Appearance of copper d9 defect centres in wide-gap CdSe nanoparticles:: A high-frequency EPR study

When preparing CdSe nanoparticles in polybutadiene-based polymer matrices, quantum confinement in the smaller particles leads to the usual increase in the band-gap energy, with associated, well defined, band-edge luminescence emission. However, for samples whose size is smaller than 3nm, new broad luminescence features appear, with the simultaneous appearance of a new paramagnetic defect centre that has not previously been reported in the literature. This work concentrates on analyzing the electron paramagnetic resonance of the new defect, which is found to be located at a trigonally-distorted tetrahedral site, with a spin S = 1/2 and g-values g(parallel to) = 2.321 +/- 0.006 and g(perpendicular to) = 2.053 +/- 0.006. With the attribution of very poorly resolved hyperfine interaction to a nucleus of spin I = 3/2, the identity of the defect is ascribed to copper in its d(9) state, which would be expected to act as a simple acceptor centre in CdSe. Conclusions are drawn to the effect that the defect normally resides within the valence band for bulk and large particles, only becoming apparent as a defect within the band-gap as the energy increases above 2.6 eV in the smaller particles.