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December 1, 2003
Volume 81, Number 48
CENEAR 81 48 p. 7
ISSN 0009-2347


Novel synthesis yields nanocrystalline PbSe films that lase in near-infrared


Tunable infrared lasers based on nanocrystals may become available soon. Researchers at Los Alamos National Laboratory in New Mexico have shown that amplified spontaneous emission--an important milestone along the road to making practical lasers--can be achieved in the near-IR region using nanocrystalline lead salts. The advance may be especially useful in optical telecommunications, remote sensing, and related applications that depend upon light in that wavelength region.

LASER PROS Los Alamos scientists (from left) Schaller, Petruska, and Klimov.
Chemical flexibility and ease of processing make nanometer-sized semiconductor crystals (quantum dots) ideal candidate materials for numerous applications involving light emission. A number of proposed applications, such as luminescent markers for biological assays and lasers and optical amplifiers for visible light, have been developed or demonstrated to be feasible. But efforts to generate infrared laser light using nanocrystals have been unsuccessful until now. And the lack of success had begun to convince some researchers that the goal was unlikely to be attained because of fundamental limitations.

But now, Los Alamos postdoctoral fellows Richard D. Schaller and Melissa A. Petruska and team leader Victor I. Klimov have demonstrated that a novel synthesis procedure for preparing lead selenide nanocrystals in a titania matrix leads to high-quality optical films capable of generating amplified spontaneous emission in the near-IR region [J. Phys. Chem. B, published online Nov. 21,].

"It's an important advance, especially for technological applications," says Alexander Efros, a staff scientist at the Naval Research Laboratory in Washington, D.C. By adjusting the size of nanocrystals synthetically, scientists can tune the light emitted from quantum dot sources, Efros explains. That type of flexibility should make it possible to adjust the output of future optical devices so that scattering and absorption interferences can be avoided.

Three years ago, the Los Alamos group and their coworkers reported that CdSe nanocrystals could be made to lase in the visible region (C&EN, Oct. 23, 2000, page 78). Motivated by this success, the Los Alamos team and other research groups sought to use nanocrystals of lead salts to produce IR laser light. But despite some indications that lead-based nanocrystals could work in that application, the lasing effect remained elusive.

Two fundamental shortcomings make it difficult to achieve lasing in PbSe. One of the problems is that several of PbSe's lowest electronic states have equivalent (degenerate) energy levels. In addition, excitations that potentially could lead to IR emission can be quenched rapidly by a fast competing process known as Auger recombination, which does not lead to light emission.

"The nonradiative mechanism steals energy from the lasing transition," explains Mark I. Stockman, a physics professor at Georgia State University, Atlanta. Stockman adds that despite these problems, the Los Alamos group has shown via "a direct and convincing demonstration" that lasing is still possible and can be achieved in a practical manner.

Klimov and coworkers credit their synthesis method, which includes colloidal chemistry techniques and amphiphilic polymers, with producing optical films of high enough quality to observe the lasing effect. According to the Los Alamos group, the results indicate that the lack of success previously in developing the required lasing conditions in nanocrystalline lead salts was due to material quality issues, not intrinsic physical limitations.

SHINE ON Irradiating films of lead selenide nanocrystals (yellow spheres) embedded in a matrix made from titania and an amphiphilic polymer triggers lasing action that produces amplified IR light.


Chemical & Engineering News
Copyright © 2003 American Chemical Society

Related Story
Quantum Dot Lasers Coming Soon
[C&EN, Oct. 23, 2000]

Related Link
Tunable Near-Infrared Optical Gain and Amplified Spontaneous Emission Using PbSe Nanocrystals
[J. Phys. Chem. B]
Web Release Date: Nov. 21, 2003
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