The problem with CNTs is essentially a quality control issue. We can make them — but aligning them in the proper direction has proven to be difficult. The problem is exacerbated when attempting to build semiconductors. Dope a group of CNTs and you wind up with a set of semiconducting CNTs and a set of metal CNTs — and separating them out from each other is extremely difficult. Past methods have included the use of a centrifuge and a gel-based sorting method that works, but tops out at a purity of 88-94%. While even that was notable, 94% purity is far too low for semiconductor applications at the cutting edge, where impurities of just a few atoms can cause problems.