Bandgap tunable colloidal Cu-based ternary and quaternary chalcogenide nanosheets via partial cation exchange

Abstract

Copper based ternary and quaternary semiconductor nanostructures are of great interest for the fabrication of low cost photovoltaics. Although well-developed syntheses are available for zero dimensional (0D) nanoparticles, colloidal synthesis of two dimensional (2D) nanosheets remains a big challenge. Here we report, for the first time, a simple and reproducible cation exchange approach for 2D colloidal Cu₂GeSe₃, Cu₂ZnGeSe₄ and their alloyed Cu₂GeS_xSe_3−x, Cu₂ZnGeS_xSe_4−x nanosheets using pre-synthesized Cu_2xSe nanosheets as a template. A mechanism for the formation of Cu_2−xSe nanosheets has been studied in detail. In situ oxidation of Cu⁺ ions to form a CuSe secondary phase facilitates the formation of Cu_2−xSe NSs. The obtained ternary and quaternary nanosheets have average lateral size in micrometers and thickness less than 5 nm. This method is general and can be extended to produce other important ternary semiconductor nanosheets such as CuIn_1−xGa_xSe₂. The optical band gap of these nanosheets is tuned from 1 to 1.48 eV, depending on their composition.