Arrays of interconnect-type carbon nanotubes (CNTs) have been grown over etched silicon substrates. These tubes are grown over trenches ranging from 200-1000 nm in width. Through control of initial parameters such as trench size, catalyst concentration and the initial parameters for the chemical vapour deposition (CVD) of the tubes (gas flow rates, gas flow times and reaction temperature) dense arrays of CNTs spanning the trenches have been formed, with densities in the region of 1.6 interconnects per micron of trench length. High proportions of branch-structure CNTs have been noted within these arrays. The cleaved sections of silicon substrate are simply treated by drop-casting and drying catalyst-containing solution prior to CVD treatment. The density of the resultant arrays can be controlled through the density of the catalyst solution.
Tailored pore size mesoporous silica, incorporating different concentrations of transition metal-based catalysts, has been used as platforms for the growth of carbon nanotubes by the catalytic chemical vapor deposition method. Both compositional surface analysis by EDX/SEM combinatory techniques and thermo gravimetric analysis were employed to characterize the samples prior to CNT growth. The CNTs produced were characterized using Raman Spectroscopy, high resolution SEM and TEM. Raman spectroscopy showed good quality highly graphitic CNTs and indicated the presence of crystalline graphitic carbon, microcrystalline graphite as well as amorphous carbon in the carbon nanotube layer. TEM and HI RES SEM images matched diameters of the carbon nanotubes to the corresponding pores of the matrices. Comparison of the carbon nanotube diameters to porous properties of the mesoporous silica confirmed probable growth from within the pores. The density of the carbon nanotubes was found to be high for higher metal concentrations for the same pore diameters. Fe and Co were confirmed to be better catalysts, compared to Ni, for growth of carbon nanotubes by the catalytic chemical vapour method.
We report the desorption characteristics of H2O, CO2, H2 and O2, and changes in electrical resistance, single walled carbon nanotube (SWNT) mats during vacuum annealing from room temperature to 873 K. H2O desorption at ~503 K coincides with a resistance decrease of ~ 2%, in agreement with theoretical calculations. CO2 and H2 desorption correspond to subsequent decreases in resistance at ~563, and 663 K respectively. Repeated gas exposures, after thermal desorption, result in reversible R-T characteristics, indicating the tunability of average electrical response of SWNT mats. Our results showing the lack of O2 desorption, and increased CO2 desorption after O2 exposure, show that oxygen is strongly chemisorbed to SWNT surfaces, and desorbs primarily as CO2 and CO. These results are of relevance for harnessing SWNT-based nano-structures for potential applications such as hydrogen storage and chemical sensors, where the resistance-temperature characteristics and gas adsorption phenomena are intimately related.