Phase Transformation and Evolution of Localized Surface Plasmon Resonance in Cu_2-xS Thin Films Deposited at 60 °c

Cu_2-xS (0 ≤ x ≤ 1) thin films deposited at low temperatures (<95 °C) have rich polymorphs due to small differences in Gibbs free energy of formation, which is critical for understanding their fundamental properties. In this work, phase transformation from djurleite to covellite is obtained by simple chemical bath deposition (CBD) without using oxidizing/reducing agents. Cu_2-xS thin films synthesized using cetyltrimethylammonium bromide as a surfactant at 60 °C for different deposition time exhibit red shift in optical absorption due to quantum size effect and blue shift in localized surface plasmon resonance in the near-infrared region originating from increased copper vacancy. The surface morphology of the Cu_2-xS thin films is influenced by the surfactant, which in turn alters the optoelectronics properties. The preferential bonding between Cu and S is determined by hydrolysis of thioacetamide to release sulfides (S^2-) and disulfides (S₂^2-) and the kinetics to reduce Cu²⁺ to Cu⁺ leading to mixed phase formation and an increase in surface to volume ratio. Through X-ray photoelectron spectroscopy and X-ray absorption near edge structure analysis, it is confirmed that both Cu²⁺ and S^2- are reduced with time of deposition and form covellite Cu-S₂^2--Cu bonds.