Generating plasmonic heterostructures by cation exchange and redox reactions of covellite CuS nanocrystals with Au3+ ions

Abstract

We demonstrate the fabrication of various types of heterostructures, including core–shells and dimers. This is achieved by reacting platelet-shaped covellite (CuS) nanocrystals (NCs) with Au³⁺ ions under various reaction conditions: the exposure of CuS NCs to Au³⁺ ions, in the presence or in the absence of ascorbic acid (AA), leads to the formation of CuS@Au core–shell nanostructures; the reaction of CuS NCs with Au³⁺ ions in the presence of oleylamine (OM) leads to the formation of CuS@Au₂S; the presence of both OM and AA leads to the formation of Au/CuS dimers. Depending on which condition is chosen, either cation exchange (CE) between gold and copper ions is predominant (leading to amorphous Au₂S) or the reduction of Au³⁺ leads to the nucleation of metallic Au domains (which are operated by the AA). In the heterostructures achieved by CE, the Au₂S shell is almost entirely amorphous, and can be converted to polycrystalline upon electron beam irradiation. Finally, when both oleylamine and AA are present in the reaction environment, Au/CuS dimers are formed due to the reduction of Au³⁺ to metallic Au domains which nucleate on top of the CuS seeds. The experimental dual plasmonic bands of the CuS@Au core–shells and Au/CuS dimers are in agreement with the theoretical optical simulations. The procedures described here enable the synthesis of core–shell nanostructures with tunable localized surface plasmon resonances (LSPRs) in the near-infrared (NIR) region, and of plasmonic metal/semiconductor heterostructures with LSPRs in both the NIR and the visible regions.