9 research outputs found
Wafer-Scale Synthesis of Graphene on Sapphire: Toward Fab-Compatible Graphene
The adoption of graphene in electronics, optoelectronics and photonics is
hindered by the difficulty in obtaining high quality material on
technologically-relevant substrates, over wafer-scale sizes and with metal
contamination levels compatible with industrial requirements. To date, the
direct growth of graphene on insulating substrates has proved to be
challenging, usually requiring metal-catalysts or yielding defective graphene.
In this work, we demonstrate a metal-free approach implemented in commercially
available reactors to obtain high-quality monolayer graphene on c-plane
sapphire substrates via chemical vapour deposition (CVD). We identify via low
energy electron diffraction (LEED), low energy electron microscopy (LEEM) and
scanning tunneling microscopy (STM) measurements the Al-rich reconstruction
root31R9 of sapphire to be crucial for obtaining epitaxial graphene. Raman
spectroscopy and electrical transport measurements reveal high-quality graphene
with mobilities consistently above 2000 cm2/Vs. We scale up the process to
4-inch and 6-inch wafer sizes and demonstrate that metal contamination levels
are within the limits for back-end-of-line (BEOL) integration. The growth
process introduced here establishes a method for the synthesis of wafer-scale
graphene films on a technologically viable basis.Comment: 15 main text pages, 4 main text figures, 13 supplementary information
pages, 12 supplementary figures, 3 supplementary table
The Role of Palladium in Iron based Fischer-Tropsch Catalysts Prepared by Flame Spray Pyrolysis
Flame spray pyrolysis (FSP) is a novel technique for the fabrication of nanostructured catalysts with far-reaching options to control structure and composition even in cases where complex composites need to be prepared. In this study, we took advantage of this technique to synthesize highly dispersed pure and Pd-doped iron oxide nanoparticles and investigated them as Fischer-Tropsch (FT) catalysts. By systematically varying the Pd content over a large range from 0.1 to 10 wt %, we were able to directly analyze the influence of the Pd content on activity and selectivity. In addition to catalytic measurements, the structure and composition of the particles were characterized before and after these measurements, using transmission electron microscopy, adsorption measurements, X-ray diffraction, and EXAFS. The comparison revealed on the one hand that small Pd clusters (diameter: 1-2 nm) evolve from initially homogeneously distributed Pd and on the other hand that the iron o xide transforms into iron carbides depending on the Pd content. The presence of Pd influences the particle size in the pristine samples (8-11 nm) resulting in specific surface areas that increase as the Pd content increases. However, after activation and reaction the specific surface areas become similar due to partial agglomeration and sintering. In a fixed bed FT reaction test, enhanced FT activity was observed with increasing Pd content while the selectivity shifts to longer chain hydrocarbons, mainly paraffins. Mechanistic implications regarding the role of Pd for the performance of the catalysts are discussed