Cu2Se nanoparticles were synthesized using the standard Schlenk line and glove box techniques, with the hot-injection method. The X-ray diffraction (XRD) analysis showed that the initial nanoparticles were formed in a stoichiometric Cu2Se phase with a cubic structure.
When the nanoparticles are exposed to air, the diffraction peaks shift to higher angles. This suggests that the nanoparticles are changed to a nonstoichiometric Cu2-δSe phase with copper vacancies. The mean size of the nanoparticles was about 7 nm. The magnetic results show
that the initial nanoparticles were diamagnetic, and after 1-week air exposure, they became paramagnetic. This dramatic change from diamagnetic to paramagnetic can be explained by the oxidation of Cu+ into Cu2+ at the nanoparticle surface. In addition, the superparamagnetic
properties were observed to have a blocking temperature of 150 K. The coercive field decreased as the temperature approached the blocking temperature, and eventually vanished above the blocking temperature.
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