Depending on their size and shape, the electronic properties of carbon nanotubes can be metallic or semiconducting. The problem scientists had faced in using carbon nanotubes as transistors was that all synthetic methods of production yield a mixture of metallic and semi conducting nanotubes which “stick together'' to form ropes or bundles.
This compromises their usefulness because only semiconducting nanotubes can be used as transistors; and when they are stuck together, the metallic nanotubes overpower the semiconducting nanotubes.
Beyond manipulating them individually, a slow and tedious process, there has been no practical way to separate the metallic and semiconducting nanotubes -- a roadblock in using carbon nanotubes to build transistors. The IBM team overcame this problem with "constructive destruction", a technique that allows the scientists to produce only semiconducting carbon nanotubes
where desired and with the electrical properties required to build computer