Doped ZnSe Nanocrystals Offer Alternative to CdSe

Daniel S. Burgess

Scientists at the University of Arkansas and at NN-Labs LLC, both in Fayetteville, Ark., have produced ZnSe nanocrystals doped with copper and with MnSe. They suggest that these alternatives to CdSe nanocrystals offer lower toxicity and higher stability for the development of biomedical labels, LEDs and lasers.

Xiaogang Peng, a professor in the university’s department of chemistry and biochemistry, noted that CdSe has been the material of choice in nanotechnology research and development CdSe nanocrystals are relatively easy to produce, and their emission wavelength is continuously tunable over the visible range by controlling their size. The toxicity of cadmium, however, is a limitation to the application of these structures.

Moreover, he noted, a reabsorption effect attributable to an overlap of absorption and emission bands and to other energy-transfer mechanisms degrades the performance of devices comprising packed CdSe nanocrystals. The material also exhibits thermal quenching of its luminescence, which might affect its suitability for use in LEDs and lasers.

The doped ZnSe nanocrystals are thermally stable, appearing here heated to more than 200 ºC. Courtesy of Xiaogang Peng.

ZnSe nanocrystals are much less toxic, but they do not emit in most of the visible part of the spectrum. Doping the wide-bandgap semiconductor with transition metal ions was considered as a potential solution, but doing so during the nucleation or growth phases of the synthesis process for trillions of nanocrystals in solution had proved difficult, Peng explained.

Decoupling the doping step from one or both of these phases appears to be the solution. The scientists have developed two approaches: nucleation-doping and growth-doping.

In the former, the dopant and host precursors are mixed during nucleation. The growth phase then is performed under slightly altered conditions such that the dopant precursors become inactive and the host overcoats the doped cores.

In the latter, nucleation takes place as in the standard synthesis process, and growth is permitted to begin. After the formation of small host nanocrystals, however, the reaction temperature is reduced to stop the growth phase. The doping step is performed, and host growth is subsequently reinitiated.

At this point, Peng said, the researchers can produce doped ZnSe nanocrystals that emit at between 450 and 600 nm, although work must be done to narrow their emission bands. The structures are environmentally and thermally stable, displaying no substantial quenching of emission upon exposure to air, pyridine, thiols, ultraviolet radiation and temperatures of more than 200 °C. Reabsorption and energy transfer do not appear to be an issue in the doped structures as a result of their relatively large Stokes shift.

The scientists will explore the relationship between the synthesis process and the performance of the doped nanocrystals and will work to develop other hosts and dopants. They also are curious to investigate how the nanocrystals will perform in various devices, Peng said.

Journal of the American Chemical Society, Dec. 21, 2005, pp. 17586-17587.