Nanocrystals are aggregates of anywhere from a few hundred to tens of thousands of atoms that combine into a crystalline form of matter known as a "cluster." Typically a few nanometers in diameter, nanocrystals are larger than molecules but smaller than bulk solids and therefore often exhibit physical and chemical properties somewhere in between. Given that a nanocrystal is virtually all surface and no interior, its properties can vary considerably as the crystal grows in size.
Prior to Alivisatos' research, all non-metal nanocrystals were dot-shaped, meaning they were essentially one-dimensional. No techniques had been reported for making two-dimensional or rod-shaped semiconductor nanocrystals that would also be of uniform size. However, in a landmark paper that appeared in the March 2, 2000 issue of the journal Nature, Alivisatos reported on techniques used to select the size but vary the shapes of the nanocrystals produced. This was hailed as a major breakthrough in nanocrystal fabrication because rod-shaped semiconductor nanocrystals can be stacked to create nano-sized electronic devices.
The rod-shaped nanocrystal research, coupled with earlier work led by Alivisatos in which it was shown that quantum dots or "qdots"–nanometer-sized crystal dots (spheres a few billionths of a meter in size)– made from semiconductors such as cadmium selenide can emit multiple colors of light depending upon the size of the crystal, opened the door to using nanocrystals as fluorescent probes for the study of biological materials, biomedical research tools and aids to diagnosis, and as light-emitting diodes (LEDs). Alivisatos went on to use his techniques to create an entirely new generation of hybrid solar cells that combined nanotechnology with plastic electronics.