Increase of Third-Order Nonlinear Optical Activity of PbS Quantum Dots in Zeolite Y by Increasing Cation Size

The third-order nonlinear optical (3NLO) activity of PbS quantum dots (QDs) encapsulated in zeolite Y has been expected to depend sensitively on the countercation of the zeolite host. However, ion exchange of the pristine countercation, H⁺, with other cations has not been possible because the framework decomposes and the QDs aggregate immediately when the PbS QD-incorporating zeolite Y with H⁺ as the countercation is exposed to the atmosphere. We now report that when H⁺ is transformed to NH₄⁺, the framework of PbS QD-containing zeolite Y does not undergo decomposition and the PbS QDs do not undergo aggregation to form larger QDs during the aqueous ion exchange of NH₄⁺ with alkali-metal ions (M_A⁺ = Li, Na⁺, K⁺, Rb⁺). The 3NLO activity of the M_A⁺-exchanged PbS QD-incorporating zeolite Y film increases with increasing size of M_A⁺. The stabilization of the surface-bound exciton by the electron-rich framework oxide and electron-poor cation is proposed to be responsible for the increase. This is the first example of a method for systematically increasing the 3NLO activity of QDs dispersed in a dielectric matrix by systematically changing its properties. These results will serve as a guideline for future research and also promote applications of QD-incorporating zeolites in various fields.