Marvin H. Wu

Square-Shaped, Single-Crystal, Monolayer Graphene Domains by Low-Pressure Chemical Vapor Deposition

Square-shaped, single-crystal, monolayer graphene domains are grown on Cu foils by low-pressure chemical vapor deposition. The domains are remarkably well-aligned along the direction of flow of the gases. Scanning electron microscopy shows that these ‘square’ domains have clean, smooth edges which permit seamless, defect-free merging of the domains. Raman spectra and transmission electron microscopy together demonstrate that individual domains are single layer, and electron diffraction reveals that these domains are single crystals. The expansion mechanism of ‘square’ graphene domains is discussed in the context of our aligned, tailored domain shapes. This work represents an important step toward realization of fabrication of larger area, single-crystal monolayer graphene sheets with controllable shape and alignment.

Hybrid 3D graphene and aligned carbon nanofiber array architectures

We report a one-step, low-pressure, chemical vapor deposition (CVD) process to obtain a three-dimensional (3D) carbon architecture consisting of vertically aligned carbon nanofiber arrays on multi-layered graphene films. The morphology and structure of the hybrid materials grown on transmission electron microscopy copper grids have been determined by electron microscopy and Raman spectroscopy.

Melamine as a single source for fabrication of mesoscopic 3D composites of N-doped carbon nanotubes on graphene

Integration of two-dimensional graphene and one-dimensional carbon nanotubes (CNTs) to create potentially useful 3D mesoscopic carbon structures with enhanced properties relative to the original materials is very desirable. Here, we report a novel and simple route using chemical vapor deposition (CVD) methods to fabricate bead-like nitrogen-doped CNT/graphene composites (NCNT/G) via a simple pyrolysis of the N-rich melamine in the presence of graphene oxide (GO) as a substrate using a Mn–Ni–Co ternary catalyst. We have characterized these structures by field-emission scanning electron microscopy, transmission electron microscopy, X-ray diffraction, Raman spectra, isothermal analyses, and X-ray photoelectron spectroscopy. The three dimensional NCNT/G hybrids have unique network structures, moderate graphitization, high specific surface area, good mesoporosity, and N doping, which makes them promising materials for applications in energy storage and conversion.

Large Area Synthesis of Vertical Aligned Metal Oxide Nanosheets by Thermal Oxidation of Stainless Steel Mesh and Foil

We report here the synthesis of metal oxide nanosheets (MONs) directly grown on stainless steel substrates by thermal oxidation in the presence of trace amounts of water. The morphology and microstructure of MONs were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected area electron diffraction (SAED), and atomic force microscopy (AFM). The composition of MONs was determined by the energy dispersive system and X-ray diffraction patterns. The results showed that the as-synthesized MONs were ultrathin, vertically aligned, and mostly transparent. They were polycrystalline and were composed primarily of Cr2O3 and (Fe, Mn)3O4. The optimal condition to synthesize the MONs with an optimal ultra-high surface atom ratio were determined by varying the temperature and time required for the growth of the MONs. It was found that the lateral size of MONs gradually increases as the temperature rises from 1000 to 1100 °C. An optimal temperature of 1100 °C is obtained in terms of the growth density, size and transparency degree growth morphology, and quality. The structure of MONs changes from two-dimensional to three-dimensional networks when the synthesis time is prolonged to more than 1 h.

A time-resolved millimeter wave conductivity (TR-mmWC) apparatus for charge dynamical properties of semiconductors

This article demonstrates a contactless, time-resolved, millimeter wave conductivity apparatus capable of measuring photoconductivity of a diverse range of materials. This cavity-less system determines the time-dependent magnitude of a samples charge carrier density-mobility product by monitoring the response of a continuous, millimeter-wave probe beam following excitation of the sample by an ultrafast laser pulse. The probe beam is tunable from 110 GHz to 170 GHz and the sample response data can be obtained over the sub-nanosecond to millisecond time interval. This system has been tested on silicon wafers, S-I GaAs, perovskite thin films, SiO2-Ge(nc), and CdSxSe1-x nanowire samples. We demonstrate a minimum detectable photoconductance change of ∼1 µS, an estimated time resolution for conductance decay of ∼100 ps, and a dynamic range greater than 57 dB. The calibration constant of the system, needed for quantitative calculation of photoconductivity from experimental data, has been determined using silicon wafers. This system has several advantages over currently used microwave and terahertz techniques, such as facile tunability of probe frequency and substantially wider time range for study of decay kinetics, while maintaining an open sample environment that enables characterization of a wide range of sample sizes under controlled environmental conditions.

One-Step Chemical Vapor Deposition Synthesis of 3D N-doped Carbon Nanotube/N-doped Graphene Hybrid Material on Nickel Foam

3D hybrid nanostructures connecting 1D carbon nanotubes (CNTs) with 2D graphene have attracted more and more attentions due to their excellent chemical, physical and electrical properties. In this study, we firstly report a novel and facile one-step process using template-directed chemical vapor deposition (CVD) to fabricate highly nitrogen doped three-dimensional (3D) N-doped carbon nanotubes/N-doped graphene architecture (N-CNTs/N-graphene). We used nickel foam as substrate, melamine as a single source for both carbon and nitrogen, respectively. The morphology and microstructure were characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, isothermal analyses, X-ray photoelectron microscopy and Raman spectra. The obtained 3D N-CNTs/N-graphene exhibits high graphitization, a regular 3D structure and excellent nitrogen doping and good mesoporosity.

Enhanced Performance of E-Bike Motive Power Lead–Acid Batteries with Graphene as an Additive to the Active Mass

The effects of both graphene nanoplatelets and reduced graphene oxide as additives to the negative active material in valve-regulated lead–acid batteries for electric bikes were investigated. Low-temperature performance, charge acceptance, cycle performance, and water loss were investigated. The test results show that the low-temperature performance, charge acceptance, and large-current discharge performance of the batteries with graphene additives were significantly improved compared to the control battery, and the cycle life under 100% depth of discharge condition was extended by more than 52% from 250 to 380 cycles. Meanwhile, the amount of water loss from the batteries with graphene changed only slightly compared with the control cells. The excellent performance of the batteries can be ascribed to the graphene promoting the negative-plate charge and discharge processes and suppressing the growth of lead sulfate crystals.