Tunable exciton-plasmon coupled resonances with Cu2+/Cu+ substitution in self-assembled CuS nanostructured films

Localized surface plasmon resonance (LSPR) coupled with exciton resonances in semiconductor nanostructures are crucial for extended spectral range of light matter interactions beyond diffraction limits. Here, we report the tunable coupling of exciton-plasmon resonances with Cu2+ substitution for Cu+ in pure covellite phase of CuS nanostructures by chemical bath deposition (CBD) method. The synthesized self-assembled multilayered nanostructures in shape of nanorods and nanoflakes are found to dramatically change their near field coupling resonances with Cu2+ substitution. The near normal incidence reflectance spectra is fitted by Drude-Lorentz oscillator model constrained with Kramers-Kronig relations to obtain various optical parameters. The substitution of Cu2+ ions found to significantly tune the complex refractive index nearby or below unity for wavelengths approaching plasmon frequency and correspondingly develop epsilon near zero (ENZ) modes in permittivity. The absorption spectrum measured with two different methods is used to distinguish the type of resonances in hybrid states. Raman spectra reveal the enhancement in intensity of A1g modes by four times. The carrier concentration and mobility were measured using Hall measurements. X-ray photoelectron spectroscopy (XPS) reveals the variable oxidation states of the constituent elements. This work demonstrates simplistic approach for efficient fabrication of quantum confined nanostructures for improved light-matter interactions.