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  To our knowledge, controlled doping of CNTs with transi- 
  tion metal atoms has so far not been achieved. It has, how- 
  ever, been found that metal atoms incorporated into the CNT 
  lattice during catalytic growth are afterwards very difﬁcult to 
  remove [30]. Furthermore, it has been shown that CNT vacan- 
  cies, which are needed for the metallic doping, may be formed 
  in a controlled way by irradiation by Ar ions [31]. This sug- 
  gests that metallic doping of CNTs should be possible. 
  
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