Nanotechnology

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1. Demoisson, F., Raes, M., Terryn, H., Guillot, J., Migeon, H. N., and Reniers, F. A., Characterization of gold nanoclusters deposited on HOPG by atmospheric plasma treatment,” Surf. Interface Anal. 40, 566 (2008). Abstract: The interest of gold nanoparticles in the field of nanocatalysis and nanosensors is growing. For example, carbon nanotubes covered with gold nanoclusters could present interesting properties in catalysis or/and in devices based on catalytic reaction such as chemical gas sensor. Unfortunately, the characterization of nanoparticles deposited onto a substrate is generally not a trivial exercise. Indeed, in many cases, the amount and size of material deposited is in the range of the sensitivity limits and the spatial resolutions of the techniques used. In that respect, our system, i.e. gold deposited onto a carbon support, is a favourable case, due to the high difference between the atomic numbers of gold and carbon.

2. Lee, S., Peng, J. W., and Liu, C. H., “Raman study of carbon nanotube purification using atmospheric pressure plasma,” Carbon 46, 2124 (2008). Abstract: Multiwalled carbon nanotubes (MWCNTs) were treated with an atmospheric pressure plasma source using an argon/water mixture. Optical emission diagnostics has shown that hydroxyl radicals (OH) were the major reactive species in the plasma. The structural changes in MWCNTs were monitored by micro-Raman spectroscopy. The observed variation of the D and G band intensity ratio and position dispersion with plasma treatment time was ascribed to the change in structural disorder on MWCNT surfaces. Scanning electron microscopic study showed that some defects can be induced in MWCNTs during plasma treatment. Results of thermogravimetric analysis indicated that atmospheric pressure OH plasma is as effective as traditional wet methods for purifying MWCNTs.

3. Chandrashekar, A., Ramachandran, S., Pollack, G., Lee, J. S., Lee, G. S., and Overzet, L J., “Forming carbon nanotube composites by directly coating forests with inorganic materials using low pressure chemical vapor deposition,” Thin Solid Films 517, 525 (2008). Abstract: Low pressure chemical vapor deposition has been used to fill carbon nanotube (CNT) forests with inorganic materials (polysilicon and silicon nitride). Forest filling proceeds by deposition around individual CNTs. As the coating thickness around each CNT increases, the free volume between adjacent nanotubes is filled and finally results in a contiguous composite film. The process maintains the forest height and alignment; however, the coating thickness around the CNTs is in general smaller at the base of the forest than it is at the top. This can cause a contiguous solid film to form at the top of the forest while the forest is only partially filled at the base. Once the top of the forest becomes filled, it prevents growth from occurring at the base. Consequently, the growth process can cap the top of the forest and leave voids between thinly coated CNTs at the base. Such composites have reduced hardness (4 GPa or less). Depositing at reduced temperatures and/or decreased precursor gas flow rates reduces the void fraction through improving the step coverage modulus. This allows one to produce thick (> 50 μm) polysilicon-CNT composite films having hardness approximately equal to that of polysilicon thin films (12.4 GPa).

4. Chandrashekar, A., Lee, J. S., Lee, G. S., Goeckner, M .J., and Overzet, L. J., “Gas-phase and sample characterizations of multiwall carbon nanotube growth using an atmospheric pressure plasma,” J. Vac. Sci. Technol. A 24, 1812 (2006). Abstract: Multiwall carbon nanotubes (CNTs) are synthesized using an atmospheric pressure rf plasma jet, with helium feed gas and acetylene gas as the precursor. The nanotubes are grown on a substrate with a thin catalyst (iron) film, with the substrate placed downstream from the plasma on a copper hot plate. In situ Fourier transformed infrared spectroscopy indicates an increase in gas temperature and a decrease in the density of the acetylene molecules at higher plasma powers. The helium metastables in the plasma break the C–H bonds in acetylene, causing molecular dissociation. It is apparent that the resultant formation of unsaturated carbon bonds causes taller and more graphitized CNT films to grow, as evident from scanning electron microscopy and Raman analyses of the samples. However, at higher substrate temperatures, taller and better quality films are obtained due to enhanced catalytic activity on the substrate surface.

5. Barankin, M. D., Creyghton, Y., and Schmidt-Ott, A., “Synthesis of nanoparticles in an atmospheric pressure glow discharge,” J. Nanoparticle Res. 8, 511 (2006). Abstract: Nanopowders are produced in a low temperature, non-equilibrium plasma jet (APPJ), which produces a glow discharge at atmospheric pressure, for the first time. Amorphous carbon and iron nanoparticles have been synthesized from Acetylene and Ferrocene/H2, respectively. High generation rates are achieved from the glow discharge at near-ambient temperature (40–80C), and rise with increasing plasma power and precursor concentration. Fairly narrow particle size distributions are measured with a differential mobility analyzer (DMA) and an aerosol electrometer (AEM), and are centered around 30–35 nm for carbon and 20–25 nm for iron. Particle characteristics analyzed by TEM and EDX reveal amorphous carbon and iron nanoparticles. The Fe particles are highly oxidized on exposure to air. Comparison of the mobility and micrograph diameters reveal  that the particles are hardly agglomerated or unagglomerated. This is ascribed to the unipolar charge on particles in the plasma. The generated particle distributions are examined as a function of process parameters.

6. Lee, J.S., Chandrashekar, A., Park, B. M., Overzet, L. J., and Lee, G. S., “Effects of oxygen plasma on optical and electrical characteristics of multiwall carbon nanotubes grown on a four-probe patterned Fe layer,” J. Vac. Sci. Tehnol. B 23, 1013 (2005). Abstract: We report on the fabrication and electrical characterization of aligned multiwall nanotubes (MWNTs) grown on a four-probe patterned catalyst layer. This structure has been designed to directly measure the electrical property of as-grown MWNTs. The temperature-resistance results show that the aligned MWNTs are semiconducting in directions perpendicular to the tube axis and follow the three-dimentional hopping conduction mechanism. Effects of oxygen plasma on the characteristics of the MWNTs are also investigated. Raman spectroscopy results indicate that oxygen plasma treatments can be used to reduce the carbonaceous material in the film. As the exposure time of oxygen plasma increases, the resistance of the aligned MWNTs increases mainly due to the suppression of current conduction through carbonaceous materials. These results suggest that oxygen plasma treatment is effective in improving the film quality of as-grown MWNTs.

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