Rings, towers, cages of ZnO A. C. Reber, S. N. Khanna, J. S. Hunjan and M. R. Beltran Eur. Phys. J. D, 43 1-3 (2007) 221-224 Published online: 24 May 2007 DOI: 10.1140/epjd/e2007-00087-7