The CVD generation technique of single-walled carbon nanotubes (SWNTs) from alcohol is discussed in terms of the use of metal catalysts. By using alcohols such as ethanol and methanol as the carbon sources in catalytic CVD, high-purity SWNTs without metal particles, amorphous carbon or MWNTs can be synthesized at relatively low reaction temperatures of 600−800oC. For the bulk generation of SWNTs, Fe/Co bimetal catalysts supported on USY zeolite is employed. The diameter and chirality distributions, which are examined by resonant Raman scatterings and near infrared fluorescence spectroscopy, can be quite narrow at lower CVD temperatures. Combined with the molecular dynamics simulation of the nanotube growth process, the determination mechanism of chirality by the nanotube cap structure is demonstrated. In addition to the bulk generation by using zeolite, Fe/Co or Co/Mo nano-particles directly located on quartz or silicon substrate by dip-coating can be used as efficient catalysts. With this latter system, detailed characterization of catalysts and the growth of vertically aligned SWNTs mat on a quartz substrate are demonstrated.
By using plasma-enhanced chemical vapor deposition (p-CVD), we grew vertically aligned multi-walled carbon nanotubes (CNTs) directly on a nickel-monosilicide layer, without any catalysts other than a metal-silicide layer. The results we obtained are quite important for practical application of CNT vias formed directly on nickel-monosilicide (NiSi) layers, which form the source, drain, and gate region of metal-oxide-semiconductor field-effect transistors (MOSFETs). By using a nickel-silicide layer as a catalyst, the nanotube diameter became smaller than that obtained with a nickel film catalyst. We have also clarified the possibility that CNT diameter can be controlled by forming various Ni-silicide phases such as Ni2Si, NiSi and NiSi2. We suggest that Ni-silicide composition plays an important role in controlling the diameter of the nanotubes. These results will enable us to fabricate CNT vias for future LSI interconnects.
Because of their unique 3 D structure, carbon nanocoils have potential applications to electromagnetic components, such as nanosolenoids and electromagnetic wave absorbers, and mechanical components, such as resonating elements and nanosprings or as a novel reinforcement in high-strain composites. The other potential application is to field emission devices. The stable synthesis of these nanocoils has been made possible by developing the catalysts containing Fe, In and Sn. Recently, the coil diameter has also been able to be controlled by the adjustment of the synthesis condition and the modification of the catalyst. The use of thin nanocoils, for example, enables us to develop the spatially-uniform field-emission-devices with the turn-on voltage as low as 30 V.
We describe the behavior of iron and cobalt catalysts on silicon substrates in chemical vapor deposition (CVD) of carbon nanotubes. Nanoparticles of iron and cobalt exhibited a melting point drop in methane ambient. Nanoparticles present after nanotube growth are identified as Fe3C and Co3C, for iron and cobalt, respectively. These findings indicate that a eutectic compound of metal and carbon is formed in the methane ambient, resulting in the phase separation of graphite (nanotubes) as the carbon uptake in the catalyst melt increases. This supports the vapor-liquid-solid mechanism for the nanotube growth by CVD. Iron- or cobalt-silicide formation causes poisoning of the catalysts. However, the coexistence of oxygen due to native oxide on the silicon surface or the metal surface causes formation of a SiO2 base that can prevent silicidation of iron particles.
Carbon nanotubes have potential applications in a variety of fields, such as composites, field-emission displays, electrodes, gas adsorbent, and catalyst supports. For putting these applications into industry, it is very important to establish the mass production of nanotubes. Here, we introduce our recent progress on the large-scale synthesis of carbon nanotubes, in particular, single-wall carbon nanotubes (SWNTs), based on the gas-phase flow reaction. By spraying the Co-Mo nanoparticles dissolved in toluene to the vertical furnace, we succeeded in selectively obtaining SWNTs and multi-wall carbon nanotubes (MWNTs). It was found that the structure and quality of nanotubes depend on the concentration of the thiophene additives, the hydrogen gas flow rate, and the injection rate of the colloidal solution. The growth mechanism of SWNTs and MWNTs are discussed in terms of catalytic effects. Other large-scale synthetic methods for SWNTs, such as high pressure CO disproportionation (HiPco) and the supported catalysts-based fluidized bed reactions are also introduced.
There are many ways to produce carbon nanotubes and nanofibers, such as arc discharge, laser vaporization, chemical vapor deposition (CVD), etc. The most promising mass production-wise method is the CVD that uses metal catalytic particles, and is known as catalytic CVD (CCVD). Its advantage lies in the ease of controlling the growth conditions to obtain nanotubes with different layers, diameters, and morphologies. In this paper, we briefly introduce the CCVD method and show various nanotubes and fibers obtained from the method.
In Ga+ primary ion TOF-SIMS, the fragment patterns from thin oxidized metal surfaces appear obeying the already proposed rule. The fragment ions MxOy (M: metal) appear in qx ≥ 2y+1 for positive ions and qx ≤ 2y+1 for negative ions (q: valence of M). This paper shows the possibility of the thickness estimation of thin oxide films (SiO2) on Si, from the relative intensity of typical fragment ions from thin oxides or the substrate as identified by the proposed rule in Ga+ primary ion TOF-SIMS spectrum.