1. Department of Physics, University of Warwick, Coventry, CV4 7AL, U.K.
2. Nanotube Research Center, National Institute of Advanced Industrial Science and Technology (AIST), Higashi 1-1-1, Tsukuba, 305-8565, Japan
3. Department of Chemistry, University of Birmingham, Edgbaston, Birmingham, B15 2TT, U.K.
4. Department of Chemistry, University of Warwick, Coventry, CV4 7AL, U.K.