Cation Exchange Mediated Synthesis and Tuning of Bimodal Plasmon in Alloyed Ternary Cu₃BiS_3-xSe_x Nanorods

We report a robust methodology for synthesizing monodisperse alloyed Bi₂S_3-xSe_x nanorods (NRs) to tune optical properties with the variation of Se concentration. The intercalation of Cu(I) into the presynthetic Bi₂S_3-xSe_x NRs converted to ternary Cu₃BiS_3-xSe_x NRs via cation exchange process. The transformation was monitored through the formation of core/shell Bi₂S_3-xSe_x/Cu₃BiS_3-xSe_x nanorod heterostructures. Shape anisotropy results in near-infrared bimodal localized surface plasmon resonance (LSPR) in Cu₃BiS_3-xSe_x nanorod over a broad range from 600 to 3500 nm with increase of Se:S ratio. The LSPR sensitivity, defined as the change in the LSPR peak wavelength per unit change in the refractive index (RI) of the medium, was estimated to be 250-350 nm/RI unit, much higher than other copper chalcogenides like Cu₂S/Cu₂Se. A fast photodetector was fabricated by Cu₃BiS_3-xSe_x NRs with high photocurrent gain value (∼125) with Se alloying. The intrinsic Cu vacancies and the effective mass of charge carriers play important roles to manipulate optical and electrical properties of Cu₃BiS_3-xSe_x NRs.