Nanosheets Might Make Microlasers

Hank Hogan

Researchers at the University of California, Davis, have created planar clusters of CdSe nanocrystals that emit polarized light in the plane of the cluster. The discovery could lead to nanostructures that emit light, such as lasers and LEDs; absorb light, such as photovoltaic cells; and route light, such as waveguides. And they may have other uses as well.

Nanocrystals of CdSe have been used in one-dimensional arrays by embedding them on microcrystal rods, and in three-dimensional arrays, by attaching them to the outside of a nanoshell. Now researchers have linked CdSe crystals and nanosheets of Ca₂Nb₃O₁₀, producing 2-nm-thick structures dotted on both sides with CdSe particles. Courtesy of Frank E. Osterloh, University of California, Davis.

“These structures could also lead to new optical polymer composites with smart optical properties,” said assistant chemistry professor Frank E. Osterloh. An example, he suggested, might be a polymer that changes color depending on the viewing angle.

When excited by the right light source, CdSe nanocrystals emit light at a size-dependent wavelength. The nanocrystals have been used in one-dimensional arrays by embedding them on microcrystal rods and in three-dimensional arrays by attaching them to a nanoshell.

In this work, the group linked CdSe nanocrystals to nanosheets of Ca₂Nb₃O₁₀ through a chemical reaction. The resulting structure consisted of 2-nm-thick nanosheets dotted on both sides with CdSe particles at a density of about 5300 par-ticles per square micron, as verified by transmission electron microscopy and other methods.

The researchers measured the optical characteristics of the attached CdSe nanocrystals with a Jobin-Yvon fluorometer, an Olympus microscope, various excitation filters, dichroic mirrors and long-pass emission filters. They used a mercury lamp for a 330- to 385-nm excitation source and a Hewlett-Packard spectrometer to collect data in the ultraviolet and visible. The emission wavelength and intensity for the attached nanocrystals were the same as for free-floating CdSe nanoparticles, with peaks at 568, 589 and 639 nm that correspond to nanoparticles of 4.8, 5.4 and 6.0 nm in diameter.

The emission was in the plane of the clusters for reasons that are not entirely clear. It may be that the clusters act like dielectric slab waveguides and prevent the light from escaping in other directions. However, that does not explain the phenomena completely.

The scientists are working to understand the optical and physical characteristics of these structures so that they can be cost-effectively synthesized and put to use in microscale lasers and integrated optics. Achieving stimulated emission is a particular goal of theirs, Osterloh said.

Journal of the American Chemical Society, online Oct. 12, 2005, doi:10.1021/ja0541377.