Preparation of a Binder-Free Three-Dimensional Carbon Foam/Silicon Composite as Potential Material for Lithium Ion Battery Anodes

We report a novel three-dimensional nitrogen containing carbon foam/silicon (CFS) composite as potential material for lithium ion battery anodes. Carbon foams were prepared by direct carbonization of low cost, commercially available melamine formaldehyde (MF, Basotect) foam precursors. The carbon foams thus obtained display a three-dimensional interconnected macroporous network structure with good electrical conductivity (0.07 S/cm). Binder free CFS composites used for electrodes were prepared by immersing the as-fabricated carbon foam into silicon nanoparticles dispersed in ethanol followed by solvent evaporation and secondary pyrolysis. In order to substantiate this new approach, preliminary electrochemical testing has been done. The first results on CFS electrodes demonstrated initial capacity of 1668 mAh/g with 75% capacity retention after 30 cycles of subsequent charging and discharging. In order to further enhance the electrochemical performance, silicon nanoparticles were additionally coated with a nitrogen containing carbon layer derived from codeposited poly­(acrylonitrile). These carbon coated CFS electrodes demonstrated even higher performance with an initial capacity of 2100 mAh/g with 92% capacity retention after 30 cycles of subsequent charging and discharging

Graphene Nanoribbons as Low Band Gap Donor Materials for Organic Photovoltaics: Quantum Chemical Aided Design

Graphene nanoribbons (GNRs) are strips of graphene cut along a specific direction that feature peculiar electronic and optical properties owing to lateral confinement effects. We show here by means of (time-dependent) density functional theory calculations that GNRs with properly designed edge structures fulfill the requirements in terms of electronic level alignment with common acceptors (namely, C₆₀), solar light harvesting, and singlet–triplet exchange energy to be used as low band gap semiconductors for organic photovoltaics

Chemical Vapor Deposition of High Quality Graphene Films from Carbon Dioxide Atmospheres

The realization of graphene-based, next-generation electronic applications essentially depends on a reproducible, large-scale production of graphene films <i>via</i> chemical vapor deposition (CVD). We demonstrate how key challenges such as uniformity and homogeneity of the copper metal substrate as well as the growth chemistry can be improved by the use of carbon dioxide and carbon dioxide enriched gas atmospheres. Our approach enables graphene film production protocols free of elemental hydrogen and provides graphene layers of superior quality compared to samples produced by conventional hydrogen/methane based CVD processes. The substrates and resulting graphene films were characterized by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and Raman microscopy, sheet resistance and transport measurements. The superior quality of the as-grown graphene films on copper is indicated by Raman maps revealing average G band widths as low as 18 ± 8 cm^–1 at 514.5 nm excitation. In addition, high charge carrier mobilities of up to 1975 cm²/(V s) were observed for electrons in transferred films obtained from a carbon dioxide based growth protocol. The enhanced graphene film quality can be explained by the mild oxidation properties of carbon dioxide, which at high temperatures enables an uniform conditioning of the substrates by an efficient removal of pre-existing and emerging carbon impurities and a continuous suppression and <i>in situ</i> etching of carbon of lesser quality being co-deposited during the CVD growth