Effects of I₂ on Cu_{2- x}S Nanoparticles: Enabling Cation Exchange but Complicating Plasmonics

The effects of I₂ oxidation on the behavior of Cu_2xS nanoparticles - their long-term localized-surface plasmon resonance (LSPR), crystal structure, composition, and ability to undergo cation exchange - were explored. I₂ has been known to induce a blue shift in the LSPR of Cu_2xS nanoparticles due to the increase in the hole concentration through the removal of Cu⁺ ions. An unexpected red shift is observed in I₂-exposed Cu_2-xS nanoparticles over the longer times required for cation exchange. Despite this complication of the LSPR behavior, the process of cation exchange from Cu⁺ to Cd²⁺ is modulated by the exposure of the Cu_2xS nanoparticle precursor to elemental iodide, I₂. This is consistent with the promotion of cation exchange through increased vacancy concentration. Treatment with I₂ induces a greater extent of cation exchange than in controls. Increasing I₂ exposure results in a greater extent of exchange and exchange at lower temperatures. The increased mobility of sulfide ions to which the red shift in LSPR is attributed also affects the resultant CdS crystal structure. Zinc-blende CdS, which has an altered anion network, is observed upon increased I₂ exposure or extended cation-exchange reaction times. This work offers insights into the mechanism of cation exchange and the modulation of the LSPR of plasmonic semiconductors.