Evolution of Surface Copper(II) Environments in Cu_2–<i>x</i>Se Nanoparticles

Cu2–xSe nanoparticles are part of a promising class of alternative plasmonic materials where the location, stability, and structural impact of charge carrier generation are crucial to their optoelectronic performance. Here, electron paramagnetic resonance spectroscopy is used to identify the location and dynamics of Cu2+ environments that form upon air-induced oxidation of Cu2–xSe nanoparticles. The results indicate the formation of two distinct Cu2+ environments at or near the surface of the nanoparticle. The first environment can be assigned to Cu2+ bound to oleylamine capping ligands, and the second can be assigned to Cu2+ located in CuO domains. In addition, these experiments indicate that the observed Cu2+ environments are consistent with vacancy formation on the Cu sublattice, which are mobile (i.e., vacancy hopping) at temperatures down to 180 K. Taken together, our results elucidate time scales of air-mediated Cu oxidation in Cu2–xSe nanoparticles, the chemical environments of the resulting Cu2+ species, and the impact of this oxidation on the overall particle crystallographic structure and optoelectronic properties