Helin Wei

Fabrication of thickness controllable free-standing sandwich-structured hybrid carbon film for high-rate and high-power supercapacitor.

Hybrid carbon films composed of graphene film and porous carbon film may give full play to the advantages of both carbon materials, and have great potential for application in energy storage and conversion devices. Unfortunately, there are very few reports on fabrication of hybrid carbon films. Here we demonstrate a simple approach to fabricate free-standing sandwich-structured hybrid carbon film composed of porous amorphous carbon film and multilayer graphene film by chemical vapor deposition in a controllable and scalable way. Hybrid carbon films reveal good electrical conductivity, excellent flexibility, and good compatibility with substrate. Supercapacitors assembled by hybrid carbon films exhibit ultrahigh rate capability, wide frequency range, good capacitance performance, and high-power density. Moreover, this approach may provide a general path for fabrication of hybrid carbon materials with different structures by using different metals with high carbon solubility, and greatly expands the application scope of carbon materials.

Controllable synthesis of large-area free-standing amorphous carbon films and their potential application in supercapacitors

A new and simple approach is proposed for the first time to fabricate free-standing amorphous carbon films (FS-ACFs) with controllable thickness by ambient pressure chemical vapor deposition (APCVD). The approach uses methane as the carbon source and Ni foil as the catalytic substrate, in which a large number of carbon atoms are trapped by controlling the experimental conditions during the APCVD growth process, and FS-ACFs are obtained by a simple corrosion of the Ni foil after APCVD growth. FS-ACFs having a uniform and continuous morphology with variable sizes over one hundred square centimeters and a controllable thickness of tens to hundreds of nanometers are synthesized by controlling the experimental conditions. Microstructure observations show that FS-ACFs have porous and transparent characteristics, which can be transferred or bent on different substrates, thus allowing for a variety of potential applications in electrochemistry. As a proof of concept, an electrochemical supercapacitor device directly assembled by using the FS-ACF exhibits an ultrashort time constant of 46 μs, a wide frequency range (∼kHz) for capacitive feature, and a good capacitance performance with an area specific capacitance of 0.28 mF cm−2 at a scan rate of 50 mV s−1. Furthermore, the FS-ACF-based supercapacitor shows a high power density with a maximum volumetric power density of 17.76 W cm−3.

Enhanced Field Emission From Aligned ZnO Nanowires Grown on a Graphene Layer With Hydrothermal Method

Zinc oxide (ZnO) nanowire arrays on silicon substrates with amorphous carbon or graphene as buffer layer for field electron emission application are obtained using hydrothermal method. It is found that graphene could optimize the structure and morphology of ZnO nanowires grown on it. Smaller nanowire diameter and larger length/diameter ratio are obtained for those grown on graphene buffer layer. ZnO nanowires grown on graphene layer has c-axis preferential orientation, while those grown on bare silicon wafer without any buffer layer has no obvious preferred growth orientation. Low turn-on electrical field of 1.59 V/μm and high field enhancement factor of 1625 are obtained from ZnO nanowires grown on graphene layer, which indicates that graphene is not only beneficial to the growth of ZnO nanowires by improving their configuration but can also enhance their field electron emission.

Fabrication of photonic crystal heterostructures by a simple vertical deposition technique

This paper describes a facile method for the fabrication of photonic crystal heterostructures (PCHSs) composed of photonic crystals (PCs) of core–shell spheres with different diameters and effective refractive indexes. The PCs are fabricated by a simple vertical deposition technique. The PCs of monodisperse polystyrene/silica core–shell (PS@SiO2) spheres or hollow silica spheres are used as substrates to fabricate PCHSs, respectively. The results indicate that the resultant PCHSs formed from PS@SiO2 spheres or hollow silica spheres have a very high quality and a good adsorbing interface between the PCs. Transmission spectra show that there are two optical stop bands of the PCHSs, and the positions of optical stop bands are controlled by tuning the size and the effective refractive index of spheres. The PCHSs formed from hollow silica spheres may facilitate the development of the potential applications due to the novel properties of hollow silica spheres, such as, low density, low refractive index, and high specific surface area.

Tunable optical stop band of silica shell photonic crystals

In this study, optical stop band of three-dimensional silica shell photonic crystals is tuned by adjusting inner diameter and shell thickness of hollow silica spheres. The silica shell photonic crystals are fabricated by sintering crystalline arrays, which are assembled from polystyrene/silica core–shell spheres by an improved vertical deposition method. The inner diameter and the shell thickness are controlled by diameter of polystyrene spheres and concentration of tetraethoxysilane. The results of transmission spectra show that there is an evident red shift of optical stop band as the inner diameter and the shell thickness increase. The red shift of optical stop band is due to variations in the inter-planar spacing and the effective refractive index of silica shell photonic crystals, which result from the variations of the inner diameter and the shell thickness of hollow silica spheres.