Intrazeolite PbS Quantum Dots with Nonlinear Properties Observed

Michael A. Greenwood

At Sogang University in Seoul, South Korea, scientists have successfully incorporated lead sulfide quantum dots into zeolite-Y films and found high levels of third-order nonlinear optical activity. The material could be used for optical switches, computers and limiters; saturable absorbers; infrared detectors for medical and military applications; and laser components.

Intrazeolite quantum dots offer several advantages: They are nearly identical in size, usually smaller than 1.5 nm and exist regularly within the crystalline hosts. Measuring their optical properties, however, has not been possible because of the difficulty of preparing ion-exchangeable zeolite films supported on optically transparent substrates such as glass and fused silica. Zeolite-Y films grown on ordinary glass or fused silica readily peel off the substrate during drying and the exchange of sodium ions with other ions. The intrazeolite quantum dots also tend to be expelled from the zeolite pores to the external surface by moisture.

To overcome this, researchers grew zeolite-Y films (with a thickness of 2 μm) on the surface of indium-tin-oxide-coated glass plates. The films remained firmly bonded to the substrates during the drying process and the exchange of sodium ions with lead ions. The formation of PbS quantum dots was carried out by treating the exchanged lead ions with H₂S gas under the anhydrous condition.

The investigators found that the nonlinear refractive and absorption coefficients of the intrazeolite PbS quantum dots were ∼20 to 330 times higher than the highest values obtained from other quantum dot-dielectric matrix systems.

Because there are many types of quantum dots and of zeolites whose pores have varying sizes, shapes, networking and cations, the researchers said that the development of materials with third-order nonlinear optical properties with much higher sensitivities that can operate at various wavelengths is inevitable.

Journal of the American Chemical Society, doi: 10.1021/ja0661966, online Nov. 4, 2006.