Magneto-optical measurements of chromophore/semiconductor nanocrystalline superstructures

The magneto-optical properties of chromophore/semiconductor nanocrystalline superstructures were examined using optically detected magnetic resonance (ODMR) and time-resolved photo-luminescence (PL) spectroscopy. The samples consisted of CdS or PbS nanocrystals separated by conjugated organic chains, forming three-dimensional superstructures. The ODMR measurements revealed that the magnetooptical properties of the superstructures are mainly controlled by the individual characteristic of the nanocrystals. The ODMR spectra were compared with simulated curves generated by the diagonalization of a spin Hamiltonian, indicating the existence of the following luminescence events. a) Recombination between electron-hole pairs trapped at a stoichiometric defect (metal or sulfur vacancies) or oxygen adatom sites at the surface of the nanocrystals. These electron-hole pairs showed anistropic g-factors and weak exchange interactions. b) Bound exciton emission, from strongly coupled electron-hole pairs trapped at intrinsic stoichiometric or structural defects at the core of the nanocrystals. The existence of the two overlapping luminescence events is further confirmed by the acceptance of biexponential PL decay processes.