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

Abstract

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 NH(4)(+), 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 NH(4)(+) 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.

Work-Function Engineering in Lead Sulfide and Cadmium Sulfide Quantum Dots incorporated into Zeolite Y using Ion Exchange

“…This is one of the unique advantages of using zeolites as the hosts for QDs; note that one cannot easily and systematically change the environment of generated QDs when using conventional dielectric matrices. The properties of zeolite frameworks can be changed by changing the counter cations of the framework, as seen in previous reports …”

“…After this complete neutralization, the structure of zeolite Y with incorporated QDs was very stable to moisture even in aqueous solutions, so this reaction allows for the full exchange with other alkali cations of the framework without any disintegration of the framework. We therefore carried out full ion exchanges of NH 4 + with Na + and then of the desired alkali cations, including Li + , Na + , K + , Rb + , and Cs + as indicated in Reaction …”

“…[ 32 ] Building on these results and insights, our group previously improved the 3rd-NLO activity of PbS QDs encapsulated in zeolite Y fi lms markedly, by exchanging cation species and hence changing the electric fi eld strength, framework donor strength, and cation acceptor strength in zeolite frameworks. [ 20 ] As noted, QDs encapsulated into zeolite Y could be expected to be profoundly infl uenced by the environment of the inner pore of the zeolite.One would expect that the electronic and optical characteristics of QDs encapsulated into the zeolite could be tuned by changing the environment of the inner pore of the zeolite. This is one of the unique advantages of using zeolites as the hosts for QDs; note that one cannot easily and systematically change the environment of generated QDs when using conventional dielectric matrices.…”

Synthesis, characterization, and third-order nonlinear optical properties of copper quantum dots embedded in sodium borosilicate glass

“…Over the past two decades, much attention has been focused on the nonlinear optical progresses of low-dimensional materials such as quantum dots and quantum wires [1][2][3]. Quantum dots whose radii are smaller than the bulk exciton Bohr radius constitute a class of materials intermediate between molecular and bulk forms of matter.…”

“…In view of these special properties, fabrication of quantum dots by various methods such as molecular beam epitaxy (MBE) [5], organic synthesis [6] and high temperature melting method [7,8] has been an emerging area of research for different optical, electronic, magnetic, and spectroscopic applications [9]. Therefore, the investigation of quantum dots has attracted enormous theoretical and experimental interest [1,2].…”