Anisotropic intrinsic lattice thermal conductivity of borophane from first-principles calculations

Advances In Borophene: Synthesis, Tunable Properties, and Energy Storage Applications

Monolayer boron nanosheet, commonly known as borophene, has garnered significant attention in recent years due to its unique structural, electronic, mechanical, and thermal properties. This review paper provides a comprehensive overview of the advancements in the synthetic strategies, tunable properties, and prospective applications of borophene, specifically focusing on its potential in energy storage devices. The review begins by discussing the various synthesis techniques for borophene, including molecular beam epitaxy (MBE), chemical vapor deposition (CVD), and chemical methods, such as ultrasonic exfoliation and thermal decomposition of boron-containing precursors. The tunable properties of borophene, including its electronic, mechanical, and thermal characteristics, are extensively reviewed, with discussions on its bandgap engineering, plasmonic behavior, and thermal conductivity. Moreover, the potential applications of borophene in energy storage devices, particularly as anode materials in metal-ion batteries and supercapacitors, along with its prospects in other energy storage systems, such as sodium-oxygen batteries, are succinctly, discussed. Hence, this review provides valuable insights into the synthesis, properties, and applications of borophene, offering much-desired guidance for further research and development in this promising area of nanomaterials science.

Hydrogenation driven ultra-low lattice thermal conductivity inβ12borophene

Borophene gathered large interest owing to its polymorphism and intriguing properties such as Dirac point, inherent metallicity, etc but oxidation limits its capabilities. Hydrogenated borophene was recently synthesised experimentally to harness its applications. Motivated by experimental work, in this paper, using first-principles calculations and Boltzmann transport theory, we study the freestandingβ12borophene nanosheet doped and functionalised with hydrogen (H), lithium (Li), beryllium (Be), and carbon (C) atoms at differentβ12lattice sites. Among all possible configurations, we screen two stable candidates, pristine and hydrogenatedβ12borophene nanosheets. Both nanosheets possess dynamic and mechanical stability while the hydrogenated sheet has different anisotropic metallicity compared to pristine sheet leading to enhancement in brittle behaviour. Electronic structure calculations reveal that both nanosheets host Dirac cones (DCs), while hydrogenation leads to shift and enhancement in tilt of the DCs. Further hydrogenation leads to the appearance of additional Fermi pockets in the Fermi surface. Transport calculations reveals that the lattice thermal conductivity changes from 12.51 to 0.22 W m-1 K-1(along armchair direction) and from 4.42 to 0.07 W m-1 K-1(along zigzag direction) upon hydrogenation at room temperature (300 K), demonstrating a large reduction by two orders of magnitude. Such reduction is mainly attributed to decreased phonon mean free path and relaxation time along with the enhanced phonon scattering rates stemming from high frequency phonon flat modes in hydrogenated nanosheet. Comparatively larger weighted phase space leads to increased anharmonic scattering in hydrogenated nanosheet contributing to ultra-low lattice thermal conductivity. Consequently, hydrogenatedβ12nanosheet exhibits a comparatively higher thermoelectric figure of merit (∼0.75) at room temperature along armchair direction. Our study demonstrates the effects of functionalisation on transport properties of freestandingβ12borophene nanosheets which can be utilised to enhance the thermoelectric performance in two-dimensional (2D) systems and expand the applications of boron-based 2D materials.

Lattice thermal conductivity of 2D nanomaterials: a simple semi-empirical approach

Extracting reliable information on certain physical properties of materials, such as thermal transport, can be computationally very demanding. Aiming to overcome such difficulties in the particular case of lattice thermal conductivity (LTC) of 2D nanomaterials, we propose a simple, fast, and accurate semi-empirical approach for LTC calculation. The approach is based on parameterized thermochemical equations and Arrhenius-like fitting procedures, thus avoiding molecular dynamics or ab initio protocols, which frequently require computationally expensive simulations. As a proof of concept, we obtain the LTC of some prototypical physical systems, such as graphene (and other 2D carbon allotropes), hexagonal boron nitride (hBN), silicene, germanene, binary, and ternary BNC lattices and two examples of the fullerene network family. Our obtained values are in good agreement with other theoretical and experimental estimations, nonetheless, being derived in a rather straightforward way, at a fraction of the usual computational cost.

Methods for Measuring Thermal Conductivity of Two-Dimensional Materials: A Review

Two-dimensional (2D) materials are widely used in microelectronic devices due to their excellent optical, electrical, and mechanical properties. The performance and reliability of microelectronic devices based 2D materials are affected by heat dissipation performance, which can be evaluated by studying the thermal conductivity of 2D materials. Currently, many theoretical and experimental methods have been developed to characterize the thermal conductivity of 2D materials. In this paper, firstly, typical theoretical methods, such as molecular dynamics, phonon Boltzmann transport equation, and atomic Green's function method, are introduced and compared. Then, experimental methods, such as suspended micro-bridge, 3ω, time-domain thermal reflectance and Raman methods, are systematically and critically reviewed. In addition, the physical factors affecting the thermal conductivity of 2D materials are discussed. At last, future prospects for both theoretical and experimental thermal conductivity characterization of 2D materials is given. This paper provides an in-depth understanding of the existing thermal conductivity measurement methods of 2D materials, which has guiding significance for the application of 2D materials in micro/nanodevices.

The Emergence and Evolution of Borophene

Neighboring carbon and sandwiched between non-metals and metals in the periodic table of the elements, boron is one of the most chemically and physically versatile elements, and can be manipulated to form dimensionally low planar structures (borophene) with intriguing properties. Herein, the theoretical research and experimental developments in the synthesis of borophene, as well as its excellent properties and application in many fields, are reviewed. The decade-long effort toward understanding the size-dependent structures of boron clusters and the theory-directed synthesis of borophene, including bottom-up approaches based on different foundations, as well as up-down approaches with different exfoliation modes, and the key factors influencing the synthetic effects, are comprehensively summarized. Owing to its excellent chemical, electronic, mechanical, and thermal properties, borophene has shown great promise in supercapacitor, battery, hydrogen-storage, and biomedical applications. Furthermore, borophene nanoplatforms used in various biomedical applications, such as bioimaging, drug delivery, and photonic therapy, are highlighted. Finally, research progress, challenges, and perspectives for the future development of borophene in large-scale production and other prospective applications are discussed.

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

We studied the thermal conductivity of Al-intercalated bilayer δ₄ borophene sheet by solving phonon Boltzmann transport equation based on density functional theory. Although the overall atomic density of Al-intercalated borophene is larger than that of δ₄ borophene, it possesses significant enhancement in in-plane thermal conductivity. With metallic atom intercalation, the armchair-direction thermal conductivity increases from 53.8 to 160.2 W m^-1 K^-1 and that along the zigzag direction increases from 115.7 to 157.2 W m^-1 K^-1. This pronounced enhancement is attributed to the bunching of the acoustic branches in the Al-intercalated borophene, which decreases the phase space for the high frequency three acoustic phonon scattering processes. In addition to the pronounced increased thermal conductivity, the Al-intercalation also tunes the in-plane anisotropy. This study illustrates the importance of metallic atom intercalation in the in-plane thermal conductivity of 2D van der Waals materials and also has practical implications for fields ranging from thermal management to thermoelectrics design.

Comparative investigation of the thermal transport properties of Janus SnSSe and SnS2 monolayers

Recently, Janus two-dimensional (2D) materials as a new member of 2D derivatives have been receiving much attention due to their novel properties. In this work, the lattice thermal conductivity κ_L of the Janus SnSSe monolayer is investigated based on first-principles calculations, while that of the SnS₂ monolayer is studied for comparison. It is found the the κ_L values of SnSSe and SnS₂ are 13.3 and 11.0 W m^-1 K^-1 at room temperature, and acoustic branches dominate their thermal transport. Weaker phonon anharmonicity in SnSSe leads to a slightly higher κ_L, though it has weaker phonon harmonicity. The smaller Grüneisen parameters of TA and LA phonons lower than 1 THz in SnSSe indicate weaker phonon anharmonicity, resulting in a higher κ_L. Finally, the size effect and boundary effect are also investigated, exhibiting that the κ_L can further decrease at the nanoscale. Our work suggests that Janus SnSSe and SnS₂ have a much lower κ_L compared with conventional transition metal dichalcogenides (TMDs) and are potential competitors in the thermoelectric field.

Borophene: New Sensation in Flatland

Borophene, a 2D allotrope of boron and the lightest elemental Dirac material, is the latest very promising 2D material owing to its unique structural and electronic characteristics of the X₃ and β₁₂ phases. The high atomic density on ridgelines of the β₁₂ phase of borophene provides a substantial orbital overlap, which leads to an excellent electron density in the conduction level and thus to a highly metallic behavior. These unique structural characteristics and electronic properties of borophene attract significant scientific interest. Herein, approaches for crystal growth/synthesis of these unique nanostructures and their potential technological applications are discussed. Various substrate-supported ultrahigh-vacuum growth techniques for borophene, such as molecular beam epitaxy, atomic layer deposition, and chemical vapor deposition, along with their challenges, are also summarized. The sonochemical exfoliation and modified Hummer's technique for the synthesis of free-standing borophene are also discussed. Solution-phase exfoliation seems to address the scalability issues and expands the applications of these unique materials to various fields, including renewable energy devices and ultrafast sensors. Furthermore, the electronic, optical, thermal, and elastic properties of borophene are thoroughly discussed and are compared with those of graphene and its "cousins." Numerous frontline applications are envisaged and an outlook is presented.

On the in-plane electronic thermal conductivity of biased nanosheet β12-borophene

The unique physical and chemical properties of β₁₂-borophene stem from the coexistence of the Dirac and triplet fermions. The metallic phase of β₁₂-borophene transitions to the semiconducting one when it is subjected to a perpendicular electric field or bias voltage. In this work, with the aid of a five-band tight-binding Hamiltonian, the Green's function approach and the Kubo-Greenwood formalism, the electronic thermal conductivity (ETC) of the semiconducting phase of β₁₂-borophene is studied. Two homogeneous (H) and inversion symmetric (IS) models are considered depending on the interaction of the substrate and boron atoms. In addition, due to the anisotropic structure of β₁₂-borophene, the swapping effect of bias poles is addressed. First of all, we find the pristine ETC^IS < ETC^H independent of the temperature. Furthermore, a decrease of 74.45% (80.62%) is observed for ETC^H (ETC^IS) when strong positive bias voltages are applied, while this is 25.2% (47.48%) when applying strong negative bias voltages. Moreover, the shoulder temperature of both models increases (fluctuates) with the positive (negative) bias voltage. Our numerical results pave the way for setting up future experimental thermoelectric devices in order to achieve the highest performance.

Electron-phonon scattering effect on the lattice thermal conductivity of silicon nanostructures

Nanostructuring technology has been widely employed to reduce the thermal conductivity of thermoelectric materials because of the strong phonon-boundary scattering. Optimizing the carrier concentration can not only improve the electrical properties, but also affect the lattice thermal conductivity significantly due to the electron-phonon scattering. The lattice thermal conductivity of silicon nanostructures considering electron-phonon scattering is investigated for comparing the lattice thermal conductivity reductions resulting from nanostructuring technology and the carrier concentration optimization. We performed frequency-dependent simulations of thermal transport systematically in nanowires, solid thin films and nanoporous thin films by solving the phonon Boltzmann transport equation using the discrete ordinate method. All the phonon properties are based on the first-principles calculations. The results show that the lattice thermal conductivity reduction due to the electron-phonon scattering decreases as the feature size of nanostructures goes down and could be ignored at low feature sizes (50 nm for n-type nanowires and 20 nm for p-type nanowires and n-type solid thin films) or a high porosity (0.6 for n-type 500 nm-thick nanoporous thin films) even when the carrier concentration is as high as 10²¹ cm^-3. Similarly, the size effect due to the phonon-boundary scattering also becomes less significant with the increase of carrier concentration. The findings provide a fundamental understanding of electron and phonon transports in nanostructures, which is important for the optimization of nanostructured thermoelectric materials.

A theoretical insight into a feasible strategy for the fabrication of borophane

This theoretical study demonstrates a feasible strategy for the fabrication of borophane through the mechanism of hydrogen decomposition on charged borophene.

Two-dimensional carbon dioxide with high stability, a negative Poisson's ratio and a huge band gap

In this work, based on density functional theory and the particle swarm optimization method, we unveil two CO₂ 2D crystals with space groups of P4̄M2 and Amm2.

Li-decorated carbon ene–yne as a potential high-capacity hydrogen storage medium

Based on comprehensive first-principles calculations, we predict that Li-decorated carbon ene–yne (CEY) can serve as a reversible and high density hydrogen storage medium.

Edge-Dependent Electronic and Magnetic Characteristics of Freestanding β 12-Borophene Nanoribbons

This work presents an investigation of nanoribbons cut from β ₁₂-borophene sheets by applying the density functional theory. In particular, the electronic and magnetic properties of borophene nanoribbons (BNR) are studied. It is found that all the ribbons considered in this work behave as metals, which is in good agreement with the recent experimental results. β ₁₂-BNR has significant diversity due to the existence of five boron atoms in a unit cell of the sheet. The magnetic properties of the ribbons are strongly dependent on the cutting direction and edge profile. It is interesting that a ribbon with a specific width can behave as a normal or a ferromagnetic metal with magnetization at just one edge or two edges. Spin anisotropy is observed in some ribbons, and the magnetic moment is not found to be the same in both edges in an antiferromagnetic configuration. This effect stems from the edge asymmetry of the ribbons and results in the breaking of spin degeneracy in the band structure. Our findings show that β ₁₂ BNRs are potential candidates for next-generation spintronic devices.