Xiaozhi Xu

Ultrafast growth of single-crystal graphene assisted by a continuous oxygen supply

Graphene has a range of unique physical properties and could be of use in the development of a variety of electronic, photonic and photovoltaic devices. For most applications, large-area high-quality graphene films are required and chemical vapour deposition (CVD) synthesis of graphene on copper surfaces has been of particular interest due to its simplicity and cost effectiveness. However, the rates of growth for graphene by CVD on copper are less than 0.4u2005μmu2005s-1, and therefore the synthesis of large, single-crystal graphene domains takes at least a few hours. Here, we show that single-crystal graphene can be grown on copper foils with a growth rate of 60u2005μmu2005s-1. Our high growth rate is achieved by placing the copper foil above an oxide substrate with a gap of ∼15u2005μm between them. The oxide substrate provides a continuous supply of oxygen to the surface of the copper catalyst during the CVD growth, which significantly lowers the energy barrier to the decomposition of the carbon feedstock and increases the growth rate. With this approach, we are able to grow single-crystal graphene domains with a lateral size of 0.3u2005mm in just 5u2005s.

Surface Monocrystallization of Copper Foil for Fast Growth of Large Single-Crystal Graphene under Free Molecular Flow

Wafer-sized single-crystalline Cu (100) surface can be readily achieved on stacked polycrystalline Cu foils via simple oxygen chemisorption-induced reconstruction, enabling fast growth of large-scale millimeter-sized single-crystalline graphene arrays under molecular flow. The maximum growth rate can reach 300 μm min-1 , several orders of magnitude higher than previously reported values for millimeter-sized single-crystalline graphene growth on Cu foils.

Direct observation of ordered configurations of hydrogen adatoms on graphene.

Ordered configurations of hydrogen adatoms on graphene have long been proposed, calculated, and searched for. Here, we report direct observation of several ordered configurations of H adatoms on graphene by scanning tunneling microscopy. On the top side of the graphene plane, H atoms in the configurations appear to stick to carbon atoms in the same sublattice. Scanning tunneling spectroscopy measurements revealed a substantial gap in the local density of states in H-contained regions as well as in-gap states below the conduction band due to the incompleteness of H ordering. These findings can be well explained by density functional theory calculations based on double-sided H configurations. In addition, factors that may influence H ordering are discussed.

Synthesis of nanoscale superconducting YBCO by a novel technique

Abstract A novel technique using citrate pyrolysis was developed to prepare nanoscale superconducting oxide materials. This paper describes the details on synthesizing nanocrystalline YBCO with a T c of ∼80 K using this method. The morphology and structure of the nanoscale products were characterized by powder X-ray diffraction and scanning electron microscopy. The obtained YBCO grains have a mean particle size of 40–60 nm (for unannealed samples) and 100–150 nm (for the annealed products). The crystalline size was only ∼20 nm. Currently there are attempts at using such fine powder to fabricate longer superconducting tapes, which should induce a technical revolution in the production of superconducting tapes.

A novel technique by the citrate pyrolysis for preparation of iron oxide nanoparticles

A novel technique by the citrate pyrolysis is developed to prepare metal oxide nanocrystals. In this paper details on synthesizing iron oxide nanoparticles through this method is described. The structure and morphology of nanoscale products have been characterized by powder X-ray diffraction (XRD) and scanning electron microscopy (SEM). The obtained iron oxide particles have two lattice types and a mean particle size distribution of 20 and 50 nm, respectively.

TEM CHARACTERIZATION OF CALCIUM-OXYGEN NANORODS

Abstract During the catalytic growth of carbon nanotubes we have found a new rod-like product with nanoscale. The TEM examination has revealed that these nanorods consist of calcium and oxygen and have an orthorhombic structure.

Superconducting HgBa2CaCu2Oy thin films growth on NdGaO3, SrTiO3, LaAlO3 and Y–ZrO2 substrates

Abstract Superconducting HgBa 2 CaCu 2 O y (Hg-1212) films have been fabricated on (110) NdGaO 3 , (100) SrTiO 3 , (100) LaAlO 3 and (100) Y–ZrO 2 substrates by a two-step process. Good reproducible superconducting properties, zero resistance temperatures ( T c )>115 K and critical current density ( J c )>4000 MA/m 2 at 77 K in zero field, are obtained. X-ray diffraction patterns indicate that the films have an epitaxial structure with the c -axis perpendicular to the surface. Scanning electron microscopy measurements show a layered growth structure with square and octagonal grains on the Hg-1212 film surfaces. (110) NdGaO 3 is demonstrated to be a good substrate for growing the Hg-cuprate thin films. A lower superconducting transition temperature is observed in films deposited on Y–ZrO 2 substrates, which can be attributed to variations of the microstructure in the films.

Preparation of superconducting HgBa2CaCu2Ox films with a zero-resistance transition temperature of 121 K

Abstract Hg-1212 films were prepared by means of annealing of mercury-free precursor films with bulk Hg-1212 pellets in an evacuated quartz tube. The mercury free precursor films were deposited by laser ablation on (100) SrTiO 3 substrates. The annealed films exhibited the (00 n ) oriented X-ray diffraction pattern of Hg-1212. The superconducting transition temperature, T c ( ϱ = 0), was 121 K.

Greatly Enhanced Anticorrosion of Cu by Commensurate Graphene Coating

Metal corrosion is a long-lasting problem in history and ultrahigh anticorrosion is one ultimate pursuit in the metal-related industry. Graphene, in principle, can be a revolutionary material for anticorrosion due to its excellent impermeability to any molecule or ion (except for protons). However, in real applications, it is found that the metallic graphene forms an electrochemical circuit with the protected metals to accelerate the corrosion once the corrosive fluids leaks into the interface. Therefore, whether graphene can be used as an excellent anticorrosion material is under intense debate now. Here, graphene-coated Cu is employed to investigate the facet-dependent anticorrosion of metals. It is demonstrated that as-grown graphene can protect Cu(111) surface from oxidation in humid air lasting for more than 2.5 years, in sharp contrast with the accelerated oxidation of graphene-coated Cu(100) surface. Further atomic-scale characterization and ab initio calculations reveal that the strong interfacial coupling of the commensurate graphene/Cu(111) prevents H2 O diffusion into the graphene/Cu(111) interface, but the one-dimensional wrinkles formed in the incommensurate graphene on Cu(100) can facilitate the H2 O diffusion at the interface. This study resolves the contradiction on the anticorrosion capacity of graphene and opens a new opportunity for ultrahigh metal anticorrosion through commensurate graphene coating.

Wafer-Scale Growth and Transfer of Highly-Oriented Monolayer MoS2 Continuous Films

Large scale epitaxial growth and transfer of monolayer MoS2 has attracted great attention in recent years. Here, we report the wafer-scale epitaxial growth of highly oriented continuous and uniform monolayer MoS2 films on single-crystalline sapphire wafers by chemical vapor deposition (CVD) method. The epitaxial film is of high quality and stitched by many 0°, 60° domains and 60°-domain boundaries. Moreover, such wafer-scale monolayer MoS2 films can be transferred and stacked by a simple stamp-transfer process, and the substrate is reusable for subsequent growth. Our progress would facilitate the scalable fabrication of various electronic, valleytronic, and optoelectronic devices for practical applications.