Stacking and freestanding borophene for lithium-ion battery application.

The growth of artificial synthesis two-dimensional (2D) materials usually demands for suitable substrate due to their rare bulk allotropies. Borophene, as a typical artificial synthetic material, has been proved its substrate-growth on metal or nonmetals and its high theoretical specific capacity (1720 mAh g-1) for next-genatration electrode material, but structural instability and transfer difficulties have hindered the development of its applications. Here, a structurally stable and freestanding AA-stacked-α'-4H-borophene sheets have been synthesized byin situlithium eutectic salt-assisted synthetic method to realize the application of borophene in lithium-ion battery. The atomic structure of AA-α'-4H-borophene with interlayer VdWs was established by comparing the experimental observation with DFT optimal calculation. Different stacking configurations (AA- and AB-) of borophene was realized by a temperature-structure-photoluminescence intensity relationship, and the AA-stacked borophene exhibits higher specific capacity than AB structure. Based on electrochemical performance, the AA-borophene exhibits excellent rate capability and cycling performance due to its non-collapsible stacking configurations, which dominates great initial coulombic efficiency of 87.3% at 200 mA g-1superior to that of black phosphorus-based and borophene/graphene. Meanwhile, it still maintains the coulombic efficiency of 99.13% after 1000 cycles. It also shows a reversible capacity of 181 mAh g-1at 10 mA g-1between the voltage window of 0.01 and 2 V, which improves the reported capacity (43 mAh g-1) of bulk boron anode by over 430%. This work brings fantastic new view of fabricating stable, stacking and freestanding borophene and provides a significative idea on applications of borophene in energy storage domain.

Multilayerα'-4H-borophene growth on gallium arsenide towards high-performance near-infrared photodetector.

Multilayer borophene was predicted to have a similar semiconductor property to its monolayer arise from the weak van der Waals interactions between the layers. Besides, multilayer borophene has a higher carrier mobility than monolayer ones, so it is placed great hopes in applications of photoelectric and photovoltaic devices. However, its preparation and application in experiments of multilayer borophene are still lacking. Here, multilayerα'-4H-borophene was synthesized on semiconductingn-type GaAs substrates using NaBH4source as precursor and hydrogen as the carrier gas under controlled temperature and pressure conditions. The experimental results of the borophene are in good agreement with those of its theoretical prediction. The borophene is a semiconductor with a bandgap of 2.48 eV. To demonstrate the device application potential of the borophene, a near-infrared photodetector composed ofp-type borophene andn-type GaAs was fabricated. The photodetector shows a high photoresponsivity of 0.31 mA·W-1, a high specific detectivity of108Jones, and a fast response or recovery speed of 117 or 109 ms under the irradiation with the wavelength of 940 nm at zero bias. The results prove that theα'-4H-borophene/GaAs photodetector can show high sensitivity and zero consumption, which is of great value in meeting the appeal of sustainable development of society.

Freestandingα-rhombohedral borophene nanosheets: preparation and memory device application.

Borophene has attracted extensive interests owing to its distinct structural, electronic and optical properties for promising potential applications. However, the structural instability and need of metal substrate for deposition of borophene seriously restrict the exploration of its exceptional physical and chemical properties and further hamper its extensive applications towards high-performance electronic and optoelectronic devices. Here, we reported the synthesis of high-quality freestandingα-rhombohedral borophene nanosheets by a facile probe ultrasonic approach in different organic solvents. The results show that the nanosheets have high-quality in ethanol solution and have an average lateral size of 0.54µm and a thickness of around 1.2 nm. Photoluminescence spectra indicate that a strong quantum confinement effect occurs in the nanosheets, which caused the increase of the band gap from 1.80 eV for boron powders and 2.52 eV for the nanosheets s. A nonvolatile memory device based on the nanosheets mixed with polyvinylpyrrolidone was fabricated, which exhibited a good rewriteable nonvolatile memory behavior and good stability.

Triclinic boron nanosheets high-efficient electrocatalysts for water splitting.

Recently, two-dimensional (2D) boron nanosheets have been predicted to exhibit exceptional physical and chemical properties, which is expected to be widely used in advanced electronics, optoelectronic, energy storage and conversion devices. However, the experimental application of 2D boron nanosheets in hydrogen evolution reactiuon (HER) has not been reported. Here, we have grown ultrathin boron nanosheets on tungsten foils via chemical vapor deposition. The prepared triclinic boron nanosheets are highly crystalline, which perfectly match the structure in the previous theoretical calculations. Notably, the boron nanosheets show excellent HER performance. The Tafel slope is only 64 mV dec-1and the nanosheets can maintain good stability under long-time cycle in acidic solution. The improvement of performance is mainly due to the metal properties and a large number of exposed active sites on the boron nanosheets, which is confirmed by first-principle calculations.

Ultraviolet photodetector based on p-borophene/n-ZnO heterojunction.

Borophene has attracted enormous attention because of its rich and unique structural and electronic properties for promising pratical applications. Although borophene sheets have been realized on different substrates in recent experiments, there are very few reports on the device application of borophene. Recently, borophene can be grown on some functional substrates, which lays a good foundation for its potential applications. Here, we report that hydrogenated borophene can be grown on the fluorine-doped tin oxide glass substrate. The phase of the obtained borophene is well consistent with the predicted semiconductingδ5-boron sheet. Furthermore, a vertical heterojunction ultraviolet detector based p-borophene/n-zinc oxide was fabricated. The photoresponsivity of the detector is 1.02 × 10-1A W-1, the specific detection rate was 1.43 × 109Jones and the response speed wasτres = 2.8 s,τrec = 3.2 s at the reversed bias of -5 V under the light excitation of 365 nm. This work will lay a foundation for further study on the attractive properties and applications of borophene in new optoelectronic devices and integrated circuits.

Ultrastable Crystalline Semiconducting Hydrogenated Borophene.

Borophene sheets have been synthesized in recent experiments, but the metallic nature and structural instability of the sheets seriously prevent emerging applications. Hydrogenated borophene has been predicted as an ideal material for nanoelectronic applications due to its high stability as well as excellent electronic and mechanical properties. However, the fabrication of hydrogenated borophene is still a great challenge. Here, we demonstrate that hydrogenated borophenes in large quantities can be prepared without any metal substrates by a stepwise in-situ thermal decomposition of sodium borohydride under hydrogen as the carrier gas. The borophenes with good crystallinity exhibit superior stability in strong acid or base solvents. The structure of the grown borophene is in good agreement with the predicted semiconducting α-boron sheet. A fabricated borophene-based memory device shows a high ON/OFF-current ratio of 3×103 and a low operating voltage of less than 0.35 V as well as good stability.

Chemical Vapor Deposition Growth of Few-Layer ß12-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 ß12-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 ß12-borophane is in good agreement with previous reports. A fabricated photodetector based on ß12-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.

Surface Modification Techniques to Produce Micro/Nano-scale Topographies on Ti-Based Implant Surfaces for Improved Osseointegration.

Titanium and titanium alloys are used as artificial bone substitutes due to the good mechanical properties and biocompatibility, and are widely applied in the treatment of bone defects in clinic. However, Pure titanium has stress shielding effect on bone, and the effect of titanium-based materials on promoting bone healing is not significant. To solve this problem, several studies have proposed that the surface of titanium-based implants can be modified to generate micro or nano structures and improve mechanical properties, which will have positive effects on bone healing. This article reviews the application and characteristics of several titanium processing methods, and explores the effects of different technologies on the surface characteristics, mechanical properties, cell behavior and osseointegration. The future research prospects in this field and the characteristics of ideal titanium-based implants are proposed.

Borophene Nanosheets as High-Efficiency Catalysts for the Hydrogen Evolution Reaction.

Borophene has been predicted to have outstanding catalytic activity owing to its extreme electron deficiency and abundant active sites. However, no experimental results have been still reported for borophene application in high-efficiency catalysis. Here, a borophene nanosheet was prepared on a carbon cloth surface via chemical vapor deposition. The boron source is sodium borohydride and the carrier gas is hydrogen gas. The crystal structure of the borophene nanosheet highly matches that of a theoretical α'-borophene nanosheet. Borophene shows good electrocatalytic hydrogen evolution reaction (HER) ability with a 69 mV/dec Tafel slope and good cycling stability in a 0.5 M H2SO4 solution. The enhanced performance is ascribed to an abundant electrocatalytic active area and low resistance of charge transfer, which results from its rich surface active sites. The improvement has been revealed by first-principles calculations, which is originated from their inherent metallicity and abundant electrocatalytic active sites on the nanosheets' surface. Borophene's extraordinarily high activity and stability give rise to extensive investigation of the application of borophene in high-efficiency energy applications such as catalysts and batteries.

van der Waals Epitaxial Growth of Borophene on a Mica Substrate toward a High-Performance Photodetector.

The emergence of borophene has triggered soaring interest in the investigation of its superior structural anisotropy, a novel photoelectronic property for diverse potential applications. However, the structural instability and need of a metal substrate for depositing borophene restrict its large-scale applications toward high-performance electronic and optoelectric devices. van der Waals epitaxy is regarded as an efficient technique for growing superb two-dimensional materials onto extensive functional substrates, but the preparation of stable and controllable borophene on nonmetallic substrates is still not reported. Here, we demonstrate that borophene films can be synthesized onto a mica substrate by van der Waals epitaxy, where hydrogen and NaBH4 are respectively used as the carrier gas and the boron source. The lattice structure of the as-synthesized borophene coincides with the predicted α'-boron sheet. The borophene-based photodetector shows an excellent photoresponsivity of 1.04 A W-1 and a specific detectivity of 1.27 × 1011 Jones at a reversed bias of 4 V under illumination of a 625 nm light-emitting diode, which are remarkably superior to those of reported boron nanosheets. This work facilitates further studies of borophene toward its attractive properties and applications in novel optoelectronic devices and integrated circuits.

Borophene: Current Status, Challenges and Opportunities.

Borophenes (2D boron sheets) have triggered a surge of interest both theoretically and experimentally because of its distinct structural, optical and electronic properties for extensive potential applications. Although theoretical efforts have guided the research directions of borophene, only few synthetic borophene sheets have been demonstrated experimentally. Borophene sheets have been successfully synthesized experimentally on metal substrates until 2015. Afterwards, more efforts were put on the controlled synthesis of crystalline and semiconducting borophene sheets as well as on the investigation of their novel and fascinating physical properties. This report provides a brief review on theoretical and experimental progress in borophene research. Some typical structures and properties of borophenes have been reviewed. The focus is laid on summarizing the experimental synthesis of borophene in recent years, and on showing some ultrastable and semiconducting borophenes which have been applied in electronic information devices. Finally, the future challenges and opportunities regarding experimental realization and practical applications of borophenes are presented.

Experimental realization of quasicubic boron sheets.

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

Crystalline Semiconductor Boron Quantum Dots.

Zero-dimensional boron structures have always been the focus of theoretical research owing to their abundant phase structures and special properties. Boron clusters have been reported extensively by combining structure searching theories and photoelectron spectroscopy (PES) experiments; however, crystalline boron quantum dots (BQDs) have rarely been reported. Here, we report the preparation of large-scale and uniform crystalline semiconductor BQDs from the expanded bulk boron powders via a facile and efficient probe ultrasonic approach in the acetonitrile solution. The obtained BQDs have 2.46 nm average lateral size and 2.81 nm thickness. Optical measurements demonstrate that a strong quantum confinement effect occurs in the BQDs, implying the increase of the band gap from 1.80 eV for the corresponding bulk to 2.46 eV for the BQDs. By injecting the BQDs into poly(vinylpyrrolidone) as an active layer, a BQD-based memory device is fabricated that shows a rewriteable nonvolatile memory effect with a low transition voltage of down to 0.5 V and a high on/off switching ratio of 103 as well as a good stability.