Two-dimensional boron: structures, properties and applications

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.

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.

Giant Expanded Porous Metallo-Hexagons

Molecular self-assembly is a widely recognized approach for fabricating biomimetic functional nanostructures. Here, we report the synthesis of two giant hollow coronoid-like supramolecular hexagons, H1 and H2. These hexagons feature large cavities, showcasing unique inner and outer hexagons fixed by specific connectivities for enhanced stability and high metal center density. H1 exhibits properties that can be transformed through the thermodynamic conversion of the metallopolymer formed by L1 and L2. With an edge length of 6.8 nm, H2 is one of the largest hexagons reported to date. 1D and 2D NMR, TEM, ESI-MS, and TWIM-MS experiments provided conclusive evidence for the composition and structure of the assembled hexagons. This work demonstrates the feasibility of constructing giant supramolecular architectures with precise control over their size and shape, opening up new possibilities for the design and synthesis of sophisticated supramolecules and nonbiological materials.

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.

Borophene: a piezocatalyst for water remediation

Borophene is an emerging two-dimensional material exhibiting exceptional piezocatalytic activity under the influence of ultrasonic vibrations, as exemplified herein by the decomposition of highly stable organic dyes in water. After 6 minutes of exposure, borophene sheets converted up to 92 percent of a mixture of dye molecules at room temperature.

Klein Tunneling in β12 Borophene

Motivated by the recent observation of Klein tunneling in 8-Pmmn borophene, we delve into the phenomenon in β12 borophene by employing tight-binding approximation theory to establish a theoretical mode. The tight-binding model is a semi-empirical method for establishing the Hamiltonian based on atomic orbitals. A single cell of β12 borophene contains five atoms and multiple central bonds, so it creates the complexity of the tight-binding model Hamiltonian of β12 borophene. We investigate transmission across one potential barrier and two potential barriers by changing the width and height of barriers and the distance between two potential barriers. Regardless of the change in the barrier heights and widths, we find the interface to be perfectly transparent for normal incidence. For other angles of incidence, perfect transmission at certain angles can also be observed. Furthermore, perfect and all-angle transmission across a potential barrier takes place when the incident energy approaches the Dirac point. This is analogous to the "super", all-angle transmission reported for the dice lattice for Klein tunneling across a potential barrier. These findings highlight the significance of our theoretical model in understanding the complex dynamics of Klein tunneling in borophene structures.

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.

Selective Interfacial Excited-State Carrier Dynamics and Efficient Charge Separation in Borophene-Based Heterostructures

Borophene-based van der Waals heterostructures have demonstrated enormous potential in the realm of optoelectronic and photovoltaic devices, which has sparked a wide range of interest. However, a thorough understanding of the microscopic excited-state electronic dynamics at interfaces is lacking, which is essential for determining the macroscopic optoelectronic and photovoltaic performance of borophene-based devices. In this study, photoexcited carrier dynamics of β12 , χ3 , and α΄ borophene/MoS2 heterostructures are systematically studied based on time-domain nonadiabatic molecular dynamics simulations. Different Schottky contacts are found in borophene/semiconductor heterostructures. The interplay between Schottky barriers, electronic coupling, and the involvement of different phonon modes collectively contribute to the unique carrier dynamics in borophene-based heterostructures. The diverse borophene allotropes within the heterostructures exhibit distinct and selective carrier transfer behaviors on an ultrafast timescale: electrons tunnel into α΄ borophene with an ultrafast transfer rate (≈29 fs) in α΄/MoS2 heterostructures, whereas β12 borophene only allows holes to migrate with a lifetime of 176 fs. The feature enables efficient charge separation and offers promising avenues for applications in optoelectronic and photovoltaic devices. This study provides insight into the interfacial carrier dynamics in borophene-based heterostructures, which is helpful in further design of advanced 2D boron-based optoelectronic and photovoltaic devices.

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.

Enhancement of multilayer lithium storage in a β12-borophene/graphene heterostructure with built-in dipoles

The combination of borophene with a supporting metallic layer is beneficial in stabilizing its structure and promoting its application in energy storage. Here, through first-principles calculations, we screen a β12-borophene/graphene (β12-B/G) heterostructure with superior structural integrity, strong interlayer binding, and high thermodynamic stability among different B/G heterostructures. Besides, it is noteworthy that β12-B/G has been recently synthesized, further opening the possibility of expanding its use in energy storage. Then the selected target is systematically investigated as an anode material for lithium-ion batteries (LIBs). Compared with each monolayer component, multiple lithium-ion adsorption is achieved in the β12-B/G heterostructure, resulting in an ultra-high theoretical specific capacity of 2267 mA h g-1. In addition, a lower diffusion energy barrier indicates faster electron transport and lithium-ion diffusion in the β12-B/G heterostructure. Notably, the multilayer lithium adsorption avoids the formation of dendritic deposits, as evidenced by complete ionization of the cationic layers. Moreover, the disparity in the work functions of the individual layers gives rise to a built-in dipole in β12-B/G, further enhancing the multilayer lithium storage and ion migration. All these results suggest that the construction of borophene-based heterostructures with built-in dipoles is a feasible way to design high-performance LIB anode materials.

Self-Termination of Borophene Edges

Due to boron's unique bonding nature, planar boron materials, including borophenes, boron nanoclusters, and nanoribbons, show very puzzling features, especially the superior stability of the free-standing planar boron edges. Here, we present a systematic investigation of the bonding configurations of various edges of borophene. Because of the flexibility of forming either three-center two-electron (3c-2e) or two-center two-electron bonds (2c-2e), an edge of borophene tends to be self-terminated by adopting a different bonding configuration at the edge from that in bulk. Among various borophene edge types, the double-chain-terminated flat edge is found to be significantly stable. As a consequence, we found that the double- and triple-chain borophene nanoribbons with a triangular lattice and wider ribbons with hexagonal holes in the central area are more stable than the quadruple-chain borophene nanoribbon. This study greatly deepens our understanding of the bonding configurations, electronic properties, and stabilities of planar boron nanostructures and paves the way for the rational design and synthesis of various boron materials.

Nanotechnology-assisted treatment of pharmaceuticals contaminated water

The presence of pharmaceutical compounds in wastewater due to an increase in industrialization and urbanization is a serious health concern. The demand for diverse types of pharmaceutical compounds is expected to grow as there is continuous improvement in the global human health standards. Discharge of domestic pharmaceutical personal care products and hospital waste has aggravated the burden on wastewater management. Further, the pharmaceutical water is toxic not only to the aquatic organism but also to terrestrial animals coming in contact directly or indirectly. The pharmaceutical wastes can be removed by adsorption and/or degradation approach. Nanoparticles (NPs), such as 2D layers materials, metal-organic frameworks (MOFs), and carbonaceous nanomaterials are proven to be more efficient for adsorption and/or degradation of pharmaceutical waste. In addition, inclusion of NPs to form various composites leads to improvement in the waste treatment efficacy to a greater extent. Overall, carbonaceous nanocomposites have advantage in the form of being produced from renewable resources and the nanocomposite material is biodegradable either completely or to a great extent. A comprehensive literature survey on the recent advancement of pharmaceutical wastewater is the focus of the present article.

Generalized Octet Rule with Fractional Occupancies for Boron

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

Semiconducting Bilayer Borophene with High Carrier Mobility

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

Influence of plasma kinetic energy during the pulsed laser deposition of borophene films on silicon (100)

Developing borophene films with good structural stability on non-metallic substrates to maximize their potential in photosensitivity, gas detection, photothermia, energy storage, and deformation detection, among others has been challenging in recent years. Herein, we succeeded in the pulsed laser deposition of multilayered borophene films on Si (100) with β12 or χ3 bonding by tuning the mean kinetic energy in the plasma during the deposition process. Raman and X-ray photoelectron spectroscopies confirm β12 and χ3 bonding in the films. Borophene films with β12 bonding were obtained by tuning a high mean kinetic energy in the plasma, while borophene with χ3 bonding required a relatively low mean kinetic energy. Atomic force microscopy (AFM) micrographs revealed a granular and directional growth of the multilayered borophene films following the linear atomic terraces from the (100) silicon substrate. AFM nanofriction was used to access the borophene surfaces and to reveal the pull-off force and friction coefficient of the films where the surface oxide showed a significant contribution. To summarize, we show that it is possible to deposit multilayered borophene thin films with different bondings by tuning the mean kinetic energy during pulsed laser deposition. The characterization of the plasma during borophene deposition accompanies our findings, providing support for the changes in kinetic energy.

Two-dimensional boron nanosheets for selective enrichment and detection of cis-diol compounds by surface-assisted laser desorption/ionization time-of-flight mass spectrometry

Surface-assisted laser desorption/ionization time-of-flight mass spectrometry (SALDI-TOF MS) is an effective method for detecting of low-mass molecules. In this study, two-dimensional boron nanosheets (2DBs) were fabricated through thermal oxidation etching and coupling liquid exfoliation technologies, and applied as a matrix and selective sorbent for detecting cis-diol compounds by SALDI-TOF MS. The outstanding nanostructure and boric acid active sites of 2DBs endow them with sensitivity for cis-diol compound detection, excellent selectivity, and low background interference for complex samples. The specific in-situ enrichment faculty of the 2DBs as a matrix was investigated by SALDI-TOF MS using glucose, arabinose, and lactose as model analytes. In the presence of 100 -fold more interfering substances, the 2DBs showed high selectivity against cis-diol compounds, and exhibited a better sensitivity and a reduced limit of detection through enrichment treatment than graphene oxide matrices. The linearity, limit of detection (LOD), reproducibility, and accuracy of the method were evaluated under optimized conditions. The results showed that the linear relationships of six saccharides remained in the range of 0.05-0.6 mM with a correlation coefficient r ≥0.98. The LODs of six saccharides were 1 nM (glucose, lactose, mannose, fructose) and 10 nM (galactose, arabinose). Sample-to-sample (n = 6) with relative standard deviations (RSDs) of 3.2% to 8.1% were observed. Recoveries (n = 5) of 87.9-104.6% were obtained at three spiked levels in the milk samples. The proposed strategy promoted the development of a matrix for use with SALDI-TOF MS detection, in which the UV absorption properties and enrichment capabilities of 2DBs were combined.

Ag+-doped boron quantum dots with enhanced stability and fluorescence enabling versatile practicality in visual detection, sensing, imaging and photocatalytic degradation

In this work, a metal-doping strategy was put forward to construct metal-doped borophene and the corresponding zero-dimensional boron. Through theoretical calculations, Ag⁺ acts as the optimal metal ions to prepare Ag⁺-doped borophene derived boron quantum dots (Ag-BQDs). As predicted theoretically, doping of Ag⁺ endows borophene with enhanced stability of electronic structures. The newly emerging Ag-BQDs were experimentally acquired from ultrasonic-assisted liquid-phase exfoliation of bulk boron and solvothermal treatments. According to theoretical and experimental studies, the improved stability and fluorescence (FL) of Ag-BQDs are due to the formation of strong B-Ag bonding to competitively suppress B-O bonding. The function enables the maximal protection of borophene electronic structures from oxidization, destruction and reconfiguration. Because of Ag-BQDs with relatively higher colloidal and FL stability over BQDs, potential applications of Ag-BQDs were further explored in promising fields toward FL visualization in aqueous solutions and on filter paper, employed as a chemosensor of Fe³⁺ for FL sensing and visual detection at the solid/liquid phases, utilized for multiple FL bio-imaging at the levels of fresh plants, live animals and live cells of fresh plants, and applied to photocatalytic degradation of organic dyes and anticancer drug. Experimental results demonstrate excellent performances of Ag-BQDs in multiple applications, including versatile FL sensing and visual detection, unique multi-channel FL bio-imaging and visible-light-driven photodegradation of organic pollutants, toxic and harmful substances. This work can promote the development of metal-ion-doped low- dimensional nanomaterials with improved stability and FL properties for significant applications.

Performance of Common Density Functionals for Excited States of Tetraphenyldibenzoperiflanthene

Time-dependent density functional theory is the method of choice to efficiently calculate excitation spectra with the functional and basis set choice allowing one to compromise between accuracy and computational cost. In this work, the performance of different functionals as well as the second-order approximate coupled cluster singles and doubles model CC2 is evaluated by comparing the results to experimental results of the example molecule tetraphenyldibenzoperiflanthene (DBP). The choice of the functional has a significant impact on the calculated spectrum of DBP. The performance of a number of different functionals was evaluated, quantified, and, where possible, discussed. The best functional, tuned-CAM-B3LYP, is used to investigate DBP on a surface of hexagonal boron nitride (h-BN). The resulting spectrum shows excellent agreement with experimental results for a monolayer of DBP on h-BN.

Chemical Vapor Deposition Growth of Few-Layer β12-Borophane on Copper Foils toward Broadband Photodetection

Borophene has drawn tremendous attention in the past decade for a wide range of potential applications owing to its unique structural, optical, and electronic properties. However, applications of borophene toward next-generation nanodevices are mostly theoretical predictions, while experimental realization is still lacking due to rapid oxidation of intrinsic borophene in an air environment. Here, we have successfully prepared structurally stable and transferrable few-layer β₁₂-borophane on copper foils by a typical two-zone chemical vapor deposition method, where bis(triphenylphosphine)copper tetrahydroborate was used as the boron source in a hydrogen-rich atmosphere to stabilize its structure through hydrogenation. The crystal structure of the as-prepared β₁₂-borophane is in good agreement with previous reports. A fabricated photodetector based on β₁₂-borophane-silicon (n-type) Schottky junction shows good photoelectric responses to light excitations in a wide wavelength range from 365 to 850 nm. Especially, the photodetector exhibits a good photoresponsivity of around 0.48 A W^-1, a high specific detectivity of 4.39 × 10¹¹ jones, a high external quantum efficiency of 162%, and short response and recovery times of 115 and 121 ms under an ultraviolet light with the wavelength of 365 nm at a reverse bias of 5 V. The results show great potential applications of borophane in next-generation nanophotonic and nanoelectronic devices.

High temperature phases of borophene: borophene glass and liquid

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

Rhombohedral Boron Monosulfide as a p-Type Semiconductor

Two-dimensional materials have wide ranging applications in electronic devices and catalysts owing to their unique properties. Boron-based compounds, which exhibit a polymorphic nature, are an attractive choice for developing boron-based two-dimensional materials. Among them, rhombohedral boron monosulfide (r-BS) has recently attracted considerable attention owing to its unique layered structure similar to that of transition metal dichalcogenides and a layer-dependent bandgap. However, experimental evidence that clarifies the charge carrier type in the r-BS semiconductor is lacking. In this study, we synthesized r-BS and evaluated its performance as a semiconductor by measuring the Seebeck coefficient and photo-electrochemical responses. The properties unique to p-type semiconductors were observed in both measurements, indicating that the synthesized r-BS is a p-type semiconductor. Moreover, a distinct Fano resonance was observed in Fourier transform infrared absorption spectroscopy, which was ascribed to the Fano resonance between the E(2) (TO) phonon mode and electrons in the band structures of r-BS, indicating that the p-type carrier was intrinsically doped in the synthesized r-BS. These results demonstrate the potential future application prospects of r-BS.

Nanointerconnect design based on edge fluorinated/hydrogenated zigzag borophene nanoribbons: an ab initio analysis

Using an ab initio framework and non-equilibrium Green's function technique, the effect of hydrogen and fluorine atom passivation on the electronic and transport properties of borophene nanoribbons (BNRs) are explored. For zigzag edge states, we have explored all potentially stable combinations of hydrogen and fluorine passivation. Fluorine passivation leads to thermodynamically stable structures with improved stability for the increased concentration of F atoms, according to our binding energy (E_b) calculations. Furthermore, density-of-states and dispersion relation (E-k structures) computations indicate that fluorine-passivated BNRs are primarily metallic in nature. We proposed these nanostructures for their use in metal interconnects because of their increased metallicity. We have used the typical two-probe setup to calculate the critical parameters like quantum resistance (R_Q), kinetic inductance (L_K), and quantum capacitance (C_Q) to evaluate their performance as metal interconnects. Because they have the lowest estimated values of L_K = 26.1 nH μm^-1, and C_Q = 399 pF cm^-1, the zigzag BNRs (ZBNRs) with two edge fluorinated (F-BNR-F) nanostructures may be considered as a promising candidate for nanoscale interconnect applications.

Iron boride boosted Fenton oxidation: Boron species induced sustainable FeIII/FeII redox couple

The regeneration of Fe(II) is the rate-limiting step in the Fenton/Fenton-like chain reactions that seriously hinder their scientific progress towards practical application. In this study, we proposed iron boride (FeB) for the first time as a new material to sustainably decompose H₂O₂ to generate hydroxyl radicals, which can non-selectively degrade a wide array of refractory organic pollutants. Fe(II) can be steadily released by the stepwise oxidation of FeB to stimulate Fenton reaction, meanwhile, B-B bonds as electron donors on the surface of FeB effectively promote the regeneration of Fe(II) from Fe(III) species and significantly accelerate the production of hydroxyl radicals. The low generation of toxic by-products and the high utilization rate of iron species validly avoid the secondary organic/metal pollution in the FeB/H₂O₂ system. Therefore, FeB mediated Fenton oxidation provides a novel strategy to realize a green and long-lasting environmental remediation.

Metal-Doped Boron Quantum Dots for Versatile Detection of Lactate and Fluorescence Bioimaging

To improve the stability and fluorescence (FL) of monoelemental boron nanomaterials, this work put forward a metal-coordination strategy to explore emerging metal-doped boron quantum dots, Co@BQDs. Through theoretical calculations, B-Co bonding as predicted can suppress the B-O reaction and protect the electronic structures of exfoliated two-dimensional (2D) boron from oxidation and decomposition upon exposure to oxygen. In experimental studies, Co²⁺ was added into a dispersion liquid of bulk boron and subjected to probe sonication to promote Co²⁺ adsorption on the surface of exfoliated 2D boron, followed by Co²⁺ coordination with exposed boron atoms. Solvothermal treatment of exfoliated 2D boron resulted in the generation of Co²⁺-doped 0D boron Co@BQDs. Experimental results confirm that Co@BQDs have higher colloidal and FL stability than BQDs as a reference. B-Co bonding formation to suppress the B-O reaction ensures the high stability of exfoliated boron structures. A dispersion liquid of Co@BQDs with stable and bright FL was used for visual FL imaging of solutions and solid substrates. Based on enzymatic and cascade oxidation-induced FL quenching of Co@BQDs, a novel FL bio-probe of lactate was explored. This bio-probe, with a broad detection range of 0.01-10 mM and a low detection limit of 3.1 μM, enables FL sensing of lactate in biosamples and shows high detection recoveries of 98.0-102.8%. Moreover, this bio-probe realized versatile FL imaging and visual detection of lactate in liquid/solid-phase systems. These results demonstrate great prospects of Co@BQDs as emerging and efficient imaging reagents for long-term tracking and bioimaging applications.

Band Gap Opening in Borophene/GaN and Borophene/ZnO Van der Waals Heterostructures Using Axial Deformation: First-Principles Study

One of the topical problems of materials science is the production of van der Waals heterostructures with the desired properties. Borophene is considered to be among the promising 2D materials for the design of van der Waals heterostructures and their application in electronic nanodevices. In this paper, we considered new atomic configurations of van der Waals heterostructures for a potential application in nano- and optoelectronics: (1) a configuration based on buckled triangular borophene and gallium nitride (GaN) 2D monolayers; and (2) a configuration based on buckled triangular borophene and zinc oxide (ZnO) 2D monolayers. The influence of mechanical deformations on the electronic structure of borophene/GaN and borophene/ZnO van der Waals heterostructures are studied using the first-principles calculations based on density functional theory (DFT) within a double zeta plus polarization (DZP) basis set. Four types of deformation are considered: uniaxial (along the Y axis)/biaxial (along the X and Y axes) stretching and uniaxial (along the Y axis)/biaxial (along the X and Y axes) compression. The main objective of this study is to identify the most effective types of deformation from the standpoint of tuning the electronic properties of the material, namely the possibility of opening the energy gap in the band structure. For each case of deformation, the band structure and density of the electronic states (DOS) are calculated. It is found that the borophene/GaN heterostructure is more sensitive to axial compression while the borophene/ZnO heterostructure is more sensitive to axial stretching. The energy gap appears in the band structure of borophene/GaN heterostructure at uniaxial compression by 14% (gap size of 0.028 eV) and at biaxial compression by 4% (gap size of 0.018 eV). The energy gap appears in the band structure of a borophene/ZnO heterostructure at uniaxial stretching by 10% (gap size 0.063 eV) and at biaxial compression by 6% (0.012 eV). It is predicted that similar heterostructures with an emerging energy gap can be used for various nano- and optoelectronic applications, including Schottky barrier photodetectors.

Boron- and phosphorus-containing molecular/nano platforms: exploiting pathological redox imbalance to fight cancer

Cancer is currently the second leading cause of death globally. Despite multidisciplinary efforts, therapies to fight various types of cancer still remain inefficient. Reducing high recurrence rates and mortality is thus a major challenge to tackle. In this context, redox imbalance is an undervalued characteristic of cancer. However, it may be targeted by boron- and phosphorus-containing materials to selectively or systemically fight cancer. In particular, boron and phosphorus derivatives are attractive building blocks for rational drug discovery due to their unique and wide regioselective chemistry, high degree of tuneability and chemical stability. Thus, they can be meticulously employed to access tunable molecular platforms to selectively exploit the redox imbalance of cancer cells towards necrosis/apoptosis. This field of research holds a remarkable potential; nevertheless, it is still in its infancy. In this mini-review, we underline recent advances in the development of boron- or phosphorus-derivatives as molecular/nano platforms for rational anticancer drug design. Our goal is to provide comprehensive information on different methodologies that bear an outstanding potential to further develop this very promising field of research.

Recent Progress in Nanotechnology-Based Approaches for Food Monitoring

Throughout the food supply chain, including production, storage, and distribution, food can be contaminated by harmful chemicals and microorganisms, resulting in a severe threat to human health. In recent years, the rapid advancement and development of nanotechnology proposed revolutionary solutions to solve several problems in scientific and industrial areas, including food monitoring. Nanotechnology can be incorporated into chemical and biological sensors to improve analytical performance, such as response time, sensitivity, selectivity, reliability, and accuracy. Based on the characteristics of the contaminants and the detection methods, nanotechnology can be applied in different ways in order to improve conventional techniques. Nanomaterials such as nanoparticles, nanorods, nanosheets, nanocomposites, nanotubes, and nanowires provide various functions for the immobilization and labeling of contaminants in electrochemical and optical detection. This review summarizes the recent advances in nanotechnology for detecting chemical and biological contaminations in the food supply chain.

Substrate-Mediated Borophane Polymorphs through Hydrogenation of Two-Dimensional Boron Sheets

The two-dimensional boron monolayer (borophene) stands out from the two-dimensional atomic layered materials due to its structural flexibility and tunable electronic and mechanical properties from a large number of allotropic materials. The stability of pristine borophene polymorphs could possibly be improved via hydrogenation with atomic hydrogen (referred to as borophane). However, the precise adsorption structures and the underlying mechanism are still elusive. Employing first-principles calculations, we demonstrate the optimal configurations of freestanding borophanes and the ones grown on metallic substrates. For freestanding borophenes, the energetically favored hydrogen adsorption sites are sensitive to the polymorphs and corresponding coordination numbers of boron atoms. With various metal substrates, the hydrogenation configurations of borophenes are modulated significantly, attributed to the overlap between B p_z and H s orbitals. These findings provide a deep insight into the hydrogenating borophenes and facilitate the stabilization of two-dimensional boron polymorphs by engineering hydrogen adsorption sites and concentrations.

The Mechanical and Antibacterial Properties of Boron Nitride/Silver Nanocomposite Enhanced Polymethyl Methacrylate Resin for Application in Oral Denture Bases

The introduction of nanomaterials into polymethyl methacrylate (PMMA) resin has been effective for mechanically reinforcing PMMA for application in oral denture bases. However, these methods cannot simultaneously improve the mechanical and antibacterial properties, which limits widespread clinical application. Here, we self-assembled binary nanocomposites of boron nitride nanosheets (h-BNNs) and silver nanoparticles (AgNPs) as nanofillers and incorporated the nanofillers into PMMA. The aim of this study was to achieve antibacterial effects while significantly improving the mechanical properties of PMMA and provide a theoretical basis for further clinical application. We employed scanning electron microscopy (SEM), X-ray diffraction (XRD), Raman spectroscopy (Raman), Ultraviolet visible spectrum (UV) and atomic force microscopy (AFM) to investigate the microscopic morphology and composition of PMMA containing nanocomposites with different mass fraction. In addition, the content of the h-BNNs/AgNPs was 1 wt%, and the compressive strength and flexural strength of pure PMMA were improved by 53.5% and 56.7%, respectively. When the concentration of the nanocomposite in the PMMA resin was 1.4 wt%, the antibacterial rate was 92.1%. Overall, synergistically reinforcing PMMA composite resin with a multi-dimensional nanocomposite structure provided a new perspective for expanding not only the application of resins in clinical settings but also the research and development of new composite resins.

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.

Single-Element 2D Materials beyond Graphene: Methods of Epitaxial Synthesis

Today, two-dimensional materials are one of the key research topics for scientists around the world. Interest in 2D materials is not surprising because, thanks to their remarkable mechanical, thermal, electrical, magnetic, and optical properties, they promise to revolutionize electronics. The unique properties of graphene-like 2D materials give them the potential to create completely new types of devices for functional electronics, nanophotonics, and quantum technologies. This paper considers epitaxially grown two-dimensional allotropic modifications of single elements: graphene (C) and its analogs (transgraphenes) borophene (B), aluminene (Al), gallenene (Ga), indiene (In), thallene (Tl), silicene (Si), germanene (Ge), stanene (Sn), plumbene (Pb), phosphorene (P), arsenene (As), antimonene (Sb), bismuthene (Bi), selenene (Se), and tellurene (Te). The emphasis is put on their structural parameters and technological modes in the method of molecular beam epitaxy, which ensure the production of high-quality defect-free single-element two-dimensional structures of a large area for promising device applications.

Aromaticity of Singlet and Triplet Boron Disk-like Clusters: A Test for Electron Counting Aromaticity Rules

Boron clusters are polyhedral boron-containing structures that have unique features and properties. The disk-like boron clusters are among the most fascinating boron cluster forms. These clusters have a molecular orbital (MO) distribution similar to the one derived from the simple particle-on-a-disk model. In this model, the MOs come in pairs except for m = 0. Disk-like boron clusters in their singlet ground state are aromatic when they reach a closed-shell structure. One could expect that disk-like aromatic boron clusters in the singlet state, when acquiring or releasing two electrons, may also be aromatic in the lowest-lying triplet state. We use magnetically induced current densities and bond current strengths to analyze the aromatic character of a series of disk-like boron clusters. Our results show that, with the exception of triplet ³B₁₉^-, the disk-like boron clusters follow Hückel and Baird's rules if one considers the different MOs grouped by their symmetry. We also found that if the lowest-lying triplet state in disk-like boron clusters is aromatic, this triplet state is the ground state for this species.

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.

Quantum-coupled borophene-based heterolayers for excitonic and molecular sensing applications

Borophene (B), with remarkably unique chemical binding in its crystallographic structural phases including anisotropic structures, theoretically has high Young's modulus and thermal conductivity. Moreover, it is metallic in nature, and has recently joined the family of two-dimensional (2D) materials and is poised to be employed in flexible hetero-layered devices and sensors in fast electronic gadgets and excitonic devices. Interfacial coupling helps individual atomic sheets synergistically work in tandem, and is very crucial in controllable functionality. Most of the microscopic and spectroscopic scans reveal surface information; however, information regarding interfacial coupling is difficult to obtain. Electronic signatures of dynamic inter-layer coupling in B/boron nitride (BN) and B/molybdenum disulfide (MoS₂) have been detected in the form of distinct peaks in differential current signals obtained from scanning tunneling spectroscopy (STS) and conducting atomic force microscopy (CAFM). These unique sets of observed peaks represent interfacial coupling quantum states. The peaks in the electronic density of states (DOS) obtained via density functional theory (DFT) band structure calculations matched well with the electronic signatures of coupling quantum states. In our calculations, we found that the DOS peak evolves when the component layers are brought to compromised distances. While B/BN exhibits green sensitivity indicating mid-gap formation, B/MoS₂ bestows red sensitivity indicating band-gap excitation of MoS₂. Molecular detection of methylene blue (MB) based on surface-enhanced Raman spectroscopy (SERS) was carried out with borophene-based hetero-layered stacks as molecular anchoring platforms.

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.

Electronic structures and optical properties of monolayer borophenes

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

A Multi-Layered Borophene-Silica-Silver Based Refractive Index Sensor for Biosensing Applications Operated at the Infrared Frequency Spectrum

We have presented the borophene based refractive index sensor for the infrared frequency spectrum of 188 to 250 THz (1.2–1.6 µm) range. The proposed structure was formed by using the Silver-borophene-silica-Ag layered structure. The behaviour of the different analyte (with a different refractive index) material is numerically calculated by placing it on the top of the structure. The behaviour of the structure is identified in terms of absorption, reflectance, physical parameter variation, and oblique angle incident conditions. The presented results provide the basic idea of selecting optimized structure dimensions to get the specific resonating response. This sensor offers the Figure of Merit (FOM) of 444 RIU−1 with high sensitivity of 660 THz/RIU (4471 nm/RIU). The refractive index sensor also provides wide-angle stability for (0° to 80°) for the wide frequency range (239 to 245 THz and 207 to 209 THz). This sensor is developed on the silver metal layer (not required to separate borophene from its origin metal deposition process) and easily fabricated using standard boron fabrication and layered deposition techniques. The results of the proposed structure make it possible to design a basic biosensor structure. This device is also applicable for various THz and biomedical applications.

Exploring the emerging applications of the advanced 2-dimensional material borophene with its unique properties

Borophene, a crystalline allotrope of monolayer boron, with a combination of triangular lattice and hexagonal holes, has stimulated wide interest in 2-dimensional materials and their applications. Although their properties are theoretically confirmed, they are yet to be explored and confirmed experimentally. In this review article, we present advancements in research on borophene, its synthesis, and unique properties, including its advantages for various applications with theoretical predictions. The uniqueness of borophene over graphene and other 2-dimensional (2D) materials is also highlighted along with their various structural stabilities. The strategy for its theoretical simulations, leading to the experimental synthesis, could also be helpful for the exploration of many newer 2D materials.

Engineering 2D Materials for Photocatalytic Water-Splitting from a Theoretical Perspective

Splitting of water with the help of photocatalysts has gained a strong interest in the scientific community for producing clean energy, thus requiring novel semiconductor materials to achieve high-yield hydrogen production. The emergence of 2D nanoscale materials with remarkable electronic and optical properties has received much attention in this field. Owing to the recent developments in high-end computation and advanced electronic structure theories, first principles studies offer powerful tools to screen photocatalytic systems reliably and efficiently. This review is organized to highlight the essential properties of 2D photocatalysts and the recent advances in the theoretical engineering of 2D materials for the improvement in photocatalytic overall water-splitting. The advancement in the strategies including (i) single-atom catalysts, (ii) defect engineering, (iii) strain engineering, (iv) Janus structures, (v) type-II heterostructures (vi) Z-scheme heterostructures (vii) multilayer configurations (viii) edge-modification in nanoribbons and (ix) the effect of pH in overall water-splitting are summarized to improve the existing problems for a photocatalytic catalytic reaction such as overcoming large overpotential to trigger the water-splitting reactions without using cocatalysts. This review could serve as a bridge between theoretical and experimental research on next-generation 2D photocatalysts.

Electronic structure, stability, and aromaticity of M2B6 (M = Mg, Ca, Sr, and Ba): an interplay between spin pairing and electron delocalization

It has been shown in previous studies that the Be₂B₆ complex exhibits a triplet ground state with double aromaticity. In this work, the stability, electronic structure, and aromaticity of the homologous series M₂B₆ (M = Mg, Ca, Sr and Ba) were examined and compared to those of Be₂B₆. At the CCSD(T)/def2-TZVP//B3LYP/def2-TZVP level of theory, the target molecules were found to be more stable in the singlet than in the triplet spin state. Magnetically induced current densities and multicentre delocalization index (MCI) were employed to assess the aromatic character of the studied complexes. Both employed methods agree that M₂B₆ (M = Mg, Ca, Sr and Ba) are π aromatic and σ nonaromatic in the singlet ground state, and double aromatic in the triplet state. It was demonstrated that the electron counting rules of aromaticity cannot be used to correctly predict the aromaticity and relative stability of the examined molecules in different spin states.

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

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

2D WO3-x Nanosheet with Rich Oxygen Vacancies for Efficient Visible-Light-Driven Photocatalytic Nitrogen Fixation

Oxygen vacancy modulation holds great promise for enhancing the photocatalytic activity for efficient nitrogen fixation under mild conditions. In this work, the two-dimensional WO_3-x nanosheets with rich oxygen vacancies were prepared using solvothermal synthesis. The WO_3-x nanosheets (rich oxygen vacancies) display nice photocatalytic activity for N₂ reduction to ammonia with a high yield rate of 82.41 μmol·g_cat^-1·h^-1 under irradiation of visible light (420 nm), which is 3.59 times higher than that of the WO_3-x nanoparticles (poor oxygen vacancies). Electron spin resonance (ESR), N₂ adsorption-desorption isotherms, and transient photocurrent responses in the N₂ or Ar atmosphere experiments proved that the rich oxygen vacancies, which are induced by the nanosheet structure, could serve as active sites for the chemisorption of N₂ and facilitate the electron transfer from unsaturated sites to activated N₂. Moreover, based on the analysis of banding energy, the oxygen vacancies not only boosted the ability of visible light harvesting but also elevated the defect energy level to the Fermi level, further inhibiting the defect relaxation effect. The findings offer an insight into the design of the efficient photocatalysts via structure engineering and defect engineering for photocatalytic N₂ fixation.

Probing the Nature of the Transition-Metal-Boron Bonds and Novel Aromaticity in Small Metal-Doped Boron Clusters Using Photoelectron Spectroscopy

Photoelectron spectroscopy combined with quantum chemistry has been a powerful approach to elucidate the structures and bonding of size-selected boron clusters (B_n^-), revealing a prevalent planar world that laid the foundation for borophenes. Investigations of metal-doped boron clusters not only lead to novel structures but also provide important information about the metal-boron bonds that are critical to understanding the properties of boride materials. The current review focuses on recent advances in transition-metal-doped boron clusters, including the discoveries of metal-boron multiple bonds and metal-doped novel aromatic boron clusters. The study of the RhB^- and RhB₂O^- clusters led to the discovery of the first quadruple bond between boron and a transition-metal atom, whereas a metal-boron triple bond was found in ReB₂O^- and IrB₂O^-. The ReB₄^- cluster was shown to be the first metallaborocycle with Möbius aromaticity, and the planar ReB₆^- cluster was found to exhibit aromaticity analogous to metallabenzenes. Expected final online publication date for the Annual Review of Physical Chemistry, Volume 73 is April 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Structural transformations in boron clusters induced by metal doping

In the last decades, experimental techniques in conjunction with theoretical analyses have revealed the surprising structural diversity of boron clusters. Although the 2D to 3D transition thresholds are well-established, there is no certainty about the factors that determine the geometry adopted by these systems. The structural transformation induced by doping usually yields a minimum energy structure with a boron skeleton entirely different from that of the bare cluster. This review summarizes those clusters no larger than 40 boron atoms where one or two dopants show a radical transformation of the structure. Although the structures of these systems are not easy to predict, they often adopt familiar shapes such as umbrella-like, wheel, tubular, and cages in various cases.

KNa0.78Eu0.27In3.80B12S12: A Novel Hexanary Thioborate Featuring B12S12 Cluster and Diverse InSx (x = 4, 5, 6) Units

Thioborates combine the advantages of sulfides and borates, are a new type of multifunctional inorganic materials with versatile structural features. Here, a thioborate KNa0.78Eu0.27In3.80B12S12 crystallizing with a new structure-type was...

Ground and Electronically Excited States of Main-Group-Metal-Doped B20 Double Rings

Ab initio coupled-cluster, electron propagator, and Møller-Plesset second-order perturbation theory calculations are utilized to analyze the low-lying electronic states of several metal-doped B₂₀. In the ground state, the presently focused AB₂₀/EB₂₀ (A = Li, Na, and K; E = Mg and Ca) consist of charge-separated A⁺B₂₀^-/E²⁺B₂₀^2- frameworks. The excited electronic states of AB₂₀ and EB₂₀⁺ were analyzed by computing the vertical electron attachment energies (VEAEs) of AB₂₀⁺ and EB₂₀²⁺. In several excited states, the radical electron is predominantly localized on the B₂₀ frames, which are counterparts of the low-lying states of bare B₂₀^-. A variety of basis sets were tested on obtaining VEAEs, and the aug-cc-pVDZ/_A,E d-aug-cc-pVDZ/_B combination provided the best accuracy-efficiency compromise on them. Furthermore, this work analyzes the Rydberg-like excited states of AB₂₀ and EB₂₀⁺ and will serve as a guide for future studies on similar metal-doped boron systems.