Growth Control And Characterizations Of High-quality Single-walled Carbon Nanotubes

As a one-dimension nanomaterial, the superior electronic and opticalproperties of single-walled carbon nanotubes (SWNTs) strongly depend on theirdiameter and chiral angle, and the bandgap energies of semiconducting SWNTsare inversely proportional to their diameters. However, the as-synthesizedSWNTs always come as a mixture of nanotubes with different chiralities,resulting in the heterogeneous performance of SWNT-based devices. Thispresents a major obstacle to many advanced applications of SWNTs. On theother hand, there are still several big challenges for low-cost, large-scalesynthesis of SWNTs by DC arc discharge method, especially for low catalyticefficiency of the existing catalysts. As a result, numerous impurities and SWNTsare produced together, which causes the difficulties in the purification andhigh-cost of SWNT products. To address the above problems, we have carriedout a lot of research work in optimizing synthesis conditions,controlled-synthesis of SWNTs, purification of SWNT samples with low-purity,and selective etching of metallic SWNTs. In the dissertation, many factors affecting SWNT synthesis during arcdischarge process have been analyzed, and we paid more attention to theinvestigation on the adding type of catalysts and the cathode diameters, whichwill influence the purity and yield of SWNT products. The experimental resultsshow that the distribution of catalysts in the anode can be improved significantlywhen the catalysts were added with metal salt, leading to higher catalyticefficiency, which will improve the purity of SWNTs in products. During the arcdischarge process, increasing the cathode diameter improves the nucleation andgrowth conditions of SWNTs, resulting in SWNT products with high purity, andthe length-controlled synthesis has been done through changing the cathodediameters from0.8cm to2.5cm.The active intervention to the nucleation and growth process of SWNTswas introduced by adding low-pressure reactive gases, which is supposed tocontrol the diameter distribution of SWNTs. Low-pressure reactive gases (CO,CO2and N2O) with different contents were mixed with buffer gas during arcdischarge process, and the increasing of the reactive gas results in theenrichment of SWNTs with big diameters. Meanwhile, the purity of SWNTs canbe improved to some extent when a little reactive gas was added.For the first time, transversal magnetic field was applied to change plasmaparameters (plasma density, electron temperature), the motions of catalysts, charged carbon atoms and electrons, which will influence the nucleation, growthprocess and deposition direction of SWNTs due to the change in the direction oforiented arc plasma. After applying a transversal magnetic field to arc plasma,the diameter distribution of SWNTs in different regions will be different witheach other. Especially for Fe/Mo as catalysts, the diameter distribution andselective deposition of SWNTs can be more effectively controlled by changingthe direction and strength of transversal magnetic field. Meanwhile, althoughLorenz forces can influence the diameter distribution of SWNTs, magnetic fieldcontrolled synthesis process of SWNTs is dominated by tailoring the sizedistribution of the catalysts in non-uniform magnetic field. In addition, thesingle batch yield of SWNT products have be increased firstly from10g to50gthrough applying a transversal magnetic field to arc plasma.For low-purity SWNT products, the fractional centrifugation, combinedwith chemical oxidization, was devoted to the purification process. In particular,air oxidization was firstly used to remove most of amorphous carbon, thenSWNT samples were dispersed uniformly into aqueous SDS solution, and theimpurities with different sizes were separated from SWNT-SDS solution usingcentrifugation method with4500,9000,12000and15000rpm, respectively.Finally, the impurity particles with small sizes were removed by H2O2refluxingand acid washing. The diameter distribution of purified SWNTs is consistent with the primary ones due to few destroy of SWNT structures.Based on the differences in electronic properties between metallic andsemiconducting SWNTs, room-temperature plasmas (Ar, H2, N2and He) wereperformed firstly to selectively etch the purified SWNTs, and the ratio changesbetween metallic and semiconducting SWNTs after selective etching werecharacterized by UV-vis-NIR absorption spectra. We find that plasma power, gaspressure and etching time play important roles in selective etching SWNTs. Bycontrolling the plasma parameters, four gas (Ar, N2, H2and He) plasmas can beused to etch preferably metallic SWNTs at room temperature, retaining thesemiconducting SWNTs. Other gas plasmas, especially for reactive gas, shouldalso be suitable for selective etching of SWNTs.