Synthesis of bilayer borophene

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.

High Transition Temperature Driven by Type-II Dirac Fermions in Topological Superconductor B7Be2B7 Nanosheet

Topological superconductors (TSCs) offer a promising avenue for delving into exotic states of matter and fundamental physics. We propose a strategy for realizing high transition temperatures (high-Tc) in TSCs by leveraging nontrivial topology alongside a high carrier density near the Fermi level in metal-doped borophenes. We identified 39 candidates with exceptional thermodynamic stability from thousands of Be-intercalated borophenes (Be1-xBx) via extensive structural searches. Seven candidates exhibit high carrier densities, with B7Be2B7 emerging as a particularly promising candidate. This nanosheet displays both type-I and type-II Dirac fermions, indicative of Z 2 topological metals, thereby positioning it as an ideal platform for high-Tc TSCs. The high-density π electrons of B7Be2B7 originating from type-II Dirac fermions, coupled with the out-of-plane vibrations of B and Be atoms, significantly enhance the electron-phonon coupling (λ = 1.42), resulting in a substantially high-Tc of 31.5 K. These findings underscore the potential of metal-doped borophenes as a cutting-edge material platform for achieving high-Tc TSCs.

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.

Borozenes: Benzene-Like Planar Aromatic Boron Clusters

ConspectusWith three valence electrons and four valence orbitals, boron (2s22p1) is an electron-deficient element, resulting in interesting chemical bonding and structures in both borane molecules and bulk boron materials. The electron deficiency leads to electron sharing and delocalization in borane compounds and bulk boron allotropes, characterized by polyhedral cages, in particular, the ubiquitous B12 icosahedral cage. During the past two decades, the structures and bonding of size-selected boron clusters have been elucidated via combined photoelectron spectroscopy and theoretical investigations. Unlike bulk boron materials, finite boron clusters have been found to possess 2D structures consisting of B3 triangles, dotted with tetragonal, pentagonal, or hexagonal holes. The discovery of the planar B36 cluster with a central hexagonal hole provided the first experimental evidence for the viability of 2D boron nanostructures (borophene), which have been synthesized on inert substrates. The B7-, B8-, and B9- clusters were among the first few boron clusters to be investigated by joint photoelectron spectroscopy and theoretical calculations, and they were all found to possess 2D structures with a central B atom inside a Bn ring. Recently, the B73- (C6v), B82- (D7h), and B9- (D8h) series of closed-shell species were shown to possess similar π bonding akin to that in the C5H5-, C6H6, and C7H7+ series, respectively, and the name "borozene" was coined to highlight their analogy to the classical aromatic hydrocarbon molecules.Among the borozenes, the D7h B82- species is unique for its high stability originating from both its double aromaticity and the fact that the B7 ring has the perfect size to host a central B atom. The B82- borozene has been realized experimentally in a variety of MB8 and M2B8 complexes. In particular, the B82- borozene has been observed to stabilize the rare valence-I oxidation state of lanthanides in LnB8- complexes, as well as a Cu2+ species in Cu2B8-. The B6 ring in B73- is too small to host a B atom, resulting in a slight out-of-plane distortion. Interestingly, the bowl-shaped B7 borozene is perfect for coordination to a metal atom, leading to the observation of a series of highly stable MB7 borozene complexes. On the other hand, the B8 ring is slightly too large to host the central B atom, such that a low-lying and low-symmetry isomer also exists for B9-. Even though most 2D boron clusters are aromatic, the B73-, B82-, and B9- borozenes are special because of their high symmetries and their analogy to the series of C5H5-, C6H6, and C7H7+ prototypical aromatic compounds. This Account discusses recent experimental and theoretical advances on the investigations of various borozene complexes. It is expected that many new borozene compounds can be designed and may be eventually synthesized.

Heterogeneous-Structured Molybdenum Diboride as a Novel and Promising Anode for Lithium-Ion Batteries

With the development of electric vehicles, exploiting anode materials with high capacity and fast charging capability is an urgent requirement for lithium-ion batteries (LIBs). Borophene, with the merits of high capacity, high electronic conductivity and fast diffusion kinetics, holds great potential as anode for LIBs. However, it is difficult to fabricate for the intrinsic electron-deficiency of boron atom. Herein, heterogeneous-structured MoB2 (h-MoB2) with amorphous shell and crystalline core, is prepared by solid phase molten salt method. As demonstrated, crystalline core can encapsulate the honeycomb borophene within two adjacent Mo atoms, and amorphous shell can accommodate more lithium ions to strengthen the lithium storage capacity and diffusion kinetics. According to theoretical calculations, the lithium adsorption energy in MoB2 is about -2.7 eV, and the lithium diffusion energy barrier in MoB2 is calculated to be 0.199 eV, guaranteeing the enhanced adsorption capability and fast diffusion kinetic behavior of Li+ ions. As a result, h-MoB2 anode presents high capacity of 798 mAh g-1 at 0.1 A g-1, excellent rate performance of 183 mAh g-1 at 5 A g-1 and long-term cyclic stability for 1200 cycles. This work may inspire ideas for the fabrication of borophene analogs and two-dimensional metal borides.

Growth of Single Crystalline 2D Materials beyond Graphene on Non-metallic Substrates

The advent of 2D materials has ushered in the exploration of their synthesis, characterization and application. While plenty of 2D materials have been synthesized on various metallic substrates, interfacial interaction significantly affects their intrinsic electronic properties. Additionally, the complex transfer process presents further challenges. In this context, experimental efforts are devoted to the direct growth on technologically important semiconductor/insulator substrates. This review aims to uncover the effects of substrate on the growth of 2D materials. The focus is on non-metallic substrate used for epitaxial growth and how this highlights the necessity for phase engineering and advanced characterization at atomic scale. Special attention is paid to monoelemental 2D structures with topological properties. The conclusion is drawn through a discussion of the requirements for integrating 2D materials with current semiconductor-based technology and the unique properties of heterostructures based on 2D materials. Overall, this review describes how 2D materials can be fabricated directly on non-metallic substrates and the exploration of growth mechanism at atomic scale.

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.

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.

An Extended Rudolph Diagram: B3H5 and B3H6+ Relate 3D-, 2D-, 1D-, and 0D-Boron Allotropes

The structural chemistry of boron goes beyond the sp, sp2, and sp3 hybridization paradigms of carbon chemistry. We relate the apparently unconnected polyhedral boranes and 3D allotropes on the one hand and 2D clusters, borophenes, and multilayer borophenes on the other hand, through an extended Rudolph diagram. All-boron equivalents of cyclopropenium cation viz the flat B3H5 and the nonplanar B3H6+ constitute the missing links. The nonplanar B3H6+ (C3v) is the starting point for construction of polyhedral boranes; e.g., fusion of two of them leads to octahedral B6H62-. On the other hand, planar B3H6+ and B3H5 relate to borophenes with hexagonal holes. These borophene sheets can be further stacked with diverse interlayer BB bonds, ranging from bilayers to infinite layers. The tendency to achieve electron sufficiency as in the parent C3H3+ dictates the preference for hexagonal holes in the constituent layers and the interlayer bonds between them in multilayer borophenes. The design principles and theoretical validations for the formation of multilayer borophenes are also presented, indicating the variety and complexities involved.

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

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

Chiral Induction in 2D Borophene Nanoplatelets through Stereoselective Boron-Sulfur Conjugation

Chirality is a structural metric that connects biological and abiological forms of matter. Although much progress has been made in understanding the chemistry and physics of chiral inorganic nanoparticles over the past decade, almost nothing is known about chiral two-dimensional (2D) borophene nanoplatelets and their influence on complex biological networks. Borophene's polymorphic nature, derived from the bonding configurations among boron atoms, distinguishes it from other 2D materials and allows for further customization of its material properties. In this study, we describe a synthetic methodology for producing chiral 2D borophene nanoplatelets applicable to a variety of structural polymorphs. Using this methodology, we demonstrate feasibility of top-down synthesis of chiral χ3 and β12 phases of borophene nanoplatelets via interaction with chiral amino acids. The chiral nanoplatelets were physicochemically characterized extensively by various techniques. Results indicated that the thiol presenting amino acids, i.e., cysteine, coordinates with borophene in a site-selective manner, depending on its handedness through boron-sulfur conjugation. The observation has been validated by circular dichroism, X-ray photoelectron spectroscopy, and 11B NMR studies. To understand how chiral nanoplatelets interact with biological systems, mammalian cell lines were exposed to them. Results showed that the achiral as well as the left- and right-handed biomimetic χ3 and β12 borophene nanoplatelets have distinct interaction with the cellular membrane, and their internalization pathway differs with their chirality. By engineering optical, physical, and chemical properties, these chiral 2D nanomaterials could be applied successfully to tuning complex biological events and find applications in photonics, sensing, catalysis, and biomedicine.

Searching for stable copper borozene complexes in CuB7- and CuB8. Phys Chem Chem Phys 2024;26:12928-12938. [PMID: 38456623 DOI: 10.1039/d4cp00296b] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Copper has been shown to be an important substrate for the growth of borophenes. Copper-boron binary clusters are ideal platforms to study the interactions between copper and boron, which may provide insight about the underlying growth mechanisms of borophene on copper substrates. Here we report a joint photoelectron spectroscopy and theoretical study on two copper-doped boron clusters, CuB7- and CuB8-. Well resolved photoelectron spectra are obtained for the two clusters at different wavelengths and are used to understand the structures and bonding properties of the two CuBn- clusters. We find that CuB8- is a highly stable borozene complex, which possesses a half-sandwich structure with a Cu+ species interacting with the doubly aromatic η8-B82- borozene. The CuB7- cluster is found to consist of a terminal copper atom bonded to a double-chain B7 motif, but it has a low-lying isomer composed of a half-sandwich structure with a Cu+ species interacting with an open-shell η7-B72- borozene. Both ionic and covalent interactions are found to be possible in the binary Cu-B clusters, resulting in different structures.

Superhard and Superconducting Bilayer Borophene

Two-dimensional superconductors, especially the covalent metals such as borophene, have received significant attention due to their new fundamental physics, as well as potential applications. Furthermore, the bilayer borophene has recently ignited interest due to its high stability and versatile properties. Here, the mechanical and superconducting properties of bilayer-δ6 borophene are explored by means of first-principles computations and anisotropic Migdal-Eliashberg analytics. We find that the coexistence of strong covalent bonds and delocalized metallic bonds endows this structure with remarkable mechanical properties (maximum 2D-Young's modulus of ~570 N/m) and superconductivity with a critical temperature of ~20 K. Moreover, the superconducting critical temperature of this structure can be further boosted to ~46 K by applied strain, which is the highest value known among all borophenes or two-dimensional elemental materials.

B63: The Most Stable Bilayer Structure with Dual Aromaticity

The emergence of a bilayer B48 cluster, which has been both theoretically predicted and experimentally observed, as well as the recent experimental synthesis of bilayer borophene sheets on Ag and Cu surfaces, has generated tremendous curiosity in the bilayer structures of boron clusters. However, the connection between bilayer boron cluster and bilayer borophene remains unknown. By combining a genetic algorithm and density functional theory calculations, a global search for the low-energy structures of the B63 cluster was conducted, revealing that the Cs bilayer structure with three interlayer B-B bonds is the most stable bilayer structure. This structure was further examined in terms of its structural stability, chemical bonding, and aromaticity. Interestingly, the interlayer bonds induce strong electronegativity and robust aromaticity. Furthermore, the dual aromaticity stems from diatropic currents originating from virtual translational transitions for both σ and π electrons. This unprecedent bilayer boron cluster is anticipated to enrich the concept of dual aromaticity and serve as a potential precursor for bilayer borophene.

Build Borophite from Borophenes: A Boron Analogue Graphite

Unlike graphene derived from graphite, borophenes represent a distinct class of synthetic two-dimensional materials devoid of analogous bulk-layered allotropes, leading to covalent bonding within borophenes instead of van der Waals (vdW) stacking. Our investigation focuses on 665 vdW-stacking boron bilayers to uncover potential bulk-layered boron allotropes through vdW stacking. Systematic high-throughput screening and stability analysis reveal a prevailing inclination toward covalently bonded layers in the majority of boron bilayers. However, an intriguing outlier emerges in δ5 borophene, demonstrating potential as a vdW-stacking candidate. We delve into electronic and topological structural similarities between δ5 borophene and graphene, shedding light on the structural integrity and stability of vdW-stacked boron structures across bilayers, multilayers, and bulk-layered allotropes. The δ5 borophene analogues exhibit metallic properties and characteristics of phonon-mediated superconductors, boasting a critical temperature near 22 K. This study paves the way for the concept of "borophite", a long-awaited boron analogue of graphite.

Electrochemical assisted enhanced nanozymatic activity of functionalized borophene for H2O2 and tetracycline detection

This study unveils the electrochemically-enhanced nanozymatic activity exhibited by borophene during the reaction of 3,3',5,5'-tetramethylbenzidine (TMB) and H2O2. Herein, the surface of the pristine borophene was first modified with the addition of thiocyanate groups to improve hydroxyl radical (•OH) scavenging activity. Then, the oxidation reaction of TMB was accelerated under applied electrochemical potential. Both factors significantly improved the detection limit and drastically decreased the detection time. DPPH testing revealed that the radical scavenging nature of borophene was more than 70%, boosting its catalytic activity. In the presence of H2O2, borophene catalyzed the oxidation of TMB and produced a blue-colored solution that was linearly correlated with the concentration of H2O2 and allowed for the detection of H2O2 up to 38 nM. The present finding was further extended to nanozymatic detection of tetracyclines (TCs) using a target-specific aptamer, and the results were colorimetrically quantifiable up to 1 μM with a LOD value of 150 nM. Moreover, transferring the principles of the discussed detection method to form a portable and disposable paper-based system enabled the quantification of TCs up to 0.2 μM. All the sensing experiments in this study indicate that the nanozymatic activity of borophene has significantly improved under electrochemical potential compared to conventional nanozyme-based colorimetric detection. Hence, the present discovery of electrochemically-enhanced nanozymatic activity would be promising for various sensitive and time-dependent colorimetric sensor development initiatives in the future.

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.

Two-Dimensional Transition Metal Boron Cluster Compounds (MBnenes) with Strain-Independent Room-Temperature Magnetism

Two-dimensional (2D) metal borides (MBenes) with unique electronic structures and physicochemical properties hold great promise for various applications. Given the abundance of boron clusters, we proposed employing them as structural motifs to design 2D transition metal boron cluster compounds (MBnenes), an extension of MBenes. Herein, we have designed three stable MBnenes (M4(B12)2, M = Mn, Fe, Co) based on B12 clusters and investigated their electronic and magnetic properties using first-principles calculations. Mn4(B12)2 and Co4(B12)2 are semiconductors, while Fe4(B12)2 exhibits metallic behavior. The unique structure in MBnenes allows the coexistence of direct exchange interactions between adjacent metal atoms and indirect exchange interactions mediated by the clusters, endowing them with a Néel temperature (TN) up to 772 K. Moreover, both Mn4(B12)2 and Fe4(B12)2 showcase strain-independent room-temperature magnetism, making them potential candidates for spintronics applications. The MBnenes family provides a fresh avenue for the design of 2D materials featuring unique structures and excellent physicochemical properties.

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.

Chemical bottom-up approach for inorganic single-atomic layers aiming beyond graphene

A chemical bottom-up approach for single-atomic-layered materials like graphene is attractive due to the possibility of introducing functions. This article includes the synthesis and properties of borophene-oxide and metalladithiolene layers, which are reported as inorganic materials. They have graphene-like two-dimensional networks that enable conjugated structures. Their atomically thin layers are also available by dissolution or synthetic methods. Their two-dimensional electronic features are evaluated from the activation energies for electrical conduction, focusing on the anisotropic features of borophene-oxide layers and the switching abilities of metalladithiolene layers.

A new type of stable borophene with flat-band-induced magnetism

Based on first-principles calculations, we propose a new type of thermally and dynamically stable magnetic borophene (B11) with a tetragonal lattice. The magnetism is found coming from spin polarization of one bonding flat band located at the Fermi level. Despite of the 'anti-molecular' behavior in the monolayer, the interactions between thepzorbitals of the B atoms in the double-octahedron structural unit lead to the formation of the flat bands with localization behaviors. One tight binding model is built to comprehend the magnetic mechanism, which can guide us to tune other nonmagnetic borophene becoming magnetic. Biaxial tensile strain (>2.1%) is found triggering a phase transition from a semimetal to a semiconductor in the B11monolayer. The mechanism is analyzed based on the orbital-resolved crystal field effect. Our work provides a new route for designing and achieving two-dimensional magnetic materials with light elements.

Density Functional Theory Study of Bilayer Borophene-Based Anode Material for Rechargeable Lithium Ion Batteries

The bilayer borophene has been successfully fabricated in experiments recently and possesses superior antioxidation and robust metallic properties, which holds great promise for the future anode materials of Li-ion batteries. Herein, using first-principles calculations, two bilayer borophenes including P6/mmm or P6̅m2 symmetry groups with or without vacancy defects are comprehensively explored and acted as electrode materials with high performance in Li-ion batteries. The charge density difference, adsorption energies, and Bader charge analysis are calculated and discussed for single lithium adsorbed on bilayer borophene. The results shown that with the increase of lithium concentration, the adsorption energies are rapidly decreased due to the repulsion of boron atoms except for the P6̅m2 systems with double side adsorption and corresponding energies remain the narrow range. Meanwhile, the partial density of states shows metallic character after lithium adsorption and indicates good conductivity for the charge-discharge process. Furthermore, small diffusion barriers, low average open-circuit voltage, can be achieved, and large storage capacity is up to 930.2 mA h/g at the lower lithium content of 0.375. These results propose that bilayer borophene might be a good choice for anode material applications in future Li-ion batteries with fast ion diffusion and high power density.

Dynamically Adjusting Borophene-Based Plasmon-Induced Transparency in a Polymer-Separated Hybrid System for Broadband-Tunable Sensing

Borophene, an emerging two-dimensional (2D) material platform, is capable of supporting highly confined plasmonic modes in the visible and near-infrared wavebands. This provides a novel building block for light manipulation at the deep subwavelength scale, thus making it well-suited for designing ultracompact optical devices. Here, we theoretically explore a borophene-based plasmonic hybrid system comprising a continuous borophene monolayer (CBM) and sodium nanostrip gratings (SNGs), separated by a polymer spacer layer. In such a structure, a dynamically tunable plasmon-induced transparency (PIT) effect can be achieved by strongly coupling dark and bright plasmonic modes, while actively controlling borophene. Here, the bright mode is generated through the localized plasmon resonance of SNGs when directly excited by TM-polarized incident light. Meanwhile, the dark mode corresponds to a propagating borophene surface plasmon (BSP) mode in the CBM waveguide, which cannot be directly excited, but requires phase matching with the assistance of SNGs. The thickness of the polymer layer has a significant impact on the coupling strength of the two modes. Owing to the BSP mode, highly sensitive to variations in the ambient refractive index (RI), this borophene-based hybrid system exhibits a good RI-sensing performance (643.8 nm/RIU) associated with a wide range of dynamically adjustable wavebands (1420-2150 nm) by tuning the electron density of borophene. This work offers a novel concept for designing active plasmonic sensors dependent on electrically gating borophene, which has promising applications in next-generation point-of-care (PoC) biomedical diagnostic techniques.

Photoelectron Spectroscopy and Theoretical Study of Di-Copper-Boron Clusters: Cu2B3- and Cu2B4. J Phys Chem A 2023. [PMID: 37235389 DOI: 10.1021/acs.jpca.3c02417] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Copper has been found to be able to mediate the formation of bilayer borophenes. Copper-boron binary clusters are ideal model systems to probe the copper-boron interactions, which are essential to understand the growth mechanisms of borophenes on copper substrates. Here, we report a joint photoelectron spectroscopy and theoretical study on two di-copper-doped boron clusters: Cu₂B₃^- and Cu₂B₄^-. Well-resolved photoelectron spectra are obtained, revealing the presence of a low-lying isomer in both cases. Theoretical calculations show that the global minimum of Cu₂B₃^- (C_2v, ¹A₁) contains a doubly aromatic B₃^- unit weakly interacting with a Cu₂ dimer, while the low-lying isomer (C_2v, ¹A₁) consists of a B₃ triangle with the two Cu atoms covalently bonded to two B atoms at two vertexes. The global minimum of Cu₂B₄^- (D_2h, ²A_g) is found to consist of a rhombus B₄ unit covalently bonded to the two Cu atoms at two opposite vertexes, whereas in the low-lying isomer (C_s, ²A'), one of the two Cu atoms is bonded to two B atoms.

Rectangular-Phase Tellurene on Ni(111) from Monolayer Films to Periodic Striped Patterns

As an emerging member of monoelemental two-dimensional (2D) materials, 2D tellurium (tellurene) has recently attracted intensive attention due to its polymorphism arising from the multivalent nature and fascinating properties such as wide-range band gaps, high carrier mobilities, etc. Herein, we predict the formation of a rectangular-phase tellurene on Ni(111) by first-principles density functional theory (DFT) calculations and realize its direct syntheses and characterizations by molecular beam epitaxy (MBE) and scanning tunneling microscopy (STM). We reveal that the monolayer rectangular tellurene and underlying Ni(111) substrate are strongly coupled, along with good lattice registry along two mutually perpendicular directions, which serves as the key driving force for the tellurene formation. We also uncover the unique morphological transitions of Te/Ni(111) from rectangular tellurene monolayer, to uniform periodic striped patterns at the second layer, and then to thick striped patterns. This work should offer valuable insights for the substrate-mediated syntheses of monoelemental 2D materials, thus propelling their phase engineering and intriguing property explorations.

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.

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.

Arsenic Monolayers Formed by Zero-Dimensional Tetrahedral Clusters and One-Dimensional Armchair Nanochains

One-dimensional (1D) arsenene nanostructures are predicted to host a variety of interesting physical properties including antiferromagnetic, semiconductor-semimetal transition and quantum spin Hall effect, which thus holds great promise for next-generation electronic and spintronic devices. Herein, we devised a surface template strategy in a combination with surface-catalyzed decomposition of molecular As₄ cluster toward the synthesis of the superlattice of ultranarrow armchair arsenic nanochains in a large domain on Au(111). In the low annealing temperature window, zero-dimensional As₄ nanoclusters are assembled into continuous films through intermolecular van der Waals and molecule-substrate interactions. At the elevated temperature, the subsequent surface-assisted decomposition of molecular As₄ nanoclusters leads to the formation of a periodic array of 1D armchair arsenic nanochains that form a (2 × 3) superstructure on the Au(111) surface. These ultranarrow armchair arsenic nanochains are predicted to have a small bandgap of ∼0.50 eV, in contrast to metallic zigzag chains. In addition, the Au-supported arsenic nanochains can be flipped to form a bilayer structure through tip indentation and manipulation, suggesting the possible transfer of these nanochains from the substrate. The successful realization of arsenic nanostructures is expected to advance low-dimensional physics and infrared optoelectronic nanodevices.

The effects of substrate and stacking in bilayer borophene

Bilayer borophene has recently attracted much interest due to its outstanding mechanical and electronic properties. The interlayer interactions of these bilayers are reported differently in theoretical and experimental studies. Herein, we design and investigate bilayer [Formula: see text] borophene, by first-principles calculations. Our results show that the interlayer distance of the relaxed AA-stacked bilayer is about 2.5 Å, suggesting a van der Waals interlayer interaction. However, this is not supported by previous experiments, therefore by constraining the interlayer distance, we propose a preferred model which is close to experimental records. This preferred model has one covalent interlayer bond in every unit cell (single-pillar). Further, we argue that the preferred model is nothing but the relaxed model under a 2% compression. Additionally, we designed three substrate-supported bilayers on the Ag, Al, and Au substrates, which lead to double-pillar structures. Afterward, we investigate the AB stacking, which forms covalent bonds in the relaxed form, without the need for compression or substrate. Moreover, phonon dispersion shows that, unlike the AA stacking, the AB stacking is stable in freestanding form. Subsequently, we calculate the mechanical properties of the AA and AB stackings. The ultimate strengths of the AA and the AB stackings are 29.72 N/m at 12% strain and 23.18 N/m at 8% strain, respectively. Moreover, the calculated Young's moduli are 419 N/m and 356 N/m for the AA and the AB stackings, respectively. These results show the superiority of bilayer borophene over bilayer [Formula: see text] in terms of stiffness and compliance. Our results can pave the way of future studies on bilayer borophene structures.

A family of superconducting boron crystals made of stacked bilayer borophenes

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

Structures, and electronic and spectral properties of single-atom transition metal-doped boron clusters MB24 - (M = Sc, Ti, V, Cr, Mn, Fe, Co, and Ni)

A theoretical study of geometrical structures, electronic properties, and spectral properties of single-atom transition metal-doped boron clusters MB₂₄ ^- (M = Sc, Ti, V, Cr, Mn, Fe, Co, and Ni) is performed using the CALYPSO approach for the global minimum search, followed by density functional theory calculations. The global minima obtained for the MB₂₄ ^- (M = Sc, Ti, V, and Cr) clusters correspond to cage structures, and the MB₂₄ ^- (M = Mn, Fe, and Co) clusters have similar distorted four-ring tubes with six boron atoms each. Interestingly, the global minima obtained for the NiB₂₄ ^- cluster tend to a quasi-planar structure. Charge population analyses and valence electron density analyses reveal that almost one electron on the transition-metal atoms transfers to the boron atoms. The electron localization function (ELF) of MB₂₄ ^- (M = Sc, Ti, V, Cr, Mn, Fe, Co, and Ni) indicates that the local delocalization of MB₂₄ ^- (M = Sc, Ti, V, Cr, and Ni) is weaker than that of MB₂₄ ^- (M = Mn, Fe, and Co), and there is no obvious covalent bond between doped metal and B atoms. The spin density and spin population analyses reveal that open-shell MB₂₄ ^- (M = Ti, Cr, Fe, and Ni) has different spin characteristics which are expected to lead to interesting magnetic properties and potential applications in molecular devices. The polarizability of MB₂₄ ^- (M = Sc, Ti, V, Cr, Mn, Fe, Co, and Ni) shows that MB₂₄ ^- (M = Mn, Fe, and Co) has larger first hyperpolarizability, indicating that MB₂₄ ^- (M = Mn, Fe, and Co) has a strong nonlinear optical response. Hence, MB₂₄ ^- (M = Mn, Fe, and Co) might be considered as a promising nonlinear optical boron-based nanomaterial. The calculated spectra indicate that MB₂₄ ^- (M = Sc, Ti, V, Cr, Mn, Fe, Co, and Ni) has different and meaningful characteristic peaks that can be compared with future experimental values and provide a theoretical basis for the identification and confirmation of these single-atom transition metal-doped boron clusters. Our work enriches the database of geometrical structures of doped boron clusters and can provide an insight into new doped boron clusters.

Macroscopic Single-Phase Monolayer Borophene on Arbitrary Substrates

A major challenge in the investigation of all 2D materials is the development of synthesis protocols and tools which would enable their large-scale production and effective manipulation. The same holds for borophene, where experiments are still largely limited to in situ characterizations of small-area samples. In contrast, our work is based on millimeter-sized borophene sheets, synthesized on an Ir(111) surface in ultrahigh vacuum. Besides high-quality macroscopic synthesis, as confirmed by low-energy electron diffraction (LEED) and atomic force microscopy (AFM), we also demonstrate a successful transfer of borophene from Ir to a Si wafer via electrochemical delamination process. Comparative Raman spectroscopy, in combination with the density functional theory (DFT) calculations, proved that borophene's crystal structure has been preserved in the transfer. Our results demonstrate successful growth and manipulation of large-scale, single-layer borophene sheets with minor defects and ambient stability, thus expediting borophene implementation into more complex systems and devices.

Recent Advances in Structure Separation of Single-Wall Carbon Nanotubes and Their Application in Optics, Electronics, and Optoelectronics

Structural control of single-wall carbon nanotubes (SWCNTs) with uniform properties is critical not only for their property modulation and functional design but also for applications in electronics, optics, and optoelectronics. To achieve this goal, various separation techniques have been developed in the past 20 years through which separation of high-purity semiconducting/metallic SWCNTs, single-chirality species, and even their enantiomers have been achieved. This progress has promoted the property modulation of SWCNTs and the development of SWCNT-based optoelectronic devices. Here, the recent advances in the structure separation of SWCNTs are reviewed, from metallic/semiconducting SWCNTs, to single-chirality species, and to enantiomers by several typical separation techniques and the application of the corresponding sorted SWCNTs. Based on the separation procedure, efficiency, and scalability, as well as, the separable SWCNT species, purity, and quantity, the advantages and disadvantages of various separation techniques are compared. Combined with the requirements of SWCNT application, the challenges, prospects, and development direction of structure separation are further discussed.

Quasi-Freestanding Bilayer Borophene on Ag(111)

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

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.

AuB8-: an Au-borozene complex

Photoelectron spectroscopy and quantum chemistry studies are used to investigate the structure and bonding of AuB₈^-. Global minimum sturctural searches show that AuB₈^- possesses a chair-like structure, which can be viewed as Au⁺ bonded to the edge of the doubly-aromatic B₈^2- borozene, Au⁺[η²-B₈^2-]. Chemical bonding analyses reveal that the AuB₈^- is a novel borozene complex with unique Au-borozene bonding.

Two-dimensional graphyne-graphene heterostructure for all-carbon transistors

Semiconducting graphyne is a two-dimensional (2D) carbon allotrope with high mobility, which is promising for next generation all-carbon field effect transistors (FETs). In this work, the electronic properties of van der Waals heterostructure consists of 2D graphyne and graphene (GY/G) were studied from first-principles calculations. It is found that the band dispersion of isolated graphene and graphyne remain intact after they were stacked together. Due to the charge transfer from graphene to graphyne, the Fermi level of the GY/G heterostructure crosses the VB of graphene and the CB of graphyne. As a result, n-type Ohmic contact with zero Schottky barrier height (SBH) is obtained in GY/G based FETs. Moreover, the electron tunneling from graphene to graphyne is found to be efficient. Therefore, excellent electron transport properties can be expected in GY/G based FETs. Lastly, it is demonstrated that the SBH in the GY/G heterostructure can be tune by applying a vertical external electric field or doping, and the transition from n-type to p-type contact can be realized. These results show that GY/G is potentially suitable for 2D FETs, and provide insights into the development of all-carbon electronic devices.

Vibrational Property of α-Borophene Determined by Tip-Enhanced Raman Spectroscopy

We report a Raman characterization of the α borophene polymorph by scanning tunneling microscopy combined with tip-enhanced Raman spectroscopy. A series of Raman peaks were discovered, which can be well related with the phonon modes calculated based on an asymmetric buckled α structure. The unusual enhancement of high-frequency Raman peaks in TERS spectra of α borophene is found and associated with its unique buckling when landed on the Ag(111) surface. Our paper demonstrates the advantages of TERS, namely high spatial resolution and selective enhancement rule, in studying the local vibrational properties of materials in nanoscale.

Bottom-up synthesized crystalline boron quantum dots with nonvolatile memory effects through one-step hydrothermal polymerization of ammonium pentaborane and boric acid

Crystalline BQDs are synthesized through a bottom-up strategy and used to fabricate a BQD–PVP memory device with nonvolatile rewritable memory effects.

Prediction of superconductivity in bilayer borophenes

Borophenes and related two-dimensional materials have exhibited many exotic properties, especially for superconductivity, although the superconductivity of single-layer borophene is suppressed by the strains or doping from its substrates. Intriguingly, bilayer (BL) borophenes can be stabilized by appropriate pillar density and hexagonal holes density, rather than being supported by Ag(111) or Cu(111) substrates. Thus, we studied the two most stable structures, namely BL-B8 and BL-B30, stabilized by the above-mentioned two methods. Within density functional theory and Bardeen-Cooper-Schrieffer theory framework, their stability, electron structures, and phonon properties, as well as possible superconductivity are systematically scrutinized. The metallic BL-B8 and BL-B30 exhibit intrinsic superconducting features with superconductivity transition temperatures (T c) of 11.9 and 4.9 K, respectively. The low frequency (below 400 cm-1) consisting of out-of-plane vibrations of boron atoms plays crucial rule in their superconductivity. In particular, a Kohn anomaly appears at the Γ point in BL-B8, leading to substantial electron-phonon coupling. Here, our findings will provide instructive clues for experimentally determining the superconductivity of borophene and will broaden the two-dimensional superconductor family.