Low Temperature Metal Free Growth of Graphene on Insulating Substrates by Plasma Assisted Chemical Vapor Deposition.

Direct growth of graphene films on dielectric substrates (quartz and silica) is reported, by means of remote electron cyclotron resonance plasma assisted chemical vapor deposition r-(ECR-CVD) at low temperature (650°C). Using a two step deposition process- nucleation and growth- by changing the partial pressure of the gas precursors at constant temperature, mostly monolayer continuous films, with grain sizes up to 500 nm are grown, exhibiting transmittance larger than 92% and sheet resistance as low as 900 Ω·sq-1. The grain size and nucleation density of the resulting graphene sheets can be controlled varying the deposition time and pressure. In additon, first-principles DFT-based calculations have been carried out in order to rationalize the oxygen reduction in the quartz surface experimentally observed. This method is easily scalable and avoids damaging and expensive transfer steps of graphene films, improving compatibility with current fabrication technologies.

Influence of the atmosphere on the growth by CVD of silicon based nanostructures.

Silica nanowires (phi = 70-80 nm) and core crystalline silicon carbide/amorphous silica coaxial nanocables (phi = 30-50 nm) have been grown on Ni (5 nm) covered silicon substrates by thermal CVD at 950 degrees C. Heating the sample in different atmospheres and the addition of methane lead to the growth of the nanostructures. The as-grown product was characterized by Scanning Electron (SEM) and Transmission Electron (TEM) Microscopies, Infrared (IR) and Glow Discharge Optical Emission (GDOES) Spectroscopies. High contents of Ni, O and Si have been detected in the surface layer for argon and nitrogen treatments and in both cases silica based nanostructures grew. However, no nanostructures were obtained in hydrogen environment probably due to the detected Ni migration inward the substrate making the nanostructures nucleation more difficult. In this work we have studied and proven that previous processes on the substrate surface affect significantly the nanostructures growth.

The key role of hydrogen in the growth of SiC/SiO(2) nanocables.

SiC/SiO(2) nanocables, consisting of a crystalline SiC core surrounded by an amorphous silica shell, have been grown by thermal chemical vapour deposition (CVD) at 950 °C on Ni-covered silicon substrates. The addition of methane to a 375 Torr hydrogen atmosphere, after heating the substrate in argon, leads to the growth of the SiC/SiO(2) nanocables, by the carbothermal reduction of silicon oxide as the initial stage. The growth mechanism follows the model previously proposed by us for a reducing medium. From the results obtained, several effects of hydrogen on the deposition process have been established: (a) reduction of the nickel nucleation sites, thus favouring the formation of SiC from the initial stage; (b) oxygen removal in the medium hindering the oxidative effect over the SiO and C species, thus promoting the nanocable growth, and (c) increase of the SiO concentration in the neighbourhood of the active nucleation sites. In addition, it is important to mention that SiC/SiO(2) nanocables, following the already proposed model, are obtained uniquely in a narrow hydrogen pressure range. At high hydrogen pressure, the unexpected formation of silica nanowires together with the SiC/SiO(2) nanocables has been detected.