title: Superparamagnetism of Cu2Se nanoparticles.
abstract: 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-deltaSe 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.
authors.affiliation: Department of Physics, Sogang University, Seoul 121-742, South Korea.
authors.forename: Soo-Whan
authors.initials: SW
authors.lastname: Kim
authors.forename: Kyu Joon
authors.initials: KJ
authors.lastname: Lee
authors.forename: Myung-Hwa
authors.initials: MH
authors.lastname: Jung
authors.forename: Yan
authors.initials: Y
authors.lastname: Li
authors.forename: Won Seek
authors.initials: WS
authors.lastname: Seo