The B35 cluster with a double-hexagonal vacancy: a new and more flexible structural motif for borophene

Observation of a metal-centered B2-Ta@B18−tubular molecular rotor and a perfect Ta@B20−boron drum with the record coordination number of twenty

A B₂-Ta@B₁₈⁻tubular molecular rotor and a Ta@B₂₀⁻boron drum with the record coordination number of twenty were observedviaa joint experimental and theoretical investigation.

Aromatic cage-like B46: existence of the largest decagonal holes in stable atomic clusters

The boron cluster B₄₆has a cage-like structure containing two hexagonal, two heptagonal and two decagonal holes. This finding presents a new family of cage-like boron clusters containing large B_Nholes withN= 6–10.

Stable sandwich structures of two-dimensional iron borides FeBxalloy: a first-principles calculation

Multilayer iron borides FeB_x(x= 4, 6, 8, 10) are wide-band-gap semiconductors; the electronic and optical properties of these semiconductors may be modulated by biaxial strains.

Wheel-like, elongated, circular, and linear geometries in boron-based CnB7−n(n = 0–7) clusters: structural transitions and aromaticity

Boron–carbon mixed clusters C_nB_7−n(n= 0–7) assume wheel-like, elongated, circular, and linear geometries, dictated by (π and σ) aromaticity and antiaromaticity.

From Quasi-Planar B56 to Penta-Ring Tubular Ca©B56: Prediction of Metal-Stabilized Ca©B56 as the Embryo of Metal-Doped Boron α-Nanotubes

Motifs of planar metalloborophenes, cage-like metalloborospherenes, and metal-centered double-ring tubular boron species have been reported. Based on extensive first-principles theory calculations, we present herein the possibility of doping the quasi-planar C_2v B₅₆ (A-1) with an alkaline-earth metal to produce the penta-ring tubular Ca©B₅₆ (B-1) which is the most stable isomer of the system obtained and can be viewed as the embryo of metal-doped (4,0) boron α-nanotube Ca©BNT_(4,0) (C-1). Ca©BNT_(4,0) (C-1) can be constructed by rolling up the most stable boron α-sheet and is predicted to be metallic in nature. Detailed bonding analyses show that the highly stable planar C_2v B₅₆ (A-1) is the boron analog of circumbiphenyl (C₃₈H₁₆) in π-bonding, while the 3D aromatic C_4v Ca©B₅₆ (B-1) possesses a perfect delocalized π system over the σ-skeleton on the tube surface. The IR and Raman spectra of C_4v Ca©B₅₆ (B-1) and photoelectron spectrum of its monoanion C_4v Ca©B₅₆⁻ are computationally simulated to facilitate their spectroscopic characterizations.

Substrate-Induced Nanoscale Undulations of Borophene on Silver

Two-dimensional (2D) materials tend to be mechanically flexible yet planar, especially when adhered on metal substrates. Here, we show by first-principles calculations that periodic nanoscale one-dimensional undulations can be preferred in borophenes on concertedly reconstructed Ag(111). This "wavy" configuration is more stable than its planar form on flat Ag(111) due to anisotropic high bending flexibility of borophene that is also well described by a continuum model. Atomic-scale ultrahigh vacuum scanning tunneling microscopy characterization of borophene grown on Ag(111) reveals such undulations, which agree with theory in terms of topography, wavelength, Moiré pattern, and prevalence of vacancy defects. Although the lattice is coherent within a borophene island, the undulations nucleated from different sides of the island form a distinctive domain boundary when they are laterally misaligned. This structural model suggests that the transfer of undulated borophene onto an elastomeric substrate would allow for high levels of stretchability and compressibility with potential applications to emerging stretchable and foldable devices.

Unveiling the atomic structure and electronic properties of atomically thin boron sheets on an Ag(111) surface

Two-dimensional (2D) boron sheets (i.e., borophene) have a huge potential as a basic building block in nanoelectronics and optoelectronics; such a situation is greatly promoted by recent experiments on fabrication of borophene on silver substrates. However, the fundamental atomic structure of borophene on the Ag substrate is still under debate, which greatly impedes further exploration of its properties. Herein, the atomic structure and electronic properties of borophene on an Ag(111) surface have been studied using first-principles calculations and ab initio molecular dynamics simulations. Our results reveal that there exist three energetically favorable borophene structures (β₅, χ₁, and χ₂) on the Ag(111) surface and their simulated STM images are in good agreement with experimental results, suggesting the coexistence of boron phases during the growth. All these stable borophene structures have a planar structure with slight surface buckling (∼0.15 Å) and relatively high hexagonal vacancy density (1/6 and 1/5) and exhibit typical metallic conductivity. These findings not only can be applied to solve the experimental controversies about the atomic structure of borophene on the Ag substrate but also provide a theoretical basis for exploring the fundamental properties and applications of 2D boron sheets.

Boron Nitride Nanostructures: Fabrication, Functionalization and Applications

Boron nitride (BN) structures are featured by their excellent thermal and chemical stability and unique electronic and optical properties. However, the lack of controlled synthesis of quality samples and the electrically insulating property largely prevent realizing the full potential of BN nanostructures. A comprehensive overview of the current status of the synthesis of two-dimensional hexagonal BN sheets, three dimensional porous hexagonal BN materials and BN-involved heterostructures is provided, highlighting the advantages of different synthetic methods. In addition, structural characterization, functionalizations and prospective applications of hexagonal BN sheets are intensively discussed. One-dimensional BN nanoribbons and nanotubes are then discussed in terms of structure, fabrication and functionality. In particular, the existing routes in pursuit of tunable electronic and magnetic properties in various BN structures are surveyed, calling upon synergetic experimental and theoretical efforts to address the challenges for pioneering the applications of BN into functional devices. Finally, the progress in BN superstructures and novel B/N nanostructures is also briefly introduced.

Comparative study on the spectral properties of boron clusters Bn(0/-1)(n = 38-40)

The all-boron fullerenes B40(-1) and B39(-1) discovered in recent experiments are characterized and revealed using photoelectron spectroscopy. Except for the photoelectron spectroscopy, one may identify such boron clusters with other spectroscopic techniques, such as infrared spectra and Raman spectra. Insight into the spectral properties of boron clusters is important to understand the boron clusters and find their potential applications. In this work, density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations are carried out to comparatively study the vibrational frequencies, infrared spectra, Raman spectra and electronic absorption spectra of boron clusters Bn(0/-1)(n = 38-40). The numerical simulations show that such boron clusters have different and meaningful spectral features. These spectral features are readily compared with future spectroscopy measurements and can be used as fingerprints to distinguish the boron clusters Bn(0/-1) with different structures (cage structure or quasi-planar structure) and with different sizes (n = 38-40).

N-H bond cleavage of ammonia on graphene-like B36 borophene: DFT studies

Ammonia N-H bond cleavage at metal-free substrates has attracted great attention because of its industrial importance. Here, we investigate the dissociative adsorption of ammonia onto the surface of a B36 borophene sheet by means of density functional theory calculations. We show that the N-H bond may be broken at the edges of B36 even at room temperature, regarding the small energy barrier of 14.1-19.3 kcal mol(-1) at different levels of theory, and more negative Gibbs free energy change. Unlike basis set size, the kind of exchange correlation functional significantly affects the electronic properties of the studied systems. Also, by increasing the percentage of Hartree Fock (HF) exchange of density functionals, the activation and adsorption energies are lowered. A linear relationship between the highest occupied molecular orbital or lowest unoccupied molecular orbital of B36 borophene and the %HF exchange of functionals is predicted. Our work reveals that pure whole boron nanosheets may be promising metal-free materials in N-H bond cleavage, which would raise the potential application of these sheets.

Lithium-Boron (Li-B) Monolayers: First-Principles Cluster Expansion and Possible Two-Dimensional Superconductivity

Recent works demonstrated that the superconductivity at two-dimensional (2-D) can be achieved in Li-decorated graphene (Nature Phys. 2012, 8, 131 and Proc. Natl. Acad. Sci. 2015, 112, 11795). Inspired by the progress made in graphene, we predict by using the first-principles calculations that Li-incorporated B monolayers (Li-B monolayers) can be alternative 2-D superconductors. First-principles cluster expansion approach was used to evaluate the structural diversity and energetic stability of the 2-D Li-B monolayers by treating them as ternary Lix⬡yB1-x-y pseudoalloys (⬡ refers to B hexagonal hole). After thoroughly exploring the Li-B configuration space, several well-ordered and stable Li-B monolayers were identified. Detailed analyses regarding the electronic structures and lattice dynamics properties of the predicted Li-B monolayers were performed. Compared with the non-superconducting pure B-sheet, some predicted Li-B monolayers can exhibit the phonon-mediated superconducting properties above the liquid helium temperature.

Endohedral charge-transfer complex Ca@B37−: stabilization of a B373−borospherene trianion by metal-encapsulation

First-principles theory investigations present the possibility of an endohedralC_sCa@B₃₇⁻which contains a 3D aromatic fullerene-likeC_sB₃₇³⁻trianion composed of interwoven double chains.

Concentric dual π aromaticity in bowl-like B30cluster: an all-boron analogue of corannulene

The bowl-like B₃₀cluster is an all-boron analogue of corannulene, featuring concentric dual π aromaticity with 6π and 14π electrons for the inner and the outer boron ribbons, respectively.

Beyond organic chemistry: aromaticity in atomic clusters

We describe joint experimental and theoretical studies carried out collaboratively in the authors' labs for understanding the structures and chemical bonding of novel atomic clusters, which exhibit aromaticity.

Aromatic character of planar boron-based clusters revisited by ring current calculations

The planarity of small boron-based clusters is the result of an interplay between geometry, electron delocalization, covalent bonding and stability.

Chemical bonding and dynamic fluxionality of a B15+cluster: a nanoscale double-axle tank tread

An elongated B₁₅⁺cluster is fluxional at 500 K. The peripheral ring rotates freely around a diamond-shaped core, akin to a tank tread.

Competition between quasi-planar and cage-like structures in the B29−cluster: photoelectron spectroscopy and ab initio calculations

B₂₉⁻and B₂₉clusters have competitive stingray-shapedversusseashell-like structures. The anion favors the former conformation slightly, whereas the neutral ion is dominated by the latter.

Planar B3S2H3−and B3S2H3clusters with a five-membered B3S2ring: boron–sulfur hydride analogues of cyclopentadiene

Boron–sulfur hydride clusters,C_2vB₃S₂H₃⁻and B₃S₂H₃, possess a five-membered B₃S₂ring as the core, which is analogous to cyclopentadiene in terms of π bonding.

Endohedral Ca@B38: stabilization of a B382−borospherene dianion by metal encapsulation

We present the viability of an endohedralC_sCa@B₃₈which contains aC_sB₃₈²⁻borospherene cage composed of interwoven boron double chains with a σ + π double delocalization bonding pattern, extending the B_n^q(q=n− 40) borospherene family fromn= 39–42 ton= 38.

Saturn-like charge-transfer complexes Li4&B36, Li5&B36+, and Li6&B362+: exohedral metalloborospherenes with a perfect cage-like B364−core

We present a first-principles theory prediction of the Saturn-like Li₄&B₃₆, Li₅&B₃₆⁺, and Li₆&B₃₆²⁺which extend the B_n^q(q=n− 40) borospherene family fromn= 38–42 ton= 36.

Boron based two-dimensional crystals: theoretical design, realization proposal and applications

The successful realization of free-standing graphene and the various applications of its exotic properties have spurred tremendous research interest for two-dimensional (2D) layered materials. Besides graphene, many other 2D materials have been successfully produced by experiment, such as silicene, monolayer MoS2, few-layer black phosphorus and so on. As a neighbor of carbon in the periodic table, element boron is interesting and many researchers have contributed their efforts to realize boron related 2D structures. These structures may be significant both in fundamental science and future technical applications in nanoelectronics and nanodevices. In this review, we summarize the recent developments of 2D boron based materials. The theoretical design, possible experimental realization strategies and their potential technical applications are presented and discussed. Also, the current challenges and prospects of this area are discussed.

Three-Dimensional Assignment of the Structures of Atomic Clusters: an Example of Au8M (M=Si, Ge, Sn) Anion Clusters

Identification of different isomer structures of atomic and molecular clusters has long been a challenging task in the field of cluster science. Here we present a three-dimensional (3D) assignment method, combining the energy (1D) and simulated (2D) spectra to assure the assignment of the global minimum structure. This method is more accurate and convenient than traditional methods, which only consider the total energy and first vertical detachment energies (VDEs) of anion clusters. There are two prerequisites when the 3D assignment method is ultilized. First, a reliable global minimum search algorithm is necessary to explore enough valleys on the potential energy surface. Second, trustworthy simulated spectra are necessary, that is to say, spectra that are in quantitative agreement. In this paper, we demonstrate the validity of the 3D assignment method using Au₈M⁻ (M = Si, Ge, Sn) systems. Results from this study indicate that the global minimum structures of Au₈Ge⁻ and Au₈Sn⁻ clusters are different from those described in previous studies.

Cobalt-centred boron molecular drums with the highest coordination number in the CoB16- cluster

The electron deficiency and strong bonding capacity of boron have led to a vast variety of molecular structures in chemistry and materials science. Here we report the observation of highly symmetric cobalt-centered boron drum-like structures of CoB₁₆⁻, characterized by photoelectron spectroscopy and ab initio calculations. The photoelectron spectra display a relatively simple spectral pattern, suggesting a high symmetry structure. Two nearly degenerate isomers with D_8d (I) and C_4v (II) symmetries are found computationally to compete for the global minimum. These drum-like structures consist of two B₈ rings sandwiching a cobalt atom, which has the highest coordination number known heretofore in chemistry. We show that doping of boron clusters with a transition metal atom induces an earlier two-dimensional to three-dimensional structural transition. The CoB₁₆⁻ cluster is tested as a building block in a triple-decker sandwich, suggesting a promising route for its realization in the solid state.

Boron is known to form a wide variety of molecular structures. Here, the authors observe the highly symmetric cobalt-centered boron drum-like structure of CoB₁₆⁻, characterized by photoelectron spectroscopy and ab initio calculations, in which the cobalt atom is sixteen-coordinate.

B28: the smallest all-boron cage from an ab initio global search

Our ab initio global searches reveal the lowest-energy cage for B28, which is built from two B12 units and prevails over the competing structural isomers such as planar, bowl, and tube. This smallest boron cage extends the scope of all-boron fullerene and provides a new structural motif of boron clusters and nanostructures.

Effects of bimetallic doping on small cyclic and tubular boron clusters: B7M2 and B14M2 structures with M = Fe, Co

Using density functional theory with the TPSSh functional and the 6-311+G(d) basis set, we extensively searched for the global minima of two metallic atoms doped boron clusters B6M2, B7M2, B12M2 and B14M2 with transition metal element M being Co and Fe. Structural identifications reveal that B7Co2, B7Fe2 and B7CoFe clusters have global minima in a B-cyclic motif, in which a perfectly planar B7 is coordinated with two metallic atoms placed along the C7 axis. The B6 cluster is too small to form a cycle with the presence of two metals. Similarly, the B12 cluster is not large enough to stabilize the metallic dimer within a double ring 2 × B6 tube. The doped B14M2 clusters including B14Co2, B14Fe2 and B14CoFe have a double ring 2 × B7 tubular shape in which one metal atom is encapsulated by the B14 tube and the other is located at an exposed position. Dissociation energies demonstrate that while bimetallic cyclic cluster B7M2 prefers a fragmentation channel that generates the B7 global minimum plus metallic dimer, the tubular structure B14M2 tends to dissociate giving a bimetallic cyclic structure B7M2 and a B@B6 cluster. The enhanced stability of the bimetallic doped boron clusters considered can be understood from the stabilizing interactions between the anti-bonding MOs of metal-metal dimers and the levels of a disk aromatic configuration (for bimetallic cyclic structures), or the eigenstates of the B14 tubular form (in case of bimetallic tubular structure).

Stabilization of fullerene-like boron cages by transition metal encapsulation

The stabilization of fullerene-like boron (B) cages in the free-standing form has been long sought after and a challenging problem. Studies that have been carried out for more than a decade have confirmed that the planar or quasi-planar polymorphs are energetically favored ground states over a wide range of small and medium-sized B clusters. Recently, the breakthroughs represented by Nat. Chem., 2014, 6, 727 established that the transition from planar/quasi-planar to cage-like Bn clusters occurs around n = ∼38-40, paving the way for understanding the intriguing chemistry of B-fullerene. We herein demonstrate that the transition demarcation, n, can be significantly reduced with the help of transition metal encapsulation. We explore via extensive first-principles swarm-intelligence based structure searches the free energy landscapes of B24 clusters doped by a series of transition metals and find that the low-lying energy regime is generally dominated by cage-like isomers. This is in sharp contrast to that of bare B24 clusters, where the quasi-planar and rather irregular polyhedrons are prevalent. Most strikingly, a highly symmetric B cage with D3h symmetry is discovered in the case of Mo or W encapsulation. The endohedral D3h cages exhibit robust thermodynamic, dynamic and chemical stabilities, which can be rationalized in terms of their unique electronic structure of an 18-electron closed-shell configuration. Our results indicate that transition metal encapsulation is a feasible route for stabilizing medium-sized B cages, offering a useful roadmap for the discovery of more B fullerene analogues as building blocks of nanomaterials.

B₈₄: a quasi-planar boron cluster stabilized with hexagonal holes

We report the finding of a bowl-shaped quasi-planar structure of a B84 cluster with four hexagonal holes and a three-chain ring all around the edges using ab initio calculations. A large number of other isomers including those explored earlier such as an empty cage, a filled cage, and a disordered structure, have been found to lie in a significantly higher energy band. A tubular structure, however, is only about 0.45 eV higher in energy. Calculations of the infrared and Raman spectra show that the quasi-planar structure is dynamically stable. These results suggest that quasi-planar structures may be among the low energy structures for larger clusters as well. Accordingly we have calculated the optimal quasi-planar structures stabilized with 2, 3, 5, 6, and 7 hexagonal holes also. The stability of quasi-planar structures is discussed in terms of multi-center two-electron bonding and it is shown that with increasing size their binding energy tends to approach the value for an α-boron sheet.