Nanocrystals Outshine Laser Dyes

Dr. James P. Smith

Semiconductor nanocrystals fabricated at Hebrew University produce a bright fluorescence radiation that can be size-tuned to the near-infrared. Chemists Uri Banin and Yun-Wie Cao's method for making the nanocrystals with an indium arsenide core and a shell of either indium phosphide or cadmium selenide was reported in Angewandte Chemie, International Edition (Vol. 38, 1999).

Small crystals of semiconductors, those in the 2- to 20-nm range, are termed nanocrystals, or quantum dots. They have different optical properties from the bulk material. As more atoms are added, the properties of the crystal vary, approaching bulk values. In particular, the bandgap increases with size.

Nanocrystals of semiconductor materials such as silicon, indium arsenide and cadmium sulfide will fluoresce, and, because their bandgaps vary with size, it is possible to size-tune the fluorescence wavelengths. Also, because the quantum yield of the fluorescence is also sensitive to surface conditions, a second, more stable semiconductor material -- cadmium selenide -- is added as a protector for the indium arsenide core. This layer also causes the particles to glow more brightly.

"As the components in semiconductors become very tiny, almost molecular in scale, the techniques used for making the components begin to look more like the techniques used for making molecules," explained professor Paul Alivisatos of the University of California at Berkeley.

"In fact, you can treat these little components as if they were molecules and even put these solid-state devices in places where molecules would otherwise go."

Typically, quantum dots are prepared with physical methods, such as epitaxy or vapor deposition. "This is the first time that colloidal indium arsenide nanocrystals with high quantum yield have been prepared from solution," Alivisatos said.

The most useful application of nanocrystals is as fluorescent tags in biological systems.

"We can attach a nanocrystal to a protein which will transport the tag to a specific cell structure," Alivisatos explained. "Instead of using dyes to decorate different parts of a cell, we can now use different sizes of inorganic nanocrystals which fluoresce different colors. These little crystals are replacing organic dyes."