Saïd Tahir

Increased chemical reactivity of single-walled carbon nanotubes on oxide substrates: In situ imaging and effect of electron and laser irradiations

We studied the oxygen etching of individual single-walled carbon nanotubes on silicon oxide substrates using atomic force microscopy and high-temperature environmental scanning electron microscopy. Our in situ observations show that carbon nanotubes are not progressively etched from their ends, as frequently assumed, but disappear segment by segment. Atomic force microscopy, before and after oxidation, reveals that the oxidation of carbon nanotubes on substrates proceeds through a local cutting that is followed by a rapid etching of the disconnected nanotube segment. Unexpectedly, semiconducting nanotubes appear more reactive under these conditions than metallic ones. We also show that exposure to electron and laser beams locally increases the chemical reactivity of carbon nanotubes on such substrates. These results are rationalized by considering the effect of substrate-trapped charges on the nanotube density of states close to the Fermi level, which is impacted by the substrate type and the exposure to electron and laser beams.

In situ study of single-walled carbon nanotube growth in an environmental scanning electron microscope

Monitoring individual single-walled carbon nanotubes (SWCNTs) during their growth is a highly sought-after goal in view of understanding the processes involved in the nucleation, elongation and termination which ultimately control the diameter and chiral selectivity. Here, we report on the first truly in situ observations of SWCNT growth in an environmental scanning electron microscope (ESEM). The CNT growth from lithographically patterned catalysts was investigated as a function of the catalyst type (Fe, Co or Ni), temperature, type of precursor (ethanol or acetylene), gas phase composition and pressure, and pretreatment conditions, and we report on the most appropriate conditions for SWCNT growth in ESEM conditions. We show that this approach allows the observation at the submicron scale of the different steps of the nanotube synthesis including the catalyst reduction, the growth and percolation of the nanotube network, and the deposition of individual nanotubes grown in the gas phase on the substrate. Despite these obvious advantages, we identified a few limitations which will need to be tackled for fully taking advantage of the approach, for instance for monitoring the growth of individual SWCNTs by ESEM, including the short lifetime of the catalyst nanoparticles, the preference for kite growth (by opposition to surface growth) and the influence of the electron beam on the nanotube growth.