Doping of CdS nanoparticles by Co2+ ions studied by NMR

Nanoparticles of the diluted magnetic semiconductor Cd0.991Co0.009S, with d iameters D between 3.5 and 29.5 nm, were studied by Cd-113 NMR spectroscopy . Two spectral features could be discerned: (1) a strong line corresponding to cadmium atoms that are removed more than four bonds from cobalt ions an d (2) a set of shifted lines resulting from transferred hyperfine (THF) int eractions between the d-electrons of Co2+ and its next nearest (2N) neighbo ring cadmium atoms. Significant changes in the Cd-113 spectrum were observe d as a function of the size of the nanoparticles. More specifically, these changes were attributed to a structural zinc blende-to-wurtzite phase trans ition that occurs around D = 8 nm. The frequency spread and the fine struct ure of the spectra indicate that most of the Co2+ impurities in the crystal s are located at the positions of cadmium sites. These paramagnetic ions ar e distributed homogeneously in the samples and the transferred hyperfine in teractions between the d-electrons of cobalt and the 2N Cd-113 nuclei are o f the same order of magnitude as in bulk samples. Inhomogeneous broadening of the lines in the spectra can be attributed to possible distortions of th e electronic polarization pathways because of surface and local disorder ef fects in the nanocrystals.