Synthesis of Ultrathin and Thickness-Controlled Cu2-xSe Nanosheets via Cation Exchange.

We demonstrate the use of cation exchange to synthesize ultrathin and thickness-controlled Cu2-xSe nanosheets (NSs) beginning with CdSe NSs. In this manner, extremely thin (i.e., 1.6 nm thickness) Cu2-xSe NSs, beyond which can be made directly, have been obtained. Furthermore, they represent the thinnest NSs produced via cation exchange. Notably, the exchange reaction preserves the starting morphology of the CdSe sheets and also retains their cubic crystal structure. The resulting nonstoichiometric and cubic Cu2-xSe NSs are stable and do not exhibit any signs of Cu or Se oxidation after exposure to air for 2 weeks. Resulting NSs also show the existence of a localized surface plasmon resonance in the infrared due to the presence of copper vacancies. Efforts to isolate intermediates during the cation exchange reaction show that it occurs via a mechanism where entire sheets are rapidly converted into the final product once the exchange reaction commences, precluding the isolation of alloyed species.