Carbon Nanotube Seperation

DWNTs has recently recognized as important CNT family because they are expected to have superior properties than SWNTs and MWNTs. For example, DWNTs demonstrate outstanding properties as SWNTs and show high carbon nanotube yield per catalysts as MWNTs. Due to their higher mechanical stability than SWCT, they are considered as one of the most luminous materials for field electron emission applications. Thin film transistor (TFT) based on SWNTs has showed the lowest sheet resistance and the highest transmittance among these types of CNTs. However, as the fact that the same weight of catalysts produces more DWNTs than SWNTs due to an intensification of activated carbon species is considered, DWNTs seems to be the best candidate for CNT-FTF applications. In addition, DWNTs enable a sophisticated understanding about a fundamental physics of CNT system. Additionally, due to DWNT's unique atomic structure, double layers of graphenes, they demonstrated the possibility of independent dopping or functionalization of inner and outer tube and DWNTs with a semiconducting outer shell and a metallic inner one showed a significant relevance for carbon-nanotube based electronic devices.
Many efforts have been placed on fabricating high-quality DWNTs for last few years. However, unavoidable side products, such as SWNTs, DWNTs and even MWNTs, still exist, and enriching DWNTs by thermal oxidation in air might degrade DWNT's quality. These obstacles prevent us from utilizing DWNTs' advantages into many applications. Therefore, the development of methods to achieve highly pure degree of DWNTs without any degradation of their properties is essential. To overcome these obstacles, high-speed centrifugation in gradient media was introduce to sort CNTs by their number of side walls. In this process, centrifugation is carried out until particles reach their buoyant density. When density of particle become equivalent to that of solution, the velocity of sedimentation is zero and the particles stay at their buoyant density. Iodixanol was used as a density gradient media which forms a gradient under the influence of the centrifugal field. By using this method, DWNTs are separated from other type of CNTs. Fundamental optical properties of solely pure-DWNTs were studied using UV-vis NIR, PL and Raman spectroscopy. SWNTs were successfully extracted from as-grown CNTs and careful (n,m) evaluation was conducted to demonstrate sorting SWNTs by their diameter. Furthermore, TFTs were fabricated to investigate the effect of CNT-type on its conductivity.
After density gradient centrifugation, surfactants were removed from separated CNTs to take TEM images by washing ethanol. As shown in figure 1, CNTs in upper layer region are mostly small diameter of SWNTs. On the other hand, relatively larger DWNTs were major species of the bottom layer region. Buyant density difference between SWNTs and DWNTs results in this separation.



Figure 1: TEM images. Left one is corresponding to the top layer, and the right one is bottom layer.


As shown in figure 2, several separated layers were clearly observed in the different position through density gradient solution where CNT's buoyant density is matched with. The 6th layer (y6) contains bigger diameter of SWNT compared to the 1st layer. In other words, the population of SWNTs whose diameter is smaller than about 0.830 nm has significantly decreased in the 6th layer. On the other hand, 9th, 12th and 14th layers are corresponding to DWNTs layers.



Figure 2: UV-vis NIR of separated layers. y1 corresponds to the first layer and y6 to the sixth one and so forth.




 

Contact Information

Dr. Jie Liu
Department of Chemistry
Duke University
2105 French Family Science Center
Durham, NC, 27708-0354
Tel: (919) 660-1549
Fax: (919)660-1605
Email: j.liu@duke.edu

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For available positions, please contact Dr. Jie Liu at j.liu@duke.edu for more information

 

 

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