Intermixing and periodic self-assembly of borophene line defects

Atomic-resolution structural and spectroscopic evidence for the synthetic realization of two-dimensional copper boride

Since the first realization of borophene on Ag(111), two-dimensional (2D) boron nanomaterials have attracted substantial interest because of their polymorphic diversity and potential for hosting solid-state quantum phenomena. Here, we use atomic-resolution scanning tunneling microscopy (STM) and field-emission resonance (FER) spectroscopy to elucidate the structure and properties of atomically thin boron phases grown on Cu(111). Specifically, FER spectroscopy reveals charge transfer and electronic states that strongly differ from the decoupled borophene phases observed on silver, suggesting that the deposition of boron on copper results in strong covalent bonding characteristic of a 2D copper boride. This conclusion is reinforced by detailed STM characterization of line defects that are consistent with density functional theory calculations for atomically thin Cu8B14. This evidence for 2D copper boride is likely to motivate future synthetic efforts aimed at expanding the relatively unexplored family of atomically thin metal boride materials.

Progress and future directions in borophene research

Borophene-an atomically thin, two-dimensional (2D) boron analogue of graphene-has attracted significant attention as a 2D synthetic platform. Since its initial experimental realization, borophene has proven to be a versatile 2D material due to its high polymorphism and amenability to heterostructure integration. Nevertheless, several synthetic challenges have hindered the practical utilization of borophene, primarily due to its high chemical reactivity and interlayer charge transfer with growth substrates. Here we discuss emerging synthesis strategies for borophene, ranging from on-surface synthesis using elemental and molecular boron sources to substrate segregation growth techniques and solution-based reactions. We also focus on the surface and interface engineering of borophene with the aim of tailoring chemical reactivity and electronic properties. Finally, we highlight the remaining unresolved synthetic challenges for borophene and suggest future directions for accelerating fundamental science and applied technology for boron in the 2D limit.

Machine learning-driven global optimization reveals nanometre-scale mixed phases of borophene on Ag(100)

Metal-supported borophene exhibits significant polymorphism and an inherently complex potential energy landscape, posing challenges to exploring its structural diversity. In this study, we integrate a neural network-driven machine learning potential with stochastic surface walking global optimization and an active learning framework to comprehensively map the potential energy surface (PES) of large-size borophene on an Ag(100) substrate. Our exhaustive search identifies 59 857 local minima across 556 distinct supercells, revealing a PES segmented into multiple energy basins and three major funnels. Among the low-energy configurations, 1391 low-energy structures extend to the nanometre scale, showcasing a diverse array of mixed-phase borophene architectures, including monolayer ribbons (β12 and χ3) and bilayer fragments (BL-α5, BL-α7, BL-α1, BL-α6, and BL-α1α6). Notably, the global minimum structures feature monolayers composed of alternating χ3 and β12 ribbons and bilayers formed from BL-α5, BL-α1α6, and BL-α1 fragments. All mixed-phase borophenes exhibit metallic properties, and their simulated scanning tunneling microscopy (STM) images are provided to facilitate future experimental validation. These findings highlight the extraordinary structural complexity and rich polymorphism of borophene on extended metal surfaces, offering valuable insight into their formation, stability, and potential for tunable electronic properties.

Cryo-Exfoliation Synthesis of Borophene and its Application in Wearable Electronics

Borophene, an anisotropic Dirac Xene, exhibits diverse crystallographic phases, including metallic β₁₂, χ₃, and semiconducting α phases, alongside exceptional properties such as high electronic mobility, superior Young's modulus, thermal conductivity, superconductivity, and ferroelasticity. These attributes position borophene as a promising material for energy storage, electrocatalysis, and wearable electronics. However, its widespread application is hindered by existing synthesis methods that are expensive, complex, and yield-limited. This study presents a novel, cost-effective, environmentally friendly cryo-exfoliation method for borophene synthesis. Crystalline boron powder is rapidly quenched in liquid nitrogen and subjected to mild sonication, producing borophene with lateral dimensions of ≈50 to 10 µm and few-layer thicknesses. Advanced characterizations, including Atomic Force Microscopy (AFM), High-Resolution Transmission Electron Microscopy (HRTEM), Raman Spectroscopy, and X-ray Photoelectron Spectroscopy (XPS), confirm structural integrity, chemical purity, and minimal surface oxidation. Molecular dynamics simulations further elucidate the weakened inter-layer coupling induced by cryo-processing. The integration of borophene into Polyvinylidene Fluoride (PVDF) nanocomposites demonstrates its potential for wearable electronics, achieving motion-sensitive devices with outstanding performance, generating output voltages up to ≈40 V. This scalable cryo-exfoliation approach paves the way for borophene-based applications in energy harvesting, sensing, and next-generation electronics.

Harnessing Dirac Semi-Metallicity in a Metal-Free Stand-alone Borophene Composite Electrode for High-Performance Supercapacitors

Borophene, a semimetallic Dirac material with exceptional mechanical and electronic properties, shows great energy storage potential but requires conductive substrates for stability. A strategy for fabricating flexible and conductive rGO-supported borophene supercapacitor electrodes is proposed, featuring a layered assembly of borophene nanosheets sandwiched between rGO layers. This architecture prevents rGO nanosheet restacking, maintaining interlayer spacing at 5.75 Å, which promotes rapid electrolyte ion diffusion and enhances electroactive site accessibility. The rGO/borophene electrode demonstrates a capacitance of 328 F g-1 at 1 A g-1 current density with excellent cycling stability. The flexible supercapacitor achieves an energy density of 24.3 Wh kg-1 at a power density of 600 W kg-1 and maintains stability even under bending angles of up to 180°. Density functional theory calculations indicate that boron and carbon contribute to an increase in electron density near the Fermi level, enabling semimetallic behavior which allows rapid electron transfer and high performance.

Roadmap for Borophene Gas Sensors

Borophene, a two-dimensional allotrope of boron, has emerged as a promising material for gas sensing because of its exceptional electronic properties and high surface reactivity. This review comprehensively overviews borophene synthesis methods, properties, and sensing applications. However, it is crucial to acknowledge the substantial gap between the abundance of theoretical literature and the limited experimental studies. While theoretical investigations have elucidated the stability and remarkable sensing capabilities of various borophene polymorphs across different gases, significant experimental challenges have hindered the translation of these theoretical predictions into practical devices. Consequently, this review carefully studies these challenges and shortcomings that are jeopardizing the practical implementation of borophene in real-world settings. Specifically, four key issues are thoroughly studied, such as superficial borophene oxidation upon exposure to the air, interference from relative humidity on gas molecule detection, lack of selectivity, and synthesis scalability. Finally, novel strategies are proposed to overcome these bottlenecks. By adopting these approaches, borophene can pave the way to drive the advancement of the next generation of sensing devices.

Borophene: Synthesis, Chemistry, and Electronic Properties

As a neighbor of carbon in the periodic table, boron exhibits versatile structural and electronic configurations, with its allotropes predicted to possess intriguing structures and properties. Since the experimental realization of two-dimensional (2D) boron sheets (borophene) on Ag(111) substrates in 2015, the experimental study of the realization and characteristics of borophene has drawn increasing interest. In this review, we summarize the synthesis and properties of borophene, which are mainly based on experimental results. First, the synthesis of borophene on different substrates, as well as borophane and bilayer borophene, featuring unique phases and properties, are discussed. Next, the chemistry of borophene, such as oxidation, hydrogenation, and its integration into heterostructures with other materials, is summarized. We also mention a few works focused on the physical properties of borophene, specifically its electronic properties. Lastly, the brief outlook addresses challenges toward practical applications of borophene and possible solutions.

Ab Initio Kinetic Pathway of Diborane Decomposition on Transition Metal Surfaces in Borophene Chemical Vapor Deposition Growth

The chemical vapor deposition (CVD) method holds promise for the scalable and controlled synthesis of high-quality borophene. However, the current lack of an atomistic understanding of intricate kinetic pathways from precursors to borophene impedes process optimization. Here, we employ first-principles simulations to systematically explore the pyrolytic decomposition pathways of the most used precursor diborane (B2H6) to borophene on various transition metal surfaces. Our results reveal that B2H6 on various metal substrates exhibits different dissociation behaviors. Meanwhile, the activity of the examined metal substrates is quite anisotropic and surface direction-dependent, where the estimated overall catalytic activity order of these metals is found to be Pd ≈ Pt ≈ Rh > Ir ≈ Ru ≈ Cu > Au ≈ Ag. Our study provides atomistic insights into the dissociation kinetics of diborane precursors on various transition metal surfaces, serving as a guide for experimental growth of borophene.

Atomic-Resolution Vibrational Mapping of Bilayer Borophene

The successful synthesis of borophene beyond the monolayer limit has expanded the family of two-dimensional boron nanomaterials. While atomic-resolution topographic imaging has been previously reported, vibrational mapping has the potential to reveal deeper insight into the chemical bonding and electronic properties of bilayer borophene. Herein, inelastic electron tunneling spectroscopy (IETS) is used to resolve the low-energy vibrational and electronic properties of bilayer-α (BL-α) borophene on Ag(111) at the atomic scale. Using a carbon monoxide (CO)-functionalized scanning tunneling microscopy tip, the BL-α borophene IETS spectra reveal unique features compared to single-layer borophene and typical CO vibrations on metal surfaces. Distinct vibrational spectra are further observed for hollow and filled boron hexagons within the BL-α borophene unit cell, providing evidence for interlayer bonding between the constituent borophene layers. These experimental results are compared with density functional theory calculations to elucidate the interplay between the vibrational modes and electronic states in bilayer borophene.

Formation of Supernarrow Borophene Nanoribbons

Borophenes have sparked considerable interest owing to their fascinating physical characteristics and diverse polymorphism. However, borophene nanoribbons (BNRs) with widths less than 2 nm have not been achieved. Herein, we report the experimental realization of supernarrow BNRs. Combining scanning tunneling microscopy imaging with density functional theory modeling and ab initio molecular dynamics simulations, we demonstrate that, under the applied growth conditions, boron atoms can penetrate the outermost layer of Au(111) and form BNRs composed of a pair of zigzag (2,2) boron rows. The BNRs have a width self-contained to ∼1 nm and dipoles at the edges to keep them separated. They are embedded in the outermost Au layer and shielded on top by the evacuated Au atoms, free of the need for post-passivation. Scanning tunneling spectroscopy reveals distinct edge states, primarily attributed to the localized spin at the BNRs' zigzag edges. This work adds a new member to the boron material family and introduces a new physical feature to borophenes.

Directional dependence of the electronic and transport properties of biphenylene under strain conditions

In this study, we investigated the electronic and electronic transport properties of biphenylene (BPN) using first-principles density functional theory (DFT) calculations combined with the non-equilibrium Green's function (NEGF) formalism. We have focused on understanding the electronic properties of BPN, and the anisotropic behavior of electronic transport upon external strain. We found the emergence of electronic stripes (ESs) on the BPN surface and the formation of type-II Dirac cone near the Fermi level. In the sequence, the electronic transport results reveal that such ESs dictate the anisotropic behavior of the transmission function. Finally, we show that the tuning of the (anisotropic) electronic current, mediated by external mechanical strain, is ruled by the energy position of the lowest unoccupied states with wave-vectors perpedicular to the ESs. This control could be advantageous for applications in nanoelectronic devices that require precise control of current direction.

Synthesis of Quantum-Confined Borophene Nanoribbons

Borophene nanoribbons (BNRs) are one-dimensional strips of atomically thin boron expected to exhibit quantum-confined electronic properties that are not present in extended two-dimensional borophene. While the parent material borophene has been experimentally shown to possess anisotropic metallicity and diverse polymorphic structures, the atomically precise synthesis of nanometer-wide BNRs has not yet been achieved. Here, we demonstrate the synthesis of multiple BNR polymorphs with well-defined edge configurations within the nanometer-scale terraces of vicinal Ag(977). Through atomic-scale imaging, spectroscopy, and first-principles calculations, the synthesized BNR polymorphs are characterized and found to possess distinct edge structures and electronic properties. For single-phase BNRs, v1/6-BNRs and v1/5-BNRs adopt reconstructed armchair edges and sawtooth edges, respectively. In addition, the electronic properties of single-phase v1/6-BNRs and v1/5-BNRs are dominated by Friedel oscillations and striped moiré patterns, respectively. On the other hand, mixed-phase BNRs possess quantum-confined states with increasing nodes in the electronic density of states at elevated biases. Overall, the high degree of polymorphism and diverse edge topologies in borophene nanoribbons provide a rich quantum platform for studying one-dimensional electronic states.

Structural Characterization of Boron Sheets beyond the Monolayer and Implication for Experimental Synthesis and Identification

The successful synthesis of quasi-freestanding bilayer borophene has aroused much attention for its superior physical properties and holds great promise for future electronic devices. Herein, we comprehensively explore six boron sheets beyond the monolayer and structurally characterize them via various methods using first-principles calculations for experimental references. On the basis of atomic models of borophenes, simulated scanning tunneling microscope (STM) images show different morphologies at different bias voltages and are explained by the partial densities of states and the height differences in the vertical direction. Simulated transmission electron microscope images further probe the internal atomic arrangement of boron sheets and compensate for the shortcomings of STM images to better distinguish different phases of boron sheets. The interlayer coupling strength is stronger in bilayer borophenes than in the three-layer system via the electron localization function and Mulliken bond population. In addition, simulated X-ray diffraction and infrared spectra show different characteristic peaks and corresponding vibrational modes to further characterize these boron sheets. These theoretical results can decrease the prime cost and provide vital guidance for the experimental synthesis and identification of boron sheets beyond the monolayer.

Generalized Octet Rule with Fractional Occupancies for Boron

The octet rule is a fundamental theory in the chemical bonding of main-group elements, which achieve stable configurations by gaining, losing, or sharing electrons. However, the conventional octet rule, as depicted through Lewis structures, is inadequate for describing the electron delocalization in boron allotropes and boron-rich compounds due to the electron deficiency of boron. To address this, we introduce the concept of fractional electron occupancies, which more accurately reflect the electron delocalization in boron systems. Based on this, we propose a generalized octet rule that provides a more comprehensive understanding of the complex bonding configurations in boron allotropes and boron-rich compounds. Importantly, our predictions for α-B12 are validated by both first-principles calculations and existing experimental data. Beyond boron, this generalized octet rule is also applicable to systems with multiple resonance structures.

Semiconducting Bilayer Borophene with High Carrier Mobility

Borophene has attracted much interest due to its rich configurations and novel properties such as Dirac fermions and superconductivity. The recently emerged bilayer borophene mitigates the oxidation problem when exposed to air, yet most studies ignore the influence of charge transfer induced by metal substrates on structural stability. Here we identified 31 monolayer borophene polymorphs that are stabilized on Au(111), Ag(111), or Cu(111) substrates through first-principle calculations. Interestingly, two novel semiconducting bilayer borophene polymorphs with band gaps of 0.37 and 0.42 eV were screened by integrating these monolayers. The formation of interlayer bonding contributed by the delocalized electrons is responsible for the semiconductivity. The predicted highest electron mobility reaches 2.01 × 104 cm2V-1 s-1, implying the possibility as a semiconductor device with a low power consumption. Moreover, light was also systemically thrown on the factors that may affect the electronic properties of bilayer borophenes and the positional preference of interlayer bonds.

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.

Theory of sigma bond resonance in flat boron materials

In chemistry, theory of aromaticity or π bond resonance plays a central role in intuitively understanding the stability and properties of organic molecules. Here we present an analogue theory for σ bond resonance in flat boron materials, which allows us to determine the distribution of two-center two-electron and three-center two-electron bonds without quantum calculations. Based on this theory, three rules are proposed to draw the Kekulé-like bonding configurations for flat boron materials and to explore their properties intuitively. As an application of the theory, a simple explanation of why neutral borophene with ~1/9 hole has the highest stability and the effect of charge doping on borophene's optimal hole concentration is provided with the assumption of σ and π orbital occupation balance. Like the aromaticity theory for carbon materials, this theory greatly deepens our understanding on boron materials and paves the way for the rational design of various boron-based materials.

Superior Thermoelectric Properties of Twist-Angle Superlattice Borophene Induced by Interlayer Electrons Transport

In this paper, the energy bands, interlayer interactions and thermoelectric effects of twisted bilayer borophene (TBB) synthesized on Ag (111) are studied theoretically. The results manifest the advantages of twistronics, where the high electrical conductivity and the large Seebeck coefficient are regulated to the same range, which lead to the significantly increase of figure of merit ZT than that of bilayer borophene (BB) without twist, where the BB without twist is successfully synthesized on Ag (111) film is recently experimental report [Nat. Mater. 2022, 21, 35]. For the TBB synthesized of on Ag (111) film, theoretical analysis demonstrates that TBB and Ag are relatively strongly coupled, and TBB becomes a metallic 2D material, where the top and bottom borophene layers are semiconducting and metallic, respectively. TBB exhibits excellent thermoelectric efficiency due to the charge transfer bonding between the layers, less electron localization, and the regulation of Seebeck coefficient, electrical conductivity, and ZT at the same region of chemical potential and the same temperature by twistronics. The structure-property relationship offers the possibility of applying TBB in thermoelectric devices.

Defect engineering of two-dimensional materials for advanced energy conversion and storage

In the global trend towards carbon neutrality, sustainable energy conversion and storage technologies are of vital significance to tackle the energy crisis and climate change. However, traditional electrode materials gradually reach their property limits. Two-dimensional (2D) materials featuring large aspect ratios and tunable surface properties exhibit tremendous potential for improving the performance of energy conversion and storage devices. To rationally control the physical and chemical properties for specific applications, defect engineering of 2D materials has been investigated extensively, and is becoming a versatile strategy to promote the electrode reaction kinetics. Simultaneously, exploring the in-depth mechanisms underlying defect action in electrode reactions is crucial to provide profound insight into structure tailoring and property optimization. In this review, we highlight the cutting-edge advances in defect engineering in 2D materials as well as their considerable effects in energy-related applications. Moreover, the confronting challenges and promising directions are discussed for the development of advanced energy conversion and storage systems.

High temperature phases of borophene: borophene glass and liquid

Borophene is a family of two-dimensional (2D) boron materials containing many isomers with different hole concentrations and distributions in a triangular lattice. Although it has been widely studied theoretically and some have been synthesized experimentally, their thermodynamic properties are still unexplored. Based on density functional theory (DFT), we developed an accurate potential for the kinetic Monte Carlo (kMC) simulations of borophene. Through extensive kMC simulations, new phases were discovered, such as the glass state of borophene, liquid borophene and borophene with large holes. A phase diagram of borophene is constructed to guide future experiments on borophene materials at high temperature.

A theoretical study on the line defects in β12-borophene: enhanced direct-current and alternating-current conductances

Using density functional theory and the non-equilibrium Green's function method, we theoretically investigated the structures, stabilities, electronic properties, and the direct-current (DC) and alternating-current (AC) transport properties of the line defects in two-dimensional material β₁₂-borophene. Our results suggest that there exist six line defects that can enhance the stability of β₁₂-borophene and the line defects have profound influences on the electronic structure of β₁₂-borophene. Along the zigzag direction, the line defects can change the atomic orbital components of the Dirac cones in perfect β₁₂-borophene, but the line defects along the armchair direction have complicated influences on the Dirac cones. In the case of DC transport, some of the line defects lead to the constant DC phenomenon and the negative differential resistance effect, and enhance the DC conductances since the line defects exhibit typical one-dimensional characteristics. In the case of AC transport, some of the line defects enhance the AC conductances in the medium-frequency and high-frequency ranges through the photon-assisted tunneling effect. The microscopic mechanisms of the enhanced DC and AC conductances are different. In addition, for a low-frequency range, the equivalent circuits of β₁₂-borophene and the line defects were also suggested, which will be beneficial for designing borophene-based functional nanodevices.

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.

2D Ladder Polyborane: An Ideal Dirac Semimetal with a Multi-Field-Tunable Band Gap

Hydrogen, a simple and magic element, has attracted increasing attention for its effective incorporation within solids and powerful manipulation of electronic states. Here, we show that hydrogenation tackles common problems in two-dimensional borophene, e.g., stability and applicability. As a prominent example, a ladder-like boron hydride sheet, named as 2D ladder polyborane, achieves the desired outcome, enjoying the cleanest scenario with an anisotropic and tilted Dirac cone, that can be fully depicted by a minimal two-band tight-binding model. Introducing external fields, such as an electric field or a circularly polarized light field, can effectively induce distinctive massive Dirac fermions, whereupon four types of multi-field-driven topological domain walls hosting tunable chirality and valley indexes are further established. Moreover, the 2D ladder polyborane is thermodynamically stable at room temperature and supports highly switchable Dirac fermions, providing an ideal platform for realizing and exploring the various multi-field-tunable electronic states.

Optical spectra of bilayer borophene synthesized on Ag(111) film

In this paper, we theoretically investigated electronic structures, density of states (DOS), optical absorption, dielectric function of bilayer borophene synthesized on Ag(111) film, stimulated by the recent experimental report [Nature materials 2022, 21:35]. The results show that there is strong coupling between the Ag film and borophene layers. In the absorption spectra of BL borophene on Ag(111) substrate, there are strong absorption peaks in visible and infrared (IR) regions, which reveals strong plexciton peaks in visible and IR regions, which is contributed from the plasmonic and excitonic coupling interaction by the hybrid between Ag film and BL borophene. Raman modes of strongest vibration directly reflects the interlayer interaction of interlayer chemical bond. Our results not only provide physical insight into BL borophene synthesized on Ag(111) film, but also propose the potential applications of BL borophene in optoelectronic devices.

Study of the Cu(111) Surface by Scanning Tunneling Microscopy: The Morphology Evolution, Reconstructions, Superstructures and Line Defects

The Cu(111) surface is an important substrate for catalysis and the growth of 2D materials, but a comprehensive understanding of the preparation and formation of well-ordered and atomically clean Cu(111) surfaces is still lacking. In this work, the morphology and structure changes of the Cu(111) surface after treatment by ion bombardment and annealing with a temperature range of 300-720 °C are investigated systematically by using in situ low-temperature scanning tunneling microscopy. With the increase of annealing temperature, the surface morphology changes from corrugation to straight edge, the number of screw dislocations changes from none to numerous, and the surface atomic structure changes from disordered to ordered structures (with many reconstructions). In addition, the changing trend of step width and step height in different stages is different (first increased and then decreased). A perfect Cu(111) surface with a step height of one atom layer (0.21 nm) and a width of more than 150 nm was obtained. In addition, two interesting superstructures and a new surface phase with a large number of line defects were found. This work serves as a strong foundation for understanding the properties of Cu(111) surface, and it also provides important guidance for the effective pretreatment of Cu(111) substrates, which are widely used.

Vacancy-engineered nodal-line semimetals

Symmetry-enforced nodal-line semimetals are immune to perturbations that preserve the underlying symmetries. This intrinsic robustness enables investigations of fundamental phenomena and applications utilizing diverse materials design techniques. The drawback of symmetry-enforced nodal-line semimetals is that the crossings of energy bands are constrained to symmetry-invariant momenta in the Brillouin zone. On the other end are accidental nodal-line semimetals whose band crossings, not being enforced by symmetry, are easily destroyed by perturbations. Some accidental nodal-line semimetals have, however, the advantage that their band crossings can occur in generic locations in the Brillouin zone, and thus can be repositioned to tailor material properties. We show that lattice engineering with periodic distributions of vacancies yields a hybrid type of nodal-line semimetals which possess symmetry-enforced nodal lines and accidental nodal lines, with the latter endowed with an enhanced robustness to perturbations. Both types of nodal lines are explained by a symmetry analysis of an effective model which captures the relevant characteristics of the proposed materials, and are verified by first-principles calculations of vacancy-engineered borophene polymorphs. Our findings offer an alternative path to relying on complicated compounds to design robust nodal-line semimetals; one can instead remove atoms from a common monoatomic material.

Elemental Two-Dimensional Materials for Li/Na-Ion Battery Anode Applications

Two-dimensional (2D) nanostructure is currently the subject in the fields of new energy storage and devices. During the past years, a broad range of 2D materials represented by graphene have been developed and endow with excellent electrochemical properties. Among them, elemental 2D materials (Xenes) are an emerged material family for Li/Na-ion battery (LIB/SIB) anodes. Compared with other 2D materials and bulk materials, Xenes may exhibit some great superiorities for Li/Na storage, including excellent conductivity, fast ion diffusion and large active sites exposure. In this review, we provide a systematic summary of the recent progress and achievements of Xenes as well as their applications in LIBs/SIBs. The broad categorization of Xenes from group IIIA to VIA has been concisely outlined, and the related details in syntheses, structures and Li/Na-ion storage properties are reviewed. Further, the latest research progress of Xenes in Li/Na ion batteries are summarized, together with mechanism discussions. Finally, the challenges and prospects of Xenes applied to Li/Na ion battery are proposed based on its current developments.

Electronic structures and optical properties of monolayer borophenes

In this paper, we theoretically investigated the electronic and optical properties of monolayer borophene, including the electronic energy band, density of states (DOS), dielectric function, and absorption spectra and the charge distribution. The calculated phonon spectra and phononic DOS confirm that the four kinds of monolayer borophene structures can stably exist. Two-dimensional (2D) borophene exhibits apparent optical anisotropy in visible and near infrared (NIR) regions. Our results provide a reliable theoretical base for the application of monolayer borophene in optoelectronic 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.

Chemically identifying single adatoms with single-bond sensitivity during oxidation reactions of borophene

The chemical interrogation of individual atomic adsorbates on a surface significantly contributes to understanding the atomic-scale processes behind on-surface reactions. However, it remains highly challenging for current imaging or spectroscopic methods to achieve such a high chemical spatial resolution. Here we show that single oxygen adatoms on a boron monolayer (i.e., borophene) can be identified and mapped via ultrahigh vacuum tip-enhanced Raman spectroscopy (UHV-TERS) with ~4.8 Å spatial resolution and single bond (B–O) sensitivity. With this capability, we realize the atomically defined, chemically homogeneous, and thermally reversible oxidation of borophene via atomic oxygen in UHV. Furthermore, we reveal the propensity of borophene towards molecular oxygen activation at room temperature and phase-dependent chemical properties. In addition to offering atomic-level insights into the oxidation of borophene, this work demonstrates UHV-TERS as a powerful tool to probe the local chemistry of surface adsorbates in the atomic regime with widespread utilities in heterogeneous catalysis, on-surface molecular engineering, and low-dimensional materials.

Probing borophene oxidation at the atomic scale

Two-dimensional boron (i.e. borophene) holds promise for a variety of emerging nanoelectronic and quantum technologies. Since borophene is synthesized under ultrahigh vacuum (UHV) conditions, it is critical that the chemical stability and structural integrity of borophene in oxidizing environments are understood for practical borophene-based applications. In this work, we assess the mechanism of borophene oxidation upon controlled exposure to air and molecular oxygen in UHV via scanning tunneling microscopy andspectroscopy, x-ray photoelectron spectroscopy, and density functional theory calculations. While borophene catastrophically degrades almost instantaneously upon exposure to air, borophene undergoes considerably more controlled oxidation when exposed to molecular oxygen in UHV. In particular, UHV molecular oxygen dosing results in single-atom covalent modification of the borophene basal plane in addition to disordered borophene edge oxidation that shows altered electronic characteristics. By comparing these experimental observations with density functional theory calculations, further atomistic insight is gained including pathways for molecular oxygen dissociation, surface diffusion, and chemisorption to borophene. Overall, this study provides an atomic-scale perspective of borophene oxidation that will inform ongoing efforts to passivate and utilize borophene in ambient conditions.

Borophene as an emerging 2D flatland for biomedical applications: current challenges and future prospects

Recently, two-dimensional (2D)-borophene has emerged as a remarkable translational nanomaterial substituting its predecessors in the field of biomedical sensors, diagnostic tools, high-performance healthcare devices, super-capacitors, and energy storage devices. Borophene justifies its demand due to high-performance and controlled optical, electrical, mechanical, thermal, and magnetic properties as compared with other 2D-nanomaterials. However, continuous efforts are being made to translate theoretical and experimental knowledge into pragmatic platforms. To cover the associated knowledge gap, this review explores the computational and experimental chemistry needed to optimize borophene with desired properties. High electrical conductivity due to destabilization of the highest occupied molecular orbital (HOMO), nano-engineering at the monolayer level, chemistry-oriented biocompatibility, and photo-induced features project borophene for biosensing, bioimaging, cancer treatment, and theragnostic applications. Besides, the polymorphs of borophene have been useful to develop specific bonding for DNA sequencing and high-performance medical equipment. In this review, an overall critical and careful discussion of systematic advancements in borophene-based futuristic biomedical applications including artificial intelligence (AI), Internet-of-Things (IoT), and Internet-of-Medical Things (IoMT) assisted smart devices in healthcare to develop high-performance biomedical systems along with challenges and prospects is extensively addressed. Consequently, this review will serve as a key supportive platform as it explores borophene for next-generation biomedical applications. Finally, we have proposed the potential use of borophene in healthcare management strategies.

Borophene as a rising star in materials chemistry: synthesis, properties and applications in analytical science and energy devices

Borophene is a two-dimensional material that has shown outstanding applications in energy storage devices and analytical chemistry.

Borophene synthesis beyond the single-atomic-layer limit

Synthetic two-dimensional (2D) materials have no bulk counterparts and typically exist as single atomic layers due to substrate-stabilized growth. Multilayer formation, although broadly sought for structure and property tuning, has not yet been achieved in the case of synthetic 2D boron: that is, borophene^1,2. Here, we experimentally demonstrate the synthesis of an atomically well-defined borophene polymorph beyond the single-atomic-layer (SL) limit. The structure of this bilayer (BL) borophene is consistent with two covalently bonded α-phase layers (termed BL-α borophene) as evidenced from bond-resolved scanning tunnelling microscopy, non-contact atomic force microscopy and density functional theory calculations. While the electronic density of states near the Fermi level of BL-α borophene is similar to SL borophene polymorphs, field-emission resonance spectroscopy reveals distinct interfacial charge transfer doping and a heightened local work function exceeding 5 eV. The extension of borophene polymorphs beyond the SL limit significantly expands the phase space for boron-based nanomaterials.

Step-Edge Epitaxy for Borophene Growth on Insulators

Borophene─a monatomic layer of boron atoms─stands out among two-dimensional (2D) materials, with its versatile properties tantalizing for physics exploration and next-generation devices. Yet its phases are all synthesized on and stay bound to metal substrates, hampering both characterization and use. Borophene growth on an inert insulator would allow postsynthesis exfoliation, but the weak adhesion to such a substrate results in a high 2D nucleation barrier, preventing clean borophene growth. This challenge can be circumvented in a strategy devised and demonstrated here with ab initio calculations. Naturally present 1D-defects, the step-edges on an h-BN substrate surface, enable boron epitaxial assembly, reduce the nucleation dimensionality, and lower the barrier by an order of magnitude (to 1.1 eV or less), yielding a v_1/9 phase. Weak borophene adhesion to the insulator makes it readily accessible for comprehensive property tests or transfer into the device setting.

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.

Angstrom-Scale Spectroscopic Visualization of Interfacial Interactions in an Organic/Borophene Vertical Heterostructure

Two-dimensional boron monolayers (i.e., borophene) hold promise for a variety of energy, catalytic, and nanoelectronic device technologies due to the unique nature of boron-boron bonds. To realize its full potential, borophene needs to be seamlessly interfaced with other materials, thus motivating the atomic-scale characterization of borophene-based heterostructures. Here, we report the vertical integration of borophene with tetraphenyldibenzoperiflanthene (DBP) and measure the angstrom-scale interfacial interactions with ultrahigh-vacuum tip-enhanced Raman spectroscopy (UHV-TERS). In addition to identifying the vibrational signatures of adsorbed DBP, TERS reveals subtle ripples and compressive strains of the borophene lattice underneath the molecular layer. The induced interfacial strain is demonstrated to extend in borophene by ∼1 nm beyond the molecular region by virtue of 5 Å chemical spatial resolution. Molecular manipulation experiments prove the molecular origins of interfacial strain in addition to allowing atomic control of local strain with magnitudes as small as ∼0.6%. In addition to being the first realization of an organic/borophene vertical heterostructure, this study demonstrates that UHV-TERS is a powerful analytical tool to spectroscopically investigate buried and highly localized interfacial characteristics at the atomic scale, which can be applied to additional classes of heterostructured materials.

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.

Self-Assembled Borophene/Graphene Nanoribbon Mixed-Dimensional Heterostructures

Atomically thin metal-semiconductor heterojunctions are highly desirable for nanoelectronic applications. However, coherent lateral stitching of distinct two-dimensional (2D) materials has traditionally required interfacial lattice matching and compatible growth conditions, which remains challenging for most systems. On the other hand, these constraints are relaxed in 2D/1D mixed-dimensional lateral heterostructures due to the increased structural degree of freedom. Here, we report the self-assembly of mixed-dimensional lateral heterostructures consisting of 2D metallic borophene and 1D semiconducting armchair-oriented graphene nanoribbons (aGNRs). With the sequential ultrahigh vacuum deposition of boron and 4,4″-dibromo-p-terphenyl as precursors on Ag(111) substrates, an on-surface polymerization process is systematically studied and refined including the transition from monomers to organometallic intermediates and finally demetallization that results in borophene/aGNR lateral heterostructures. High-resolution scanning tunneling microscopy and spectroscopy resolve the structurally and electronically abrupt interfaces in borophene/aGNR heterojunctions, thus providing insight that will inform ongoing efforts in pursuit of atomically precise nanoelectronics.

Structural Defects, Mechanical Behaviors, and Properties of Two-Dimensional Materials

Since the success of monolayer graphene exfoliation, two-dimensional (2D) materials have been extensively studied due to their unique structures and unprecedented properties. Among these fascinating studies, the most predominant focus has been on their atomic structures, defects, and mechanical behaviors and properties, which serve as the basis for the practical applications of 2D materials. In this review, we first highlight the atomic structures of various 2D materials and the structural and energy features of some common defects. We then summarize the recent advances made in experimental, computational, and theoretical studies on the mechanical properties and behaviors of 2D materials. We mainly emphasized the underlying deformation and fracture mechanisms and the influences of various defects on mechanical behaviors and properties, which boost the emergence and development of topological design and defect engineering. We also further introduce the piezoelectric and flexoelectric behaviors of specific 2D materials to address the coupling between mechanical and electronic properties in 2D materials and the interactions between 2D crystals and substrates or between different 2D monolayers in heterostructures. Finally, we provide a perspective and outlook for future studies on the mechanical behaviors and properties of 2D materials.

Polymorphism in Post-Dichalcogenide Two-Dimensional Materials

Two-dimensional (2D) materials exhibit a wide range of atomic structures, compositions, and associated versatility of properties. Furthermore, for a given composition, a variety of different crystal structures (i.e., polymorphs) can be observed. Polymorphism in 2D materials presents a fertile landscape for designing novel architectures and imparting new functionalities. The objective of this Review is to identify the polymorphs of emerging 2D materials, describe their polymorph-dependent properties, and outline methods used for polymorph control. Since traditional 2D materials (e.g., graphene, hexagonal boron nitride, and transition metal dichalcogenides) have already been studied extensively, the focus here is on polymorphism in post-dichalcogenide 2D materials including group III, IV, and V elemental 2D materials, layered group III, IV, and V metal chalcogenides, and 2D transition metal halides. In addition to providing a comprehensive survey of recent experimental and theoretical literature, this Review identifies the most promising opportunities for future research including how 2D polymorph engineering can provide a pathway to materials by design.

Nanoscale Probing of Image-Potential States and Electron Transfer Doping in Borophene Polymorphs

Because synthetic 2D materials are generally stabilized by interfacial coupling to growth substrates, direct probing of interfacial phenomena is critical for understanding their nanoscale structure and properties. Using field-emission resonance spectroscopy with an ultrahigh vacuum scanning tunneling microscope, we reveal Stark-shifted image-potential states of the v_1/6 and v_1/5 borophene polymorphs on Ag(111) with long lifetimes, suggesting high borophene lattice and interface quality. These image-potential states allow the local work function and interfacial charge transfer of borophene to be probed at the nanoscale and test the widely employed self-doping model of borophene. Supported by apparent barrier height measurements and density functional theory calculations, electron transfer doping occurs for both borophene phases from the Ag(111) substrate. In contradiction with the self-doping model, a higher electron transfer doping level occurs for denser v_1/6 borophene compared to v_1/5 borophene, thus revealing the importance of substrate effects on borophene electron transfer.

Realization of Regular-Mixed Quasi-1D Borophene Chains with Long-Range Order

The polymorphism of borophene makes it a promising system to realize tunable physical or chemical properties. Various pure borophene phases consisting of quasi-1D boron chains with different widths have been commonly obtained in experimental studies. Here, it is shown that, due to a substrate mediation effect, artificial long-range ordered phases of borophene consisting of different combinations of boron chains seamlessly joined together can be achieved on Ag(100). Scanning tunneling microscopy measurements and theoretical calculations reveal that mixed-chain phases are more stable than the pure phase, and interact only weakly with the substrate. The mixed-chain phases with various proportions of different chains can be well separated based on the crystal direction of the substrate. The successful growth of mixed-chain phases is expected to deepen the impact of substrate tailored synthesis of borophene.

Highly efficient photothermal heating via distorted edge-defects in boron quantum dots

Quantum dots (QDs) are increasingly being utilized as near infrared (NIR) active photothermal agents for cancer diagnosis and therapy, with the main emphasis of current research being the enhancement of photothermal conversion efficiencies. Herein, we report the facile synthesis of 2-3 nm boron quantum dots (B QDs), which demonstrated a remarkable photothermal conversion efficiency of 57% under NIR excitation. This outstanding performance can be attributed to the alteration of the electronic structure, which was a result from the distorted edge-effect induced by the unique empty orbit of B atoms in the B QDs. These results can be verified by B K-edge near edge X-ray absorption fine structure (NEXAFS), high-resolution transmission electron microscopy (HR-TEM) and density functional theory (DFT) calculations. The results demonstrate that B QDs represent a promising new and non-toxic agent for both multimodal NIR-driven cancer imaging and photothermal therapy. This work thus identifies B QDs as an exciting new and theranostic agent for cancer therapy. Furthermore, the synthetic strategy used here to synthesize the B QDs was simple and easily scalable.

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.

Electric field tuned anisotropic to isotropic thermal transport transition in monolayer borophene without altering its atomic structure

Thermal anisotropy/isotropy is one of the fundamental thermal transport properties of materials and plays a critical role in a wide range of practical applications. Manipulation of anisotropic to isotropic thermal transport or vice versa is in increasing demand. However, almost all the existing approaches for tuning anisotropy or isotropy focus on structure engineering or materials processing, which is time and cost consuming and irreversible, while little progress has been made with an intact, robust, and reversible method. Motivated by the inherent relationship between interatomic interaction mediated phonon transport and electronic charges, we comprehensively investigate the effect of external electric field on thermal transport in two-dimensional (2D) borophene by performing first-principles calculations along with the phonon Boltzmann transport equation. Under external electric field, the lattice thermal conductivity of borophene in both in-plane directions first increases significantly to peak values with the maximum augmentation factor of 2.82, and the intrinsic anisotropy (the ratio of thermal conductivity along two in-plane directions) is boosted to the highest value of 2.13. After that, thermal conductivities drop down steeply and anisotropy exhibits oscillating decay. With the electric field increasing to 0.4 V Å^-1, the thermal conductivity is dramatically suppressed to 1/40 of the original value at no electric field. More interestingly, the anisotropy of the thermal conductivity decreases to the minimum value of 1.25, showing almost isotropic thermal transport. Such abnormal anisotropic to isotropic thermal transport transition stems from the large enhancement and suppression of phonon lifetime at moderate and high strength of electric field, respectively, and acts as an amplifying or reducing factor to the thermal conductivity. We further explain the tunability of phonon lifetime of the dominant acoustic mode by an electron localization function. By comparing the electric field-modulated thermal conductivity of borophene with the dielectric constant, it is found that the screened potential resulting from the redistributed charge density leads to phonon renormalization and the modulation of phonon anharmonicity and anisotropy through electric field. Our study paves the way for robust tuning of anisotropy of phonon transport in materials by applying intact, robust, and reversible external electric field without altering their atomic structure and would have a significant impact on emerging applications, such as thermal management of nanoelectronics and thermoelectric energy conversion.