Growth of High-Crystalline, Single-Layer Hexagonal Boron Nitride on Recyclable Platinum Foil

Hexagonal boron nitride (h-BN) is gaining significant attention as a two-dimensional dielectric material, along with graphene and other such materials. Herein, we demonstrate the growth of highly crystalline, single-layer h-BN on Pt foil through a low-pressure chemical vapor deposition method that allowed h-BN to be grown over a wide area (8 × 25 mm²). An electrochemical bubbling-based method was used to transfer the grown h-BN layer from the Pt foil onto an arbitrary substrate. This allowed the Pt foil, which was not consumed during the process, to be recycled repeatedly. The UV–visible absorption spectrum of the single-layer h-BN suggested an optical band gap of 6.06 eV, while a high-resolution transmission electron microscopy image of the same showed the presence of distinct hexagonal arrays of B and N atoms, which were indicative of the highly crystalline nature and single-atom thickness of the h-BN layer. This method of growing single-layer h-BN over large areas was also compatible with use of a sapphire substrate

Catalyst-Free Synthesis of Si-SiO_<i>x</i> Core-Shell Nanowire Anodes for High-Rate and High-Capacity Lithium-Ion Batteries

Si-SiO_<i>x</i> core-shell nanowires (NWs) ranging from 10 to 30 nm in diameter are prepared by a simple evaporation of silicon monoxide and control of substrate temperatures without any catalyst. The Si-SiO_<i>x</i> NWs grown at 735 and 955 °C are strongly anchored to the Cu current collector by forming copper silicide at the interface between Si and Cu, and subsequently used as anodes in lithium-ion batteries, in which no binder or conducting materials are used. The Si-SiO_<i>x</i> NWs anodes show excellent electrochemical performances in terms of capacity retention and rate capability. In particular, the Si-SiO_<i>x</i> NW anode grown at 955 °C shows a reversible capacity of ∼1000 mAh g^–1 even at a high-rate of 50 C. This catalyst-free synthetic route of Si-SiO_<i>x</i> NWs that are strongly anchored to the Cu current collector opens up an effective process for fabricating other high-capacity anodes in lithium-ion batteries (LIBs)

Synthesis and Characterization of Patronite Form of Vanadium Sulfide on Graphitic Layer

With the exploding interest in transition metal chalcogenides, sulfide minerals containing the dianion S₂^2–, such as pyrite (FeS₂), cattierite (CoS₂), and vaesite (NiS₂), have recently attracted much attention for potential applications in energy conversion and storage devices. However, the synthesis of the patronite structure (VS₄, V⁴⁺(S₂^2–)₂) and its applications have not yet been clearly demonstrated because of experimental difficulties and the existence of nonstoichiometric phases. Herein, we report the synthesis of VS₄ using a simple, facile hydrothermal method with a graphene oxide (GO) template and the characterization of the resulting material. Tests of various templates such as CNT, pyrene, perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA), and graphite led us to the conclusion that the graphitic layer plays a role in the nucleation during growth of VS₄. Furthermore, the VS₄/rGO hybrid was proved to be a promising functional material in energy storage devices

Oxidation Resistance of Iron and Copper Foils Coated with Reduced Graphene Oxide Multilayers

Protecting the surface of metals such as Fe and Cu from oxidizing is of great importance due to their widespread use. Here, oxidation resistance of Fe and Cu foils was achieved by coating them with reduced graphene oxide (rG-O) sheets. The rG-O-coated Fe and Cu foils were prepared by transferring rG-O multilayers from a SiO₂ substrate onto them. The oxidation resistance of these rG-O-coated metal foils was investigated by Raman spectroscopy, optical microscopy, and scanning electron microscopy after heat treatment at 200 °C in air for 2 h. The bare metal surfaces were severely oxidized, but the rG-O-coated metal surfaces were protected from oxidation. This simple solution process using rG-O is one advantage of the present study

Flexible Thermochromic Window Based on Hybridized VO₂/Graphene

Large-scale integration of vanadium dioxide (VO₂) on mechanically flexible substrates is critical to the realization of flexible smart window films that can respond to environmental temperatures to modulate light transmittance. Until now, the formation of highly crystalline and stoichiometric VO₂ on flexible substrate has not been demonstrated due to the high-temperature condition for VO₂ growth. Here, we demonstrate a VO₂-based thermochromic film with unprecedented mechanical flexibility by employing graphene as a versatile platform for VO₂. The graphene effectively functions as an atomically thin, flexible, yet robust support which enables the formation of stoichiometric VO₂ crystals with temperature-driven phase transition characteristics. The graphene-supported VO₂ was capable of being transferred to a plastic substrate, forming a new type of flexible thermochromic film. The flexible VO₂ films were then integrated into the mock-up house, exhibiting its efficient operation to reduce the in-house temperature under infrared irradiation. These results provide important progress for the fabrication of flexible thermochromic films for energy-saving windows

Probing Evolution of Twist-Angle-Dependent Interlayer Excitons in MoSe₂/WSe₂ van der Waals Heterostructures

Interlayer excitons were observed at the heterojunctions in van der Waals heterostructures (vdW HSs). However, it is not known how the excitonic phenomena are affected by the stacking order. Here, we report twist-angle-dependent interlayer excitons in MoSe₂/WSe₂ vdW HSs based on photoluminescence (PL) and vdW-corrected density functional theory calculations. The PL intensity of the interlayer excitons depends primarily on the twist angle: It is enhanced at coherently stacked angles of 0° and 60° (owing to strong interlayer coupling) but disappears at incoherent intermediate angles. The calculations confirm twist-angle-dependent interlayer coupling: The states at the edges of the valence band exhibit a long tail that stretches over the other layer for coherently stacked angles; however, the states are largely confined in the respective layers for intermediate angles. This interlayer hybridization of the band edge states also correlates with the interlayer separation between MoSe₂ and WSe₂ layers. Furthermore, the interlayer coupling becomes insignificant, irrespective of twist angles, by the incorporation of a hexagonal boron nitride monolayer between MoSe₂ and WSe₂