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  Fundamental Studies towards (n,m) Controlled CVD Growth of SWCNTs  
Prof. Esko I. Kauppinen:Aalto University School of Science
    We start by reviewing the literature what is known experimentally regarding the chirality i.e. (n,m) as well as chiral angle distributions of SWNTs made via arc discharge, laser and both supported as well as floating catalyst CVD growth. Then we discuss in detail our current understanding of floating catalyst CVD synthesis of SWCNTs from CO using Fe catalyst clusters made via direct evaporation using hot wire generator as well as via thermal decomposition of ferrocene, and with the addition of trace amounts for CO2, ammonia and water vapor. Here both the tube diameter as well as length can be tailored by changing the reactor temperature profile as well as CO2 concentration. Chiral i.e. (n,m) distributions as determined with ED/TEM are biased towards large angles with the maximum population at about 23 degrees. Then we proceed to explore the effect of carbon source gas, by adding C2H4 together with CO and looking at the effect of temperature when producing catalysts via ferrocene decomposition.
  
  To study the effect of Fe catalyst cluster size and concentration in the floating catalyst synthesis, we have developed a novel catalyst particle production method via physical vapor deposition, based on arc discharge between two electrodes i.e. the spark generator. This methods allows to control separately both the catalyst particle size and concentration when fed into the floating catalyst SWCNT synthesis reactor. Recent results show that when reducing catalyst particle gas phase number concentration, the bundle size of the produced tubes also is reduced, and we reduce the tube diameter below 1 nm and narrow the chiral angle distribution towards armchair when reducing synthesis temperature and CO concentration.
  
  To further understand the growth mechanisms, we have carried out parallel studies on SWCNT growth from carbon monoxide (CO) using supported CVD methods, both at ambient CO pressure in the in-situ Raman microscope as well as at 7 mbar pressure inside the dedicated, Cs-corrected environmental TEM (ETEM). When using supported bimetallic Fe-Cu catalysts, narrow chiral distribution SWCNTs were produced at ambient pressure growth. In addition, epitaxial formation of single crystal cobalt (Co) nanoparticles from CoxMg1-xO solid solution reduction (when deposited into MgO via impregnation) in CO enables to grow SWCNTs also with a narrow diameter distribution, and with predominantly (6,5) tubes [1]. (ETEM) studies at reduced pressure reveal that the Co nanoparticles remain in metallic state and their epitaxial contact with MgO support remains coherent during SWCNT root growth process. Interestingly, when depositing Co via atomic layer deposition (ALD) onto MgO surface, we observe tip growth of SWCNTs inside the ETEM at 700℃, with the Co catalyst nanoparticle shape as well as the tube growth direction fluctuating during the growth process.
 
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