Hmm. A tube is composed of many carbon atoms in a ring. Add one more atom, you increase the circumference by 0.34nm but the radius only by 0.34/2pi=0.054nm. So

Hmm. A tube is composed of many carbon atoms in a ring. Add one more atom, you increase the circumference by 0.34nm but the radius only by 0.34/2pi=0.054nm. So still looks infeasible.