Epitaxial single-crystal hexagonal boron nitride multilayers on Ni (111).

Large-area single-crystal monolayers of two-dimensional (2D) materials such as graphene1-3, hexagonal boron nitride (hBN)4-6 and transition metal dichalcogenides7,8 have been grown. hBN is considered to be the 'ideal' dielectric for 2D-materials-based field-effect transistors (FETs), offering the potential for extending Moore's law9,10. Although hBN thicker than a monolayer is more desirable as substrate for 2D semiconductors11,12, highly uniform and single-crystal multilayer hBN growth has yet to be demonstrated. Here we report the epitaxial growth of wafer-scale single-crystal trilayer hBN by a chemical vapour deposition (CVD) method. Uniformly aligned hBN islands are found to grow on single-crystal Ni (111) at early stage and finally to coalesce into a single-crystal film. Cross-sectional transmission electron microscopy (TEM) results show that a Ni23B6 interlayer is formed (during cooling) between the single-crystal hBN film and Ni substrate by boron dissolution in Ni. There are epitaxial relationships between hBN and Ni23B6 and between Ni23B6 and Ni. We also find that the hBN film acts as a protective layer that remains intact during catalytic evolution of hydrogen, suggesting continuous single-crystal hBN. This hBN transferred onto the SiO2 (300 nm)/Si wafer acts as a dielectric layer to reduce electron doping from the SiO2 substrate in MoS2 FETs. Our results demonstrate high-quality single-crystal  multilayered hBN over large areas, which should open up new pathways for making it a ubiquitous substrate for 2D semiconductors.

Single-crystal, large-area, fold-free monolayer graphene.

Chemical vapour deposition of carbon-containing precursors on metal substrates is currently the most promising route for the scalable synthesis of large-area, high-quality graphene films1. However, there are usually some imperfections present in the resulting films: grain boundaries, regions with additional layers (adlayers), and wrinkles or folds, all of which can degrade the performance of graphene in various applications2-7. There have been numerous studies on ways to eliminate grain boundaries8,9 and adlayers10-12, but graphene folds have been less investigated. Here we explore the wrinkling/folding process for graphene films grown from an ethylene precursor on single-crystal Cu-Ni(111) foils. We identify a critical growth temperature (1,030 kelvin) above which folds will naturally form during the subsequent cooling process. Specifically, the compressive stress that builds up owing to thermal contraction during cooling is released by the abrupt onset of step bunching in the foil at about 1,030 kelvin, triggering the formation of graphene folds perpendicular to the step edge direction. By restricting the initial growth temperature to between 1,000 kelvin and 1,030 kelvin, we can produce large areas of single-crystal monolayer graphene films that are high-quality and fold-free. The resulting films show highly uniform transport properties: field-effect transistors prepared from these films exhibit average room-temperature carrier mobilities of around (7.0 ± 1.0) × 103 centimetres squared per volt per second for both holes and electrons. The process is also scalable, permitting simultaneous growth of graphene of the same quality on multiple foils stacked in parallel. After electrochemical transfer of the graphene films from the foils, the foils themselves can be reused essentially indefinitely for further graphene growth.

Corrosion resistant three-dimensional nanotextured silicon for water photo-oxidation.

We demonstrate the ability to chemically transform bulk silicon into a nanotextured surface that exhibits excellent electrochemical stability in aqueous conditions for water photo-oxidation. Conformal defective graphene coatings on nanotextured silicon formed by thermal treatment enable over 50× corrosion resistance in aqueous electrolytes based upon Tafel analysis and impedance spectroscopy. This enables nanotextured silicon as an effective oxygen-evolution photoanode for water splitting with saturation current density measured near 35 mA cm(-2) under 100 mW cm(-2) (1 sun) illumination. Our approach builds upon simple and scalable processing techniques with silicon to develop corrosion resistant electrodes that can benefit a broad range of catalytic and photocatalytic applications.

Engineered porous silicon counter electrodes for high efficiency dye-sensitized solar cells.

In this work, we demonstrate for the first time, the use of porous silicon (P-Si) as counter electrodes in dye-sensitized solar cells (DSSCs) with efficiencies (5.38%) comparable to that achieved with platinum counter electrodes (5.80%). To activate the P-Si for triiodide reduction, few layer carbon passivation is utilized to enable electrochemical stability of the silicon surface. Our results suggest porous silicon as a promising sustainable and manufacturable alternative to rare metals for electrochemical solar cells, following appropriate surface modification.

A multifunctional load-bearing solid-state supercapacitor.

A load-bearing, multifunctional material with the simultaneous capability to store energy and withstand static and dynamic mechanical stresses is demonstrated. This is produced using ion-conducting polymers infiltrated into nanoporous silicon that is etched directly into bulk conductive silicon. This device platform maintains energy densities near 10 W h/kg with Coulombic efficiency of 98% under exposure to over 300 kPa tensile stresses and 80 g vibratory accelerations, along with excellent performance in other shear, compression, and impact tests. This demonstrates performance feasibility as a structurally integrated energy storage material broadly applicable across renewable energy systems, transportation systems, and mobile electronics, among others.

Assessing the improved performance of freestanding, flexible graphene and carbon nanotube hybrid foams for lithium ion battery anodes.

We demonstrate the fabrication of three-dimensional freestanding foams of hybrid graphene-single-walled carbon nanotube nanomanufactured materials with reversible capacities of 2640 mA h g(-1) at 0.186 A g(-1) and 236 mA h g(-1) at 27.9 A g(-1). The Li storage behavior of this material is compared against other nanostructures in similar flexible foam platforms including graphene, ultra-thin graphite, and single-walled carbon nanotubes (SWNTs), and we elucidate the improved hybrid material performance due to the decoupling of lithium storage reaction energetics dictated by the SWNTs from the total storage capacity of the hybrid material. This work demonstrates a route to develop mechanically robust all-carbon electrodes with the potential for reversible Li-ion storage capacity approaching silicon, power capability of the best supercapacitors, and based on a material simultaneously usable as a charge collector and anode.

Uniform, homogenous coatings of carbon nanohorns on arbitrary substrates from common solvents.

We demonstrate a facile technique to electrophoretically deposit homogenous assemblies of single-walled carbon nanohorns (CNHs) from common solvents such as acetone and water onto nearly any substrate including insulators, dielectrics, and three-dimensional metal foams, in many cases without the aid of surfactants. This enables the generation of pristine film-coatings formed on time scales as short as a few seconds and on three-dimensional templates that enable the formation of freestanding polymer-CNH supported materials. As electrophoretic deposition is usually only practical on conductive electrodes, we emphasize our observation of efficient deposition on nearly any material, including nonconductive substrates. The one-step versatility of deposition on these materials provides the capability to directly assemble CNH materials onto functional surfaces for a broad range of applications. In this manner, we utilized as-deposited CNH films as conductometric gas sensors exhibiting better sensitivity in comparison to equivalent single-walled carbon nanotube sensors. This gives a route toward scalable and inexpensive solution-based processing routes to manufacture functional nanocarbon materials for catalysis, energy, and sensing applications, among others.

Surface engineered porous silicon for stable, high performance electrochemical supercapacitors.

Silicon materials remain unused for supercapacitors due to extreme reactivity of silicon with electrolytes. However, doped silicon materials boast a low mass density, excellent conductivity, a controllably etched nanoporous structure, and combined earth abundance and technological presence appealing to diverse energy storage frameworks. Here, we demonstrate a universal route to transform porous silicon (P-Si) into stable electrodes for electrochemical devices through growth of an ultra-thin, conformal graphene coating on the P-Si surface. This graphene coating simultaneously passivates surface charge traps and provides an ideal electrode-electrolyte electrochemical interface. This leads to 10-40X improvement in energy density, and a 2X wider electrochemical window compared to identically-structured unpassivated P-Si. This work demonstrates a technique generalizable to mesoporous and nanoporous materials that decouples the engineering of electrode structure and electrochemical surface stability to engineer performance in electrochemical environments. Specifically, we demonstrate P-Si as a promising new platform for grid-scale and integrated electrochemical energy storage.

Iridium and ruthenium catalyzed syntheses, hydroborations, and metathesis reactions of alkenyl-decaboranes.

The selective syntheses of new classes of 6,9-dialkenyl- and 6-alkenyl-decaboranes and 6-alkyl-9-alkenyl-decaboranes have been achieved via iridium and ruthenium catalyzed decaborane and 6-alkyl-decaborane alkyne-hydroborations. Reactions employing [Cp*IrCl2]2 and [RuCl2(p-cymene)]2 precatalysts gave ß-E-alkenyl-decaboranes, while the corresponding reactions with [RuI2(p-cymene)]2 gave the α-alkenyl-decaborane isomers, with the differences in product selectivity suggesting quite different mechanistic steps for the catalysts. The alkenyl-decaboranes were easily converted to other useful derivatives, including coupled-cage and functionally substituted compounds, via iridium-catalyzed hydroborations and ruthenium-catalyzed homo and cross olefin-metathesis reactions.

Metal-catalyzed decaborane-alkyne hydroboration reactions: efficient routes to alkenyldecaboranes.

Transition-metal-catalyzed decaborane-alkyne hydroboration reactions have been developed that provide high-yield routes to the previously unknown di- and monoalkenyldecaboranes. These alkenyl derivatives should be easily modified starting materials for many biomedical and/or materials applications. Unusual catalyst product selectivity was observed that suggests quite different mechanistic steps, with the reactions catalyzed by the [RuCl(2)(p-cymene)](2) and [Cp*IrCl(2)](2) complexes giving the beta-E alkenyldecaboranes and the corresponding reactions with the [RuI(2)(p-cymene)](2) complex giving the alpha-alkenyldecaborane isomers.

Double-walled boron nitride nanotubes grown by floating catalyst chemical vapor deposition.

One-dimensional nanostructures exhibit quantum confinement which leads to unique electronic properties, making them attractive as the active elements for nanoscale electronic devices. Boron nitride nanotubes are of particular interest since, unlike carbon nanotubes, all chiralities are semiconducting. Here, we report a synthesis based on the use of low pressures of the molecular precursor borazine in conjunction with a floating nickelocene catalyst that resulted in the formation of double-walled boron nitride nanotubes. As has been shown for carbon nanotube production, the floating catalyst chemical vapor deposition method has the potential for creating high quality boron nitride nanostructures with high production volumes.