X. L. Ma

Large-area high-quality 2D ultrathin Mo2C superconducting crystals

Transition metal carbides (TMCs) are a large family of materials with many intriguing properties and applications, and high-quality 2D TMCs are essential for investigating new physics and properties in the 2D limit. However, the 2D TMCs obtained so far are chemically functionalized, defective nanosheets having maximum lateral dimensions of ∼10 μm. Here we report the fabrication of large-area high-quality 2D ultrathin α-Mo2C crystals by chemical vapour deposition (CVD). The crystals are a few nanometres thick, over 100 μm in size, and very stable under ambient conditions. They show 2D characteristics of superconducting transitions that are consistent with Berezinskii-Kosterlitz-Thouless behaviour and show strong anisotropy with magnetic field orientation; moreover, the superconductivity is also strongly dependent on the crystal thickness. Our versatile CVD process allows the fabrication of other high-quality 2D TMC crystals, such as ultrathin WC and TaC crystals, which further expand the large family of 2D materials.

CdS–mesoporous ZnS core–shell particles for efficient and stable photocatalytic hydrogen evolution under visible light

A mesoporous ZnS shell is in situ grown on a CdS core through a one-pot surfactant-free hydrothermal route. Due to the mesoporous ZnS shell and the unique spatial distribution of the photoexcited charge carriers in the CdS–ZnS core–shell particles, the hydrogen evolution rate over CdS–ZnS is 169 and 56 times higher than that of ZnS and CdS under visible light, respectively. Moreover, the core–shell particles show excellent photocatalytic stability over 60 h.

Repeated and Controlled Growth of Monolayer, Bilayer and Few-Layer Hexagonal Boron Nitride on Pt Foils

Atomically thin hexagonal boron nitride (h-BN), as a graphene analogue, has attracted increasing interest because of many fascinating properties and a wide range of potential applications. However, it still remains a great challenge to synthesize high-quality h-BN with predetermined number of layers at a low cost. Here we reported the controlled growth of h-BN on polycrystalline Pt foils by low-cost ambient pressure chemical vapor deposition with ammonia borane as the precursor. Monolayer, bilayer and few-layer h-BN domains and large-area films were selectively obtained on Pt by simply changing the concentration of ammonia borane. Moreover, using a bubbling method, we have achieved the nondestructive transfer of h-BN from Pt to arbitrary substrates and the repeated use of the Pt for h-BN growth, which not only reduces environmental pollution but also decreases the production cost of h-BN. The monolayer and bilayer h-BN obtained are very uniform with high quality and smooth surfaces. In addition, we found that the optical band gap of h-BN increases with decreasing number of layers. The repeated growth of large-area, high-quality monolayer and bilayer h-BN films, together with the successful growth of graphene, opens up the possibility for creating various functional heterostructures for large-scale fabrication and integration of novel electronics.

Scalable Clean Exfoliation of High-Quality Few-Layer Black Phosphorus for a Flexible Lithium Ion Battery.

Few-layer black phosphorus (BP) nanosheets that are clean and of high quality, are efficiently produced by exfoliating bulk BP crystals, which are prepared by a scalable gas-phase catalytic transformation method in water. They are stable enough in water for further processing and applications. As an example, these BP nanosheets are combined with graphene to give high-performance flexible lithium-ion batteries.

Edge-controlled growth and kinetics of single-crystal graphene domains by chemical vapor deposition.

Significance Controlled synthesis of wafer-sized single crystalline high-quality graphene is a great challenge of graphene growth by chemical vapor deposition because of the complicated kinetics at edges that govern the growth process. Here we report the synthesis of single-crystal graphene domains with tunable edges from zigzag to armchair via a growth–etching–regrowth process. Both growth and etching of graphene are strongly dependent on the edge structure. This growth/etching behavior is well explained at the atomic level, given the concentrations of kinks on various edges, and allows control of graphene edges and morphology according to the classical kinetic Wulff construction theory. This work provides a deep understanding of the fundamental problems that limit graphene growth by chemical vapor deposition. The controlled growth of large-area, high-quality, single-crystal graphene is highly desired for applications in electronics and optoelectronics; however, the production of this material remains challenging because the atomistic mechanism that governs graphene growth is not well understood. The edges of graphene, which are the sites at which carbon accumulates in the two-dimensional honeycomb lattice, influence many properties, including the electronic properties and chemical reactivity of graphene, and they are expected to significantly influence its growth. We demonstrate the growth of single-crystal graphene domains with controlled edges that range from zigzag to armchair orientations via growth–etching–regrowth in a chemical vapor deposition process. We have observed that both the growth and the etching rates of a single-crystal graphene domain increase linearly with the slanted angle of its edges from 0° to ∼19° and that the rates for an armchair edge are faster than those for a zigzag edge. Such edge-structure–dependent growth/etching kinetics of graphene can be well explained at the atomic level based on the concentrations of the kinks on various edges and allow the evolution and control of the edge and morphology in single-crystal graphene following the classical kinetic Wulff construction theory. Using these findings, we propose several strategies for the fabrication of wafer-sized, high-quality, single-crystal graphene.

Characterization of zinc oxide crystal whiskers grown by thermal evaporation

Abstract ZnO crystal whiskers were prepared via a simple thermal evaporation of a mixture of ZnS and Fe(NO 3 ) 3 . The growth process was carried out in an alumina tube at 1300°C using Ar mixed with 5% H 2 as the carrier gas. The ZnO whiskers obtained had diameters ranging from 200 to 500 nm and lengths up to several hundreds of micrometers. The photoluminescence spectrum of as-grown ZnO whiskers was studied. The influences of reaction temperature, time, and carrier gases on the formation of the whiskers were investigated. A preliminary growth mechanism of the ZnO crystal whiskers was proposed.

Large-area synthesis of high-quality and uniform monolayer WS2 on reusable Au foils.

Large-area monolayer WS2 is a desirable material for applications in next-generation electronics and optoelectronics. However, the chemical vapour deposition (CVD) with rigid and inert substrates for large-area sample growth suffers from a non-uniform number of layers, small domain size and many defects, and is not compatible with the fabrication process of flexible devices. Here we report the self-limited catalytic surface growth of uniform monolayer WS2 single crystals of millimetre size and large-area films by ambient-pressure CVD on Au. The weak interaction between the WS2 and Au enables the intact transfer of the monolayers to arbitrary substrates using the electrochemical bubbling method without sacrificing Au. The WS2 shows high crystal quality and optical and electrical properties comparable or superior to mechanically exfoliated samples. We also demonstrate the roll-to-roll/bubbling production of large-area flexible films of uniform monolayer, double-layer WS2 and WS2/graphene heterostructures, and batch fabrication of large-area flexible monolayer WS2 film transistor arrays.

High sensitivity of positive magnetoresistance in low magnetic field in perovskite oxide p–n junctions

Large positive magnetoresistance (MR) and high MR sensitivity in low magnetic fields have been discovered in the Sr-doped LaMnO3 and Nb-doped SrTiO3 p-n junctions fabricated by laser molecular-beam epitaxy. The MR ratios, defined as DeltaR/R-0, DeltaR = R-H-R-0, are observed as large as 11 % in 5 Oe, 23 % in 100 Oe, and 26 % in 1000 Oe at 290 K; 53% in 5 Oe, 80 % in 100 Oe, and 94 % in 1000 Oe at 255 K. The MR sensitivities are 85 Omega/Oe at 290 K, 246 Omega/Oe at 255 K, and 136 Omega/Oe at 190 K, respectively, with the applied magnetic field changed from 0 to 5 Oe. The positive MR ratios and high MR sensitivities of the p-n junctions are very different from that of the LaMnO3 compound family

Phosphorene as a Polysulfide Immobilizer and Catalyst in High‐Performance Lithium–Sulfur Batteries

Theoretical and experimental studies together show phosphorene as a highly potent polysulfide immobilizer for lithium-sulfur batteries, enabling a high capacity, good rate capability, and excellent cycling stability.

Decagonal quasicrystal and related crystalline phases in slowly solidified Al-Co alloys

A two-dimensional decagonal quasicrystal (DQC) has been found in binary Al-Co alloys in the composition range Al11Co4 to Al10Co4 after slow solidification from the melt. The crystalline phase found most frequently coexisting with this DQC is two new structure variants, oneC-centered and the other primitive, of the monoclinic layer compound Al13Co4. Theira andc parameters are roughly τ2 times (τ = (1 + √5)/2 = 1.6180…) larger than the corresponding ones in Al13Co4. Moreover, a new orthorhombic phase Al3Co or Al11Co4 (Pnmn, a = 1.25 nm,b = 0.81 nm, andc = 1.46 nm) has also been found. As shown by the characteristic electron diffraction patterns (EDPs), these crystalline phases can be considered as approximate structures of the DQC. The lattice relationship between these phases has been discussed.