Geometric imaging of borophene polymorphs with functionalized probes

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.

Optical Spectroscopic Probing and Atomic Visualization of the Motion of N-Heterocyclic Carbenes on Ag(111)

The application of N-heterocyclic carbenes (NHCs) as versatile anchors for planar surface modifications has been well documented over the past decade. Despite its fundamental importance to the formation of self-assembled NHC monolayers on surfaces, the microscopic mechanism behind the mobility of NHCs has primarily been explored through theoretical studies; an atomic-level experimental understanding of NHC motion on surfaces remains elusive. Here, we combine tip-enhanced Raman spectroscopy (TERS) and scanning tunneling microscopy (STM) to investigate the mobility of a model NHC on Ag(111). Two distinct molecular behaviors are observed, depending on substrate preparation. Room-temperature deposition leads to diffusing NHC-Ag adatom complexes exhibiting a ballbot-like motion, chemically identified by TERS through their spectroscopic fingerprint. By contrast, NHCs deposited at low temperature are stabilized on Ag(111) as isolated single molecules directly bound to the substrate. Significantly, a desorption/readsorption scenario is suggested for the displacement of NHCs by moving otherwise immobile single NHCs deposited at low temperature via STM manipulation, with their trajectory traced to atomic precision. This study provides chemical and atomic-level insights into the mobility of NHCs, which will advance the understanding of the fundamental properties of NHC-based surface modifications at the spatial limit.

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.

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.

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.

Probing charge redistribution at the interface of self-assembled cyclo-P5 pentamers on Ag(111)

Phosphorus pentamers (cyclo-P5) are unstable in nature but can be synthesized at the Ag(111) surface. Unlike monolayer black phosphorous, little is known about their electronic properties when in contact with metal electrodes, although this is crucial for future applications. Here, we characterize the atomic structure of cyclo-P5 assembled on Ag(111) using atomic force microscopy with functionalized tips and density functional theory. Combining force and tunneling spectroscopy, we find that a strong charge transfer induces an inward dipole moment at the cyclo-P5/Ag interface as well as the formation of an interface state. We probe the image potential states by field-effect resonant tunneling and quantify the increase of the local change of work function of 0.46 eV at the cyclo-P5 assembly. Our experimental approach suggest that the cyclo-P5/Ag interface has the characteristic ingredients of a p-type semiconductor-metal Schottky junction with potential applications in field-effect transistors, diodes, or solar cells.

Molecular temperature descriptors as a novel approach for QSPR analysis of Borophene nanosheets

Borophene nanosheets appear in various sizes and shapes, ranging from simple planar structures to complicated polyhedral formations. Due to their unique chemical, optical, and electrical properties, Borophene nanosheets are theoretically and practically attractive and because of their high thermal conductivity, boron nanosheets are suitable for efficient heat transmission applications. In this paper, temperature indices of borophene nanosheets are computed and these indices are employed in QSPR analysis of attributes like Young's modulus, Shear modulus, and Poisson's ratio of borophene nanosheets and borophene β12 sheets. The regression model for the F-Temperature index is discovered to be the best fit for shear modulus, the reciprocal product connectivity temperature index is discovered to be fit for Poisson's ratio and the second hyper temperature index is discovered to be fit for Young's modulus based on the correlation coefficient.

Unraveling the Mechanism of Doping Borophene

We elucidate the doping mechanism of suitable elements into borophene with first-principles density functional theory calculation. During doping with nitrogen (N), the sp2 orbitals are responsible for arranging themselves to accommodate the electron of the N atom. Doping dramatically changes structure and electronic properties from corrugated and metallic borophene to flat and insulating h-BN with 100 % N-doping. We extend the mechanism of N-doping in borophene to doping of non-metallic and metallic ad-atoms on borophene. Our findings will help to design boron-based 2D materials.

Monolayer Borophene Formation on Cu(111) Surface Triggered by ⟨ 1 1 ¯ 0 ⟩ $\langle {1\bar{1}0} \rangle $ Step Edge

Borophene, a promising material with potential applications in electronics, energy storage, and sensors, is successfully grown as a monolayer on Ag(111), Cu(111), and Au(111) surfaces using molecular beam epitaxy. The growth of two-dimensional borophene on Ag(111) and Au(111) is proposed to occur via surface adsorption and boron segregation, respectively. However, the growth mode of borophene on Cu(111) remains unclear. To elucidate this, scanning tunneling microscopy in conjunction with theoretical calculations is used to study the phase transformation of boron nanostructures under post-annealing treatments. Results show that by elevating the substrate temperature, boron nanostructures undergo an evolution from amorphous boron to striped-phase borophene (η = 1/6) adhering to the Cu⟨ 1 1 ¯ 0 ⟩ $\langle {1\bar{1}0} \rangle $ step edge, and finally to irregularly shaped β-type borophene (η = 5/36) either on the substrate surface or embedded in the topmost Cu layer. dI/dV spectra recorded near the borophene/Cu lateral interfaces indicate that the striped-phase borophene is a metastable phase, requiring more buckling and electron transfer to stabilize the crystal structure. These findings offer not only an in-depth comprehension of the β-type borophene formation on Cu(111), but also hold potential for enabling borophene synthesis on weakly-binding semiconducting or insulating substrates with 1D active defects.

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.

Unidirectional Nano-modulated Binding and Electron Scattering in Epitaxial Borophene

A complex interplay between the crystal structure and the electron behavior within borophene renders this material an intriguing 2D system, with many of its electronic properties still undiscovered. Experimental insight into those properties is additionally hampered by the limited capabilities of the established synthesis methods, which, in turn, inhibits the realization of potential borophene applications. In this multimethod study, photoemission spectroscopies and scanning probe techniques complemented by theoretical calculations have been used to investigate the electronic characteristics of a high-coverage, single-layer borophene on the Ir(111) substrate. Our results show that the binding of borophene to Ir(111) exhibits pronounced one-dimensional modulation and transforms borophene into a nanograting. The scattering of photoelectrons from this structural grating gives rise to the replication of the electronic bands. In addition, the binding modulation is reflected in the chemical reactivity of borophene and gives rise to its inhomogeneous aging effect. Such aging is easily reset by dissolving boron atoms in iridium at high temperature, followed by their reassembly into a fresh atomically thin borophene mesh. Besides proving electron-grating capabilities of the boron monolayer, our data provide comprehensive insight into the electronic properties of epitaxial borophene which is vital for further examination of other boron systems of reduced dimensionality.

Effect of modulation by adsorption and doping on the quantum capacitance of borophene

Electric double-layer supercapacitors (EDLCs) have attracted much attention in the energy storage field due to their advantages such as high output power, long service life, safety and high efficiency. However, their low energy density limits their application. Aiming at the problem of the low energy density of EDLCs, improving quantum capacitance (CQ) of electrode materials is an effective strategy. In this paper, we systematically studied the effects of vacancy, doping, and metal atom adsorption on the CQ of borophene using first-principles calculations. The results show that S and N doping greatly enhance the charge accumulation of borophene at positive and negative potential, respectively. The maximum CQ values of S-doped and N-doped borophene are 157.3 μF cm-2 (0.38 V) and 187.8 μF cm-2 (-0.24 V), respectively. Both of them can serve as ideal candidates for the positive (S-doped one) and negative (N-doped one) electrodes of EDLCs. Besides, metal Al atom-adsorbed borophene can also effectively enhance the CQ, with a maximum value of 109.1 μF cm-2.

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.

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.

Two-dimensional Be2Al and Be2Ga monolayer: anti-van't Hoff/Le Bel planar hexacoordinate bonding and superconductivity

Because of the electron deficiency of boron, a triangular network with planar hexacoordination is the most common structural and bonding property for isolated boron clusters and two-dimensional (2D) boron sheets. However, this network is a rule-breaking structure and bonding case for all other main-group elements. Herein, the Be₂M (M = Al and Ga) 2D monolayer with P6/mmm space group was found to be the lowest-energy structure with planar hexacoordinate Be/Al/Ga motifs. More interestingly, Be₂Al and Be₂Ga were observed to be intrinsic phonon-mediated superconductors with a superconducting critical temperature (T_c) of 5.9 and 3.6 K, respectively, where compressive strain could further enhance their T_c. The high thermochemical and kinetic stability of Be₂M make a promising candidate for experimental realization, considering its high cohesive energy, absence of soft phonon modes, and good resistance to high temperature. Moreover, the feasibility of directly growing Be₂M on the electride Ca₂N substrate was further demonstrated, where its intriguing electronic and superconducting properties were well maintained in comparison with the freestanding monolayer. The Be₂M monolayer with rule-breaking planar hexacoordinate motifs firmly pushes the ultimate connection of the "anti-van't Hoff/Le Bel" structure with promising physical properties.

Monoelemental two-dimensional boron nanomaterials beyond theoretical simulations: From experimental preparation, functionalized modification to practical applications

During the past decade, there is an explosive growth of theoretical and computational studies on 2D boron-based nanomaterials. In terms of extensive predictions from theoretical simulations, borophene, boron nanosheets and 2D boron derivatives show excellent structural, electronic, photonic and nonlinear optical characteristics, and potential applications in a wide range of fields. In recent years, previous studies have reported the successful experimental preparations, superior properties, multi-functionalized modifications of various 2D boron and its derivatives, which show many practical applications in significant fields. To further promote the ever-increasing experimental studies, this present review systematically summarizes recent progress on experimental preparation methods, functionalized modification strategies and practical applications of 2D boron-based nanomaterials and multifunctional derivatives. Firstly, this review summarizes the experimental preparation methods, including molecular beam epitaxy, chemical vapor deposition, liquid-phase exfoliation, chemical reaction, and other auxiliary methods. Then, various strategies for functionalized modification are introduced overall, focusing on borophene derivatives, boron-based nanosheets, atom-introduced, chemically-functionalized borophene and boron nanosheets, borophene or boron nanosheet-based heterostructures, and other functionalized 2D boron nanomaterials. Subsequently, various potential applications are discussed in detail, involving energy storage, catalysis conversion, photonics, optoelectronics, sensors, bio-imaging, biomedicine therapy, and adsorption. We comment the state-of-the-art related studies concisely, and also discuss the current status, probable challenges and perspectives rationally. This review is timely, comprehensive, in-depth and highly attractive for scientists from multiple disciplines and scientific fields, and can facilitate further development of advanced functional low-dimensional nanomaterials and multi-functionalized systems toward high-performance practical applications in significant fields.

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.

Extraction of Hidden Science from Nanoscale Images

Scanning probe microscopies and spectroscopies enable investigation of surfaces and even buried interfaces down to the scale of chemical-bonding interactions, and this capability has been enhanced with the support of computational algorithms for data acquisition and image processing to explore physical, chemical, and biological phenomena. Here, we describe how scanning probe techniques have been enhanced by some of these recent algorithmic improvements. One improvement to the data acquisition algorithm is to advance beyond a simple rastering framework by using spirals at constant angular velocity then switching to constant linear velocity, which limits the piezo creep and hysteresis issues seen in traditional acquisition methods. One can also use image-processing techniques to model the distortions that appear from tip motion effects and to make corrections to these images. Another image-processing algorithm we discuss enables researchers to segment images by domains and subdomains, thereby highlighting reactive and interesting disordered sites at domain boundaries. Lastly, we discuss algorithms used to examine the dipole direction of individual molecules and surface domains, hydrogen bonding interactions, and molecular tilt. The computational algorithms used for scanning probe techniques are still improving rapidly and are incorporating machine learning at the next level of iteration. That said, the algorithms are not yet able to perform live adjustments during data recording that could enhance the microscopy and spectroscopic imaging methods significantly.

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.

Synthesis of bilayer borophene

As the nearest-neighbour element to carbon, boron is theoretically predicted to have a planar two-dimensional form, named borophene, with novel properties, such as Dirac fermions and superconductivity. Several polymorphs of monolayer borophene have been grown on metal surfaces, yet thicker bilayer and few-layer nanosheets remain elusive. Here we report the synthesis of large-size, single-crystalline bilayer borophene on the Cu(111) surface by molecular beam epitaxy. Combining scanning tunnelling microscopy and first-principles calculations, we show that bilayer borophene consists of two stacked monolayers that are held together by covalent interlayer boron-boron bonding, and each monolayer has β₁₂-like structures with zigzag rows. The formation of a bilayer is associated with a large transfer and redistribution of charge in the first boron layer on Cu(111), which provides additional electrons for the bonding of additional boron atoms, enabling the growth of the second layer. The bilayer borophene is shown to possess metallic character, and be less prone to being oxidized than its monolayer counterparts.

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.

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.

First-Principles Study of Linear and Nonlinear Optical Properties of Multi-Layered Borophene

Anisotropic materials are of great interest due to their unique direction-dependent optical properties. Borophene, the two-dimensional analog of graphene consisting of boron atoms, has attracted immense research interest due to its exciting anisotropic electronic and mechanical properties. Its synthesis in several structural polymorphic configurations has recently been reported. The present work reports the layer-dependent optical absorption and hyperpolarizabilities of the buckled borophene (δ6-borophene). The results, based on density functional theory, show that multilayer borophene is nearly transparent with only a weak absorbance in the visible region, reflecting its anisotropic structural characteristics. The static first-order hyperpolarizability significantly increases with the number of layers, due mainly to interactions among the frontier orbitals in multilayer borophene. Transparency in the visible region combined with enhanced nonlinear optical properties makes the multilayer borophene important for future photonics technologies.

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.

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.

Borophene with Large Holes

In two-dimensional (2D) borophene, the structural transition from triangular lattice to hexagonal lattice with an increase in vacancy concentration is a basic principle of constructing various borophene isomers. Here, by performing an extensive structural search of 4239 borophene isomers with both hexagonal holes (HHs) and large holes (LHs), we show that the structural transformation from triangular lattice to borophene with large holes is energetically more favorable. Borophene isomers with LHs are more stable than those with only HHs at high vacancy concentrations (>20%) and are just slightly less stable than those with only HHs at low vacancy concentrations. This discovery greatly expands the family of 2D borophene and opens a route for synthesizing new borophene isomers.

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.

Microscopic Theory of Plasmons in Substrate-Supported Borophene

We compute the dielectric properties of freestanding and metal-supported borophene from first-principles time-dependent density functional theory. We find that both the low- and high-energy plasmons of borophene are fully quenched by the presence of a metallic substrate at borophene-metal distances smaller than ≃9 Å. Based on these findings, we derive an electrodynamic model of the interacting, momentum-dependent polarizability for a two-dimensional metal on a model metallic substrate, which quantitatively captures the evolution of the dielectric properties of borophene as a function of metal-borophene distance. Applying this model to a series of metallic substrates, we show that maximizing the plasmon energy detuning between borophene and substrate is the key material descriptor for plasmonic performance.

Ultrathin boron nanosheets as an emerging two-dimensional photoluminescence material for bioimaging

Two-dimensional (2D) metal-free sheets with atomic thickness have been highly considered as promising candidates for fluorescent probes, due to their intriguing characteristics. In this work, 2D ultrathin boron nanosheets (B NSs) with a surface defect nanolayer can be effectively prepared by modified liquid phase exfoliation. The as-prepared ultrathin B NSs show blue fluorescence characteristics even with a quantum yield efficiency of up to 10.6%. Such luminescent behavior originates from the quantum confinement effect and the existence of a surface defect layer. In light of the advantages of being environmentally friendly, having high photostability and good biocompatibility, for the first time we have shown that ultrathin B NSs can be used as an emerging fluorescent probe for application in cellular bioimaging. It is believed that this work will open new avenues for ultrathin B NSs in biomedical fields, and it will also inspire the development of other elemental 2D nanomaterials.

Experimental realization of quasicubic boron sheets

Boron atoms possess a short covalent radius and the flexibility to adopt sp² hybridization, which favour the formation of diverse two-dimensional allotropes of boron. Several examples of such boron sheets with metallic nature have been reported recently. However, a semiconducting boron crystal with a direct bandgap is rarely reported either in bulk boron crystals or in two-dimensional boron sheets. Here, the boron sheets with a direct bandgap are synthesized on a Ni foil substrate by chemical vapor deposition. The boron sheets with 48 boron atoms per unit cell have a quasicubic structure, and they are semiconducting and have a direct bandgap of around 2.4 eV, which are verified by combining theoretical and experimental investigations. The result greatly expands the known allotropy of the fifth element and opens vast opportunities to design 2D boron sheets with tunable optical, electronic, magnetic and chemical properties.

Borophene Concentric Superlattices via Self-Assembly of Twin Boundaries

Due to its in-plane structural anisotropy and highly polymorphic nature, borophene has been shown to form a diverse set of linear superlattice structures that are not observed in other two-dimensional materials. Here, we show both theoretically and experimentally that concentric superlattice structures can also be realized in borophene via the energetically preferred self-assembly of coherent twin boundaries. Since borophene twin boundaries do not require the creation of additional lattice defects, they are exceptionally low in energy and thus easier to nucleate and even migrate than grain boundaries in other two-dimensional materials. Due to their high mobility, borophene twin boundaries naturally self-assemble to form novel phases consisting of periodic concentric loops of filled boron hexagons that are further preferred energetically by the rotational registry of borophene on the Ag(111) surface. Compared to defect-free borophene, concentric superlattice borophene phases are predicted to possess enhanced mechanical strength and localized electronic states. Overall, these results establish defect-mediated self-assembly as a pathway to unique borophene structures and properties.

Two-Dimensional Borophene: Properties, Fabrication, and Promising Applications

Monoelemental two-dimensional (2D) materials (Xenes) aroused a tremendous attention in 2D science owing to their unique properties and extensive applications. Borophene, one emerging and typical Xene, has been regarded as a promising agent for energy, sensor, and biomedical applications. However, the production of borophene is still a challenge because bulk boron has rather intricate spatial structures and multiple chemical properties. In this review, we describe its excellent properties including the optical, electronic, metallic, semiconducting, photoacoustic, and photothermal properties. The fabrication methods of borophene are also presented including the bottom-up fabrication and the top-down fabrication. In the end, the challenges of borophene in the latest applications are presented and perspectives are discussed.

Single-Phase Borophene on Ir(111): Formation, Structure, and Decoupling from the Support

Artificial two-dimensional (2D) materials, which host electronic or spatial structure and properties not typical for their bulk allotropes, can be grown epitaxially on atomically flat surfaces; the design and investigation of these materials are thus at the forefront of current research. Here we report the formation of borophene, a planar boron allotrope, on the surface of Ir(111) by exposing it to the flux of elemental boron and consequent annealing. By means of scanning tunneling microscopy and density functional theory calculations, we reveal the complex structure of this borophene, different from all planar boron allotropes reported earlier. This structure forms as a single phase on iridium substrate in a wide range of experimental conditions and may be then decoupled from the substrate via intercalation. These findings allow for production of large, defect-free borophene sheets and advance theoretical understanding of polymorphism in borophene.

Quantitative determination of atomic buckling of silicene by atomic force microscopy

The atomic buckling in 2D "Xenes" (such as silicene) fosters a plethora of exotic electronic properties such as a quantum spin Hall effect and could be engineered by external strain. Quantifying the buckling magnitude with subangstrom precision is, however, challenging, since epitaxially grown 2D layers exhibit complex restructurings coexisting on the surface. Here, we characterize using low-temperature (5 K) atomic force microscopy (AFM) with CO-terminated tips assisted by density functional theory (DFT) the structure and local symmetry of each prototypical silicene phase on Ag(111) as well as extended defects. Using force spectroscopy, we directly quantify the atomic buckling of these phases within 0.1-Å precision, obtaining corrugations in the 0.8- to 1.1-Å range. The derived band structures further confirm the absence of Dirac cones in any of the silicene phases due to the strong Ag-Si hybridization. Our method paves the way for future atomic-scale analysis of the interplay between structural and electronic properties in other emerging 2D Xenes.

Two novel triangular borophenes B3H and B6O: first-principles prediction

By means of density functional theory calculations, we successfully predict two stable 2D triangular borophenes, namely B₃H and B₆O. Our results indicate that B₃H is a Dirac material and its cone point is located at the K point of the Brillouin zone (BZ). B₆O is identified as having a node-line ring and Dirac cones together. Its node-line ring formed by the intersection of the extended energy band from the two Dirac cones located on K point. This modified 2D borophene has great thermal and dynamic stability due to the electron transfer from the triangular boron lattice to the O atoms. The electronic structure of B₆O nanofilm demonstrates novel properties such as two Dirac cones, more than 1.3 eV linear dispersion bands at some points of the BZ, as well as excellent transport properties for the extremely high mobility brought by the combination of the node-line semimetal and Dirac cones. Our study may motivate potential applications of 2D materials in nanoelectronics.

Borophene-graphene heterostructures

Integration of dissimilar two-dimensional (2D) materials is essential for nanoelectronic applications. Compared to vertical stacking, covalent lateral stitching requires bottom-up synthesis, resulting in rare realizations of 2D lateral heterostructures. Because of its polymorphism and diverse bonding geometries, borophene is a promising candidate for 2D heterostructures, although suitable synthesis conditions have not yet been demonstrated. Here, we report lateral and vertical integration of borophene with graphene. Topographic and spatially resolved spectroscopic measurements reveal nearly atomically sharp lateral interfaces despite imperfect crystallographic lattice and symmetry matching. In addition, boron intercalation under graphene results in rotationally commensurate vertical heterostructures. The rich bonding configurations of boron suggest that borophene can be integrated into a diverse range of 2D heterostructures.

Near-equilibrium growth from borophene edges on silver

Two-dimensional boron, borophene, was realized in recent experiments but still lacks an adequate growth theory for guiding its controlled synthesis. Combining ab initio calculations and experimental characterization, we study edges and growth kinetics of borophene on Ag(111). In equilibrium, the borophene edges are distinctly reconstructed with exceptionally low energies, in contrast to those of other two-dimensional materials. Away from equilibrium, sequential docking of boron feeding species to the reconstructed edges tends to extend the given lattice out of numerous polymorphic structures. Furthermore, each edge can grow via multiple energy pathways of atomic row assembly due to variable boron-boron coordination. These pathways reveal different degrees of anisotropic growth kinetics, shaping borophene into diverse elongated hexagonal islands in agreement with experimental observations in terms of morphology as well as edge orientation and periodicity. These results further suggest that ultrathin borophene ribbons can be grown at low temperature and low boron chemical potential.