Iron substitution in CdSe nanoparticles: Magnetic and optical properties

Chemically synthesized, thiol capped undoped, and Fe doped CdSe nanoparticles (NPs) have been investigated using a variety of physicochemical techniques. The electron spin resonance spectra exhibit two distinct signals at g similar to 4 and g similar to 2 characteristic of Fe(3+) ions occupying highly asymmetric and nearly symmetric lattice sites, respectively. The room temperature Moumlssbauer studies show a broad asymmetric doublet with isomer shift similar to 0.35 mm/sec, quadrupole splitting similar to 0.76 mm/sec characteristic of high spin ferric ions occupying the Cd(2+) sites with associated changes in local lattice environment. The room temperature photoluminescence spectra show transition from excited state Fe(3+) ((4)T(1), (4)T(2), and (4)E) to the ground-state Fe(3+) ((6)A(1)) and also suggest that photoexcited electrons are preferentially transferred to iron ion induced trapping centers in CdSe nanoparticles. Room-temperature ferromagnetism is also observed in thiol capped Fe doped CdSe nanoparticles. Significant changes in saturation magnetization value with increasing iron concentration have been observed. The origin of room temperature ferromagnetism and significant change in M(s) value are discussed in terms of F-center exchange mechanism (bound magnetic polarons).