CdS/ZnS Nanocrystals Display Blue Lasing
Daniel S. Burgess
Scientists at Massachusetts Institute of Technology in Cambridge have observed room-temperature amplified spontaneous emission tunable between approximately 450 and 500 nm and lasing centered near 480 nm from optically pumped CdS/ZnS composite nanocrystals suspended in a silica film. The work suggests an economical means of fabricating short-wavelength emitters that the investigators propose are much more stable than dye-based systems.
CdS/ZnS composite nanocrystals form the basis of an optically pumped blue laser. This artistic rendering illustrates a CdSe/ZnS whispering gallery mode laser. Image by Preston T. Snee.
According to team members Preston T. Snee and Yinthai Chan, lasers based on semiconductor nanocrystals offer several unique advantages, including narrow gain profiles, ease of tunability and low production costs, a result of recent developments in wet chemical synthesis. Although nanocrystals have the crystalline structure of the bulk material, they display discrete energy levels that are dependent on the size of the crystals. By creating nanocrystals of the appropriate size and by adding capping materials to further tune their behavior, it is possible to obtain lasing at wavelengths anywhere between the blue and near-IR regions of the spectrum.
Snee and Chan explained that optical gain in semiconductor nanocrystals is accomplished by producing two excitons in each crystal. The problem is that interaction among the excitons increases in smaller-diameter crystals that otherwise would emit at shorter wavelengths. This interaction results in Auger recombination, an ultrafast nonradiative effect that had limited gain to longer wavelengths in larger-diameter nanocrystals at lower temperatures -- 540 nm in CdSe at 77 K.
To address this, they said, they turned to CdS, which has a higher bandgap energy than CdSe and therefore enables shorter-wavelength emission from larger-diameter nanocrystals by avoiding Auger recombination. To suppress deep-trap emission, which leads to spectral broadening, they covered the CdS cores with ZnS. The shells also increase the quantum yield of the CdS and enable a higher tolerance to chemical processing.
To fabricate the emitters, the researchers chemically treated the CdS/ZnS nanocrystals so that they were dispersible in ethanol and combined them with a silica precursor to form a sol. They added 20-µm-diameter silica microspheres to the mixture and spin-coated it onto a glass substrate.
Annealing at 100 °C for five minutes and at 200 °C for another two minutes yielded a thin film with an rms surface roughness of 4.6 nm that contained uniformly dispersed nanocrystals in a silica matrix.
A regeneratively amplified Ti:sapphire laser system producing 100-fs pulses of 400-nm light at 1 kHz served as the optical pump for the demonstrations of amplified spontaneous emission and lasing. Snee and Chan noted that the choice of the pump source was limited by the nonradiative Auger relaxation time -- on the order of 100 ps for CdSe nanocrystals. Excitons must be generated in the crystals faster than this to achieve gain. They said, however, that they also have observed room-temperature amplified spontaneous emission in a slab of CdS/ZnS-silica using 16-ns pulses of 355-nm radiation from a tripled Nd:YAG laser operating at 50 Hz, a more economical approach than the Ti:sapphire system.
Four years ago, Victor I. Klimov of Los Alamos National Laboratory in New Mexico told Photonics Spectra that moving to electrical pumping was a significant hurdle in the development of these devices (see "Let's Get Small," July 2001, page 102). Since then, he and his colleagues have described an indirect-injection technique by which electrically pumped quantum wells might generate unbound excitons and transfer them to nearby CdSe/ZnS nanocrystals capped with organic molecules (see "Electrically Driven Nanocrystal Emitters Proposed," Photonics Spectra, July 2004, page 109). Snee and Chan noted that their team is pursuing a different strategy to identify materials that would enable the sufficiently rapid electrical injection of excitons into nanocrystals.
Another avenue of study for the group involves optical nonlinearities associated with CdS/ZnS; specifically, emission involving more than two excitons per nanocrystal. The scientists' previous investigations involving CdSe/ZnS in titania films revealed that it was possible to achieve room-temperature lasing from states higher in energy than the band edge transition. They plan to explore whether the same is true for CdS/ZnS nanocrystals.
MORE FROM PHOTONICS MEDIA