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

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.