摘要
单壁碳纳米管(SWNTs)是一种典型的一维纳米材料，具有与其直径、手性紧密相关的电学、光学以及化学等方面的优异性能，且半导体性SWNTs的带隙与其直径成倒数关系。然而，目前所制备的SWNTs样品中绝大多数是由多种具有不同直径和手性的SWNTs组成的，且难以分离，因此无法保证SWNTs基器件性能的均一性，这已经成为阻碍SWNTs大规模应用于纳电子器件的科学难题之一。此外，当前在利用电弧放电法进行高效率、低成本、大规模制备SWNTs方面还存在诸多问题，尤其表现在催化剂的催化效率低下方面，一方面导致产物中含有大量杂质，需要经过繁琐的后续提纯处理，另一方面导致SWNTs制备效率不高，难以大幅度降低SWNTs的生产成本。针对上述问题，本论文在对电弧放电过程进行深入分析的基础上开展了优化制备工艺、调控SWNTs成核、生长过程、低纯度样品提纯及选择性刻蚀金属性SWNTs等方面的研究工作。
首先对电弧放电过程中各影响因素进行了深入细致的分析，重点研究了催化剂的引入方式、不同阴极尺寸等因素对所制备SWNTs的纯度和产量的影响。实验结果表明，催化剂以金属盐的方式引入时能够显著地改善其在阳极石墨棒的分布均匀性，有利于提高催化效率，大大提高了产物中SWNTs的纯度。适当增加阴极直径有利于改善SWNTs成核、生长环境，提高产物中SWNTs的纯度，通过调控阴极直径（0.8cm～2.5cm）可以实现对SWNTs长度的调控。
提出了通过引入低压反应性气体对SWNTs成核、生长过程进行主动干预来实现调控SWNTs直径分布的思想。在电弧放电过程中，引入的低压反应性气体（CO、CO2和N2O）能够影响SWNTs的成核、生长过程。增加反应性气体在缓冲气体中的含量能够实现抑制小直径SWNTs的形成，而使SWNTs直径分布向大直径SWNTs方向移动。同时，引入少量反应性气体在一定程度上可以提高产物中SWNTs的纯度。
首次利用横向磁场控制磁性催化剂在电弧等离子体中的运动行为；利用电磁场来调控电弧等离子体参数(等离子体密度、电子温度)、带电碳原子簇的运动行为、电子运动方向等。通过改变磁场强度、方向能够控制电弧等离子体的喷射方向，也即控制了催化剂以及带电碳原子簇的运动状态，最终达到对SWNTs成核、生长以及沉积方向的调控。施加横向磁场后，在定向电弧等离子体前、后、左、右等四个区域收集的SWNTs样品具有不同的直径分布，尤其当Fe/Mo作为催化剂时，改变横向磁场强度与方向能够对SWNTs直径分布与定向沉积方向进行更加有效地调控。同时，研究结果表明磁场对SWNTs制备过程的调控主要是通过影响催化剂粒子在非均匀磁场中的尺寸分布来实现的，洛伦兹力改变尽管能够调控SWNTs的直径分布但并未起到决定性作用。另外，首次利用施加横向磁场可诱导SWNTs定向沉积这一特点将非连续单反应室SWNTs的单批次制备产量由10克提高到50克以上。
针对低纯度SWNTs样品提纯这一难题，提出利用分步离心逐级提高SWNTs纯度与湿法氧化相结合的综合方法进行SWNTs提纯处理。具体来讲，首先通过空气氧化去除大部分无定形碳成分；然后将样品均匀分散形成SWNTs-SDS水溶液，然后分别在4500、9000、12000和15000rpm等四种转速下离心分离逐步将杂质沉淀分离出来；接着采用H2O2加热回流的方式氧化去除残余的超细杂质粒子，并通过稀酸酸洗反应掉残余的催化剂粒子，最后获得洁净的SWNTs样品。该提纯方法对SWNTs结构产生的破坏很小，能很好地保留了初始样品中SWNTs直径分布的信息，这为研究直径可控性生长及后续的测试分析提供保障。
利用金属性与半导体性SWNTs在电学性能上的差异，我们首次分别采用Ar、H2、N2和He四种室温等离子体对提纯后的SWNTs进行选择性刻蚀，利用UV-vis-NIR吸收光谱对等离子体刻蚀前后金属性与半导体性SWNTs比例的变化进行表征。结果表明，通过控制等离子体功率、气压和刻蚀时间等参数可实现对金属性SWNTs的选择性刻蚀，而剩下半导体性SWNTs。该方法还适用于其他气体等离子体，尤其是反应性气体等离子体。
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.
引文
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