Colloidal Nanoparticles of a Metastable Copper Selenide Phase with Near-Infrared Plasmon Resonance

Copper selenide nanoparticles are important materials with desirable properties for a broad scope of applications. Copper selenides are additionally known to adopt several different crystal structures and compositions. Here, we report the direct solution-phase synthesis of colloidal Cu2–xSe nanoparticles that adopt a structure distinct from other known copper selenide phases. This Cu2–xSe phase was determined to be structurally related to weissite Cu2–xTe, which is a layered compound containing alternating Cu-rich and Cu-deficient layers sandwiching distorted hexagonal layers of chalcogen atoms. When the weissite-like Cu2–xSe nanoparticles were annealed in solution, they converted to the more stable cubic berzelianite phase, indicating that they are metastable. Optical characterization of the weissite-like Cu2–xSe nanoparticles showed a broad plasmon absorption band centered around 1550 nm, and the position of this absorption band shifted only slightly when subjected to reductive and oxidative conditions.