M. Cantoro

Effects of catalyst film thickness on plasma-enhanced carbon nanotube growth

A systematic study is presented of the influence of catalyst film thickness on carbon nanostructures grown by plasma-enhanced chemical-vapor deposition from acetylene and ammonia mixtures. We show that reducing the Fe∕Co catalyst film thickness below 3nm causes a transition from larger diameter (>40nm), bamboolike carbon nanofibers to small diameter (∼5nm) multiwalled nanotubes with two to five walls. This is accompanied by a more than 50 times faster growth rate and a faster catalyst poisoning. Thin Ni catalyst films only trigger such a growth transition when pretreated with an ammonia plasma. We observe a limited correlation between this growth transition and the coarsening of the catalyst film before deposition. For a growth temperature of ⩽550°C, all catalysts showed mainly a tip growth regime and a similar activity on untreated silicon, oxidized silicon, and silicon nitride support.

Raman scattering on silicon nanowires : The thermal conductivity of the environment determines the optical phonon frequency

We studied the Raman spectra of silicon nanowires as a function of excitation power for various ambient gases. For a given excitation power, we find that the gas thermal conductivity determines the wire temperature, which can be detected by a change in phonon frequency. This shows that the redshift of the optical phonon in silicon nanowires compared to bulk silicon is mainly due to the lower thermal conductivity of nanowires and an increase in laser heating. The spectra of nanowires allow distinguishing gases on the basis of their thermal conductivity.

Submicron patterning of Co colloid catalyst for growth of vertically aligned carbon nanotubes

Applications of carbon nanotubes such as field emission or microelectrode sensor arrays require a patterning of vertically aligned carbon nanotubes over large areas. A highly purified and concentrated monodisperse cobalt colloid was produced for use as a catalyst for growth of carbon nanotubes. Nanocontact printing was employed to deposit the cobalt nanoparticles in regular patterns with feature sizes at the 100?nm scale onto silicon wafers at low cost over large areas. Vertically aligned carbon nanotubes were grown by direct current plasma enhanced chemical vapour deposition at temperatures ranging from 300 to 640??C.

Plasma restructuring of catalysts for chemical vapor deposition of carbon nanotubes

The growth of multiwalled carbon nanotubes and carbon nanofibers by catalytic chemical vapor deposition at lower temperatures is found to be aided by a separate catalyst pretreatment step in which the catalyst thin film is restructured into a series of nanoparticles with a more active surface. The restructuring is particularly effective when carried out by an ammonia plasma. The nature of the restructuring is studied by atomic force microscopy, transmission electron microscopy, x-ray photoelectron spectroscopy, and Raman. We find that as the growth temperature decreases, there is a limiting maximum catalyst thickness, which gives any nanotube growth. Plasmas are found to restructure the catalyst by a combination of physical etching and chemical modification. Large plasma powers can lead to complete etching of thin catalyst films, and hence loss of activity. Ni is found to be the better catalyst at low temperatures because it easily reduced from any oxide form to the catalytically active metallic state. On...

Synthesis of Silicon Nanowires

We present alternative routes for the production of silicon nanowires (SiNWs). We have successfully synthesized SiNWs by plasma enhanced chemical vapor deposition, high temperature annealing, and thermal evaporation. Selective growth is achieved by using a patterned catalyst. Bulk production of SiNWs can be achieved by thermal evaporation.

Low temperature growth of carbon nanotubes and nanofibres

This paper reviews work on low temperature growth of carbon nanotubes, on Si, on plastic, on carbon cloth, using sputtered and colloidal catalysts, and with nano-imprinted patterning.

Direct Growth of Carbon Nanofibers on Plastic Substrates

Carbon nanofibers were successfully grown onto Cr covered thin polyimide plastic substrates by plasma‐enhanced chemical vapour deposition. Growth is visible at a temperature as low as 200 °C, without detectable substrate degradation. Patterned 100 nm‐size Ni catalyst features allow selective deposition. We propose that surface carbon transport on the metal catalyst is the low activation energy pathway in low temperature nanofiber growth, as shown by activation energy measurements and first principles density functional calculations.

Raman spectroscopy of silicon nanowires: Phonon confinement and anharmonic phonon processes

We calculate the effects of phonon confinement on the Raman spectra of Silicon nanowires. The theoretical predictions are checked by measuring the Raman spectra of SiNWs selectively grown by plasma enhanced chemical vapor deposition (PECVD) employing gold as a catalyst. In order to fully account for the measured spectra and their variation as a function of laser power, the standard confinement theory is extended to include anharmonic phonon processes.