Direct evidence of Cd vacancies in CdSe nanoparticles: positron annihilation studies S. K. Sharma, K. Sudarshan, P. Maheshwari, D. Dutta, P. K. Pujari, C. P. Shah, M. Kumar and P. Bajaj Eur. Phys. J. B, 82 3-4 (2011) 335-340 Published online: 05 August 2011 DOI: 10.1140/epjb/e2011-20363-y