Controllable transformation from rhombohedral Cu_1.8S nanocrystals to hexagonal CuS clusters: Phase- and composition-dependent plasmonic properties

Because of the rich polymorphs and lower diffusion energy barriers of copper chalcogenide systems, the phase transformation of colloidal Cu _2-xS (0 ≤ x ≤ 1) nanocrystals is critical for understanding their fundamental properties and designing convenient synthetic routes. In this work, high quality digenite Cu_1.8S nanocrystals with rhombohedral structure were synthesized at gram-scale. The as-prepared colloidal nanocrystals undergo an in situ phase transformation from rhombohedral Cu_1.8S nanocrystals to hexagonal CuS clusters upon keeping the resulting colloidal solution for a few days. The observed transformation was explored by a combination of structural and spectroscopic analyses, including powder X-ray diffraction, transmission electron microscopy, energy dispersive spectroscopy, and X-ray photoelectron spectroscopy characterizations. A possible mechanism is proposed and thoroughly discussed. We further determined the evolution of plasmonic absorption spectra during the transformation. The Cu_1.8S nanocrystals and CuS clusters exhibit composition-dependent local surface plasmon resonance absorption (LSPR) in the near-infrared region, which are in good agreement with calculated extinction spectra based on Mie-Drude model. Combined experimental and theoretical analyses demonstrated that both the phase induced dielectric constant change and the composition induced carrier concentration variation account for the spectroscopic evolution.