynamics simulation indicate that the new structure is dynamically and thermally stable. Furthermore, the structure is mechanically stable and has a satisfactory elastic modulus. Our results show that the B₃S monolayer is a semiconductor with strong visible-light optical absorption. More importantly, the electronic properties of the structure are tunable via surface functionalization. For example, hydrogenation or fluorination could transform the monolayer from the semiconducting to metallic state. On the other hand, surface oxidation could significantly enhance both carrier mobility and near-infrared optical absorption. Furthermore, we also discovered that the monolayer possesses satisfactory storage capacity for H₂.

Two-dimensional group-III nitrides and devices: a critical review

As third-generation semiconductors, group-III nitrides are promising for high power electronic and optoelectronic devices because of their wide bandgap, high electron saturation mobility, and other unique properties. Inspired by the thickness-dependent properties of two-dimensional (2D) materials represented by graphene, it is predicted that the 2D counterparts of group-III nitrides would have similar properties. However, the preparation of 2D group-III nitride-based materials and devices is limited by the large lattice mismatch in heteroepitaxy and the low rate of lateral migration, as well as the unsaturated dangling bonds on the surfaces of group-III nitrides. The present review focuses on theoretical and experimental studies on 2D group-III nitride materials and devices. Various properties of 2D group-III nitrides determined using simulations and theoretical calculations are outlined. Moreover, the breakthroughs in their synthesis methods and their underlying physical mechanisms are detailed. Furthermore, devices based on 2D group-III nitrides are discussed accordingly. Based on recent progress, the prospect for the further development of the 2D group-III nitride materials and devices is speculated. This review provides a comprehensive understanding of 2D group-III nitride materials, aiming to promote the further development of the related fields of nano-electronic and nano-optoelectronics.

Schottky barrier heights in two-dimensional field-effect transistors: from theory to experiment

Over the past decade, two-dimensional semiconductors (2DSCs) have aroused wide interest due to their extraordinary electronic, magnetic, optical, mechanical, and thermal properties, which hold potential in electronic, optoelectronic, thermoelectric applications, and so forth. The field-effect transistor (FET), a semiconductor gated with at least three terminals, is pervasively exploited as the device geometry for these applications. For lack of effective and stable substitutional doping techniques, direct metal contact is often used in 2DSC FETs to inject carriers. A Schottky barrier (SB) generally exists in the metal-2DSC junction, which significantly affects and even dominates the performance of most 2DSC FETs. Therefore, low SB or Ohmic contact is highly preferred for approaching the intrinsic characteristics of the 2DSC channel. In this review, we systematically introduce the recent progress made in theoretical prediction of the SB height (SBH) in the 2DSC FETs and the efforts made both in theory and experiments to achieve low SB contacts. From the comparison between the theoretical and experimentally observed SBHs, the emerging first-principles quantum transport simulation turns out to be the most powerful theoretical tool to calculate the SBH of a 2DSC FET. Finally, we conclude this review from the viewpoints of state-of-the-art electrode designs for 2DSC FETs.

The exceptionally high thermal conductivity after 'alloying' two-dimensional gallium nitride (GaN) and aluminum nitride (AlN)

Alloying is a widely employed approach for tuning properties of materials, especially for thermal conductivity which plays a key role in the working liability of electronic devices and the energy conversion efficiency of thermoelectric devices. Commonly, the thermal conductivity of an alloy is acknowledged to be the smallest compared to the parent materials. However, the findings in this study bring some different points of view on the modulation of thermal transport by alloying. The thermal transport properties of monolayer GaN, AlN, and their alloys of Ga _x Al_1-x N are comparatively investigated by solving the Boltzmann transport equation (BTE) based on first-principles calculations. The thermal conductivity of Ga_0.25Al_0.75N alloy (29.57 Wm^-1 K^-1) and Ga_0.5Al_0.5N alloy (21.49 Wm^-1 K^-1) are found exceptionally high to be between AlN (74.42 Wm^-1 K^-1) and GaN (14.92 Wm^-1 K^-1), which violates the traditional knowledge that alloying usually lowers thermal conductivity. The mechanism resides in that, the existence of Al atoms reduces the difference in atomic radius and masses of the Ga_0.25Al_0.75N alloy, which also induces an isolated optical phonon branch around 18 THz. As a result, the scattering phase space of Ga_0.25Al_0.75N is largely suppressed compared to GaN. The microscopic analysis from the orbital projected electronic density of states and the electron localization function further provides insight that the alloying process weakens the polarization of bonding in Ga_0.25Al_0.75N alloy and leads to the increased thermal conductivity. The exceptionally high thermal conductivity of the Ga _x Al_1-x N alloys and the underlying mechanism as revealed in this study would bring valuable insight for the future research of materials with applications in high-performance thermal management.

Structure and electronic bandgap tunability of m-plane GaN multilayers

Two-dimensional (2D) gallium nitride (GaN) has attracted a lot of attention due to its promising applications in photoelectric nano-devices. Most previous research studies have focused on polar c-plane 2D structures. Here, by employing first principles calculations, we systematically investigate the structural and electronic properties of non-polar m-plane GaN with different numbers of atomic layers. The results show a layer-dependent structure transition and electronic band variation for m-plane GaN. It is found that the monolayer keeps a planar hexagonal structure due to sp2 hybridization, whereas the multilayers are formed by stacking of buckled hexagonal monolayers with unsaturated coordination number at the surface sublayer and bulk-like inner layers. These discrepancies in the structure further induce an indirect to direct transition of the band gap type when the layer number reaches twelve. By carefully examining the relationship between the structure and electronic bandgap, we find that the indirect bandgap comes from the unsaturated surface with a planar like structure. On surface modification, saturation of the surface dangling bonds results in an indirect to direct band gap transition.

Lateral and flexural thermal transport in stanene/2D-SiC van der Waals heterostructure

Thermal management is one of the key challenges in nanoelectronic and optoelectronic devices. The development of a van der Waals heterostructure (vdWH) using the vertical positioning of different two-dimensional (2D) materials has recently appeared as a promising way of attaining desirable electrical, optical, and thermal properties. Here, we explore the lateral and flexural thermal conductivity of stanene/2D-SiC vdWH utilizing the reverse non-equilibrium molecular dynamics simulation and transient pump-probe technique. The effects of length, area, coupling strength and temperature on the thermal transport are studied systematically. The projected lateral thermal conductivity of a stanene/2D-SiC hetero-bilayer is found to be 66.67 [Formula: see text], which is greater than stanene, silicene, germanene, MoSe₂ and even higher than some hetero-bilayers, including MoS₂/MoSe₂ and stanene/silicene. The lateral thermal conductivity increases as the length increases, while it tends to decrease with increasing temperature. The computed flexural interfacial thermal resistance between stanene and 2D-SiC is 3.0622 [Formula: see text] [Formula: see text] K.m² W^-1, which is close to other 2D hetero-bilayers. The interfacial resistance between stanene and 2D-SiC is reduced by 70.49% and 50.118% as the temperature increases from 100 K to 600 K and the coupling factor increases from [Formula: see text] to [Formula: see text], respectively. In addition, various phonon modes are evaluated to disclose the fundamental mechanisms of thermal transport in the lateral and flexural direction of the hetero-bilayer. These results are important in order to understand the heat transport phenomena of stanene/2D-SiC vdWH, which could be useful for enhancing their promising applications.

Universal growth of ultra-thin III-V semiconductor single crystals

Ultra-thin III-V semiconductors, which exhibit intriguing characteristics, such as two-dimensional (2D) electron gas, enhanced electron-hole interaction strength, and strongly polarized light emission, have always been anticipated in future electronics. However, their inherent strong covalent bonding in three dimensions hinders the layer-by-layer exfoliation, and even worse, impedes the 2D anisotropic growth. The synthesis of desirable ultra-thin III-V semiconductors is hence still in its infancy. Here we report the growth of a majority of ultra-thin III-V single crystals, ranging from ultra-narrow to wide bandgap semiconductors, through enhancing the interfacial interaction between the III-V crystals and the growth substrates to proceed the 2D layer-by-layer growth mode. The resultant ultra-thin single crystals exhibit fascinating properties of phonon frequency variation, bandgap shift, and giant second harmonic generation. Our strategy can provide an inspiration for synthesizing unexpected ultra-thin non-layered systems and also drive exploration of III-V semiconductor-based electronics.

Vibrational hierarchy leads to dual-phonon transport in low thermal conductivity crystals

Many low-thermal-conductivity (κL) crystals show intriguing temperature (T) dependence of κL: κL ∝ T-1 (crystal-like) at intermediate temperatures whereas weak T-dependence (glass-like) at high temperatures. It has been in debate whether thermal transport can still be described by phonons at the Ioffe-Regel limit. In this work, we propose that most phonons are still well defined for thermal transport, whereas they carry heat via dual channels: normal phonons described by the Boltzmann transport equation theory, and diffuson-like phonons described by the diffusion theory. Three physics-based criteria are incorporated into first-principles calculations to judge mode-by-mode between the two phonon channels. Case studies on La2Zr2O7 and Tl3VSe4 show that normal phonons dominate low temperatures while diffuson-like phonons dominate high temperatures. Our present dual-phonon theory enlightens the physics of hierarchical phonon transport as approaching the Ioffe-Regel limit and provides a numerical method that should be practically applicable to many materials with vibrational hierarchy.

The effect of non-analytical corrections on the phononic thermal transport in InX (X = S, Se, Te) monolayers

We investigate the effect of non-analytical corrections on the phonon thermal transport properties in two-dimensional indium chalcogenide compounds. The longitudinal optical (LO) and transverse optical (TO) branches in the phonon dispersion are split near the Γ-point. The lattice thermal conductivity of monolayer InS is increased by 30.2% under non-analytical corrections because of the large LO-TO splitting at Γ-point. The predicted lattice thermal conductivities with non-analytical corrections at room temperature are 57.1 W/mK, 44.4 W/mK and 33.1 W/mK for the monolayer InS, InSe and InTe, respectively. The lattice thermal conductivity can be effectively reduced by nanostructures because the representative mean free paths are found very large in these monolayers. By quantifying the relative contribution of the phonon modes to the lattice thermal conductivity, we predict that the longitudinal acoustic branch is the main contributor to the lattice thermal conductivity. Due to the low lattice thermalconductivities of these monolayers, they can be useful in the nanoscale thermoelectric devices.

High thermal conductivity driven by the unusual phonon relaxation time platform in 2D monolayer boron arsenide

The cubic boron arsenide (BAs) crystal has received extensive research attention because of its ultra-high thermal conductivity comparable to that of diamond. In this work, we performed a comprehensive study on the diffusive thermal properties of its two-dimensional (2D) counterpart, the monolayer honeycomb BAs (h-BAs), through solving the phonon Boltzmann transport equation combined with first-principles calculation. We found that unlike the pronounced contribution from out-of-plane acoustic phonons (ZA) in graphene, the high thermal conductivity (181 W m⁻¹ K⁻¹ at 300 K) of h-BAs is mainly contributed by in-plane phonon modes, instead of the ZA mode. This result is explained by the unique frequency-independent ‘platform’ region in the relaxation time of in-plane phonons. Moreover, we conducted a comparative study of thermal conductivity between 2D h-BAs and h-GaN, because both of them have a similar mass density. The thermal conductivity of h-BAs is one order of magnitude higher than that of h-GaN (16 W m⁻¹ K⁻¹), which is governed by the different phonon scattering process attributed to the opposite wavevector dependence in out-of-plane optical phonons. Our findings provide new insight into the physics of heat conduction in 2D materials, and demonstrate h-BAs to be a new thermally conductive 2D semiconductor.

The cubic boron arsenide (BAs) crystal has received extensive research attention because of its ultra-high thermal conductivity comparable to that of diamond.

Modulated thermal conductivity of 2D hexagonal boron arsenide: a strain engineering study

On-going prediction and synthesis of two-dimensional materials attract remarkable attention to engineer high performance intended devices. Through this, comprehensive and detailed uncovering of the material properties could be accelerated to achieve this goal. Hexagonal boron arsenide (h-BAs), a graphene counterpart, is among the most attractive 2D semiconductors. In this work, our objective is to explore the mechanical, electronic, and thermal properties of h-BAs. We found that this novel 2D material can show a high elastic modulus of 260 GPa, which is independent of the loading direction. We also observed that this system shows a direct and narrow band-gap of 1.0 eV, which is highly desirable for electronic applications. The focus of our investigation is to gain an in-depth understanding of the thermal transport along the monolayer h-BAs and further tune the thermal conductivity by strain engineering. In this regard, the thermal conductivity of a stress-free and pristine monolayer was predicted to be 180.2 W m-1 K-1, which can be substantially enhanced to 375.0 W m-1 K-1 and 406.2 W m-1 K-1, with only 3% straining along the armchair and zigzag directions, respectively. The underlying mechanism for such a remarkable boosting of thermal conductivity in h-BAs was correlated to the fact that stretching makes the flexural out-of-plane mode the dominant heat carrier. Our results not only improve the understanding concerning the heat transfer in h-BAs nanosheets but also offer possible new routes to drastically improve the thermal conductivity, which can play critical roles in thermal management systems.

Anomalous temperature dependent thermal conductivity of two-dimensional silicon carbide

Recently, two-dimensional silicon carbide (2D-SiC) has attracted considerable interest due to its exotic electronic and optical properties. Here, we explore the thermal properties of 2D-SiC using reverse non-equilibrium molecular dynamics simulation. At room temperature, a thermal conductivity of ∼313 W mK^-1 is obtained for 2D-SiC which is one order higher than that of silicene. Above room temperature, the thermal conductivity deviates the normal 1/T law and shows an anomalous slowly decreasing behavior. To elucidate the variation of thermal conductivity, the phonon modes at different length and temperature are quantified using Fourier transform of the velocity auto-correlation of atoms. The calculated phonon density of states at high temperature shows a shrinking and softening of the peaks, which induces the anomaly in the thermal conductivity. On the other hand, quantum corrections are applied to avoid the freezing effects of phonon modes on the thermal conductivity at low temperature. In addition, the effect of potential on the thermal conductivity calculation is also studied by employing original and optimized Tersoff potentials. These findings provide a means for better understating as well as designing the efficient thermal management of 2D-SiC based electronics and optoelectronics in near future.

High Thermal Boundary Conductance across Bonded Heterogeneous GaN-SiC Interfaces

High-power GaN-based electronics are limited by high channel temperatures induced by self-heating, which degrades device performance and reliability. Increasing the thermal boundary conductance (TBC) between GaN and SiC will aid in the heat dissipation of GaN-on-SiC devices by taking advantage of the high thermal conductivity of SiC substrates. For the typical growth method, there are issues concerning the transition layer at the interface and low-quality GaN adjacent to the interface, which impedes heat flow. In this work, a room-temperature bonding method is used to bond high-quality GaN to SiC directly, which allows for the direct integration of high-quality GaN with SiC to create a high TBC interface. Time-domain thermoreflectance is used to measure the GaN thermal conductivity and GaN-SiC TBC. The measured GaN thermal conductivity is larger than that of grown GaN-on-SiC by molecular beam epitaxy. High TBC is observed for the bonded GaN-SiC interfaces, especially for the annealed interface (∼230 MW m^-2 K^-1, close to the highest value ever reported). Thus, this work provides the benefit of both a high TBC and higher GaN thermal conductivity, which will impact the GaN-device integration with substrates in which thermal dissipation always plays an important role. Additionally, simultaneous thermal and structural characterizations of heterogeneous bonded interfaces are performed to understand the structure-thermal property relation across this new type of interface.

Ultrahigh and anisotropic thermal transport in the hybridized monolayer (BC2N) of boron nitride and graphene: a first-principles study

Heat removal has become a significant challenge in the miniaturization of electronic devices, especially in power electronics, so semiconducting materials with suitable band gaps and high lattice thermal conductivity are highly desired. Here, through first-principles calculations, we theoretically predict an ultra-high and anisotropic lattice thermal conductivity in monolayer BC₂N. The predicted values of lattice thermal conductivity at room-temperature are 893.90 W m^-1 K^-1 and 1275.79 W m^-1 K^-1 along the armchair and zigzag directions, respectively. These values are probably the highest that have ever been reported for two-dimensional semiconducting materials. Such high lattice thermal conductivities are attributed to the high vibrational frequencies, large phonon group velocities, long phonon lifetime, low phonon anharmonicity, and strong bonding in monolayer BC₂N. We also calculate the electrical and electronic thermal conductivities, which are also very high. Based on these theoretical findings, we expect monolayer BC₂N to be an adequate candidate for thermal management in nanoelectronic 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.

First-principles study of the layered thermoelectric material TiNBr

TiNBr: novel layered thermoelectric material with high ZT value. Physics behind were investigated using mode level analysis.

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.

2D AlN Layers Sandwiched Between Graphene and Si Substrates

Due to the superior thickness-dependent properties, 2D materials have exhibited great potential for applications in next-generation optoelectronic devices. Despite the significant progress that has been achieved, the synthesis of 2D AlN remains challenging. This work reports on the epitaxial growth of 2D AlN layers via utilizing physically transferred graphene on Si substrates by metal-organic chemical vapor deposition. The 2D AlN layers sandwiched between graphene and Si substrates are confirmed by annular bright-field scanning transmission electron microscopy and the effect of hydrogenation on the formation of 2D AlN layers is clarified by theoretical calculations with first-principles calculations based on density functional theory. Moreover, the bandgap of as-grown 2D AlN layers is theoretically predicted to be ≈9.63 eV and is experimentally determined to be 9.20-9.60 eV. This ultrawide bandgap semiconductor shows great promise in deep-ultraviolet optoelectronic applications. These results are expected to support innovative and front-end development of optoelectronic devices.