Engineering the work function of buckled boron α-sheet by lithium adsorption: a first-principles investigation

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.

Effects of adatom species on the structure, stability, and work function of adatom-α-borophene nanocomposites

Work function-tunable borophene-based electrode materials are of significant importance because they promote efficient carrier extraction/injection, thereby enabling electronic devices to achieve maximum energy conversion efficiency. Accordingly, determining the work function of adatom-borophene nanocomposites within a series wherein the adatom is systematically changed will facilitate the design of such materials. In this study, we theoretically determined that the M-B bond length, binding energy, electron transfer between adatoms and BBP, and work function (ϕ) are linearly dependent on the ionization potential (IP) and electronegativity for thermodynamically and kinetically stable adatom-α-borophene (M/BBP) systems involving a series of alkali (earth) metal/BBP (M = Li-Cs; Be-Ba) and halogen/BBP (M = F-I), respectively. However, the binding energies of Li/BBP and Be/BBP deviate from these dependencies owing to their super small adatoms and the resulting significantly enhanced effective M-B bonding areas. By interpreting the electron transfer picture among the different parts of M/BBP, we confirmed that metallic M/BBP possesses ionic sp-p and dsp-p M-B bonds in alkali (earth) metal/BBP but covalent-featured ionic p-p interactions in halogen/BBP. In particular, the direct proportionality between IP and ϕ for alkali (earth) metal/BBP originates from the synergistic effect of charge rearrangement and the increased induced dipole moment; however, the inverse proportionality between electronegativity and ϕ for halogen/BBP arises from the adsorption induced charge redistribution. Our results provide guidance for experimental efforts toward the realization of work function-tunable borophene-based electrodes as well as insight into the bonding rules between various adatoms and α-borophene.

Imparting α-Borophene with High Work Function by Fluorine Adsorption: A First-Principles Investigation

Increasing the work function of borophene over a large range is crucial for the development of borophene-based anode materials for highly efficient electronic devices. In this study, the effect of fluorine adsorption on the structures and stabilities, particularly on the work function, of α-borophene (BBP), was systematically investigated via first-principles density functional theory. The calculations indicated that BBP was well-stabilized by fluorine adsorption and the work functions of metallic fluorine-adsorbed BBPs (F_n-BBPs) sharply increased with increasing fluorine content. Moreover, the work function of F-BBP was close to that of the frequently used anode material Au and even, for other F_n-BBPs, higher than that of Pt. Furthermore, we have comprehensively discussed the factors, including substrate deformation, charge transfer, induced dipole moment, and Fermi and vacuum energy levels, affecting the improvement of work function. Particularly, we have demonstrated that the charge redistribution of the substrate induced by the bonding interaction between fluorine and the matrix predominantly contributes to the observed increase in the work function. Additionally, the effect of fluorine adsorption on the increase in the work function of BBP was significantly stronger than that of silicene or graphene. Our results concretely support the fact that F_n-BBPs can be extremely attractive anode materials for electronic device applications.

Enhanced field-emission properties of buckled α-borophene by means of Li decoration: a first-principles investigation

In this study, the structures and field-emission properties of Li-decorated buckled α-borophene (BBP) were investigated by first-principles density functional theory at the PW91 level. Using the computed binding energies, Hirshfeld- and electrostatic potential-derived charges, induced dipole moments, densities of states, and ionization potentials, we evaluated the influence of an applied electric field on the structural stability, work function, and field-emission current of the Li-decorated BBP nanostructures. Furthermore, we also explored the quantitative dependence of the emission current on the electric field, Li concentration, and molecular orbitals. The computed results indicated that increasing the electric field and Li concentration has a considerably positive effect on the field-emission performance of the Li-decorated BBPs. Besides advantages including small work functions and low ionization potentials, most remarkably, the field-emission current can be as high as 48.81 μA in Li4/BBP (supercell with 36 atoms only) under a rather small applied electric field of 0.05 V Å-1, which rivals the highest value of the graphene-BN nanocomposite among all the theoretical nanostructures presented to date. Our results highly support the fact that Li-decorated BBPs can be appealing field-emission cathode materials with an extremely high emission current.

Heterogeneous activation of peroxymonosulfate by amorphous boron for degradation of bisphenol S

Recently, tremendous efforts have been devoted to developing carbon-based metal-free catalysts as an alternative to metal-based catalysts for remediation of emerging contaminants. However, further investigations have demonstrated that the durability of carbocatalysts is poor. Therefore, it is extremely desirable to seek a novel metal-free catalyst with high efficiency and superb stability. Herein, we first discovered that amorphous boron (A-boron) can be used as a metal-free catalyst for peroxymonosulfate (PMS) activation to produce free radicals for effective degradation of bisphenol S (BPS), which is a newly-occurring estrogenic endocrine-disrupting chemical. It exhibited outstanding catalytic activity and superior stability as comparing to metal-based and metal-free carbon-based catalysts. Moreover, many other typical organic pollutants in water such as bisphenol F, sulfamethoxazole, rhodamine B and methyl orange can also be effectively decomposed in A-boron/PMS oxidative system. The effects of reaction parameters on BPS degradation were systematically investigated. The catalytic oxidation mechanism was proposed. The intriguing catalytic feature of A-boron discovered in this study will provide new opportunities for the future development of A-boron based materials with promising applications in water remediation.

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.

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.