Three-Fold Enhancement of In-Plane Thermal Conductivity of Borophene through Metallic Atom Intercalation

Chemical Intercalation of Layered Materials: From Structure Tailoring to Applications

The intercalation of layered materials offers a flexible approach for tailoring their structures and generating unexpected properties. This review provides perspectives on the chemical intercalation of layered materials, including graphite/graphene, transition metal dichalcogenides, MXenes, and some particular materials. The characteristics of the different intercalation methods and their chemical mechanisms are discussed. The influence of intercalation on the structural changes of the host materials and the structural change how to affect the intrinsic properties of the intercalation compounds are discussed. Furthermore, a perspective on the applications of intercalation compounds in fields such as energy conversion and storage, catalysis, smart devices, biomedical applications, and environmental remediation is provided. Finally, brief insights into the challenges and future opportunities for the chemical intercalation of layered materials are provided.

Boron nanosheets boosting solar thermal water evaporation

Hydrogel-based solar vapour generators (SVGs) are promising for wastewater treatment and desalination. The performance of SVG systems is governed by solar thermal conversion and water management. Progress has been made in achieving high energy conversion efficiency, but the water evaporation rates are still unsatisfactory under 1 sun irradiation. This study introduced novel two-dimensional (2D) boron nanosheets as additives into hydrogel-based SVGs. The resulting SVGs exhibit an outstanding evaporation rate of 4.03 kg m-2 h-1 under 1 sun irradiation. This significant improvement is attributed to the 2D boron nanosheets, which leads to the formation of a higher content of intermediate water and reduced water evaporation enthalpy to 845.11 kJ kg-1. The SVGs into which boron nanosheets were incorporated also showed high salt resistance and durability, demonstrating their great potential for desalination applications.

Regulating interfacial thermal conductance with commensurate-incommensurate transitions at atomic-scale silicon/silicon interfaces

Interfacial thermal conductance (ITC) between two contact surfaces is an important factor in accurately measuring energy transfer and heat dissipation at the interface; however, it is still not fully resolved how to more effectively modulate the ITC and unravel the related inner mechanisms. In this study, the contribution of commensurability and normal load to ITC at the atomic-scale silicon/silicon interface is disclosed. The results manifest that the ITC gradually reduces with the transition from commensurability to incommensurability. This is because the reduced force constant at the incommensurate interface decreases the transmittance of phonons, leading to the suppression of high-frequency phonon excitation and a red shift in the phonon spectrum, thereby weakening the ITC. We further discovered that increasing the normal loads can significantly enhance the ITC in both contact states, and the reason is that the interlayer distance decreases with increasing normal loads, which strengthens the interfacial potential and force constant, consequently resulting in greater heat transfer efficiency. This paper reveals that interfacial thermal transport can be regulated by applying normal loads and changing the interfacial contact states.

Atomic-Scale Surface Engineering for Giant Thermal Transport Enhancement Across 2D/3D van der Waals Interfaces

Heat dissipation in two-dimensional (2D) material-based electronic devices is a critical issue for their applications. The bottleneck for this thermal issue is inefficient for heat removal across the van der Waals (vdW) interface between the 2D material and its supporting three-dimensional (3D) substrate. In this work, we demonstrate that an atomic-scale thin amorphous layer atop the substrate surface can remarkably enhance the interfacial thermal conductance (ITC) of the 2D-MoS₂/3D-GaN vdW interface by a factor of 4 compared to that of the untreated crystalline substrate surface. Meanwhile, the ITC can be broadly manipulated through adjusting substrate surface roughness. Phonon dynamic and heat flux spectrum analyses show that this giant enhancement is attributed to the increased phonon densities and channels at the interfaces and enhanced phonon coupling. The slight surface fluctuation in MoS₂ and the increased diffuse interfacial scattering facilitate energy transfer from MoS₂'s in-plane phonons to its out-of-plane phonons and then to the substrate. In addition, it is further found that the substrate and its surface topology can dramatically influence the thermal conductivity of MoS₂ due to the reduction of phonon relaxation time, especially for low-frequency acoustic phonons. This study elucidates the effects of the amorphous surface of the substrate on thermal transport across 2D/3D vdW interfaces and provides a new dimension to aid in the heat dissipation of 2D-based electronic devices via atomic-scale surface engineering.

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.

Exploring the emerging applications of the advanced 2-dimensional material borophene with its unique properties

Borophene, a crystalline allotrope of monolayer boron, with a combination of triangular lattice and hexagonal holes, has stimulated wide interest in 2-dimensional materials and their applications. Although their properties are theoretically confirmed, they are yet to be explored and confirmed experimentally. In this review article, we present advancements in research on borophene, its synthesis, and unique properties, including its advantages for various applications with theoretical predictions. The uniqueness of borophene over graphene and other 2-dimensional (2D) materials is also highlighted along with their various structural stabilities. The strategy for its theoretical simulations, leading to the experimental synthesis, could also be helpful for the exploration of many newer 2D materials.

Nonequilibrium Green's function method for phonon heat transport in quantum system

Phonon heat transport property in quantum devices is of great interesting since it presents significant quantum behaviors. In the past few decades, great efforts have been devoted to establish the theoretical method for phonon heat transport simulation in nanostructures. However, modeling phonon heat transport from wavelike coherent regime to particlelike incoherent regime remains a challenging task. The widely adopted theoretical approach, such as molecular dynamics, semiclassical Boltzmann transport equation, captures quantum mechanical effects within different degrees of approximation. Among them, Non-equilibrium Green's function (NEGF) method has attracted wide attention, as its ability to perform full quantum simulation including many-body interactions. In this review, we summarized recent theoretical advances of phonon NEGF method and the applications on the numerical simulation for phonon heat transport in nanostructures. At last, the challenges of numerical simulation are discussed.

Remarkable decrease in lattice thermal conductivity of transition metals borides TiB2by dimensional reduction

Two-dimensional transition metals borides Ti_xB_xhave excellent magnetic and electronic properties and great potential in metal-ion batteries and energy storage. The thermal management is important for the safety and stability in these applications. We investigated the lattice dynamical and thermal transport properties of bulk-TiB₂and its two-dimensional (2D) counterparts based on density functional theory combined with solving phonon Boltzmann transport equation. The Poisson's ratio of bulk-TiB₂is positive while it changes to negative for monolayer TiB₂. We found that dimension reduction can cause the room-temperature in-plane lattice thermal conductivity decrease, which is opposite the trend of MoS₂, MoSe₂, WSe₂and SnSe. Additionally, the room temperature thermal conductivity of mono-TiB₂is only one sixth of that for bulk-TiB₂. It is attributed to the higher Debye temperature and stronger bonding stiffness in bulk-TiB₂. The bulk-TiB₂has higher phonon group velocity and weaker anharmonic effect comparing with its 2D counterparts. On the other hand, the room temperature lattice thermal conductivity of mono-Ti₂B₂is two times higher than that of mono-TiB₂, which is due to three-phonon selection rule caused by the horizontal mirror symmetry.

Hydrogen-Intercalated 2D Magnetic Bilayer: Controlled Magnetic Phase Transition and Half-Metallicity via Ferroelectric Switching

Electrically controlled magnetism in two-dimensional (2D) multiferroics is highly desirable for both fundamental research and the future development of low-power nanodevices. Herein, inspired by the recently experimentally realized 2D antiferromagnetic MnPSe₃ [ Nat. Nanotechnol. 2021, 16 (7), 782] and guided by a heteromagnetic structural design, we engineer strong magnetoelectric coupling in a hydrogen-intercalated 2D MnPSe₃ bilayer. Hydrogen functionalization breaks the centrosymmetry of bilayer MnPSe₃, leading to out-of-plane ferroelectricity. Moreover, there is a phase transition from antiferromagnetic semiconductor to ferromagnetic half-metal in the H-bonded MnPSe₃ layer, while the other remains antiferromagnetic and semiconducting. When reversing the electrical polarization, the intercalated H atom can flip between the top and bottom layers with an ultralow switching barrier, which allows one to tune the magnetic order and conductivity of the individual layers via an external electric field. Our results pave a new avenue to realize strong magnetoelectric coupling in single-phase multiferroic material. The ferroelectricity-controlled magnetic phase transition and half-metallicity offer promising applications in nanoscale spintronics such as electrically written and magnetically read memories.

Borophene: Two-dimensional Boron Monolayer: Synthesis, Properties, and Potential Applications

Borophene, a monolayer of boron, has risen as a new exciting two-dimensional (2D) material having extraordinary properties, including anisotropic metallic behavior and flexible (orientation-dependent) mechanical and optical properties. This review summarizes the current progress in the synthesis of borophene on various metal substrates, including Ag(110), Ag(100), Au(111), Ir(111), Al(111), and Cu(111), as well as heterostructuring of borophene. In addition, it discusses the mechanical, thermal, magnetic, electronic, optical, and superconducting properties of borophene and the effects of elemental doping, defects, and applied mechanical strains on these properties. Furthermore, the promising potential applications of borophene for gas sensing, energy storage and conversion, gas capture and storage applications, and possible tuning of the material performance in these applications through doping, formation of defects, and heterostructures are illustrated based on available theoretical studies. Finally, research and application challenges and the outlook of the whole borophene's field are given.

Promising Thermoelectric Performance in Two-Dimensional Semiconducting Boron Monolayer

A heavy element is a special character for high thermoelectric performance since it generally guarantees a low lattice thermal conductivity. Here, we unexpectedly found a promising thermoelectric performance in a two-dimensional semiconducting monolayer consisting of a light boron element. Using first-principles combined with the Boltzmann transport theory, we have shown that in contrast to graphene or black phosphorus, the boron monolayer has a low lattice thermal conductivity arising from its complex crystal of hexagonal vacancies. The conduction band with an intrinsic camelback shape leads to the high DOS and a high n-type Seebeck coefficient, while the highly degenerate valence band along with the small hole effective mass contributes to the high p-type power factor. As a result, we obtained the p-type thermoelectric figure of merit up to 0.96 at 300 K, indicating that the boron monolayer is a promising p-type thermoelectric material.

Electronic and optical properties of boron-based hybrid monolayers

Anisotropic 2D Dirac cone materials are important for the fabrication of nanodevices having direction-dependent characteristics since the anisotropic Dirac cones lead to different values of Fermi velocities yielding variable carrier concentrations. In this work, the feasibility of the B-based hybrid monolayers BX (X = As, Sb, and Bi), as anisotropic Dirac cone materials is investigated. Calculations based on density functional theory and molecular dynamics method find the stability of these monolayers exhibiting unique electronic properties. For example, the BAs monolayer possesses a robust self-doping feature, whereas the BSb monolayer carries the intrinsic charge carrier concentration of the order of 10¹²cm^-2which is comparable to that of graphene. Moreover, the direction-dependent optical response is predicted in these B-based monolayers; a high IR response in thex-direction is accompanied with that in the visible region along they-direction. The results are, therefore, expected to help in realizing the B-based devices for nanoscale applications.

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.

The Impact of Interlayer Rotation on Thermal Transport Across Graphene/Hexagonal Boron Nitride van der Waals Heterostructure

Graphene/hexagonal boron nitride (h-BN) van der Waals (vdW) heterostructure has aroused great interest because of the unique Moiré pattern. In this study, we use molecular dynamics simulation to investigate the influence of the interlayer rotation angle θ on the interfacial thermal transport across graphene/h-BN heterostructure. The interfacial thermal conductance G of graphene/h-BN interface reaches 509 MW/(m²K) at 500 K without rotation, and it decreases monotonically with the increase of the rotation angle, exhibiting around 50% reduction of G with θ = 26.33°. The phonon transmission function reveals that G is dominantly contributed by the low-frequency phonons below 10 THz. Upon rotation, the surface fluctuation in the interfacial graphene layer is enhanced, and the transmission function for the low-frequency phonon is reduced with increasing θ, leading to the rotation angle-dependent G. This work uncovers the physical mechanisms for controlling interfacial thermal transport across vdW heterostructure via interlayer rotation.

Spectroscopic, computational and molecular docking study of Cu(ii) complexes with flavonoids: from cupric ion binding to DNA intercalation

Copper(ii) complexes with flavonoids as perspective therapeutic agents with DNA as a target molecule.