Borophene Concentric Superlattices via Self-Assembly of Twin Boundaries

Enhanced electron transport and self-similarity in quasiperiodic borophene nanoribbons with line defects

Recent experiments have revealed multiple borophene phases of distinct lattice structures, suggesting that the unit cells of ν_1/6 and ν_1/5 boron sheets, namely α and β chains, serve as building blocks to assemble into novel borophene phases. Motivated by these experiments, we present a theoretical study of electron transport along two-terminal quasiperiodic borophene nanoribbons (BNRs), with the arrangement of the α and β chains following the generalized Fibonacci sequence. Our results indicate that the energy spectrum of these quasiperiodic BNRs is multifractal and characterized by numerous transmission peaks. In contrast to the Fibonacci model that all the electronic states should be critical, both delocalized and critical states appear in the quasiperiodic BNRs, where the averaged resistance saturates at the inverse of one conductance quantum for the delocalized states in the large length limit and contrarily exhibits a power-law dependence on the nanoribbon length for the critical states. Besides, the self-similarity is observed from the transmission spectrum, where the conductance curves overlap at different energy regions of two quasiperiodic BNRs of different Fibonacci indices and the resistance curves are analogous to each other at different energy scales of a single quasiperiodic BNR. These results complement previous studies on quasiperiodic systems where the multifractal energy spectrum and the self-similarity are observed by generating quasiperiodic potential energies, suggesting that borophene may provide an intriguing platform for understanding the structure-property relationships and exploring the physical properties of quasiperiodic systems.

Retrieving Grain Boundaries in 2D Materials

The coalescence of randomly distributed grains with different crystallographic orientations can result in pervasive grain boundaries (GBs) in 2D materials during their chemical synthesis. GBs not only are the inherent structural imperfection that causes influential impacts on structures and properties of 2D materials, but also have emerged as a platform for exploring unusual physics and functionalities stemming from dramatic changes in local atomic organization and even chemical makeup. Here, recent advances in studying the formation mechanism, atomic structures, and functional properties of GBs in a range of 2D materials are reviewed. By analyzing the growth mechanism and the competition between far-field strain and local chemical energies of dislocation cores, a complete understanding of the rich GB morphologies as well as their dependence on lattice misorientations and chemical compositions is presented. Mechanical, electronic, and chemical properties tied to GBs in different materials are then discussed, towards raising the concept of using GBs as a robust atomic-scale scaffold for realizing tailored functionalities, such as magnetism, luminescence, and catalysis. Finally, the future opportunities in retrieving GBs for making functional devices and the major challenges in the controlled formation of GB structures for designed applications are commented.

A family of superconducting boron crystals made of stacked bilayer borophenes

Monolayer borophenes tend to be easily oxidized, while thicker borophenes have stronger antioxidation properties. Herein, we proposed four novel metallic boron crystals by stacking the experimentally synthesized borophenes, and one of the crystals has been reported in our previous experiments. Bilayer units tend to act as blocks for crystals as determined by bonding analyses. Their kinetic, thermodynamic and mechanical stabilities are confirmed by our calculated phonon spectra, molecular dynamics and elastic constants. Our proposed allotropes are more stable than the boron α-Ga phase below 1000 K at ambient pressure. Some of them become more stable than the α-rh or γ-B₂₈ phases at appropriate external pressure. More importantly, our calculations show that three of the proposed crystals are phonon-mediated superconductors with critical temperatures of about 5-10 K, higher than those of most superconducting elemental solids, in contrast to typical boron crystals with significant band gaps. Our study indicates a novel preparation method for metallic and superconducting boron crystals dispensing with high pressure.

Structure of χ3-Borophene Studied by Total-Reflection High-Energy Positron Diffraction (TRHEPD)

We have investigated the structure of χ₃-borophene on Ag(111), a monolayer material of boron atoms, via total-reflection high-energy positron diffraction (TRHEPD). By comparing the experimental rocking-curves with ones for several structures calculated by using dynamical diffraction theory, we confirmed that the χ₃-borophene layer has a flat structure. The distance from the topmost layer of the metal crystal is 2.4 Å, which is consistent with results reported by X-ray standing wave-excited X-ray photoelectron spectroscopy. We also demonstrated that the in-plane structure of χ₃-borophene is compatible with the theoretical predictions. These structural properties indicate that χ₃-borophene belongs to a group of epitaxial monolayer sheets, such as graphene, which have weak interactions with the substrates.

Quasi-Freestanding Bilayer Borophene on Ag(111)

The lattice structure of monolayer borophene depends sensitively on the substrate yet is metallic independent of the environment. Here, we show that bilayer borophene on Ag(111) shares the same ground state as its freestanding counterpart that becomes semiconducting with an indirect bandgap of 1.13 eV, as evidenced by an extensive structural search based on first-principles calculations. The bilayer structure is composed of two covalently bonded v_1/12 boron monolayers that are stacked in an AB mode. The interlayer bonds not only localize electronic states that are otherwise metallic in monolayer borophene but also in part decouple the whole bilayer from the substrate, resulting in a quasi-freestanding system. More relevant is that the predicted bilayer model of a global minimum agrees well with recently synthesized bilayer borophene on Ag(111) in terms of lattice constant, topography, and moiré pattern.

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.

Ultraviolet photodetector based on p-borophene/n-ZnO heterojunction

Borophene has attracted enormous attention because of its rich and unique structural and electronic properties for promising pratical applications. Although borophene sheets have been realized on different substrates in recent experiments, there are very few reports on the device application of borophene. Recently, borophene can be grown on some functional substrates, which lays a good foundation for its potential applications. Here, we report that hydrogenated borophene can be grown on the fluorine-doped tin oxide glass substrate. The phase of the obtained borophene is well consistent with the predicted semiconductingδ₅-boron sheet. Furthermore, a vertical heterojunction ultraviolet detector based p-borophene/n-zinc oxide was fabricated. The photoresponsivity of the detector is 1.02 × 10^-1A W^-1, the specific detection rate was 1.43 × 10⁹Jones and the response speed wasτ_res = 2.8 s,τ_rec = 3.2 s at the reversed bias of -5 V under the light excitation of 365 nm. This work will lay a foundation for further study on the attractive properties and applications of borophene in new optoelectronic devices and integrated circuits.

Polymorphism of Segmented Grain Boundaries in Two-Dimensional Transition Metal Dichalcogenides

Grain boundaries (GBs) are vital to crystal materials and their applications. Although GBs in bulk and two-dimensional materials have been extensively studied, the segmented GBs observed in transition metal dichalcogenide monolayers by a sequence of folded segments remain a mystery. We visualize the large-area distribution of the segmented GBs in MoSe₂ monolayers and unravel their structural origin using ab initio calculations combined with high-resolution atomic characterizations. Unlike normal GBs in two-dimensional materials with commonly one type of dislocation cores, the segmented GBs consist of two basic elements-4|8 and 4|4|8 cores, whose alloying results in structural diversity and distinctly high stability due to relieved stress fields nearby. The defective polygons can uniquely migrate along the segmented GBs via the movement of single molybdenum atoms, unobtrusively endowing a given GB with variable appearances. Furthermore, the segmented GBs can achieve useful functionalities such as intrinsic magnetism and highly active electrocatalysis.

Fully Boron-Sheet-Based Field Effect Transistors from First-Principles: Inverse Design of Semiconducting Boron Sheets

High-performance two-dimensional (2D) field effect transistors (FETs) have a broad application prospect in future electronic devices. The lack of an ideal material system, however, hinders the breakthrough of 2D FETs. Recently, phase engineering offers a promising solution, but it requires both semiconducting and metallic phases of materials. Here we suggest borophenes as ideal systems for 2D FETs by theoretically searching semiconducting phases. Using multiobjective differential optimization algorithms implemented in the IM²ODE package and the first-principles calculations, we have successfully identified 16 new semiconducting borophenes. Among them, the B₁₂-1 borophene is the most stable semiconducting phase, whose total energy is lower than any other known semiconducting borophenes. By considering not only the band alignments but also the lattice matches between semiconducting and metallic borophenes, we then have theoretically proposed several device models of fully boron-sheet-based 2D FETs. Our work provides beneficial ideas and attempts for discovering novel borophene-based 2D FETs.

The grain boundary effect on mechanical and electronic transport properties of a striped borophene

The grain boundary (GB) effect on the mechanical and electronic transport properties of a striped borophene are investigated based on first principles calculations. Three GBs, (1,2)|(1,2), (2,1)|(2,1) and (3,1)|(3,1), constructed using the translation vector method are verified to possess low formation energy and stability at room temperature. The presence of GBs does not destroy the metallic nature of borophene, but results in the accumulation of charge densities. The mechanical strength of borophene is decreased due to the introduction of GBs. Their fracture behaviors are more complex, accompanied by reconstructions in the GB region. The transport current is also degraded, which is mainly caused by GBs in the borophene giving rise to backscattering. The degree of these reductions rely on the specific structure of GBs.

Borophene: Current Status, Challenges and Opportunities

Borophenes (2D boron sheets) have triggered a surge of interest both theoretically and experimentally because of its distinct structural, optical and electronic properties for extensive potential applications. Although theoretical efforts have guided the research directions of borophene, only few synthetic borophene sheets have been demonstrated experimentally. Borophene sheets have been successfully synthesized experimentally on metal substrates until 2015. Afterwards, more efforts were put on the controlled synthesis of crystalline and semiconducting borophene sheets as well as on the investigation of their novel and fascinating physical properties. This report provides a brief review on theoretical and experimental progress in borophene research. Some typical structures and properties of borophenes have been reviewed. The focus is laid on summarizing the experimental synthesis of borophene in recent years, and on showing some ultrastable and semiconducting borophenes which have been applied in electronic information devices. Finally, the future challenges and opportunities regarding experimental realization and practical applications of borophenes are presented.

Growth and Grain Boundaries in 2D Materials

Grain boundaries (GBs) are a kind of lattice imperfection widely existing in two-dimensional materials, playing a critical role in materials' properties and device performance. Related key issues in this area have drawn much attention and are still under intense investigation. These issues include the characterization of GBs at different length scales, the dynamic formation of GBs during the synthesis, the manipulation of the configuration and density of GBs for specific material functionality, and the understanding of structure-property relationships and device applications. This review will provide a general introduction of progress in this field. Several techniques for characterizing GBs, such as direct imaging by high-resolution transmission electron microscopy, visualization techniques of GBs by optical microscopy, plasmon propagation, or second harmonic generation, are presented. To understand the dynamic formation process of GBs during the growth, a general geometric approach and theoretical consideration are reviewed. Moreover, strategies controlling the density of GBs for GB-free materials or materials with tunable GB patterns are summarized, and the effects of GBs on materials' properties are discussed. Finally, challenges and outlook are provided.